X‐ray studies of thermal expansion and phase transitions in (NH₄)₂SbF₅ crystals

Abstract

X‐ray diffraction studies of phase transitions in (NH4)2SbF3 single crystals have been made by using the Bond‐type diffractometer for high‐precision measurements of the unit‐cell dimensions in a function of temperature. The principal linear thermal expansion coefficients and the linear Eulerian strain tensor have been calculated in the temperature interval 110 – 298 K. The phase transition diagram was proposed and the sequence of the phases has been compared with the results of other experimental techniques (NMR, NQR, DTA, electrophysical methods).

Optical Investigations of Phase Transitions in (NH4)2SbF5Crystal

“…3) resembles a diffused phase transition at Tc3 P:. 132 K. This behaviour is in qualitative agreement with temperature dependences of Cp [10] and dilatation [6] of APFA. The similarity of temperature dependences of d(δ∆)/dT and Cp is shown in Fig.…”

“…1.5 and 1 1.5 mm the average relative error of determination of double optical thickness variations is equal to 7 x 10-5 . Using dependence of δ∆/∆ = f(T) (obtained in this experiment) and temperature dependences of lattice constant [6] (obtained from Bond's method), the temperature changes of relative refractive indices δn/n of APFA were obtained according to the formula The used method is characterized by high temperature sensitivity. 350-380 interference extrema (number of points on plots) correspond to 110-310 K temperature range and 1.5 mm sample thickness.…”

“…Successive phase transitions were reported in (NH4)2SbF5 crystal [1][2][3][4][5][6][7][8]. However, there are discrepancies related to the number of the phase transitions in this compound.…”

“…However, there are discrepancies related to the number of the phase transitions in this compound. In a paper [6] the proposed phase transitions and symmetry of particular phases are recollected. In the temperature range 110-330 K four phase transitions at 138, 165, 257 and 294 K are reported.…”

“…Additionally the unusual behaviour of resonance frequency below 169 K is observed. Temperature dependences of lattice parameters [6] confirmed two phase transitions and proved the (611) change of symmetry from orthorhombic to monoclinic at about 294 K. Optical observations [8] showed the ferroelastic character of the phase transition at 292 K. Dielectric measurements showed the change of slope of e' at 169 K and its decrease in the temperature range 169-120 K. The aim of the presented paper is to obtain additional information related to the phase transitions sequence and description of phase transitions character.…”