Covalency-driven structural instability and spin-phonon coupling in barium cobalt oxychloride

Our combined experimental and theoretical study reveals unusually large cobalt-oxygen covalency in CoO4 tetrahedral unit of a barium cobalt oxychloride compound. This drives significant charge redistribution, resulting into large hole density on tetrahedral oxygens, which effectively behave as "positively charged" anions. These positively charged oxygens form local dipoles with dopant chloride anions, situated in the same atomic plane, which gets manifested in associated structural distortions. The spatial freezing of these local dipoles below certain temperature is found to produce concomitant effects on dielectric and magnetic responses, coupled via exchange-striction driven spin-phonon interaction. Our study should form the basis for designing new functional oxide materials using the concept of covalency-driven charge redistribution.