Structural and transport properties of a new class of oxide ion conductors:: Nd4[Ga2(1-x)M2xO7+x▭1-x]O2 (M = Ti, Ge)

The Nd-4[Ga2(1-x)M(2x)O(7+x)square(1-x)]O-2 (M =Ti, Ge) compounds have been prepared by solid state reaction. Nd-4(Ga(2)O(7)square)O-2 and Nd-4(Ti2O8)O-2 both exhibit a cuspidine-like structure and they form a complete solid solution. Upon substituting Ge for Ga, the cuspidine structure is maintained up to x = 0.3. Although both Ti and Ge substitutions for Ga induce a concomitant filling of oxygen vacancies, opposite effects are observed on the oxide ion conductivity: Ti plays a detrimental role whereas the Ge improves the conductivity. A possible interpretation is that additional oxygen atoms are trapped in the coordination polyhedron of Ti4+ whereas they are forced to increase the coordination number of Ga3+ when Ge4+ is introduced in the structure. The highest conductivity at 800 degreesC, sigma approximate to 0.5 x 10(-3) S/cm, is obtained for Nd-4[Ga(1.8)Ge(0.2)O(7.1)square(0.9)]O-2. (C) 2002 Editions scientifiques et medicales Elsevier SAS. All rights reserved.