Structure and Properties of Mixed StrontiumâManganese

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Structure and Properties of Mixed Strontium-Manganese Metaphosphate Glasses Ioannis Konidakis,† Christos-Platon E. Varsamis,*,† Efstratios I. Kamitsos,† Doris Mo¨ncke,† and Doris Ehrt‡ Theoretical & Physical Chemistry Institute, National Hellenic Research Foundation, 48 Vassileos Constantinou AVenue, 11635 Athens, Greece, and Otto-Schott-Institut, Friedrich-Schiller-UniVersita¨t, Fraunhoferstr. 6, D-07743, Jena, Germany ReceiVed: February 26, 2010; ReVised Manuscript ReceiVed: April 13, 2010

Structural, optical, and physical properties of glasses prepared by melt reduction in the mixed Sr-Mn metaphosphate system xMnO-(1 - x)SrO-P2O5, 0 e x e 1, have been investigated by vibrational, optical, EPR, and thermal techniques. Mn ions were found mostly in the +2 oxidation state and in sites of octahedral symmetry. Such sites are formed by neighboring Mn-oxygen polyhedra, where the covalent character of Mn-O bonding increases with cation mixing. The phosphate structure was found to consist predominantly of metaphosphate tetrahedral species (Q2) with a minority of pyrophosphate (Q1) and neutral (Q3) phosphate tetrahedra, whose relative abundance changes nonlinearly with MnO content. The symmetric stretching vibration of terminal PO2- units in Q2 species was employed to probe the influence of mixed Sr/Mn environments on phosphate structure, and the results suggested a deviation from the homogeneous distribution of metal cations. This was attributed to the coordination numbers of Sr and Mn ions (i.e., 8 and 6, respectively) which exceed the available number of terminal oxygen atoms per metal ion, M (i.e., 4), and thus require the formation of neighboring M-oxygen polyhedra which are connected by P-O-M-O-M-O-P linkages. Nevertheless, each metal ion was found to form its own M-O site and to retain the identity of its site in both single and mixed cation glasses. While density and molar volume follow a linear decrease with MnO content, glass transition temperature Tg, thermal expansion coefficient, refractive index, and optical dispersion exhibit clear deviations from additivity. The increasing trend of Tg with cation mixing was attributed to a combination of the different cross-linking abilities by P-O-M-O-M-O-P bridges of the Sr and Mn ions with the relative proportion of metaphosphate Q2 units. The composition dependence of optical dispersion, as expressed by the Abbe number, was correlated with the average electronic band gap obtained from refractive-index dispersion data using the Wemple-DiDomenico single oscillator model. While all glasses in the Sr-Mn system were found to exhibit low optical dispersion, cation mixing was shown to increase dispersion because of increased covalency in Mn-O bonding. 1. Introduction Phosphate glasses are technologically important materials due to their attractive optical properties, e.g., low dispersion, high ultraviolet transparency, photoluminescence, laser, and amplifier effects.1-5 Such properties depend strongly on the nature of metal oxides which modify the phosphate structure and on the ratio of metal oxide content to P2O5. Due to their technological importance, phosphate glasses have been the subject of continuous investigations over the years with the purpose of understanding structure-property correlations that would facilitate the design of new materials with advanced properties. Van Wazer6 and Kordes and co-workers7 were the first to set the basis for systematic studies of phosphate glasses. For example, the study of binary Mg- and Zn-phosphate glasses suggested correlations between glass properties and coordination numbers of the metal cation modifiers.7 Even though this model was not supported by subsequent diffraction and spectroscopic studies,8-11 it initiated numerous structural investigations in phosphate glasses. Earlier studies, reviewed in excellent articles by Martin12 and Brow,13 and later works in * Corresponding author. E-mail: [email protected]. Tel.: +302107273826. Fax: +302107273794. † National Hellenic Research Foundation. ‡ Friedrich-Schiller-Universita¨t.

the field relied on the application of infrared and Raman,14-22 solid state NMR,23-27 neutron, and X-ray diffraction28-31 techniques to probe phosphate glass structures. While the majority of studies on phosphate glasses12,13 refers to binary phosphates xM2On-(1 - x)P2O5 (where n is the valence of the metal cation, Mn+), mixed cation phosphate glasses relevant to this work have been investigated to a lesser extent. An early study on mixed alkali metaphosphates, xM2O-(1 - x)M′2O-P2O5 where M2O and M′2O are different network-modifying alkali metal oxides, was reported by Rouse et al.32 in a search for structural origins of the mixed cation effect (MCE). This effect, known also as mixed alkali effect when two different alkali ions coexist in the glass, has been associated with the nonlinear variation of glass properties with composition, x. The MCE is manifested mainly by properties which depend on ionic mobility such as electrical conductivity, ionic diffusion, internal friction, viscosity, and glass transition temperature, while other bulk properties including molar volume, elastic moduli, and refractive index either vary linearly or show small deviations from linearity with cation mixing.33-35 By employing Raman and far-infrared spectroscopy, Rouse et al.32 concluded that alkali ions are homogeneously distributed in the vitreous matrix without altering the basic phosphate structure.

10.1021/jp101750t  2010 American Chemical Society Published on Web 04/28/2010

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They showed also that each alkali ion forms its favorable site and retains this characteristic site in both single and mixed alkali glasses. Similar conclusions were reached by Swenson et al.30,31 in more recent studies of mixed alkali metaphosphate glasses by neutron and X-ray scattering, Raman spectroscopy, and reverse Monte Carlo simulations. They concluded that the short-range order (SRO) structure of the phosphate network is almost independent of alkali cation substitution and that alkali ions tend to preserve their local structural environments regardless of glass composition, i.e., degree of alkali mixing. On these grounds, they suggested that the MCE results from a large energy mismatch for ionic jumps to dissimilar cation sites, in line with the dynamic structure model of Maass and co-workers.36 The existence of distinct alkali cation sites in mixed Li-Na phosphate glasses is supported also by two-dimensional NMR spectroscopy where, in addition, multiple sites for Na ions were detected.37 Studies of mixed alkali and alkali-alkaline earth metaphosphate glasses by Shelby38 showed that the composition dependence of bulk properties is influenced strongly by the nature of the mixed cation pair. Thus, while the electrical conductivity of mixed alkali glasses demonstrates a well-pronounced MCE effect, the same property either varies linearly or exhibits a weak MCE with composition in mixed alkaline earth glasses. The glass density was found to vary also linearly with composition, contrary to the glass transition temperature, Tg, which exhibits negative departures from additivity.38 A similar Tg behavior was reported by Walter et al.39 for mixed alkali-alkaline earth metaphosphate glasses. However, the glasses investigated showed either linear changes or negative deviations from additivity in the composition dependence of molar volume, Vm. The authors discussed these results in terms of spatial changes in medium range order (MRO) structures, which are formed by interconnected PO4 tetrahedral units with MOn polyhedra (M ) alkali or alkaline earth metal). Besides mixed alkaline earth glasses, mixed divalent cation phosphate glasses were investigated in the Ba-Zn40 and Pb-Zn41 systems. Measured Vm and Tg properties in the xZnO-(0.5 - x)BaO-0.5P2O5 system were found to show large negative deviations from linearity, and this effect was attributed to an abrupt change in Zn ion coordination.40 However, the infrared spectra of the same glasses appear to vary in a rather smooth way with substitution of zinc for barium ions. Similarly, the analysis of infrared and Raman spectra of glasses xPbO-(0.6 - x)ZnO-0.4P2O5 showed no significant changes in the nature of the SRO phosphate units with substitution of zinc by lead ions.41 This was not the case though for mixed pyrophosphate glasses, 2xM2O-2(1 - x)ZnO-P2O5 (M ) Li, Na), for which infrared spectroscopy showed that substitution of alkali oxide for ZnO leads to changes in the relative population of the SRO phosphate units.42 Mixed strontium-manganese metaphosphate glasses xMnO-(1 - x)SrO-P2O5, 0 e x e 1, have recently attracted interest because of their photoluminescence properties which make them promising materials for special optical applications.43-45 Considering the complex dependence of physical properties of mixed cation phosphates on glass composition, we have investigated and report here optical and thermal properties of glasses in the Sr-Mn system, including refractive index and optical dispersion, glass transition temperature, thermal expansion coefficient, and density. In addition, the same glasses have been studied by optical, electron paramagnetic resonance, infrared, and Raman techniques to reveal the composition

Konidakis et al. dependence of local structure and to understand the effect of composition on glass properties. 2. Experimental Section Mixed metaphosphate glasses xMnO-(1 - x)SrO-P2O5 were prepared from appropriate amounts of high purity raw materials, Sr(PO3)2, MnCO3, and dry crystalline P2O5, which were thoroughly mixed and melted in covered high purity silica crucibles. Small amounts of sugar were added in the melts as a reducing agent to maintain manganese in the Mn2+ oxidation state. Glasses were prepared in batches of 100 g which were melted in a resistant heated furnace at 1300 °C. The melts were quenched by pouring into preheated graphite molds at 550 °C, and the glasses were subsequently annealed from 550 °C to room temperature at a rate 3-5 °C/min. All measurements were performed on annealed glass samples. To characterize the optical performance of the prepared glasses, their dispersion was measured using the reciprocal dispersive power or Abbe number, νe, defined by

Ve )

ne - 1 nF′ - nC′

(1)

where ne, nF′, and nC′ are the refractive indices at the mercury green line (546.07 nm) and at the cadmium blue (479.99 nm) and red (643.85 nm) lines, respectively. Refractive indices were measured on a Pulfrich refractometer with an error of (2 × 10-5. Optical spectra of glasses were recorded in the 190-3200 nm range on a conventional double beam UV-vis spectrometer using polished glass plates 1-2 mm thick. The density, d, was measured by the Archimedes method with an accuracy of (0.005 g/cm3, and the glass molar volume, Vm, was determined from the relation Vm ) MW/d where MW is the glass molecular weight corresponding to the formula xMnO-(1 - x)SrO-P2O5. Glass transition temperature, Tg, was measured by differential thermal analysis at 10 K/min with accuracy (2 °C. Thermal expansion coefficient, TEC, measurements were performed in the temperature range 100-300 °C on a conventional dilatometer. The heating rate was 5 K/min, and the accuracy of TEC values is (2 × 10-7 K-1. Electron paramagnetic resonance (EPR) spectra were recorded at an X-field frequency band of ν ∼ 9.8 GHz, and the spin standard diphenylpicrylhydrazyl (dpph) was added for spectral calibration. Raman spectra were collected on a micro-Raman spectrometer using for excitation the 488 nm line of an argon ion laser. Infrared (IR) measurements were performed on polished glass samples using a vacuum Fourier transform spectrometer. The IR spectra were measured in the reflectance mode at quasinormal incidence (∼11°) in the range 30-7000 cm-1 with 2 cm-1 resolution, and each spectrum represents the average of 200 scans. The reflectance spectra were analyzed by Kramers-Kronig transformation as detailed elsewhere,46,47 to yield the absorption coefficient spectra, R(ν), from the expression R(ν) ) 4πνk(ν) where k(ν) is the imaginary part of the complex refractive index and ν is the infrared frequency in inverse centimeters. 3. Results 3.1. Oxidation State and Coordination Number of Mn Ions in Glass. 3.1.1. Optical Spectra. Previous studies on Mncontaining glasses showed that Mn ions are present in the +2 or +3 oxidation states.48-54 The presence of Mn3+ ions results in colored glasses, with color ranging from light to dark purple

Properties of Mixed Sr-Mn Metaphosphate Glasses

Figure 1. Optical spectra of xMnO-(1 - x)SrO-P2O5 glasses prepared without melt reduction for x ) 0, 0.3, 0.5, and 1 (solid lines) and with melt reduction for x ) 0.5 (dash dotted line). The arrow denotes the shift of the charge-transfer band.

depending on the Mn3+ content. This color is due to the 530 nm absorption band in the visible range attributed to Mn3+ ions. Therefore, prior to further studies in the Sr-Mn metaphosphate system it is important to estimate the amount of Mn3+ present in the glass matrix. In Figure 1, the optical spectra of glasses prepared without melt reduction for x ) 0, 0.3, 0.5, and 1 are compared with that of the x ) 0.5 glass developed by melt reduction. While the Mn-free glass (x ) 0) shows no absorption in the visible range, introduction of MnO leads to development of absorption bands at 410 and 530 nm which result from d-d electronic transitions of Mn2+ (d5) and Mn3+ (d4) ions, respectively.48-59 Also, the glass absorption edge shifts gradually to the visible range with increasing MnO content as a result of very intense charge-transfer excitations from oxygen to Mn2+ and Mn3+ ions and of the intervalence charge-transfer excitations between Mn2+ and Mn3+ ions at high manganese contents.48,49,52 Comparison in Figure 1 of the optical spectrum of glass x ) 0.5 prepared without reduction with those obtained under reducing conditions shows weakening of the 530 nm Mn3+ band and shifting of the absorption edge to the ultraviolet range in the reduced sample. The intensity of the 530 nm band can be used to estimate the Mn3+ content in glass according to the Lambert-Beer law, Α(λ)t ) ε(λ)c, where A(λ) and ε(λ) denote the absorbance and molar extinction coefficient at wavelength λ, respectively; t is the thickness of the glass plate; and c is the concentration of Mn3+ ions. For the molar extinction coefficient of the 530 nm transition, we used ε(Mn3+) ) 25 L mol-1 cm-1, this value being established as a lower limit in phosphate glasses.54 The Mn3+ content is given in Table 1 for the reduced glasses investigated here, and it is found to vary between 0.07 and 0.75% of the total Mn content. It should be noted that even for the binary Mn-metaphosphate glass (x ) 1) melted under nonreduced conditions (which produces a dark purple color) the Mn3+ concentration does not exceed 1.5% of the total Mn content. Our findings are consistent with the results of analytical titration for the more basic xMnO-(1 - x)NaPO3 glass system where the Mn3+content did not exceed 3 mol %, even though Mn cations were introduced in the trivalent state as Mn2O3.60 Having obtained the Mn3+ content, we can calculate the molar extinction coefficient for the 410 nm absorption band of Mn2+

J. Phys. Chem. C, Vol. 114, No. 19, 2010 9127 ions. The obtained value is ε(Mn2+) ) 0.1 ((0.02) L mol-1 cm-1, which is within the reported range of 0.04-0.4 L mol-1 cm-1.49,55,57 The fact that our result for ε(Mn2+) is closer to the minimum reported value indicates that Mn2+ ions in Sr-Mn glasses are present predominantly in octahedral coordination. Indeed, the selection rules for centrosymmetric octahedral coordination predict weak electronic transitions in line with the weak absorption bands observed in Figure 1. On the contrary, in the noncentrosymmetric tetrahedral coordination the Laporte rule can be relaxed by mixing metal 3d with metal 4p or ligand p orbitals, resulting in more intense absorption bands. The optical spectra were deconvoluted into Gaussian component bands to explore further the coordination environments assumed by Mn ions in Sr-Mn glasses. A typical example of deconvolution is demonstrated in Figure 2 for the melt-reduced glass with composition x ) 0.5. In general, all d-d excitations of transition metal ions are of low intensity since they are parity forbidden by the Laporte rule, which allows transitions whenever ∆1 ) ( 1, where 1 is the orbital angular momentum quantum number.48-52 The d5 Mn2+ ion has the ground level configuration 6 A1g(6S) in both tetrahedral and octahedral coordination, while transitions to the excited states, either quartet or doublets, are spin forbidden by the equal multiplicity selection rule which allows transitions with ∆S ) 0. In that sense, the doubly spin forbidden doublet transitions can be omitted due to their negligible intensity. On the other hand, the d4 Mn3+ ion has the ground level configuration 5D which, in an octahedral field, splits into 5Eg and 5T2g states. These two states split further by Jahn-Teller distortion, i.e., 5Eg into 5B1g and 5A1g and 5T2g into 5 B2g and 5Eg. This additional splitting gives rise to three spin allowed transitions with the 5B1gf5B2g transition being the strongest one at 530 nm.48-50,52-57 On these grounds, the excitations of Mn3+ ions are expected to give much stronger bands in the optical spectra compared to those of Mn2+ ions. Along these lines, the molar extinction coefficient ε(Mn2+) for Mn2+ transitions is much lower, i.e., 0.04-0.4 L mol-1 cm-1,49,55,57 compared to the ε(Mn3+) which varies from 20 to 180 L mol-1 cm-1.48-53,56 The assignment of component bands resolved in Figure 2 is summarized in Table 2. From the positions of the absorption component bands and their best fit in the Tanabe-Sugano diagram,48 the magnitudes of the ligand field splitting Dq and the Racah interelectronic repulsion parameter B can be obtained. The calculated values Dq ) 845 cm-1 and B ) 756 cm-1 are in close agreement with previously reported values for mixed Ba-Mn phosphate glasses.59 It is noted that Dq values between 800 and 1000 cm-1 are typical for octahedral coordination of divalent cations, while Dq for tetrahedral coordination varies from 350 to 500 cm-1.48,55 These findings show that Mn2+ ions are mostly in octahedral coordination in the studied Sr-Mn metaphosphate glasses. In addition, previous fluorescence measurements on these glasses showed emission maxima in the orange to red range, i.e., 590-680 nm, which are typical for octahedral coordination,43-45,58 while green emission indicates Mn2+ ions in tetrahedral coordination. In conclusion, the analysis of the optical spectra of the reduced Sr-Mn metaphosphate glasses shows that Mn ions are predominantly in the +2 oxidation state and occupy octahedral sites in the glass matrix. 3.1.2. EPR Spectra. The EPR spectra shown in Figure 3 for glasses xMnO-(1 - x)SrO-P2O5 are characterized by a strong signal centered close to the free electron value of g ) 2.0023 for Mn2+ ions in nearly perfect octahedral symmetry.61,62 It is noted that the EPR spectra of Sr-Mn glasses at room temperature are sensitive only to the bonding environment of the

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TABLE 1: Effect of Cation Substitution on Properties of Glasses xMnO-(1 - x)SrO-P2O5, Refractive Indices (nF′, ne, nC′), Abbe Number (νe), Density (d), Molar Volume (Vm), Glass Transition Temperature (Tg), Thermal Expansion Coefficient (TEC), Relative Content in Trivalent Manganese, Mn3+, and Total Mn Content glass property

x)0

x ) 0.1

x ) 0.3

x ) 0.5

x ) 0.7

x ) 0.9

x)1

color nF′ (478 nm), (2 × 10-5 ne (546 nm), (2 × 10-5 nC′ (644 nm), (2 × 10-5 νe, (0.5 d (g/cm3), (0.005 Vm (cm3/mol), (0.2 Tg (°C), (2 TEC (10-7 K-1), (2 [Mn3+]/([Mn3+]+[Mn2+]) (%) Mn content (1021 ions/cm3)

colorless 1.56342 1.55913 1.55501 66.5 3.18 77.2 485 130 0 0

weak yellow 1.56789 1.56352 1.55931 65.7 3.15 76.9 500 117 0.44 0.8

yellow 1.57420 1.56964 1.56526 63.7 3.09 76.3 490 111 0.49 2.4

yellow 1.57970 1.57501 1.57050 62.5 3.08 74.4 480 112 0.07 4.0

yellow 1.57872 1.57394 1.56937 61.4 3.00 74.2 477 97 0.13 5.7

yellow orange 1.58211 1.57721 1.57262 60.8 2.95 73.2 475 97 0.75 7.4

yellow orange 1.58246 1.57702 1.57300 61.0 2.92 72.9 469 115 0.04 8.3

to-peak (App) or trough-to-trough (Att) as indicated in Figure 3. This increase is caused by the strain broadening of the individual hyperfine lines. Nevertheless, a constant value of 9 mT is found for all lines in the x ) 0.001 and 0.01 spectra when the average distance between the lines is calculated by averaging the differences of peak-to peak (∆pp) and trough-to-trough (∆tt) distances64

Aav )

Figure 2. Deconvolution of the optical spectrum (solid line) of the 0.5MnO-0.5SrO-P2O5 glass prepared by melt reduction into Gaussian bands. Component bands are shown by solid lines and the simulated spectrum by open circles. Bands labeled 1-8 refer to Mn2+ d-d transitions, and those denoted A-D correspond to Mn3+ d-d transitions. For band assignments, see text and Table 2.

paramagnetic Mn2+ ions. The observed EPR spectra consist of two convoluted signals: one broad response from Mn2+ ions involved in dipole-dipole type interactions and a six-line pattern obtained for the two glass samples having the lowest manganese content. The weak features seen for very low Mn contents at g ) 3.3 and at g ) 4.3 can be associated with Mn2+ ions in distorted sites,62 indicating that Mn ions in Sr-Mn glasses occupy predominantly undistorted octahedral sites in agreement with optical spectroscopy. The main resonance at g ≈ 2.0 is due to the allowed M 1/2 T -1/2 magnetic dipolar transitions (∆mS ) (1, ∆mI ) (0) from the 6S5/2 ground state of Mn2+, while the hyperfine splitting (hfs) of 2I + 1 ) 6 lines arises from the interaction between the spin of the unpaired 3d5 electrons (S ) 5/2) and the spin of the 55Mn nucleus, I(55Mn, 100%) ) 5/2. The satellites observed in the six-line pattern of the x ) 0.001 spectrum are due to “forbidden” transitions for the simultaneous change of the electronic and nuclear spin (∆mS ) (1, ∆mI ) ( 1).48,61-64 The hfs-sextet is a sign of isolated Mn2+ ions in a highly ordered environment close to the octahedral symmetry. The measured hyperfine splitting increases from 7 to 12 mT with increasing magnetic field and differs when measured either peak-

[

∆Mpp + ∆Mtt ∆Ipp + ∆Itt 1 ∆Opp + ∆Ott + + 6 5 3 1

]

(2)

where ∆O, ∆M, and ∆I refer to the differences of the outer (first and sixth), middle (second and fifth), and inner (third and fourth) lines, respectively. The value of 9 mT is indicative of an ionic environment for Mn2+ ions in glasses with x ) 0.001 and 0.01, which is also consistent with the measured negative shifts of ghfs values compared to the electron value of g ) 2.0023.63 While for x ) 0.001 and 0.01 the hfs-sextet is superimposed on a singlet centered at g ≈ 2.00, which is responsible for the observed downward slope of the sextet, for higher MnO contents, i.e., x > 0.01, the hyperfine structure merges into one broad resonance due to the increased dipolar interactions between the Mn2+ ions. The line width of the resonance depends on the dipole-dipole, exchange, and spin-orbit interactions, as well as on the distribution of the slightly distorted octahedral sites in glasses. Inspection of Figure 3 shows that the line width of the resonance at g ≈ 2.00 increases drastically from 50 (x ) 0.001) to 100 mT at x ) 1. This extensive broadening for x > 0.5 cannot be explained by considering only the interaction mechanisms; instead, it can be interpreted as indicating the formation of neighboring Mn-oxygen polyhedra in the glass matrix. This is consistent with the EPR results of Menzel et al.65 for Mn-doped BaCl2 crystals, where the broadening and the shift of the g ≈ 2.00 resonance was correlated with the formation of Mn-Cl clusters upon increasing Mn content. Besides the broadening of the g ≈ 2.00 central peak with MnO content, there is a shift of this EPR peak toward lower magnetic field. Since the g value is inversely proportional to the magnetic field, it follows that g increases with increasing MnO content. The dependence of g on the MnO content is illustrated in the inset of Figure 3 and manifests a positive deviation from additivity upon Sr/Mn mixing. This trend is indicative of an increased degree of covalent character in Mn2+-O bonding in mixed Sr-Mn glasses.63 3.2. Physical Properties. Having determined that manganese ions are present mainly in the Mn2+ oxidation state and, thus,


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TABLE 2: Assignments of Bands Deconvoluted in the Optical Spectrum of Glass 0.5SrO-0.5MnO-P2O5 Prepared by Melt Reduction (Figure 2)a band

assignment

observed (cm-1)

calculated (cm-1)

ref. values (cm-1)

18 435 22 121

18 870b 23 120 24 960 25 275 27 980 29 750 32 960 40 810

Mn2+ 1 2 3 4 5 6 7 8

6

A1g(S)f4T1g(4G) 6 A1g(S)f4T2g(4G) 6 A1g(S)f4A1g(4G) 6 A1g(S)f4Eg(4G) 6 A1g(S)f4T2g(4D) 6 A1g(S)f4Eg(4D) 6 A1g(S)f4T1g(P) 6 A1g(S)f4A2g(F)

18 467 22 890 23 993 24 487 27 782 29 079 30 506 32 051

A B C D

5

16 668 19 724 21 326 26 326

24 493 27 885 29 785 30 005 33 907 Mn3+

B1gf5A1g 5 B1gf5B2g 5 B1gf5Eg 5 B1gf5Egc

16 000 19 300 24 000

15 500 18 500 23 500c

a Calculated frequency values refer to the Tanabe-Sugano diagram using the values Dq ) 845 cm-1 and B ) 756 cm-1. Calculated frequency values for Mn3+ correspond to a tetragonal ligand field with ∆ ) 14 000 cm-1, B ) 780 cm-1, C/B ) 4.6, δ1 ) 16 000 cm-1, δ1/δ2 ) 2.5, and Dq ) 1880 cm-1, as stated by Ko¨hler et al.57 b From Heidt et al.55 for Mn(H2O)6, Dq ) 848 cm-1, B ) 671 cm-1. c From Ko¨hler et al.57 for CsMnF4 with ∆ ) 13 400 cm-1, B ) 780 cm-1, δ1 ) 3800 cm-1, and δ2 ) 1300 cm-1; for SrMnF5 eg splits even further into a1 and b1.

Figure 3. EPR spectra of selected xMnO-(1 - x)SrO-P2O5 glasses. The inset shows the variation of the g ≈ 2 value with the MnO content.

that Mn3+ ions can be practically ignored in further discussions of Sr-Mn glasses, we report measured physical properties in Table 1 and show their dependence on the MnO mole fraction in Figure 4. It is clear that glass transition temperature and thermal expansion coefficient vary nonlinearly with MnO content, the fist having a positive and the second a negative departure from linearity. To the best of our knowledge, these Sr-Mn glasses constitute the first known mixed cation phosphate system exhibiting a positive deviation from additivity in the composition dependence of Tg. On the contrary, density and molar volume vary linearly with x as found in many mixed cation glasses.33-35 Their decrease upon increasing MnO content is consistent with the gradual replacement of strontium ions by the lighter and smaller manganese ions. The optical properties show well-defined deviations from additivity, negative for the Abbe number, νe, and positive for the refractive index, ne. It can be easily verified from the values

Figure 4. Physical properties of xMnO-(1 - x)SrO-P2O5 glasses as a function of MnO content: refractive index, ne, Abbe number, νe, density, d, molar volume, Vm, glass transition temperature, Tg, and thermal expansion coefficient, TEC. The experimental error is within the size of the symbol unless error bars are added, while the lines are drawn to guide the eye.

reported in Table 1 that this inverse relationship between Abbe number and refractive index holds also for refractive indices nF′ and nC′. 3.3. Infrared Spectroscopy. The measured reflectance, R(ν), and the calculated absorption coefficient, R(V), spectra of mixed Sr-Mn metaphosphate glasses are shown in Figures 5 and 6, respectively. Both R(ν) and R(V) spectra exhibit complex spectral envelopes in the entire frequency range. Differences in shape and peak frequencies between reflectance and absorption coefficient spectra are due to the fact that R(ν) depends on both real, n(ν), and imaginary, k(ν), parts of the complex refractive index of glass, whereas R(ν) is a function only of k(ν). Henceforth, peak frequencies discussed here will be those of the absorption coefficient spectra to allow comparison with reported infrared spectra.

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Figure 7. Schematic representation of Qi phosphate tetrahedral units in phosphate glasses.

Figure 5. Reflectance spectra of xMnO-(1 - x)SrO-P2O5 glasses. Spectra have been offset by 0.1 to allow comparison.

Figure 6. Calculated absorption coefficient spectra, R(ν), of xMnO-(1 - x)SrO-P2O5 glasses. Spectra have been offset by 5000 cm-1 to allow comparison.

Bands in the midinfrared region, i.e., above ca. 300 cm-1, originate mainly from stretching and bending vibrations of structural arrangements built up by the four phosphate tetrahedral units shown schematically in Figure 7.13 Usually, the phosphate tetrahedral units are labeled using the Qi terminology where i is the number of bridging oxygen atoms per phosphate tetrahedron. For the metaphosphate composition, M(PO3)n where n is the valence of the metal cation, previous studies12-32 have revealed the formation of polymer-like glass networks formed by linked phosphate tetrahedral units with the majority of them being of the Q2 type, i.e., (PØ2O2)- with Ø and O denoting bridging and terminal oxygen atoms, respectively. This seems to be the case also for Sr-Mn glasses since their most intense infrared band appearing at 1275 cm-1 (Figure 6) originates from the asymmetric stretching vibration of terminal PO2- groups in Q2 units, νas(PO2-),12-22,66,67 without excluding some contribution

in this range from the stretching of PdO double bonds, ν(PdO), in Q3 units.18-20,22 For x ) 0, the 850-1200 cm-1 envelope shows three distinct features at 910, 1012, and 1095 cm-1. The bands at 910 and 1012 cm-1 can be attributed to asymmetric stretching modes of P-O-P bridges, νas(P-O-P), connecting Q2 units in chain and ring formations, respectively.17,22,68 The third peak at 1095 cm-1 corresponds to the asymmetric stretching modes of nonbridging oxygen atoms of Q1 type units, νas(PO32-), terminating metaphosphate chains.15-22,68 Besides these bands, a shoulder appears at ca. 1160 cm-1, and it can be attributed12-22,66,67 to the weak infrared activation of the symmetric stretching mode of the PO2- group in Q2 entities, νs(PO2-). As will be presented in the following section, this is the strongest mode of metaphosphate sites, Q2, active in the Raman spectrum. The weak features at ca. 705 and 780 cm-1 may result from coupling of the symmetric stretch of P-O-P bridges, νs(P-O-P), with the bending motion of bridging oxygen, whereas the stronger envelope in the 400-600 cm-1 range results mainly from the bending motion of bridging oxygen atoms.66 As observed in Figure 6, the midinfrared spectra are influenced by glass composition. The most apparent changes include the enhancement of the Q2 band at 1275 cm-1 relative to the Q1 feature at 1095 cm-1, the broadening and downshift of the Q1 band to 1082 cm-1, and the upshift of the 910 cm-1 band to 930 cm-1 as x increases to x ) 1. These spectral changes indicate a progressive variation of the SRO phosphate structure with increasing MnO content, and they will be discussed in a following section. A relatively weak band envelope is present below 300 cm-1 in the far-infrared region. For x ) 0, the band peaks at ca. 180 cm-1 and shifts progressively to ca. 240 cm-1 for x ) 1. Previous farinfrared studies of metal cation-containing phosphate,14-16,22 borate,17,46,69 and germanate70 glasses have shown that the vibration of metal ions against their oxygen cage (site) is active in the far IR region. The frequency of such cation-motion modes, -1/2 where mM is the mass of νM-O, was found to scale with mM the metal cation. Thus, the 180 cm-1 band (x ) 0) can be assigned to νSr-O and the one at 240 cm-1 (x ) 1) to νMn-O. Earlier transmission measurements by Exarhos and co-workers15,16 on metaphosphate glasses prepared as thin films have identified the cation-motion bands for Sr2+ and Mn2+ ions at 169 and 232 cm-1, respectively. Figure 6 shows that as x increases νM-O shifts progressively to higher frequencies, and the band becomes broader until it reaches the characteristics of the Mn-O band in the binary x ) 1 glass.


Figure 8. Raman spectra of xMnO-(1 - x)SrO-P2O5 glasses.

Figure 9. Composition dependence of peak frequency (a) and full bandwidth, fwhm (b), for the symmetric stretching mode of the PO2group, νs(PO2-), as determined from the Raman spectra of xMnO-(1 - x)SrO-P2O5 glasses. The experimental error is within the size of symbols. Solid lines denote a linear composition dependence, and the dash-dot line is a guide to the eye.

3.4. Raman Spectroscopy. The Raman spectra of Sr-Mn metaphosphate glasses (Figure 8) appear simpler than their corresponding IR spectra in Figure 6. All Raman spectra are dominated by a strong band situated at ca. 1175 cm-1, which is assigned to the symmetric stretching vibration of terminal PO2groups in Q2 units, νs(PO2-).12-22,66,67 The frequency of this band increases gradually from 1167 to 1181 cm-1 as Sr is substituted by Mn, and it manifests also a negative departure from additivity as shown in Figure 9a. The relatively small width of this band is consistent with the out-of-chain, i.e., localized, nature of this vibration.66 Cation mixing in the Sr-Mn system was found to affect the width at half-maximum of the νs(PO2-) mode in a rather linear fashion as shown in Figure 9b. The second strongest Raman band appears at 686 cm-1 for x ) 0 and shifts to 694 cm-1 for x ) 1. This band is due to the symmetric stretching vibration of P-O-P bridges, Vs(P-O-P), resulting in the bending motion of the bridging oxygen.12-22,66,67 Bending of

J. Phys. Chem. C, Vol. 114, No. 19, 2010 9131 PO2- groups and in-chain or ring O-P-O bending motions give rise to weak bands at ca. 384 and 315 cm-1, respectively.68 Besides the above bands, very weak features are observed at ca. 1080 and 1015 cm-1. In line with the IR spectra, these features can be associated with Q1 type terminal units of metaphosphate chains, i.e., 1080 cm-1 to νas(PO32-) and 1015 cm-1 to νs(PO32-). The broad bands at ca. 1260 cm-1 (x ) 0) and 1250 cm-1 (x ) 1) could be associated with the feature calculated at 1260 cm-1 which results from the coupling of stretching motions of terminal (out-of-chain) and bridging (inchain) oxygen atoms in metaphosphate units.66 The weak highfrequency shoulders at ca. 1305 cm-1 (x ) 0) and 1325 cm-1 (x ) 1) can be attributed to the stretching of PdO bonds, ν(PdO), in Q3 units.18-21 The presence of small amounts of doubly charged Q1 units requests that neutral Q3 units are also part of the phosphate structure for reasons of mass balance and charge neutrality. In pure vitreous P2O5, which contains only Q3 units, the ν(PdO) mode was measured at 1380 cm-1 in both Raman and infrared spectra.18-20 Differences in ν(PdO) frequency may arise from local bonding variations in Q3 environments between V-P2O5 and Sr-Mn glasses, i.e., different connections of Q3 units with their neighboring phosphate tetrahedra. Indeed, the Raman spectra of xZnO-(1 - x)P2O5 glasses demonstrate a shift of ν(PdO) from 1380 cm-1 for x ) 0 to 1330 cm-1 for x ) 0.5 (metaphosphate).18 Shifts of ν(PdO) as large as 130 cm-1 have been reported also for Li- and Na-phosphate glasses and have been attributed to π-bond delocalization on Q3 units that effectively elongate the PdO bond with increasing alkali content.21 4. Discussion 4.1. Short-Range Order Structure. The consideration of infrared and Raman spectra of Sr-Mn glasses has shown that Q2 species are the majority SRO units which build up a phosphate network in chain and ring arrangements. Replacement of strontium by manganese ions was found to affect mainly the symmetric stretching mode of the terminal PO2- unit in Q2 tetrahedra, νs(PO2-), with only one band being measured for νs(PO2-) in the entire composition range, 0 e x e 1. Similar findings were reported by Rouse et al.32 and Swenson et al.30 for mixed alkali metaphosphate glasses and by Nelson and Exarhos16 for mixed alkali-alkaline earth metaphosphates. In these earlier studies, the peak frequency of νs(PO2-) was found to vary nearly linearly with cation mixing.15,16 Considering the localized nature of the νs(PO2-) mode, this result was interpreted to indicate that cations are homogeneously distributed, i.e., that there is no cation clustering at the molecular level. Therefore, the negatively charged PO2- units are associated with an average cation environment whose effect on the νs(PO2-) mode varies linearly with x.15,16,30 The present Raman spectra show also that Q2 tetrahedra are influenced by a mixed Sr/Mn cation environment; however, the interactions of such environments with PO2units vary nonlinearly with MnO content (see Figure 9a). This trend indicates a deviation from the homogeneous distribution of cations, and this is consistent with the Mn2+ EPR results, which suggest the formation of neighboring Mn-oxygen polyhedra in Sr-Mn glasses. For mixed alkali metaphosphate glasses, Rouse et al.32 modeled νs(PO2-) and νas(PO2-) using a simple vibrational model and found that both frequencies depend mainly on the O-P-O angle of the PO2- unit (φ) and on the force constant of the M-O vibration (FM-O). Their results showed that νs(PO2-) increases as φ decreases or as FM-O increases, whereas

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νas(PO2-) decreases as φ decreases and increases as FM-O increases. While there are no available data for the effect of Sr2+ and Mn2+ ions on φ, it is reasonable to assume that φ would decrease as the larger Sr2+ ion (r ) 1.16 Å) is replaced by the smaller Mn2+ ion (r ) 0.67 Å).71 The force constant FM-O can be calculated from the expression 2 FM-O ) 4πc2µM-OVM-O

(3)

where c is the speed of light; µM-O is the reduced mass of cation vibration against its oxygen cage; and νM-O is the frequency of this vibration. The evaluation of µM-O requires knowledge of the nature of oxygen cages (sites) occupied by metal ions. For Mn ions, we consider 6-fold coordination with oxygen, based on optical spectroscopy and EPR results of this work, while for Sr ions we take the 8-fold coordination found in an earlier neutron and X-ray scattering study of Sr-metaphosphate glass.72 The reduced masses corresponding to octahedral and cubic sites of Mn and Sr ions, respectively, are given by

µ6,M-O )

µ8,M-O

mMmO mM + 2mO

3mMmO ) 3mM + 8mO

(4a)

(4b)

where mM and mO are the masses of cation and oxygen, respectively.73 Using the above expressions and the far-infrared data, νSr-O ) 180 cm-1 for x ) 0 and νMn-O ) 240 cm-1 for x ) 1 (Figure 6), we obtain FSr-O ) 2.0 × 104 dyn/cm and FMn-O ) 3.4 × 104 dyn/cm. Considering the Rouse et al. model,32 the FM-O results, and the assumption made for the cation dependence of φ, it is expected that νs(PO2-) should increase from x ) 0 to x ) 1. Indeed, this expectation is fulfilled by the Raman spectra of Sr-Mn glasses as shown in Figures 8 and 9a. The deviation of νs(PO2-) vs MnO content from linearity may reflect a tendency for Mn-oxygen clustering in Sr-Mn glasses. On the other hand, FM-O and φ will affect the frequency of the asymmetric stretching mode in the opposite way: νas(PO2-) will increase from the Sr- to the Mn-glass because of FM-O, but it will decrease in parallel because of φ. It appears from the infrared spectra in Figure 6 that the net effect of FM-O and φ is an almost composition-independent νas(PO2-) at 1275 cm-1. While Q2 units constitute the majority phosphate sites in Sr-Mn glasses, the infrared and Raman spectra have suggested the presence of minor contributions from Q3 and Q1 sites. NMR studies of Mg-phosphate26 and Zn-phosphate27 glasses have shown that Q2 accounts for almost 90% of the metaphosphate structure, and Q3 and Q1 species are created through the disproportionation reaction

2Q2 T Q3 + Q1

(5)

As discussed by Fletcher et al.,74 the above equilibrium shifts to the right with increasing field strength of the modifier cation, i.e., charge/radius ratio which is proportional to the polarizing power of the metal cation. Therefore, an important difference between the Na- and Ca-metaphosphate glasses is that the NMR spectrum of the Na glass shows only the presence of Q2

Figure 10. Typical deconvolution of the 800-1400 cm-1 range of the absorption coefficient spectra of xMnO-(1 - x)SrO-P2O5 glasses for x ) 0.3 and 0.5. Experimental spectra and Gaussian components are shown by solid lines, and the simulated spectrum is shown by open circles. For band assignments, see Table 3.

TABLE 3: Assignments of Bands Obtained from Deconvolution of the 800-1400 cm-1 Region of the Infrared Spectra (see Figure 10 and Text for Details) band

frequency (cm-1)

assignment

1 2 3 4 5 6 7

895-922 950-967 1020-1040 1090-1100 1146-1172 1255-1270 1302-1320

νas(P-O-P), Q2 in chains νas(P-O-P), Q2 in large rings νas(P-O-P), Q2 in small rings νas(PO32-), Q1 end groups νs(PO2-), Q2 νas(PO2-), Q2 ν(PdO), Q3

tetrahedra, whereas that of the Ca glass shows weak peaks of Q3 and Q1 units in addition to the main Q2 signal.74 An earlier study of the infrared and Raman spectra of alkaline earth metaphosphate glasses22 has shown that equilibrium 5 shifts to the right as the cation field strength increases from Ba to Ca in accordance with the Fletcher et al. study.74 However, this trend was found inverted for the Mg-metaphosphate glass which exhibits a larger tendency for Q2 formation than Ca-metaphosphate. This peculiarity of Mg2+ can be traced to its tendency to develop a considerable degree of covalent character in the Mg-O bonding,16 as compared to the purely ionic M-O interactions for the rest of the alkaline earth cations. To search further for the effect of cation mixing on the SRO network structure in light of equilibrium 5, we have deconvoluted the 800-1400 cm-1 infrared range since it shows measurable spectral signatures of various Qi tetrahedral species. It was found that a satisfactory description of the experimental infrared profiles is obtained by using at least seven Gaussian components as illustrated in Figure 10 for x ) 0.3 and 0.5. For all deconvolutions, the peak frequency and bandwidth values were kept constant to within 2% and 10%, respectively, of the values obtained by fitting the spectrum of the x ) 0 glass. With these constraints, the only free parameters of fitting were the integrated intensities of the component bands. Following the above discussion of infrared spectra, Table 3 summarizes the assignments for the bands resulting from spectral deconvolution. Bands 4 and 7 are taken as typical of Q1 and Q3 tetrahedral species, respectively, whereas the rest of the bands


Figure 11. Normalized integrated intensity of bands characteristic of Qi phosphate units as resulting from the deconvolution of the 800-1400 cm-1 infrared range of xMnO-(1 - x)SrO-P2O5 glasses (Figure 10). Lines are drawn to guide the eye.

are associated with Q2 species. The normalized integrated intensities for phosphate units Q1, Q2, and Q3 are shown in Figure 11 and demonstrate a clear nonadditive dependence of the SRO structure on cation mixing. In particular, substitution of Sr by Mn leads to a preference for Q2 units with a pronounced positive departure from linearity, while Q1 and Q3 units exhibit the opposite trend. Thus, cation mixing induces rearrangements in the relative population of SRO phosphate units, with their composition dependence reminiscent of a structural mixed cation effect. The occurrence of similar effects has been observed in mixed alkali borate75 and thiogermanate76 glasses. Since Mn2+ has a smaller effective ionic radius than Sr2+, i.e., 0.67 vs 1.16 Å,71 the average cation field strength is expected to increase as the glass composition changes from x ) 0 to x ) 1. According to Fletcher et al.,74 this would shift eq 5 to the right with increasing MnO content, but this expectation is opposite to what is found in Figure 11. Therefore, Mn2+ and Mg2+ ions appear to behave similarly with respect to their effect on eq 5. This finding suggests some degree of covalency in Mn-O bonding, and this is consistent with the fact that Mn2+ is a transition metal ion. As seen in Figure 11, the preference for Q2 formation shows maximum deviation from additivity at maximum mixing (x ) 0.5), suggesting an increasing degree of covalency in Mn-O bonding upon Sr/Mn mixing. This trend is consistent with the composition dependence of the g-value obtained from the EPR spectra (inset of Figure 3) and with the tendency for Mn-oxygen clustering suggested by the νs(PO2-) mode in the Raman spectra (Figure 9a). 4.2. Metal Cation-Oxygen Site Interactions. Having discussed the effect of cation mixing on the SRO phosphate structure, we examine here the composition dependence of cation sites formed by oxygen atoms of Qi units. Information about the nature of cation-hosting sites can be extracted from far-IR spectra where the vibrations of cations against their sites are active.14,16-22 As discussed above, for the Sr (x ) 0) and Mn (x ) 1) metaphosphate glasses, the cation vibrations give rise to quite symmetric bands at ca. 180 and 240 cm-1 (Figure 6), which are narrower compared to the broad and asymmetric far-IR profiles reported for alkali and alkaline earth borate glasses.69 The comparison with borates suggests a rather narrow distribution of cation sites in metaphosphate glasses. This is in


Figure 12. Typical deconvolution of far-infrared absorption coefficient spectra of glasses xMnO-(1 - x)SrO-P2O5 for x ) 0, 0.3, 0.7, and 1. Experimental spectra and Gaussian components are shown by solid lines and simulated spectra by open circles.

agreement with the dominating presence of Q2 units which form a narrow distribution of anionic environments for coordination with the metal cations. Figure 12 shows the far-IR region in an expanded frequency and intensity scale for representative compositions in the Sr-Mn series. As MnO is introduced in the glass, a shoulder at ca. 240 cm-1 develops on the 180 cm-1 band and grows eventually into the main feature of the x ) 1 glass. This observation indicates that the far-IR response of mixed Sr-Mn glasses can be regarded, on a first approximation, as resulting mainly from the superposition of Sr2+ and Mn2+ cation-motion bands. Because of the strong overlapping of these bands, we explore this aspect further by deconvoluting the far-IR profiles into Gaussian components as shown in Figure 12. The x ) 0 spectrum could be described by four bands at 77, 327, 417, and 180 cm-1, where the last feature is the Sr-motion band having a full width at half-maximum intensity ∆ν ) 94 cm-1. The two bands above 300 cm-1 can be related to O-P-O bending modes of rings and of terminal PO2- groups,68 while the low-frequency band at 77 cm-1 can be attributed to relative rotations of metaphosphate tetrahedra.66 Keeping the frequency and bandwidth of the 327 and 417 cm-1 bands fixed to within 1.5% and 10%, respectively, and those for the 77 cm-1 band to within 10%, and letting their relative intensity vary, the x ) 1 spectrum was deconvoluted and found to be dominated by the Mn-motion band at ν ) 240 cm-1 with ∆ν ) 135 cm-1. Deconvolution of mixed-glass spectra was done similarly with five component bands, where the frequency and bandwidth of the Sr2+- and Mn2+-oxygen site vibrations were fixed to their values in the end-member spectra. We note that even though the Mn-motion band is considerably broader than that of Sr, both bands have similar relative bandwidths, ∆ν/ν ) 0.52-0.56, indicating similar disorder in the nature of the cation-hosting sites. The ability to fit well the spectra of mixed Sr-Mn glasses with Sr2+- and Mn2+-motion bands having the characteristics of the end members (x ) 0, 1) indicates that metal cations form their own sites and retain the identity of such sites in mixedcation glasses. This is in agreement with key conclusions of

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Figure 13. Normalized integrated intensity of the 180 and 240 cm-1 far-infrared component bands attributed to Sr2+- and Mn2+-cation motions, respectively. Solid lines are fits to eq 7, and dotted lines are guides to the eye.

earlier far-IR studies on mixed alkali glasses including phosphates,32 silicates,77 and borates,78 with the dynamic structure model developed for the mixed alkali effect,36 as well as with recent molecular dynamics results on mixed alkali borate glasses.79 When the mass and/or charge difference of cations is large enough to allow for a good separation of the two cation-motion bands, it was found both experimentally16,78 and by molecular dynamics79 that the frequency of cation-motion bands varies with composition (x). However, the Sr-Mn pair is not suitable for exploring this aspect because of the small difference in cation-motion frequencies (60 cm-1) compared to the corresponding bandwidths (94 and 135 cm-1). Despite this fact, the intensity of the Sr2+- and Mn2+-motion bands depends strongly on glass composition as shown in Figure 12. The relative integrated intensity of both bands has been obtained from the deconvoluted spectra and is found to manifest a clear nonlinear dependence on MnO content (see Figure 13). To understand the origin of this trend, we consider the integrated intensity of each cation-motion band, AM, given by the expression

AM )

[

]

2 1 ε∞ + 2 2 NMqM 3 nMmM 4c2ε0

(6)

NM is the number of cation oscillators per unit volume with effective charge qM and effective mass of vibration mM; nM is the index of refraction at the peak frequency of the cation-motion band; c is the speed of light; ε0 is the permittivity of free space; and ε∞ is the high frequency dielectric constant of glass.80 Then, the relative integrated intensity, , of the Mn2+-motion band is obtained by

〈AMn〉 )

2 AMn NMnqMn /nMnmMn ) 2 2 AMn + ASr NMnqMn /nMnmMn + NSrqSr /nSrmSr (7)

while a similar equation applies for the Sr2+-motion band.

The numbers of metal cations in eq 7 can be replaced by their mole fractions; the effective charge of each cation is approximated by the nominal charge of +2 and the effective mass of vibration by the bare cation mass. The values of the refractive indices nMn and nSr have been obtained from the Kramers-Kronig transformation of the reflectance spectra for glasses x ) 0 and x ) 1 and are nMn ) 1.96 and nSr ) 1.80. The relative integrated intensities are then calculated from eq 7, and the results are included in Figure 13. The excellent agreement between calculation and data derived from spectral deconvolution suggests that the nonlinear dependence of the relative integrated intensities on MnO content reflects mainly the mass difference of Sr and Mn cations and their relative content. 4.3. Structure-Property Correlations. 4.3.1. Thermal and Static Properties. We proceed now to correlate glass properties with structural changes induced by cation mixing. As found in Figures 4a and 4b, thermal expansion coefficient, TEC, and glass transition temperature, Tg, exhibit opposite deviations from linearity. This would be expected since these properties are related in an opposite manner with the network rigidity; an enhanced Tg manifests an increased network rigidity and, thus, a reduced ability for expansion (i.e., lower value of TEC). To account for differences in Tg in the two end-member glasses (x ) 0, 1), we consider the structural model of Hoppe81 that gives the number of terminal oxygen atoms, NTO, per modifying metal cation

NTO )

n x

(8)

where n is the valence of the metal ion in glass xM2/nO-(1 x)P2O5. It was discussed by Hoppe that there will be two structurally different composition ranges depending on the relation between NTO and the coordination number of the metal ion, CN(M). When NTO > CN(M) (region I) the metal ions form isolated sites (coordination polyhedra) within the phosphate network. For NTO < CN(M) (region II), there are not enough individual terminal oxygen (TO) atoms to satisfy the coordination environment for each metal ion, and thus, metal ions share TOs to form their coordination polyhedra. This TO-sharing in region II leads to formation of P-O-M-O-M-O-P linkages which act as bridges between neighboring Q2 units and, thus, increase the overall network connectivity.10,13,82 For glasses in the Sr-Mn system, eq 8 gives NTO ) 4. Also, the optical and EPR spectra suggest that CN(Mn) ) 6, while the diffraction study of Sr-metaphosphate glass has given CN(Sr) ) 8.2.72 According to the Hoppe model, Sr-Mn glasses belong to region II, with a substantial contribution to structure and bonding by P-O-M-O-M-O-P linkages. Due to their larger coordination number, Sr ions are expected to have greater need for P-O-M-O-M-O-P linkages relative to Mn ions. In this context, the higher Tg of the x ) 0 glass compared to x ) 1 reflects an increased population of P-O-M-O-M-O-P links in the Sr-metaphosphate glass. It is recalled at this point that Raman spectroscopy indicated a departure from the homogeneous distribution of metal cations, which is consistent with formation of P-O-M-O-M-O-P linkages, while EPR spectroscopy pointed also toward formation of P-O-Mn-OMn-O-P bridges connecting neighboring Mn-O polyhedra. When Sr is gradually replaced by Mn, Tg exhibits a positive departure from additivity (Figure 4) despite the fact that the average metal ion-coordination number should normally decrease. This suggests that additional structural elements should

Properties of Mixed Sr-Mn Metaphosphate Glasses be considered upon cation mixing. As found in Figure 11, the relative amount of Q2 units also shows a positive deviation from additivity with a leveling off after maximum mixing. The observed increase in Q2 population suggests an increased polymerization and entanglement of the phosphate network,82 and this could be associated with the initial increase in Tg in the range 0 e x e 0.3 (Figure 4). However, larger MnO contents lead to a reduction in Tg even though the relative population of Q2 units remains high. This shows that toward the Mn-phosphate glass (0.3 < x e 1) the effect of decreasing average metal ioncoordination number cancels out gradually the effect of Q2 population on Tg. Besides the static structural aspects considered above, one may invoke dynamic processes to trace further the origins of the unusual Tg trend in the Sr-Mn system. In this context, we recall that the dynamic structure model36 for mixed glasses relies on the presence of distinct cation sites which provide appropriate vacancies for cation migration through the glass network. When cations enter “wrong” sites (i.e., sites appropriate for the other cation), mismatch energy is created. In addition, the concept of matrix mediated coupling proposed by Ingram et al.83 implies that coupling between the movements of unlike cations occurs and provides a way to dissipate the mechanical stresses created from cation mismatch. Such synergetic cation movements are responsible for several mixed cation trends83 including the usual decrease of Tg encountered in many mixed cation glasses.38,39 When divalent cations are mixed, it was found that the depression of Tg is less profound compared to the one observed in monovalent mixed cation glass systems.84 This effect was attributed to the lower ability of divalent cations to be decoupled from the network, which leads to a decreased ability to couple with the foreign cations and to participate in network distortions responsible for the decrease in Tg. In the case of Sr-Mn phosphate glasses, one needs to consider the ability of Mn cations to bond tightly with phosphate segments, resulting from the enhanced covalent character of Mn-O bonding with cation mixing. Thus, Mn ions become unable to decouple from the glass network and to participate in relaxation coupling with the Sr cations. This is mainly the case when small amounts of Mn are introduced in the glass, i.e., for 0 e x e 0.3. On the basis of the above discussion, we attribute the unusual behavior of Tg to show positive departure from linearity in Sr-Mn glasses to both static and dynamic structural aspects. From the static point of view, we recall the composition dependence of two structural elements, i.e., the population of P-O-M-O-M-O-P linkages which provide the crosslinking between various phosphate units and the population of metaphosphate Q2 units. As for the dynamics, the tight bonding of Mn ions to the phosphate network does not allow them to participate significantly in network distortion processes occurring near Tg that could lead eventually to a decrease of Tg. Compared to Tg and TEC, the molar volume and density show an almost linear dependence on composition (Figures 4c and 4d) as was found in many mixed cation glasses.33-35 The decrease of density and molar volume with MnO content in the Sr-Mn system is compatible with the gradual replacement of strontium ions by the lighter and smaller manganese ions. 4.3.2. Optical Properties. As noted in the Introduction, the low dispersion of phosphate glasses makes them important materials for optics. Low dispersion is associated with small values of partial dispersion of the refractive index (nF′ - nC′), which according to eq 1 results in large Abbe numbers, νe. Glasses with νe > 50 are characterized by low optical dispersion,2 and in this context the νe values in Table 1 show that all Sr-Mn


Figure 14. Dependence of refractive index (n) on photon energy (E) for glasses in the system xMnO-(1 - x)SrO-P2O5. Linear lines are fits to the Wemple-DiDomenico relationship, eq 9.

TABLE 4: Dispersion Parameters for Glasses xMnO-(1 x)SrO-P2O5: Average Electronic Band Gap (E0) and Optical Transition Strength (Ed) glass x x x x x x x

) ) ) ) ) ) )

0 0.1 0.3 0.5 0.7 0.9 1

E0 (eV)

Ed (eV)

12.92 12.85 12.64 12.51 12.41 12.35 12.36

17.92 17.98 17.90 17.92 17.71 17.75 17.78

glasses studied here are low-dispersion materials. It is also clear that the x ) 0 Sr glass shows lower dispersion than the x ) 1 Mn glass, while the coexistence of Mn and Sr ions in glass introduces additional dispersion as manifested by the negative deviation from additivity of the Abbe number (Figure 4e). Wemple85 associated the dispersion in glasses with changes in the electronic band gap, E0, with composition. This energy gap can be pictured as the average excitation energy for electronic transitions between the “centers of gravity” of the valence and conduction bands,86 and it can be estimated by fitting refractive index dispersion data to the Wemple and DiDomenico87 single oscillator expression

n2(E) - 1 )

EdE0 E20

- E2

(9)

where n(E) is the value of the refractive index at the photon energy E and Ed is the optical transition strength or dispersion energy parameter. It is easily deduced from eq 9 that the parameters E0 and Ed can be determined from the slope, -1/ EdE0, and intercept, E0/Ed, of the linear plots of 1/(n2 - 1) vs (E)2. As shown in Figure 14, the refractive-index dispersion of Sr-Mn metaphosphate glasses follows well the WempleDiDomenico model, in agreement with earlier studies in oxide85,88 and chalcogenide89,90 glasses. The E0 and Ed values obtained from the linear fits are given in Table 4 and are plotted in Figure 15 vs MnO content. The values of E0 found for Sr-Mn phosphate glasses are comparable to those reported for modified

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Nc ) 4.7x + 5.4(1 - x)

(11)

The values of β resulting from eqs 10 and 11 are shown in Figure 15c vs x and are consistent with the ionic nature of the present glasses; the slightly lower β values compared to those of crystalline compounds can be attributed to the presence of voids in the glassy state.85 The parameter β exhibits also a rather linear increase with MnO content, indicating slightly increasing average bond covalency with manganese content. Therefore, we suggest that the combined effect of increasing β and decreasing Nc with MnO content is the origin of the observed Ed trend in Sr-Mn metaphosphate glasses. 5. Conclusions

Figure 15. Average electronic energy gap (E0), electronic oscillator strength (Ed), and parameter (β) as a function of composition in glasses xMnO-(1 - x)SrO-P2O5. Lines are drawn to guide the eye.

silicate glasses with alkali and/or alkaline-earth oxides,85 but they are considerably larger than those of chalcogenide glasses or glasses containing cations with strong electronic transitions, e.g., Tl+, Pb2+, and Bi3+.85,89,90 The presence of metal cations with considerable covalency in the M-O bonding, e.g., Nb-O, leads also to smaller values of E0.88 On these grounds, the trends of E0 (Figure 15(a)) and Abbe number (Figure 4e) with MnO content should reflect the composition dependence of the degree of covalency in Mn-O bonding, as manifested also by EPR spectroscopy (inset of Figure 3). The Mn-free glass (x ) 0) has the largest E0 value (12.92 eV) in line with the ionic character of Sr-O bonding. The optical band gap, Eopt, for the x ) 0 glass can be estimated from the empirical relationship86 Eopt ) E0/1.9 which gives Eopt ) 6.8 eV. This is identical to the optical band gap of the Sr-metaphosphate glass derived from vacuum-ultraviolet spectroscopic measurements.91 The optical transition strength, Ed, is found to exhibit a more complicated composition dependence. It decreases from the average value of 17.93 eV in the range 0 e x e 0.5 to ca. 17.75 eV for 0.7 e x e 1 (Figure 15b). Wemple85 related changes in cation coordination number, Nc, with variations in Ed through the empirical relation

Ed (eV) ) βNcZaNe

(10)

where β is a constant; Za is the formal valency of the anion; and Ne is the number of valence electrons per anion. The value of β was found to depend on chemical nature of materials; β ) 0.37 ( 0.04 eV for covalent materials (e.g., GaP, ZnS) and β ) 0.26 ( 0.03 eV for more ionic materials (e.g., NaCl, Al2O3). Equation 10 was established originally for single crystalline compounds, but its validity was confirmed for glassy materials as well.85,88-90 Oxygen is the only anion species in Sr-Mn glasses, and thus Za ) 2 and Ne ) 8. Also, EPR and optical spectroscopy suggest that the coordination number of manganese is CN(Mn) ) 6. If we assume that strontium retains its coordination number in the x ) 0 glass, i.e., CN(Sr) ) 8.2,72 and since CN(P) ) 4, we can write for the average cation coordination number in the Sr-Mn series

Mixed strontium-manganese metaphosphate glasses MnO-(1 - x)SrO-P2O5, 0 e x e 1, were developed by melt reduction and quenching, and their optical, thermal, and structural properties were investigated. Optical spectroscopy showed that Mn ions are present predominantly in the +2 oxidation state and occupy octahedral sites in the phosphate matrix. The nature of Mn2+ sites was confirmed by EPR spectroscopy, which suggested also the formation of neighboring Mn-oxygen polyhedra linking phosphate units by bridges like P-O-MnO-Mn-P. In addition, the g ≈ 2.00 central EPR resonance was found to depend on MnO content and its position to indicate an increased covalent character in Mn-O bonding upon cation mixing. The study of infrared and Raman spectra showed that Q2 tetrahedral units are the majority short-range-order phosphate species in Sr-Mn metaphosphate glasses, with Q2 units contributing about 75% of the total infrared absorption due to P-O stretching modes. The rest of the infrared activity originates from contributions of Q3 and Q1 species, which coexist with Q2 because of the disproportionation reaction 2Q2 T Q3 + Q1. Cation mixing was found to affect the phosphate speciation, with the relative Qi infrared activities exhibiting nonlinear dependence on MnO content. Because of its localized nature, the Raman active symmetric stretching mode of PO2terminal groups, νs(PO2-), of Q2 units was used to probe the influence of glass composition on the distribution of metal ions. It was found that νs(PO2-) varies nonlinearly on x, and this was attributed to deviation from a homogeneous distribution of Sr and Mn cations, in agreement with the EPR result concerning Mn ions. The far-infrared absorption arising from cation-oxygen vibrations could be analyzed in terms of two bands attributed to Sr-O and Mn-O vibrations in sites hosting the metal ions. The spectra of mixed Sr-Mn glasses could be fitted with Sr-O and Mn-O oscillators which retain their characteristics (i.e., resonance frequency and bandwidth) in the corresponding single cation glasses. Also, the composition dependence of the relative strength of the two cation-oxygen oscillators could be well explained in terms of the mass difference and relative content of Sr and Mn cations. These far-infrared results provide strong evidence that each metal ion forms its own favorable site in both single and mixed cation glasses. The studied Sr-Mn glasses constitute the first known mixedcation phosphate system with glass transition temperature Tg exhibiting positive deviation from linearity. This unusual behavior of Tg and the opposite trend of the thermal expansion coefficient were attributed to a combination of different factors including the cross-linking abilities of the two cations via P-O-M-O-M-O-P linkages, the composition dependence of the population of metaphosphate Q2 units, and the tight

Properties of Mixed Sr-Mn Metaphosphate Glasses bonding of Mn ions to the phosphate due to increased covalency in Mn-O bonding upon cation mixing. The density and molar volume were found to decrease linearly with MnO content in accordance with the mass and size difference of the two divalent cations. Refractive index measurements showed that all glasses in the Sr-Mn system have low optical dispersion with Abbe numbers exceeding 60. Replacement of Sr by Mn was found to increase dispersion at a rate greater than expected from a linear dependence on MnO content. The origin of this trend was traced to the composition dependence of the average electronic band gap, which was obtained by fitting refractive index data to the Wemple-DiDomenico single oscillator model. The corresponding optical transition strength was found to reflect changes in the average cation coordination number and bond covalency with MnO content. Acknowledgment. We thank R. Atzrodt and other colleagues from the Otto-Schott-Institut, Jena, for glass preparation and for assistance with measurements of optical properties, and M. Friedrich for assistance with EPR measurements. One of us (I.K.) acknowledges the help of Dr. N. Makris with spectroscopic measurements and financial support from the EC (grant HPMD-CT-2000-00033) and the Theoretical and Physical Chemistry Institute of the NHRF. References and Notes (1) Weber, M. J. J. Non-Cryst. Solids 1990, 123, 208–222. (2) Vogel, W. Glass Chemistry, 2nd ed.; Springer-Verlag: Berlin, 1994. (3) Ebendorff-Heidepriem, H.; Seeber, W.; Ehrt, D. J. Non-Cryst. Solids 1995, 183, 191–200. (4) Campbell, J. H.; Suratwala, T. I. J. Non-Cryst. Solids 2000, 263&264, 318–341. (5) (a) Ehrt, D.; Ebeling, P.; Natura, U. J. Non-Cryst. Solids 2000, 263&264, 240–250. (b) Ehrt, D. C. R. Chimie 2002, 5, 679–692. (6) Van Wazer, J. R. Phosphorus and Its Compounds; Interscience: New York, 1958. (7) Kordes, E.; Vogel, W.; Feterowsky, R. Z. Elektrochem. 1953, 57, 282. Kordes, E.; Navarrete, J. Glastech. Ber. 1973, 46, 113–119. (8) Kanazawa, T. J. Non-Cryst. Solids 1982, 52, 187–194. (9) Brow, R. K.; Tallant, D. R.; Myers, S. T.; Phifer, C. C. J. NonCryst. Solids 1995, 191, 45–55. (10) Hoppe, U.; Walter, G.; Kronold, R.; Stachel, D.; Barz, A. J. NonCryst. Solids 1995, 192-193, 28–31. (11) Brow, R. K.; Click, C. A.; Alam, T. M. J. Non-Cryst. Solids 2000, 274, 9–16. (12) Martin, S. W. Eur. J. Solid State Chem. 1991, 28, 163–205. (13) Brow, R. K. J. Non-Cryst. Solids 2000, 263&264, 1–28. (14) Exarhos, G. J.; Risen, W. M. Chem. Phys. Lett. 1971, 10, 484– 486. (15) Exarhos, G. J.; Miller, P. J.; Risen, W. M. J. Chem. Phys. 1974, 60, 4145–4155. (16) Nelson, B. N.; Exarhos, G. J. J. Chem. Phys. 1979, 71, 2739–2747. (17) Kamitsos, E. I.; Kapoutsis, J. A.; Chryssikos, G. D.; Hutchinson, J. M.; Pappin, A. J.; Ingram, M. D.; Duffy, J. A. Phys. Chem. Glasses 1995, 36, 141–149. (18) Meyer, K. J. Non-Cryst. Solids 1997, 209, 227–239. (19) (a) Meyer, K.; Barz, A.; Stachel, D. J. Non-Cryst. Solids 1995, 191, 71–78. (b) Meyer, K. Phys. Chem. Glasses 2001, 42, 79–87. (20) Hudgens, J. J.; Brow, R. K.; Tallant, D. R.; Martin, S. W. J. NonCryst. Solids 1998, 223, 21–31. (21) Brow, R. K.; Tallant, D. R.; Mayers, S. T.; Phifer, C. C. J. NonCryst. Solids 1995, 191, 45–55. (22) Velli, L. L.; Varsamis, C. P. E.; Kamitsos, E. I.; Mo¨ncke, D.; Ehrt, D. Phys. Chem. Glasses 2005, 46, 178–181. (23) Ba¨renwald, U.; Dubiel, D.; Matz, W.; Ehrt, D.; Vogel, W. J. NonCryst. Solids 1988, 103, 311–318. (24) Brow, R. K.; Kirkpatrick, R. J.; Turner, G. L. J. Non-Cryst. Solids 1990, 116, 39–45. (25) Bruni, S.; Cariati, F.; Corrias, A.; Gaskell, P. H.; Lai, A.; Musinu, A.; Piccaluga, G. J. Phys. Chem. 1995, 99, 15229–15235. (26) Fayon, F.; Massiot, D.; Suzuya, K.; Price, D. L. J. Non-Cryst. Solids 2001, 283, 88–94. (27) Tischendorf, B.; Otaigbe, J. U.; Wiench, J. W.; Pruski, M.; Sales, B. C. J. Non-Cryst. Solids 2001, 282, 147–158.

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