Thermal Change of Nickel Potassium Phosphate Monohydrate

Abstract

KNiPO₄. H₂O was synthesized by the reaction between Ni₃ (PO₄) ₂.8 H₂O, known as a raw material of nickel yellow and K₂HPO₄ in water at 96°C, and its thermal change was investigated.
KNiPO₄?H₂O with platy crystal of light greenish yellow (P_e : 19. 0%, Y : 88. 1%, λ_d : 569.4 nm) changed to a new phase of low-temperature-type KNiPO₄ (pale yellow ; P_e : 22. 8%, Y : 59. 0%, λ_d : 579. 5 nm) by dehydration at 263°C, and to KNiPO₄ at the transition temperature of 590°C. The color of KNiPO₄ obtained by heating at 900°C was brilliant yellowish red (P_e : 58. 0%, Y : 12. 0%, Ad : 592. 5 nm) because of the five-coordinated Ni²⁺ion.
The color of M^I NiPO₄ (M^I : Li, K, Rb, Cs) synthesized by the solidstate reaction was measured. Their colors turned from yellow to yellowish red, reddish purple and bluish purple with increasing ion radii of alkaline metals and with decreasing coordination number of Ni²⁺ ion.