Sodium calcium orthovanadate, NaCa₄(VO₄)₃

Single crystals of sodium tetra­calcium trivanadium dodeca­oxide were prepared by melting a powder sample of NaCa₄(VO₄)₃ at 1673 K, followed by slow cooling to room temperature. The compound crystallizes in the Pnma space group and is isostructural with the mineral silicocarnotite, Ca₅(PO₄)₂SiO₄. The structure is composed of isolated VO₄ tetra­hedra linked by sodium and calcium cations disordered over eight- and seven-coordinated sites.

S1. Comment

Many different structures have been observed for orthovanadates with the AB₄(VO₄)₃ stoichiometry (A = Li, Na, K and Rb, and B = Ca, Mg and Cd). In the ACd₄(VO₄)₃ series, three different structural types are observed. The structure of LiCd₄(VO₄)₃ (Gaudin et al., 2004) is a modulated variant of the Na₂CrO₄ type (Nimmo, 1981), NaCd₄(VO₄)₃ crystallizes with the maricite structure (Abrahams et al., 1983), and KCd₄(VO₄)₃ (Eddahby et al., 1997) and RbCd₄(VO₄)₃ (Mueller-Buschbaum & Mertens, 1997) adopt the scheelite structure. With increasing size of the A cation, its coordination number increases from 4 to 6 with associated structural changes. Moreover, disorder is often observed between the A and B cations. In the incommensurate structure of LiCd₄(VO₄)₃, or (Li_1/3Cd_1/3□_1/3)CdVO₄ (□ = vacancy), the Li/Cd disorder induces strong steric strains in the tetrahedral sites and a strong modulation of their occupancies (Gaudin et al., 2004, 2005). In NaCd₄(VO₄)₃, or (Na_1/3Cd_1/3□_1/3)CdVO₄, an Na/Ca/vacancy disorder is observed in strongly distorted tetrahedral sites derived from an octahedral site (Le Page & Donnay, 1977). In KCd₄(VO₄)₃ (Eddahby et al., 1997) and RbCd₄(VO₄)₃ (Mueller-Buschbaum & Mertens, 1997), all sites are fully occupied but a small Cd/alkali mixing is observed in some sites. In the AMg₄(VO₄)₃ series, only LiMg₄(VO₄)₃ (Tyutyunnik et al., 2004) and NaMg₄(VO₄)₃ (Murashova et al., 1988) have been observed, and both crystallize with the same structure, different from that of the Cd analogues. No A/Mg disorder is observed in these structures; the Mg atoms occupy octahedral sites and the alkali cations occupy sites with eightfold coordination. For the ACa₄(VO₄)₃ series, only NaCa₄(VO₄)₃ has been reported (Krasnenko et al., 1987) but without detailed structural data. Its structure has now been determined and is described here. NaCa₄(VO₄)₃ is isostructural with the mineral siliocarnotite, Ca₅(PO₄)₂SiO₄ (Dickens et al., 1971), and is the first example of an orthovanadate adopting this structure type.

The NaCa₄(VO₄)₃ structure (Fig. 1) is very similar to those of Ca₃Y₂(SiO₄)₃ (Yamane et al., 1997) and NaCd₄(PO₄)₃ (Ben Amara et al., 1982), with a silicocarnotite-type structure. The structure can be described as a three-dimensional network of isolated VO₄ tetrahedra linked by calcium and sodium cations. The Ca and Na cations are randomly distributed over three distinct crystallographic positions, Ca1/Na1, Ca2/Na2 and Ca3/Na3, coordinated to seven, eight and seven O atoms, respectively.

The VO₄ tetrahedra are almost regular with mean V1—O and V2—O distances of 1.707 and 1.704 Å, respectively, in good agreement with the Shannon (1976) radii. The bond-valence sums, equal to 5.18 and 5.23 for atoms V1 and V2, respectively (Brown, 1996), are close to the +5 oxidation state expected for vanadium. The Ca1/Na1 and Ca3/Na3 sites are occupied by 77 and 80% Ca, and 23 and 20% Na, respectively. The (Na/Ca)—O distances range from 2.2724 (14) to 2.6834 (19) Å and are consistent with the Shannon radii [r(Ca²⁺) = 1.06 Å and r(Na⁺) = 1.12 Å for a coordination number of 7]. The bond-valence sums are 1.97, 1.84, 1.26 and 1.18 for Ca1, Ca3, Na1 and Na3, respectively. By comparison with the expected values of +2 and +1, these bond-valence sums show that sites 1 and 3 are more favorable for Ca²⁺ than Na⁺ ions, since the Na⁺ ions are overbonded. In contrast, one would expect site 2 to be more favorable for Na⁺ ions, as suggested by the bond-valence sums of 1.68 for Ca2 and 1.08 for Na2. The Ca2/Na2—O distances range from 2.4365 (18) to 2.780 (3) Å and are longer than for the two other Ca/Na sites. Surprisingly, the Na2/Ca2 ratio of 0.14 is smaller than those for the two other sites, viz. 0.31 for Na1/Ca1 and 0.26 for Na3/Ca3. In the case of the isotypic compound NaCd₄(PO₄)₃ (Ben Amara et al., 1982), no Cd/Na disorder has been observed. The Na atoms occupy the crystallographic site corresponding to the Ca2/Na2 site of the title compound where the coordination number is the highest. The disorder between calcium and sodium over the three sites can be attributed to their close ionic radii and, moreover, calcium unlike cadmium is often observed with a high coordination number.

S2. Experimental

NaCa₄(VO₄)₃ was obtained by the solid-state reaction of a stoichiometric mixture of Na₂CO₃, CaCO₃ and V₂O₅. The reagents were ground in an agate mortar and placed into a platinum crucible. The temperature was raised slowly to 873 K in an oxygen flow and maintained for 12 h to decompose the carbonates. A second heat treatment at 1123 K for 48 h with intermediate grindings was needed to ensure a total reaction. The purity of the powder product was confirmed by powder X-ray diffraction. Single crystals of NaCa₄(VO₄)₃ were obtained by heating 1 g of the the corresponding powder at 1673 K in a platinum crucible in air for 1 h, followed by slow cooling at 10 k h⁻¹ to 1473 K and at 20 K h-1 to ambient temperature. A product consisting of the major phase NaCa₄(VO₄)₃ and two impurities, NaCaVO₄ and Ca₃(VO₄)₂, was recovered. Electron microprobe quantitative analyses revealed the presence of these impurities and confirmed the stoichiometry of the NaCa₄(VO₄)₃ crystals. No impurities were observed in the powder of NaCa₄(VO₄)₃ before crystallization.

S3. Refinement

An initial model with sites 1 and 3 fully occupied by Ca atoms and site 2 by Na atoms gave a reliability index R of 6.2% (wR = 14.9%) and a non-positive definite value for the Na2 atomic displacement parameter. The disordering of Na and Ca over the three cation positions led to the best result. In a first step, only constraints of fully occupied sites were used, and the chemical formula calculated from the refined Na and Ca site occupancies was found to be equal to the expected NaCa₄(VO₄)₃ composition. Then, in the final step, the total Ca and Na contents were fixed to match this ideal stoichiometry.

Computing details

Data collection: Collect (Nonius, 2004); cell refinement: EVALCCD (Duisenberg et al., 2003); data reduction: Jana2000 (Petricek & Dusek, 2000); program(s) used to solve structure: SIR97 (Altomare et al., 1999); program(s) used to refine structure: Jana2000; molecular graphics: Diamond (Brandenburg, 1999); software used to prepare material for publication: Jana2000.

Figures

Fig. 1. : A view of the structure of NaCa4(VO4)3 along the a axis. The VO4 tetrahedra are drawn in grey and white for V1 and V2, respectively. Displacement ellipsoids are shown at the 90% probability level.

sodium tetracalcium trivanadiun dodecaoxide

Crystal data

NaCa4(VO4)3	F(000) = 1024
Mr = 528.1	Dx = 3.120 (1) Mg m^−3
Orthorhombic, Pnma	Mo Kα radiation, λ = 0.71069 Å
Hall symbol: -P 2ac 2n	Cell parameters from 236 reflections
a = 6.7703 (10) Å	θ = 5–35°
b = 16.0954 (11) Å	µ = 4.37 mm^−1
c = 10.3136 (5) Å	T = 295 K
V = 1123.88 (19) Å^3	Block, colourless
Z = 4	0.09 × 0.07 × 0.07 mm

Data collection

Nonius KappaCCD diffractometer	2433 independent reflections
Radiation source: fine-focus sealed X-ray tube	1927 reflections with I > 2σ(I)
Horizonally mounted graphite crystal monochromator	Rint = 0.088
Detector resolution: 9 pixels mm^-1	θmax = 35.0°, θmin = 11.5°
ϕ and ω frames scans	h = −10→10
Absorption correction: gaussian (Jana2000; Petricek & Dusek, 2000)	k = −25→25
Tmin = 0.643, Tmax = 0.795	l = −16→16
25346 measured reflections

Refinement

Refinement on F^2	99 parameters
R[F^2 > 2σ(F^2)] = 0.035	Weighting scheme based on measured s.u.'s w = 1/[σ^2(I) + 0.0016I^2]
wR(F^2) = 0.083	(Δ/σ)max = 0.001
S = 1.16	Δρmax = 0.89 e Å^−3
2433 reflections	Δρmin = −0.57 e Å^−3

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

	x	y	z	Uiso*/Ueq	Occ. (<1)
Ca1	0.37639 (7)	0.09230 (3)	0.06696 (4)	0.01321 (15)	0.765 (7)
Na1	0.37639 (7)	0.09230 (3)	0.06696 (4)	0.01321 (15)	0.235 (7)
Ca2	0.04490 (11)	0.25	0.17790 (8)	0.0215 (3)	0.878 (12)
Na2	0.04490 (11)	0.25	0.17790 (8)	0.0215 (3)	0.122 (12)
Ca3	0.31152 (8)	0.10699 (3)	0.67177 (4)	0.01493 (15)	0.796 (9)
Na3	0.31152 (8)	0.10699 (3)	0.67177 (4)	0.01493 (15)	0.204 (9)
V1	0.35932 (5)	0.07258 (2)	0.37321 (3)	0.00824 (8)
V2	0.02550 (8)	0.25	0.57421 (4)	0.01112 (12)
O1	0.2740 (4)	0.25	0.5762 (3)	0.0254 (7)
O2	0.1881 (2)	0.04082 (9)	0.48481 (13)	0.0117 (3)
O3	0.2689 (3)	0.15011 (10)	0.27731 (15)	0.0161 (4)
O4	0.5435 (2)	0.11710 (11)	0.46136 (16)	0.0175 (4)
O5	0.4327 (3)	−0.00606 (11)	0.27284 (16)	0.0196 (4)
O6	−0.0449 (3)	0.15894 (10)	0.64634 (15)	0.0194 (4)
O7	−0.0736 (4)	0.25	0.4219 (2)	0.0206 (6)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Ca1	0.0120 (3)	0.0123 (3)	0.0154 (3)	0.00096 (15)	0.00299 (15)	−0.00030 (16)
Na1	0.0120 (3)	0.0123 (3)	0.0154 (3)	0.00096 (15)	0.00299 (15)	−0.00030 (16)
Ca2	0.0170 (4)	0.0091 (4)	0.0384 (5)	0	−0.0023 (3)	0
Na2	0.0170 (4)	0.0091 (4)	0.0384 (5)	0	−0.0023 (3)	0
Ca3	0.0180 (3)	0.0148 (3)	0.0120 (2)	0.00291 (17)	−0.00509 (16)	−0.00288 (16)
Na3	0.0180 (3)	0.0148 (3)	0.0120 (2)	0.00291 (17)	−0.00509 (16)	−0.00288 (16)
V1	0.00819 (14)	0.00900 (14)	0.00753 (13)	0.00096 (11)	0.00128 (10)	0.00059 (10)
V2	0.0151 (2)	0.00757 (19)	0.0107 (2)	0	−0.00102 (16)	0
O1	0.0170 (12)	0.0165 (10)	0.0426 (15)	0	−0.0024 (10)	0
O2	0.0100 (6)	0.0147 (6)	0.0104 (6)	−0.0017 (5)	0.0018 (5)	0.0011 (5)
O3	0.0210 (8)	0.0134 (6)	0.0138 (6)	0.0048 (6)	−0.0024 (6)	0.0019 (5)
O4	0.0104 (7)	0.0224 (8)	0.0196 (7)	−0.0002 (6)	−0.0021 (5)	−0.0033 (6)
O5	0.0241 (8)	0.0190 (7)	0.0157 (7)	0.0066 (7)	0.0043 (6)	−0.0035 (6)
O6	0.0312 (10)	0.0117 (7)	0.0153 (6)	−0.0007 (6)	0.0061 (6)	0.0037 (5)
O7	0.0329 (14)	0.0155 (10)	0.0133 (9)	0	−0.0041 (9)	0

Geometric parameters (Å, º)

Ca1/Na1—O2^i	2.3292 (14)	Ca3/Na3—O1	2.5168 (13)
Ca1/Na1—O2^ii	2.3451 (15)	Ca3/Na3—O2	2.3561 (13)
Ca1/Na1—O3	2.4701 (13)	Ca3/Na3—O4	2.6834 (19)
Ca1/Na1—O4^iii	2.3075 (15)	Ca3/Na3—O5^vi	2.4418 (19)
Ca1/Na1—O5	2.676 (2)	Ca3/Na3—O5^vii	2.5410 (19)
Ca1/Na1—O6^i	2.5041 (13)	Ca3/Na3—O6	2.567 (2)
Ca1/Na1—O7^i	2.5633 (6)	Ca3/Na3—O6^viii	2.2724 (14)
Ca2/Na2—O3	2.4365 (18)	V1—O2	1.7116 (13)
Ca2/Na2—O3^iii	2.5081 (19)	V1—O3	1.7059 (15)
Ca2/Na2—O3^iv	2.5081 (19)	V1—O4	1.7017 (17)
Ca2/Na2—O3^v	2.4365 (18)	V1—O5	1.7097 (19)
Ca2/Na2—O4^iii	2.5768 (18)	V2—O1	1.683 (3)
Ca2/Na2—O4^iv	2.5768 (18)	V2—O6	1.7117 (16)
Ca2/Na2—O7	2.641 (2)	V2—O6^v	1.7117 (16)
Ca2/Na2—O7^i	2.780 (3)	V2—O7	1.708 (2)
O2^i—Ca1—O2^ii	76.16 (5)	O3^v—Ca2—O4^iv	71.88 (5)
O2^i—Ca1—O3	127.03 (6)	O3^v—Ca2—O7	77.77 (6)
O2^i—Ca1—O4^iii	156.56 (6)	O3^v—Ca2—O7^i	65.01 (5)
O2^i—Ca1—O5	80.92 (5)	O4^iii—Ca2—O4^iv	112.23 (7)
O2^i—Ca1—O6^i	76.01 (5)	O4^iii—Ca2—O7	122.01 (5)
O2^i—Ca1—O7^i	104.06 (7)	O4^iii—Ca2—O7^i	78.29 (4)
O2^ii—Ca1—O2^i	76.16 (5)	O4^iv—Ca2—O4^iii	112.23 (7)
O2^ii—Ca1—O3	127.34 (5)	O4^iv—Ca2—O7	122.01 (5)
O2^ii—Ca1—O4^iii	86.01 (6)	O4^iv—Ca2—O7^i	78.29 (4)
O2^ii—Ca1—O5	76.85 (5)	O7—Ca2—O7^i	129.41 (8)
O2^ii—Ca1—O6^i	96.51 (5)	O7^i—Ca2—O7	129.41 (8)
O2^ii—Ca1—O7^i	161.00 (6)	O1—Ca3—O2	93.27 (7)
O3—Ca1—O4^iii	76.02 (7)	O1—Ca3—O4	71.76 (7)
O3—Ca1—O5	64.41 (5)	O1—Ca3—O5^vi	140.49 (8)
O3—Ca1—O6^i	132.29 (5)	O1—Ca3—O5^vii	132.80 (8)
O3—Ca1—O7^i	68.07 (6)	O1—Ca3—O6	64.37 (7)
O4^iii—Ca1—O5	110.01 (7)	O1—Ca3—O6^viii	91.74 (8)
O4^iii—Ca1—O6^i	91.29 (7)	O2—Ca3—O4	64.74 (5)
O4^iii—Ca1—O7^i	87.90 (7)	O2—Ca3—O5^vi	98.19 (6)
O5—Ca1—O6^i	156.91 (6)	O2—Ca3—O5^vii	79.39 (6)
O5—Ca1—O7^i	122.12 (6)	O2—Ca3—O6	74.34 (5)
O6^i—Ca1—O7^i	65.63 (6)	O2—Ca3—O6^viii	173.59 (6)
O3—Ca2—O3^iii	87.89 (6)	O4—Ca3—O5^vi	79.31 (6)
O3—Ca2—O3^iv	144.03 (5)	O4—Ca3—O5^vii	138.69 (6)
O3—Ca2—O3^v	82.59 (6)	O4—Ca3—O6	116.60 (5)
O3—Ca2—O4^iii	71.88 (5)	O4—Ca3—O6^viii	113.26 (6)
O3—Ca2—O4^iv	141.70 (6)	O5^vi—Ca3—O5^vii	86.59 (6)
O3—Ca2—O7	77.77 (6)	O5^vi—Ca3—O6	155.10 (6)
O3—Ca2—O7^i	65.01 (5)	O5^vi—Ca3—O6^viii	75.39 (7)
O3^iii—Ca2—O3	87.89 (6)	O5^vii—Ca3—O5^vi	86.59 (6)
O3^iii—Ca2—O3^iv	79.75 (6)	O5^vii—Ca3—O6	68.79 (6)
O3^iii—Ca2—O3^v	144.03 (5)	O5^vii—Ca3—O6^viii	100.06 (6)
O3^iii—Ca2—O4^iii	64.39 (5)	O6—Ca3—O6^viii	111.53 (6)
O3^iii—Ca2—O4^iv	129.21 (6)	O6^viii—Ca3—O6	111.53 (6)
O3^iii—Ca2—O7	66.31 (5)	O2—V1—O3	111.43 (8)
O3^iii—Ca2—O7^i	139.49 (4)	O2—V1—O4	105.23 (8)
O3^iv—Ca2—O3	144.03 (5)	O2—V1—O5	112.51 (8)
O3^iv—Ca2—O3^iii	79.75 (6)	O3—V1—O4	105.36 (8)
O3^iv—Ca2—O3^v	87.89 (6)	O3—V1—O5	107.15 (8)
O3^iv—Ca2—O4^iii	129.21 (6)	O4—V1—O5	115.01 (9)
O3^iv—Ca2—O4^iv	64.39 (5)	O1—V2—O6	105.85 (8)
O3^iv—Ca2—O7	66.31 (5)	O1—V2—O6^v	105.85 (8)
O3^iv—Ca2—O7^i	139.49 (4)	O1—V2—O7	113.83 (14)
O3^v—Ca2—O3	82.59 (6)	O6—V2—O6^v	117.88 (7)
O3^v—Ca2—O3^iii	144.03 (5)	O6—V2—O7	106.86 (6)
O3^v—Ca2—O3^iv	87.89 (6)	O6^v—V2—O6	117.88 (7)
O3^v—Ca2—O4^iii	141.70 (6)	O6^v—V2—O7	106.86 (6)

Symmetry codes: (i) x+1/2, y, −z+1/2; (ii) −x+1/2, −y, z−1/2; (iii) x−1/2, y, −z+1/2; (iv) x−1/2, −y+1/2, −z+1/2; (v) x, −y+1/2, z; (vi) −x+1, −y, −z+1; (vii) −x+1/2, −y, z+1/2; (viii) x+1/2, y, −z+3/2.