Flux syntheses and single-crystal structures of CsNa10 M 4(AsO4)9 (M = Zr, Hf)

The isostructural title compounds arose as unexpected single-crystal products from the reactions of Na2CO3, MO2 (M = Zr, Hf) and As2O5 in a NaCl/CsCl eutectic flux.

In an attempt to grow single crystals of the possible new KTP analogues NaZrOAsO 4 and NaHfOAsO 4 by reacting Na 2 CO 3 , MO 2 (M = Zr, Hf) and As 2 O 5 in a lowmelting flux of NaCl and CsCl, the isostructural title compounds CsNa 10 Zr 4 (AsO 4 ) 9 (I) and CsNa 10 Hf 4 (AsO 4 ) 9 (II) were the unexpected result and their crystal structures are now described.

Structural commentary
Compounds (I) and (II) are isostructural and crystallize in the rhombohedral space group R3c (No. 167) with an unusually long c unit-cell parameter of nearly 77 Å . This is of course partly a consequence of our choosing the hexagonal (Rcentred) setting of the unit cell [the equivalent primitive rhombohedral lattice for (I) has a = b = c ' 26.21 Å and = = ' 20.3 ] but even so, it is notable that the l index runs well into three figures for (I) in the R-centred setting. This description will focus on the structure of (I) and note significant differences for (II) where applicable.
The asymmetric unit of (I), expanded to show the full coordination polyhedra of the zirconium and arsenic atoms, is shown in Fig. 1. It consists of two zirconium atoms (both with site symmetry 3 on Wyckoff site 12c), two arsenic atoms [As1 on a general position (36f) and As2 with site symmetry 2 (18e)] and six oxygen atoms, one of which is disordered over two adjacent sites (all lying on general positions, 36f), which leads to the unusual 4:9 stoichiometry for the Zr IV and AsO 4 3moieties with a net charge of À11. The structure of (I) is completed by a Cs + ion (site symmetry 3, 6b) and four partly occupied sodium cations [one on a general position (36f), one with site symmetry 2 (18e) and two with site symmetry 3 (12c)]. To maintain charge balance, the four sodium ions must have a total occupancy of 10 based on Z = 6 (full occupancy of the four sites would give 13 sodium ions per caesium ion).

Figure 2
The unit-cell of (I) in polyhedral representation viewed approximately down [110]. A single O atom at the average location of O6A and O6B in the asymmetric unit has been used to construct the As2 tetrahedron. Colour code: Zr1O 6 octahedra blue, Zr2O 6 octahedra green, As1O 4 tetrahedra peach, As2O 4 tetrahedra rose, Cs sky blue, Na yellow, O (polyhedral corners) red. The caesium ion in (I) adopts a grossly squashed octahedral coordination to six O1 atoms with Cs1-O1 = 3.235 (4) Å : the cis O-Cs-O bond angles are compressed to 62.30 (10) or expanded to 117.70 (10) : the Cs1 BVS of 0.61 compared to an expected value of 1.00 suggests significant underbonding. The interpretation of the sodium-ion coordination polyhedra are complicated by the positional disorder of atom O6 but can be described as distorted trigonal bipyramidal (Na1), very distorted tetrahedral (Na2), square-based pyramidal (Na3) and squashed trigonal pyramidal (Na4). It is notable that Na4 is only three coordinate but similar NaO 3 geometries have been observed in dehydrated sodium aluminosilicate zeolites (Adams et al., 1982).
The extended structure of (I) (Fig. 2) can be conceptually broken down into two different types of layers lying parallel to (001). The first layer (type 'A') occurs at z ' 0, 1/6, 1/3, 1/2, 2/3 and 5/6 with adjacent A-layers laterally displaced by 1/3 in x and 2/3 in y and consists of the Zr2 and As1 centred polyhedra as well as the caesium ions. Fig. 3 shows that each Zr2O 6 octahedron is connected by two As1O 4 tetrahedra (via O2 and O4) to result in a 'honeycomb' array of polyhedral 12-rings (six octahedra and 12 tetrahedra) encapsulating the Cs + ions. Atom O3 of the arsenate group provides the link to the type 'B' layers on either side of the A layer. This inter-octahedral connectivity via O3 leads to a distinctive 'lantern' motif ( Fig. 4) in which three tetrahedra link two octahedra [Zr1Á Á ÁZr2  (Norberg, 2002) structure types but they differ from (I) because all the vertices of the constituent tetrahedra in these structures link to adjacent octahedra, hence their 2:3 M:X ratios compared to the 4:9 ratio for (I). View down [001] of an 'A'-type layer in the structure of (I) in polyhedral representation. Atom and polyhedron colours as in Fig. 2 except O3 is blue.

Figure 4
Detail of the extended structure of (I) showing a Zr 2 As 3 O 18 'lantern' motif of Zr1 and Zr2 octahedra linked by three As1 tetrahedra via atoms O2 and O3. In (I), this motif has crystallographically imposed threefold symmetry about a rotation axis passing through the zirconium atoms. Symmetry codes: (i) 1 À y, 1 + x À y, z; (ii) y À x, 1 À x, z.

Figure 5
View down [001] of a 'B'-type layer in the structure of (I) in polyhedral representation. Atom and polyhedron colours as in Fig. 2 except O3 is blue.
The B layers in (I) (Fig. 5) lie at z ' 1/12, 1/4, 5/12, 7/12, 3/4 and 11/12 and are associated with the Zr1 and As2 species. These also feature polyhedral 12-rings (six octahedra and six tetrahedra) but only one As2 tetrahedron (with two terminal As2-O6 bonds) links adjacent Zr1 octahedra via atom O5. There are numerous sodium sites associated with the B layers. The disorder of the sodium ions in the vicinities of the B layers and possible small [110] channels (see Fig. 2) suggests the possibility of ionic conductivity (Norberg, 2002). An analysis of the stucture with PLATON (Spek, 2020) with the sodium ions removed indicated that there was 119.4 Å 3 of free space per unit cell ($2.1%).

Synthesis and crystallization
Compound (I) was prepared by mixing 1.00 g of Na 2 CO 3 , 0.581 g of ZrO 2 and 1.399 g of As 2 O 5 (Na:Zr:As molar ratio ' 4:1:3) in an agate mortar: 1.00 g of this mixture was added to 3.0 g of a eutectic-melt mixture (T melt ' 500 C) of NaCl/CsCl ($0.35:0.65 mol) and placed in a flat-bottom alumina crucible. The crucible was rapidly heated to 500 C in a muffle furnace and then ramped at 12 C min À1 to 700 C and cooled at the same rate to 400 C and then removed from the furnace and left to cool. The gummy white product was washed with copious amounts of hot water followed by acetone to result in a mass of tiny colourless rods of (I). Compound (II) was made in the same way starting from a pre-mixture of 1.00 g Na 2 CO 3 , 1.12 g HfO 2 and 1.57 g As 2 O 5 and tiny colourless rods of (II) were the result.
Caution! Arsenic compounds are highly toxic and carcinogenic. Take all appropriate safety precautions, especially with respect to dust contamination.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1. The crystal chosen for data collection for (I) was found to be twinned over its rhombohedral obverse and reverse settings (Herbst-Irmer & Sheldrick, 2002) in a 0.797 (3):0.203 (3) ratio, which was processed as a SHELXL HKLF 5 refinement. To ensure charge balance, the occupancies of the four partially occupied sodium sites must sum to 10.0 Na per caesium ion and this was achieved by using a SUMP card (linear free variable restraint) in SHELXL, as unrestrained refinements tended to drift to a collective occupancy of above 10 (full occupancy of the four sodium sites would give 13 Na to 1 Cs). This needed cautious 740 William T. A. Harrison CsNa 10 Zr 4 (AsO 4 ) 9 and CsNa 10 Hf 4 (AsO 4 ) 9 Acta Cryst. (2022). E78, 737-741 research communications damped refinement cycles to begin with, but as the refinement converged, the damping could be removed to give refined fractional site occupancies of Na1 = 0.852 (5), Na2 = 0.860 (9), Na3 = 0.731 (12) and Na4 = 0.423 (11) for (I) and Na1 = 0.887 (7), Na2 = 0.846 (11), Na3 = 0.735 (16) and Na4 = 0.337 (14) for (II). The final difference map for (II) features electron density peaks of $2 e Å À3 near some of the sodium ions, perhaps suggesting that they are localizing over split multiple sites at low temperatures, but efforts to model this did not lead to satisfactory refinements. The value of U eq for Na4 is small, which might indicate partial occupancy of caesium on this site (i.e., a formula of Cs 1+x Na 10-x Hf 4 (AsO 4 ) 9 , but attempts to model this were inconclusive. to refine structure: SHELXL2018/3 (Sheldrick, 2015); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012) and ATOMS (Dowty, 2005); software used to prepare material for publication: SHELXL2018/3 (Sheldrick, 2015) and publCIF (Westrip, 2010).

Caesium decasodium tetrazirconium nonaarsenate (I)
Crystal data 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. Refined as a 2-component obverse/reverse twin.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. ( 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.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )