Structural studies of KNiPO4 at high temperature

The structure of the high temperature phases of KNiPO4 has been studied by X-ray powder diffraction and polarised light microscopy. This compound undergoes two phase transitions. At 486 degrees C it undergoes an isomorphous first order phase transition where the Ni2+ ions change their coordination from 5 to 4. At 581 degrees C the crystals transform into a non-polar phase with space group Pnma.