Linear birefringence studies of incommensurate systems: II. KFeF₄

Abstract

The temperature dependence of the linear birefringence (LB), Anc, is measured in the range 5 K 5 T 5 500 K on samples of KFeF,, which originate from hydrothermal, flux and Bridgman growth techniques.Pronounced anomalies are found at the orthorhombic-orthorhomic phase transition at r, -400 K. It is weakly discontinuous with a near-tricritical exponent p -0.2. Weak anomalies near -T, + (25.. .40 K) seem to indicate a transition into anjntermediate incommensurate phase. Its XY-model character is reflected by the critical LB exponent / 3 = 0.8 & 0.1. A smooth LB anomaly below 200 K is due to 2-dimensional ferromagnetic spin-order.

Electron paramagnetic resonance and x-ray studies of the critical behaviour of the layered compound

“…This is consistent with the fact that the coexistence of two ordered phases is not clearly proved. Therefore, we can suppose the coexistence of partially disordered phases, or following Bianchi and Saint Grégoire [4,11], the presence of an intermediate modulated phase between phase II and phase III. However, satellite reflections characteristic of the incommensurable phase have never been observed.…”

“…Many studies in the KFeF 4 isotype compound have shown that the mechanisms of the phase transition are complex. Indeed, according to Bianchi et al [4] an intermediate modulated phase between the phase II and the phase III is possible. Furthermore, the recent studies on this latter compound have shown that the phase transition II → III is quasi-continuous or it is weak to first order [5].…”