The sodium ytterbium orthophosphate Na3(1+x)Yb(2-x)(PO4)3

A new sodium ytterbium orthophosphate with general formula Na_3(1+x)Yb_(2-x)(PO₄)₃ (0.07 ? x ? 0.50) has been prepared and characterized. Its crystal structure has been determined from a single crystal for x = 0.50. The space group is R3?c, the lattice constants are : $\text{a}\text{= 9.12(1)}\text{A}\text{?}$, $\text{c}\text{= 21.81(6)}\text{A}\text{?}$. The structure of Na_4.50Yb_1.50(PO₄)₃ is related to that of NaZr₂(PO₄)₃. The PO₄ tetrahedra and the (Yb,Na)O₆ octahedra form a three-dimensional skeleton in which the remaining sodium atoms are inserted. This structural type is also found for the phases Na_4.50Ln_1.50(PO₄)₃ (Ln = Tm, Lu) and Na_4.50Ln_1.50(AsO₄)₃ (Ln = Er, Tm, Yb, Lu).