Thermal,optical, and dielectric properties of fluoride Rb2TaF7

Physics of the Solid State - The thermal, optical, and dielectric properties of fluoride Rb2TaF7 were investigated. It was observed that the variation in chemical pressure in fluorides A 2 +TaF7...     

Vibrational spectra of disordered oxyfluoride (NH<Subscript>4</Subscript>)<Subscript>3</Subscript>WO<Subscript>3</Subscript>F<Subscript>3</Subscript>

Comparative analysis of the IR absorption and Raman scattering spectra of a polycrystalline sample of perovskite-type oxyfluoride (NH₄)₃WO₃F₃ has been performed in the frequency range 370–4000 cm^?1 at temperatures from 92 to 303 K, including the transition between the orientationally disordered cubic and low-symmetry phases. The conformation of WO₃F₃ octahedral groups is established and transitional anomalies of the internal modes of these groups and ammonium ions are revealed. Comparative analysis of the IR and Raman spectra suggests that the phase transition under study is mainly related to the ordering of octahedral groups and formation of W-O…H-N hydrogen bonds.     

Thermophysical studies of the phase transitions in (NH<Subscript>4</Subscript>)<Subscript>3</Subscript>NbOF<Subscript>6</Subscript> crystals

The thermophysical properties of oxyfluoride (NH₄)₃NbOF₆ were studied in detail over wide ranges of temperatures and pressures. At atmospheric pressure, a sequence of four structural phase transitions was established with the following changes in entropy: ΔS ₁ = Rln 2.7, δS ₂ = Rln38.3, ΔS ₃ = 0.08R, and ΔS ₄ = 0.17R. An external hydrostatic pressure was found to narrow the region of existence of the initial cubic phase. A triple point was detected in the p-T diagram; at a pressure above 0.07 GPa, the transition between the tetragonal and monoclinic phases occurs through a distorted high-pressure phase.     

Thermodynamic Properties of Vanadium Oxypentafluoride (IV) (NH4)3VOF5

Physics of the Solid State - The (NH4)3VOF5 crystals have been synthesized and their homogeneity and single-phase structure has been established by the X-ray diffraction, energy dispersive...     

Effect of cation substitution in fluorine-oxygen molybdates (NH4)2 ? xAxMoO2F4

Thermophysical and structural studies of (NH₄)_{2 − x} A _x MoO₂F₄ (A = Cs, Rb, K) solid solutions of oxyfluorides have been performed. The character of the influence of cation substitutions on the stability of the initial phase (space group Cmcm) and on the mechanism of phase transitions has been elucidated.     

Thermodynamic properties and structure of oxyfluorides Rb<Subscript>2</Subscript>KMoO<Subscript>3</Subscript>F<Subscript>3</Subscript> and K<Subscript>2</Subscript>NaMoO<Subscript>3</Subscript>F<Subscript>3</Subscript>

According to the results of calorimetric and structural studies, the Fm{ie1202-1}m phase in K₂NaMoO₃F₃ remains stable at least to 100 K. No ferroelectric transformation assumed earlier has been revealed in a series of Rb₂KMoO₃F₃ samples prepared using various technologies. Only a phase transition of nonferroelectric origin has been observed near 195 K, and its thermodynamic characteristics have been determined. An analysis of the stability of the cubic structure of molybdenum fluorine-oxygen elpasolites-cryolites has been performed in the framework of the hypothesis on strengths of interatomic bonds. The barocaloric effect in Rb₂KMoO₃F₃ has been estimated.     

ChemInform Abstract: Structural Transformation Between Two Cubic Phases of (NH4)3SnF7.

Single crystals of (NH₄)₃SnF₇ are obtained from aqueous solutions of (NH₄)₂SnF₆ and excess NH₄F during solvent evaporation.     

Optical studies of phase transitions in oxyfluoride (NH4)2NbOF5

Polarization-optical studies and measurements of the birefringence Δn and the angle of rotation of the optical indicatrix for the (NH₄)₂NbOF₅ crystal have been carried out in the temperature range 100–350 K. Two anomalies of the birefringence have been revealed at the temperatures T ₀₁ = 258 K and T ₀₂ ≈ 219 K. According to the twinning pattern, the crystal undergoes successive changes in symmetry: orthorhombic ↔ monoclinic 1 ↔ monoclinic 2. The twofold axis of the monoclinic phases (or the normal to the plane) is directed along [001]_or. The effect of the uniaxial compression along [011]_or and the electric field E ≈ 25 kV/cm along [100]_or on the twin structure has been studied. The ferroelastic phase transition at T ₀₁ is due to the appearance of the shear deformation x ₄(T) and is accompanied by significant anomalies of the birefringence. Strong pretransition phenomena mask the jumps in the birefringence Δn(T) and in the angle of rotation of the indicatrix φ(T) at T ₀₁.     

Optical and x-ray diffraction studies of the symmetry of distorted phases of the (NH<Subscript>4</Subscript>)<Subscript>3</Subscript>ZrF<Subscript>7</Subscript> crystal

(NH₄)₃ZrF₇ single crystals were grown, and polarization-optical and x-ray diffraction studies were performed on powders and crystalline plates of various cuts over a wide temperature range. Phase transitions are revealed at temperatures T _1↑ = 280 K, T _2↑ = 279.6 K, T _3↑ = 260–265 K, and T _4↑ = 238 K on heating and at T _1↓ = 280 K, T _2↓ = 269–270 K, T _3↓ = 246 K, and T _4↓ = 235 K on cooling. The sequence of changes in symmetry is established to be as follows: O _h ⁵ (Z = 4) ? D _2h ²⁵ (Z = 2) ? C _2h ³ (Z = 2) ? C _i ¹ (Z = 108) ? monoclinic²(Z = 216).     

Mechanism and nature of phase transitions in the (NH4)3MoO3F3 oxyfluoride

The temperature dependences of the heat capacity, the unit cell parameter, and the permittivity for the (NH₄)₃MoO₃F₃ cryolite (space group Fm $ \overline 3 The temperature dependences of the heat capacity, the unit cell parameter, and the permittivity for the (NH₄)₃MoO₃F₃ cryolite (space group Fm m) are investigated. It is revealed that the compound undergoes ferroelectric and ferroelastic structural phase transitions at temperatures of 297 and 205 K, respectively. The mechanism of structural distortions is discussed in terms of the entropy parameters, pressure-temperature phase diagrams, and electron density maps for critical atoms. An analysis is made of the influence of the cation size and shape on the phase transitions in oxyfluorides of the general formula A ₂ A′MO₃ (A,A′ = NH₄, K; M = Mo, W). Original Russian Text ? I.N. Flerov, V.D. Fokina, A.F. Bovina, E.V. Bogdanov, M.S. Molokeev, A.G. Kocharova, E.I. Pogorel’tsev, N.M. Laptash, 2008, published in Fizika Tverdogo Tela, 2008, Vol. 50, No. 3, pp. 497–506.