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

A new NASICON-type phase, Na0.72(Cr0.48,Al1.52)(Mo2.77,Al0.23)O12, was synthesized by solid-state reaction. The structural unit consists of one octahedron M1O6 (M1 = Cr1/Al2) and one tetrahedron M2O4 (M2 = Mo1/Al1) sharing corners. The charge compensation is provided by Na+ cations.


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 solidstate 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 (XO 4 ) 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 Na 0.72 (Cr 0.48 ,Al 1.52 )-(Mo 2.77 ,Al 0.23 )O 12 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.

Figure 2
Projection of an M1 2 M2 3 O 18 unit along the a axis.

Figure 1
Structural unit of Na 0.72 (Cr 0.48 ,Al 1.52 )(Mo 2.77 ,Al 0.23 )O 12 . Displacement ellipsoids are drawn at the 50% probability level. [Symmetry codes:  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 cat , which measures the deviation of the calculated cationic charges, is equal to 0.3% (

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 R3c space group with similar unit-cell parameters. When compared to NaZr 2 (AsO 4 ) 3 (Coquerel et al., 1983) and Na 4 Co 3 Mo 22.33 O 72 (Chakir et al., 2003), the only difference observed is the occupancy of the sites 6b, 12c and 18e. In NaZr 2 (AsO 4 ) 3 , the sites are fully occupied, whereas in Na 4 Co 3 Mo 22.33 O 72 , 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

Figure 5
Ionic pathway valence curves of the title compound.

Figure 6
Modelling of the Na + pathway in Na 0.72 (Cr 0.48 ,Al 1.52 )(Mo 2.77 , Al 0.23 )O 12 . 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 R3c 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 -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 Na 0.72 (1) (Cr 0.48 (1) ,Al 1.52 (2) )-(Mo 2.77 (3) ,Al 0.23 (2) )O 12 . 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. 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

Sodium chromium/aluminium molybdenum/aluminium dodecaoxide
Crystal data where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 0.23 e Å −3 Δρ min = −0.42 e Å −3 Extinction correction: SHELXL2014 (Sheldrick, 2015), Fc * =kFc[1+0.001xFc 2 λ 3 /sin(2θ)] -1/4 Extinction coefficient: 0.00046 (6) sup-2 Acta Cryst. (2018). E74, 406-409 Special details 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 F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2sigma(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.