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

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 ABX 3 antiferroelectric perovskites SrTiO 3 , KMnF 3 , RbCaF 3 and CsPbCl 3 in their cubic phase. The background and motivation of the work, and some first results for CsPbCl 3 , 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 KMnF 3 and RbCaF 3 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 SrTiO 3 to KMnF 3 to RbCaF 3 to CsPbCl 3 : for the O-ion distribution in SrTiO 3 no anharmonic parameter (i.e. above second order) is significantly non-zero, even at T c +5K, whilst the Cl-ion distribution in CsPbCl 3 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.