Thermal Expansion of (CH₃NH₃)₅Bi₂X₁₁(X=Cl, Br) Crystals in the Temperature Range from 80 to 300 K

Abstract

Thermal expansion studies on (C H3NH3)5 Biz C111 and (CH3NH3)5Bi2Br11 in the temperature range 80-350 K are reported. The dilatometric measurements revealed a considerable similarity with respect to the anisotropy of dilatations of both isomorphous crystals. A diffused thermal anomaly was found around i70 K in (CH3NH3)5Bi2Cl11 crystal.

Strain–polarisation coupling and elastic constant determined from polarisation and strain curves in order–disorder uniaxial ferroelectrics

“…The saturation values of the strain are about ε sat = 0.0056 at τ sat = −0.35 for MAPCB and ε sat = 0.0125 at τ sat = −0.55 for MAPBB. The decrease and the sudden increase in the strain ε below τ sat = −0.35 for ferroelectric MAPCB is connected with an anomaly and an ordering of another methylammonium cation at 170 K 10, 35, 36, 44–46. The similar anomaly occurs at 77 K for ferroelectric MAPBB 36, 44, 47.…”

“…Figure 1(a) and (b) present the experimental spontaneous strain ε for MAPCB and MAPBB, respectively. The strain ε is extracted from the dilatometric measurements 44. The spontaneous “pancake” strain ε , defined in Ref.…”

Study of Order Parameters Coupling in Uniaxial Ferroelectrics on the Basis of Pyroelectric and Dilatometric Measurements

“…The spontaneous "pancake" strain ε, defined in Ref. [26] as ε ≡ −(2ε 33 − ε 11 − ε 22 )/ √ 3 turns out to be the strongest spontaneous strain appearing in the low temperature phase [16], has been extracted from the dilatometric measurements [27]. The decrease and the sudden increase of the strain ε below τ sat = −0.35 for ferroelectric MAPCB is connected with an anomaly and an ordering of another methylammonium cations at 170 K [16,19,20,26,28,29].…”

Pyroelectric and Dilatometric Studies on the Low Temperature Ferroelectric Phase Transition of (CH3NH3)5Bi2Br11

“…The linear thermal expansion studies on the MAPBB crystals have been limited to the high temperature ferroelectric transition [18,19]. To throw more light on the character of the phase transition at 77 K and the linear thermal expansion in the wide temperature region of MAPBB we decided to perform the dilatometric studies of the compound in the temperature range 350-60 K. This paper also contains new pyroelectric measurements on MAPBB along three crystallographic directions (the a-, b-, and c-axis of the orthorhombic system) from 315 K to 30 K, with a special inspection of the lowest temperature phase transition at 77 K.…”