X-Ray Study on the Phase Transition of Ferroelectric Rb₂Cd₂(SO₄)₃

Abstract

An X-ray study has been made for Rb₂Cd₂(SO₄)₃ crystals and the superlattice reflections are found in both ferroelectric phases. The crystal symmetries of the two ferroelectric phases were determined to be monoclinic C₂²(P2₁) and triclinic C₁¹(P1) as the temperature was decreased. The lattice parameters were measured as a function of temperature by the Bond method. The cell parameters are determined without consideration of the superlattice structure as a=10.3482±0.0005Å, b=10.3430±0.0005Å, c=10.3475±0.0005Å, β=90.04±0.006° and α=γ=90±0.006° at −150°C, and a=10.3392±0.0005Å, b=10.3330±0.0005Å, c=10.3382±0.0005Å, β=90.038±0.006°, α=89.988±0.006° and γ=89.979±0.006° at −190°C. The phase transition T⁴–C₂² is accompanied by the lattice modulations of two types the periods of which are 2 times and 3 times the spacing.