Temperature‐dependent Raman scattering studies of Na2MoO4

The mode assignment of the cubic phase of anhydrous Na₂MoO₄ was carried out on the basis of lattice dynamic calculation using the classical rigid‐ion model. Temperature‐dependent studies indicate that this crystal remains in the cubic structure in the 15–773 K range and undergoes a phase transition at around 783 K. The behavior of the Raman modes indicates that this transition is strongly first‐order in nature and the phase above 773 K may have an orthorhombic symmetry. This transition is connected with tilting and/or rotations of the MoO₄ tetrahedra, which lead to a disorder at the MoO₄ sites. Our results give also evidence that the Mo O bond lengths decrease in the high‐temperature phase. Copyright © 2008 John Wiley & Sons, Ltd.     

Raman spectroscopy study of Na2MoO4·2H2O and Na2MoO4 under hydrostatic pressure

Raman spectroscopy measurements of polycrystalline Na₂MoO₄·2H₂O (NMHO) and Na₂MoO₄ (NM) under hydrostatic pressure (from 0 to 10 GPa) were performed. This study allowed us to monitor the stretching and bending vibrations of MoO₄ ions as well as the translational modes as a function of pressure. The pressure dependence of the wavenumbers of the modes indicates that the Na₂MoO₄·2H₂O undergoes two phase transitions at about ∼3 and ∼4 GPa. When releasing pressure, we have observed that the original spectrum is recovered, thereby pointing to a reversible process. The Na₂MoO₄ (NM) starting phase was found to be stable up to 10 GPa. The pressure‐dependent Raman data for NM did not reveal any structural modification. The influence of the pressure‐transmitting medium on the phase transitions is also discussed. Copyright © 2009 John Wiley & Sons, Ltd.     

Temperature‐dependent Raman scattering studies on Na2Mo2O7 disodium dimolybdate

Temperature–dependent Raman studies of disodium dimolybdate (Na₂Mo₂O₇) crystal are reported. Lattice dynamical calculation was used to predict both wavenumbers and atomic displacements (eigenvectors) for the vibrational modes. These calculations were based on the classical rigid‐ion model. The high‐temperature Raman scattering study of the crystal showed that it remains in the orthorhombic structure in the 8–848 K range and undergoes a structural phase transition between 848 and 854 K. This phase transition is most likely connected with weak tiltings and/or rotations of both MoO₄ (tetrahedra) and MoO₆ (octahedra) units, which lead to a disorder in the oxygen sublattice. Copyright © 2010 John Wiley & Sons, Ltd.     

Raman spectroscopic study of the phase transitions in the Cs2NH4WO3F3 oxyfluoride

The Raman spectra of Cs₂NH₄WO₃F₃ elpasolite crystals are studied in the temperature range 93–373 K at pressures of up to 6.3 GPa. No indication of a phase transition is revealed from the Raman spectra as the temperature decreases to 93 K. An analysis of the Raman spectra measured under pressure demonstrates that the Cs₂NH₄WO₃F₃ elpasolite crystals undergo a phase transition at a pressure of 2.58 GPa. Judging from the behavior of the pressure dependences of the vibrational frequencies, the revealed phase transition is associated with the lowering of the symmetry of the WO₃F₃ octahedra.     

Variations of phase transition temperature and enthalpy in the systems Na2Mo1− x W x O4 and Na2Mo1− y S y O4

The samples in two material systems, Na₂Mo_{1? x} W _x O₄ and Na₂Mo_{1? y} S _y O₄, were prepared by using conventional solid reactions and characterized by X-ray diffractometer (XRD), energy dispersive X-ray spectroscopy (EDX), and difference scanning calorimetry (DSC). The XRD and EDX data indicated that all the samples in both systems were in the solid solution range. The DSC data indicated that in the system Na₂Mo_{1? x} W _x O₄, the solid–solid transition temperatures increased and in the system Na₂Mo_{1? y} S _y O₄, the solid–solid transition temperatures decreased. The total enthalpy (ΔH _total) of the solid–solid transitions in the system Na₂Mo_{1? x} W _x O₄, decreased much less than that in the system Na₂Mo_{1? y} S _y O₄. This is probably because similar to Na₂MoO₄, the solid–solid transition of Na₂WO₄ has relatively large ΔH _total, but Na₂SO₄ has much smaller solid–solid transition enthalpy. In order to modify the transition temperatures of Na₂MoO₄ and also to keep its relatively large ΔH _total, it is necessary to choose a doping material with large ΔH _total.     

Low-temperature vibrational properties of KGd(WO4)2: (Er, Yb) single crystals studied by Raman spectroscopy

Temperature-dependent polarized Raman spectra of KGd(WO₄)₂: (Er, Yb) single crystals have been analyzed over the 77-292 K temperature range. The A_g and B_g spectra obtained are discussed in terms of factor group analysis. The spectra have been found to reveal the bands related to internal and external vibrations of WO₄²⁻, WOW and WOOW molecular groups. Strong depolarization of the majority of the Raman bands has been observed in the whole temperature range. Some anomalies in the spectral parameters of selected Raman bands below 175 K have been discussed in terms of the local distortion of WO₄²⁻ ions in KGd(WO₄)₂: (Er, Yb) crystals.     

A Raman spectroscopic and TEM study on the structural evolution of Na2Ti3O7 during the transition to Na2Ti6O13

The synthesis of sodium hexatitanate from sodium trititanate was characterized by Raman spectroscopy, X‐ray diffraction (XRD) and high‐resolution transmission electron microscopy (HRTEM). The structural evolution from trititanate to hexatitanate was studied using Raman spectra, XRD and HRTEM techniques. It was found that the Raman bands at 279 cm⁻¹ corresponding to very long Ti O bonds and at 883 cm⁻¹ corresponding to the very short Ti O bonds decrease in intensity and finally disappear during the transition from sodium trititanate to sodium hexatitanate. The band at 922 cm⁻¹ corresponding to an intermediate‐length Ti O bond was observed to become stronger with the increase in temperature, indicating that there is no terminal oxygen atom in the crystal structure of Na₂Ti₆O₁₃ and that all the oxygen atoms become linearly coordinated by two titanium atoms. Furthermore, the TiO₆ octahedron in Na₂Ti₆O₁₃ are more regular because the very long (2.2 Å) or very short (1.7 Å) Ti O bonds disappear. It is revealed that the phase transition from trititanate to hexatitanate is a step‐by‐step slipping process of the TiO₆ octahedral slabs with the loss of sodium cations, and a new phase with formula Na_1.5H_0.5Ti₃O₇ has been discovered as an intermediate phase to interlink Na₂Ti₃O₇ and Na₂Ti₆O₁₃. Copyright © 2010 John Wiley & Sons, Ltd.     

Temperature‐dependent vibrational studies of [N(C2H5)4]2MnCl4

Room‐temperature polarized Raman spectra of a single crystal and IR spectra of a polycrystalline sample were measured for [N(C₂H₅)₄]₂MnCl₄ and the assignment of the observed bands to the respective modes has been proposed. Temperature‐dependent Raman and far‐IR studies were also performed for the polycrystalline sample in order to obtain information on changes occurring in this material as a result of phase transitions at T₁ = 227 K and at T₂ = 199 K. These studies revealed that the higher‐temperature ferroelastic phase transition is associated with significant modification of vibrational properties due to ordering of tetraethylammonium groups. The lower‐temperature phase transition does not lead to any clear changes in the spectra. However, our results suggest that disorder of MnCl₄²⁻ ions decreases with decreasing temperature. Copyright © 2007 John Wiley & Sons, Ltd.     

Phase transitions in orthorhombic oxyfluoride (NH4)2MoO2F4

(NH₄)₂MoO₂F₄ single crystals were grown and studied using polarization-optical methods, and the birefringence was measured in the temperature range 90–350 K. The following sequence of phase transitions is revealed: G ₀ ? G ₁ ? G ₂. It is established that the phase transition at T ₀₁ ≈ 267 K is of the first order and exhibits thermal hysteresis δT ₀₁ ≈ 0.6 K. A weak anomaly is found in Δn(T) at T ₀₂ ≈ 180 K. The crystals are shown to retain the orthorhombic symmetry during the phase transitions.     

Electronic structure and lattice dynamics of Li2Ni(WO4)2

We combined spectroscopic ellipsometry and Raman scattering measurements to explore the electronic structure and lattice dynamics in Li₂Ni(WO₄)₂. The optical absorption spectrum of Li₂Ni(WO₄)₂ measured at room temperature presents a direct optical band gap at 2.25 eV and two bands near 5.2 and 6.0 eV, which are attributed to charge-transfer transitions from oxygen 2p states to nickel 3d or tungsten 5p states. The Raman scattering spectrum of Li₂Ni(WO₄)₂ measured at room temperature presents seventeen phonon modes at approximately 112, 143, 193, 222, 267, 283, 312, 352, 387, 418, 451, 476, 554, 617, 754, 792, and 914 cm⁻¹. When the temperature is decreased to 20 K, the frequency, linewidth, and normalized intensity of all phonon modes exhibited almost no temperature dependence. Upon cooling across 13 K, which is the antiferromagnetic phase transition temperature, the oxygen octahedra stretching mode at 914 cm⁻¹ exhibited a softening and an increase in intensity, thus suggesting a coupling between the magnetic and lattice degrees of freedom. The spin-phonon coupling constant was estimated to be 0.94 mRy/Ų, indicating a weak spin-phonon interaction in Li₂Ni(WO₄)₂.     

Joint Raman spectroscopic and quantum chemical analysis of the vibrational features of Cs2RuO4

The Raman spectroscopic characterization of the orthorhombic phase of Cs₂RuO₄ was carried out by means of group theory and quantum chemical analysis. Multiple models based on ruthenate (VI+) tetrahedra were tested, and characterization of all the active Raman modes was achieved. A comparison of Raman spectra of Cs₂RuO₄, Cs₂MoO₄, and Cs₂WO₄ was also performed. Raman laser heating induced a phase transition from an ordered to a disordered structure. The temperature‐phase transition was calculated from the anti‐Stokes/Stokes ratio and compared with the ones measured at macroscopic scale. The phase transition is connected with tilting and/or rotations of RuO₄ tetrahedra, which lead to a disorder at the RuO₄ sites. © 2015 The Authors. Journal of Raman Spectroscopy published by John Wiley & Sons Ltd.     

Electrical conductivity of superionic solid solutions of Na2SO4 with Mx (XO4)y -[M=Na,K, Rb,Cd, GdandX=W,Mo, S,Si;x=1, 2, 4 andy=1, 3]

Electrical conductivity data are reported for solid solutions of Na₂SO₄, K₂WO₄, Na₂WO₄, Na₂MoO₄, Rb₂SO₄, Na₄SiO₄ and Gd₂ (SO₄)₃. In all cases, except K₂SO₄, we observed an increase in Na⁺ conductivity effected by lattice expansion and/or incorporation of ion vacancies in addition to a structural transformation. Boundary conditions were shown to exist for these factors to yield a limiting Na⁺ conductivity with a constant fraction of Na⁺ based on a percolation model of transport. The higher conductivity data observed for the larger radius isovalent WO^2-₄ and aliovalent SiO^4-₄ doped Na₂SO₄ show conclusively that the anion-rotation ”cogwheel” mechanism does not contribute to the cationic conductivity in Na₂SO₄.     

Laser Raman study of phase transformations in [N(CH3)4]2ZnCI4 single crystals

We report temperature-dependent Raman studies on single crystals of [N(CH₃)₄]₂ZnCI₄ from 300 to 10 K. The observed spectral features suggest that both the N(CH₃)₄ ⁺ and ZnCl^2- ₄ ions are distorted from their regular tetrahedral structure and occupy sites of C_s symmetry in the lattice at room temperature. From the variation of line width of some selected Raman bands and other spectral changes as a function of temperature, it is inferred that both the ZnCl^2- ₄ and—CH₃ groups have high motional freedom at room temperature and the different phase transitions up to 160 K are triggered by the gradual freezing-in of orientational freedom of these groups, while the N—C₄ tetrahedra do not play any significant role in these phase transitions. The monoclinic to orthorhombic superlattice phase transitions at 159 K is triggered by freezing-in of the orientational motions of both the ZnCl^2- ₄ and N(CH₃)⁺ ₄ groups in the lattice.     

Synthesis and phonon properties of nanosized Aurivillius phase of Bi2MoO6

Nanosized crystallites Bi₂MoO₆ of a platelet shape were synthesized by a mild hydrothermal crystallization process. The effect of particle size on the structure and properties of Bi₂MoO₆ was studied by X‐ray diffraction, transmission electron microscopy, as well as Raman and infrared (IR) spectroscopies. Moreover, temperature‐dependent Raman spectra were collected for bulk and nanocrystalline Bi₂MoO₆. These studies showed that the thickness of the smallest crystallites is about 8–11 nm. Raman and IR studies have revealed that the damping and the intensity of Raman and IR bands are significantly modified with decreasing particle size and temperature. Moreover, some bands experience significant shifts both towards lower and higher wavenumbers. The obtained results indicate that, similar to the isostructural Bi₂WO₆, the orthorhombic distortion decreases with decreasing particle size. However, the structure of nanocrystalline Bi₂MoO₆ remains orthorhombic and noncentrosymmetric, and the structural changes are not the same as those observed in Bi₂WO₆. Copyright © 2010 John Wiley & Sons, Ltd.     

Spin dynamics and structural phase transitions in quasi-2D antiferromagnets R2CuO4 (R=Pr, Sm, and Eu)

Spin and lattice dynamics of R₂CuO₄ (R=Pr, Sm, and Eu) crystals were studied over the frequency and temperature ranges 20–250 GHz and 5–350 K, respectively. The absorption coefficients of the R₂CuO₄ crystals (R=Pr, Sm, and Eu) were found to change dramatically at temperatures of, respectively, 20, 80, and 150 K over a broad frequency range above 120 GHz. The absorption jumps were caused by the structural phase transitions. Broad spin-wave bands were observed in the high-temperature phases of all crystals studied. Absorption lines due to lattice dynamics were observed near the temperatures of structural phase transition over a broad frequency range, including the frequencies corresponding to the spin-wave bands.     

Spatially resolved Raman investigation on phase separations of mixed Na2SO4/MgSO4 droplets

By using spatially resolved Raman spectroscopy together with in situ microscopy, mixed Na₂SO₄/MgSO₄ aerosol particles with molar ratios of 1:1 and 2:1 deposited on a quartz substrate were carefully examined in their evaporation processes. Upon decreasing the relative humidity (RH), phase separations were found to occur for these droplets. For Na₂SO₄/MgSO₄ droplets with a molar ratio of 1:1, two paths of phase separation were identified, and a large amount of small crystals were observed to disorderly distribute in the residual solutions. By comparisons with the known Raman spectra of crystals, it was concluded that the scattered crystals in the two paths were anhydrous Na₂SO₄ in metastable phase III and the double salt of Na₂SO₄· MgSO₄ · 4H₂O, respectively. For Na₂SO₄/MgSO₄ aerosol droplets with a molar ratio of 2:1, only anhydrous Na₂SO₄ in metastable phase III precipitated with the decrease of RH. Copyright © 2008 John Wiley & Sons, Ltd.     

The temperature dependence of the Raman spectra of chromium‐doped titanite (CaTiOSiO4)

The temperature dependence of the Raman bands of Cr⁴⁺ modes which show enhanced intensities due to pre‐resonance effects is reported from 293 to 673 K in chromium‐doped titanite (CaTiOSiO₄). Some aspects of the temperature dependence of Raman bands in pure, synthetic titanite which have not been previously published are also included in this study. Two Raman‐active components of A_g and B_g symmetry, respectively, of the symmetric Si–O mode in titanite are predicted under P2₁/a symmetry and also been identified in this phase for the first time. The one component of B_g symmetry disappears just above the antiferroelectric–paraelectric transition at ~500 K in accordance with the predictions under A2/a symmetry for the high‐temperature phase. Two resonance‐enhanced components of the Cr⁴⁺–O stretch are also evident in the P2₁/a phase and only one could be identified in the A2/a phase, again in accordance with group‐theoretical predictions. These observations can be used to characterize the P2₁/a and A2/a phases of pure synthetic and chromium‐doped titanite. The temperature dependence of the Cr⁴⁺–O modes can be approximated by two‐dimensional Ising behavior with the critical exponent β ≈ 1/8 below 450 K. Between 450 and 498 K, anomalous behavior is observed and this could be due to the appearance of mobile anti‐phase boundaries (APBs). Anomalous behavior also persists to temperatures above 500 K. The half‐width of the Ti–O stretching mode reflects the influence of the order parameter (Ti–O displacements) as well as mobile anti‐phase boundaries. No evidence could be found of the existence of other ions such as Cr⁴⁺‐ions in Ti‐sites and/or Cr³⁺‐ions also in Ti‐positions in Cr‐doped titanite in the Raman spectra using different laser lines to excite the spectra. Copyright © 2013 John Wiley & Sons, Ltd.     

Evaluation of stoichiometric rare-earth molybdate and tungstate compounds as laser materials

Photoluminescence spectra, photoluminescence decay curves and Raman scattering spectra have been investigated for stoichiometric rare-earth molybdate and tungstate compounds. NaNd(MoO₄)₂ and NaNd(WO₄)₂ show emissions due to the transition ⁴F_3/2→⁴I_9/2 in Nd³⁺. A possibility of laser oscillation in NaNd(MoO₄)₂ is pointed from comparisons of the emission intensity and the decay time constant with NaNd(WO₄)₂ where laser oscillations have been reported. In NaLa(MoO₄)₂ and NaLa(WO₄)₂, observed emissions which are not related to La³⁺ are probably due to the transitions in MoO₄^2- and WO₄^2- molecular ions, respectively, in scheelite crystal. Raman spectra of these compounds are similar, probably related to the same crystal structure. LiEr(MoO₄)₂ shows the emissions due to transitions ²H_11/2→⁴I_15/2, ⁴S_3/2→⁴I_15/2 and ⁴F_9/2→⁴I_15/2 in Er³⁺, respectively, which are believed to be observed for the first time.     

Polarization properties of Li2−x Na x Ge4O9 (0.2 ≤ x ≤ 0.3) crystals

The polarization switching in sinusoidal fields and the pyroelectric properties of Li_2?x Na _x Ge₄O₉ (0.2 ≤ x ≤ 0.3) crystals are measured in the temperature range T _c ?T ≤ 40 K. The behavior of the P?E hysteresis loops with variations in temperature is investigated for crystals with phase transition temperatures T _c < 300 K and T _c > 300 K. It is shown that, for crystals with phase transition temperatures T _c < 300 K, the temperature dependence of the hysteresis loop exhibits a behavior typical of crystals with second-order phase transitions. The crystals with phase transition temperatures T _c > 300 K are characterized by double hysteresis loops in the temperature range T _c ?T ₁ ≈ 30 K. The correlation between the polarization properties and possible structural transformations of the Li_2?x Na _x Ge₄O₉ crystals due to the change in the concentration ratio of Na and Li ions is discussed.     

Pressure-induced amorphization of Gd2(MoO4)3: A high pressure Raman investigation

High pressure Raman spectroscopic studies on Gd₂(MoO₄)₃(GMO) have been carried out at ambient temperature in the diamond cell to 10 GPa hydrostatic pressure. These experiments have revealed pressure-induced phase transitions in GMO near 2 GPa and 6.0 GPa. The first transition is from Pba2(β′) phase to another undetermined crystalline phase, designated as phase II, and the second transition is to an amorphized state. On releasing pressure there is a partial reversion to the crystalline state. The Raman data indicate that the amorphization is due to disordering of the MoO₄ tetrahedral units. Further, it is inferred from the nature of the Raman bands in the amorphized material that the Mo-O bond lengths and bond angles have a range of values, instead of a few set values. The results of the present study as well as previous high pressure-high temperature quenching experiments strongly support that pressure-induced amorphization in GMO is a consequence of the kinetically impededβ toα phase transition. The system in frustration becomes disordered. The rare earth trimolybdates crystallizing in theβ′ structure are all expected to undergo similar pressure-induced amorphization.