Ferroelastic order parameter and W' domain walls in A(4)LiH(3)(SO4)(4) (A=NH4, Rb, K)

High-resolution neutron powder diffraction was used to study the lattice parameters of three crystals: Rb4LiH3(SO4)(4) (RLHS), deuterated ammonium (NH4)(4)LiH3(SO4)(4) (ALHS), and K4LiH3(SO4)(4) (KLHS) in a temperature range from 5 to 250 K. For RLHS and ALHS a continuous ferroelastic phase transition 4F2 was detected at 134 and 234 K, respectively. KLHS was found to show monoclinic symmetry in the whole temperature region studied. Using the structural data obtained, it was possible to calculate the temperature behavior of spontaneous strain es and its components (e(11)-e(22)) and e(12). This allowed the calculation of the possible temperature changes in the orientation of W' domain walls. The ''pinning energy'' modification of the free energy expansion was introduced to explain the discrepancy between the measured temperature dependence of the order parameter and that expected in the Landau approximation.