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LazTi20 7 with the monoclinic space group P21 transforms its structure into one with the orthorhombic space group Cmc2~ at approximately 1053 K. The orthorhombic structure at 1173 + 10 K with a =-: 3.954 (2), b = 25.952 (8) and c = 5.607 (2) A and Z = 4 has been determined from three-dimensional X-ray diffraction data and refined to a conventional R value of 0.066 for 766 observed reflexions. The crystal is essentially isostructural with that of Sr2Nb20 7, in which * To whom correspondence should be addressed.0567-7408/82/020368-05501.00 all atoms are on the mirror plane at x = 0 or 0.5 and the corner-shared BO 6 octahedra and A atoms with twelve coordination form perovskite-type slabs. The structural change between the orthorhombic and monoclinic modifications is characterized by displacements of La atoms taking place within the respective planes perpendicular to the a axis on which the relevant La atoms lie, and by rotations of TiO 6 octahedra around axes parallel to the b axis and running through the respective Ti atoms. No significant displacement of the Ti positions occurs. Half of the mirror planes in the orthorhombic modification are lost on the phase transition and the remaining half survive as local mirror

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