Crystal-Like Atomic Arrangement and Optical Properties of 25La2O3–75MoO3 Binary Glasses Composed of Isolated MoO42–

Transparent and brown La2O3–MoO3 binary glasses were prepared in bulk form using a levitation technique. The glass-forming range was limited, with the primary composition being approximately 25 mol % La2O3. The 25La2O3–75MoO3 glass exhibited a clear crystallization at 546 °C, while determining its glass transition temperature was difficult. Notably, despite its amorphous nature, the glass possessed a density and packing density comparable to those of crystalline La2Mo3O12. X-ray absorption fine structure and Raman scattering analyses revealed that the glass structure closely resembles La2Mo3O12 due to the presence of isolated MoO42– units, whereas disordered atomic arrangement around La atoms was confirmed. The glass demonstrated transparency ranging from 378 to 5500 nm, and the refractive index at 1.0 μm was estimated to be 2.0. The optical bandgap energy was 3.46 eV, which was slightly smaller than that of La2Mo3O12. Additionally, the glass displayed a transparent region ranging from 6.5 to 8.0 μm. This occurrence results from the decreased diversity of MoOn units and connectivity of Mo–O–Mo, which resulted in the reduced overlap of multiphonon absorption. This glass formation, with its departure from conventional glass-forming rules, resulted in distinctive glasses with crystal-like atomic arrangements.


■ INTRODUCTION
Oxide glasses are constructed on a three-dimensional network of MO n polyhedra where element M and oxygen (O) are bonded covalently.The constraints in glass formation are that the oxygen coordination number, n, should be as small as three or four, and that the MO n polyhedra are connected to each other not by edge-or face-sharing but by corner-sharing.SiO 2 , P 2 O 5 , etc., can form corner-sharing MO 4 tetrahedral networks, and they are thus referred to as network former oxides.Modifier oxides, such as alkali metal oxides, alkaline earth metal oxides, and rare earth oxides, typically break the network and introduce nonbridging oxygens.Al 2 O 3 , TiO 2 , Nb 2 O 5 , and other intermediate oxides act as either a network former or modifier, depending on their composition in a glass. 1,2In addition, among the intermediate oxides that cannot vitrify without additives, Al 2 O 3 , Ga 2 O 3 , MoO 3 , and WO 3 can vitrify relatively easily, even at a binary composition, in combination with a specific modifier or intermediate.They are especially called conditional glass formers. 3The glass-forming region of the binary system that includes conditional glass formers is generally narrow.Nevertheless, an additional small amount of network former oxide added to a binary system drastically improves the glass-forming ability.MoO 3 is a conditional glass former, and glass formation in some of its binary systems has been reported.However, compared to Al 2 O 3 -and Ga 2 O 3 -based binary glasses, the number of reports is considerably small.Nassau et al. prepared Li 2 O−MoO 3 binary glasses during a search for glasses with high ionic conductivity in the 1980s. 4However, glass formation required the use of the twin-roller quenching method with a very high cooling rate (10 4 −10 6 K/s).Ag 2 O− MoO 3 binary glasses with a thickness of approximately 1 mm were obtained by a conventional melt-quench method to investigate their ionic conductivity. 5Recent structural analysis combined with X-ray and neutron diffraction revealed that the coordination number of Mo decreased from 5.4 to 4.2 with increasing Ag 2 O content and a variety of MoO n polyhedra (n = 4, 5, and 6) connected to each other to develop the network structure. 6−9 They succeeded in the glass formation of the 10La 2 O 3 −10Nd 2 O 3 −80MoO 3 and 10R 2 O 3 −90MoO 3 systems (R = La or Nd) using slow cooling rates (10 2 K/s).It was reported that the 10La 2 O 3 −90MoO 3 glass had the glass transition temperature at around 325 °C and crystallization temperature at 410 °C.From structural analysis using Fourier transform infrared (FT-IR) and X-ray absorption fine structure (XAFS) spectroscopies, it was suggested that the glass network was build up with cornersharing MoO 6 or MoO 5 without short isolated Mo=O bonds. 9 levitation technique is an effective method to vitrify a composition without a sufficient network former oxide in bulk form despite its low glass-forming ability.−13 New glass compositions and the expansion of the glass-forming region were reported in systems with conditional glass formers as the main component.−19 In this study, La 2 O 3 −MoO 3 binary glasses were successfully obtained by a levitation technique in the bulk form.Physical properties of the binary glasses, including thermal and optical properties, were measured.Structural analysis using Mo L 3 -edge, K-edge, and La L 3 -edge XAFS and Raman scattering spectroscopies were performed to investigate the atomic arrangement of the glass by comparing it with the crystalline phase structure with the same composition as the glass.

■ EXPERIMENTAL PROCEDURE
Glass syntheses of the (100−x)La 2 O 3 −xMoO 3 binary system were examined by a levitation technique.High purity La 2 O 3 and MoO 3 were stoichiometrically mixed and pressed into pellets.The pellets were sintered at 600 °C for 12 h in air and then were crushed to target pieces for the aerodynamic levitation (ADL) furnace.A piece of the target was placed on the nozzle of the ADL furnace and levitated by an O 2 gas flow.A CO 2 laser was used to melt the levitated sample for several seconds.The melt was rapidly cooled to room temperature by turning off the laser power, and then it solidified.The cooling rate was estimated to be approximately 500 °C/sec.Glass formation was confirmed by Cu Kα X-ray diffraction measurements (XRD: Rigaku, MiniFlex600).Although the size of the fabricated glasses was approximately 2 mm, the thermal, optical, and structural properties could be sufficiently investigated.Crystalline La 2 Mo 3 O 12 as a reference was prepared by conventional solid-state reaction.High purity La 2 O 3 and MoO 3 were stoichiometrically mixed and pressed into a pellet, and it was sintered in air at 800 °C for 24 h several times with intermediate pulverization.
The glass transition temperature (T g ) and crystallization temperature (T x ) for the glasses were investigated by using differential scanning calorimetry (DSC: NETZSCH, DSC 404 F1 Pegasus).The temperature was raised from room temperature to 700 °C with a heating rate of 10 °C/min in a N 2 gas flow.Prior to measurement of the physical properties, the glasses were annealed at 475 °C for 5 min to remove any internal stresses.Furthermore, post annealing of the glass in an oxidized atmosphere was conducted to diminish Mo 5+ and improve transparency at the 400−600 nm region.The glasses were annealed in an O 2 gas flow at 400 °C for 24 h.The density of the glasses was measured using a He-gas pycnometer (micromeritics, AccupycII 1340).More than a dozen glass beads were placed in a 0.1 cc sample cell to reduce experimental errors.
Mo L-edge XAFS measurements were performed at the soft X-ray beamline, BL-10, in the SR Center of Ritsumeikan University. 20The incident X-ray was monochromated with a double crystal using Ge(111) planes.Powdered samples of glasses and reference crystals were put on the carbon seal on the sample holder in a high vacuum chamber.The X-ray absorption spectra were recorded with the fluorescence yield method.Photon energy was calibrated at the energy of a peak (2481.69eV) of white line of S K-edge XANES spectra of K 2 SO 4 standard samples.
−23 The incident X-ray was monochromated with a double crystal by using Si(111) planes.The samples were ground into powders and mixed with a high purity hexagonal boron nitride powder to form pellet specimens.Energy calibration for Mo K-edge spectra was performed by setting the inflection point of the spectra of a metallic Mo foil as E = 20003.9eV and that for the La L 3 -edge was done by setting the energy of the preedge peak of the Ti K-edge of a metallic Ti foil as E = 4966.0eV.XAFS data analysis was conducted using the Athena and Artemis programs in the Demeter software package. 24The k 3 -weighting EXAFS spectra were Fourier-transformed (FT) to real space.Structural parameters regarding the first peak in the FT magnitude, such as the oxygen coordination number of Mo and La, the cation− oxygen distance, and the Debye−Waller (DW) factor, including both structural and thermal disorder, were evaluated by curve fitting analysis using phases and amplitude functions calculated using the FEFF8 code.
Unpolarized Raman scattering spectra of the glasses and crystalline La 2 Mo 3 O 12 were obtained in a 180°-scattering geometry by a confocal Raman microscope (JASCO, NRS-4500).The incident source was a 532 nm semiconductor laser.
For transmittance measurements, both sides of the glass samples were optically polished to a thickness of approximately 500 μm.Transmittance spectra were obtained using an ultraviolet−visible (UV−vis) spectrometer (Shimadzu, UV3600 plus) in the range of 250−2000 nm and an FTIR spectrometer (Shimadzu, IRAffinity-1S) in the range of 2000−10000 nm.The diffuse reflectance spectra of powder samples were also obtained using an integrating sphere installed in a UV3600 plus.BaSO 4 was used as a reference.the inset.The glass-forming region of the La 2 O 3 −MoO 3 binary system is similar to that of the La 2 O 3 −WO 3 binary system. 19he brown color of the La 2 O 3 −MoO 3 glasses is a unique characteristic compared to that of colorless La 2 O 3 −WO 3 glasses.The diameter of the spheric glasses was approximately 2 mm.At x = 50, 70, 85, 90, and 95, violent evaporation occurred during melting, and the targets were not vitrified.The glass formation in this study is clearly different from the previous report that showed glass formation with x = 90 using the conventional melt-quench method. 7,8,9Glass formation at x = 90 rarely succeeded in our levitation experiments, but only where the melting time was long and the melting temperature was high.This may be due to composition deviation, resulting in an increase of La 2 O 3 content because of MoO 3 volatilization.
Figure 2 shows the DSC curve of the x = 75 glass.The crystallization temperature, T X , was found at 546 °C, and the crystallization peak was remarkably sharp.T X of the x = 75 glass was lower than those of 20La 2 O 3 −80WO 3 glass, which was at 594 °C.Determining the glass transition temperature was challenging due to minimal change in heat flow.Previous reports based on differential thermal analysis (DTA) data of the x = 90 glass indicated a crystallization peak temperature at 410 °C, and a subtle change, assumed to be T g , was observed at approximately 325 °C. 7,9However, no significant signal was observed in the temperature range from 300 °C to T X in this study.Two possible reasons explain why the glass transition temperature was not detectable.One possibility is that T g might be so close to T X that T g is obscured by the onset of the crystallization peak.Another possibility is that the difference in specific heat between the glassy state and supercooled liquid might be too small to be detected.
Figure 3 shows XRD profiles of the as-melted sample of x = 75, the crystallized sample after DSC measurement, and a calculated profile using the crystal structure of La 2 Mo 3 O 12 . 26he profile of the as-melted sample does not have any peaks, indicative of an amorphous nature., all of whose oxygen atoms are nonbridging, and La 3+ coordinates it as a charge compensator.
The density of the x = 75 glass was measured to be 4.67 g/ cm 3 , indicating a 4.1% deviation from the calculated value of 4.87 g/cm 3 of La 2 Mo 3 O 12 .The packing density (PD) was calculated using the equation PD = V P /V m , where V P is the sum of the volume of component ions and V m is the molar volume of the glass.Assuming that ions in the glass are spherical, the volume of the ith ion is expressed as 4πr i 3 /3, where r i represents the ionic radius of the ith ion.Shannon's ionic radii were utilized for volume calculations: 1.16 Å for La 3+ (VIII), 0.41 Å for Mo 6+ (IV), and 1.35 Å for O 2− (II). 28he PD of the 25La 2 O 3 −75MoO 3 glass was determined to be 0.510, closely resembling 0.532 of the La 2 Mo 3 O 12 crystal.Such similarity in PD between glass and crystalline phase, often observed in glasses prepared by a levitation technique, 29 suggests that the glass structure is highly packed, akin to the crystalline phase.
X-ray absorption near edge spectroscopy (XANES) spectra mainly originate from a superposition of electronic transitions and provide information on valence and chemical states of the elements to be measured.−32 XANES analyses for Mo were reported for silicate glasses and melts by comparing a variety of reference crystals. 33XANES spectral shape and the pre-edge peak of the reference crystals strongly depend on the atomic arrangements and the valence state of Mo.Linear correlation was clearly shown between the average valence state of Mo and the Mo Kedge position at a normalized absorbance equal to one.The XANES spectra of silicate glasses also showed this variation; however, the degree of difference is much smaller than the case of crystalline phases.It was concluded that the valence state of Mo was six in silicate glasses prepared in ambient conditions, and the reduction of Mo requires a strong reduced condition.The coordination number of Mo estimated from the pre-edge peaks is almost four; however, there are some glasses with higher coordination numbers, ranging from 4 to 6 even in silicate glasses. 33o K-edge XAFS spectra of the La 2 O 3 −MoO 3 binary system were clearly different from silicate glasses, and they provide clear evidence of the similarity of local structure around Mo atoms between glassy and crystalline phases.Figure  Compared to MoO 3 , the pre-edge peak of the glass is higher, and the position shifted to the lower energy side, indicating that Mo forms MoO 4 tetrahedra as well as La 2 Mo 3 O 12 .From the absorption edge energy, Mo in the glass is estimated to be 6+, as well as that in the two reference crystals.
Mo L 3 -edge XANES spectrum, in which the peaks are mainly ascribed to the transition from 2p to 4d states, is helpful to predict the oxygen coordination number of Mo as well as Kedge XANES spectrum. 34Mo L 3 -edge XANES spectra of the glass and reference crystals are shown in the inset of Figure 4a.Two peaks were clearly observed in the spectra of MoO 3 .The peak at the higher energy side was less intense than that on the lower energy side, implying that they are associated with the e g and t 2g empty states of octahedral MoO 6 .The spectra of the glass and crystalline La 2 Mo 3 O 12 were almost the same shape, which is evidence of the local structural similarity between them.The main peak consists of two peaks with an energy difference smaller than that of the MoO 3 spectrum.The intensity ratio of the peak at the higher energy side and lower side was the opposite of that of MoO 3 .These results suggest that the two peaks are attributed to the electron transition in the tetrahedral crystal field splitting.
Figure 4b shows the k 3 -weighted EXAFS spectra, k 3 χ(k) of the 25La 2 O 3 −75MoO 3 glass and crystalline La 2 Mo 3 O 12 .The oscillation was observed clearly in a high k range.The glass and crystal are almost the same in shape and intensity.This means the homogeneity and uniformity of the local structure around Mo in the glass.Figure 4c shows the FT amplitude of the k 3weighted EXAFS spectra with k max = 14.0 Å −1 .They are almost the same; however, a relatively smaller peak height was observed for the glass, indicating a slightly smaller coordination number or disorder in atomic arrangement around Mo. Furthermore, there is no correlation at 3.8−4.0Å for the second nearest shell in the glass, while a small but clear correlation is observed in the crystal, showing disorder in the glass.A correlation smaller than expected from the FEFF calculation is often observed in molybdate crystalline phases, such as Na 2 MoO 4 and Ag 2 MoO 4 . 33,35n order to more quantitatively investigate the local structure around the Mo, fitting of the K-edge EXAFS spectra was performed.Prior to the fitting of EXAFS spectra for the glass, it is necessary to determine the inelastic multielectronic losses, S 0 2 , of the EXAFS formula in this experiment from the fitting result of a reference crystal.The fitting for La 2 Mo 3 O 12 was performed in the r range from 1 to 2 Å with a fixed value of 4.0 for the coordination number, N. The value of S 0 2 was 0.885.The Mo−O distance, R, and DW factor, σ 2 , are 1.778 ± 0.002 and 0.0022 ± 0.0002 Å 2 , respectively, as shown in Table 1.The fitting of the glass was performed by using the S 0 2 value.The coordination numbers of Mo, R, and σ 2 were 3.9 ± 0.1, 1.778 ± 0.002 Å, and 0.0022 ± 0.0002 Å 2 , which are almost the same as that of La 2 Mo 3 O 12 .The coordination number of the glass certainly smaller than that of the crystal but they are almost the same within an error.It should be noted that there is almost no difference in the structural parameters N, R, and σ 2 between the glass and crystal, which strongly indicates the similarity in the local structure of the Mo atom in the first coordination shell.Therefore, it is highly estimated that Mo in the glass forms regular tetrahedra with four nonbridging oxygens as in the case of La 2 Mo 3 O 12 .
Figure 5 shows La L 3 -edge XAFS spectra of 25La 2 O 3 − 75MoO 3 glass and crystalline La 2 Mo 3 O 12 .In contrast to Mo Kedge XAFS, there is a clear difference between the glass and the crystal.The peak height of the white line in the glass XANES spectrum is clearly smaller than that of the crystal, and the oscillation above 5.50 eV for the glass is not obvious compared to the crystal (Figure 5a).This is indicative of the disordered structure of the glass.Figure 5b shows EXAFS spectra of the glass and the crystal.The intensity of the glass is much smaller than the crystal; however, the oscillation periods are similar to each other, indicative of local environment similarity around the La atoms.Because the La L 2 -edge absorption is nearly located, there is a limited k range available  for FT of the EXAFS spectra.Figure 5c shows the FT magnitudes of the glass and the crystal.Because of the limited range of k (3 Å −1 ≤ k ≤ 8.4 Å −1 ), it is difficult to estimate structural parameters by fitting.From the simple fitting of crystalline spectra with a fixed coordination number value of eight, the inelastic multielectronic loss, S 0 2 , is estimated to be 0.95, and the La−O distance R and DW factor σ 2 are 2.48 ± 0.02 Å and 0.007 ± 0.001 Å 2 , respectively.By using the S 0 2 value, the coordination number of La, R, and σ 2 for the glass were 6.4 ± 0.1, 2.46 ± 0.01 Å, and 0.009 ± 0.002 Å 2 , respectively.The La−O bond length of the glass is slightly shorter than that of the crystal.The apparent small coordination number of glass might be due to disorder in the atomic arrangement and does not indicate that the number of oxygen atoms around La in the glass is smaller than that of the crystal.The disorder sometimes diminishes the longer side peak by averaging the distribution in amorphous materials.Accordingly, XAFS analysis using Mo K-edge and La L 3 -edge absorptions indicates structural similarity around Mo and La between the glass and crystal, although disorder is obvious, especially around La in the glass.
Further evidence of the structural similarity between the glass and the crystal is clearly shown in the vibration spectra.Figure 6 shows Raman scattering spectra of the glass and the crystal.It should be noted that the spectrum of glass clearly resembles the crystalline spectrum in peak position and peak height.It seems that broadening the spectrum of the crystal matches well with that of the glass.The agreement of the glass and the crystal strongly suggests that the local environment around the Mo atoms, such as the bonding distance to oxygen, the oxygen coordination number, and the oxygen−cation− oxygen bond angles, is almost the same.−38 The bands at 880−970 cm −1 and 750−880 cm −1 are attributed to the Mo−O symmetric stretching mode and antisymmetric stretching modes in MoO 4 tetrahedra, respectively.The band at 320 cm −1 is attributed to the O− Mo−O bending mode in MoO 4 tetrahedra.
Finally, the optical properties of the glass are shown in Figure 7. Figure 7a shows the transmittance spectra of the x = 75 glasses in the UV−vis region.Compared with the spectrum of the as-melted glass, the transparency of the glass annealed in the O 2 gas flow was improved in the visible range.The UV absorption edge is approximately 378 nm, from which the optical bandgap energy is estimated to be 3.40 eV.The small absorption at 400−700 nm is obvious, which might be due to the d−d transition in Mo 5+ .The amount of Mo 5+ should be very small because it was difficult to find evidence of a lower valence state of Mo in the Mo K-edge XANES spectra.The maximum transmittance was 80% at 1000 nm, which can be attributed to reflection on both sides of the surface during the measurement.Assuming that there is only surface reflection and no light scattering inside the glass, the refractive index, n, of the glass is estimated to be 2.0 using the equation where T max is the maximum transmittance and R′= [(n − 1)/(n + 1)] 2 .The estimated refractive index at 1000 nm is slightly larger than that of 20La 2 O 3 −80WO 3 glass. 19Since the optical bandgap energy of the x = 75 glass is not so large, the wavelength dispersion of the refractive index will be large, which is not suitable for lenses in the visible region.Nevertheless, the La 2 O 3 −MoO 3 glasses are possible   candidates for optical applications using high refractive index in the near-infrared region.
The optical absorption edge of the glass can be compared to that of the powdered crystalline phase using diffuse reflectance spectroscopy.The Kubelka−Munk function, F(R), is obtained from the reflectance spectra.The Tauc method equation is transformed to eq 1 by substituting F(R) for the absorption coefficient, α: where h is the Planck constant, ν is the photon frequency, E g is the optical band gap energy, and B is a constant.The factor m depends on the nature of the electron transition and is equal to 1/2 or 2 for direct or indirect transition band gaps, respectively.The inset of Figure 7a shows the Tauc plot with m = 1/2 by using the Kubelka−Munk function.The Tauc plot curve of the glass clearly shifts to the lower energy side compared to that of the crystalline sample.The x-axis intersection point of the linear fit of the Tauc plot gives an optical band gap energy.The estimated E g of the glass is 3.46 eV, which is in good agreement with the value obtained from transmittance spectra.The E g of the crystalline sample is 3.60 eV. 39The bandgap of the glass is slightly smaller than that of the crystal, which is reasonable considering the disorder in atomic arrangements characteristic to amorphous materials.Figure 7b shows the infrared transmittance spectra of the glass.The glass was transparent to 5.5 μm, and the absorption at 2.9 μm is attributed to the presence of the O−H vibration.Compared with conventional oxide glasses, the O−H absorption of the glass is considerably small, indicating a scarcity of OH groups.Furthermore, there is the additional transmittance window in the range 6.5−8.0 μm.−42 The additional transmittance in the R-rich borate glasses was explained by considering the local structure around the B atoms.B atoms in the glass form isolated BO 3 units without any threedimensional network structure.The simple environment around the B atoms resulted in the suppression of the overlap of the absorption bands of the B−O multiple vibrations and their overtones observed in conventional borate glasses.As a result, the absorption bands discretize, and then an additional transmittance window emerges.The local structure around the Mo atom in the 25La 2 O 3 −75MoO 3 glass is confirmed to be the isolated MoO 4 2− from XAFS and Raman scattering spectra, which is seen in crystalline La 2 Mo 3 O 12 .Therefore, the additional transmittance window at 6.5−8.0 μm can be explained by suppression of the overlap of the absorption bands of Mo−O multiple vibrations and their overtones because of the simple environment around Mo as in the case of crystalline La 2 Mo 3 O 12 .
Crystallization of the glass, XAFS and Raman scattering spectra, and the additional infrared transmittance as mentioned above strongly indicate that the atomic arrangement of the 25La 2 O 3 −75MoO 3 glass is almost the same as that of the crystalline La 2 Mo 3 O 12 , even though structural disorder in the glass was confirmed in the La L 3 -edge XAFS and the absorption edge in the UV−vis region.MoO 3 is classified as a conditional network former, and it has been reported that MoO n (n = 4, 5, or 6) formed a network in MoO 3 -based glasses. 5,6Contrary to previously reported glasses, there is no network in the La 2 O 3 −MoO 3 glasses and Mo forms completely isolated MoO 4 2− .Furthermore, the La 2 O 3 −MoO 3 glasses are highly packed and comparable to those of the crystalline phase.The highly dense packed structure and the structural similarity of the crystal are unique and distictive in glass science; however, similar results have been reported recently in glasses prepared by a levitation technique. 12,40As suggested in the previous study, the structural similarity of the glass and the crystalline phases might enhance the glass-forming ability of the densely packed glass system.Assuming that R 3+ and O 2− are arranged in the nearly closest packed structure and small cations, such as Mo 6+ ions, are inserted into interstitial sites (tetrahedral sites in the case of La 2 O 3 −MoO 3 binary system) in the glass, the amorphous nature is produced by the slight displacement from the atomic arrangement of the crystalline phase.The small displacement enables a fall in metabasins in the free energy landscape 43,44 and thus loses long-range order in the case of densely packed glass systems. 45In this case, glass formation becomes possible even when the three-dimensional regular tetrahedral networks are not developed.

■ SUMMARY
Bulk glass formation of the La 2 O 3 −MoO 3 binary system was achieved using the levitation technique.The glass-forming region was narrow at the vicinity of 25-mol % La 2 O 3 .The fabricated glasses were brownish but transparent in the visible region.The physical and structural properties of the 25La 2 O 3 − 75MoO 3 glass were investigated because competing crystalline phase of La 2 Mo 3 O 12 exists at the same composition.The glass transition temperature was not detected due to minimal change in heat flow, while the crystallization temperature was 546 °C.After the sharp crystallization peak, a single phase of La 2 Mo 3 O 12 crystallized.Mo L 3 -edge and K-edge XAFS and Raman scattering spectra revealed structural similarity between the glass and the crystal.The glass is formed by isolated MoO 4 2− tetrahedra without any network formation.La L 3 -edge XAFS and optical bandgap in the UV−vis range indicate the disorder in the atomic arrangement of the glass.An additional infrared transparent window at 6.5−8.0 μm was explained by the simple environment of the local structure around Mo, which has been commonly observed in highly packed glasses prepared by the levitation technique.The estimated refractive index of 2.0 at 1.0 um and broad infrared transparency open up the possibility of optical applications in the infrared range.Furthermore, it was suggested that the glass formation of the La 2 O 3 −MoO 3 system was caused by slight movement from the ordered La 2 Mo 3 O 12 structure and, thus, it does not require a three-dimensional tetrahedral network.

■Figure 1 .
Figure 1.Glass-forming range indicated on the (100−x)La 2 O 3 − xMoO 3 phase diagram.Circles and crosses indicate glass formation and crystallization, respectively.The inset shows a picture of the 25La 2 O 3 −MoO 3 glass annealed in an O 2 gas flow at 400 °C in 24 h.
It is clearly shown that the profile of the crystallized sample is almost identical to the calculated profile of La 2 Mo 3 O 12 .This implies that the atomic arrangement of x = 75 glass and crystalline La 2 Mo 3 O 12 may resemble each other.The crystal structure of La 2 Mo 3 O 12 is shown in the inset of Figure 3.There are five Mo sites and three La sites in the La 2 Mo 3 O 12 unit cell.The Mo atom forms highly isolated MoO 4 , without any Mo−O−Mo connections, while the La atom is coordinated by eight oxygens.It can be seen that the MoO 4 tetrahedra connect at all four oxygen with LaO 8 by corner-sharing.Each of the six oxygens in LaO 8 is shared with MoO 4 at the corner, and each of the remaining two oxygens is shared by MoO 4 and two edge-shared LaO 8 .Alternatively, Mo 6+ forms isolated MoO 4 2−

Figure 3 .
Figure 3. XRD profiles of the as-melted 25La 2 O 3 −75MoO 3 sample and the crystallized sample after DSC measurement.The calculated profile of La 2 Mo 3 O 12 is shown at the bottom.The inset shows the crystal structure of La 2 Mo 3 O 12 drawn using VESTA software. 27Purple and green polyhedra in the inset represent MoO 4 and LaO 8 , respectively.

Figure 4 .
Figure 4. Mo K-edge XAFS spectra of 25La 2 O 3 −75MoO 3 glass and crystalline La 2 Mo 3 O 12 .(a) XANES spectra, (b) k 3 -weighted EXAFS spectra, and (c) the FT magnitude of EXAFS oscillation.Inset of (a) shows Mo L 3 -edge XANES spectra.Inset of (c) shows the fitting result of the glass.

Table 1 . 12 N
Parameters Used to Fit the Mo K-edge EXAFS Spectra of 25La 2 O 3 −75MoO 3 Glass and Crystalline La 2 Mo 3 O