Neutron Scattering Study on the Incommensurate Phases in Ferroelectric Rb₂ZnCl₄ and K₂ZnCl₄

Structural study of three modulated phases in Rb2ZnBr4

“…The phase-transition temperatures T i and T~ are 553 and 403 K for K2ZnCI 4 (Gesi &Iizumi, 1979), 303 and194 K for RbEZnCl 4 (Quilichini &Pannetier, 1983), and 129 and 94 K for KESeO 4 (lizumi, Axe, Shirane & Shimaoka, 1977). In addition, many other A2BX 4 compounds show incommensurately and commensurately modulated structures related to the modulated structure of Rb2ZnBr 4 (Hogervorst, 1986).…”

ESR INVESTIGATIONS OF INCOMMENSURATE Rb_{2}ZnCl_{4}:Mn^{2+}

“…Here the amplitude ρ is assumed spatially independent whereas the phase ϕ varies linearly along the modulation axis ϕ = q i z + ϕ 0 . According to neutron scattering data [2], the phase shift between Cl − ions, forming (ZnCl 4 ) tetrahedra, does not exceed ∼ 2 0 just below T i and further decreases on cooling. So, it may be expected, that displacements of Mn 2+ nearest neighbouring Cl − ions are nearly in phase and, hence, "local" approximation [5,6] should be well adapted to the description of ESR spectra in the temperature region adjoining T i .…”

“…Since in the paraelectric phase (ab) represents the mirror plane for paramagnetic centers point symmetry group, the linear term in expansion (1) vanishes for the orientation considered. Hence quadratic coupling between resonance fields and the order parameter is realized and line positions have to be described by expressions (2). The distance between singularities according to (2) equals ∆H = H Q2 − H Q1 = (B/2)∆T 2β .…”

“…Below T i =303 K Rb 2 ZnCl 4 undergoes the phase transition from high temperature paraelectric phase (space group D 16 2h -Pnam) to the incommensurate phase with wave vector of structural modulation q i =(1/3-δ)a * directed along a axis [2]. As it is well known a considerable progress in studying the incommensurate phases has been achieved by using radiospectroscopic techniques and in particular via ESR of Mn 2+ probe in Rb 2 ZnCl 4 crystals [3].…”

Low-temperature commensurate phase of potassium tetrachlorozincate, K2ZnCl4

“…K2ZnCI4 is a ferroelectric compound that performs the normal-incommensurate and the incommensurate-commensurate transitions (Gesi, 1978;Gesi & Iizumi, 1979), as does KzSeO4 (Iizumi, Axe, Shirane & Shimaoka, 1977). The crystal structures of the normal phase above 553 K and the incommensurate phase between 403 and 553 K have been analyzed by Quilichini, Bernede, Lefebvre & Schweiss (1990).…”