Crystal Structure and Photoconductivity of Cæsium Plumbohalides

WELLS¹ and his collaborators have shown that crystals of the general compositions CsPbX₃ with X = Cl, Br or I, and Cs₄PbX₆ with X = Cl or Br, may be prepared from aqueous solutions. I have investigated the structures of these crystals and also prepared a few more. I have found² that CsPbCl₃ and CsPbBr₃ have the perovskite structure. At room temperature they are tetragonally or monoclinically distorted. Both of them, however, show transition to pure cubic perovskite structure, at 47° C. and 130° C. respectively and with cell dimensions a = 5.605 A. for CsPbCl₃, a = 5.874 A. for CsPbBr₃. Whereas no extra or forbidden X-ray reflexions can be observed above these transition temperatures, CsPbBr₃ below 130° C.-and presumably also CsPbCl₃ below 47° C.-exhibit a new structure corresponding to a doubling of the cell dimensions. The transitions are likely to be of second order as ΔV = 0 within the limits of accuracy of my measurements.