I. P. Aleksandrova

Detection of the incommensurate modulation of the hydrogen-bond double-minimum potential in the antiferroelectric phase of NH4HSeO4

Abstract An incommensurate antiferroelectric phase in the NH 4 HSeO 4 crystal was found by 77 Se high-resolution NMR in the temperature range 250–262 K. An anomalous line-shape continuum limited by two-edge singularities is observed in this region and described by plane-wave modulation. A model where the asymmetry of proton occupation in the H-bond system is modulated along a certain direction is discussed. The incommensurate structure is metastable.

The study of ferroelectric and modulated phase of ammonium hydrogen selenate

The pulse NMR methods, the measurements of the conductivity and dielectric permittivity were used to study the sequence of the phase transitions in NH4HSeO4. In the temperature region between ferroelectric and paraelectric phases (Tcl ÷ Ti) the spontaneous asymmetry of the occupation of the H-bond double well potential is homogeneous in the ab-plan, whereas it is modulated along the c-direction. The deuteration drastically increases the incommensurate region. The long-time processes leading to the decay of the long-range order in the structure above Tcl are discussed both in the terms of the topological phase transition and at the assumption of first-order polymorphic transformation.

The study of phase transitions in crystals of ammonium hydrogen selenate family

Abstract The 2D, 77Se NMR spectra, relaxation times and conductivity in NH4HSeO4, ND4DSeO4 and RbHSeO4 have been measured. It was found that the phase transitions in these crystals at 417 and 446 K correspondingly are transitions to superionic plastic phases, the high conductivity in which is due to the fast cation diffusion. The nature of other transitions is considered.

Polymorphism of the highly deuterated ammonium hydrogen selenate

Abstract A polymorphic phase transition from orthorhombic to monoclinic phase in the highly deuterated ammonium hydrogen selenate crystal has been found at 339 K by the 77Se NMR and dielectric measurements.

Dtslocations and time-dependent dynamics of the incommensurate phase of NH4HSeO4

Abstract The instability of a modulated structure in time scale has been discovered in NH4HSeO4. The time-dependent kinetics of the NMR spectra has been studied under different thermodynamic conditions.

The Incommensurate Metastable Phase, Nonequilibrium Processes and Memory Effect in the Ferroelectric NH4HSeO4

The instability of the incommensurate phase in NH4HSeO4 was revealed using 77Se high resolution NMR. The nonequilibrium processes accompaning the long-range order destruciton and memory effect were investigated using both NMR and dielectric measurements. The three-dimensional version of the topological phase transition accounting the interaction of three-dimensional solitons with the point defect is developed to explain the experimental data.

The model of phase transitions in the crystals of the NH4HSeO4 type

Abstract NH4HSeO4 crystals and their deuterated analogs undergo phase transitions and posess a number of interesting properties. Experimental investigations of the crystals by NMR, X-ray diffraction, optical and other methods were realized in 1-4: All crystals have a high temperature phase of still not defined symmetry. In this phase an increase of proton conductivity is observed 2. With temperature decrease the NH4HSeO4 crystals undergo the successive phase transitions: into paraelectric phase (space group I1) at Tc1=417 K; into incommensurate phase at Tci=261 K, into ferroelectric phase (space group I2) at Tc3=98 K. There are three molecules per unit cell in monoclinic and triclinci phases 1. Monoclinic phase at temperatures below T=271 K, incommensurate and apprently ferroelectric phases are unstable since after being for several hours in this temperature region the crystal undergoes phase transition into orthorhombic phase (P212121) and turns to have four molecules per unit cell 3,4. The orthorhombic ...

The study of the phase transitions in mixed crystals of NH4HSeO4 group

Abstract The study of the mixed system Mex(NH4)1-xHSeO4, where Me = K, Rb, Cs and of the system NH4H(SxSe1-x)O4 have been carried out using NMR cross-polarization method, dielectric and X-ray measurements. The region of x where the structure of ammonium hydrogen selenate (AHSe) is maintained for different substitutes was determined. The temperature shift of the transition points T i and T c have been found in the mixed crystals. The selective sensitivity of the different type of hydrogen bonds in the monoclinic structure of AHSe to the substitutions and the appearance of the frozen local electric dipole moments affecting the transition temperatures T i and T c are discussed.