Calcium nitrate tetrahydrate (Ca(NO3)2•4H2O) (CNT) (Junsei Chemicals Co., Ltd., S. Korea) and potassium dihydrogenphosphate (KDP) (Kanto Chemical Co. Inc., Japan) were used as starting calcium and phosphorous precursors. Ammonia (NH3, Daejung Chemicals and Metals Co. Ltd., S. Korea) was used for adjusting the pH of the solution