Isostructural Phase Transition by Point Defect Reorganization in the Binary Type-I Clathrate Ba _7.5Si ₄₅

A competition of point defect configurations is inherent to crystalline phases of increased structural complexity. Symmetry preserving phase transitions between them belong to the special class of isostructural transitions. Type-I silicon clathrates are crystalline complex phases whose unit cell containing a fifties of atoms consists of a 3D covalent silicon framework of face-sharing polyhedral cages encapsulating guest cations. At high pressure, an intriguing structural transition is associated to an abrupt volume reduction with no indication for any symmetry breaking. By means of isothermal high-pressure X-ray diffraction performed on single crystal of the simplest representative type-I silicon clathrates, the binary Ba8Si46, we confirm the isostructural character of the transition and identify the associated mechanism. A detailed analysis of the atomic structural parameters across the transition in combination with ab initio studies allow us to pinpoint a microscopic mechanism driven by a defect rearrangement initially present in the structure. An analysis based on the Landau theory allows to give a coherent description of the experimental observations. A discussion on the analogy between this transformation and liquid-liquid transitions is proposed.