Effect of hydrostatic pressure on the structural phase transitions in ferroelectric (CH₃NH₃)₂AlX₅·6H₂O (X: Cl, Br)

Abstract

The effect of hydrostatic pressure on the structural phase transitions in the ferroelectric crystals of (CH₃NH₃)₂AlX ₅·6H₂O (X: Cl, Br) has been studied. The I–II transition temperatures (307 K in the Cl-salt, 250 K in the Br-salt at 1 atm) linearly increases with increasing pressure. The pressure coefficients of the I–II transition temperatures are 0.072 and 0.068 K/MPa for the Cl-salt and the Br-salt, respectively. The pressure coefficients of the ferroelectric Curie temperatures are slight both for the Cl- and for the Br-salts. The results are consistent with the fact that the I–II transitions in the (CH₃NH₃)₂AlX ₅·6H₂O (X: Cl, Br) group crystals are caused by an order-disorder process.

Key Words:

• Ferroelectricity
• phase transition
• methylammonium
• aluminum
• halogen
• high pressure