Theory of the low-temperature phases in boracites: Latent antiferromagnetism, weak ferromagnetism, and improper magnetostructural couplings

Cubic structure of chromium iodine boracite

“…Cr,BTO13 I,Mr=566.56,cubic,F713c,a =12.214(1)~, V=1822.1(5) /~ 3, Z=8, Introduction. Boracites, M3B 7013 X (M = bivalent metal ion, X = halogen ion), tend to undergo structural phase transitions (Schmid, 1965;Nelmes, 1974;Toledano, Schmid, Clin & Rivera, 1985). A structural feature of particular interest is the environment of the metal ions because it is relatively invariant with respect to the substitution of other metal and halogen ions whereas it changes strongly during phase transitions.…”

Magnetic ferroelectric materials

“…The simultaneous occurrence of ferroelectricity and ferromagnetism has reliably been proved in the boracite family (Tolédano et al 1985) and recently in the phosphate KNiP04. (Lujan et al 1994).…”

“…This compound is a member of a large crystal structure family with the gênerai formula M3B7O13X, where M stands for a bivalent cation of Mg, Cr, Mn, Fe, Ce, Ni, Cu, Zn, etc and X for a monovalent anion like OH", F~, Cl", Br~, I~, or NO3". Nearly ail the boracite compositions with paramagnetic métal ions undergo phase transitions from a cubic high-temperature phase to fuUy ferroelectric/fully ferroelastic phases (for nomenclature see Aizu 1970), most of which undergo at low températures a transition to a (weakly) ferromagnetic phase with orthorhombic, monoclinic or tricHnic symmetry (Tolédano et al 1985;Rivera et al 1985).…”

Multiferroic Materials: Physics and Properties

“…For most of these structures, the ferroelectric ordering takes place well above room temperature whereas the magnetic ordering (often weakly ferromagnetic) occurs at temperatures lower than 65 K (see Tolédano et al 46 for a review). Many of these materials undergo complex sequences of structural transitions on cooling.…”