X-ray study of proper ferroelastic phase transition in lithium ammonium tartrate crystal

Abstract

The space group in the ferroelectric — ferroelastic phase of lithium ammonium tartrate LiNH₄C₄H₄O₆ · H₂O crystal has been examined with X-ray diffraction, and found to be the monoclinic P12₁ 1, just as considered from previous studies. The temperature dependence of the lattice constants α(T), b(T), c(T) and the deviation Δβ(T) = β(T) − 90° of the monoclinic angle β(T) from 90° has been measured from 300 K to 10 K. All the 4 spontaneous strains x _1s (T), x _2s (T), x _3s (T) and x _5s (T) are found to increase monotonically and tend to saturate around 20 K, as temperature is lowered.

Key Words:

• Lithium ammonium tartrate
• proper ferroelastic phase transition
• space group
• X-ray diffraction
• lattice constants
• spontaneous strain