Acta Cryst. (2008). E64, i42 [ doi:10.1107/S1600536808015328 ]
Praseodymium calcium dimolybdenum heptaoxide, Pr1.37Ca0.63Mo2O7, crystallizes in the cubic pyrochlore-type structure. In the crystal structure, MoO6 octahedra are linked together by common corners, forming a three-dimensional [Mo2O6] network. The Pr and Ca atoms and the remaining O atoms are located in the voids of the [Mo2O6] network. The Pr and Ca atoms are distributed statistically over the same 16c crystallographic position with site-occupancy factors of 0.684 (3) and 0.316 (3), respectively. They are surrounded by eight O atoms forming a ditrigonal scalenohedron. All atoms lie on special positions. The (Pr, Ca) and Mo atoms are, respectively in the 16c and 16d positions with 
m symmetry, and the O atoms in the 48f or 8a positions with mm or 
3m site symmetry, respectively.
Single crystals of Pr1.37Ca0.63Mo2O7 were prepared from a mixture of Pr6O11 (Rhone Poulenc, 99.99%), CaMoO4, MoO3 (Cerac, 99.95%) and Mo (Plansee, 99.9999%) with the nominal composition PrCaMo16O28. Before use, Mo powder was reduced under a flow of H2 gas at 1273 K for ten hours in order to eliminate any trace of oxygen. CaMoO4 was prepared by heating a stoichiometric mixture of CaCO3 and MoO3 in an open porcelain crucible at 1073 K for 24 h. The initial mixture (ca 5 g) was cold pressed and loaded into a molybdenum crucible, which was sealed under a low argon pressure using an arc welding system. The charge was heated at a rate of 300 K/h up to 2223 K, and the temperature was held for 5 min., then cooled at 100 K/h to 1373 K and finally furnace cooled. The final product was multiphasic with Pr1.37Ca0.63Mo2O7 and Pr1 - xCaxMo10O16, isomorphous with the RMo5O8 compounds (R = La to Gd; Gougeon et al., 2003), as predominant phases. The crystals thus obtained were of irregular shape.
The structure was solved by direct methods using SIR97 (Altomare et al., 1999). The second setting, with the origin at 3 m of the Fd3m space group, was chosen. Initial refinement with full occupancy for the Pr1 site resulted in an R factor of about 0.30. Refinement of the site-occupancy factor of the Pr1 atoms lowered the R factor to 0.0274 with an occupation factor of 0.74. As qualitative microanalyses using a Jeol JSM-35 CF scanning electron microscope equipped with a Tracor energy-dispersive-type X-ray spectrometer indicated the presence of calcium in the crystals, we surmised that the deficiency observed on the Pr1 site resulted from the presence of calcium. Refinements taking into account an occupation of the deficient Pr1 site simultaneously by Pr and Ca atoms with no constraint on the site-occupancy factors of the Pr1 and Ca1 atoms led to an over-occupation of the 16 d position. Consequently, the sum of the site occupancy factors was constrained to unity, and the ADPs of the Pr1 and Ca1 atoms were constrained to be equal. Refinement of the occupancy factor of the O2 atom in 8a position which frequently exhibits partial or total deficiency, indicates full occupation of this position.
Data collection: COLLECT (Nonius, 1998); cell refinement: COLLECT (Nonius, 1998); data reduction: EVALCCD (Duisenberg et al., 2003); program(s) used to solve structure: SIR97 (Altomare et al., 1999); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 2001); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008).
![[Figure 1]](./pk2092fig1thm.gif)
| Fig. 1. : View of Pr1.37Ca0.63Mo2O7 along the [110] direction. Displacement ellipsoids are drawn at the 97% probability level. |
| Pr1.37Ca0.63Mo2O7 | Z = 8 |
| Mr = 522.18 | F000 = 1867.4 |
| Cubic, Fd3m | Dx = 6.109 Mg m−3 |
| Hall symbol: -F 4vw 2vw | Mo Kα radiation λ = 0.71070 Å |
| a = 10.4329 (3) Å | Cell parameters from 1172 reflections |
| b = 10.4329 (3) Å | θ = 3.4–45.3º |
| c = 10.4329 (3) Å | µ = 16.45 mm−1 |
| α = 90º | T = 293 (2) K |
| β = 90º | Irregular block, black |
| γ = 90º | 0.16 × 0.14 × 0.12 mm |
| V = 1135.57 (6) Å3 |
| Nonius KappaCCD diffractometer | 266 independent reflections |
| Radiation source: fine-focus sealed tube | 167 reflections with I > 2σ(I) |
| Monochromator: horizontally mounted graphite crystal | Rint = 0.053 |
| Detector resolution: 9 pixels mm-1 | θmax = 45.3º |
| T = 293(2) K | θmin = 3.4º |
| φ scans (κ = 0) + additional ω scans | h = 1→20 |
| Absorption correction: analytical (de Meulenaer & Tompa, 1965) | k = 0→14 |
| Tmin = 0.093, Tmax = 0.125 | l = 0→13 |
| 771 measured reflections |
| Refinement on F2 | Secondary atom site location: difference Fourier map |
| Least-squares matrix: full | w = 1/[σ2(Fo2) + (0.0207P)2 + 4.885P] where P = (Fo2 + 2Fc2)/3 |
| R[F2 > 2σ(F2)] = 0.027 | (Δ/σ)max < 0.001 |
| wR(F2) = 0.086 | Δρmax = 2.51 e Å−3 |
| S = 1.12 | Δρmin = −1.55 e Å−3 |
| 266 reflections | Extinction correction: SHELXL97 (Sheldrick, 2008), Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4 |
| 12 parameters | Extinction coefficient: 0.00256 (19) |
| Primary atom site location: structure-invariant direct methods |
| Pr1.37Ca0.63Mo2O7 | γ = 90º |
| Mr = 522.18 | V = 1135.57 (6) Å3 |
| Cubic, Fd3m | Z = 8 |
| a = 10.4329 (3) Å | Mo Kα |
| b = 10.4329 (3) Å | µ = 16.45 mm−1 |
| c = 10.4329 (3) Å | T = 293 (2) K |
| α = 90º | 0.16 × 0.14 × 0.12 mm |
| β = 90º |
| Nonius KappaCCD diffractometer | 266 independent reflections |
| Absorption correction: analytical (de Meulenaer & Tompa, 1965) | 167 reflections with I > 2σ(I) |
| Tmin = 0.093, Tmax = 0.125 | Rint = 0.053 |
| 771 measured reflections |
| R[F2 > 2σ(F2)] = 0.027 | 12 parameters |
| wR(F2) = 0.086 | Δρmax = 2.51 e Å−3 |
| S = 1.12 | Δρmin = −1.55 e Å−3 |
| 266 reflections |
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. |
Refinement. Refinement of F2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F2, conventional R-factors R are based on F, with F set to zero for negative F2. The threshold expression of F2 > σ(F2) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F2 are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger. |
| x | y | z | Uiso*/Ueq | Occ. (<1) | |
| Pr1 | 0.0000 | 0.0000 | 0.0000 | 0.00947 (18) | 0.685 (3) |
| Ca1 | 0.0000 | 0.0000 | 0.0000 | 0.00947 (18) | 0.315 (3) |
| Mo1 | 0.5000 | 0.5000 | 0.5000 | 0.00539 (19) | |
| O1 | 0.4247 (3) | 0.1250 | 0.1250 | 0.0160 (5) | |
| O2 | 0.1250 | 0.1250 | 0.1250 | 0.0100 (8) |
| U11 | U22 | U33 | U12 | U13 | U23 | |
| Pr1 | 0.00947 (18) | 0.00947 (18) | 0.00947 (18) | −0.00154 (5) | −0.00154 (5) | −0.00154 (5) |
| Ca1 | 0.00947 (18) | 0.00947 (18) | 0.00947 (18) | −0.00154 (5) | −0.00154 (5) | −0.00154 (5) |
| Mo1 | 0.00539 (19) | 0.00539 (19) | 0.00539 (19) | −0.00013 (5) | −0.00013 (5) | −0.00013 (5) |
| O1 | 0.0239 (14) | 0.0120 (6) | 0.0120 (6) | 0.000 | 0.000 | −0.0026 (8) |
| O2 | 0.0100 (8) | 0.0100 (8) | 0.0100 (8) | 0.000 | 0.000 | 0.000 |
| Pr1—O2i | 2.2588 | Mo1—Ca1xvii | 3.68859 (11) |
| Pr1—O2 | 2.2588 | Mo1—Ca1xviii | 3.68859 (11) |
| Pr1—O1ii | 2.5930 (18) | Mo1—Ca1xix | 3.68859 (11) |
| Pr1—O1iii | 2.5930 (18) | Mo1—Ca1xx | 3.68859 (11) |
| Pr1—O1iv | 2.5930 (18) | Mo1—Ca1xxi | 3.68859 (11) |
| Pr1—O1v | 2.5930 (18) | Mo1—Ca1xxii | 3.68859 (11) |
| Pr1—O1vi | 2.5930 (18) | O1—Mo1xxiii | 2.0046 (10) |
| Pr1—O1vii | 2.5930 (18) | O1—Mo1xxiv | 2.0046 (10) |
| Pr1—Pr1vii | 3.68859 (11) | O1—Pr1vii | 2.5930 (18) |
| Pr1—Ca1viii | 3.68859 (11) | O1—Ca1vii | 2.5930 (18) |
| Pr1—Ca1ix | 3.68859 (11) | O1—Ca1xxv | 2.5930 (18) |
| Pr1—Ca1x | 3.68859 (11) | O1—Pr1xxv | 2.5930 (18) |
| Mo1—O1xi | 2.0046 (10) | O2—Ca1xxv | 2.2588 |
| Mo1—O1xii | 2.0046 (10) | O2—Pr1vii | 2.2588 |
| Mo1—O1xiii | 2.0046 (10) | O2—Pr1ix | 2.2588 |
| Mo1—O1xiv | 2.0046 (10) | O2—Pr1xxv | 2.2588 |
| Mo1—O1xv | 2.0046 (10) | O2—Ca1vii | 2.2588 |
| Mo1—O1xvi | 2.0046 (10) | O2—Ca1ix | 2.2588 |
| O2i—Pr1—O2 | 180.0 | O1xii—Mo1—Ca1xvii | 42.51 (5) |
| O2i—Pr1—O1ii | 79.93 (4) | O1xiii—Mo1—Ca1xvii | 137.49 (5) |
| O2—Pr1—O1ii | 100.07 (4) | O1xiv—Mo1—Ca1xvii | 90.0 |
| O2i—Pr1—O1iii | 79.93 (4) | O1xv—Mo1—Ca1xvii | 137.49 (5) |
| O2—Pr1—O1iii | 100.07 (4) | O1xvi—Mo1—Ca1xvii | 42.51 (5) |
| O1ii—Pr1—O1iii | 117.01 (2) | O1xi—Mo1—Ca1xviii | 42.51 (5) |
| O2i—Pr1—O1iv | 79.93 (4) | O1xii—Mo1—Ca1xviii | 90.0 |
| O2—Pr1—O1iv | 100.07 (4) | O1xiii—Mo1—Ca1xviii | 137.49 (5) |
| O1ii—Pr1—O1iv | 117.01 (2) | O1xiv—Mo1—Ca1xviii | 137.49 (5) |
| O1iii—Pr1—O1iv | 117.01 (2) | O1xv—Mo1—Ca1xviii | 90.0 |
| O2i—Pr1—O1v | 100.07 (4) | O1xvi—Mo1—Ca1xviii | 42.51 (5) |
| O2—Pr1—O1v | 79.93 (4) | Ca1xvii—Mo1—Ca1xviii | 60.0 |
| O1ii—Pr1—O1v | 180.00 (8) | O1xi—Mo1—Ca1xix | 42.51 (5) |
| O1iii—Pr1—O1v | 62.99 (2) | O1xii—Mo1—Ca1xix | 137.49 (5) |
| O1iv—Pr1—O1v | 62.99 (2) | O1xiii—Mo1—Ca1xix | 90.0 |
| O2i—Pr1—O1vi | 100.07 (4) | O1xiv—Mo1—Ca1xix | 137.49 (5) |
| O2—Pr1—O1vi | 79.93 (4) | O1xv—Mo1—Ca1xix | 42.51 (5) |
| O1ii—Pr1—O1vi | 62.99 (2) | O1xvi—Mo1—Ca1xix | 90.0 |
| O1iii—Pr1—O1vi | 180.00 (8) | Ca1xvii—Mo1—Ca1xix | 120.0 |
| O1iv—Pr1—O1vi | 62.99 (2) | Ca1xviii—Mo1—Ca1xix | 60.0 |
| O1v—Pr1—O1vi | 117.01 (2) | O1xi—Mo1—Ca1xx | 137.49 (5) |
| O2i—Pr1—O1vii | 100.07 (4) | O1xii—Mo1—Ca1xx | 90.0 |
| O2—Pr1—O1vii | 79.93 (4) | O1xiii—Mo1—Ca1xx | 42.51 (5) |
| O1ii—Pr1—O1vii | 62.99 (2) | O1xiv—Mo1—Ca1xx | 42.51 (5) |
| O1iii—Pr1—O1vii | 62.99 (2) | O1xv—Mo1—Ca1xx | 90.0 |
| O1iv—Pr1—O1vii | 180.00 (8) | O1xvi—Mo1—Ca1xx | 137.49 (5) |
| O1v—Pr1—O1vii | 117.01 (2) | Ca1xvii—Mo1—Ca1xx | 120.0 |
| O1vi—Pr1—O1vii | 117.01 (2) | Ca1xviii—Mo1—Ca1xx | 180.0 |
| O2i—Pr1—Pr1vii | 144.7 | Ca1xix—Mo1—Ca1xx | 120.0 |
| O2—Pr1—Pr1vii | 35.3 | O1xi—Mo1—Ca1xxi | 90.0 |
| O1ii—Pr1—Pr1vii | 135.34 (4) | O1xii—Mo1—Ca1xxi | 137.49 (5) |
| O1iii—Pr1—Pr1vii | 81.87 (4) | O1xiii—Mo1—Ca1xxi | 42.51 (5) |
| O1iv—Pr1—Pr1vii | 81.87 (4) | O1xiv—Mo1—Ca1xxi | 90.0 |
| O1v—Pr1—Pr1vii | 44.66 (4) | O1xv—Mo1—Ca1xxi | 42.51 (5) |
| O1vi—Pr1—Pr1vii | 98.13 (4) | O1xvi—Mo1—Ca1xxi | 137.49 (5) |
| O1vii—Pr1—Pr1vii | 98.13 (4) | Ca1xvii—Mo1—Ca1xxi | 180.0 |
| O2i—Pr1—Ca1viii | 35.3 | Ca1xviii—Mo1—Ca1xxi | 120.0 |
| O2—Pr1—Ca1viii | 144.7 | Ca1xix—Mo1—Ca1xxi | 60.0 |
| O1ii—Pr1—Ca1viii | 44.66 (4) | Ca1xx—Mo1—Ca1xxi | 60.0 |
| O1iii—Pr1—Ca1viii | 98.13 (4) | O1xi—Mo1—Ca1xxii | 137.49 (5) |
| O1iv—Pr1—Ca1viii | 98.13 (4) | O1xii—Mo1—Ca1xxii | 42.51 (5) |
| O1v—Pr1—Ca1viii | 135.34 (4) | O1xiii—Mo1—Ca1xxii | 90.0 |
| O1vi—Pr1—Ca1viii | 81.87 (4) | O1xiv—Mo1—Ca1xxii | 42.51 (5) |
| O1vii—Pr1—Ca1viii | 81.87 (4) | O1xv—Mo1—Ca1xxii | 137.49 (5) |
| Pr1vii—Pr1—Ca1viii | 180.0 | O1xvi—Mo1—Ca1xxii | 90.0 |
| O2i—Pr1—Ca1ix | 144.7 | Ca1xvii—Mo1—Ca1xxii | 60.0 |
| O2—Pr1—Ca1ix | 35.3 | Ca1xviii—Mo1—Ca1xxii | 120.0 |
| O1ii—Pr1—Ca1ix | 81.87 (4) | Ca1xix—Mo1—Ca1xxii | 180.0 |
| O1iii—Pr1—Ca1ix | 81.87 (4) | Ca1xx—Mo1—Ca1xxii | 60.0 |
| O1iv—Pr1—Ca1ix | 135.34 (4) | Ca1xxi—Mo1—Ca1xxii | 120.0 |
| O1v—Pr1—Ca1ix | 98.13 (4) | Mo1xxiii—O1—Mo1xxiv | 133.86 (14) |
| O1vi—Pr1—Ca1ix | 98.13 (4) | Mo1xxiii—O1—Pr1vii | 105.99 (3) |
| O1vii—Pr1—Ca1ix | 44.66 (4) | Mo1xxiv—O1—Pr1vii | 105.99 (3) |
| Pr1vii—Pr1—Ca1ix | 60.0 | Mo1xxiii—O1—Ca1vii | 105.99 (3) |
| Ca1viii—Pr1—Ca1ix | 120.0 | Mo1xxiv—O1—Ca1vii | 105.99 (3) |
| O2i—Pr1—Ca1x | 35.3 | Mo1xxiii—O1—Ca1xxv | 105.99 (3) |
| O2—Pr1—Ca1x | 144.7 | Mo1xxiv—O1—Ca1xxv | 105.99 (3) |
| O1ii—Pr1—Ca1x | 98.13 (4) | Pr1vii—O1—Ca1xxv | 90.68 (8) |
| O1iii—Pr1—Ca1x | 98.13 (4) | Ca1vii—O1—Ca1xxv | 90.68 (8) |
| O1iv—Pr1—Ca1x | 44.66 (4) | Mo1xxiii—O1—Pr1xxv | 105.99 (3) |
| O1v—Pr1—Ca1x | 81.87 (4) | Mo1xxiv—O1—Pr1xxv | 105.99 (3) |
| O1vi—Pr1—Ca1x | 81.87 (4) | Pr1vii—O1—Pr1xxv | 90.68 (8) |
| O1vii—Pr1—Ca1x | 135.34 (4) | Ca1vii—O1—Pr1xxv | 90.68 (8) |
| Pr1vii—Pr1—Ca1x | 120.0 | Pr1—O2—Ca1xxv | 109.5 |
| Ca1viii—Pr1—Ca1x | 60.0 | Pr1—O2—Pr1vii | 109.5 |
| Ca1ix—Pr1—Ca1x | 180.0 | Ca1xxv—O2—Pr1vii | 109.5 |
| O1xi—Mo1—O1xii | 94.97 (9) | Pr1—O2—Pr1ix | 109.5 |
| O1xi—Mo1—O1xiii | 94.97 (9) | Ca1xxv—O2—Pr1ix | 109.5 |
| O1xii—Mo1—O1xiii | 94.97 (9) | Pr1vii—O2—Pr1ix | 109.5 |
| O1xi—Mo1—O1xiv | 180.0 | Pr1—O2—Pr1xxv | 109.5 |
| O1xii—Mo1—O1xiv | 85.03 (9) | Pr1vii—O2—Pr1xxv | 109.5 |
| O1xiii—Mo1—O1xiv | 85.03 (9) | Pr1ix—O2—Pr1xxv | 109.5 |
| O1xi—Mo1—O1xv | 85.03 (9) | Pr1—O2—Ca1vii | 109.5 |
| O1xii—Mo1—O1xv | 180.0 | Ca1xxv—O2—Ca1vii | 109.5 |
| O1xiii—Mo1—O1xv | 85.03 (9) | Pr1ix—O2—Ca1vii | 109.5 |
| O1xiv—Mo1—O1xv | 94.97 (9) | Pr1xxv—O2—Ca1vii | 109.5 |
| O1xi—Mo1—O1xvi | 85.03 (9) | Pr1—O2—Ca1ix | 109.5 |
| O1xii—Mo1—O1xvi | 85.03 (9) | Ca1xxv—O2—Ca1ix | 109.5 |
| O1xiii—Mo1—O1xvi | 180.0 | Pr1vii—O2—Ca1ix | 109.5 |
| O1xiv—Mo1—O1xvi | 94.97 (9) | Pr1xxv—O2—Ca1ix | 109.5 |
| O1xv—Mo1—O1xvi | 94.97 (9) | Ca1vii—O2—Ca1ix | 109.5 |
| O1xi—Mo1—Ca1xvii | 90.0 |
| Symmetry codes: (i) −x, −y, −z; (ii) −y, z−1/4, x−1/4; (iii) z−1/4, x−1/4, −y; (iv) x−1/4, y−1/4, −z; (v) y, −z+1/4, −x+1/4; (vi) −z+1/4, −x+1/4, y; (vii) −x+1/4, −y+1/4, z; (viii) −x−1/4, −y−1/4, z; (ix) x, −y+1/4, −z+1/4; (x) x, −y−1/4, −z−1/4; (xi) −y+1/2, −z+1/2, −x+1; (xii) −z+1/2, −x+1, −y+1/2; (xiii) −x+1, −y+1/2, −z+1/2; (xiv) y+1/2, z+1/2, x; (xv) z+1/2, x, y+1/2; (xvi) x, y+1/2, z+1/2; (xvii) −x+1/4, −y+3/4, z+1/2; (xviii) y+1/4, −x+1/2, z+3/4; (xix) x+1/2, −y+1/4, −z+3/4; (xx) y+3/4, −x+1/2, z+1/4; (xxi) −x+3/4, −y+1/4, z+1/2; (xxii) x+1/2, −y+3/4, −z+1/4; (xxiii) x, −y+3/4, −z+3/4; (xxiv) x, y−1/2, z−1/2; (xxv) y+1/4, −x, z+1/4. |
Intensity data were collected on the Nonius KappaCCD X-ray diffactometer system of the `Centre de diffractométrie de l'Université de Rennes I' (www.cdifx.univ-rennes1.fr).
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An attempt to synthesize PrCaMo16O28, a compound with the PrMo8O14 type structure (Kerihuel & Gougeon, 1995), was unsuccessful, resulting in a multiphase product. However, the formation of the new compound, Pr1.37Ca0.63Mo2O7 was achieved. A survey of the literature related to the rare earth molybdates R2Mo2O7 with the database SciFinder Scholar (American Chemical Society, 2007) shows that these compounds only form for the rare-earths from Nd to Lu. To our knowledge, no quaternary molybdate pyrochlore has thus far been reported.