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Acta Cryst. (2012). E68, i92    [ doi:10.1107/S1600536812045138 ]

The pyrochlore-type molybdate Pr2Mo1.73Sc0.27O7

P. Gall and P. Gougeon

Abstract top

Dipraseodymium molybdenum scandium hepta­oxide, Pr2Mo1.73Sc0.27O7, crystallizes in the cubic pyrochlore-type structure. In the crystal, (Mo,Sc)O6 octa­hedra are linked together by common corners, forming a three-dimensional [(Mo,Sc)2O6] framework. The Pr atom and another O atom atom are located in the voids of this framework. The Mo and the Sc atom are distributed statistically over the same 16d crystallographic position, with site-occupancy factors of 0.867 (3) and 0.133 (3), respectively. The Pr3+ ions are surrounded by six O atoms from the MoO6 octa­hedra and by two other O atoms, forming a ditrigonal scalenohedron. All atoms lie on special positions. The Pr and the statistically distributed (Mo,Sc) sites are in the 16c and 16d positions with .-3m symmetry, and two O atoms are in 48f and 8a positions with and -43m site symmetry, respectively.

Comment top

An attempt to synthesize ScPr9Mo16O35, a compound with the LiNd9Mo16O35 type structure (Gougeon et al., 2011), was unsuccessful, resulting in a multiphase product. However, the formation of the new compound, Pr2Mo1.73Sc0.27O7 was achieved. Recently, we presented the crystal structure of the pseudo-quaternary pyrochlore Pr1.37Ca0.63Mo2O7 (Gall & Gougeon, 2008) in which the Pr and Ca toms occupy statistically the A site. The existence of the latter two phases clearly suggests the unknown Pr2Mo2O7 could be synthesized. However, our attempts to obtain a single- phase powder sample as well as single-crystals of Pr2Mo2O7 were unsuccessful up to now.

Related literature top

For pyrochlore-type molybdates, see, for example: Hubert (1974); Subramanian et al. (1983); Gall & Gougeon (2008). For the physical properties of some rare-earth molybdate pyrochlores, see: Hill et al. (1989); Ali et al. (1989); Miyoshi et al. (2001, 2003). An attempt to synthesize ScPr9Mo16O35, a compound with the LiNd9Mo16O35 type structure (Gougeon et al., 2011), was unsuccessful, resulting in a multiphase product with Pr2Mo1.73Sc0.27O7 and Pr16Mo21O56 Gougeon & Gall (2011) as predominant phases.

Experimental top

Single crystals of Pr2Mo1.73Sc0.27O7 were prepared from a mixture of Pr6O11 (Rhone Poulenc, 99.99%), Sc2O3 (Strem Chemicals, 99.99%), MoO3 (Cerac, 99.95%) and Mo (Plansee, 99.9999%) with the nominal composition ScPr9Mo16O35. Before use, Mo powder was reduced under H2 flowing gas at 1273 K during ten hours in order to eliminate any trace of oxygen. 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 the rate of 300 K/h up to 2223 K, temperature which was held for 5 min., then cooled at 100 K/h down to 1373 K and finally furnace cooled. The final product was multiphasic with Pr2Mo1.73Sc0.27O7 and Pr16Mo21O56 (Gougeon & Gall, 2011), as predominant phases. The crystals thus obtained were of irregular shape.

Refinement top

The structure was solved by direct method using SIR97. The second setting, with the origin at 3 m of the Fd3m space group, was chosen. The first refinements taking into account a full occupancy of the Pr1 and Mo1 sites resulted in a R factor of about 0.033. Refinements of the site-occupancy factors of the Pr1 and Mo1 atoms show that the first site was fully occupied while the second one was slightly deficiency. 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 scandium in the crystals, we could expect that the deficiency observed on the Mo1 site results from the presence of scandium. Refinements taking into account an occupation of the deficient Mo1 site simultaneously by Mo and Sc atoms with no constraint on the site-occupancy factors of the Mo1 and Sc1 atoms led to an occupation of 1.03 (9) of the 16 d position. Consequently, the sum of the site occupancy factors was constrained to the unity, and the ADPs of the Mo1 and Sc1 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 a quasi-full occupation of this position (0.97 (2) %).

Computing details top

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).

Figures top
[Figure 1] Fig. 1. : View of Pr2Mo1.73Sc0.27O7 along the [110] direction. Displacement ellipsoids are drawn at the 97% probability level.
Dipraseodymium molybdenum scandium heptaoxide top
Crystal data top
Pr2Mo1.73Sc0.27O7Mo Kα radiation, λ = 0.71070 Å
Mr = 571.69Cell parameters from 5283 reflections
Cubic, Fd3mθ = 5.5–50°
a = 10.5271 (3) ŵ = 20.33 mm1
V = 1166.61 (6) Å3T = 293 K
Z = 8Irregular block, black
F(000) = 20180.1 × 0.06 × 0.05 mm
Dx = 6.510 Mg m3
Data collection top
Nonius KappaCCD
326 independent reflections
Radiation source: fine-focus sealed tube269 reflections with I > 2σ(I)
Horizontally mounted graphite crystal monochromatorRint = 0.044
Detector resolution: 9 pixels mm-1θmax = 50.0°, θmin = 5.5°
ϕ scans (κ = 0) + additional ω scansh = 1922
Absorption correction: analytical
(de Meulenaar & Tompa, 1965)
k = 1222
Tmin = 0.302, Tmax = 0.461l = 2222
9208 measured reflections
Refinement top
Refinement on F2Primary atom site location: structure-invariant direct methods
Least-squares matrix: fullSecondary atom site location: difference Fourier map
R[F2 > 2σ(F2)] = 0.018 w = 1/[σ2(Fo2) + (0.0144P)2 + 3.2607P]
where P = (Fo2 + 2Fc2)/3
wR(F2) = 0.047(Δ/σ)max < 0.001
S = 1.33Δρmax = 0.79 e Å3
326 reflectionsΔρmin = 0.81 e Å3
13 parametersExtinction correction: SHELXL97 (Sheldrick, 2008), Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4
0 restraintsExtinction coefficient: 0.00067 (7)
Crystal data top
Pr2Mo1.73Sc0.27O7Z = 8
Mr = 571.69Mo Kα radiation
Cubic, Fd3mµ = 20.33 mm1
a = 10.5271 (3) ÅT = 293 K
V = 1166.61 (6) Å30.1 × 0.06 × 0.05 mm
Data collection top
Nonius KappaCCD
326 independent reflections
Absorption correction: analytical
(de Meulenaar & Tompa, 1965)
269 reflections with I > 2σ(I)
Tmin = 0.302, Tmax = 0.461Rint = 0.044
9208 measured reflections
Refinement top
R[F2 > 2σ(F2)] = 0.01813 parameters
wR(F2) = 0.0470 restraints
S = 1.33Δρmax = 0.79 e Å3
326 reflectionsΔρmin = 0.81 e Å3
Special details top

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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/UeqOcc. (<1)
Pr10.00000.00000.00000.00874 (8)
Mo10.50000.50000.50000.00591 (13)0.867 (6)
Sc10.50000.50000.50000.00591 (13)0.133 (6)
O10.41969 (18)0.12500.12500.0132 (3)
O20.12500.12500.12500.0072 (6)0.972 (16)
Atomic displacement parameters (Å2) top
Pr10.00874 (8)0.00874 (8)0.00874 (8)0.00193 (2)0.00193 (2)0.00193 (2)
Mo10.00591 (13)0.00591 (13)0.00591 (13)0.00007 (4)0.00007 (4)0.00007 (4)
Sc10.00591 (13)0.00591 (13)0.00591 (13)0.00007 (4)0.00007 (4)0.00007 (4)
O10.0175 (7)0.0110 (4)0.0110 (4)0.0000.0000.0014 (5)
O20.0072 (6)0.0072 (6)0.0072 (6)0.0000.0000.000
Geometric parameters (Å, º) top
Pr1—O2i2.2792Mo1—O1xiii2.0440 (8)
Pr1—O22.2792Mo1—O1xiv2.0440 (8)
Pr1—O1ii2.5795 (13)Mo1—O1xv2.0440 (8)
Pr1—O1iii2.5795 (13)Mo1—O1xvi2.0440 (8)
Pr1—O1iv2.5795 (13)O1—Sc1xvii2.0440 (8)
Pr1—O1v2.5795 (13)O1—Mo1xvii2.0440 (8)
Pr1—O1vi2.5795 (13)O1—Sc1xviii2.0440 (8)
Pr1—O1vii2.5795 (13)O1—Mo1xviii2.0440 (8)
Pr1—Pr1v3.7219 (1)O1—Pr1v2.5795 (13)
Pr1—Pr1viii3.7219 (1)O1—Pr1viii2.5795 (13)
Pr1—Pr1ix3.7219 (1)O2—Pr1viii2.2792
Pr1—Pr1x3.7219 (1)O2—Pr1v2.2792
Mo1—O1xi2.0440 (8)O2—Pr1x2.2792
Mo1—O1xii2.0440 (8)
O2i—Pr1—O2180.0O1vi—Pr1—Pr1ix43.83 (3)
O2i—Pr1—O1ii79.09 (3)O1vii—Pr1—Pr1ix136.17 (3)
O2—Pr1—O1ii100.91 (3)Pr1v—Pr1—Pr1ix180.0
O2i—Pr1—O1iii100.91 (3)Pr1viii—Pr1—Pr1ix120.0
O2—Pr1—O1iii79.09 (3)O2i—Pr1—Pr1x144.7
O1ii—Pr1—O1iii180.00 (6)O2—Pr1—Pr1x35.3
O2i—Pr1—O1iv79.09 (3)O1ii—Pr1—Pr1x82.59 (3)
O2—Pr1—O1iv100.91 (3)O1iii—Pr1—Pr1x97.41 (3)
O1ii—Pr1—O1iv116.506 (19)O1iv—Pr1—Pr1x136.17 (3)
O1iii—Pr1—O1iv63.494 (19)O1v—Pr1—Pr1x43.83 (3)
O2i—Pr1—O1v100.91 (3)O1vi—Pr1—Pr1x82.59 (3)
O2—Pr1—O1v79.09 (3)O1vii—Pr1—Pr1x97.41 (3)
O1ii—Pr1—O1v63.494 (19)Pr1v—Pr1—Pr1x60.0
O1iii—Pr1—O1v116.506 (19)Pr1viii—Pr1—Pr1x60.0
O1iv—Pr1—O1v180.00 (6)Pr1ix—Pr1—Pr1x120.0
O2i—Pr1—O1vi79.09 (3)O1xi—Mo1—O1xii83.22 (6)
O2—Pr1—O1vi100.91 (3)O1xi—Mo1—O1xiii96.78 (6)
O1ii—Pr1—O1vi116.506 (19)O1xii—Mo1—O1xiii180.0
O1iii—Pr1—O1vi63.494 (19)O1xi—Mo1—O1xiv83.22 (6)
O1iv—Pr1—O1vi116.506 (19)O1xii—Mo1—O1xiv96.78 (6)
O1v—Pr1—O1vi63.494 (19)O1xiii—Mo1—O1xiv83.22 (6)
O2i—Pr1—O1vii100.91 (3)O1xi—Mo1—O1xv96.78 (6)
O2—Pr1—O1vii79.09 (3)O1xii—Mo1—O1xv83.22 (6)
O1ii—Pr1—O1vii63.494 (19)O1xiii—Mo1—O1xv96.78 (6)
O1iii—Pr1—O1vii116.506 (19)O1xiv—Mo1—O1xv180.0
O1iv—Pr1—O1vii63.494 (19)O1xi—Mo1—O1xvi180.0
O1v—Pr1—O1vii116.506 (19)O1xii—Mo1—O1xvi96.78 (6)
O1vi—Pr1—O1vii180.00 (6)O1xiii—Mo1—O1xvi83.22 (6)
O2i—Pr1—Pr1v144.7O1xiv—Mo1—O1xvi96.78 (6)
O2—Pr1—Pr1v35.3O1xv—Mo1—O1xvi83.22 (6)
O1ii—Pr1—Pr1v82.59 (3)Sc1xvii—O1—Mo1xvii0.0
O1iii—Pr1—Pr1v97.41 (3)Sc1xvii—O1—Sc1xviii131.13 (9)
O1iv—Pr1—Pr1v82.59 (3)Mo1xvii—O1—Sc1xviii131.13 (9)
O1v—Pr1—Pr1v97.41 (3)Sc1xvii—O1—Mo1xviii131.13 (9)
O1vi—Pr1—Pr1v136.17 (3)Mo1xvii—O1—Mo1xviii131.13 (9)
O1vii—Pr1—Pr1v43.83 (3)Sc1xviii—O1—Mo1xviii0.0
O2i—Pr1—Pr1viii144.7Sc1xvii—O1—Pr1v106.64 (2)
O2—Pr1—Pr1viii35.3Mo1xvii—O1—Pr1v106.64 (2)
O1ii—Pr1—Pr1viii136.17 (3)Sc1xviii—O1—Pr1v106.64 (2)
O1iii—Pr1—Pr1viii43.83 (3)Mo1xviii—O1—Pr1v106.64 (2)
O1iv—Pr1—Pr1viii82.59 (3)Sc1xvii—O1—Pr1viii106.64 (2)
O1v—Pr1—Pr1viii97.41 (3)Mo1xvii—O1—Pr1viii106.64 (2)
O1vi—Pr1—Pr1viii82.59 (3)Sc1xviii—O1—Pr1viii106.64 (2)
O1vii—Pr1—Pr1viii97.41 (3)Mo1xviii—O1—Pr1viii106.64 (2)
Pr1v—Pr1—Pr1viii60.0Pr1v—O1—Pr1viii92.34 (6)
O1ii—Pr1—Pr1ix97.41 (3)Pr1viii—O2—Pr1v109.5
O1iii—Pr1—Pr1ix82.59 (3)Pr1—O2—Pr1x109.5
O1iv—Pr1—Pr1ix97.41 (3)Pr1viii—O2—Pr1x109.5
O1v—Pr1—Pr1ix82.59 (3)Pr1v—O2—Pr1x109.5
Symmetry codes: (i) x, y, z; (ii) z1/4, x1/4, y; (iii) z+1/4, x+1/4, y; (iv) x1/4, y1/4, z; (v) x+1/4, y+1/4, z; (vi) y, z1/4, x1/4; (vii) y, z+1/4, x+1/4; (viii) y+1/4, x, z+1/4; (ix) x1/4, y1/4, z; (x) x, y+1/4, z+1/4; (xi) y+1/2, z+1/2, x+1; (xii) x, y+1/2, z+1/2; (xiii) x+1, y+1/2, z+1/2; (xiv) z+1/2, x, y+1/2; (xv) z+1/2, x+1, y+1/2; (xvi) y+1/2, z+1/2, x; (xvii) x, y+3/4, z+3/4; (xviii) x, y1/2, z1/2.

Experimental details

Crystal data
Chemical formulaPr2Mo1.73Sc0.27O7
Crystal system, space groupCubic, Fd3m
Temperature (K)293
a (Å)10.5271 (3)
V3)1166.61 (6)
Radiation typeMo Kα
µ (mm1)20.33
Crystal size (mm)0.1 × 0.06 × 0.05
Data collection
DiffractometerNonius KappaCCD
Absorption correctionAnalytical
(de Meulenaar & Tompa, 1965)
Tmin, Tmax0.302, 0.461
No. of measured, independent and
observed [I > 2σ(I)] reflections
9208, 326, 269
(sin θ/λ)max1)1.078
R[F2 > 2σ(F2)], wR(F2), S 0.018, 0.047, 1.33
No. of reflections326
No. of parameters13
Δρmax, Δρmin (e Å3)0.79, 0.81

Computer programs: COLLECT (Nonius, 1998), EVALCCD (Duisenberg et al., 2003), SIR97 (Altomare et al., 1999), SHELXL97 (Sheldrick, 2008), DIAMOND (Brandenburg, 2001).

Selected bond lengths (Å) top
Pr1—O22.2792Mo1—O1ii2.0440 (8)
Pr1—O1i2.5795 (13)
Symmetry codes: (i) z1/4, x1/4, y; (ii) x, y+1/2, z+1/2.
Acknowledgements top

Intensity data were collected at the Centre de diffractométrie de l'Université de Rennes I (