A new structural modification of superconducting La2−xMxCuO4

La1.9Ba0.1CuO4has been observed to undergo the following sequence of transformations upon cooling from 300 K: tetragonal (I4/mmm) equation orthorhombic (Cmca) equation tetragonal (P42/ncm). The newly discovered low-temperature tetragonal structure can be understood geometrically as arising from a coherent superposition of the two domain modifications of the orthorhombic structure. Dynamically, it results from a second instability in the twofold degenerate soft modes of the high-temperature tetragonal phase. Energetically, the system can be modeled as an XY-spin system with temperature-dependent quartic anisotropy ν(T), and the low-temperature transformation coincides with an isotropic point ν(T1) = 0. The relationship of the newly discovered transformation to other anomalous properties and to superconductivity is discussed briefly.