Polarization optical study of a new orthorhombic-monoclinic phase transition in chromium-chlorine boracite, Cr3B7013Cl

Abstract

Abstract In a previous study1 an electric field/temperature phase diagram investigation of Cr-Cl boracite suggested the existence of a phase transition in zero electric field between about 143 and 183K. However, the continuous temperature variation of spontaneous birefringence on (100)-cuts perpendicular to the orthorhombic c-axis failed to prove such a transition between 133 and 264K (TC)1. In the present study a first order phase transition, in zero electric field, from the orthorhombic (mm2) to a monoclinic (m) ferroelectric phase has indeed been identified, both by differential scanning calorimetry and polarized ligth studies on (110) and (111)-cuts. The passage from mm2 to m is accompanied by the formation of a domain pattern of extremely fine lamellae, similar to that of analogous transitions found in Fe-Cl, Co-Cl, Zn-Cl1 and Fe-I boracites1–2. The transition has a small enthalpy change (ΔH ≈ 0.12kJmole-1) and is sluggish : large transition range and thermal hysteresis (visual : on upheat 175–187K, ...