Effect of Ca on HoPO 4 ·nH 2 O (n = 1 and 2) crystallization from phosphoric acid solution

• Published in: Crystal research and technology (1979) 2012-07, Vol.47 (7), p.804-809
• Main Authors: Kijkowska, R., Malina, D., Sobczak‐Kupiec, A., Wzorek, Z., Orlicki, R.
• Format: Article
• Language: English
• ISSN: 0232-1300; 1521-4079
• DOI: 10.1002/crat.201200083

Ca‐substituted holmium phosphate, isomorphic with tetragonal form of HoPO 4 ·H 2 O was synthesized by crystallization from boiling phosphoric acid (2 M H 3 PO 4 ) solution containing 0.02 M 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 HoPO 4 ·H 2 O and orthorhombic HoPO 4 ·2H 2 O. Ca incorporation in the solid according to substitution mechanism: Ca 2+ + H + ↔ Ho 3+ was limited to ∼3 wt.% and was coupled with simultaneous incorporation of HPO 4 2− . Ca for Ho substitution caused an expansion of the tetragonal unit cell of HoPO 4 ·H 2 O, resulted from differences in the ionic radii (r Ca > r Ho ). Effects of thermal treatment at 900 °C were as follows: (i) the orthorhombic admixture of HoPO 4 ·2H 2 O re‐crystallized into tetragonal anhydrous HoPO 4 , (ii) Ca–at first dissolved in crystal structure of HoPO 4 ·H 2 O was expelled from it during re‐crystallization to form Ca(PO 3 ) 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 HoPO 4 . (iii) HPO 4 2− present in the solids as prepared underwent condensation according to reaction 2HPO 4 2− → P 2 O 7 4− + H 2 O. Scanning electron micrographs revealed significant changes in size and morphology of the crystals ranging from spherical globules of HoPO 4 ·H 2 O formed in Ca‐free H 3 PO 4 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.