FERROTOROIC PHASE TRANSITION
Institute of Crystallography, Russia
|v|||@The third dipole moment
(in addition to the polarization Pi and magnetisation Mi), which is called
the toroidal moment Ti, was comparatively recently discovered in macroscopic
Borasites, M_3B_7O_13X, where M is a bivalent metal and X is a halogen, are the first reliable example of ferrotoroics, i.e. the crystals with ferrotoroic phase transitions (components of Ti are order paramers).
These transitions at the same time are improper ferroelectric and ferroelastic transitions and also week ferromagnetic transitions. From the thermodynamic potential the spontaneous components of Ti, Mi, and Pi and also the susceptibilites: dielectric dPi/dEj, magnetic dMi/dHj, and magnetoelectric dPi/dHj=dMj/dEi, which show specific temperature anomalies, are derived.