A low-temperature structural phase transition in CsPbF3

The structural behaviour of caesium lead fluoride, CsPbF3, has been investi gated as a function of temperature in the range 148(2) less than or equal t o T (K) less than or equal to 276(2). The presence of a structural phase tr ansition at similar to 190 K reported from previous nuclear magnetic resona nce measurements (Bouznik V M, Moskvich Yu N and Voronov V N 1976 Chem. Phy s, Lett. 37 464) has been confirmed by impedance spectroscopy and powder ne utron diffraction studies. The former show that there is no significant dis continuity in the ionic conductivity sigma at the transition temperature, o nly a change in the slope d sigma /dT at T = 185(2) K. On cooling, the neut ron diffraction data indicate a transition from the cubic perovskite struct ure (Pm (3) over barm) to a rhombohedrally distorted perovskite arrangement (space group R3c) at T = 187(5) K. The transition is discontinuous, with a small volume change DeltaV/ V similar to 0.113(7)%. In the low-temperature phase there is clear evidence of parallel displacements of the cations awa y from the centres of their anion polyhedra, indicative of ferroelectric be haviour. The implications of this finding for the wider topic of structural systematics within perovskite-structured compounds are briefly discussed.