Modulated Structures in Oxidized Cerium Niobates

Three previously reported oxidized cerium niobate phases CeNbO_4+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 Ce^IIINbO₄structure (I2/a,a=5.5342 (2) Åb=11.4016(6) Å,c=5.1583 (3) Å,β=94.600(5)°). Thex=0.08 phase with parent unit cell (I2/a,a=5.3029(8) Å,b=11.483(2) Å,c=5.2515(8) Å,β=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) Å,b=11.374(3) Å,c=5.116(1) Å,β=93.34(2)°) is a commensurately modulated superstructure phase characterized by the reciprocal space unit cell a^*_r=112[402]^*_p, b^*_r=14[020]^*_p, and c^*_r=13[101]^*_p(r for resultant). Thex=0.33 phase with parent unit cell (I1,a=5.4374(8) Å,b=11.189(2) Å,c=5.1458(8) Å,α=90.56(1),β=94.37(1),γ=88.19(1)°) is again commensurately modulated with q=13[101]^*_p. The close structural relationship between the three oxidized phases and possible interstitial oxygen sites in the Ce^IIINbO₄structure are discussed.