X-ray diffraction study of spontaneous strain in superconducting Ba0.6K0.4BiO3

The lattice parameter of Ba0.6K0.4BiO3, which is a well-studied conventional superconductor, was measured by the X-ray Rietveld method between 10 K and room temperature. A very small change in the lattice parameter could be detected in the superconducting phase. The change could be attributed to a spontaneous strain generated in the superconducting phase by the coupling between a superconducting order parameter and the strain. Previously published data on Y Ba2Cu3O6.5 and MgB2 were analyzed, and the present results were compared with our previously obtained results on La1.85Sr0.15CuO4. It was concluded that this coupling phenomenon is common to all superconductors.