research papers
The crystal structure of the defect perovskite Ce1/3NbO3, cerium niobium oxide, has been re-examined by neutron powder and electron diffraction. The results of a powder neutron Rietveld refinement indicate that the structure is monoclinic: space group P2/m with Z = 4, a = 5.5267 (3), b = 7.8824 (2), c = 5.5245 (3) Å, β = 90.294 (1)°, V = 240.67 (2) Å3 at 298 K with χ2 = 2.570. Previous reports have described the Ce1/3NbO3 structure in a smaller (V/2) orthorhombic cell based solely upon X-ray powder diffraction data. The presence of weak reflections in the electron diffraction pattern provides conclusive evidence for a monoclinic superstructure of the orthorhombic cell. While these superlattice reflections are barely detectable with X-ray radiation, they are clearly visible in the neutron diffraction experiments. The superlattice reflections are shown to arise from a tilting of the NbO6 octahedra which results in the reduction of symmetry from orthorhombic to monoclinic. It is also found that the Ce3+ and Nb5+ cations are displaced from the centres of their respective polyhedra to accommodate the bond-valence requirements of the crystal structure. It is likely that distortions of this type are present in other Ln1/3NbO3 and Ln1/3TaO3 defect perovskites.
Supporting information
Crystallographic Information File (CIF) https://doi.org/10.1107/S0108768199012951/br0090sup1.cif | |
Rietveld powder data file (CIF format) https://doi.org/10.1107/S0108768199012951/br0090sup2.rtv |
Computing details top
Cell refinement: GSAS; data reduction: GSAS; program(s) used to refine structure: GSAS.
cerium niobium oxide top
Crystal data top
Ce0.33333NbO3 | V = 240.67 (2) Å3 |
Mr = 187.61 | Z = 4 |
Monoclinic, P2/m | Dx = 5.173 Mg m−3 |
Hall symbol: -P 2y | neutron radiation |
a = 5.5267 (3) Å | µ = 0.14 (1) mm−1 |
b = 7.8824 (2) Å | T = 298 K |
c = 5.5245 (3) Å | brown |
β = 90.294 (1)° | Specimen preparation: Prepared at 1200 K |
Data collection top
SEPD diffractometer | Absorption correction: part of the refinement model (ΔF) Debye-Scherrer absorption correction provided in the GSAS powder refinement package (useful for TOF data). Reference: Hewat (1979) A35, 248. NOTE: This value is REFINED (not calculated based upon composition) |
Radiation source: spallation | |
None monochromator | 2θfixed = 145 |
Specimen mounting: Vanadium can | Distance from source to specimen: 14m mm |
Data collection mode: transmission | Distance from specimen to detector: 1.5m mm |
Scan method: time of flight |
Refinement top
Rp = 0.035 | 60 parameters |
Rwp = 0.057 | (Δ/σ)max = 0.01 |
Excluded region(s): 0-4, 30-1000(s) | Background function: power series |
Profile function: Convolution exponentials and a Gaussian |
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
x | y | z | Uiso*/Ueq | Occ. (<1) | |
Ce | 0.255 (2) | 0.0 | 0.756 (2) | 0.0120 (6)* | .66667 |
Nb | 0.2458 (8) | 0.2613 (2) | 0.2527 (7) | 0.01624 | |
O1 | 0.271 (1) | 0.5 | 0.286 (1) | 0.02033 | |
O2 | 0.224 (1) | 0.0 | 0.212 (1) | 0.01881 | |
O3 | 0.5 | 0.2059 (5) | 0.5 | 0.0154 | |
O4 | 0.0 | 0.223 (1) | 0.5 | 0.01346 | |
O5 | 0.5 | 0.242 (1) | 0.0 | 0.02488 | |
O6 | 0.0 | 0.2626 (5) | 0.0 | 0.02277 |
Atomic displacement parameters (Å2) top
U11 | U22 | U33 | U12 | U13 | U23 | |
Ce | 0.0120 (6) | 0.0120 (6) | 0.0120 (6) | 0.00 | 0.00 | 0.00 |
Nb | 0.0131 (3) | 0.0206 (7) | 0.015 (3) | 0.007 (1) | −0.0008 (5) | −0.003 (1) |
O1 | 0.023 (4) | 0.008 (1) | 0.030 (4) | 0.00 | −0.001 (2) | 0.00 |
O2 | 0.017 (3) | 0.009 (1) | 0.031 (4) | 0.00 | −0.003 (2) | 0.00 |
O3 | 0.008 (4) | 0.023 (2) | 0.015 (4) | 0.00 | −0.008 (2) | 0.00 |
O4 | 0.012 (4) | 0.024 (3) | 0.005 (3) | 0.00 | 0.005 (2) | 0.00 |
O5 | 0.033 (5) | 0.028 (4) | 0.014 (4) | 0.00 | 13 (2) | 0.00 |
O6 | 0.033 (5) | 0.023 (2) | 0.012 (4) | 0.00 | −13 (2) | 0.00 |
Geometric parameters (Å, º) top
Nb—O1 | Cei—O2 | ||
Nb—O2 | Ceii—O2 | ||
Nb—O3 | Ceiii—O2 | ||
Nb—O4 | Ce—O3 | ||
Nb—O5 | Ce—O4 | ||
Nb—O6 | Cei—O5 | ||
Ce—O2 | Cei—O6 | ||
O2—Ce—O2i | 172.6 (6) | O4—Ce—O5i | 93.54 (11) |
O2—Ce—O2ii | 90.7 (5) | O4—Ce—O5iv | 176.4 (6) |
O2—Ce—O2iii | 96.6 (3) | O4—Ce—O6i | 60.56 (15) |
O2—Ce—O3 | 58.26 (19) | O4—Ce—O6v | 118.1 (5) |
O2—Ce—O4 | 55.9 (3) | O5v—Ce—O5iv | 90.0 (6) |
O2—Ce—O5i | 121.72 (19) | O5i—Ce—O6i | 59.86 (13) |
O2—Ce—O6i | 116.5 (4) | O5i—Ce—O6v | 121.8 (4) |
O2—Ce—O2ii | 81.9 (3) | O6i—Ce—O6v | 93.3 (3) |
O2i—Ce—O3 | 126.4 (4) | O1—Nb—O2 | 179.1 (5) |
O2i—Ce—O4 | 119.4 (2) | O1—Nb—O3 | 95.7 (3) |
O2i—Ce—O5i | 62.6 (3) | O1—Nb—O4 | 97.7 (5) |
O2i—Ce—O6i | 59.4 (2) | O1—Nb—O5 | 95.4 (5) |
O2ii—Ce—O2iii | 172.7 (6) | O1—Nb—O6 | 96.6 (2) |
O2ii—Ce—O3 | 124.8 (4) | O2—Nb—O3 | 84.26 (16) |
O2ii—Ce—O4 | 60.6 (3) | O2—Nb—O4 | 83.2 (4) |
O2ii—Ce—O5i | 117.8 (3) | O2—Nb—O5 | 83.7 (5) |
O2ii—Ce—O6i | 58.2 (2) | O2—Nb—O6 | 83.4 (2) |
O2iii—Ce—O3 | 60.0 (3) | O3—Nb—O4 | 88.69 (17) |
O2iii—Ce—O4 | 124.0 (3) | O3—Nb—O5 | 88.09 (18) |
O2iii—Ce—O5i | 58.0 (3) | O3—Nb—O6 | 167.6 (2) |
O2iii—Ce—O6i | 117.9 (4) | O4—Nb—O5 | 166.8 (2) |
O3—Ce—O3iv | 79.1 (3) | O4—Nb—O6 | 90.98 (19) |
O3—Ce—O4 | 64.18 (16) | O5—Nb—O6 | 89.43 (15) |
O3—Ce—O4v | 114.1 (4) | Nb—O1—Nbvi | 166.0 (4) |
O3—Ce—O5i | 63.88 (17) | Nb—O2—Nbvii | 165.7 (3) |
O3—Ce—O5iv | 117.4 (5) | Nb—O3—Nbiii | 154.8 (3) |
O3—Ce—O6i | 93.75 (9) | Nb—O4—Nbii | 162.3 (5) |
O3—Ce—O6v | 172.7 (3) | Nb—O5—Nbviii | 171.2 (6) |
O4—Ce—O4v | 82.9 (6) | Nb—O6—Nbix | 179.4 (3) |
Symmetry codes: (i) x, y, z+1; (ii) −x, y, −z+1; (iii) −x+1, y, −z+1; (iv) −x+1, −y, −z+1; (v) −x, −y, −z+1; (vi) x, −y+1, z; (vii) x, −y, z; (viii) −x+1, y, −z; (ix) −x, y, −z. |