High-resolution studies of cubic perovskites by elastic neutron diffraction. II. SrTiO3, KMnF3, RbCaF3and CsPbCl3

Abstract

For pt.I see ibid., vol.12, p.5393 (1979). A programme of elastic neutron-diffraction experiments has been undertaken to derive (and compare) atomic distribution functions, with high spatial resolution, for the ABX₃ antiferroelectric perovskites SrTiO₃, KMnF₃, RbCaF₃ and CsPbCl₃ in their cubic phase. The background and motivation of the work, and some first results for CsPbCl₃, have been published in paper I. Data collected 5-10K above the cubic to tetragonal transition temperature, T_c, have been used in least-squares refinements of parameters describing anharmonic distributions for the metal and X ions. For KMnF₃ and RbCaF₃ the same parameters have been refined from data collected approximately 100K above T_c-at room temperature. The metal ions are found to have relatively small anharmonicity, which decreases on cooling. By contrast the X ions have an anharmonicity which becomes anomalously enhanced near T_c. This effect increases strongly through the series SrTiO₃ to KMnF₃ to RbCaF₃ to CsPbCl₃: for the O-ion distribution in SrTiO₃ no anharmonic parameter (i.e. above second order) is significantly non-zero, even at T_c+5K, whilst the Cl-ion distribution in CsPbCl₃ at T_c+5K has some significant sixth-order parameters. The form and temperature dependence of the X-ion anharmonicity are examined in relation to current attempts to understand some structural phase transitions in terms of 'precursor' order (clusters), already present in the high-temperature phase.