Effects of Compression on the Local Iodine Environment in Dipotassium Zinc Tetraiodate(V) Dihydrate K2Zn(IO3)4·2H2O

Abstract

Combining X-ray diffraction with density-functional theory and electron topology calculations, we found that pressure substantially modifies the bonding in K₂Zn(IO₃)₄·2H₂O. We discovered that under compression, there is a progressive change from primary covalent I–O bonds and secondary halogen I···O interactions toward O–I–O electron-deficient multicenter bonds. Because of this, iodine hypercoordination converts IO₃ trigonal pyramids toward IO₆ units. The formation of these IO₆ units breaks the typical isolation of iodate molecules, forming an infinite two-dimensional iodate network. Hypercoordination influences the hydrogen atoms too, such that multicenter O–H–O bonds are also promoted with increasing pressure. We have determined that K₂Zn(IO₃)₄·2H₂O is one of the most compressible iodates studied to date, with a bulk modulus of 22(3) GPa. The pressure-induced structural changes strongly modify the electronic structure as shown by optical-absorption measurements and band-structure calculations. The band gap energy closes from 4.2(1) eV at ambient pressure to 3.4(1) eV at 20 GPa.