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 and electron diffraction. All three phases display structures which are modulated variants of a parent fergusonite-type CeIIINbO4structure (I2/a,a=5.5342 (2) ?b=11.4016(6) ?,c=5.1583 (3) ?,=94.600(5)°). Thex= 0.08 phase with parent unit cell (I2/a,a=5.3029(8) ?,b=11.483(2) ?,c=5.2515(8) ?,=91.32(2)°) is a two-dimensional, incommensurately modulated phase characterized by incommensurate primary modulation wavevectors q1~[0.345, 0, 0.138] *pand q2~[-0.069, 0, 0.172]*p(p for parent). Thex=0.25 phase with parent unit cell (I2/a,a=5.3522(8) ?,b=11.374(3) ?,c= 5.116(1) ?,=93.34(2)°) is a commensurately modulated superstructure phase characterized by the reciprocal space unit cell a*r= [402]*p, b*r= [020]*p, and c*r= [ 01]*p(r for resultant). Thex=0.33 phase with parent unit cell (I1,a=5.4374(8) ?,b= 11.189(2) ?,c=5.1458(8) ?,=90.56(1),=94.37(1),=88.19(1)°) is again commensurately modulated with Q= [101]*p. The close structural relationship between the three oxidized phases and possible interstitial oxygen sites in the CeIIINbO4structure are discussed.