PREPARATION AND STRUCTURAL STUDY OF SODIUM GERMANIUM PHOSPHATE-SODIUMTITANIUM PHOSPHATE SOLID-SOLUTIONS - EVOLUTION OF STRUCTURE WITH COMPOSITION

Compounds NaM2IV(PO4)3 with M = Ge, Ti present the [NZP]-type structur e, with a low thermal expansion. The space group at room temperature f or the compound NaGe2(PO4)3 is R3BAR, whereas for the compound NaTi2(P O4)3 is R3cBAR. Evolution of the structure with composition in NaGe2(P O4)3 - NaTi2(PO4)3 system is discussed. The space group and lattice pa rameters were determined for NaGe2-xTix(PO4)3 (0 < x < 2) solid soluti ons, prepared by ceramic method. Compositions rich in Ge(IV) (0 less-t han-or-equal-to x less-than-or-equal-to 1) show the R3BAR space group, whereas for those rich in Ti(IV) (1.2 less-than-or-equal-to x less-th an-or-equal-to 2) the space group is R3cBAR. The variation of the latt ice parameters with composition agreed with these results since it sho wed a change in its tendency between compositions with x = 1 and 1.2. Structure of NaGeTi(PO4)3 (x = 1) was determined by Rietveld method. I t presents a R3BAR symmetry with an ordered distribution of the Ge(IV) and Ti(IV) atoms over two types of octahedra.