Thermodynamics of the incommensurate state in Rb_2WO_4: on the Lifshitz point in A`A``BX_4 compounds

I. Luk'yanchuk; A. Jorio; P. Saint-Gregoire

doi:10.1103/PhysRevB.61.3147

paper

Thermodynamics of the incommensurate state in Rb_2WO_4: on the Lifshitz point in A`A``BX_4 compounds

arXiv:cond-mat/9902138 · doi:10.1103/PhysRevB.61.3147

Abstract

We consider the evolution of the phase transition from the parent hexagonal phase $P 6_{3} / mm c$ to the orthorhombic phase $P m c n$ that occurs in several compounds of $A^{'} A^{''} B X_{4}$ family as a function of the hcp lattice parameter $c / a$ . For compounds of $K_{2} S O_{4}$ type with $c / a$ larger than the threshold value 1.26 the direct first-order transition $P m c n - P 6_{3} / mm c$ is characterized by the large entropy jump $R l n 2$ . For compounds $R b_{2} W O_{4}$ , $K_{2} M o O_{4}$ , $K_{2} W O_{4}$ with $c / a < 1.26$ this transition occurs via an intermediate incommensurate $(I n c)$ phase. DSC measurements were performed in $R b_{2} W O_{4}$ to characterize the thermodynamics of the $P m c n - I n c - P 6_{3} / mm c$ transitions. It was found that both transitions are again of the first order with entropy jumps $0.2 R l n 2 an d$ 0.3Rln2 $. T h er e f or e, a t$ c/a ~ 1.26 $t h e$ A'A''BX_{4} $co m p o u n d sr e v e a l an u n u s u a l L i f s hi t z p o in tw h er e t h r ee f i r s t or d er t r an s i t i o n l in es m ee t . W e p r o p ose t h eco u pl in g o f cr y s t a l e l a s t i c i t y w i t h$ BX_{4}$ tetrahedra orientation as a possible source of the transitions discontinuity.

arXiv comment13 pages,1 Postscript figure. Submitted as Brief Report to Phys. Rev. B, this paper reports a new work in Theory and Experiment, directed to Structural Phase Transitions

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Basal-plane Incommensurate Phases in HCP Structures

Thermodynamics of the incommensurate state in Rb_2WO_4: on the Lifshitz point in A`A``BX_4 compounds

Abstract

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