Effect of Ca on HoPO4·nH2O (n = 1 and 2) crystallization from phosphoric acid solution

Abstract

AbstractCa‐substituted holmium phosphate, isomorphic with tetragonal form of HoPO4·H2O was synthesized by crystallization from boiling phosphoric acid (2 M H3PO4) solution containing 0.02M of Ho and 0–0.2 M of Ca. Calcium concentration, higher than 0.2 M Ca in solution resulted in biphasic solid crystallization: tetragonal HoPO4·H2O and orthorhombic HoPO4·2H2O. Ca incorporation in the solid according to substitution mechanism: Ca2+ + H+ ↔ Ho3+ was limited to ∼3 wt.% and was coupled with simultaneous incorporation of HPO42−. Ca for Ho substitution caused an expansion of the tetragonal unit cell of HoPO4·H2O, resulted from differences in the ionic radii (rCa > rHo). Effects of thermal treatment at 900 °C were as follows: (i) the orthorhombic admixture of HoPO4·2H2O re‐crystallized into tetragonal anhydrous HoPO4, (ii) Ca–at first dissolved in crystal structure of HoPO4·H2O was expelled from it during re‐crystallization to form Ca(PO3)2, and that was associated with a contraction of the unit cell; a ‐ and c‐ axes went down to the level of Ca‐free anhydrous tetragonal form of HoPO4. (iii) HPO42− present in the solids as prepared underwent condensation according to reaction 2HPO42− → P2O74− + H2O. Scanning electron micrographs revealed significant changes in size and morphology of the crystals ranging from spherical globules of HoPO4·H2O formed in Ca‐free H3PO4 with increasing diameter in the presence of lower Ca concentration to rod‐like crystals organized in bundles resembling the “scheaf of wheat”, while crystallized from phosphoric acid solution with higher than 0.2 M Ca.