Sodium scandium arsenate, Na₃Sc₂(AsO₄)₃

Nasicon-type trisodium discandium tris­(arsenate), Na₃Sc₂(AsO₄)₃, contains a polyhedral network of vertex-sharing octahedral ScO₆ and tetrahedral AsO₄ units [d_av(Sc—O) = 2.089 (2) Å and d_av(As—O) = 1.672 (2) Å] encapsulating two types of Na⁺ species. The sodium site occupancies are similar to those of the equivalent species in β-Na₃Sc₂(PO₄)₃.

S1. Comment

Based on the present refinement, Na₃Sc₂(AsO₄)₃ adopts a centrosymmetric rhombohedral Nasicon structure (Sljukic et al., 1969; Mesquelier et al., 2000), as indicated earlier on the basis of X-ray powder data (Winand et al., 1990). However, a superstructure or other symmetry reduction, perhaps associated with sodium cation ordering, cannot be completely ruled out.

The stretched octahedral Na1 position (Wyckoff site 6 b) with site symmetry 3 is fully occupied within experimental error. Its bond valence sum (BVS), calculated by the Brown (1996) formalism, of 0.98 is in good agreement with the expected value of 1.00. Na2 occupies Wyckoff site 18 e (symmetry 2) with an occupancy of 0.626 (11). This site is usually described as eight-coordinate (Mesquelier et al., 2000) to framework O atoms. In the title compound, two of these links are exceptionally long (d > 3.16 Å) and the Na2 coordination could equally well be described as sixfold. Bond valence sums for Na2 indicate significant underbonding if either six-coordinate (BVS = 0.76) and eight-coordinate (BVS = 0.81) is assumed.

The overall sodium unit-cell content of 17.27 (12) based on the refinement is in adequate agreement with the value of 18 required for charge-balancing purposes. Other Nasion phases show a similar apparent reduced cation content which may be rationalized on the basis of the mobility of these species in the extra-framework channel system (de la Rochère et al., 1983). The sodium content of 18 can be formally achieved by full occupancy of the Na1 (6 b) site and 2/3 occupancy of the Na2 (18 e) site (Susman et al., 1983).

The site occupancies of the sodium cations in Na₃Sc₂(AsO₄)₃ are similar to those of the equivalent species in β-Na₃Sc₂(PO₄)₃ (Collin et al., 1986), where the 6 b and 18 e Wyckoff sites are 0.92 (2) and 0.693 occupied, respectively (restrained total unit-cell sodium content = 17.99). At 473 K, Na₃Fe₂(PO₄)₃ shows a similar occupation pattern for the sodium cations, although this effect is strongly temperature dependent (de la Rochère et al., 1983; Masquelier et al., 2000).

The Sc1 atom adopts its usual octahedral coordination, with d_av(Sc1—O) = 2.089 (2) Å, and As1 is tetrahedral [d_av(As1—O) = 1.672 (2) Å]. The polyhedral linkage of the ScO₆ and AsO₄ moieties [θ_av(Sc—O—As) = 145.3°] to form the Nasicon framework has been described in detail elsewhere (Subramanian et al., 1985; Masquelier et al., 2000).

S2. Experimental

Initially, Na₂CO₃ (4.01 g, 37.8 mmol, Aldrich), Sc₂O₃ (0.55 g, 4 mmol, Varlacoid) and NH₄H₂AsO₄ (8.58 g, 54 mmol, prepared by oxidizing As₂O₃ with H₂O₂ and titrating with NH₄OH) were heated in a Pt crucible at 1193 K for 16 h. Further Na₂CO₃ (4.01 g) and NH₄H₂AsO₄ (8.58 g) were added and the mixture heated to 1193 K for another 24 h. The crucible was removed from the furnace to cool to ambient temperature, and was then returned to the 1193 K furnace for 4 h. The flux was decanted whilst hot and, upon cooling the crucible once more, the remaining solids were worked up in water. This resulted in a mixture of white powder and a few small single crystals of Na₃Sc₂(AsO₄)₃ (1.47 g; yield based on Sc = 64%).

S3. Refinement

During data reduction, a handful of weak (I < 1% of I_max) reflections violating the R-centering condition were apparent. These could be accounted for by assuming a primitive trigonal cell of the same dimensions as that reported in the results section. However, no convincing model could be established in a primitive cell. The atomic parameters showed high correlations and oscilations when refined, suggesting that the primitive model symmetry was too low. Rhombohedral-symmetry breaking and superstructure reflections have been seen in other Nasicons (d'Yvoire et al., 1983). These have tentatively been associated with an ordering of the extra-framework cations, although no detailed structural models to account for this behaviour have yet been developed. For the present refinement, the largest difference peak is 0.69 Å from Na1.

Computing details

Data collection: SMART (Bruker, 1997); cell refinement: SMART; data reduction: SMART; program(s) used to refine structure: CRYSTALS (Watkin et al., 1997); molecular graphics: ORTEP-3 (Farrugia, 1997) and ATOMS (Shape Software, 1999); software used to prepare material for publication: CRYSTALS.

Figures

	Fig. 1. Fragment of the Na3Sc2(AsO4)3 structure (50% displacement ellipsoids) showing the bonding environments of the Sc and As cations. [Symmetry codes: (v) 1 + x-y, 1 - y, 1/2 - z; (vi) 1/3 - x + y, 2/3 + y, z + 7/6–1; (ix) -x, -x + y, 1/2 - z; (x) y, 1 - x, 1/2 - z; (xi) 1/3 + x, 2/3 + x-y, z + 7/6–1; (xii) 1/3 - y, 2/3 - x, z + 7/6–1.]
	Fig. 2. Polyhedral view down [110] of the Na3Sc2(AsO4)3 structure (ScO6 octahedra are shown with light shading, AsO4 tetrahedra with dark shading, Na1 as small circles and Na2 as large circles).

(I)

Crystal data

Na3Sc2(AsO4)3	Dx = 3.41 Mg m^−3
Mr = 575.64	Melting point: not measured K
Trigonal, R3c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -R 3 2"c	Cell parameters from 3531 reflections
a = 9.2760 (3) Å	θ = 2.7–32.5°
c = 22.4640 (8) Å	µ = 10.22 mm^−1
V = 1673.9 Å^3	T = 298 K
Z = 6	Rod, colourless
F(000) = 1617.08	0.05 × 0.03 × 0.03 mm

Data collection

Bruker SMART 1000 diffractometer	599 reflections with I > σ(I)
ω scans	Rint = 0.054
Absorption correction: multi-scan (SADABS; Bruker, 1999)	θmax = 32.5°, θmin = 1.0°
Tmin = 0.536, Tmax = 0.695	h = −14→14
16306 measured reflections	k = −14→14
673 independent reflections	l = −33→33

Refinement

Refinement on F	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Chebychev polynomial with 3 parameters (Carruthers & Watkin, 1979) 0.225 0.116 0.141
R[F^2 > 2σ(F^2)] = 0.027	(Δ/σ)max = 0.0004
wR(F^2) = 0.025	Δρmax = 1.14 e Å^−3
S = 1.12	Δρmin = −0.78 e Å^−3
599 reflections	Extinction correction: Larson (1967)
37 parameters	Extinction coefficient: 7 (2)
Primary atom site location: structure-invariant direct methods

Special details

Experimental. Collection of duplicate frames at the end of the data collection indicated that negligible crystal decay had occurred.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq	Occ. (<1)
Na1	0.0000	0.0000	0.0000	0.0545	1.028 (18)
Na2	0.0373 (3)	0.3333	0.0833	0.0254	0.626 (11)
Sc1	0.3333	0.6667	0.31236 (3)	0.0075
As1	0.0000	0.29458 (3)	0.2500	0.0114
O1	−0.1703 (2)	0.3136 (2)	0.24968 (8)	0.0174
O2	0.0162 (3)	0.2021 (3)	0.18869 (13)	0.0413

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Na1	0.075 (2)	0.075 (2)	0.0141 (15)	0.0373 (12)	0.0000	0.0000
Na2	0.0189 (12)	0.0143 (13)	0.0415 (17)	0.0071 (6)	0.0053 (5)	0.011 (1)
Sc1	0.00744 (18)	0.00744 (18)	0.0078 (3)	0.00372 (9)	0.0000	0.0000
As1	0.00866 (15)	0.00788 (13)	0.01803 (16)	0.00433 (7)	−0.00517 (11)	−0.00259 (5)
O1	0.0142 (7)	0.0251 (9)	0.0192 (8)	0.0144 (7)	−0.0042 (6)	−0.0011 (7)
O2	0.0318 (12)	0.0362 (12)	0.0567 (16)	0.018 (1)	−0.0077 (11)	−0.0348 (12)

Geometric parameters (Å, º)

Na1—O1^i	2.4752 (18)	Na2—O2^x	3.164 (3)
Na1—O1^ii	2.4752 (18)	Na2—O2^xi	3.164 (3)
Na1—O1^iii	2.4752 (18)	Sc1—O1^xii	2.1303 (17)
Na1—O1^iv	2.4752 (18)	Sc1—O1^xiii	2.1303 (17)
Na1—O1^v	2.4752 (18)	Sc1—O1^xiv	2.1303 (17)
Na1—O1^vi	2.4752 (18)	Sc1—O2^xv	2.047 (2)
Na2—O1^vii	2.539 (3)	Sc1—O2^xvi	2.047 (2)
Na2—O1^vi	2.539 (3)	Sc1—O2^xvii	2.047 (2)
Na2—O1^ii	2.552 (2)	As1—O1	1.6755 (17)
Na2—O1^viii	2.552 (2)	As1—O1^xiii	1.6755 (17)
Na2—O2	2.624 (3)	As1—O2	1.668 (2)
Na2—O2^ix	2.624 (3)	As1—O2^xiii	1.668 (2)
O1^i—Na1—O1^ii	180.0	O1^ii—Na2—O2^xi	54.12 (7)
O1^i—Na1—O1^iii	68.62 (6)	O1^vi—Na2—O2^xi	113.11 (7)
O1^ii—Na1—O1^iii	111.38 (6)	O1^viii—Na2—O2^xi	110.10 (9)
O1^i—Na1—O1^iv	111.38 (6)	O1^vii—Na2—O2^xi	154.02 (7)
O1^ii—Na1—O1^iv	68.62 (6)	O2—Na2—O2^x	85.27 (6)
O1^iii—Na1—O1^iv	180.0	O2—Na2—O2^ix	161.91 (16)
O1^i—Na1—O1^v	68.62 (6)	O2^x—Na2—O2^ix	108.87 (9)
O1^ii—Na1—O1^v	111.38 (6)	O2—Na2—O2^xi	108.87 (9)
O1^iii—Na1—O1^v	68.62 (6)	O2^x—Na2—O2^xi	79.90 (13)
O1^iv—Na1—O1^v	111.38 (6)	O2^ix—Na2—O2^xi	85.27 (6)
O1^i—Na1—O1^vi	111.38 (6)	O1^xii—Sc1—O1^xiii	81.83 (7)
O1^ii—Na1—O1^vi	68.62 (6)	O1^xii—Sc1—O1^xiv	81.83 (7)
O1^iii—Na1—O1^vi	111.38 (6)	O1^xiii—Sc1—O1^xiv	81.83 (7)
O1^iv—Na1—O1^vi	68.62 (6)	O1^xii—Sc1—O2^xv	87.7 (1)
O1^v—Na1—O1^vi	180.0	O1^xiii—Sc1—O2^xv	167.24 (11)
O1^ii—Na2—O1^vi	66.48 (8)	O1^xiv—Sc1—O2^xv	89.5 (1)
O1^ii—Na2—O1^viii	161.81 (14)	O1^xii—Sc1—O2^xvi	89.5 (1)
O1^vi—Na2—O1^viii	131.71 (9)	O1^xiii—Sc1—O2^xvi	87.7 (1)
O1^ii—Na2—O1^vii	131.71 (9)	O1^xiv—Sc1—O2^xvi	167.24 (11)
O1^vi—Na2—O1^vii	65.2 (1)	O2^xv—Sc1—O2^xvi	99.55 (12)
O1^viii—Na2—O1^vii	66.48 (8)	O1^xii—Sc1—O2^xvii	167.24 (11)
O1^ii—Na2—O2	68.01 (7)	O1^xiii—Sc1—O2^xvii	89.5 (1)
O1^vi—Na2—O2	69.42 (8)	O1^xiv—Sc1—O2^xvii	87.7 (1)
O1^viii—Na2—O2	115.10 (6)	O2^xv—Sc1—O2^xvii	99.55 (12)
O1^vii—Na2—O2	94.97 (9)	O2^xvi—Sc1—O2^xvii	99.55 (12)
O1^ii—Na2—O2^x	110.10 (9)	O1—As1—O1^xiii	109.51 (13)
O1^vi—Na2—O2^x	154.02 (7)	O1—As1—O2	112.50 (11)
O1^viii—Na2—O2^x	54.12 (7)	O1^xiii—As1—O2	105.22 (12)
O1^vii—Na2—O2^x	113.11 (7)	O1—As1—O2^xiii	105.22 (12)
O1^ii—Na2—O2^ix	115.10 (6)	O1^xiii—As1—O2^xiii	112.50 (11)
O1^vi—Na2—O2^ix	94.97 (9)	O2—As1—O2^xiii	112.0 (2)
O1^viii—Na2—O2^ix	68.01 (7)	Sc1^xviii—O1—As1	139.4 (1)
O1^vii—Na2—O2^ix	69.42 (8)	Sc1^xix—O2—As1	151.2 (2)

Symmetry codes: (i) x+1/3, y−1/3, z−1/3; (ii) −x−1/3, −y+1/3, −z+1/3; (iii) −y+1/3, x−y+2/3, z−1/3; (iv) y−1/3, −x+y−2/3, −z+1/3; (v) −x+y−2/3, −x−1/3, z−1/3; (vi) x−y+2/3, x+1/3, −z+1/3; (vii) −y+2/3, −x+1/3, z−1/6; (viii) −x+y−1/3, y+1/3, z−1/6; (ix) x−y+1/3, −y+2/3, −z+1/6; (x) y−1/3, −x+y+1/3, −z+1/3; (xi) x−1/3, x−y+1/3, z−1/6; (xii) x−y+1, −y+1, −z+1/2; (xiii) −x, −x+y, −z+1/2; (xiv) y, x+1, −z+1/2; (xv) −x+y+1/3, y+2/3, z+1/6; (xvi) x+1/3, x−y+2/3, z+1/6; (xvii) −y+1/3, −x+2/3, z+1/6; (xviii) x−y, −y+1, −z+1/2; (xix) −x+y−1/3, y−2/3, z−1/6.