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

Ca-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.