S. E. Kichanov

High-pressure effect on the ferroelectric-paraelectric transition in PbTiO3

The crystal structure of lead titanate PbTiO3 was investigated by energy dispersive X-ray diffraction at high pressures up to 4 GPa in a temperature range of 300–950 K. At the ambient conditions, the PbTiO3 structure is tetragonal with the space group P4mm (ferroelectric phase). A structural phase transition into the cubic phase with a space group

High-pressure effect on the crystal and magnetic structures of the frustrated antiferromagnet YMnO3

Pm\bar 3m

Investigation of structural features of the Y3Al5O12: Ce3+/Lu2O3 crystal phosphors formed by the colloidal chemical method

is observed at T = 747 K. It was found that the phase transition temperature decreases upon applying the high pressure with the coefficient dTC/dP = -65 K/GPa. Dependences of parameters and volume of the unit cell on the pressure and temperature was found, and the bulk modulus and thermal expansion coefficients for the tetragonal and cubic phases of lead titanate have been calculated.

Investigation of Structural Transformations in Pyridine Nitrate at Low Temperatures and High Pressures

The magnetic structures of hexagonal manganites YMnO3 and LuMnO3 have been studied by powder neutron diffraction up to 6 GPa in the temperature range 10‐295 K. At ambient pressure, a triangular antiferromagnetic (AFM) state of a � 1 irreducible representation is stable below TN = 70 K in YMnO3. Upon the application of high pressure, a spin reorientation is induced and the triangular AFM structure evolves from � 1 to � 1 + � 2 representations. On the other hand, in LuMnO3 the triangular AFM state of a � 2 irreducible representation with TN ≈ 90 K remains stable over the entire pressure range investigated. The ordered magnetic moment values decrease under pressure with dM/dP =− 0.35 μB GPa −1 in YMnO3 and −0.08 μB GPa −1 in LuMnO3. Simultaneously, a considerable increase in diffuse scattering intensity was found in YMnO3, while it was much less pronounced for LuMnO3. Both features indicate the enhancement of spin fluctuations due to geometrical frustration effects and an increase in the volume fraction of the spin-liquid state coexisting with the ordered AFM phase. The characteristic spin correlation length is weakly affected by pressure. The relationship between the pressureinduced behaviour of magnetic structure and the structural characteristics of the quasi-two-dimensional (2D) triangular network formed by Mn and O ions in hexagonal RMnO3 is analysed.

The Polymorphic Phase Transformations in the Chlorpropamide under Pressure

The high-pressure (to 5 GPa) effect on the crystal and magnetic structures of the hexagonal manganite YMnO3 is studied by neutron diffraction in the temperature range 10–295 K. A spin-liquid state due to magnetic frustration on the triangular lattice formed by Mn ions is observed in this compound at normal pressure and T > TN = 70 K, and an ordered triangular antiferromagnetic state with the symmetry of the irreducible representation Γ1 arises at T < TN. The high-pressure effect leads to a spin reorientation of Mn magnetic moments and a change in the symmetry of the antiferromagnetic structure, which can be described by a combination of the irreducible representations Γ1 and Γ2. In addition, it is observed that the ordered magnetic moment of Mn ions decreases from 3.27 μB (5 GPa) to 1.52 μB (5 GPa) at T = 10 K and diffuse scattering is enhanced at temperatures close to TN. These effects can be explained within the model of the coexistence of the ordered antiferromagnetic phase and the spin-liquid state, whose volume fraction increases with pressure due to the enhancement of frustration effects.

Effect of high pressure on the crystal and magnetic structure and on the raman spectra in Pr0.7Ba0.3MnO3 manganite

The crystal and magnetic structures of La0.75Ca0.25MnO3 manganite are studied under high pressures up to 4.5 GPa in the temperature range 12–300 K by the neutron diffraction method. At normal pressure and temperature TC = 240 K, a ferromagnetic state is formed in La0.75Ca0.25MnO3. At high pressures P ≥ 1.5 GPa and at temperatures T < TN ≈ 150 K, a new A-type antiferromagnetic state appears. A further increase in pressure leads to an increase in the volume fraction of the antiferromagnetic phase, which coexists with the initial ferromagnetic phase. The effect of high pressure causes a considerable increase in TC with the slope dTC/dP ≈ 12 K/GPa. Calculations performed in the framework of the double exchange model with allowance for the electron-phonon interaction make it possible to explain this pressure dependence of TC on the basis of experimental data.

Structural Aspects of the Antiferroelectric-Paraelectric Phase Transition in Double Perovskite Pb 2 MgWO 6 at High Pressures and Temperatures

The specific features of the crystal structure of crystal phosphors Y3Al5O12: Ce3+/Lu2O3 (Lu2O3: Ce) synthesized by the colloidal chemical method have been investigated by neutron diffraction at room temperature. The influence of the method used for introducing Lu2O3 into the system on the structure and luminescence properties of the samples has been analyzed. The investigation has revealed that the spectra of the samples prepared under the most nonequilibrium conditions are characterized by the Stokes shift and high photoluminescence intensity. This has been explained by the disorder of their crystal structure due to the formation of stable associates of defects.

A multisectional annular thermal-neutron detector for the study of diffraction on microsamples in axial geometry

The structural transformations in pyridine nitrate PyHNO3 (C5D5NHNO3) are investigated by neutron diffraction in the temperature range 16–300 K at normal pressure and in the high-pressure range 0–3.5 GPa at room temperature. A new high-pressure phase with a monoclinic structure (space group P21/c) is revealed in the PyHNO3 compound at pressures P > Ptr ∼ 1 GPa. The geometry of hydrogen bonds and the coordination of the PyH+ and NO3− ions in the structure of the PyHNO3 compound are studied as a function of the temperature and pressure.