Behaviour of iron, cobalt and nickel fluosilicate hehahydrates under pressure

Abstract The effect of hydrostatic pressure up to 120MPa on the stability boudaries of crystalline phases and phase transition behaviour observed in MSiF 6 ·6H 2 O (M = Fe, Co, Ni) in the temperature range 20–300 K, is studied by X-ray diffractometry and the differential thermal analysis technique. Existence of critical pressures and the destabilization of the low-temperature monoclinic phase in FeSiF 6 ·6H 2 O and CoSiF 6 ·6H 2 O are established. P-T diagrams of the investigated fluosilicate hexahydrates showing the unique variety of phases and phase transitions are plotted. It is shown that the P-T phase diagram of crystalline states NiSiF 6 ·6H 2 O has the same form as the diagram region CoSiF 6 ·6H 2 O corresponding to pressures which exceed the critical pressure for the stability of the monoclinic phase.