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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
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Abstract

We consider the evolution of the phase transition from the parent hexagonal phase $P6_{3}/mmc$ to the orthorhombic phase $Pmcn$ that occurs in several compounds of $A'A''BX_{4}$ family as a function of the hcp lattice parameter $c/a$. For compounds of $K_{2}SO_{4}$ type with $c/a$ larger than the threshold value 1.26 the direct first-order transition $Pmcn-P6_{3}/mmc$ is characterized by the large entropy jump $Rln2$. For compounds $Rb_{2}WO_{4}$, $K_{2}MoO_{4}$, $K_{2}WO_{4}$ with $c/a<1.26$ this transition occurs via an intermediate incommensurate $(Inc)$ phase. DSC measurements were performed in $Rb_{2}WO_{4}$ to characterize the thermodynamics of the $Pmcn-Inc-P6_{3}/mmc$ transitions. It was found that both transitions are again of the first order with entropy jumps $0.2Rln2 and $0.3Rln2$. Therefore, at $c/a ~ 1.26$ the $A'A''BX_{4}$ compounds reveal an unusual Lifshitz point where three first order transition lines meet. We propose the coupling of crystal elasticity with $BX_{4}$ tetrahedra orientation as a possible source of the transitions discontinuity.

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I. Luk'yanchuk, A. Jorio, P. Saint-Gregoire. 1999-02-10. Thermodynamics of the incommensurate state in Rb_2WO_4: on the Lifshitz point in A`A``BX_4 compounds. https://doi.org/10.1103/physrevb.61.3147

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