Modulated structures in oxidized cerium niobates

Three previously reported oxidized cerium niobate phases CeNbO4+x (x = 0.08 , 0.25, and 0.33) have been synthesized and characterized by X-ray powder a nd electron diffraction. All three phases display structures which are modu lated variants of a parent fergusonite-type (CeNbO4)-Nb-III structure (I2/a , a = 5.5342 (2) Angstrom b = 11.4016(6) Angstrom, c = 5.1583 (3) Angstrom, beta = 94.600(5)degrees). The x = 0.08 phase with parent unit cell (I2/a, a = 5.3029(8) Angstrom, b = 11.483(2) Angstrom, c = 5.2515(8) Angstrom, bet a = 91.32(2)degrees) is a two-dimensional, incommensurately modulated phase characterized by incommensurate primary modulation wavevectors q(1) simila r to [0.345, 0, 0.1381](p)* and q(2) similar to [-0.069, 0, 0.172](p)* (p f or parent). The x = 0.25 phase with parent unit cell (I2/a, a = 5.3522(8) A ngstrom, b = 11.374(3) Angstrom, c = 5.116(1) Angstrom, beta = 93.34(2)degr ees) is a commensurately modulated superstructure phase characterized by th e reciprocal space unit cell a(r)* = 1/12[402](p)*, b(r)* = 1/4[020](p)*, a nd c(r)* = 1/3[(1) over bar 01](p)* (r for resultant). The x = 0.33 phase w ith parent unit cell (I1, a = 5.4374(8) Angstrom, b = 11.189(2) Angstrom, c = 5.1458(8) Angstrom, alpha = 90.56(1), beta = 94.37(1), gamma = 88.19(1)d egrees) is again commensurately modulated with q = 1/3[101](p)*. The close structural relationship between the three oxidized phases and possible inte rstitial oxygen sites in the (CeNbO4)-Nb-III structure are discussed. (C) 1 999 Academic Press.