The Phase Diagrams of Na2O and K2O with Nb2O5 and the Ternary System Nb2O5-Na2O-Yb2O3.

Abstract The Nb 2 O,-Na 2 O and Nb 2 -K 2 O systems were re-investigated between 50 and 100 mol% Nb 2 O 5 by thermal analysis, dilatometry and X-ray techniques. Five compounds were found in the investigated region of the first system: NaNbO 3 , Na 2 Nb 4 O 11 , Na 1± x Nb 3± x O 8±2 x ' NaNb 7 O 18 and NaNb 13 O 33 . NaNbO 3 melts congruently, Na 2 Nb 4 O 11 decomposes peritectoidally and the three others decompose peritectically. NaNb 13 O 33 , NaNb 7 O 18 and Na 1± x Nb 3± x O 8±2 x are high temperature compounds. The system Nb 2 O 5 -K 2 O contains eight compounds in the investigated region: KNbO 3 , K 4 Nb 6 O 17 , K 5.75 Nb 10.85 O 30 , K 3 Nb 7 O 19 KNb 3 O 8 , K 2 Nb 8 O 21 , KNb 7 O 18 and KNb 13 O 33 . K 4 Nb 6 O 17 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 Nb 2 O 5 -Na 2 O-Yb 2 O 3 were studied at 1273 and 1523 K. No ternary oxide was detected.