M(m)H(n)(XO4)((m+n)/2) crystals: structure, phase transitions, hydrogen bonds, conductivity. I. K9H7(SO4)8·H2O crystals--a new representative of the family of solid acid conductors.

: To reveal the structural conditionality for anomalies in physical properties, including dielectric and optical properties, diffraction studies of K9H7(SO4)8·H2O crystals were performed using synchrotron radiation in the temperature range 293-450 K and determined at 405 K, taking H atoms into account. The results indicate that the occurrence of high conductivity in K9H7(SO4)8·H2O crystals with a temperature increase is associated with the outward diffusion of water molecules, hydrogen-bond network rearrangement and the formation of channels for the possible motion of K(+) ions. A rearranged system of hydrogen bonds consists of permanent bonds and partly of dynamically disordered bonds. Hydrogen-bond rearrangement and the hindered back diffusion of water to the crystal bulk stabilize the high-temperature crystal structure right down to room temperature.