Isostructural Transition and Mixed Valence Merging in Cervantite under High Pressure Conditions

Abstract:

Antimony can form a series of oxides, including Sb₂O₃, Sb₂O₄, Sb₂O₅, Sb₆O₁₃, Sb₂O and gaseous SbO. Among them, diantimony tetroxide (Sb₂O₄) has mineral name of cervantite, which was first described in 1850 for an occurrence in Cervantes, Sierra de Ancares, Lugo, Galicia, Spain, and named for the locality. As a well-known mixed valence (Sb³⁺ and Sb⁵⁺) oxide mineral, cervantite has interesting structures and properties, which is used as conventional flame retardant and catalyst. Using synchrotron X-ray diffraction combined with diamond anvil cell techniques, the structural evolution of cervantite was studied at room temperature. An isostructural transition happened above 36.9 GPa. The corresponding bulk moduli of β-Sb₂O₄ and β`-Sb<sub>2</sub>O<sub>4</sub> changed abruptly from 100.9 GPa to 321 GPa. The coordination numbers of antimony ions, from 4 and 6 in β-Sb<sub>2</sub>O<sub>4</sub> structure, merge to roughly as 6 in β`-Sb₂O₄ phase with the increasing pressure at around 70 GPa, which was caused by the disappearance of the lone pair electrons in Sb³⁺ ion and formed the extra distorted octahedra local structure with the valence merging as Sb⁵⁺ in the high pressure phase.