Hand Milling Induced Phase Transition for Marcasite-type Carbodiimide

While high-pressure phase transitions have been widely studied in inorganic compounds such as oxides and chalcogenides, relatively little attention has been given to compounds containing molecular anions, such as carbodiimides and cyanamides. This study investigates the phase transition of marcasite-type orthorhombic Ba_0.9M_0.1NCN carbodiimides, where a transformation to a CsCl-type tetragonal phase was observed at room temperature under hydrostatic pressure of 0.8 GPa (M = Ca) and 0.3 GPa (M = Sr). This transition, accompanied by an increase in the metal coordination under from 6 to 8, occurs at pressures significantly lower than those required for the high-pressure phase transitions of rock-salt-type metal halides and marcasite-type metal diantimonides. Remarkably, partial phase transitions were also induced by hand milling, a process that applies localized shear forces, distinct from the particle-crushing effects of high-energy ball milling. The transition mechanism, analyzed via variable-cell nudged elastic band (VCNEB) calculations, revealed that while the linear NCN^2- anions remain stable, the shear-sliding of Ba²⁺ cations and the rotation of NCN^2- anions are critical to the structural transformation. These findings underscore the potential of hand milling to effectively induce phase transitions in compounds containing linear molecular anions, offering new strategies for predicting and controlling such transitions in similar materials.