Polymorphism in BaPb1-xBixO3 at the superconducting composition

Using a combination of high-resolution and high-sensitivity neutron and x-ray diffraction measurements, it is shown that BaPb(1-x)BixO(3) is dimorphic for the critical superconducting composition regime of x between 0.2 and 0.3. The system is not chemically phase separated; the simultaneous existence of tetragonal and orthorhombic polymorphs of the same composition is demonstrated. The polymorphs differ fundamentally in the distortion and tilting of the octahedra, such that they cannot be obtained from each other via a second-order phase transition. It is established that the tetragonal symmetry polymorph is the superconducting phase.