Mechanism of phase transitions in the (NH4)2WO2F4 ferroelastic

Precision studies of the thermophysical properties and structure of an (NH₄)₂WO₂F₄ crystal have been performed. It was established reliably that there is a sequence of two phase transitions at T ₁ = 201 K and T ₂ = 160 K characterized by wedging out of an intermediate phase with an increase in pressure. The role of tetrahedral and octahedral ionic groups in the mechanism of the structural transitions was determined.     

On reasons for varied reversibility of different kinds of nickel oxide electrodes in alkaline-zincate electrolytes

Significant difference between the working capacities in zincate electrolytes of reversible nickel oxide electrodes of two types, lamellar with a graphite additive and metal-ceramic, was noted. It was demonstrated that the electrochemical reversibility of the active substance of nickel oxide electrodes is strongly impaired in the simultaneous presence of the carbon (graphite) component and oxygen in the reacting electrodes and zincate in the electrolyte. In operation of electrodes of this kind in alkaline nickel-zinc batteries, the oxygen-involving process is of side (parallel) nature.     

The influence of deuteration on the phase transitions in (NH4)3 Me 3+F6 cryolites (Me 3+=Sc and Ga)

The heat capacity of partially deuterated crystals with a cryolite structure, namely, (NH₄)₃ScF₆ and (NH₄)₃GaF₆, is measured in the temperature range from 80 to 370 K. The p-T phase diagrams of these compounds are investigated at pressures up to p=0.6 GPa. It is revealed that the deuteration does not affect the sequences of phase transitions observed in the proton-containing ammonium cryolites studied earlier. The isotope effect most clearly manifests itself in significant changes in the thermodynamic parameters of the $I12/m1 - P\bar 1$ low-temperature transformation in scandium cryolite.     

Heat capacity study of double perovskite-like compounds BaTi<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Zr<Subscript>x</Subscript>O<Subscript>3</Subscript>

The temperature dependence of the heat capacity of two compositions in the solid solution system BaTi_1?xZr_xO₃ (x = 0.25, 0.35) was measured using adiabatic calorimetry. In the T-x phase diagram, these compounds occupy positions near the crossover from conventional ferroelectric behavior to the relaxor state. Both compounds reveal diffuse heat capacity anomalies: two anomalies in the temperature ranges 250–350 and 150–200 K at x = 0.35 and one anomaly within the range ～150–320 K at x = 0.25. The results obtained are discussed together with structural and dielectric measurements.     

Ferroelastic phase transitions in fluorides with cryolite and elpasolite structures

The ferroelastic phase transitions are investigated in several series of fluoride crystals belonging to the elpasolite and cryolite families (space group \(Fm\bar 3m\)) with the general formula A₂BB′ F₆. The influence of the size and shape of cations and anions on the entropy and the mechanism of structural distortions is discussed.     

Thermal behavior of ammonium-containing titanium and germanium fluoride complexes (NH4)4Li2(AF6)3 and NH4NaAF6 (A = Ge or Ti)

The compounds (NH₄)₄Li₂A₃F₁₈ and NH₄NaAF₆ (A = Ge or Ti) were studied thermogravimetrically. These fluoro complexes are thermolyzed in the range 230–450°C. Measurements on a differential scanning microcalorimeter revealed reversible phase transitions in NH₄NaAF₆ at T ₁ = 126°C and T ₂ = 111°C. Original Russian Text ? T.F. Antokhina, I.N. Flerov, N.N. Savchenko, T.A. Kaidalova, E.B. Merkulov, L.N. Ignat’eva, V.D. Fokina, 2008, published in Zhurnal Neorganicheskoi Khimii, 2008, Vol. 53, No. 4, pp. 639–643.     

Investigation of thermal expansion,phase diagrams,and barocaloric effect in the (NH4)2WO2F4 and (NH4)2MoO2F4 oxyfluorides

The thermal expansion along the principal crystallographic axes of the (NH₄)₂WO₂F₄ and (NH₄)₂MoO₂F₄ oxyfluorides has been studied. The anomalous behavior of α _i (T) due to the phase transitions has been revealed at T ₁ = 271.4 K and T ₂ ≈ 180 K for the molybdate and at T ₁ = 201.5 K and T ₂ ≈ 161 K for the tungstate. The quantities dT/dp and dT/dσ _i , which characterize the dependence of the phase transition temperatures on the hydrostatic and uniaxial pressures, have been determined from analyzing the results of studies of the thermal expansion and heat capacity with the use of the Pippard relations. The p-T and α _i -T phase diagrams reflect different characters of the influence of the pressure on the stability of the initial and distorted phases of the oxyfluorides. The magnitudes of the extensive and intensive barocaloric effects determined in the vicinity of the structural phase transitions are as follows: ΔS _BCE varies from approximately −10 to −17 J/mol K and ΔT _AD ≈ 8−17 K for the molybdate and ΔS _BCE varies from approximately −10 to −17 J/mol K and ΔT _AD ≈ 8−13 K for the tungstate.     

Thermal and physical properties of sodium niobate ceramics over a wide temperature range

The temperature dependences of the heat capacity C _p (T) and thermal expansion coefficient α(T) of NaNbO₃ ceramic samples have been investigated in the temperature range from 2 to 800 K. In addition to the anomalies associated with the known phase transitions at temperatures T ₆ ≈ 265 K, T ₅ ≈ 638 K, T ₄ ≈ 760 K, and T ₃ ≈ 793 K, anomalies in the behavior of C _p (T) and α(T) have been observed near T _5″ ≈ 500 K and T _5′ ≈ 600 K. It has been found that all the observed structural transformations, according to the values of the entropy change, are not related to the ordering of structural elements. It has been shown that, with an increase in the temperature, the unit cell volume during the phase transitions near 265, 515, 604, and 638 K decreases. The specific features of the transition to the phase R3c have been examined. Two possible scenarios of the sequence of phase transformations in the temperature range between T ₅ and T ₆ have been analyzed.     

Heat capacity and the p-T phase diagram of Pb2MgTeO6 elpasolite

The heat capacity of Pb₂MgTeO₆ is measured in the temperature range 80–300 K. It is found that the heat capacity exhibits an anomaly associated with the phase transition at T ₀=186.9 K. The thermodynamic parameters of the structural transformation are determined. The effect of hydrostatic pressure up to 0.5 GPa on the phase transition temperature is examined.     

Entropy and the mechanism of phase transitions in elpasolites

The phase transitions in series of crystals with the general formulas A₂BB’X₆ (X=F, Cl, Br, or CN) and Pb₂BB’O₆ that belong to the elpasolite family (space group $Fm\bar 3m$ ) are analyzed. The influence of the size and the shape of cations and anions on the entropy and the mechanism of structural distortions is discussed.