Phase Transitions in (CH₃NH₃)₅Bi₂X₁₁(X=Cl, Br) Crystals

Abstract

Electric properties of (CΗ3ΝΗ3)5 Bi2C111 and (CΗ3ΝΗ3)5Βi2Br11 single crystals in the direction of the crystallographic b-axis were studied. The samples tested were natural crystal seeds in the form of thin plates, whose crystallographic b-axis coincides with the normal to the plate. Measurements of the pyroelectric coefficient were performed near the high temperature phase transition and spontaneous polarization from the paraelectric phase down to helium temperature. The attempt of explanation of the anomalous electric properties of these crystals has been presented.

Dielectric Properties of the (CH3NH3)5Bi2(1- x)Sb2xCl11Mixed Crystals

“…At increasing temperature the crystal undergoes a transition from the ferroelectric to paraelectric phase with a symmetry P cab at temperature T c = 308 K. The dielectric, pyroelectric [1], and dilatometric studies indicate a continuous character of the phase transition. Due to the high value of the pyroelectric coefficient, p, the crystals should be taken into account as a material for infrared pyroelectric detector.…”

“…Alkylammonium halogenobismuthate(III) (CH 3 NH 3 ) 5 Bi 2 Cl 11 crystallizes at room temperature in the orthorhombic system, space group P ca2 1 . At increasing temperature the crystal undergoes a transition from the ferroelectric to paraelectric phase with a symmetry P cab at temperature T c = 308 K. The dielectric, pyroelectric [1], and dilatometric studies indicate a continuous character of the phase transition.…”

Influence of Defects on Dielectric Properties of (CH3NH3)5Bi2X11(x=Cl, Br) Crystals

“…MAPCB and MAPBB crystals are very sensitive to the influence of various kind of defects. For example, MAPBB crystals grown from aqueous solution with some excess of HBr have strong internal bias field Eb which significantly modifles temperature dependence of Ps a n d ε ( T ) , e s p e c i a l l y i n t h e v i c i n i t y o f t h e p h a s e transition at 77 Κ [8].…”