THE PHASE-DIAGRAMS OF NA2O AND K2O WITH NB2O5 AND THE TERNARY-SYSTEM NB2O5-NA2O-YB2O3

The Nb2O5-Na2O and Nb2O5-K2O systems were re-investigated between 50 and 100 mol% Nb2O5 by thermal analysis, dilatometry and X-ray techniques. Five compounds were found in the investigated region of the first system: NaNbO3, Na2Nb4O11, Na(1+/-x)Nb(3+/-x)O8+/-2x, NaNb7O18 and NaNb13O33. NaNbO3 melts congruently, Na2Nb4O11 decomposes peritectoidally and the three others decompose peritectically. NaNb13O33, NaNb7O18 and Na1 +/- (x)Nb3 +/- (x)O8 +/- 2x are high temperature compounds. The system Nb2O5K2O contains eight compounds in the investigated region: KNbO3, K4Nb6O17, K5.75Nb10.85O30, K3Nb7O19, KNb3O8, K2Nb8O21, KNb7O18 and KNb13O33. K4Nb6O17 melts congruently, all the others decompose peritectically or peritectoidally. Linear thermal expansion coefficients between 350 and 1250 K are given for the sodium and potassium niobates. Two isothermal sections of the ternary oxide system Nb2O5-Na2O-Yb2O3 were studied at 1273 and 1523 K. No ternary oxide was detected.