A low-temperature structural phase transition in CsPbF₃

The structural behaviour of caesium lead fluoride, CsPbF 3 , has been investigated as a function of temperature in the range 148(2)⩽ T (K)⩽276(2). The presence of a structural phase transition at ~190 K reported from previous nuclear magnetic resonance measurements (Bouznik V M, Moskvich Yu N and Voronov V N 1976 Chem. Phys. Lett. 37 464) has been confirmed by impedance spectroscopy and powder neutron diffraction studies. The former show that there is no significant discontinuity in the ionic conductivity σ at the transition temperature, only a change in the slope dσ/d T at T = 185(2) K. On cooling, the neutron diffraction data indicate a transition from the cubic perovskite structure ( Pm m ) to a rhombohedrally distorted perovskite arrangement (space group R 3 c ) at T = 187(5) K. The transition is discontinuous, with a small volume change Δ V / V ~0.113(7)%. In the low-temperature phase there is clear evidence of parallel displacements of the cations away from the centres of their anion polyhedra, indicative of ferroelectric behaviour. The implications of this finding for the wider topic of structural systematics within perovskite-structured compounds are briefly discussed.