High-resolution synchrotron x-ray powder diffraction studies on samples of La2−xMxCuO4 (M = Sr,Ba) prepared by standard ceramic techniques show that macroscopic compositional inhomogeneities may exist that are unlikely to be revealed by conventional x-ray diffraction methods. Rietveld refinement of neutron data collected at 200, 50, and 11 K from one such sample, nominally La1.8Sr0.2CuO4, gave satisfactory fits to a tetragonal structure of K2NiF4 type at all three temperatures. However, careful individual peak fits revealed that part of the sample transforms to orthorhombic between 200 and 50 K. It is suggested that this multiphase character has an important influence on the superconducting properties.
