Investigation of CuI-Containing Molybdophosphate Glasses by

Article pubs.acs.org/JPCC

Investigation of CuI-Containing Molybdophosphate Glasses by Infrared Reflectance Spectroscopy Christos-Platon E. Varsamis,† Efstratios I. Kamitsos,*,† Tsutomu Minami,‡ and Nobuya Machida§ †

Theoretical & Physical Chemistry Institute, National Hellenic Research Foundation, 48 Vassileos Constantinou Avenue, 11635 Athens, Greece ‡ Osaka Prefecture University, Osaka 599-85 31, Japan § Department of Chemistry, Konan University, 8-9-1 Okamoto, Higashinada-ku, Kobe 658, Japan ABSTRACT: Superionic glasses xCuI−(l − x)[Cu2MoO4−CuPO3] were prepared and studied by infrared reflectance spectroscopy to investigate the structure of the oxyanion matrix and the type of sites occupied by copper ions. The study was complemented by the consideration of glasses 0.67CuI−0.33[Cu2MoO4−Cu3PO4] and xCuI−(l − x)[Cu2O-nP2O5] with n = 0.33, 0.50, 1.0. The oxyanion structure of glasses xCuI−(l − x)[Cu2MoO4−CuPO3] was found to involve discrete PO43‑, P2O74‑ and MoO42‑ units, where P and Mo are 4-fold coordinated to oxygen, and molybdate octahedral species which have the MoO3 stoichiometry and are linked by Mo−O−Mo bridging bonds. Despite the nominal metaphosphate composition (CuPO3) of these glasses the spectra gave no signature for metaphosphate structures based on the PO3− unit. Also, increasing amounts of CuI were found to favor the creation of PO43‑ and MoO3 species at the expense of P2O74‑ and MoO42‑. These findings were explained by the acidity order P2O5 > MoO3 and the need to accommodate the bulky CuI in the glassy matrix, a process facilitated by condensed MoO3 octahedral species. Cu ions were found to be present as monovalent cations and to occupy oxide and iodide sites. The latter sites organize into CuI-like pseudophases at high CuI contents, in agreement with the conduction pathway model for superionic glasses.

1. INTRODUCTION Metal molybdates constitute an important family of inorganic materials for applications as diverse as photoluminescence, sensors, ion conductors and catalysts.1 Concerning the particular field of electrical conduction, it was shown over the years that mixing of metal molybdates with P2O5 facilitates greatly the formation of glassy electrolytes which combine high ionic conductivity and improved thermal stability against devitrification.2−30 Unmodified molybdophosphate glasses can be intercalated with Li or Na ions and, thus, they can function as positive electrolyte materials.2 On the other hand, the doping of molybdophosphate glasses with monovalent metal oxides such as Li2O and Ag2O can lead to dc ionic conductivity values as high as 10−2 Sm−1 at room temperature.4−7 These functionalities make molybdophosphates promising materials for electrochemical cells which combine a modified glass as the electrolyte and an unmodified glass as the cathode.6 The use of a common glassy network for both ion diffusion and intercalation eliminates interfacial chemical reactions and minimizes overpotentials.3,6 The introduction of metal halide salts to molybdophosphate glasses was found to improve further ionic conduction and give dc conductivity values as high as 10° Sm−1 in the system CuX− Cu2O−MoO3−P2O5 (X = I, Br, Cl).10−15 It was suggested that the conductivity enhancement is closely related to structural changes induced by the mixing of two glass formers,14,15 but the © 2012 American Chemical Society

exact correlation of ion transport and glass structure is still a subject of investigation. The structure of unmodified molybdophosphate glasses, MoO3−P2O5, was studied by infrared spectroscopy and was proposed to consist of MoO6 octahedral and PO4 tetrahedral units.8 The motif of connection of these units in two- and three-dimensional networks depends on the MoO3 content, and the resulting cross-link density of the glass network is correlated with elastic properties.8,9 Addition of metal oxides introduces negative charges on the MoO3−P2O5 network by breaking P−O−P, Mo−O−Mo, and P−O−Mo bridging bonds, a process leading to increased populations of terminal Mo−O and P−O bonds. Glasses in the system M2O−MoO3−P2O5 (M = Na, K) were studied by infrared spectroscopy, and physical properties were measured in broad composition ranges.18−20 The results supported a structural model based on corner sharing Mo(O1/2)6 octahedra and PO(O1/2)3 tetrahedra (O1/2=bridging and O = terminal oxygen atom), and suggested that addition of M2O breaks the bridging bonds in the order Mo−O−Mo > Mo−O−P > P−O−P and creates terminal Mo− O and P−O bonds.18−20 A study of glasses Li2O−MoO3−P2O5 by X-ray photoelectron spectroscopy (XPS) showed that phosphate groups are preferentially modified by Li2O compared to molybdate groups.7 Also, the formation of Mo− Received: March 26, 2012 Revised: May 4, 2012 Published: May 4, 2012 11671

dx.doi.org/10.1021/jp302876u | J. Phys. Chem. C 2012, 116, 11671−11681

The Journal of Physical Chemistry C

Article

Following this procedure, four glass compositions were prepared in the molybdophosphate system xCuI−(l − x)[Cu2MoO4−CuPO3] where x is the mole fraction of CuI (x = 0.18, 0.33, 0.57 and 0.67), and three phosphate glasses in the system xCuI−(l − x)[Cu2O−nP2O5] with stoichiometries x = 0.40 and n = 1.0 (meta), x = 0.33 and n = 0.5 (pyro), x = 0.30 and n = 0.33 (ortho). The orthomolybdate-orthophosphate glass 0.67CuI−0.33[Cu2MoO4−Cu3PO4] was also prepared to facilitate spectral assignments. 2.2. Infrared Measurements and Data Analysis. Infrared measurements were performed at room temperature in the specular reflectance mode at quasi-normal incidence (11°) in the spectral range 30−5000 cm−1, using a vacuum Fourier-transform spectrometer (Bruker IFS 113 V). Each spectrum is the average of 200 scans with a resolution of 2 cm−1. The measured infrared reflectance spectra were analyzed using two different procedures as detailed in earlier studies.24,31 First, the reflectance curves were fitted using the classical dispersion theory to model the dielectric function:

O−P bridges was shown to be preferred relative to Mo−O− Mo and P−O−P bridges. Studies of glasses in the system NaPO3−MoO3 by NMR, infrared and Raman spectroscopy demonstrated a strong dependence of the network structure on glass composition.26 At low MoO3 contents the replacement of P−O−P by P−O− Mo bridges transforms the one-dimensional metaphosphate chain structure of NaPO3 into a three-dimensional network of interlinked PO4 tetrahedra and MoO6 octahedra. High MoO3 contents lead to formation of neutral phosphate tetrahedral species, which are connected to anionic molybdate species containing nonbridging (terminal) oxygen atoms. Similar results were obtained by NMR and Raman spectroscopy on glasses in the systems MO-MoO3−P2O5 where MZn and Pb.27,28 The structure of cuprous halide-containing molybdate/ phosphate glasses has been studied to a lesser extent, and appears to depend on the nature of halide ion. For example, the incorporation of CuCl in molybdophosphate glasses has been studied by infrared spectroscopy and found to have no affect on the structure of phosphate and molybdate units,21 indicating that this salt acts simply as a plasticizer in facilitating cooperative ionic motion.17,21 While a plasticizing effect was also suggested for CuI in glasses, this was found to be accompanied by structural reorganizations between isomeric tetrahedral and octahedral molybdate species.24 This work presents results by infrared reflectance spectroscopy on the structure of superionic glasses having the orthomolybdate-metaphosphate composition xCuI−(l − x)[Cu2MoO4−CuPO3], where the CuI content varies from x = 0.18 to x = 0.67. To assist identifying structural trends in this system, we have studied in parallel the orthomolybdateorthophosphate glass 0.67CuI−0.33[Cu2MoO4−Cu3PO4] and phosphate glasses xCuI−(l − x)[Cu2O-nP2O5] with n = 0.33, 0.5, and 1.0. As reported elsewhere,24 the reflectance technique is advantageous over transmittance techniques employed in the literature for studies of glasses by infrared. Besides the better quality and correct line shapes of the measured spectra, the proper analysis of reflectance spectra can yield information both on the nature and relative population of the structure-building units. The analysis of far-infrared spectra measured by reflectance can elucidate the nature of sites occupied by metal ions, and the spectroscopic parameters derived for such sites can be employed to calculate the effective charge of the hosted metal ions. This type of knowledge is of particular importance for the family of glasses investigated in this work because the monovalent Cu+ ion may easily oxidize in the melt to the divalent Cu2+ state or it can disproportionate spontaneously to the Cu0 and Cu2+ states.10,11,16 Such processes would have direct implications on the ion transport properties of glass.

ε (̃ ν) = ε′(ν) + iε″(ν) = ε∞ +

∑ j

Δεjνj2 νj2 − ν 2 − iν Γj

(1)

where each Lorentzian oscillator j is characterized by its resonance frequency νj, the bandwidth Γj, and the dielectric strength Δεj. The oscillator parameters νj, Γj, and Δεj and the high-frequency dielectric constant ε∞ constitute the fitting parameters. The second procedure involves Kramers−Kronig analysis of the reflectance data to obtain the absorption coefficient spectra α(ν), and the subsequent deconvolution of α(ν) into Gaussian component bands: α (ν ) =

∑ j

⟨A⟩j π /2 Δνj

exp[−2(ν − νj)2 /Δνj2] (2)

where the adjustable parameters ⟨A⟩j, νj and Δνj are the integrated intensity, resonance frequency and bandwidth, respectively, of the Gaussian component j. The minimization program used in this work to analyze the experimental data is a modified version of the MINUIT routine.32

3. RESULTS 3.1. Infrared Spectra of CuI-containing Phosphate Glasses. For reasons of simplicity we start with the CuIcontaining phosphate glasses because they contain only one glass former oxide, P2O5. The measured infrared reflectance spectra shown in Figure 1 are characterized by complex profiles in the mid-infrared region (above ∼300 cm−1), where infrared activity results from vibrational modes of the network-building phosphate units. In the far-infrared region, below ∼300 cm−1, the measured reflectivity arises from the rattling motions of Cu ions against their sites in glass.24 It is observed that the highestfrequency reflectance band of each spectrum shifts gradually to lower frequency values from the meta-phosphate to the orthophosphate composition, for example, as the degree of network depolymerization increases. This result is suggestive of a progressive charge localization on the oxygen atoms, which renders the P−O bonding more ionic. As a consequence, the average frequency of stretching vibration of P−O bonds decreases from the meta- to the ortho-phosphate glass composition. A similar trend is observed in Figure 2 for the absorption coefficient spectra, α(ν), of the CuI-phosphate glasses, where

2. EXPERIMENTAL SECTION 2.1. Glass Preparation. Reagent-grade Cul, Cu2O, MoO3 and P2O5 were used as raw materials for glass preparation. The raw materials were weighed and mixed thoroughly in a glovebox filled with dry nitrogen gas. In order to preserve the Cu+ oxidation state during melting, all mixtures of fresh starting materials were melted in silica tubes under dry nitrogen (1 atm) for 1 h at 650−750 °C and then quenched in ice water as reported in earlier studies.10,11 Glass plates with dimensions 0.8 × 0.8 × 0.3 cm3 were removed from the silica tubes and were cut and polished for infrared reflectance measurements. 11672



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measured in the absorption spectra of silver metaphosphate glasses in the system yAgI−(l − y)[Ag2O−P2O5].23 Glass stoichiometry suggests that the phosphate network should consist mainly of metaphosphate tetrahedral units [P(O1/2)2O2]−. Indeed, the strong band at 1255 cm−1 is typical of the asymmetric stretching vibration of PO2− groups in [P(O1/2)2O2]− units, νas(PO2−), while the band at 910 cm−1 is related to the asymmetric stretching vibration of P−O−P bridges, νas(P−O−P), connecting [P(O1/2)2O2]− units in metaphosphate chains.23,33−36 Weaker bands at ca. 1145 cm−1 (shoulder) and 715 and 775 cm−1 can be assigned to the symmetric stretching modes νs(PO2−) and νs(P−O−P), respectively, while bending modes of bridging and terminal oxygen atoms, δ(O−P−O) and δ(PO2−), give rise to the broad band envelope peaking at ∼510 cm−1.37,38 The band at 1070 cm−1 can be attributed to the asymmetric stretching mode of terminal PO32− groups, νas(PO32−), in short metaphosphate chains.23,36,37 This assignment is consistent with the absorption spectrum of the pyrophosphate glass 0.33CuI−0.67[Cu2O− 0.5P2O5] which exhibits a strong band at 1078 cm−1 (Figure 2), and with previously reported infrared spectra of pyrophosphate crystalline compounds and glasses which show strong νas(PO32‑) modes in the frequency range 1050−1150 cm−1.39,40 Besides the strong νas(PO32−) mode at 1078 cm−1, the glass 0.33CuI−0.67[Cu2O−0.5P2O5] gives a strong band at 930 cm−1 due to νas(P−O−P) in pyrophosphate units, P2O74‑. Weaker bands at 733 cm−1 and 1023 cm−1 (shoulder) arise from the infrared activity of the symmetric stretching modes of P−O−P bridges and PO32− groups, respectively, and the features at ca. 515 and 570 cm−1 from deformation modes, δ(PO32−), of the terminal groups in P2O74− species.39,40 Assignments of infrared bands for the studied CuI-containing phosphate glasses are summarized in Table 1. The orthophosphate glass 0.30CuI−0.70[Cu2O−0.33P2O5] gives a strong infrared envelope in the range 850−1100 cm−1, and weaker bands from 500 to 700 cm−1. The stoichiometry of this glass indicates that the phosphate species should be present as isolated PO43− units. Previous studies on crystalline orthophosphate compounds39,41 suggest that the component

Figure 1. Infrared reflectance spectra of CuI-containing phosphate glasses. (a) 0.40CuI−0.60[Cu2O−P2O5], (b) 0.33CuI−0.67[Cu2O− 0.5P2O5], and (c) 0.30CuI−0.70[Cu2O−0.33P2O5].

bands are marked for convenience. The α(ν) spectrum of the metaphosphate glass 0.4CuI−0.6[Cu2O−P2O5] exhibits strong bands at ∼1255, 1070, 910, and 510 cm−1. Similar features were

Table 1. Frequencies and Assignments of Infrared Absorption Bands of CuI-Containing Phosphate Glasses xCuI−(l − x)[Cu2O−nP2O5] with meta- (n = 1), pyro- (n = 0.5), and ortho-Phosphate (n = 0.33) Composition (See Figure 2 and Text for Details) glass composition 0.4CuI−0.6[Cu2O− P2O5] band (cm−1)

band (cm−1)

band (cm−1)

assignment

νas(PO2−)

1120, 1078

νas(PO32−)

1145 1070 910

νs(PO2−) νas(PO32−) νas(P−O− P) νs(P−O−P)

1023

νs(PO32−)

930

νas(P−O− P) νs(P−O−P)

∼120 11673

0.31CuI−0.69[Cu2O− 0.33P2O5]

1255

715, 775 ∼510

Figure 2. Absorption coefficient spectra of CuI-containing phosphate glasses: (a) 0.40CuI−0.60[Cu2O−P2O5], (b) 0.33CuI−0.67[Cu2O− 0.5P2O5], and (c) 0.30CuI−0.70[Cu2O−0.33P2O5]. The spectra were calculated by Kramers−Kronig analysis of the reflectance curves shown in Figure 1, and band assignments are summarized in Table 1.

assignment

0.33CuI−0.67[Cu2O− 0.5P2O5]

δ(PO2−) δ(O−P−O) ν(Cu+-site)

733 515, 570

δ(PO32−)

∼120

ν(Cu+-site)

1075, 1008, 980 942

assignment ν3(PO43−) ν1(PO43−)

650, 605, 560

ν4(PO43−)

∼120

ν(Cu+-site)



Article

bands at ca. 1075, 1008, and 980 cm−1 should be attributed to the triply degenerate asymmetric stretching vibration of orthophosphate units, ν3(PO43−), and those at 550−650 cm−1 to the triply degenerate bending mode ν4(PO43−) of the same species (Table 1). Besides these features, a band is observed at 942 cm−1 and is due to the symmetric stretching vibration of PO43− units, ν1(PO43−). The infrared activation of the totally symmetric ν1(PO43−) mode indicates the reduction of the tetrahedral symmetry of PO43− units in glass and justifies the observed splitting of modes ν3 and ν4. 3.2. Infrared Spectra of CuI-containing Molybdophosphate Glasses. Figure 3 shows the reflectance spectra of

Cu3PO4]. The values of the adjustable parameters obtained by best fitting the reflectivity data are given in Table 2. The error in resonance frequency νj is ±2 cm−1 for the stronger bands and ±5 cm−1 for the weaker bands. The corresponding accuracies for the bandwidths Γj are ±5 cm−1 and ±10 cm−1, and for the oscillator strengths Δεj are ±2% and ±8%, respectively. Inspection of Table 2 shows that within experimental error the resonance frequencies and bandwidths of oscillators remain unaffected by CuI content for glasses xCuI−(l − x)[Cu2MoO4−CuPO3], while the corresponding dielectric strengths change with increasing CuI content. The calculated absorption coefficient spectra, α(ν), of molybdophosphate glasses are shown in Figure 4. Increasing CuI content in glasses xCuI−(l − x)[Cu2MoO4−CuPO3] causes a clear evolution of absorption features in the range 750−900 cm−1, while the stronger band is observed at ∼1020 cm−1 as in the case of glass 0.67CuI−0.33[Cu2MoO4− Cu3PO4]. To quantify the effect of CuI on the structure of molybdophosphate glasses, the α(ν) spectra were deconvoluted into Gaussian component bands. Examples are shown in Figure 5 for glass 0.67CuI−0.33[Cu2MoO4−Cu3PO4] and in Figure 6 for glass xCuI−(l − x)[Cu2MoO4−CuPO3] with the x = 0.57. Deconvolution of the α(ν) spectrum of the first glass (ortho− ortho composition) required thirteen components, and that of the orthomolybdate-metaphosphate glass sixteen components. The resulted parameters of Gaussian components are given in Table 3. The errors in determining νj and Δνj are similar to those for the reflectivity fitting procedure, and the error in integrated intensity ⟨A⟩j is ±3% and ±7% for the stronger and weaker bands, respectively. The differences observed between resonance frequencies and bandwidths of corresponding oscillators in Tables 2 and 3 are attributed to the different nature of fitting functions used in analyzing infrared reflectivity and absorption data, that is, Lorentzian versus Gaussian line shapes. In addition, the absorption coefficient α(ν) depends only on the imaginary part k(ν) of the complex refractive index, α(ν) = 4πνk(ν), while the dielectric function ε̃(ν) depends on both imaginary and real part n(ν) of the complex refractive index. Thus, the dispersion of n(ν) influences only the reflectance fitting procedure.

Figure 3. Infrared reflectance spectra of glasses xCuI−(l − x)[Cu2MoO4−CuPO3] with CuI contents x = 0.18−0.67. The spectrum labeled ortho−ortho corresponds to the orthomolybdateorthophosphate glass 0.67CuI−0.33[Cu2MoO4−Cu3PO4]. Solid lines show the experimental spectra and open circle symbols the reflectance curve fitting with eq 1.

4. DISCUSSION 4.1. On the Network Structure of Molybdophosphate Glasses. The structure of orthomolybdate-orthophosphate glass 0.67CuI−0.33[Cu2MoO4−Cu3PO4] will be considered first. An earlier study of glasses CuI−Cu2MoO4−Cu3PO4, using infrared transmission on glass powders dispersed in nujol mulls, suggested the presence of monomeric PO43− and MoO42− ortho-oxoanions.10 The oxide part of this orthomolybdateorthophosphate glass can be rewritten in the form 62.5Cu2O− 25.0MoO3−12.5P2O5. For such an oxide composition the proposed structural modeling18−20 suggests the presence of molybdate octahedral units Mo(O1/2)4O22− with four bridging (O1/2) and two terminal (O) oxygen atoms, in addition to the majority PO43− and MoO42− species. A coexistence of monomeric molybdate tetrahedral units with interconnected molybdate octahedral species would influence glass properties (e.g., glass transition temperature) without affecting the molybdate content of glass as these units are chemical isomers:

orthomolybdate-metaphosphate glasses xCuI−(l − x)[Cu2MoO4−CuPO3] for CuI contents x = 0.18−0.67, in comparison to the spectrum of the orthomolybdate-orthophosphate glass 0.67CuI−0.33[Cu2MoO4−Cu3PO4]. Although the spectra of orthomolybdate-metaphosphate glasses are characterized by similar profiles, changes are observed upon increasing CuI content and include the development of a band at ∼870 cm−1 and the narrowing of the band at 990 cm−1. It is noted that the orthomolybdate-orthophosphate glass shows its high-frequency band also at 990 cm−1 despite its different nominal phosphate content compared to orthomolybdate-metaphosphate glasses. The simulated reflectance curves obtained by best fitting of eq 1 to the experimental spectra are shown also in Figure 3. It was found that a reasonable description of the experimental data is obtained by using a minimum number of sixteen oscillators for glasses xCuI−(l − x)[Cu2MoO4−CuPO3] and thirteen oscillators for glass 0.67CuI−0.33[Cu 2 MoO 4−

Mo(O1/2 )4 O2 2 − ⇔ MoO4 2 − 11674

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Table 2. Fitting Parameters of Eq 1 to the Reflectivity Data of Glasses xCuI−(1 − x)[Cu2MoO4−CuPO3] (x = 0.18−0.67) and 0.67CuI−0.33[Cu2MoO4−Cu3PO4] x = 0.18 j 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

x = 0.33

νj (cm−1)

Γj (cm−1)

Δεj

65 105 170 276 370 502 578 630 670 720 787 888 940 985 1011 1065 ε∞ = 3.68

90 105 125 161 133 121 102 79 85 76 145 89 103 58 90 99

3.240 0.327 0.442 0.840 0.153 0.070 0.116 0.092 0.002 0.020 0.258 0.026 0.018 0.022 0.064 0.074

x = 0.57

νj (cm−1)

Γj (cm−1)

Δεj

νj (cm−1)

65 101 170 275 363 502 581 628 660 729 779 882 939 991 1010 1065 ε∞ = 3.55

81 105 118 170 130 115 107 71 85 79 142 100 120 69 86 90

1.917 0.627 0.346 0.730 0.168 0.061 0.140 0.074 0.005 0.024 0.238 0.042 0.034 0.023 0.100 0.055

55 108 176 278 366 502 581 628 660 720 780 874 930 995 1017 1067 ε∞ = 3.75

0.67CuI−0.33[Cu2MoO4− Cu3PO4]

x = 0.67

Γj (cm−1)

Δεj

82 120 127 159 140 130 106 70 95 90 136 95 120 70 98 99

3.047 1.340 0.329 0.714 0.206 0.077 0.154 0.079 0.011 0.033 0.215 0.046 0.022 0.030 0.120 0.033

νj (cm−1)

Γj (cm−1)

Δεj

58 118 182 280 369 505 579 627 665 731 785 881 929 990 1020 1070 ε∞ = 3.99

90 117 125 160 140 122 109 75 95 89 130 89 105 70 100 92

2.683 1.328 0.245 0.590 0.199 0.065 0.143 0.091 0.024 0.044 0.166 0.055 0.003 0.038 0.139 0.003

νj (cm−1)

Γj (cm−1)

Δεj

57 115 171 285 364 489 581 623 670

90 137 114 147 127 112 77 64 100

3.724 1.817 0.127 0.259 0.114 0.027 0.087 0.044 0.004

789 870 929

101 68 148

0.261 0.015 0.002

1014

76

0.133

ε∞ = 3.95

Figure 5. Deconvolution of the α(ν) spectrum of glass 0.67CuI− 0.33[Cu2MoO4−Cu3PO4] (solid black line) into Gaussian component bands (solid colored lines). The simulated spectrum is shown by open circles.

Figure 4. Infrared absorption coefficient spectra of glasses xCuI−(l − x)[Cu 2 MoO 4 −CuPO 3 ] (x = 0.18−0.67) and 0.67CuI−0.33[Cu2MoO4−Cu3PO4] (ortho−ortho), calculated by Kramers−Kronig analysis of the reflectance spectra shown in Figure 3.

(Table 1). In addition, the very weak component at 936 cm−1 can be associated with ν1(PO43−), while ν2(PO43−) is expected to contribute to the 485 cm−1 band.39−41 Molybdate species should be present mainly as orthomolybdate tetrahedra (MoO42−), as suggested by the strong band at 797 cm−1 and the weaker feature at 885 cm−1 corresponding to the asymmetric ν3(MoO42−) and to the symmetric ν1(MoO42−) stretching modes, respectively.23,25 The bending vibrations of MoO42− units should contribute intensity to the component bands at 368 and 277 cm−1 through modes ν2(MoO42−) and ν4(MoO42−),42,43 while rotation of the discrete MoO42− and PO43− tetrahedra are expected to contribute intensity to the band at 175 cm−1.43 The discussed band assignments are summarized in Table 4.

Therefore, it is of interest to explore the nature of the structurebuilding units in the orthomolybdate-orthophosphate glass. The structural assessment of this glass will rely on the vibrational characteristics of the phosphate and molybdate constituents. For the phosphate species we refer to band assignments summarized in Table 1, while assignments for molybdate species were discussed in our earlier study of CuIcontaining orthomolybdate glasses xCuI−(l − x)Cu2MoO4.24 The presence of orthophosphate tetrahedral units (PO43−) is clearly evident from Figure 5 by the dominant contribution of the 1024 cm−1 band arising from ν3(PO43−). This is also consistent with the three weaker components in the 550−650 cm−1 range where contribution from ν4(PO43−) is expected 11675



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Table 4. Assignments of Bands Obtained by Deconvoluting the Infrared Spectrum of Glass 0.67CuI−0.33[Cu2MoO4− Cu3PO4] (See Figure 5, Tables 2 and 3 and Text for Details) band (cm−1)

assignment

1024 936 885 797 687, 630, 583 485 368 277 175 113 61

ν3(PO4 ) ν1(PO43−) ν1(MoO42−) ν3(MoO42−) ν4(PO43−) ν2(PO43−) ν2(MoO42−) ν4(MoO42−) Cu+-oxide site vibration + rotation (PO43−, MoO42−) Cu+-iodide site vibration Cu+-oxide site vibration 3−

preted in terms of vibrational modes of the PO43− and MoO42− monomers. Therefore, this glass appears to have a completely ionic structure with no phosphate or molybdate network connectivity, in agreement with previous findings for glasses prepared from melts of two or three ortho-oxo borate, silicate or phosphate compositions.44 It is noted though that glass formation in such all-oxide systems requires very rapid quenching by twin-roller apparatus, while the presence of CuI in the studied glass prevents crystallization of the ortho-oxo composition and allows vitrification at considerably lower quenching rates. We turn now our attention to orthomolybdate-metaphosphate glasses, xCuI−(l − x)[Cu2MoO4−CuPO3]. Consideration of stoichiometry alone would suggest a structure consisting of isolated orthomolybdate MoO42− units and chains based on bonded metaphosphate [P(O1/2) 2O2]− units. Inspection of Figure 6 shows that MoO42− tetrahedra are indeed part of glass structure as testified by the typical ν3(MoO42−) mode at 796 cm−1. However, this is not true for metaphosphate [P(O1/2)2O2]− species, because the characteristic νas(PO2−) mode at ∼1250 cm−1 (Figure 2) is clearly absent from Figure 6. Instead, the strong infrared component at

Figure 6. Deconvolution of the α(ν) spectrum of glass 0.57CuI− 0.43[Cu2MoO4−CuPO3] (solid black line) into Gaussian component bands (solid colored lines). The simulated spectrum is shown by open circles.

Molybdate octahedra Mo(O1/2)4O22− would manifest their presence by a strong infrared band in the range 940−980 cm−1 due to stretching of the terminal Mo−O bonds, ν(Mo−O).24 Also, medium-to-strong bands would be expected in the range 775−960 cm−1, due to ν(Mo−O1/2), and in ranges 630−640 and 420−445 cm−1 due to bending modes δ(O−Mo−O) and δ(O1/2−Mo−O1/2), respectively. The absence from Figure 5 of a clear signature for ν(Mo−O) in the range 940−980 cm−1 suggests that octahedral Mo(O1/2)4O22‑ species should be rather excluded from the structure of this glass, at least in terms of sensitivity of the infrared reflectance spectroscopy. Based on the previous discussion and the inspection of Table 4 and Figure 5, we suggest that the key infrared signatures of glass 0.67CuI−0.33[Cu2MoO4−Cu3PO4] can be fully inter-

Table 3. Deconvolution Parameters of the Absorption Coefficient Spectra of Glasses xCuI−(1 − x)[Cu2MoO4−CuPO3] (x = 0.18−0.67) and 0.67CuI−0.33[Cu2MoO4−Cu3PO4] x = 0.18

x = 0.33

x = 0.57

0.67CuI−0.33[Cu2MoO4− Cu3PO4]

x = 0.67

j

νj (cm−1)

Δνj (cm−1)

⟨A⟩j (104 cm−2)

νj (cm−1)

Δνj (cm−1)

⟨A⟩j (104 cm−2)

νj (cm−1)

Δνj (cm−1)

⟨A⟩j (104 cm−2)

νj (cm−1)

Δνj (cm−1)

⟨A⟩j (104 cm−2)

νj (cm−1)

Δνj (cm−1)

⟨A⟩j (104 cm−2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

70 105 175 274 370 484 580 634 699 739 799 882 942 990 1027 1080

56 70 104 100 100 100 91 70 72 56 103 68 65 59 68 84

1.95 1.70 9.20 15.30 15.80 14.28 22.82 16.61 14.31 5.50 50.7 18.00 20.81 20.49 31.01 40.02

70 106 176 274 370 480 584 632 699 740 795 882 949 995 1030 1082

56 65 84 100 100 97 98 69 66 58 91 73 68 55 68 80

1.94 2.59 4.49 13.25 12.54 7.65 21.81 16.52 11.39 7.01 43.5 21.11 20.01 16.3 39.01 33.00

65 110 180 275 370 480 581 637 699 741 796 883 949 995 1031 1084

45 69 81 100 99 99 91 65 61 50 88 74 70 62 72 80

1.62 4.64 6.14 15.87 14.16 13.52 25.44 15.6 14.10 7.00 42.80 28.20 17.50 19.50 43.01 23.51

65 114 181 277 372 485 583 640 693 739 797 886 945 988 1030 1088

46 71 87 98 99 95 92 69 50 60 92 73 63 63 78 84

1.52 5.07 6.32 14.35 14.16 11.10 26.61 15.97 4.91 12.33 38.10 32.24 6.87 14.39 57.01 13.99

61 113 175 277 368 485 582 630 687

46 62 86 100 100 105 85 51 60

2.26 4.13 5.46 7.51 7.11 7.31 11.71 7.49 7.76

797 886 936

90 60 42

57.77 9.71 2.61

1024

80

63.41

11676



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1030 cm−1 points to the presence of isolated orthophosphate units, as this band can be related to the ν3(PO43−) mode. In addition, there are component bands resolved at ∼740, 945, and 1085 cm−1 (see Tables 2 and 3) which can be assigned to stretching modes of the pyrophosphate P2O74− anion (see Table 4). This surprising result shows an overmodification of the phosphate component in comparison to the nominal metaphosphate composition of glass. As a consequence, one expects a corresponding under-modification of the molybdate component in glass with respect to its nominal orthomolybdate composition. When molybdate and phosphate species coexist in glass-forming melts they can influence each other’s level of modification by reactions like those given below: MoO4 2 − + 2PO3− → P2O7 4 − + MoO3

(4)

MoO4 2 − + PO3− → PO4 3 − + MoO3

(5)

Table 5. Assignments of Bands Obtained by Deconvoluting the Infrared Spectrum of Glass 0.57CuI−0.43[Cu2MoO4− CuPO3] (see Figure 6, Tables 2 and 3 and Text for Details) frequency (cm−1) 1085 1031 994 949 882 795 740 698, 638, 581 480 370 275 180 110 59

Reaction 4 presents a mild overmodification of the metaphosphate species, while operation of reaction 5 leads to the complete phosphate modification. The overmodification of PO3− species at the expense of MoO42−, that is, the preferential bonding of nonbridging oxygen atoms to the phosphate than the molybdate groups, is consistent with the suggested order of acidity of the glass-forming oxides: P2O5 > MoO3.7,11 Indeed, the optical basicity values assigned to these oxides are Λ(P2O5) = 0.4045 and Λ(MoO3) = 1.04,46 showing that P2O5 is much more acidic than MoO3. In addition to the infrared signatures discussed above for the discrete P2O74− and PO43− anions, the validity of processes (4) and (5) requires evidence for the presence of neutral molybdate MoO3 species. Such species could exist in the form of linked molybdate octahedra similar to those in crystalline MoO3. This compound contains distorted MoO6 octahedra in which one oxygen atom is doubly bonded to the molybdenum atom (MoO), two of the oxygen atoms are common to two octahedra (i.e., Mo2−O bridges or Mo−O1/2 bonds) and the remaining three oxygen atoms are common to three molybdate octaherda (i.e., Mo3−O bridges or Mo−O1/3 bonds).47 Bands in the infrared spectrum of crystalline MoO3 have been assigned to vibrations of different molybdenum−oxygen bonds and involve: the stretching of the shorter MoO bond, ν(MoO), which gives47 a strong infrared band at 985 or 995 cm−1,48,49 the asymmetric stretching of Mo2−O bridges, νas(Mo−O−Mo), giving a strong band at 870 cm−1 and the stretching of Mo3−O bridges, ν(Mo3−O), at 565 cm−1.48,49 Also, bending vibrations of MoO6 octahedra, δ(MoO6), have been reported at 460−490 cm−1 and at 378 cm−1.48−50 It is noted that the Raman spectrum of crystalline MoO3 is dominated by bands at 820 and 996 cm−1 due to νs(Mo−O− Mo) and ν(MoO), respectively.26 Tables 2 and 3 and Figure 6 show a strong component at 985−995 cm −1 , where ν(MoO) of distorted MoO 6 octahedra would be expected. In addition, the enhanced intensity of the ∼885 cm−1 band in Figure 6 and in Tables 2 and 3 cannot be justified by the weak activation in the infrared of the ν1(MoO42‑) alone. On the other hand, this enhanced intensity can be explained by an additional contribution originating from the νas(Mo−O−Mo) mode of distorted MoO6 octahedra (Table 5). Thus, the infrared characteristics of glasses xCuI−(l − x)[Cu2MoO4−CuPO3] are supportive of the chemical reactions 4 and 5. Besides neutral MoO3 species, pyromolybdate Mo2O72− dimers are also modified to a lesser degree than the isolated

assignment νas(PO32−) ν3(PO43−) ν(MoO) νas(P−O−P) + ν1(PO43−) νas(Mo−O−Mo) + ν1(MoO42−) ν3(MoO42−) νs(P−O−P) ν4(PO43−) + ν(Mo3−O) ν2(PO43−) + δ(MoO6) ν2(MoO42−) + δ(MoO6) ν4(MoO42−) Cu+-oxide site vibration + rotation (PO43−, MoO42−) Cu+-iodide site vibration Cu+-oxide site vibration

orthomolybdate MoO42− tetrahedra. A previous spectroscopic study of glasses in the system AgI−Ag2O−MoO3 with Ag2O/ MoO3 < 1 showed that these glasses have complex structures, which consist of both discrete MoO42− monomers and condensed molybdate macroanions.25 It was suggested that the structure of the polynuclear molybdate species is similar to the chain structure in crystalline sodium-pyromolybdate (Na2Mo2O7) which involve MoO6 octahedra linked to MoO4 tetrahedra, instead of the discrete Mo2O72− dimer species. This indicates that dimolybdate anions disproportionate easily in the melt, that is, Mo2O72− → MoO42− + MoO3, in accordance with the coexistence of MoO42− and MoO3 species in the orthomolybdate-metaphosphate glasses studied here (see eqs 4 and 5). 4.2. Effect of CuI on the Structure of Molybdophosphate Glasses. Increasing the amount of CuI in glasses xCuI− (l − x)[Cu2MoO4−CuPO3] causes progressive changes in the infrared spectra shown in Figure 4, where the most noticeable effect is the development of a band at ∼880 cm−1 at higher CuI contents. Additional changes are observed in Tables 2 and 3 and concern the oscillator strength or integrated intensity of the component bands, while resonance frequencies and bandwidths are practically unaffected by CuI content. To quantify the effect of CuI on the orthomolybdatemetaphosphate glass structure we consider bands 10−16 (Tables 2 and 3) because the stretching modes of phosphate and molybdate species show a smaller degree of overlapping compared to their bending modes (Table 5). Following the assignments in Table 5, the evolution of PO43− species was probed using the intensity of band 15, ν3(PO43−), increased by the contribution of mode ν1(PO43−) to band 13. Using the data for glass 0.67CuI−0.33[Cu2MoO4−Cu3PO4] in Table 2 we take the contribution of ν1(PO43−) to be ∼2% of that of ν3(PO43−). For P2O74− species, we consider the sum of intensities of bands 10, 13, and 16, reduced by 2% of ν3(PO43−) (band 15). In accordance to orthophosphate PO43− species, we considered for orthomolybdates MoO42− the intensity of band 11, ν3(MoO42−), increased by 6% of ν3(MoO42−) to account for the contribution of ν1(MoO42−) at ∼885 cm−1 (band 12). Along these lines, the total intensity of bands 12 and 14 reduced by 6% of ν3(MoO42−) (band 11) was assigned to neutral molybdate octahedral species (denoted by MoO3). 11677



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reflects the effect of CuI on glass fictive temperature. In conclusion, the need for accommodation of the bulky CuI salt in the glass matrix drives the formation of more condensed oxyanion species as this leaves more room to be occupied by CuI. 4.3. Copper Ion Sites in Molybdophosphate Glasses. The nature of copper ion sites in glasses under investigation can be deduced by analyzing the far-infrared region, as shown in Figure 8 for glasses xCuI−(l − x)[Cu2MoO4−CuPO3]. It was

Since the extinction coefficients of the various modes considered above are not known, we restrict our calculation to the relative infrared intensity of bands characterizing the phosphate and molybdate species. The relative intensities were obtained using the data of Table 2 and are shown in Figure 7 vs

Figure 7. Effect of CuI on the structure of glasses xCuI−(1 − x)[Cu2MoO4−CuPO3] probed by the relative intensities of bands characterizing orthophospahe tetrahedral, PO43−, pyrophosphate dimeric, P2O74−, orthomolybdate tetrahedral, MoO42−, and molybdate octahedral, MoO3, species. Lines through data points are drawn to guide the eye. For details see text. Figure 8. Far-infrared absorption coefficient spectra of glasses xCuI− (l−x)[Cu2MoO4−CuPO3] (solid black lines) and their deconvolution into Gaussian component bands (solid colored lines and dash dotted black lines). The simulated spectra are shown by open circles.

CuI content. It is clear that addition CuI causes the progressive increase of the relative abundance of orthophosphate, PO43−, and neutral molybdate octahedral species, MoO3, at the expense of MoO42− and P2O74− units. Similar trends are obtained on the basis of data in Table 3. These findings suggest that reaction 5 prevails over reaction 4 at increasing CuI contents. The redistribution of negative charge on the oxyanion species drives the phosphate units to nearly complete depolymerization, while part of the molybdate component in the structure transforms to molybdate octahedra which are cross-linked by Mo−O−Mo bridges. The observed effect of cuprous iodide on the oxyanion glass matrix takes place despite the fact that CuI is known to be an inert salt. Structural rearrangements induced by AgI and CuI salts were reported before for borate23,51,52 and molybdate24 glasses. These findings point toward a general trend in superionic glasses, which is related to the progressive decrease of glass transition temperature as the metal halide salt is added in increasing loads to a fixed oxyanion composition.13,24,51 The nature and relative abundance of the oxyanion units will reflect naturally the structure arrested at the fictive temperature, as the supercooled liquid is “frozen” into the glassy state. It is expected that structures arrested at lower fictive temperatures at constant pressure would involve on the average more condensed oxyanion species to absorb the effect of decreasing temperature (Le Chatelier’s Principle), which is analogous to increasing pressure at a constant temperature.53 Assuming that fictive temperature has composition dependence similar to that of glass transition temperature, Figure 7 shows a progressive change of Mo coordination number from four to six and

found that spectral analysis of the far-infrared profiles requires at least four Gaussian components for the absorption spectra (Table 3) or four Lorentzian components for the reflectance spectra (Table 2). Vibrational contributions of species MoO42− and PO43− to bands at ∼275 and 180 cm−1 have been presented above and summarized in Table 5. The remaining activity in the far-infrared region can be discussed in terms of Cu ion-site vibrations on the basis of earlier findings for AgI-containing metaphosphate glasses.23 The far-infrared spectra of the latter glasses could be best fitted with three bands at ∼50, 95, and 135 cm−1 and were attributed to Ag ion-site vibrations in oxide (50 and 135 cm−1) and iodide (95 cm−1) environments. For these CuI-containing glasses, the frequencies of corresponding Cu ion-site vibrations are expected to shift to higher values because of the lower mass of Cu ion relative to the Ag. In particular, if we assume that the reduced mass of metal ion-site oscillation is approximated by the bare metal ion mass and the force constant of the corresponding Cu ion-site and Ag ion-site modes remains nearly constant, then the frequency of Cu ionsite oscillations would be proportional to those of Ag ion-sites with a proportionality factor equal to (mAg/mCu)1/2 ≈ 1.3. On the basis of these assumptions, it is reasonable to associate the contribution of Cu ion-oxide site vibrations with bands at ∼65 and 175 cm−1 and that of Cu ion-iodide sites with the band at ∼110 cm−1.23,24 It should be noted that the formation of some mixed Cu ion-oxyiodide sites cannot be excluded entirely. 11678



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of conduction pathways along the metal iodide domains in glass.56 The relative population of Cu ion-oxide sites in glass reduces progressively with CuI content, Figure 9a, in accordance with the decrease in oxide content of glass. However, the frequency for these oxide sites is practically unaffected by addition of CuI, Figure 9b, implying that the average negative charge on oxides sites does not vary with glass composition.57 Indeed, reactions 4 and 5 maintain the total negative charge of oxide sites even though CuI was found to induce structural rearrangements within the phosphate and molybdate parts of the oxyanion structure (Figure 7). So far we have discussed copper ion sites in molybdophosphate glasses with no reference to the effective charge of Cu ions. As shown elsewhere,24 proper analysis of the far-infrared profiles can provide information about effective ionic charges. This is particularly important for the present glasses for reasons stated in the Introduction. Knowledge of the oscillator strength,Δεjν2j , or of the integrated intensity, ⟨A⟩j, of cationsite vibrations allows for the evaluation of the associated effective charge qeff through the following equations:

As shown in Figure 8, the far-infrared profiles are affected by the CuI content in glass. This is manifested mainly by the enhancement of the ca. 110 cm−1 band (Figure 8 and Figure 9a), in agreement with the assignment made above. The

Δεjνj2 =

⟨A⟩j = Figure 9. Effect of CuI on the far-infrared spectra of glasses xCuI−(1 − x)[Cu2MoO4−CuPO3]. (a) Relative intensity of the ca. 110 cm−1 band attributed to Cu−I sites, and total relative intensity of bands at ca. 60 and 180 cm−1 associated with Cu−O sites. (b) Band frequencies as a function of CuI content. Lines through data points are drawn to guide the eye. For details see text.

2 1 ⎛ ε∞ + 2 ⎞ Nj 2 ⎜ ⎟ q 4π 2c 2e0 ⎝ 3 ⎠ meff eff

(6)

2 1 ⎛ ε∞ + 2 ⎞ Nj 2 ⎜ ⎟ q 4c 2e0 ⎝ 3 ⎠ njmeff eff

(7)

where Nj is the concentration of ions with effective charge qeff and effective mass of vibration meff; nj is the index of refraction at the resonance frequency; e0 is the permittivity of free space and c is the speed of light.58,59 The high frequency dielectric constant of glasses, ε∞, was determined by the fitting procedure (Table 2), and meff was approximated by the mass of copper. To calculate the concentration of Cu ions in oxide and iodide sites we assumed that Cu ions introduced by CuI occupy only iodide sites. Subsequently, for glasses xCuI−(l − x)[Cu2MoO4−CuPO3] the concentration of copper ions in oxide, NCu−O, and iodide environments, NCu−I, was calculated from the expressions

frequency of the Cu ion-iodide site vibration, Figure 9b, remains lower than the value of 124 cm−1 found for crystalline CuI and attributed to vibrations of Cu ions against their tetrahedral iodide sites.54 This result may suggest the existence of distorted CuI-like sites in the molybdophosphate glassy matrix, in agreement with the earlier anomalous X-ray scattering study of glass 0.46CuI−0.54Cu2MoO4 glass.55 The slight increase of the Cu ion-iodide site vibration frequency with CuI content, Figure 9b, may indicate a tendency toward aggregation of CuI-like sites into CuI-like pseudophases in glass. Similar formation of AgI-like sites and their clustering into AgI-like domains was reported for superionic borate glasses,23,51,52 in line with earlier propositions for the creation

NCuO =

NCuI =

3(1 − x) NA Vmol

(8a)

x NA Vmol

(8b)

Table 6. Density, d, Molar Volume, Vmol, Concentration of Cu Ions in Oxide, NCu−O, and Iodide, NCu−I, Environments and Corresponding Effective Charges, qCu−O and qCu−I, in Glasses xCuI−(1 − x)[Cu2MoO4−CuPO3] Where x is the Mole Fraction of CuIa

a

x

d (g cm−3)

Vmol (cm3 mol−1)

NCu−O (1028 m−3)

NCu−I (1028 m−3)

0.18

4.80 ± 0.01

80.51 ± 0.04

1.840 ± 0.004

0.135 ± 0.001

0.33

4.84 ± 0.01

72.44 ± 0.05

1.671 ± 0.003

0.274 ± 0.001

0.57

4.95 ± 0.01

59.24 ± 0.06

1.312 ± 0.002

0.580 ± 0.001

0.67

4.99 ± 0.01

53.97 ± 0.06

1.105 ± 0.002

0.748 ± 0.001

qCu−O (e) 0.99 0.65 0.91 0.54 0.99 0.64 0.98 0.68

± ± ± ± ± ± ± ±

0.01(R) 0.01(A) 0.01(R) 0.01(A) 0.01(R) 0.01(A) 0.01(R) 0.01(A)

qCu−I (e) 0.96 0.92 0.93 0.83 0.97 0.75 0.91 0.67

± ± ± ± ± ± ± ±

0.01(R) 0.01(A) 0.01(R) 0.01(A) 0.01(R) 0.01(A) 0.01(R) 0.01(A)

Note that R and A denote effective charges calculated through eqs 6 and 7, respectively. 11679



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where NA is Avogadro’s number and Vmol is the molar volume of glass determined from its density (Table 6). The effective charge of Cu ions in iodide sites, qCu−I, was calculated using the spectroscopic parameters (Δεj, νj) and (⟨A⟩j, nj) of the ∼110 cm−1 component band and eqs 6 and 7, respectively. The corresponding parameters of bands at ∼60 and 175 cm−1 were considered together to calculate an average effective charge of Cu ions in oxide sites, qCu−O, by ignoring any contribution of PO43− and MoO42− rotations to the ∼175 cm−1 band. The calculated effective charges are reported in Table 6 and show that the formal oxidation state of Cu ions is +1 in the studied glasses, in agreement with earlier results on other CuIcontaining glass compositions studied by XPS13 and infrared spectroscopy.24 Therefore, we conclude that the preparation procedure employed in this work gives glasses of monovalent Cu ions. We note that effective charges calculated using eq 7 are usually lower than those obtained from eq 6. This effect can be attributed to the different functional dependence of absorption and reflectance spectra on the real and imaginary part of the complex refractive index, and also to the different fitting functions used to analyze these spectra. Considering these aspects and the approximations made in using eqs 6 and 7, the results in Table 6 show that charges qCu−O and qCu−I are practically unaffected by CuI content despite the effect of CuI on the oxyanion glass matrix as manifested by the mid-infrared spectra.

coordination from four to six manifests the readjustment of the glassy matrix to accommodate the bulky CuI salt. This is effected by formation of condensed molybdate octahedral species which leaves more room to be occupied by CuI. The nature of Cu ion sites was investigated by analyzing the far-infrared spectral profiles. It was found that Cu ions occupy oxide and iodide sites with relative populations consistent with glass composition. The calculation of effective charges of copper ions in oxide and iodide sites showed that they remain monovalent under the employed glass preparation technique. The vibration frequency of Cu ions against their iodide sites was found to increase with CuI content toward the characteristic frequency of crystalline CuI, suggesting the organization of iodide sites into CuI-like pseudophases.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +30 2107273828. Fax: +30 2107273794. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported partially by the “Excellence in the Research Institutes” program, supervised by the General Secretariat for Research and Technology in Greece, and by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan.

5. CONCLUSIONS Infrared reflectance spectroscopy was employed to investigate the structure of the glassy matrix and the nature of Cu ion sites in CuI-containg glasses with compositions: orthomolybdatemetaphosphate xCuI−(l − x)[Cu2MoO4−CuPO3] (x = 0.18− 0.67), orthomolybdate-orthophosphate 0.67CuI−0.33[Cu2MoO4−Cu3PO4] and phosphate xCuI−(l − x)[Cu2O− nP2O5] (n = 0.33, 0.5, 1). The reflectance spectra were analyzed by curve fitting and by Kramers−Kronig transformation and both methods of analysis were found to give comparable results. Consideration of the mid-infrared spectra showed that the majority of oxyanion species in the phosphate glasses respect the nominal stoichiometry, that is, orthophosphate PO43− monomers for n = 0.33, pyrophosphate P2O74− dimers for n = 0.5, and metaphosphate chains based on the PO3− unit for n = l. Similarly, the spectrum of orthomolybdate-orthophosphate glass was found consistent with stoichiometry; in particular with the presence of orthomolybdate MoO42− and orthophosphate PO43− tetrahedral species. The situation is drastically different for orthomolybdate-metaphosphate glasses xCuI−(l − x)[Cu2MoO4−CuPO3] because no evidence was found for the presence of metaphosphate PO3− units. Instead, spectral analysis revealed the formation of PO43− and P2O74− units, molybdate octahedral species with stoichiometry MoO3 and tetrahedral MoO42‑ units. Therefore, the phosphate groups are overmodified and the molybdate groups under-modified with respect to the nominal orthomolybdate-metaphosphate composition. This result is in agreement with the order of acidity of the glass-forming oxides, P2O5 > MoO3, which leads to nonbridging oxygen rearrangements between phosphate and molybdate species through chemical reactions operating in the melt. It was found that increasing the amount of CuI in glasses xCuI−(l − x)[Cu2MoO4−CuPO3] causes the progressive formation of PO43− and MoO3 species at the expense of P2O74− and MoO42− units. The gradual change of Mo

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