Na₃Ca₂TaO₆, a rock-salt superstructure phase with a fully ordered cation arrangement

The title quaternary oxide, trisodium dicalcium tantalum hexaoxide, is isostructural with Li₃Ni₂TaO₆, a partially ordered rock-salt phase. The Na, Ca and Ta atoms occupy octahedral sites in an orderly manner and form a cation-ordered superstructure.

S1. Comment

Crystal structures of a series of quaternary compounds containing lithium, i.e. Li₃M₂XO₆ (M = Mg, Co, Ni; X = Nb, Ta), were analyzed by X-ray and/or neutron powder diffraction (Fletcher et al., 1994; Mather et al., 1995). These compounds have a rock-salt superstructure with partial cation order. There are four fully occupied octahedral cation sites in the structure; three of these are occupied statistically with Li and M atoms with different occupancies. Na₃Ca₂TaO₆ has the same crystal structure and is the first compound with a fully ordered cation arrangement (Table 1).

Bond-valence sums were calculated with the bond distances listed in Table 2, using the bond-valence parameters presented by Brese & O'Keeffe (1991). The values of 1.00 for the Na1 site, 1.03 for the Na2 site and 2.18 for the Ca site support the full occupation of Na and Ca atoms at each octahedral site in Na₃Ca₂TaO₆. The bond-valence sums calculated with a Ni^II—O parameter for the three partially ordered octahedral sites in Li₃Ni₂TaO₆, for example, were in the range 1.6–1.8. These values suggest partial substitution of Li^II for Ni^II.

As shown in Fig. 1, the ordering sequence of cations along the c axis is –Ca–Na1–Na2–Na1–Ca–Ta–. The CaO₆ octahedron elongates along the c axis (Fig. 2). The lengths of six the Ca—O bonds in the octahedron are in a narrow range [2.3142 (10)–2.353 (3) Å]. The oxygen octahedron of the Na2 site, adjacent to the Ta site along the a axis, is most distorted. The Na2—O distance along the b axis is 2.735 (4) Å, while the others are 2.366 (3) Å. The displacement ellipsoid of the Na2 atom is also elongated along the b axis. On the other -hand, the Ta—O bond distances in the TaO₆ octahedron are almost the same [2.012 (4)–2.027 (3) Å] and the O—Ta—O angles are close to the ideal value for a regular octahedron. The Ta—O bond distances are consistent with those observed in Li₃Ni₂TaO₆ [1.989 (2)–2.015 (2) Å; Mather et al., 1995].

The polycrystalline sintered sample of Na₃Ca₂TaO₆ was an insulator at room temperature and conductive at high temperature. Evidence of electrode-polarization effects was seen in impedance plots above 700 K. The conductivity was 1.65 × 10⁻⁶ S cm⁻¹ at 573 K and 7.54 × 10⁻⁵ Scm⁻¹ at 670 K. The activation energy, E_a, was 130 kJ mo⁻¹l in the temperature region from 570 to 670 K. Above 700 K, E_a changed to about 40 kJ mol⁻¹. The conductivity was 8.13 × 10⁻⁴ S cm⁻¹ at 773 K and 1.53 × 10⁻³ S cm⁻¹ at 873 K. The two-probe dc (?) measurement with the Au^I ion blocking electrodes at 673 K, also showed a polarization effect. These data suggested ionic conduction of Na₃Ca₂TaO₆. Ionic conduction was not detected on Li₃Ni₂TaO₆ with the partial cation ordering (Fletcher et al., 1994).

S2. Experimental

Na₂CO₃ (extra pure grade, Wako Pure Chemical Industries Ltd.), CaCO₃ (99.99% purity, Rare Metallic Co. Ltd), Ta₂O₅ (99.99% purity, Rare Metallic Co. Ltd) were weighted to give a 6:3:1 atomic ratio of Na:Ca:Ta. The powders were mixed in an agate mortar and then pressed into a pellet. The pellet was heated at 1273 K for 1 h in air on a Pt plate. After heating, the sample was cooled to 1073 K at a rate of 3 K h⁻¹. Below this temperature, the sample was cooled in a furnace by shutting off the electric power. Grain growth was observed in the obtained pellet sample. A colorless transparent granule of Na₃Ca₂TaO₆ with a size of about 100 µm was picked from the crushed sample and used for the single-crystal X-ray diffraction analysis. Semi-quantitative energy dispersive X-ray analysis (EDAX Kevex) was carried out for the granule on a scanning electron microscope (Hitachi X-60). The Na:Ca:Ta atomic ratio obtained was 3.0:1.9:0.7, which was close to the ideal ratio of 3:2:1. EDAX analysis did not detect any other impurity elements. For conductivity measurements, a sintered polycrystalline pellet of Na₃Ca₂TaO₆ was prepared at 1137 K for 12 h from the mixture of starting materials with the stoichiometric composition. The electric impedance at ambient temperature was sensitive to moisture in air. Thus, the measurements were made under an argon atmosphere with Au electrodes, using an impedance analyzer (HP 4194 A).

Computing details

Data collection: SMART and SAINT (Bruker, 1997); cell refinement: SMART and SAINT; data reduction: EXPREP (Bruker, 1997); program(s) used to solve structure: SIR92 (Altomare et al., 1994); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ATOMS (Dowty, 1999).

Figures

	Fig. 1. The structure of Na3Ca2TaO6 showing the oxygen octahedra and 99% probability displacement ellipsoids.
	Fig. 2. The structure of the octahedral coordinations in Na3Ca2TaO6.

trisodium dicalcium tantalum hexoxide

Crystal data

Na3Ca2TaO6	F(000) = 1552
Mr = 426.08	Dx = 4.604 Mg m^−3
Orthorhombic, Fddd	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -F 2uv 2vw	Cell parameters from 1501 reflections
a = 6.5948 (17) Å	µ = 19.73 mm^−1
b = 9.493 (3) Å	T = 298 K
c = 19.640 (5) Å	Granule, colourless
V = 1229.5 (6) Å^3	0.10 × 0.09 × 0.05 mm
Z = 8

Data collection

CCD area-detector diffractometer	361 independent reflections
Radiation source: fine-focus sealed tube	305 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.024
ϕ and ω scans	θmax = 27.5°, θmin = 3.9°
Absorption correction: multi-scan (SADABS; Sheldrick, 1996)	h = −8→8
Tmin = 0.197, Tmax = 0.295	k = −12→6
1891 measured reflections	l = −24→25

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.015	w = 1/[σ^2(Fo^2) + (0.0195P)^2] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.035	(Δ/σ)max < 0.001
S = 1.13	Δρmax = 0.93 e Å^−3
361 reflections	Δρmin = −1.51 e Å^−3
32 parameters	Extinction correction: SHELXL97, Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
0 restraints	Extinction coefficient: 2E-5 (3)

Special details

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 (Ų)

	x	y	z	Uiso*/Ueq
Na1	0.1250	0.1250	0.46177 (10)	0.0110 (4)
Na2	0.1250	0.1250	0.6250	0.0138 (6)
Ca	0.1250	0.1250	0.29439 (5)	0.0088 (2)
Ta	0.1250	0.1250	0.1250	0.00649 (12)
O1	0.3407 (4)	0.1287 (3)	0.05153 (14)	0.0094 (5)
O2	0.1250	0.3369 (4)	0.1250	0.0102 (7)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Na1	0.0123 (11)	0.0091 (10)	0.0117 (10)	−0.003 (3)	0.000	0.000
Na2	0.0108 (14)	0.0202 (17)	0.0106 (14)	0.000	0.000	0.000
Ca	0.0105 (5)	0.0070 (5)	0.0088 (5)	0.0028 (13)	0.000	0.000
Ta	0.00736 (17)	0.00529 (17)	0.00683 (17)	0.000	0.000	0.000
O1	0.0100 (12)	0.0079 (14)	0.0105 (12)	0.0007 (13)	0.0005 (10)	−0.0009 (11)
O2	0.0112 (17)	0.0086 (17)	0.0108 (17)	0.000	0.0003 (16)	0.000

Geometric parameters (Å, º)

Na1—O2^i	2.3986 (16)	Ca—O2^ii	2.3142 (10)
Na1—O2^ii	2.3986 (16)	Ca—O2^i	2.3142 (10)
Na1—O1^iii	2.433 (3)	Ca—O1^xi	2.353 (3)
Na1—O1^iv	2.433 (3)	Ca—O1^xii	2.353 (3)
Na1—O1^v	2.573 (3)	Ca—O1^xiii	2.361 (3)
Na1—O1^vi	2.573 (3)	Ca—O1^xiv	2.361 (3)
Na2—O1^vii	2.366 (3)	Ta—O2^xv	2.012 (4)
Na2—O1^vi	2.366 (3)	Ta—O2	2.012 (4)
Na2—O1^viii	2.366 (3)	Ta—O1^xv	2.027 (3)
Na2—O1^v	2.366 (3)	Ta—O1^xiv	2.027 (3)
Na2—O2^ix	2.735 (4)	Ta—O1^xiii	2.027 (3)
Na2—O2^x	2.735 (4)	Ta—O1	2.027 (3)
O2^i—Na1—O2^ii	89.45 (8)	O2^ii—Ca—O2^i	93.66 (5)
O2^i—Na1—O1^iii	97.86 (12)	O2^ii—Ca—O1^xi	100.53 (12)
O2^ii—Na1—O1^iii	73.18 (13)	O2^i—Ca—O1^xi	74.67 (13)
O1^iii—Na1—O1^iv	167.67 (16)	O1^xi—Ca—O1^xii	173.16 (14)
O2^i—Na1—O1^v	171.67 (11)	O2^ii—Ca—O1^xiii	167.83 (10)
O2^ii—Na1—O1^v	89.08 (6)	O2^i—Ca—O1^xiii	96.56 (7)
O1^iii—Na1—O1^v	89.57 (10)	O1^xi—Ca—O1^xiii	88.56 (10)
O1^iv—Na1—O1^v	98.90 (9)	O1^xii—Ca—O1^xiii	96.90 (11)
O1^v—Na1—O1^vi	93.53 (14)	O1^xiii—Ca—O1^xiv	74.14 (14)
O1^vii—Na2—O1^vi	178.30 (15)	O2^xv—Ta—O2	180.0
O1^vii—Na2—O1^viii	104.84 (14)	O2^xv—Ta—O1^xv	89.01 (9)
O1^vi—Na2—O1^viii	75.19 (14)	O2—Ta—O1^xv	90.99 (9)
O1^vii—Na2—O2^ix	89.15 (7)	O1^xv—Ta—O1^xiv	178.01 (17)
O1^viii—Na2—O2^ix	90.85 (7)	O1^xv—Ta—O1^xiii	90.82 (15)
O2^ix—Na2—O2^x	180.0	O1^xiv—Ta—O1^xiii	89.21 (15)

Symmetry codes: (i) −x, −y+1/2, −z+1/2; (ii) x+1/4, y−1/4, −z+1/2; (iii) −x+1/2, −y, −z+1/2; (iv) x−1/4, y+1/4, −z+1/2; (v) −x+3/4, −y+1/4, z+1/2; (vi) x−1/2, y, z+1/2; (vii) −x+3/4, y, −z+3/4; (viii) x−1/2, −y+1/4, −z+3/4; (ix) −x+1/4, −y+3/4, z+1/2; (x) x, y−1/2, z+1/2; (xi) x−1/4, −y+1/2, z+1/4; (xii) −x+1/2, y−1/4, z+1/4; (xiii) −x+1/4, y, −z+1/4; (xiv) x, −y+1/4, −z+1/4; (xv) −x+1/4, −y+1/4, z.