ESR and ENDOR Investigations of the Ferroelectric BaMgF4

Abstract

ESR and ENDOR measurements for Mn 2+ ions doped in the ferroelectric BaMgF 4 crystal were made. The Mn 2+ ions are substitutional for the Mg 2+ ions in the distorted MgF 6 octahedra. The temperature dependence of crystal field tensor indicates that one MgF 6 octahedron rotates clockwise about the c -axis and another adjacent octahedron rotates counterclockwise as the temperature decreases from the hypothetical Curie temperature. It suggests that two sites become equivalent in the hypothetical phase and the phase has A mam symmetry. The critical exponent value for the rotation angle of the crystal field tensor is 0.5. The normal modes of the lattice vibration in the hypothetical phase were analyzed and the structure in the ferroelectric phase was discussed in connection with the frozen B 1u modes in the hypothetical phase.