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Abstract: For the first time, structural phase transitions induced by the temperature were found in A x M x IIM(1−x) IIIF3 fluorides with the defect pyrochlore structure (Fd m, Z=8). The room temperature structure of CsFe2F6 was determined using X-ray powder diffraction technique. This phase was found to be ordered with the Pnma space group. The study of the temperature stability of orthorhombic structure by differential scanning calorimeter between 100K and 700K has shown a succession of phase transitions. The Pnma (Z=4)→Imma (Z=4)→I41/amd (Z=4)→Fd m (Z=8) structural sequence was proposed to occur within a rather narrow temperature range 500–560K. The mechanism of structural transition has been mainly associated with the rotation of (MF6) octahedra and small displacements of some Fe atoms. This assumption is in good agreement with the low experimental entropy value, which is characteristic for displacive transformations. [Copyright &y& Elsevier]

Band structure calculations in p-AlxGa1-xAs/GaAs1-yPy/n-AlxGa1-xAs heterostructure under uni-axial compression in the 1 1 0 direction indicates an increase in the optical energy gap with dEph/dP ≈ 85 meV GPa-1, a decrease in the quantum well barriers and light hole-heavy hole crossover at uniaxial stress P ≈ 450-500 MPa. The observed increase in electroluminescence intensity and photon energy shift under uni-axial compression are explained by numerical calculation data. [ABSTRACT FROM AUTHOR]

We present new results on the influence of uniaxial stress up to P=400 MPa along [110] direction on the electroluminescence (EL) spectra of p-AlxGa1-xAs/GaAs1-yPy/n-AlxGa1-xAs double heterostructures. With increasing stress, the emission spectra demonstrate a blue shift of up to 20 meV at a pressure of P=400 MPa, while the EL intensity increases under compression. The results are discussed in terms of changes in the band structure under uniaxial compression. [ABSTRACT FROM AUTHOR]

The study of transient relaxation processes of photoconductivity in p GaAs/Al0.5Ga0.5As:Be in the dark after illumination has been undertaken for verification of the previously proposed model for explanation of low-temperature thermoactivated photoconductivity in these heterostructures. Relaxa-tions of 2D hole concentration at a different magnitude of uniaxial stress show strong temperature dependence and may be well described in terms of kinetic phenomena developed for materials with deep centers, which give an opportunity for thermoactivation barrier EB evolution. The magnitude EB=2±0.3 meV, which was calculated from experimental data, is well comparable with EB=3±0.5 meV obtained previously from dynamic equilibrium measurements under illumination and does not reveal dependence on uniaxial stress. [ABSTRACT FROM AUTHOR]

Highlights: [•] (NH4)3ZrF7 undergoes the succession of six ferroelastic phase transitions. [•] The transformation between two cubic phases Fm-3m↔F23 is suggested at T 0. [•] Excess entropy does not correspond to strong disorder model of cubic structure. [•] The stability of all phases except G4 increases with pressure increase. [ABSTRACT FROM AUTHOR]

Abstract: Calorimetric, X-ray, dielectric and DTA under pressure measurements have been performed on oxyfluoride (NH4)3Ta(O2)2F4. The succession of nonferroelectric phase transitions was found associated with the order-disorder processes. The comparative analysis tantalate with related niobate has revealed the important role of the central atom in the physical properties behavior, mechanism of structural distortions and barocaloric effect in oxyfluorides with the eight-coordinated anionic polyhedra. [Copyright &y& Elsevier]

By numerical calculation the Fermi surface of two-dimensional (2D) holes at p-GaAs/AlxGa1−xAs heterointerface is found to become strongly anisotropic under the application of in-plane uniaxial compression. This uniaxial stress-induced anisotropy of the energy spectrum reveals in more that two times increase of 2D hole mobility anisotropy (at uniaxial compression about 5 kbar), that is experimentally detected in such heterostructures. It leads also to considerable anisotropy of far-infrared absorption spectrum; absorption of light with polarization perpendicular to the direction of compression is smaller that the absorption of light with polarization parallel to the direction of compression. [Copyright &y& Elsevier]