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.     

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.     

Specific heat of the elpasolite Pb2MgWO6

The specific heat of Pb₂MgWO₆ has been measured in the temperature interval 83–370 K. An anomaly in the specific heat associated with the phase transition at T ₀=312.8K has been discovered. The thermodynamic parameters of the structural phase transition Fm3m-Pmcn have been determined. Fiz. Tverd. Tela (St. Petersburg) 41, 1686–1688 (September 1999)     

Investigation into phase diagrams of the fluorine-oxygen system: Ferroelastic-antiferroelectric (NH4)2WO2F4-(NH4)2MoO2F4

Thermal, physical, structural, optical, and dielectric investigations have been performed for oxyfluoride solid solutions (NH₄)₂W_{1 — x} Mo _x O₂F₄ (x = 0, 0.1, 0.2, 0.3, 0.4, 0.6, 0.8, 1). The character of the influence of the chemical and hydrostatic pressures on the stability of the parent (space group Cmcm) and distorted ferroelastic and antiferroelectric phases has been determined by analyzing the temperature-pressure, unit cell volume-composition, and temperature-composition phase diagrams. The specific features of the nature and mechanism of the phase transitions have been discussed using the available data on the structural, entropy, and dielectric parameters.     

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.     

Experimental study of mean and pulsation flow characteristics in the 2D/3D supersonic boundary layer behind flat roughness elements

For the first time, detailed experiments aimed at evaluation of the influence of flat microroughness elements (adhesive tape stickers attached to the streamlined surface) on the mean and pulsation flow characteristics in flat-plate and swept-wing boundary layers were carried out under comparable dimensional conditions. At Mach number M = 2, the sticker shape was found to have almost no influence on the amplitude of mass-flow pulsations in the wake of sticker. The relative receptivity of 3D boundary layer to stationary disturbances was obtained to be approximately 1.5 times higher than the receptivity of 2D boundary layer. It was found that the presence of a sticker at the line of the source of controlled disturbance changes the pulsation spectra in 2D and 3D boundary layers in comparison with unperturbed flow. In the wake behind the center of a sticker, the flow can be most unstable.     

Wave forerunners of streamwise structures in a swept wing boundary layer

In a model experiment wave packets (forerunners) have been detected for the first time in the flow regions preceding the fronts of streamwise structures in a swept wing boundary layer. The characteristics of the wave packets and the generating streamwise structures and the dynamics of their downstream development are investigated. It is shown that the forerunners can transform into turbulent spots thus leading to laminar-turbulent transition. Certain components of the forerunners are compared with a periodic instability wave.     

Heat capacity of the PbFe1/2Ta1/2O3 perovskite-like compound

Physics of the Solid State - The heat capacity C p(T) of the complex perovskite PbFe1/2Ta1/2O3 has been studied using adiabatic calorimetry in the temperature range 150–370 K. Three diffuse...