Assignment of prototype symmetry for the Y-Ba-Cu-O superconductor and some predictions based on this assignment

Using the available information about the occurrence of various twin walls in the high-Tc superconductor YBa₂Cu₂O_7-x, the prototype symmetry group for this crystal is determined as Pm3m. The room-temperature orthorhombic phase is assigned to the ferroelastic Aizu species m3mFmmm(ss). The spontaneous-strain tensor for this phase has both tensile and shear components, the magnitudes of which are computed as 0.127 and 0.007, respectively. The magnitude of spontaneous strain is found to be insensitive to temperature change over a very large temperature range. The wave vector of the order parameter for the sequence of phase transitions in this crystal is tentatively identified as ({}, {}, ±{} ⅓).