Phase transitions in the oxyfluoride (NH<Subscript>4</Subscript>)<Subscript>3</Subscript>NbOF<Subscript>6</Subscript>

(NH₄)₃NbOF₆ single crystals were grown, polarization-optical studies were performed, and birefringence was measured over the temperature range 90–500 K. A sequence of first-order structural phase transitions was found at temperatures T _1↓ = 259.7 K and T _2↓ = 257.7 K with temperature hysteresis δT ₁ = 0.9 K and δT ₂ = 1.9 K. The transitions are accompanied by twinning and the following change in the crystal symmetry: cubic ? tetragonal ? monoclinic. Optical second harmonic generation is found to occur at room temperature, which indicates that the cubic phase is not centrosymmetric. It is assumed that the phase transitions are ferroelastic and ferroelectric in nature.     

Heat capacity,structure, and <Emphasis Type="Italic">p-T</Emphasis> phase diagram of elpasolite (NH<Subscript>4</Subscript>)<Subscript>2</Subscript>KMoO<Subscript>3</Subscript>F<Subscript>3</Subscript>

Thermophysical and structural studies of an (NH₄)₂KMoO₃F₃ crystal show that this crystal belongs to the family of elpasolites (space group \(Fm\bar 3m\)) and undergoes an order-disorder phase transition at T ₀ = 241.5 K. Under hydrostatic pressure, this phase transition splits into two consecutive transitions at the tricritical point with parameters T _tr = 232.5 K and p _tr=0.21 GPa. It was found that anomalous hysteresis and relaxation phenomena accompany the transitions from the cubic to both distorted phases. The results are analyzed taking into account the data on the phase transition in the related elpasolite (NH₄)₂KWO₃F₃.     

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...     

Vibrational spectroscopy of alkaline tungsten oxyfluoride crystals: structure,lattice dynamics,ordering processes,and phase transitions

Raman scattering (RS) and infrared absorption (IR) of ammonium oxyfluoride crystals (NH₄)₃WO₃F₃, (NH₄)₂KWO₃F₃, and Cs₂(NH₄)WO₃F₃are compared. Conformation of the WO₃F₃ octahedral groups has been established; anomalies have been found close to the transition temperatures in the internal vibrational regions of ammonium and WO₃F₃ groups. The phase transition in (NH₄)₃WO₃F₃ is associated mostly with the ordering of octahedral groups and formation of W O···H N hydrogen bonds. In (NH₄)₂KWO₃F₃ crystal, the transition is not related to the ordering processes; Cs₂(NH₄)WO₃F₃ retains its disordered structure down to 10 K. Copyright © 2010 John Wiley & Sons, Ltd.     

Phase transitions in the (NH<Subscript>4</Subscript>)<Subscript>2</Subscript>NbOF<Subscript>5</Subscript> oxyfluoride

The thermal and dielectric properties of the (NH₄)₂NbOF₅ oxyfluoride have been investigated. It has been established that the structural phase transitions Cmc2₁ → C2 → Ia observed at the temperatures T ₁ = 258.0 K and T ₂ = 218.9 K exhibit a nonferroelectric nature. The hydrostatic pressure, which stabilizes the initial phase and destabilizes the low-temperature phase, hardly affects the temperature range of stability of the intermediate phase. The model of sequential ordering of the structural elements due to phase transitions has been analyzed using experimental data on the entropies of the phase transitions.     

Influence of atmospheric humidity on the symmetry and phase transitions of layered potassium oxyfluorides K2NbOF5 · H2O

Crystals of K₂NbOF₅ · H₂O have been grown, polarization optical investigations have been performed, and the birefringence and rotation angle of the optical indicatrix have been measured in the temperature range 100–400 K. It has been found that, depending on the degree of atmospheric humidity, the layered K₂NbOF₅ · H₂O crystal at room temperature can be in three states, namely, A, B, and C, which differ in symmetry and properties of the crystal. The K₂NbOF₅ · H₂O crystal in the A state exists at a relative humidity RH = 90–100% and undergoes a first-order improper ferroelastic phase transition P $\bar 1$ ? C2/m, which is accompanied by strong anomalies of the optical characteristics, twinning, and shear strain x ₆ at temperatures T ₂ ^↓ = 308 K and T ₂ ^↑ = 313 K. The most stable state of the K₂NbOF₅ sdH₂O crystal is the B state (RH = 20–90%), which retains the monoclinic symmetry C2/m in the temperature range 100–370 K. In a dry atmosphere (RH = 0–20%) or at T ₁ ≈ 370 K, the crystal becomes anhydrous (K₂NbOF₅) with the symmetry P4/nmm (the C state). The difference between the crystals in the states A and B is explained by the presence or absence of water molecules in interlayer spaces.     

Phase transitions in oxyfluoride (NH4)2WO2F4

(NH₄)₂WO₂F₄ single crystals are grown, and their polarization-optical, calorimetric, and birefringence properties are studied in the temperature range 90–350 K. A first-order structural phase transition is found to occur at T ₀₁↑ = 202 K with thermal hysteresis of ΔT ₀₁ ≈ 6–12 K. The phase transition is accompanied by twinning and modification of the symmetry . An additional weak anomaly in the differential scanning calorimeter signal is found at T ₀₂ ≈ 170 K. The total thermal effect of both anomalies is ∑ΔH _i = 3200 ± 400 J/mol and ∑ΔS _i = 16.5 ± 2.0 J/mol K. The phase transition at T ₀₁ is of the order-disorder type.     

On the identification of oxygen and fluorine atoms in disordered inorganic oxyfluoride compounds

It is considered practically impossible to differentiate between oxygen and fluorine atoms by X-ray diffraction in disordered structures of oxyfluoride compounds due to the similarity of their ionic radii and diffusion factors. Indeed, many transition metal oxyfluoride compounds containing polar pseudo-octahedral MO_xF_6–x (x = 1-3) anions form crystal structures without any fluorine-oxygen (F/O) ordering owing to a large number of local anion configurations. Because of this static disorder, it is impossible to determine the positions of O and F atoms and find the real geometry of the polyhedron. However, this becomes possible in the case of dynamic disorder of oxyfluoride anions when the central atom is displaced from the center of the octahedron toward a vertex, edge, or face (depending on the number of oxygen atoms in the polyhedron), which enables the identification of O and F atoms owing to inherent differences between M–O and M–F bonding. On cooling, such compounds undergo phase transitions of the order–disorder type with substantial changes in the entropy. The examples of static and dynamic orientational disorder in oxyfluoride compounds of d⁰ transition metals are given.     

Heat capacity, p-T phase diagram, and structure of Rb2KTiOF5

Elpasolite Rb₂KTiOF₅ (space group, Fm $ \bar 3 Elpasolite Rb₂KTiOF₅ (space group, Fm m, Z = 4) was synthesized using a solid-phase reaction method. The temperature dependences of the heat capacity, the unit cell parameters, the structure, the permittivity, the response to an external pressure, and the Raman spectra were studied. A nonferroelectric phase transition was revealed at T ₀ = 215 K; the transition is accompanied by a tetragonal distortion of the unit cell (space group I4/m, Z = 10) and a change in the entropy (ΔS ₀ = Rln8), which is anomalously large for perovskite-like oxyfluorides with atomic cations. The specific features of the mechanism of structure distortion are discussed in combination with the previous experimental data obtained for cryolite (NH₄)₃TiOF₅ and elpasolite Rb₂KGaF₆. Original Russian Text ? V.D. Fokina, I.N. Flerov, M.S. Molokeev, E.I. Pogorel’tsev, E.V. Bogdanov, A.S. Krylov, A.F. Bovina, V.N. Voronov, N.M. Laptash, 2008, published in Fizika Tverdogo Tela, 2008, Vol. 50, No. 11, pp. 2084–2092.     

Heat capacity, structural disorder, and the phase transition in cryolite (NH4)3Ti(O2)F5

The heat capacity, T-p phase diagrams, and unit cell parameters of cryolite (NH₄)₃Ti(O₂)F₅ were studied over a wide temperature range. A phase transition was found near 226 K, and its thermodynamic characteristics and their dependence on the crystallization conditions were determined. The coordinates and thermal parameters of atoms in the Fm3m phase were refined. An analysis of the electron density distribution and the transition entropy showed that the mechanism of the structural transition involves, above all, rotation of the Ti(O₂)F₅ octahedra. Possible models of disordering of tetrahedral ammonium groups are considered.