Ferroelectric and ferroelastic properties of Mg-Cl-Boracite

Mg-Cl-Boracite, Mg3B7013Cl, transforms from a piezoelectric cubic phase to a slightly orthorhombically-distorted ferroelectric-ferroelastic phase at Tc = 541°K, represented by IIPP(43m)Fc.Fc.P(mm2). The orthorhombic phase is highly twinned, due to domain rotations of 9044 about an orthorhombic [110], causing [110] to become parallel with [001], Boracite is detwinned by cooling through the transition under uniaxial compressive stress applied along a cubic [110]. The lattice constants at 298°K are a = 8.54169 ± 0.00004, b = 8.55049 ± 0.00003, c = 12.09774 ± 0.00004 Å. The spontaneous strain is 5.51 ± 10-4. A coercive stress of 120 MNm-2 ferroelastically interchanges the a and b axes at room temperature. Ferroelastic reorientation is accompanied by atomic displacements as large as 0.60 Å and a reversal of the sense of the polar c-axis. The temperature dependence of Ps has been measured from 95° to 555°K: Ps passes through a minimum of 0.04 ± 10-2Cm-2 at 413°K rising to 0.09 ± 10-2 Cm-2 at 540°K and thereafter falling to zero. The temperature dependence of the pyroelectric coefficient at Tc corresponds to that of a first order phase transition. A model for the small spontaneous polarization of this three-dimensional ferroelectric is given.