“…Firstly, dopant candidates of rare earth elements were screened, including Sc, Y, Nd, Sm, Gd, Ho, Er, Tm and Yb, but only limited compositions showed single phase: La 2 (Nb 0.5 Sc 0.5 ) 2 O 7− δ , La 2 (Nb 1− x Y x ) 2 O 7− δ (LNY, x = 0.60, 0.65, 0.70), 94 and La 2 (Nb 0.45 Yb 0.55 ) 2 O 7− δ (LNYb55). 95 Since there are no extrinsic oxygen vacancies in La 2 (Nb 0.5 Sc 0.5 ) 2 O 7− δ , the conductivity of this sample showed no response to moisture, indicating that there is no proton conduction. The crystal structure of La 2 (Nb 1− x Y x ) 2 O 7− δ (LNY) depends on the composition, and is pyrochlore and fluorite, when x = 0.60 and 0.65 and x = 0.70, respectively.…”