Ordering behaviour of the mineral lawsonite

Abstract

Lawsonite crystals have been investigated by dilatometry and dynamic mechanical analysis as well as by measurements of the optical birefringence and the dielectric constant in the temperature range 90K ≤T ≤ 633K. Two successive phase transitions have been found at low temperatures. The analysis of the spontaneous strain and the excess birefringence data reveals that the transition around 273 K (T ₁) is tricritical. The other transition around 120K. (T ₂) is of second order and proper ferroelectric. The mineral shows remarkable pretransitional effects in a temperature range up to 200 K. above T ₁. The data below T ₂ are interpreted in terms of a mutual interference of the two different ordering processes associated with T ₁ and T ₂, respectively.

Key Words:

• Ordering
• Phase transitions
• Lawsonite