Preparation of an acid niobium phosphate and alkali-metal niobium phosphates, having NbOPO4.cntdot.H2O layers condensed with the phosphate group

New alkali-metal niobium phosphates and an acid niobium phosphate were prepared under low temperature hydrothermal conditions. Their chemical formulas were deduced to be NaNb 2 (OH) 2 (PO 4 ) 3 • 2.5H 2 O, KNb 2 (OH) 2 (PO 4 ) 3 •nH 2 O, and HNb 2 (O-H) 2 (PO 4 ) 3 •H 2 O, in the monoclinic system, in the triclinic system, respectively