Pressure-induced hypercoordination of iodine and dimerization of I2O6H in strontium di-iodate hydrogen-iodate (Sr(IO3)2HIO3)

In this work, we report evidence of pressure-induced changes in the crystal structure of Sr(IO3)2HIO3 connected to changes the coordination of the iodine atom and the of the configuration of HIO3 and IO3 units. The changes favor iodine hypercoordination and happen in two steps on sample compression. Firstly, at 2.5 GPa, [HIO3]-[IO3] complexes are formed, and secondly, at 4.5 GPa, these complexes form dimers of [HIO3]-[IO3]-[IO3]-[HIO3]. The evidence is obtained from a combined experimental and theoretical study performed up to 20 GPa. Synchrotron powder X-ray diffraction, Raman spectroscopy, and optical-absorption experiments have been complemented with density-functional theory calculations, including the study of the topology of the electron density. The changes observed in the crystal structure are related to the transformation of secondary (halogen) bonds into electron-deficient multicenter bonds. The paper also discusses the effect of pressure on the compressibility of the Sr(IO3)2HIO3 crystal structure, its phonons, the electronic band gap, and the refractive index. Sr(IO3)2HIO3 was found to be highly compressible with an anisotropic compressibility. The softening of the internal I-O vibrations of IO3 units was also observed, together with a decrease of the band-gap energy (from 4.1 eV at 0 GPa to 3.7 eV at 20 GPa), a band-gap crossing, and a change in the topology of the band structure, with Sr(IO3)2HIO3 transforming from a direct gap semiconductor at 0 GPa to an indirect gap semiconductor beyond 6 GPa.