A NEW HIGH PRESSURE PHASE OF MNTIO3 AND ITS MAGNETIC PROPERTY

A high pressure phase of MnTiO3 II is newly synthesized at high pressures and temperatures. An equilibrium phase boundary between an atmospheric pressure phase of MnTiO3 I and MnTiO3 II is P(kb) = 110 - 0.044 T(°C). X-ray analysis of powder specimens has revealed that the high pressure phase MnTiO3 II has the disordered ilmenite structure with the hexagonal lattice parameters of a = 5.2051 Å, c = 13.699 Å, c a = 2.632 and V = 321.4 Å[3]. The transformation from MnTiO3 1 to MnTiO3 II causes an expansion of the a axis by 1.3 per cent and a contraction of the c axis by 4.1 per cent, resulting a volume decrease of 1.6 per cent. Both Mössbauer effect and paramagnetic ESR measurements suggest that MnTiO3 II is antiferromagnetic below 24° ± 1°K. The magnetic susceptibility obeys the Curie-Weiss law almost down to the Néel temperature and slightly deviates from it below TN, in marked contrast with that of MnTiO3 I. A possibility that the covalency of Mn2+ ions plays an important role both in the magnetic interactions and the crystal stability of manganese ilmenites is suggested. © 1969.