“…In the system material, the common basic structural unit is PO 4 group, and the varied condensation of PO 4 groups results in several structural families with the general formula M I LnP 2 O 7 [5], M I Ln(PO 3 ) 4 [6], and M I 3 LnðPO 4 Þ 2 [7] (M I = alkali metal, Ln = rare earth metal), etc. Recently, to our knowledge, structures and optical properties of some polyphosphates MLn(PO 3 ) 4 (M = Li, Na, K, Ln = La, Eu, Tb, Ho, Yb, Y, Dy) [8][9][10][11][12][13] were successively reported. We have also made efforts to synthesize MLn(PO 3 ) 4 (M = K, Cs, Ln = La, Ce, Eu) [14][15][16].…”