NaAl(MoO₄)₂: a rare structure type among layered yavapaiite-related AM(XO₄)₂ compounds

NaAl(MoO₄)₂, sodium aluminium dimolybdate(VI), represents a rare pseudo-orthorhombic structure type among AM(XO₄)₂ compounds with yavapaiite-related sheet structures. It is isostructural with NaFe(MoO₄)₂. Infinite (001) sheets composed of AlO₆ octahedra, corner-linked to MoO₄ tetrahedra, are separated by eight-coordinated Na atoms. Average Na-O, Al-O and Mo-O bond lengths are 2.648, 1.890, and 1.762 Å, respectively. All atoms are on general positions except for Na and Al (both of which have site symmetry ).

Key indicators

• Single-crystal X-ray study
• T = 293 K
• Mean [sigma](Al-O) = 0.002 Å
• R factor = 0.016
• wR factor = 0.043
• Data-to-parameter ratio = 17.0

checkCIF results

No syntax errors found

ADDSYM reports no extra symmetry

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S1. Comment

The crystal structures of both natural and synthetic kröhnkite [Na₂Cu(SO₄)₂·2H₂O]-type oxy-salts and related compounds are currently being investigated and classified (Fleck et al., 2003, Fleck, Kolitsch & Hertweck, 2002). In our recent research (Fleck & Kolitsch, 2002), we also provide a brief review of related yavapaiite [KFe(SO₄)₂]-type sheet structures of AM(XO₄)₂ compounds, including a discussion of the rod group symmetry of the underlying kröhnkite-chain building unit. 12 different space groups have been found for these sheet structures (Fleck & Kolitsch, 2002). The previously unknown structure of NaAl(MoO₄)₂, reported here, is isotypic to that of NaFe(MoO₄)₂, and represents a rare structure type among these 12 types. [Note that previously reported space group symmetries for α-NH₄Fe(CrO₄)₂ (P2₁; structure determination by Gravereau et al., 1977) and CsTa(PO₄)₂ (P1; structure determination by Nikolaev et al., 1983) are obviously incorrect and should be revised to P2₁/n and C2/m, respectively; for details, see Fleck & Kolitsch, 2002.]

NaAl(MoO₄)₂ and other related alkali metal(III) molybdates(VI) and tungstates(VI) have attracted interest due to a series of ferroelastic phase transitions observed (e.g. Dudnik et al., 1976; Otko et al., 1978, 1979, 1997; Zapart & Zapart, 1993; Maczka, Kojima & Hanuza, 1999; Maczka, Hanuza et al., 1999; Hermanowicz et al., 2000; Maczka et al., 2001). On cooling, the space-group symmetry is lowered from P3m1 to C2/c, C2/m or P1. The low-temperature modifications are ferroelastic and, on heating, transform into the paraelastic P3m1 high-temperature modification. In some crystals, transitions to simultaneously incommensurate and ferroelastic phases have been reported.

Klevtsov et al. (1975) reported that single crystals of the molybdates NaM(MoO₄)₂ (M = Al, Cr, Fe) are all monoclinic at room temperature, with space group C2/c or Cc. For NaAl(MoO₄)₂, the unit-cell parameters a = 9.59, b = 5.37, c = 13.14 Å and β = 90.1° were given (Klevtsov et al., 1975). Subsequently, fairly contradictory reports on the space-group symmetry of these three molybdates were published. The crystal structure of NaFe(MoO₄)₂ was determined by Klevtsova (1975) and refined in space group C2/c to R = 0.094 (note that these structures are not included in the current edition of the ICSD [please provide reference]). In contrast, Dudnik et al. (1986) suggested that NaFe(MoO₄)₂ has space group C2/m, a conclusion based on studies using polarized-light microscopy and X-ray diffraction. Sinyakov et al. (1978) assumed, on the basis of group-theoretical considerations, that the room-temperature ferroelastic modifications of both NaAl(MoO₄)₂ and NaFe(MoO₄)₂ are isostructural with the ferroelastic phase of KFe(MoO₄)₂, and therefore would have space group P2₁/c [the paraelastic high-temperature phase of KFe(MoO₄)₂ has space group P3c1; Klevtsova & Klevtsov, 1970]. However, a different space group was also reported for ferroelastic KFe(MoO₄)₂; Otko et al. (1984) suggested space group C2/m for this compound, and a pseudo-orthorhombic unit cell with cell parameters related to those of NaAl(MoO₄)₂. From detailed spectroscopic evidence, Maczka, Kojima & Hanuza (1999) and Maczka, Hanuza et al. (1999) concluded that NaAl(MoO₄)₂ has space group C2/c at room temperature.

The present structure determination of the title compound at room temperature confirms the pseudo-orthorhombic unit cell reported by Klevtsov et al. (1975). It also shows that the compound is centrosymmetric, with space group C2/c, and isotypic to NaFe(MoO₄)₂ (Klevtsova, 1975). The crystal structure of NaAl(MoO₄)₂ is formed by infinite sheets composed of AlO₆ octahedra corner-linked to slightly distorted MoO₄ tetrahedra (Figs. 1 and 2). These sheets are stacked parallel to (001) and are separated from each other by eight-coordinated interlayer Na atoms (Fig. 1). Average Na—O, Al—O and Mo—O bond lengths are 2.648, 1.890, and 1.762 Å, respectively.

Bond-valence sums for the Na, Al and Mo atoms, calculated using the parameters of Brese & O'Keeffe (1991), are 0.94, 3.15 and 5.94 v.u. (valence units). Although the longest Na—O bond, Na—O2 at 2 × 2.9957 (17) Å, seems very weak, it must be considered as a bond because it increases the bond-valence sum of the Na⁺ cation from 0.866 to 0.944 v.u.. The four O atoms O1–O4 have bond-valence sums of 2.04, 2.05, 2.09, and 1.80 v.u., respectively. Although atom O4 seems to be somewhat underbonded, the bond distance Mo—O4 is the shortest of all four Mo—O bonds; however, according to experience, proposed bond-valence parameters for Mo—O bonds are not flexible enough to describe distorted Mo^VIO_x polyhedra.

NaAl(MoO₄)₂ is neither isostructural with NaAl(SO₄)₂ [monoclinic, C2/m, Pannetier et al. (1972); ICDD-PDF 27–631; yavapaiite-type structure, but not refined yet], nor KAl(MoO₄)₂ (trigonal, P3m1; Klevtsova & Klevtsov, 1970), nor K(Cr_0.8Al_0.2)(MoO₄)₂ (monoclinic, C2/c; Sedello & Müller-Buschbaum, 1994), although the latter has the same space group. The atomic arrangement of NaAl(MoO₄)₂ represents one of 12 structure types found by Fleck & Kolitsch (2002) for sheet structures identical or closely related to that of yavapaiite, KFe(SO₄)₂ (Graeber & Rosenzweig, 1971). Evidently, the different size ratios of the cations and anions in AM(XO₄)₂ compounds result in both flexible stretching of the sheets and enlargements of the interlayer spacings to accommodate the interlayer cation, thereby producing a large number of different, although topologically similar crystal structure types. For a more detailed discussion of the 12 individual types, the reader is referred to Fleck & Kolitsch (2002).

High-temperature Raman studies on NaAl(MoO₄)₂ (Maczka, Kojima & Hanuza, 1999) have shown that it does not transform into the paraelastic P3m1 modification up to its melting point, although the monoclinic distortion from the parent trigonal structure is relatively small at high temperatures and increases continuously with decreasing temperature. This is in agreement with earlier data by Dudnik et al. (1976) who observed optically biaxial behavior up to 873 K, and Velikodnyi & Trunov (1977) who stated that NaAl(MoO₄)₂ at high temperatures is possibly orthorhombic but can be indexed as monoclinic. A determination of the unit-cell parameters of NaAl(MoO₄)₂ at 100 K during the present study indicated no symmetry change in the structure. As expected from the previous observations of Maczka, Kojima & Hanuza (1999), the monoclinic distortion has slightly increased upon cooling below room temperature: at 100 K the monoclinic angle has widened to a value of 90.3°.

S2. Experimental

Crystals of the title compound were grown by heating a mixture of Na₂CO₃, Al₂O₃ and MoO₃ placed in a platinum crucible to 993 K, keeping at this temperature for 20 h and cooling at a rate 2 K h⁻¹ to ambient temperature. The composition of the mixture corresponded to a 1:1 ratio between NaAl(MoO₄)₂ and the Na₂Mo₂O₇ solvent. The grown colorless translucent to transparent crystals of pseudo-hexagonal block shape were separated from the solidified solvent by washing in hot water. All crystals were found to be non-merohedrally twinned, albeit to a variable degree. For the structure refinement, a nearly untwinned crystal fragment was used.

S3. Refinement

The deepest hole in the difference map is at a distance of 0.87 Å from the Mo site.

Computing details

Data collection: COLLECT (Nonius, 2002); cell refinement: HKL SCALEPACK (Otwinowski & Minor, 1997); data reduction: HKL DENZO (Otwinowski & Minor, 1997) and SCALEPACK; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ATOMS (Shape Software, 1999) and ORTEP-3 for Windows (Farrugia, 1997); software used to prepare material for publication: SHELXL97 (Sheldrick, 1997).

Figures

	Fig. 1. The layered crystal structure of NaAl(MoO4)2 in views (a) along [010] and (b) along [001]. AlO6 octahedra are corner-connected to MoO4 tetrahedra to form sheets separated by eight-coordinated Na atoms (dark-blue spheres). The pseudo-orthorhombic unit cell is outlined.
	Fig. 2. View of the seven atoms in the asymmetric unit of NaAl(MoO4)2, shown with ellipsoids at the 70% probability level.

sodium aluminium dimolybdate(VI)

Crystal data

NaAl(MoO4)2	F(000) = 688
Mr = 369.85	Dx = 3.638 Mg m^−3
Monoclinic, C2/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -C 2yc	Cell parameters from 1065 reflections
a = 9.621 (2) Å	θ = 2.0–30.0°
b = 5.339 (1) Å	µ = 3.91 mm^−1
c = 13.146 (3) Å	T = 293 K
β = 90.01 (3)°	Irregular fragment, colorless
V = 675.3 (2) Å^3	0.07 × 0.06 × 0.02 mm
Z = 4

Data collection

Nonius KappaCCD diffractometer	986 independent reflections
Radiation source: fine-focus sealed tube	959 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.014
ψ and ω scans	θmax = 30.0°, θmin = 3.1°
Absorption correction: multi-scan (HKL SCALEPACK; Otwinowski & Minor, 1997)	h = −13→13
Tmin = 0.771, Tmax = 0.926	k = −7→7
1863 measured reflections	l = −18→18

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)] = 0.017	w = 1/[σ^2(Fo^2) + (0.02P)^2 + P] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.043	(Δ/σ)max < 0.001
S = 1.14	Δρmax = 0.50 e Å^−3
986 reflections	Δρmin = −1.17 e Å^−3
58 parameters	Extinction correction: SHELXL97, Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
0 restraints	Extinction coefficient: 0.0068 (4)

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
Na	0.0000	−0.0685 (3)	0.2500	0.0232 (3)
Al	0.0000	0.0000	0.0000	0.00759 (17)
Mo	0.169680 (14)	0.46923 (3)	0.122124 (11)	0.00768 (9)
O1	0.08830 (14)	0.7070 (3)	0.05141 (11)	0.0160 (3)
O2	0.34454 (14)	0.4289 (3)	0.08134 (11)	0.0156 (3)
O3	0.07332 (13)	0.1828 (3)	0.11055 (10)	0.0128 (3)
O4	0.16751 (19)	0.5629 (3)	0.24701 (12)	0.0204 (3)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Na	0.0329 (7)	0.0205 (6)	0.0164 (6)	0.000	0.0050 (5)	0.000
Al	0.0060 (4)	0.0078 (3)	0.0089 (4)	−0.0001 (2)	−0.0006 (3)	0.0008 (3)
Mo	0.00691 (11)	0.00757 (12)	0.00857 (12)	−0.00013 (4)	0.00015 (7)	0.00000 (4)
O1	0.0141 (6)	0.0104 (6)	0.0234 (7)	0.0014 (5)	−0.0037 (5)	0.0032 (5)
O2	0.0095 (6)	0.0181 (7)	0.0193 (7)	−0.0005 (5)	0.0034 (5)	0.0053 (6)
O3	0.0151 (6)	0.0104 (6)	0.0131 (6)	−0.0037 (5)	−0.0024 (5)	0.0003 (5)
O4	0.0249 (8)	0.0223 (8)	0.0139 (7)	−0.0001 (6)	−0.0008 (6)	−0.0059 (6)

Geometric parameters (Å, º)

Na—O3^i	2.3789 (16)	Al—O3	1.8874 (13)
Na—O3	2.3789 (16)	Al—O1^iii	1.9040 (13)
Na—O4^ii	2.5440 (19)	Al—O1^viii	1.9040 (13)
Na—O4^iii	2.5440 (19)	Al—Na^vii	3.3068 (8)
Na—O2^iv	2.6744 (17)	Mo—O4	1.7164 (17)
Na—O2^v	2.6744 (17)	Mo—O1	1.7575 (14)
Na—O1^iii	2.9957 (17)	Mo—O2	1.7789 (14)
Na—O1^ii	2.9957 (17)	Mo—O3	1.7946 (14)
Na—Al^i	3.3068 (8)	Mo—Na^ix	3.4035 (11)
Na—Al	3.3068 (8)	Mo—Na^x	3.6006 (10)
Na—Mo^iii	3.4035 (11)	O1—Al^ix	1.9040 (13)
Na—Mo^ii	3.4035 (11)	O1—Na^ix	2.9957 (17)
Al—O2^vi	1.8776 (14)	O2—Al^x	1.8776 (14)
Al—O2^v	1.8776 (14)	O2—Na^x	2.6744 (17)
Al—O3^vii	1.8874 (13)	O4—Na^ix	2.5440 (19)
O3^i—Na—O3	111.32 (9)	O2^v—Al—O3^vii	92.08 (6)
O3^i—Na—O4^ii	103.68 (6)	O2^vi—Al—O3	92.08 (6)
O3—Na—O4^ii	129.51 (5)	O2^v—Al—O3	87.92 (6)
O3^i—Na—O4^iii	129.51 (5)	O3^vii—Al—O3	180.00 (5)
O3—Na—O4^iii	103.68 (5)	O2^vi—Al—O1^iii	90.74 (6)
O4^ii—Na—O4^iii	78.65 (9)	O2^v—Al—O1^iii	89.26 (6)
O3^i—Na—O2^iv	61.96 (4)	O3^vii—Al—O1^iii	90.87 (6)
O3—Na—O2^iv	118.42 (5)	O3—Al—O1^iii	89.13 (6)
O4^ii—Na—O2^iv	109.71 (6)	O2^vi—Al—O1^viii	89.26 (6)
O4^iii—Na—O2^iv	69.79 (5)	O2^v—Al—O1^viii	90.74 (6)
O3^i—Na—O2^v	118.42 (5)	O3^vii—Al—O1^viii	89.13 (6)
O3—Na—O2^v	61.96 (4)	O3—Al—O1^viii	90.87 (6)
O4^ii—Na—O2^v	69.79 (5)	O1^iii—Al—O1^viii	180.00 (10)
O4^iii—Na—O2^v	109.71 (6)	O4—Mo—O1	106.85 (8)
O2^iv—Na—O2^v	179.40 (9)	O4—Mo—O2	109.59 (8)
O3^i—Na—O1^iii	168.93 (6)	O1—Mo—O2	110.45 (7)
O3—Na—O1^iii	57.99 (4)	O4—Mo—O3	108.84 (7)
O4^ii—Na—O1^iii	83.32 (6)	O1—Mo—O3	109.91 (6)
O4^iii—Na—O1^iii	59.82 (5)	O2—Mo—O3	111.09 (7)
O2^iv—Na—O1^iii	124.18 (5)	Mo—O1—Al^ix	168.51 (9)
O2^v—Na—O1^iii	55.53 (4)	Mo—O1—Na^ix	87.39 (6)
O3^i—Na—O1^ii	57.99 (4)	Al^ix—O1—Na^ix	81.61 (5)
O3—Na—O1^ii	168.93 (6)	Mo—O2—Al^x	154.25 (9)
O4^ii—Na—O1^ii	59.82 (5)	Mo—O2—Na^x	106.16 (7)
O4^iii—Na—O1^ii	83.32 (6)	Al^x—O2—Na^x	91.47 (6)
O2^iv—Na—O1^ii	55.53 (4)	Mo—O3—Al	134.34 (7)
O2^v—Na—O1^ii	124.18 (5)	Mo—O3—Na	124.65 (7)
O1^iii—Na—O1^ii	132.83 (7)	Al—O3—Na	101.00 (6)
O2^vi—Al—O2^v	180.00 (12)	Mo—O4—Na^ix	104.36 (8)
O2^vi—Al—O3^vii	87.92 (6)

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