Transitions Between Lanthanum Cuprates: Crystal Structures of T′, Orthorhombic, and K₂NiF₄‐type La₂CuO₄

Abstract

Abstract Through low‐temperature synthesis in CsOH flux, lanthanum cuprate La 2 CuO 4 can be obtained in a metastable form, the so‐called T′ modification (tetragonal, I 4/ mmm , no. 139, a = 400.95(2) pm, c = 1254.08(7) pm). When heated, this T′ phase transforms into a K 2 NiF 4 ‐type modification, whose crystal structure was now refined from X‐ray powder data (tetragonal, I 4/ mmm , no. 139, a = 383.29(3) pm, c = 1331.3(2) pm at T = 1073 K). The well‐known orthorhombic phase (s.g. Cmce , no. 64, a = 536.14(3) pm, b = 1315.53(8) pm, c = 540.20(3) pm) – usually obtained via conventional solid state synthesis – was observed to form upon cooling from the K 2 NiF 4 ‐type modification. High‐temperature powder diffractometry allowed crystal structure refinements for all of the three phases.