Fluorination and reduction of CaCrO3 by topochemical methods

Abstract

Topochemical reactions between CaCrO₃ and polyvinylidene difluoride yield the new fluorinated phase CaCrO_2.5F_0.5, which was characterized by powder synchrotron X-ray diffraction, X-ray photoemission spectroscopy, and magnetic susceptibility measurements. The reaction proceeds via reduced oxide intermediates, CaCrO_2.67 and CaCrO_2.5, in which CrO₆ octahedral and CrO₄ tetrahedral layers are stacked in a different manner along the c axis of CaCrO₃. These two intermediate phases can be selectively synthesized by the carbothermal reduction with g-C₃N_4. Both CaCrO₃ and CaCrO_2.5F_0.5 adopt the same orthorhombic space group, Pbnm; however, the fluorinated phase has decreased Cr–O–Cr bond angles as compared to the parent compound in both the ab plane and along the c-direction, which indicates an increased orthorhombic distortion due to the fluorination. While the oxygen vacancies are ordered in both intermediate phases, CaCrO_2.67 and CaCrO_2.5, a site preference for fluorine in the oxyfluoride phase cannot be confirmed. CaCrO₃ and CaCrO_2.5F_0.5 undergo antiferromagnetic phase transitions involving spin canting, where the fluorination causes the transition temperature to increase from 90 K to 110 K, as a result of the competition between the increased octahedral tilting and the enhancement of superexchange interactions involving Cr³⁺ ions in the CaCrO_2.5F_0.5 structure.