Optical properties of BaMnF4

Optical properties of pyroelectric and antiferromagnetic BaMnF4 were studied by using a high-accuracy universal polarimeter in the temperature range including the incommensurate phase transition point Ti. Both phases above and below Ti were found to be optically active. The active gyration tensor components gij were consistent with the reported symmetry of the crystal, i.e., C2v and C2 above and below Ti. Temperature dependences of all the gyration components were measured. It is striking that g23, which exists commonly in both phases, manifested itself a sharp peak at Ti. This divergence of g23 makes the transition perfectly second order. g11 along the incommensurate a axis develops conspicuously. The present results of the birefringences did not perfectly agree with previous reports: Although the temperature dependence of nc coincided with the result of Schäfer et al., those of na and nb were quite different in their report. Rotation of the indicatrix takes place only around the a axis of the low-temperature phase. This fact confirms the validity of the point group C2 of the low-temperature phase. The rotation angle of the indicatrix was found to be one order of magnitude larger than that of the crystallographic axes. The rotation angle of the gyration surface was also evaluated. It is approximately two orders of magnitude larger than the rotation angle of the indicatrix. This fact clearly indicates that gyrations are very sensitively affected by the change of the order parameter.