Elaboration, structural study and validation of a new NASICON-type structure, Na0.72(Cr0.48,Al1.52)(Mo2.77,Al0.23)O12.

The title compound, sodium chromium/aluminium molybdenum/aluminium dodeca-oxide, Na0.72Cr0.48Al1.74Mo2.77O12, was prepared by solid-state reaction. Its crystal structure is related to NaSICON-type compounds. The framework is built up from M1O6 (M1 = Cr/Al) octa-hedra and M2O4 (M2 = Mo/Al) tetra-hedra inter-connected by corners. The three-dimensional framework contains cavities in which sodium cations are located. Two validation models (BVS and CHARDI) were used to confirm the proposed structural model. The mobility of Na+ ions in the structure has been investigated by theoretical means.

Chemical context

The search for new and better solid electrolyte materials has grown considerably in recent years because of their amazing properties and their diverse applications in the field of solid-state chemistry. Indeed, many new molybdate phases with high ionic conductivity have been synthesized and structurally characterized by X-ray diffraction. A large number belong to the NASICON (‘Na super ionic conductor’) family, e.g. phosphate (Tkachev et al., 1984 ▸; Catti et al., 2004 ▸), arsenate (Harrison & Phillips, 2001 ▸), sulfate (Slater & Greaves, 1994 ▸) and molybdate (Sun et al., 2012 ▸; Kozhevnikova & Imekhenova, 2006 ▸) based systems. The NASICON family groups together a set of phases of the same structural type with the general formula AM 2(XO4)3 (A = alkali, M = Ti, Fe, V, Co, Ni and X = P, As, Mo, W, S; Prabaharan et al., 2004 ▸). Apart from their superionic properties, a number of NASICON compounds have considerable potential for use in laser engin­eering, optics and electronics owing to their non-linear optical, electrical, magnetic and luminescent properties. It is in this context that we chose to explore A–Cr–Mo–O systems (A = monovalent ion). A new phase Na0.72(Cr0.48,Al1.52)(Mo2.77,Al0.23)O12 was synthesized by solid-state reaction. We present here its crystal structure and its validation by the CHARDI (charge distribution) and BVS (bond-valence-sum) methods.

Structural commentary

The structural unit of Na0.72(Cr0.48,Al1.52)(Mo2.77,Al0.23)O12 consists of one octa­hedron M1O6 (M1 = Cr1/Al2) and one tetra­hedron M2O4 (M2 = Mo1/Al1) that share corners. The charge compensation is provided by Na+ cations (Fig. 1 ▸). The main construction unit in the anionic framework of the compound Na0.72(Cr0.48,Al1.52)(Mo2.77,Al0.23)O12 is formed by two M1O6 octa­hedra inter­connected by three M2O4 tetra­hedra located along the c axis. Geometrical parameters are given in Table 1 ▸. This assembly forms M12 M23O18 units (Fig. 2 ▸). The junction between these units, provided by the formation of mixed bridges of the M1–O–M2 type, leads to a three-dimensional framework with cavities parallel to the [100] and [010] directions in which the Na+ cations are located (Fig. 3 ▸). Indeed, each M1O6 octa­hedron share its six corners with different M2O4 tetra­hedra, leading to M1M2 6O24 clusters (Fig. 4 ▸). The two validation models BVS (Brown & Altermatt, 1985 ▸; Brown, 2002 ▸; Adams, 2003 ▸) and CHARDI (Hoppe et al., 1989 ▸; Nespolo et al., 2001 ▸; Nespolo, 2001 ▸) confirm the proposed structural model, in particular the distribution at mixed sites. The calculated load values Q(i) and valences V(i) are in good agreement with the oxidation degrees weighted by the occupancy rates. The dispersion factor σcat, which measures the deviation of the calculated cationic charges, is equal to 0.3% (Table 2 ▸). The variation of the bond-valence sum of sodium as a function of the distance travelled in different directions shows that the [011] and [111] directions are the most favorable directions for the mobility of sodium. The paths along these directions have the same shape when the distance travelled is about 13.5 Å and the maximum valence is about 1.4 valence units (Fig. 5 ▸). The representation of the Na migration path in the direction [011] is shown in Fig. 6 ▸.

Figure 1

Structural unit of Na0.72(Cr0.48,Al1.52)(Mo2.77,Al0.23)O12. Displacement ellipsoids are drawn at the 50% probability level. [Symmetry codes: (i)  − x,  − y,  − z; (ii) − + x − y, − + x,  − z; (iii) − + y,  − x + y,  − z; (iv) −x + y, −x, z; (v) −x, x − z, z; (vi) x − y, −y,  − z.]

Table 1

Selected geometric parameters (Å, °)

Mo1—O2i	1.7358 (15)	Cr1—O2v	1.9721 (16)
Mo1—O2	1.7359 (15)	Cr1—O2vi	1.9721 (16)
Mo1—O1	1.7540 (16)	Na1—O1ii	2.4987 (15)
Mo1—O1i	1.7540 (15)	Na1—O1vii	2.4987 (15)
Cr1—O1ii	1.9668 (16)	Na1—O1iii	2.4987 (15)
Cr1—O1iii	1.9668 (16)	Na1—O1viii	2.4987 (15)
Cr1—O1iv	1.9669 (16)	Na1—O1iv	2.4987 (15)
Cr1—O2	1.9720 (16)	Na1—O1ix	2.4987 (15)
O2i—Mo1—O2	109.56 (11)	O1iv—Cr1—O2vi	88.68 (7)
O2i—Mo1—O1	107.85 (8)	O2—Cr1—O2vi	91.35 (7)
O2—Mo1—O1	111.40 (8)	O2v—Cr1—O2vi	91.35 (7)
O2i—Mo1—O1i	111.40 (8)	O1ii—Na1—O1vii	180.0
O2—Mo1—O1i	107.86 (8)	O1ii—Na1—O1iii	65.74 (6)
O1—Mo1—O1i	108.80 (11)	O1vii—Na1—O1iii	114.26 (6)
O1ii—Cr1—O1iii	87.18 (7)	O1ii—Na1—O1viii	114.26 (6)
O1ii—Cr1—O1iv	87.18 (7)	O1vii—Na1—O1viii	65.74 (6)
O1iii—Cr1—O1iv	87.18 (7)	O1iii—Na1—O1viii	180.0
O1ii—Cr1—O2	92.79 (7)	O1ii—Na1—O1iv	65.74 (6)
O1iii—Cr1—O2	88.68 (7)	O1vii—Na1—O1iv	114.26 (6)
O1iv—Cr1—O2	175.86 (7)	O1iii—Na1—O1iv	65.74 (6)
O1ii—Cr1—O2v	88.68 (7)	O1viii—Na1—O1iv	114.26 (6)
O1iii—Cr1—O2v	175.86 (7)	O1ii—Na1—O1ix	114.26 (6)
O1iv—Cr1—O2v	92.79 (7)	O1vii—Na1—O1ix	65.74 (6)
O2—Cr1—O2v	91.35 (7)	O1iii—Na1—O1ix	114.26 (6)
O1ii—Cr1—O2vi	175.85 (7)	O1viii—Na1—O1ix	65.74 (6)
O1iii—Cr1—O2vi	92.79 (7)	O1iv—Na1—O1ix	180.0

Symmetry codes: (i) ; (ii) ; (iii) ; (iv) ; (v) ; (vi) ; (vii) ; (viii) ; (ix) .

Figure 2

Projection of an M12 M23O18 unit along the a axis.

Figure 3

Projection of Na0.72(Cr0.48,Al1.52)(Mo2.77,Al0.23)O12 along the a axis.

Figure 4

Projection of Na0.72(Cr0.48,Al1.52)(Mo2.77,Al0.23)O12 along the c axis.

Table 2

CHARDI and BVS analyses for the cations in the Na0.72Cr0.48Al1.76Mo2.77O12 compound

q(i) = formal oxidation number; sof(i) = site occupancy; CN(i) = classical coordination number; Q(i) = calculated charge; V(i) = calculated valence; ECoN(i) = coordination number; d mean(i) = mean distance; d med(i) = median distance.

Cation	q(i)·sof(i)	Q(i)	V(i)	CN(i)	ECoN(i)	d mean	d med
Mo1/Al1	5.78	5.77	5.8426	4	4.00	1.7448	1.7443
M(Cr1/Al2)	3.000	2.99	2.9397	6	6.00	1.9694	1.9696
Na1	0.72	0.71	0.6893	6	6.00	2.4989	2.4989

σcat is the dispersion factor for cationic charges where σcat = [Σ(q  − Q )2/N − 1]1/2 = 0.025.

Figure 5

Ionic pathway valence curves of the title compound.

Figure 6

Modelling of the Na+ pathway in Na0.72(Cr0.48,Al1.52)(Mo2.77, Al0.23)O12.

Database survey

A comparison between the structures of the title compound and those of other NASICON-type compounds reveals that other compounds also crystallize in the R c space group with similar unit-cell parameters. When compared to NaZr2(AsO4)3 (Coquerel et al., 1983 ▸) and Na4Co3Mo22.33O72 (Chakir et al., 2003 ▸), the only difference observed is the occupancy of the sites 6b, 12c and 18e. In NaZr2(AsO4)3, the sites are fully occupied, whereas in Na4Co3Mo22.33O72, the 6b site is not totally occupied, and the 12c site is occupied by both Co and Mo. In the title compound, the 6b site is partially occupied and the 12c and 18e sites are mixed Cr/Al and Mo/Al sites, respectively.

Synthesis and crystallization

During the investigation of the A–Mo–Cr–O phase diagrams (A = Li, Na, Ag), the new compound Na0.72(Cr0.48,Al1.52)(Mo2.77,Al0.23)12 was established. The crystals were obtained by grinding in an agate mortar the reagents NaNO3, Cr(NO3)3·9H2O and (NH4)6Mo7O24·4H2O in a Na:Cr:Mo molar ratio of 1:1:4, respectively. The resulting mixture was calcined at 673 K to remove volatiles including NO2, NH3 and H2O. The residual powder thus obtained was finely ground and then returned to the oven at a temperature close to the melting point at 973 K for three days to promote germination and crystal growth. After cooling, crystals of parallelepipedal shape and optimal size for data collection were obtained. A crystal of a good quality, selected under a polarizing microscope, was used for intensity measurements

Refinement

Crystal data, data collection and structure refinement details are summarized in Table 3 ▸. After processing the data, the structure was solved successfully in the R c space group, using direct methods implemented in the SHELXS97 program (Sheldrick, 2008 ▸). The molybdenum, chromium and oxygen atoms were located first. At this stage, an empirical ψ-scan correction (North et al., 1968 ▸) was applied. Difference-Fourier syntheses using the program SHELXL97 (Sheldrick, 2008 ▸), allowed the rest of the atoms in the cell to be localized. We obtained intense peaks close to Cr and Mo; the liberation of the occupancy factors led to values different from the full site occupancy (0.62530 for Mo and 0.24035 for Cr). The EDX analysis (Fig. 7 ▸) of the sample confirmed the presence of aluminium and we then used EADP and EXYZ constraints as well as SUMP to refine Al1 with the Mo1 site and Al2 with the Cr1 site. After refinement and verification of the electrical neutrality, the final formula was Na0.72 (1)(Cr0.48 (1),Al1.52 (2))(Mo2.77 (3),Al0.23 (2))O12. The remaining maximum and minimum electron densities in the difference-Fourier map are acceptable and are at 0.78 Å from the Mo1 site and at 0.89 Å from the Mo2, respectively.

Table 3

Experimental details

Crystal data
Chemical formula	Na0.72(Cr0.48·Al1.52)(Mo2.77·Al0.23)O12
M r	546.34
Crystal system, space group	Trigonal, R c
Temperature (K)	298
a, c (Å)	9.217 (2), 22.646 (2)
V (Å3)	1666.1 (7)
Z	6
Radiation type	Mo Kα
μ (mm−1)	3.74
Crystal size (mm)	0.24 × 0.21 × 0.18
Data collection
Diffractometer	Enraf–Nonius CAD-4
Absorption correction	ψ scan (North et al., 1968 ▸)
T min, T max	0.491, 0.599
No. of measured, independent and observed [I > 2σ(I)] reflections	2878, 414, 401
R int	0.026
(sin θ/λ)max (Å−1)	0.637
Refinement
R[F 2 > 2σ(F 2)], wR(F 2), S	0.013, 0.024, 1.25
No. of reflections	414
No. of parameters	35
No. of restraints	2
Δρmax, Δρmin (e Å−3)	0.23, −0.43

Computer programs: CAD-4 EXPRESS (Duisenberg, 1992 ▸; Macíček & Yordanov, 1992 ▸), XCAD4 (Harms & Wocadlo, 1995 ▸), SHELXS97 (Sheldrick, 2008 ▸), SHELXL2014 (Sheldrick, 2015 ▸), DIAMOND (Brandenburg, 2006 ▸), WinGX (Farrugia, 2012 ▸) and publCIF (Westrip, 2010 ▸).

Figure 7

EDX analysis of the sample confirming the presence of aluminium in Na0.72(Cr0.48,Al1.52)(Mo2.77, Al0.23)O12.

Crystal structure: contains datablock(s) I. DOI: 10.1107/S2056989018003031/vn2134sup1.cif

Structure factors: contains datablock(s) I. DOI: 10.1107/S2056989018003031/vn2134Isup2.hkl

CCDC reference: 1824957

Additional supporting information: crystallographic information; 3D view; checkCIF report

Na0.72(Cr0.48·Al1.52)(Mo2.77·Al0.23)O12	Dx = 3.267 Mg m−3
Mr = 546.34	Mo Kα radiation, λ = 0.71073 Å
Trigonal, R3c	Cell parameters from 25 reflections
a = 9.217 (2) Å	θ = 12.1–14.8°
c = 22.646 (2) Å	µ = 3.74 mm−1
V = 1666.1 (7) Å3	T = 298 K
Z = 6	Prism, red
F(000) = 1526	0.24 × 0.21 × 0.18 mm

Enraf–Nonius CAD-4 diffractometer	Rint = 0.026
Radiation source: fine-focus sealed tube	θmax = 26.9°, θmin = 3.1°
ω/2θ scans	h = −11→11
Absorption correction: ψ scan (North et al., 1968)	k = −2→11
Tmin = 0.491, Tmax = 0.599	l = −28→28
2878 measured reflections	2 standard reflections every 120 reflections
414 independent reflections	intensity decay: 0.8%
401 reflections with I > 2σ(I)

Refinement on F2	2 restraints
Least-squares matrix: full	w = 1/[σ2(Fo2) + 4.1215P] where P = (Fo2 + 2Fc2)/3
R[F2 > 2σ(F2)] = 0.013	(Δ/σ)max = 0.001
wR(F2) = 0.024	Δρmax = 0.23 e Å−3
S = 1.25	Δρmin = −0.42 e Å−3
414 reflections	Extinction correction: SHELXL2014 (Sheldrick, 2015), Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4
35 parameters	Extinction coefficient: 0.00046 (6)

Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds 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 > 2sigma(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.

	x	y	z	Uiso*/Ueq	Occ. (<1)
Mo1	0.28488 (2)	0.0000	0.7500	0.01214 (9)	0.921 (6)
Al1	0.28488 (2)	0.0000	0.7500	0.01214 (9)	0.080 (10)
Cr1	0.0000	0.0000	0.63854 (3)	0.0109 (2)	0.238 (11)
Al2	0.0000	0.0000	0.63854 (3)	0.0109 (2)	0.761 (19)
Na1	0.0000	0.0000	0.5000	0.0259 (9)	0.724 (8)
O1	0.48491 (19)	0.17851 (19)	0.74736 (7)	0.0326 (4)
O2	0.1686 (2)	−0.0152 (2)	0.68761 (6)	0.0376 (5)

	U11	U22	U33	U12	U13	U23
Mo1	0.01265 (11)	0.01229 (13)	0.01137 (12)	0.00614 (6)	0.00085 (4)	0.00169 (8)
Al1	0.01265 (11)	0.01229 (13)	0.01137 (12)	0.00614 (6)	0.00085 (4)	0.00169 (8)
Cr1	0.0117 (3)	0.0117 (3)	0.0092 (3)	0.00587 (14)	0.000	0.000
Al2	0.0117 (3)	0.0117 (3)	0.0092 (3)	0.00587 (14)	0.000	0.000
Na1	0.0322 (11)	0.0322 (11)	0.0132 (12)	0.0161 (6)	0.000	0.000
O1	0.0254 (8)	0.0218 (8)	0.0442 (9)	0.0071 (7)	−0.0038 (6)	0.0003 (7)
O2	0.0364 (9)	0.0446 (11)	0.0278 (8)	0.0172 (8)	−0.0093 (7)	0.0015 (7)

Mo1—O2i	1.7358 (15)	Na1—O1ii	2.4987 (15)
Mo1—O2	1.7359 (15)	Na1—O1vii	2.4987 (15)
Mo1—O1	1.7540 (16)	Na1—O1iii	2.4987 (15)
Mo1—O1i	1.7540 (15)	Na1—O1viii	2.4987 (15)
Cr1—O1ii	1.9668 (16)	Na1—O1iv	2.4987 (15)
Cr1—O1iii	1.9668 (16)	Na1—O1ix	2.4987 (15)
Cr1—O1iv	1.9669 (16)	Na1—Al2x	3.1373 (7)
Cr1—O2	1.9720 (16)	Na1—Cr1x	3.1373 (7)
Cr1—O2v	1.9721 (16)	O1—Al2iii	1.9668 (16)
Cr1—O2vi	1.9721 (16)	O1—Cr1iii	1.9668 (16)
Cr1—Na1	3.1374 (7)	O1—Na1xi	2.4987 (15)
O2i—Mo1—O2	109.56 (11)	O1iii—Na1—O1iv	65.74 (6)
O2i—Mo1—O1	107.85 (8)	O1viii—Na1—O1iv	114.26 (6)
O2—Mo1—O1	111.40 (8)	O1ii—Na1—O1ix	114.26 (6)
O2i—Mo1—O1i	111.40 (8)	O1vii—Na1—O1ix	65.74 (6)
O2—Mo1—O1i	107.86 (8)	O1iii—Na1—O1ix	114.26 (6)
O1—Mo1—O1i	108.80 (11)	O1viii—Na1—O1ix	65.74 (6)
O1ii—Cr1—O1iii	87.18 (7)	O1iv—Na1—O1ix	180.0
O1ii—Cr1—O1iv	87.18 (7)	O1ii—Na1—Al2x	141.20 (4)
O1iii—Cr1—O1iv	87.18 (7)	O1vii—Na1—Al2x	38.80 (4)
O1ii—Cr1—O2	92.79 (7)	O1iii—Na1—Al2x	141.19 (4)
O1iii—Cr1—O2	88.68 (7)	O1viii—Na1—Al2x	38.81 (4)
O1iv—Cr1—O2	175.86 (7)	O1iv—Na1—Al2x	141.19 (4)
O1ii—Cr1—O2v	88.68 (7)	O1ix—Na1—Al2x	38.81 (4)
O1iii—Cr1—O2v	175.86 (7)	O1ii—Na1—Cr1x	141.20 (4)
O1iv—Cr1—O2v	92.79 (7)	O1vii—Na1—Cr1x	38.80 (4)
O2—Cr1—O2v	91.35 (7)	O1iii—Na1—Cr1x	141.19 (4)
O1ii—Cr1—O2vi	175.85 (7)	O1viii—Na1—Cr1x	38.81 (4)
O1iii—Cr1—O2vi	92.79 (7)	O1iv—Na1—Cr1x	141.19 (4)
O1iv—Cr1—O2vi	88.68 (7)	O1ix—Na1—Cr1x	38.81 (4)
O2—Cr1—O2vi	91.35 (7)	Al2x—Na1—Cr1x	0.0
O2v—Cr1—O2vi	91.35 (7)	O1ii—Na1—Cr1	38.80 (4)
O1ii—Cr1—Na1	52.76 (5)	O1vii—Na1—Cr1	141.20 (4)
O1iii—Cr1—Na1	52.76 (5)	O1iii—Na1—Cr1	38.81 (4)
O1iv—Cr1—Na1	52.76 (5)	O1viii—Na1—Cr1	141.19 (4)
O2—Cr1—Na1	124.30 (5)	O1iv—Na1—Cr1	38.81 (4)
O2v—Cr1—Na1	124.30 (5)	O1ix—Na1—Cr1	141.19 (4)
O2vi—Cr1—Na1	124.30 (5)	Al2x—Na1—Cr1	180.0
O1ii—Na1—O1vii	180.0	Cr1x—Na1—Cr1	180.0
O1ii—Na1—O1iii	65.74 (6)	Mo1—O1—Al2iii	144.66 (9)
O1vii—Na1—O1iii	114.26 (6)	Mo1—O1—Cr1iii	144.66 (9)
O1ii—Na1—O1viii	114.26 (6)	Al2iii—O1—Cr1iii	0.0
O1vii—Na1—O1viii	65.74 (6)	Mo1—O1—Na1xi	126.81 (8)
O1iii—Na1—O1viii	180.0	Al2iii—O1—Na1xi	88.43 (6)
O1ii—Na1—O1iv	65.74 (6)	Cr1iii—O1—Na1xi	88.43 (6)
O1vii—Na1—O1iv	114.26 (6)	Mo1—O2—Cr1	158.35 (10)