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The crystals of (CH,NH,),Bi,Br, have been grown which at room temperature are isomorphous with Cs,Bi,Br, and (CH,NH,),Sb,Br,.The space group is P3m1, a = 0.807, c = 1.008 nm, V = 0.5685 nm3, Z = 1 . The crystals reveal, when cooling, three phase transitions at 188f0.5, 140f1, and 101.5 f 0.5 K. The high temperature phase (I) behaves as a plastic one with respect to a freedom of rotation of methylammonium cations. The two high temperature transitions I+ I1 and II+ 111 can be related, similarly to (CH,NH,),Sb,Br,, to a freezing of reorientation of those cations. At 101.5 K a transition to the improper ferroelectric phase IV was discovered. The features of this phase are non-typical and the nature of the transition is unknown.

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(CH3NH3)3Sb2Cl9 single crystals are grown which appear to be isomorphous with Cs3Sb2Cl9 crystals of β‐modification. They are characterized by the space group Pmcn with the lattice constants: a = 0.7920(5), b = 1.3334(11), and c = 1.966(2) nm. In the crystal one can distinguish the poly‐anionic (Sb2Cl9)3‐ stacks directed along the a‐axis with three crystallographically inequivalent CH3NH 3+ cations. The DSC, dielectric, and PMR studies reveal a structural phase transition at 208 K which is related to a freezing of rotational motion of CH3NH 3+ cations.

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