Multinuclear Solid State NMR Investigations of the System - American

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J. Phys. Chem. C 2009, 113, 3322–3331

Structure-Property Relations in Mixed-Network Glasses: Multinuclear Solid State NMR Investigations of the System xAl2O3:(30 - x)P2O5:70SiO2 Bruce G. Aitken* and Randall E. Youngman SP-FR-05, Corning Incorporated, Corning, New York 14831

Rashmi R. Deshpande and Hellmut Eckert* 2 Institut fu¨r Physikalische Chemie, Westfa¨lische Wilhelms-UniVersita¨t Mu¨nster, Corrensstrasse 30, D-48149 Mu¨nster, Germany

ReceiVed: October 17, 2008; ReVised Manuscript ReceiVed: December 29, 2008

New mixed-network glasses along the composition line xAl2O3-(30 - x)P2O5-70SiO2 have been prepared and characterized in terms of their density, thermal expansion coefficient, refractive index, and characteristic temperatures. The compositional changes in these macroscopic properties have been correlated with structural information, obtained via Raman spectroscopy and state-of-the-art solid state NMR techniques, including 27 Al, 29Si, and 31P magic-angle spinning (MAS) NMR, 27Al triple quantum MAS NMR, as well as static 31P spin echo decay spectroscopy. In addition, the extent of P-O-Al connectivity has been quantified on the basis of 27Al{31P} rotational echo double resonance (REDOR) and 31P{27Al} rotational echo adiabatic passage double resonance (REAPDOR) measurements. Both the macroscopic and the structural properties show nonlinear dependences on x, including abrupt changes at a nominal Al/P ratio of 1 (x ) 0.15), where no glasses can be formed by melt quenching under the conditions used in this study. The structure of phosphorusrich glasses (Al/P < 1) is characterized by four-, five- and six-coordinated Al species, whose second coordination sphere is dominated by phosphorus. 31P static and MAS NMR spectra suggest the presence of at least three distinct phosphorus environments, corresponding to silicon-bonded P(3) units, anionic metaphosphate P(2) species interacting with octahedral aluminum, and tetrahedral PO4/2 groups (P(4) units) bonded similarly as in AlPO4. (In this P(n) nomenclature, the superscript denotes the number of bridging oxygen atoms attached to a P atom.) The latter species is also the dominant phosphorus environment in the Al-rich glasses (Al/P > 1), where the alumina component is involved in Al-O-P, Al-O-Si, and possibly also Al-O-Al linkages. All of these results indicate that the structure of these glasses is dominated by the strong mutual affinity of the phosphorus oxide and alumina components. To quantify this affinity, the experimental REDOR and REAPDOR results have been compared with a cluster model assuming that both components react completely under formation of aluminum phosphate-like domains, thereby maximizing the number of Al-O-P linkages. Both the REDOR and the REAPDOR results show, however, clear deviations from such a structural scenario, supporting a more homogeneous glass structure with a certain degree of connectivity randomization. Introduction Despite the fact that SiO2, P2O5 and Al2O3 are well-known glass formers or, at least, conditional glass formers, the extent of glass formation in the Al2O3-P2O5-SiO2 system, as well as the compositional dependence of the physical properties of these ternary network glasses, is poorly known. A diagram of the glass-forming region in this system by Syritskaya based on melting experiments at 1550 °C shows only a band of glass extending from the Al2O3-P2O5 binary sideline near Al(PO3)3 toward higher silica compositions, but always on the P-rich side of the AlPO4-SiO2 join, and terminating on the SiO2-P2O5 sideline.1 A similar diagram of glass formation was proposed by Sedmalis et al. who also noted that the presence of Al2O3 resulted in significant improvement to chemical durability and suggested that coupled AlO4/2 and PO4/2 tetrahedra constituted some of the structural elements of ternary glasses.2 However, without a melting temperature limitation, it is clear that glass * Corresponding authors: (B.G.A.) [email protected]; (H.E.) [email protected].

formation is much more extensive than indicated above, especially for SiO2-rich compositions. Binary SiO2-P2O5 glasses are well-known, although compositions near pure SiO2 are typically made by chemical vapor deposition (CVD),3,4 and binary Al2O3-SiO2 glasses with Al2O3 concentrations as high as 40 wt % have also been made by conventional melt-quench experiments, albeit using melting temperatures of 1800 °C or higher.5 Moreover, in the quest for novel optical waveguide materials, ternary glasses with SiO2 contents ranging from 78 to 96% have also been synthesized by CVD.6 Compositions on the AlPO4-SiO2 join are of particular interest in this system owing to the fact that the various crystalline polymorphs of SiO2 and AlPO4 are isostructural.7 Consequently, AlPO4 glass or mixed AlPO4-SiO2 glasses are expected to comprise three-dimensional tetrahedral networks similar to that of fused silica. Attempts to synthesize AlPO4 glass by conventional melting methods were unsuccessful.8 However, Kosinski et al. reported the synthesis of a 50AlPO4: 50SiO2 glass upon melting a mixture of AlPO4 and SiO2 at 1800 °C for 100 h under Ar.9 More recently, pure AlPO4 glass as

10.1021/jp809208m CCC: $40.75  2009 American Chemical Society Published on Web 02/02/2009

Mixed-Network Glasses well as binary glasses with AlPO4:SiO2 ratios ranging from 0.5 to 4 have been prepared by a sol-gel technique.10,11 These results, coupled with the postulated ∼1400 °C eutectic near ∼70% SiO2 on this join,12 suggested that it should therefore be possible to synthesize melt-quenched 30AlPO4:70SiO2 glass. However, conventional melting trials carried out at temperatures as high as 1800 °C for the latter composition proved unsuccessful (this study). Nevertheless, further experimentation revealed a field of Al-rich glasses roughly centered about the 20Al2O3:10P2O5:70SiO2 composition that could be made by conventional methods using melting temperatures as low as 1650 °C.13 The existence of this compositional area of relatively low temperature glass formation is associated with a ternary eutectic involving the intersecting liquidus fields of mullite, SiO2 and AlPO4.13 In this paper we investigate the structure-property relations of these novel Al-rich phosphosilicate glasses. In order to compare them to some of the conventional P-rich glasses mentioned above, this study examines the effect of progressively substituting Al2O3 for P2O5 in glasses with a constant SiO2 content of 70%. Various macroscopic properties of these glasses are correlated with structural information, obtained on the basis of vibrational spectroscopy and multinuclear solid state NMR techniques. Experimental Section Sample Preparation and Characterization. Glasses containing 20-23% Al2O3 were prepared by melting mixtures of Al2O3, SiO2, and Al(PO3)3 in covered Pt crucibles at 1650 °C. In order to minimize P volatilization losses, P-rich glasses containing 2.5-10% Al2O3 were made as above, but from mixtures of Al2O3, SiO2 and powdered samples of a 30P2O5: 70SiO2 glass previously prepared by melting a calcined mixture of SiO2 and H3PO4 at 1550 °C in a covered silica crucible. Samples containing 25 and 27.5% Al2O3 were made by melting mixtures of Al2O3, SiO2 and powdered sample of a 22.5Al2O3: 7.5P2O5:70SiO2 glass in Pt/Rh crucibles at 1800 °C. Finally, glasses containing 12.5 and 17.5% Al2O3 were prepared from mixtures of powdered 30P2O5:70SiO2 and 22.5Al2O3:7.5P2O5: 70SiO2 glasses. All melts were quenched to glass by pouring onto a steel plate; glass samples were subsequently annealed by heating near the glass transition temperature (Tg) followed by slow cooling to room temperature. For representative samples, chemical analysis was done by either inductively coupled plasma emission spectroscopy or electron microprobe analysis. Refractive index (n) was measured to (0.001 by the Becke line method using powdered sample and standard immersion oils. Density (F) was measured in water to ( 0.001 gcm-3 by the Archimedes method. Viscosity in the annealing, softening range was measured by beam-bending, and parallel plate viscometry, respectively. Data from the former range permitted determination of the strain (Tstr) and annealing points (Ta), corresponding to viscosities of ∼1014 and 1013 P, respectively, to ( 2 °C. The softening point (Ts), corresponding to a viscosity of 107.6 P, was measured with similar precision from the parallel plate measurements. The thermal expansion coefficient (R) was determined to ( 0.01 ppm/°C by dilatometry and referenced to a fused silica standard. Raman spectra (parallel polarized) were collected on an Instruments SA T64000 system using a monochromator and holographic notch filter for Rayleigh scatter rejection. Scattering was collected in the 90° transmission configuration using 514 nm radiation from an Ar+ laser source. The spectral resolution of the detector was (3 cm-1. In addition,

J. Phys. Chem. C, Vol. 113, No. 8, 2009 3323 solid state FTIR spectra were obtained on powdered materials using a Nicolet Avatar 320 spectrometer. A diamond attenuated total reflectance accessory was used. Solid State NMR. All the measurements were carried out on a Bruker DSX 500 spectrometer at a magnetic field strength of 11.7 T unless indicated otherwise. 31P magic angle spinning (MAS) spectra were acquired with pulse lengths of 3 µs (90° flip angle) and a pulse delay of 75s at the resonance frequency of 202.5 MHz using a 4 mm MAS NMR probe operated at the spinning frequency of 15 kHz. 27Al MAS and triple quantum (TQ) NMR spectra14 were measured at 11.7 T (130.2 MHz resonance frequency) using a Chemagnetics Infinity 500 spectrometer and a 2.5 mm MAS NMR probe with sample spinning of 25 kHz. MAS NMR spectra were recorded with π/12 pulses of 0.6 µs length and a relaxation delay of 2 s. The triple-quantum MAS NMR data were recorded using the three-pulse, zeroquantum filtering method,15 using similar conditions as reported previously.5 The resulting two-dimensional correlation plots were analyzed in terms of the isotropic chemical shift δiso and the second-order quadrupolar effect (SOQE ) CQ(1 + η2/3)1/2) by comparing the centers of gravity of the projection along the F1 and F2 dimensions.5 Static 31P wide-line NMR spectra were measured by the Hahn spin echo method, recorded with 180° pulses of 5.8 µs length, a 50 µs interpulse delay, and a relaxation delay of 300 s. Where appropriate, spectral line shape deconvolutions were carried out using the DMFIT software.16 On selected samples, the 31P spin echo decay was monitored using variable interpulse delays and analyzed in terms of homonuclear van Vleck second moment data17 as described previously.18,19 The 27Al and 31P chemical shifts were referenced to an aqueous solution of aluminum nitrate (1M) and to 85% H3PO4 respectively. 29Si MAS NMR spectra were measured on a Bruker CXP300 spectrometer at 59.6 MHz using a 7 mm MAS NMR probe operated at 4 kHz, using 90° pulses of 4 µs length and a recycle delay of 600 s. Chemical shifts are referenced externally to tetramethylsilane. In order to probe the extent of heteronuclear dipolar interaction between P and Al, 27Al{31P} rotational echo double resonance (REDOR)20,21 and 31P{27Al} rotational echo adiabatic passage double resonance (REAPDOR)22,23 methods were used and analyzed in terms of dipolar second moments17 using procedures described previously.24-26 The 27Al{31P}REDOR measurements were carried out on a 4 mm double resonance probe, operated at spinning speeds of 13 and 15 kHz, using the compensated REDOR method24 for correcting the effect of small pulse imperfections. A 90° pulse length of 3 µs was used for both 27Al and 31P with a relaxation delay of 1s for 27Al. Optimum π pulse lengths for the decoupling channel were set by maximizing the REDOR difference signal (∆S) for a chosen dephasing time. The π pulses on the 31P channel were phase cycled according to the XY-4 scheme.27 M2 values were extracted from parabolic fits to the data within the range ∆S/So e 0.2. 31P{27Al} REAPDOR measurements were done at the spinning speed of 15 kHz, using rf amplitudes corresponding to nutation frequencies of 83 kHz for 31P and 46 kHz for 27Al. The π pulses on the 31P channel were phase cycled according to the XY-8 scheme.27 The width of the adiabatic-passage pulse was set to 22 µs, corresponding to 1/3 of the rotor period. Typically, 64 transients were accumulated for each measurement with a relaxation delay of 70 s, following presaturation by a saturation comb to ensure reproducible stationary magnetization.

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TABLE 1: Chemical Analysis Data (in mol %) and Physical Properties of xAl2O3:(30 - x)P2O5:70SiO2 Glasses R nom. % anal. % anal % anal. % Tstr Ta Ts (ppm/ Al2O3 Al2O3 P2O5 SiO2 (°C) (°C) (°C) °C) 2.5 5.0 7.5 10.0 20.0 22.0 22.5 23.0 25.0 27.5

3.35 5.70 8.22 10.5 20.6 22.2

27.2 24.4 22.6 19.8 9.57 8.05

69.5 69.9 69.2 69.7 69.9 69.8

23.2

7.03

69.8

n

592 649 880 4.24 1.504 626 684 947 3.94 1.499 646 708 1071 3.66 1.489 1.473 1.481 1197 0.99 1.490 858 915 1181 1.03 1.491 864 917 1178 1.01 1.493 1.509 1.518

F (g/cm3) 2.430 2.415 2.372 2.287 2.287 2.337 2.348 2.353

Results, Data Analysis, and Interpretation Chemical Composition and Physical Properties. Clear, colorless glasses were formed from compositions containing 2.5-10% and 17.5-27.5% Al2O3, although the 10% Al2O3 sample yielded an opalescent glass when cooled slowly in the crucible instead of being quenched. The 12.5% Al2O3 sample cooled to a translucent glass regardless of quench rate. Attempts to synthesize glasses with x ) 15% resulted in incomplete melts that quenched to a mixture of glass and crystalline AlPO4. Chemical analysis results (Table 1) indicate some loss of P2O5 during glass preparation. However, in all cases, the amount lost is less than 5% of the total nominal P2O5 concentration and, therefore, in the following, glass samples are referred to using the batch compositions. Data on selected physical properties, including n, F, Tstr, Ta, Ts, and R for the investigated glasses are reported in Table 1 and Figure 1. For glasses with Al/P < 1, both n and F decrease with increasing Al2O3 content, whereas they increase with increasing x for Al/P > 1. This compositional dependence suggests that both quantities might have minimum values for a hypothetical glass with Al/P ) 1, i.e., for a glass on the AlPO4-SiO2 join. The characteristic temperatures increase, while R decreases monotonically with rising Al2O3 content. Vibrational Spectra. The Raman spectra of selected glasses are illustrated in Figure 2a. The broad Raman P-O stretching band at 1100-1200 cm-1 weakens and shifts to lower frequency as Al2O3 replaces P2O5. A shoulder near ∼1220 cm-1 can be assigned to anionic P(2) species; its intensity decreases as x increases from 2.5 to 10% and it is absent in the spectra of

Figure 1. Dependence of the refractive index (n, () and density (F, •) of Al2O3:(30 - x)P2O5:70SiO2 glasses on the analyzed Al2O3 concentration. Smooth curves through data points are guides to the eye.

Figure 2. Vibrational spectra of Al2O3:(30 - x)P2O5:70SiO2 glasses: (a) Raman spectra of selected samples; (b) solid state FTIR spectra.

glasses with Al/P > 1. The same development is seen for the weak P(2) P-O-P stretching band at ∼730 cm-1. A similar compositional dependence is observed for the sharp high frequency band at 1350 cm-1 attributed to the terminal PdO groups of P(3) units. In contrast to the situation in binary phosphosilicate glasses,28 the Raman spectra of the present system offer no clear distinction between various OdP(OP)3-x(OSi)x environments. The FTIR spectra, summarized in Figure 2b, are consistent with these conclusions. The existence of P(3) species in the Al/P < 1 glasses is clearly confirmed by a band near 1320 cm-1; the relative contribution of which decreases with x. In addition, the spectra of these glasses evidence the metaphosphate units by a clearly defined shoulder around 1220 cm-1. 29 Si NMR. Figure 3 shows the 29Si MAS NMR spectra of some representative glasses, revealing significant changes in average isotropic chemical shifts as a function of x. 29Si chemical shifts are known to be sensitive to the nature of next-nearest neighbor environments: while Si-O-P linking produces chemical shifts more negative than those measured for the Q(4) resonance in glassy silica,29,30 Si-O-Al linking is known to produce the opposite effect.5 Thus, the spectra obtained in the present system reflect the distribution of Si-O-Si, Si-O-P, and Si-O-Al linkages. While distinct resonances might be expected for different Si(OSi)4-n(OP)n or Si(OSi)4-n(OAl)n

Mixed-Network Glasses

Figure 3. glasses.

J. Phys. Chem. C, Vol. 113, No. 8, 2009 3325

Si MAS NMR spectra of xAl2O3:(30 - x)P2O5:70SiO2

29

TABLE 2: M2(27Al-31P) Values ((10%) Obtained for Each of the Site Resolved Al units, Average Isotropic 29Si and 31P Chemical Shifts (Center of Gravity of the MAS-NMR Peak) δCG (( 0.5ppm) and M2(31P-31P) Values ((10%) Measured in xAl2O3:(30 - x)P2O5:70SiO2 Glasses M2(27Al{31P}) [106 rad2 s-2]

x [%]

AlIV

AlV

AlVI

2.5 5.0 7.5 10 17.5 20.0 22.5 25.0 27.5 AlPO4

nd 4.0 4.3 3.9 2.6a 2.3a 1.6a 0.8a 0.4a 4.7

4.0 5.0 5.1 4.4

4.8 5.5 5.0 4.3

29

Si δCG [ppm]

-117.5 -116.3 -115.6 -112.1 -107.2 -106.1 -105.3 -104.0 -103.9

31

P δCG [ppm]

-40.2 -38.7 -36.4 -34.3 -31.0 -29.8 -29.4 -29.0 -28.3 -25.8

M2(31P-31P) [106 rad2 s-2] 4.9 4.6 4.7 4.5 1.7 nd nd 1.3 0.6 3.3

a

Average value measured for all Al species because of insufficient resolution.

configurations in crystalline systems, in the present glasses only broad and poorly resolved signals can be observed, presumably because of multiple environments as well as other effects such as bond angle distributions. In spite of this limitation, Table 2 reveals a clear compositional trend for the center of gravity δCG(29Si). For x-values corresponding to Al/P ratios 1. As the alumina content increases beyond 17.5 mol %, both δiso (AlIV) and the fractional contributions of the higher-coordinated aluminum sites increase monotonically. The monotonic chemical shift trend observed for the AlIV species suggests that the Al present in these glasses is involved in Al-O-Si linkages to an increasing extent, while the appearance of AlV and AlVI also suggest the possibility of Al-O-Al linking. The alterations of the aluminum local environment are further reflected in different SOQE values. At high alumina contents the spectra are comparable to those observed in Al-rich binary SiO2-Al2O3 glasses.5 Table 3 lists approximate relative peak areas, measured by integration of the MAS-NMR spectra. This procedure is certainly a simplification as the intensities of the TQMAS peaks are strongly dependent on the strength of the nuclear electric quadrupolar interaction. Nevertheless, in the present study, the differences in SOQE values for the AlIV, AlV, and AlVI components in a given sample are found to be relatively small (see Table 3), hence justifying our approach. More quantitative information regarding the medium range order of the aluminum is available from the 27Al{31P} REDOR decays shown in Figure 6. The M2 values extracted from these data using previously described procedures24 are summarized in Table 2. Again, the data show a distinct dependence on the Al/P ratio: for Al/P < 1 the M2(27Al{31P}) values remain more or less constant, adopting values similar to that measured in glassy AlPO4.10,31 Qualitatively, this result indicates that the aluminum environment in all of its coordination states is mostly dominated by phosphorus in these glasses. In contrast, the M2 values decrease linearly at higher Al2O3 contents, reflecting the expected decrease in average P-O-Al connectivity at Al/P ratios > 1, where the amount of phosphorus is insufficient for the complete realization of Al(OP)4 units. In conjunction with the 27Al isotropic chemical shift trend mentioned above, the REDOR data reflect the increasing participation of Al-O-Si and/or Al-O-Al linkages in the local aluminum coordination environments. 31 P NMR and 31P{27Al} REAPDOR. The 31P MAS NMR spectra (Figure 4b) reveal single broad resonance lines, reflecting a distribution of local environments. The average isotropic chemical shift, as determined from the center of gravity δCG, displays a distinct dependence on the Al/P ratio: for Al/P < 1, δCG(31P) increases monotonically, reflecting a continuous change in connectivity distribution. In contrast, for Al/P > 1, the chemical shift depends much less on composition, suggesting

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Figure 4. (a) 27Al and (b) 31P MAS NMR spectra of xAl2O3:(30 - x)P2O5:70SiO2 glasses.

the formation of an almost constant local environment of the P atoms. The center of gravity near -29 ppm is close to where it has been found previously (at -25.8 ppm) in sol-gel prepared glassy AlPO410 and AlPO4-SiO2 glasses,11 where each phosphate unit is linked to four AlO4- groups. Nevertheless the 3 ppm discrepancy suggests a subtle difference to amorphous sol-gel prepared AlPO4 and AlPO4-SiO2 glasses (see 31P{27Al} REAPDOR data below). The dramatic change in the 31P local environments in the Al/P < 1 glasses is more clearly seen in the wide-line spectra shown in Figure 7a. The rather complex spectra indicate several distinct line shape components. At the lowest x values an axially symmetric shielding tensor dominates, which is centered near -47 ppm. On the basis of spectra of SiO2-P2O5 glasses,34 we can assign this resonance to neutral P(3) units involved in linkages to silicon (such as OdP(OSi)3 groups). In the glasses with 2.5% e x e 7.5% a second, nonaxially symmetric component becomes increasingly prominent. Based on literature values for various metaphosphates35 we assign this line shape to anionic P(2) units. Further justification of this assignment comes from the observation (see below) that the concentrations of these species are correlated with the total amounts of AlV and AlVI in these glasses. Probably these higher-coordinated aluminum species interact with the P(2) units, analogous to the situation in crystalline and glassy aluminum metaphosphate. Finally, a sharp isotropic Gauss-Lorentz-shaped resonance (G/L ) 0.7) near -30 ppm gains increasing intensity with x and already dominates the spectrum of the x ) 10% sample. This line shape is assigned to tetrahedral PO4/2 units interacting with AlIV similar to the situation in AlPO4. As these P atoms are linked to four bridging oxygen atoms, they will henceforth be denoted P(4) units. In all the glasses with Al/P > 1 this component is observed exclusively and invariably found at -29 ppm with a constant line width of 40 ppm (8100 Hz). The origins of this line width are comprised of magnetic dipoledipole interactions, unresolved chemical shift anisotropy and a distribution of chemical shift parameters. While simulations based on these three components reproduce the main features of the experimental lineshapes observed in glasses with Al/P < 1, the low-frequency-shoulder of these spectra appears significantly broadened, suggesting that the δ| values of the P(3) sites are subject to a wide distribution. Substantial improvement between the experimental and the simulated spectra can be achieved by introducing a fourth

Figure 5. Triple quantum 27Al NMR spectra of xAl2O3:(30 - x)P2O5: 70SiO2 glasses for x ) 5, 17.5, and 27.5.

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J. Phys. Chem. C, Vol. 113, No. 8, 2009 3327

TABLE 3: 27Al Chemical Shifts δiso((1 ppm) and SOQE Values ((0.5 MHz) Obtained from 27Al TQMAS, for xAl2O3:(30 - x)P2O5:70SiO2 Glassesa AlIV

AlV

AlVI

x [%]

δiso [ppm]

SOQE [MHz]

RA [%]

δiso [ppm]

SOQE [MHz]

RA [%]

δiso [ppm]

SOQE [MHz]

RA [%]

2.5 5 7.5 10 17.5 20 22.5 25 27.5

40.1 40.4 40.0 41.3 47.4 50.9 54.9 61.5

4.0 4.4 4.5 5.9 6.6 6.4 6.7 6.3

2.5 14 38 62 100 95 90 77 46

9.4 7.5 7.9 6.8

4.0 4.4 4.6 4.6

-19.2 -20.2 -20.3 -20.4

4.0 3.7 3.7 3.9

33.7 35.0

6.1 6.0

26 41 42 31 0 5 10 18 45

5.0

5.0

72 45 20 6 0 0 0 4 8

a

Included are relative signal areas (RA, ( 5%) measured by peak integration of the 27Al MAS-NMR spectra.

Figure 6. (a) 27Al{31P}REDOR dephasing curves of xAl2O3:(30 x)P2O5:70SiO2 glasses. (b) Parabolic fit to the data for the x ) 17.5% sample, analyzed within the data range ∆S/So e 0.2 to yield an M2 value of 2.6 × 106 rad2/s2.

component located near -70 to -80 ppm with a larger chemical shift anisotropy. Based on the small deviation from axial symmetry this fourth component might be tentatively attributed to P(3) units of the type O)P(OSi)2(OP). However, it might also be considered an artificial component mimicking the abovementioned distribution affecting the δ| values for the P(3) sites. The deconvolution results obtained for the static 31P NMR spectra are all summarized in Figure 7b and Table 4. We note

that in view of the limited resolution in these static spectra, there are certainly multiple possibilities in arriving at satisfactory four-component fits. Therefore, fitting was done manually, attempting to minimize sample-to-sample variations of the shift tensor components for a given type of phosphorus site. Figure 8 summarizes the 31P spin echo decays of selected samples. Owing to insufficient resolution, it was not possible to distinguish between individual line shape components. Thus, Table 2 summarizes only average homonuclear 31P-31P second moments M2(31P-31P) obtained from a Gaussian analysis of the spin echo decay at evolution times 2t1 e 300-400 µs.18,19 In the Al/P < 1 glasses these M2 values change little with composition, and suggest a substantial contribution from P-O-P linkages. In the Al/P > 1 glasses the dipolar couplings are significantly weaker and decrease progressively with decreasing phosphorus content. As they are significantly smaller than in glassy AlPO4, we can further rule out the formation of AlPO4 clusters. The distinct dependence of the phosphorus local environment on the Al/P ratio is also revealed by the 31P{27Al} REAPDOR results. Figure 9a shows the dephasing curves for two glasses with Al/P < 1. The experimental data are compared to some simulated curves, calculated by using the SIMPSON code.36 In these simulations the REAPDOR dephasing of the 31P magnetization is calculated in the dipolar field of one, two, and three 27 Al nuclei arranged at the vertices of a tetrahedron. A 31P-27Al internuclear distance of 3.1 Å was chosen based on the average Al-O-P distance obtained for crystalline AlPO4. The simulations are based on the specific REAPDOR measurement conditions employed, and on the average 27Al nuclear electric quadrupolar coupling constant measured via TQMAS for the P-bonded aluminum species. For calculating the curves in Figure 9a, an average CQ value of 4.2 MHz was used. Figure 9b shows the analogous plot for three glasses with Al/P > 1. For these glasses, the simulations are based on a relevant CQ value of 5.9 MHz, corresponding to the experimental value obtained via TQMAS for the AlIV species in the x ) 17.5% sample. This value is also assumed to be the appropriate one for the P-bonded Al species of the simulations for the glasses with higher Al contents.37 Parts a and b of Figure 9 show that the dipolar field created by the 27Al nuclei at the 31P sites shows the expected gain as the Al2O3 content increases from 5% to 10%. In contrast, the dipolar field changes very little in glasses with Al/P > 1, suggesting that the extent of P-O-Al connectivity remains more or less constant. In all of these glasses, the dipolar dephasing is weaker than in sol-gel prepared glassy AlPO4, suggesting a somewhat lower degree of average P-O-Al connectivity compared to the latter reference material.

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Aitken et al. TABLE 4: 31P Isotropic Chemical Shifts ((2 ppm), Axiality δax (( 10 ppm) and Asymmetry Parameter η ((0.2), Line Broadening Parameter LB ((100 Hz), as Well as Fractional Contributions from the Deconvolution of the Static 31P NMR Line Shape in xAl2O3:(30 - x)P2O5:70SiO2 glasses (x e 10) x [%] 2.5

5.0

7.5

10.0

(3)

P -I P(3)-II P(2) P(4) P(3)-I P(3)-II P(2) P(4) P(3)-I P(3)-II P(2) P(4) P(3)-I P(2) P(4)

δiso [ppm]

δax [ppm]

η

LB [Hz]

RA [%]

-46.9 -78.0 -25.7 -25.6 -47.8 -81.0 -34.7 -24.0 -35.3 -81.1 -32.0 -33.5 -31.3 -31.0 -33.0

-169 -253 -111

0.07 0.01 0.79

-169 -250 -107

0.07 0.05 0.74

-150 -256 -73

0.05 0.02 0.90

-150 -68

0.0 0.95

1900 3600 11000 7700 2200 4600 14100 8700 4600 4600 31300 13200 6700 22800 11100

36 31 28 5 22 17 54 7 21 7 59 13 13 50 37

dependences with a distinct discontinuity marked near an Al/P ratio of unity. For Al/P < 1, the 27Al spectroscopic observables (27Al chemical shifts and SOQE values as well as dipolar coupling to 31P) show a more or less constant mode of binding of the aluminum species, except for changes in the relative populations of AlIV, AlV, and AlVI, while the spectral parameters of phosphorus (isotropic chemical shifts and dipolar coupling strength to 27Al) reveal monotonic changes. In contrast, for Al/P > 1, the 31P spectroscopic parameters remain more or less constant, whereas continuous changes are evident in the aluminum environment. For both domains, the constant binding modes seen for the respective minority component signify a highly nonrandom structural organization, which is dominated by a preferential interaction between the alumina and the phosphorus oxide components. This preferential interaction is also evident from the fact that for Al/P ) 1 (x ) 0.15), the formation of glasses is completely impeded by the crystallization of AlPO4. In the Al/P < 1 region even more specific evidence for this preferential binding comes from a detailed inspection of the phosphorus and aluminum speciations as a function of composition. As illustrated in Figure 10, the fraction of P atoms present as P(2) units, extracted from the static spin echo spectra, is close to that predicted from the total amounts of AlV and AlVI present in these glasses (based on the chemical analysis results summarized in Table 1), assuming that each AlVI binds to three P(2) units (as in crystalline Al(PO3)3) and each AlV binds

Figure 7. 31P wide-line NMR spectra of xAl2O3:(30 - x)P2O5: 70SiO2 glasses: (a) compositional dependence; (b) deconvolution of representative spectra into distinct line shape components.

Discussion Compositional Evolution of Spectroscopic Parameters and Site Speciations. The Raman and NMR spectroscopic results reported above convey a consistent picture of the structural organization of xAl2O3:(30 - x)P2O5:70SiO2 glasses, which can in turn serve to rationalize the observed property trends. All of the spectroscopic observables show nonlinear compositional

Figure 8. glasses.

P spin echo decay curves of Al2O3:(30 - x)P2O5:70SiO2

31

Mixed-Network Glasses

Figure 9. (a) 31P{27Al} REAPDOR dephasing curves of two representative Al2O3:(30-x)P2O5:70SiO2 glasses with Al/P < 1, corresponding to x ) 5% and 10%. Solid curves represent SIMPSON simulations of the REAPDOR curves for 31P interacting with one, two, and three 27Al neighbors at a distance of 310 pm (2-spin system, three-spin system, and four-spin system, respectively). Simulations are based on a 27Al SOQE of 4.2 MHz in accordance with the TQMAS result for the fourcoordinated Al sites. Experimental data for glassy sol-gel-prepared AlPO4 glass are also included. (b) 31P{27Al} REAPDOR curve for three representative Al2O3:(30 - x)P2O5:70SiO2 glasses with Al/P > 1, corresponding to x ) 17.5%, 22.5%, and 27.5%. The experimental data are compared with simulations based on P interacting with one, two, and three 27Al neighbors (two-spin, three-spin, and four-spin system, respectively). Experimental data for glassy sol-gel-prepared AlPO4 glass are also included.

to 2.5 anionic P(2) species. Likewise, there seems to be a close connection between the total amounts of AlIV species and the fraction of tetrahedral P(4) species obtained from these data. While there are certainly many possibilities of arriving at satisfactory fits to these spectra, the fact that the lineshapes of all four Al/P < 1 glasses can be simulated with reasonably consistent sets of parameters and with phosphorus site distributions close to those predicted from the aluminum speciations, provides substantial support to the structural model delineated above. We note that the behavior of aluminum in the present glasses with Al/P < 1 is in contrast to the behavior of boron in xB2O3: (30 - x)P2O5:70SiO2 glasses, where B coordination in the analogous P-rich glasses is fully tetrahedral38 and suggests that, at least for glasses where the concentration of P2O5 is well in excess of the group-III oxide, the driving force for the formation of BPO4-like domains is larger than the driving force for the formation of AlPO4-like units in the present system. This tendency agrees with observations made in the Al2O3B2O3-P2O5 system, where the formation of four-coordinated boron takes precedence over formation of four-coordinate aluminum.39,40 All of these results reflect the tendency of

J. Phys. Chem. C, Vol. 113, No. 8, 2009 3329 aluminum, an intermediate oxide, to act as a network modifier at low concentrations. For P/Al < 1, only P(4) units are detectable by 31P NMR. However, the δCG values measured by MAS NMR, the continuous decrease of M2(31P-31P) with increasing Al content, and the detailed analysis of the 31P{27Al} REAPDOR curves (see below) suggest that these units are bound in a somewhat different way than in amorphous AlPO4 or in AlPO4-SiO2 glasses. To characterize these differences further, we quantify the extent of P-O-Al connectivity in the present glasses on the basis of 27Al-31P double resonance NMR experiments. Quantification of Al-O-P and P-O-Al Connectivity. The average number of P atoms linked to aluminum, nP(Al), is available from the 27Al{31P} REDOR data. For the AlIV species we obtain this number by comparing the experimental M2(27Al{31P}) values with those measured for sol-gel prepared AlPO4 glass, in which each aluminum atom is tetrahedrally surrounded by four P atoms. The M2 value of this reference material was redetermined as 4.7 × 106 rad2/s2, using the same experimental conditions as employed for the glasses reported here. As Table 2 indicates, the experimental M2(27Al{31P}) values of the low-aluminum glasses (Al/P < 1) are of similar magnitude, albeit always by a factor β ) 0.8-0.85 smaller, suggesting that nP(AlIV) always remains somewhat below the maximum value of four. In a similar vein, the M2(27Al{31P}) values measured for the AlV and AlVI species are comparable to, but somewhat lower than, those measured in other aluminophosphate glasses with Al/P ratios 1, no such site resolved second moment information is available; thus nP(Al) is approximated by the formula

nP(Al) ) 4 × M2(27Al{31P}exp)/M2(27Al{31P}AlPO4) (1b) Analogously the average number of Al atoms linked to phosphorus, nAl(P), can be extracted from Figure 9, parts a and b, by interpolating the simulated REAPDOR curves so as to make a satisfactory fit to the experimental data. Following this approach, nAl(P) is close to one in the sample with x ) 5%, and about 2.3 for the sample with x ) 10%. For higher x-values, nAl(P) remains approximately constant close to 3, i.e. distinctly below the number of 4 expected for AlPO4 glass. As parts a and b of Figure 9 illustrate, the REAPDOR dephasing of the latter reference material is indeed significantly stronger than for any of the present glasses. Table 5 summarizes the experimental data for nP(Al) and nAl(P) obtained from these data analyses. Included are the total numbers of P-O-Al linkages, nP-O-Al, obtained either by multiplying nP(Al) with the aluminum content 2x, or by multiplying nAl(P) with the phosphorus content 2(30 - x). Note that these values, obtained via two independent experiments, are in excellent agreement with each other, thereby validating the above analysis. Comparison with a Cluster Scenario. We now compare the nP-O-Al with a clustering scenario, which maximizes the number of Al-O-P linkages. On the basis of the MAS NMR results presented above, different calculations have to be done depending on the Al/P ratio. For the Al/P < 1 region, this

3330 J. Phys. Chem. C, Vol. 113, No. 8, 2009

Aitken et al.

TABLE 5: Experimental P-O-Al Connectivity Data ((10%) in xAl2O3:(30 - x)P2O5:70SiO2 Glasses x (anal.) % Al2O3 3.35 5.7 8.22 10.5 17.5 20.6 22.2 25.0 27.5

nP (Al) 4.8 4.5 4.1 3.7 2.2 2.0 1.4 0.7 0.34

nAl (P) 1 2.3 2.9 3.3 3.1

nAl-O-P from REDOR

nP-O-Al from REAPDOR

32 51 67 78 77 82 62 35 19

49 91 73 53 16

scenario involves the quantitative reaction of the P2O5 constituent with the Al2O3 constituent to yield amorphous aluminum ´ phosphate-like domains, based on AlIV, AlV, or AlVI units (with the respective concentrations determined via MAS NMR) linked to phosphate ligands. In calculating this scenario, each AlVI unit is assumed to bind three P(2) species (which in turn are linked to two Al), while each AlV unit binds to 2.5 such metaphosphate species. Finally, each AlIV unit binds to one P(4) species which in turn is linked to four AlIV units. Thus we obtain

nP-O-Al(cluster) ) 2x × (f(IV) × 4 + f(V) × 5 + f(VI) × 6)

(2a) For the Al/P > 1 region, the cluster scenario is calculated by assuming that the minority species phosphorus is entirely present as a P(4) species, which binds to four AlIV units. Thus we obtain

nP-O-Al(cluster) ) 2 × (30 -x) × 4

(2b)

Figure 11 plots the number nP-O-Al calculated on the basis of this scenario as a function of x (based on chemical analysis, solid line). As the comparison with the experimental data from Table 5 illustrates, the numbers nP-O-Al are always consistently lower than those predicted by the cluster scenario. The difference is particularly large in the sample with x ) 17.5, suggesting a particularly high degree of bonding disorder in this glass. Overall, Figure 11 documents the clear preference for P-O-Al connectivity in these glasses, but indicates that the glass structure is more homogeneous than that predicted from an AlPO4 segregation scenario. Correlation with Macroscopic Properties. The structural variation described above in which the gradual replacement of

Figure 11. Total number of P-O-Al linkages nP-O-Al as extracted from 27Al{31P}REDOR (solid squares) and 31P{27Al} REAPDOR analysis (open triangles). See text for further details. Solid curves illustrate the prediction from the extreme aluminum phosphate cluster scenario.

P2O5 by Al2O3 in a 70% SiO2 phosphosilicate glass results in the progressive conversion of octahedral and ultimately 5-coordinated Al species on the one hand and P3-type OdPO3/2 groups on the other into associated aluminum phosphate-like groups clearly corresponds to an increase in the degree of polymerization of these glasses. The increasing characteristic temperatures and decreasing R of the P-rich glasses with rising Al2O3 concentration are consistent with this growth in network connectivity. Moreover, this decrease in the concentration of the P(3) type PdO species, which is likely to be the greatest contributor to n, serves to explain the concomitant decrease in the magnitude of this property. For the present data set, network polymerization is maximized for the 17.5Al2O3:12.5P2O5:70SiO2 sample, although the experimental data on sol-gel AlPO4-SiO2 glasses suggests that 70%SiO2 glasses containing ∼15-18% Al2O3 may well be fully polymerized.11 This compositional range corresponds well with the projected minima in n and F shown in Figure 1. Phase separation is particularly evident in the Al/P < 1 region at glass compositions near Al/P ) 1. Thus, glass with x ) 12.5% is visibly translucent and the SEM micrograph (Figure S2, Supporting Information) of a slowly cooled sample of a 10%Al2O3 glass suggests that this material consists of AlPO4- and SiO2-rich domains. With further substitution of Al2O3 for P2O5, the monotonic increase in the concentration of 5-coordinated Al is expected to result in a less-pronounced depolymerization of the glass network and, consequently, one would expect characteristic temperatures or R to decrease or increase, respectively, with yet increased Al2O3 content. Unfortunately, the more limited experimental data for these properties in the Al-rich regime do not permit a firm conclusion. Nevertheless, the increase in n with rising Al2O3 concentration in this compositional range is consistent with the increased presence of nonbridging O arising from 5-coordinated Al. Conclusions

Figure 10. Fractions of P(2) units (blue), of P(3) units (black), and of P(4) groups (red) measured via 31P line shape deconvolution (open symbols) and predicted from the total number of AlIV, AlV, and AlVI species (closed symbols, see text). The solid curve is a guide to the eye connecting the calculated values.

In conclusion, the multinuclear NMR results of the present study illustrate clearly that the structure of Al2O3-P2O5-SiO2 glasses is strongly dominated by preferred interactions between the P2O5 and the Al2O3 components. In the Al/P < 1 region, this interaction produces AlVI and AlV units interacting with metaphosphate (P(2)) species, as well as AlIV units interacting with P(4) groups similar to the situation in AlPO4. Excess P2O5 forms P(3) units linked to silicon and/or phosphate species. In the Al/P > 1 region, the P(4) units constitute the sole phosphorus

Mixed-Network Glasses species, while excess alumina forms AlIV, AlV, and AlVI units linked to the silica component. While the alumina-phosphorus oxide interaction is strong, a detailed analysis of the NMR data obtained in the present study shows that the number of P-O-Al linkages is always lower than the maximum number possible at a given composition. Apparently, the resulting connectivity disorder suffices to suppress phase segregation phenomena, except in the region near Al/P ) 1. In the Al/P < 1 region, additional disorder is generated by the tendency of aluminum to function as a network modifier species (formation of AlV and AlVI species), while in the Al/P > 1 region the decrease of the 31 P-31P dipolar interaction strength with decreasing phosphorus content indicates that the phosphate species are widely distributed spatially and not concentrated in AlPO4-domains. These conclusions are consistent with the compositional dependences of various macroscopic properties. Acknowledgment. Financial support by the SFB 458 network is most gratefully acknowledged. R.R.D. thanks the NRW Graduate School of Chemistry for a doctoral stipend. C. W. Ponader is thanked for the acquisition of Raman spectra. Supporting Information Available: Text dicussing the error considerations in the calculations and figures showing the 31 P{27Al} REAPDOR curve for three representative glasses, and a SEM photomicrograph of a sample of 10Al2O3:20P2O5:70SiO2 glass. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Syritskaya, Z. M. Stekloobraznoe Sostoyanie 1964, 3, 8. (2) Sedmalis, U.; Bolschi, J.; Sedmale, G.; Lagzdina, S. Proc. XI Int. Congr. Glass, Prague 1977, 1, 123. (3) Demskaya, E. L.; Komarova, L. A.; Prokhorova, T. I. Fiz. Khim. Stekla 1989, 15, 579. (4) Wong, J. J. Non-Cryst. Solids 1976, 20, 83. (5) Sen, S.; Youngman, R. A. J. Phys. Chem. B 2004, 108, 7557. (6) DiGiovanni, D. J.; MacChesney, J. B.; Kometani, T. Y. J. NonCryst. Solids 1989, 113, 58. (7) Floerke, O. W. Sci. Ceram. 1965, 3, 13. (8) Dietzel, A. J. Non-Cryst. Solids 1985, 73, 47. (9) Kosinski, S. G.; Krol, D. M.; Duncan, T. M.; Douglass, D. C.; MacChesney, J. B.; Simpson, J. R. J. Non-Cryst. Solids 1988, 105, 45.

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