Ba5(IO6)2: crystal structure evolution from room temperature to 80 K

The structure of Ba5(IO6)2 has been redetermined at two different temperatures, namely 298 and 80 K, with a high crystalline single-crystal. In comparison with previous determinations based on powder diffraction patterns, the present redetermination results are of greatly improved precision of the structural parameters. The ambiguity of the space-group assignment was eliminated with three-dimensional patterns from a single-crystal sample.

The crystal structure of Ba 5 (IO 6 ) 2 , pentabarium bis(orthoperiodate), has been re-investigated at room temperature based on single-crystal X-ray diffraction data. In comparison with a previous crystal structure determination by the Rietveld method, an improved precision of the structural parameters was achieved. Additionally, low-temperature measurements allowed the crystal structure evolution to be studied down to 80 K. No evidence of structural transition was found even at the lowest temperature. Upon cooling, the lattice contraction is more pronounced along the b axis. This contraction is found to be inhomogeneous along different crystallographic axes. The interatomic distances between different Ba atoms reduce drastically with lowering temperature, resulting in a closer packing around the IO 6 octahedra, which remain largely unaffected.

Database survey
No structural model was found before 2014, even though the compound had been known for a long time. Two different records with structural models from Rietveld refinement against polycrystalline diffraction patterns were found in both the ICSD (426639 & 237982) and PDF4 v2020 (04-021-7777 & 01-083-9132). However, those models have no information about the ADPs. No similar structure can be found in COD before April 2021. Obviously, the high-quality model presented herein will be very useful for research activities related to iodate materials.

Chemical context
Orthoperiodates are compounds based on the (IO 6 ) 5À anion in which iodine is in the oxidation state +7. Pentasodium orthoperiodate was the first synthesized by oxidation of sodium iodide in air in the presence of Na 2 O (Zintl & Morawietz, 1940). About 30 years later, ammonium orthoperiodates were studied for their antiferroelectric properties (Grä nicher et al., 1968). Pentacalcium orthoperiodate was foreseen as a stable nutritional complement of iodine for bovines (Moss &Miller, 1970). In the past three years, orthoperiodates have regained interest. They act as a stabilizing agent of Pickering solution, which is used to harvest cellulose nanocrystals (Liu et al., 2018;Liu et al., 2020) and chitin nanocrystals (Liu et al., 2021) in high yield from microcrystalline samples. When used as ligands, orthoperiodates were found to enhance the stability of water oxidation catalysts (Chakraborty et al., 2018).

Structural commentary
According to the comprehensive description given by Hummel et al., 2015, Ba 5 (IO 6 ) 2 contains two crystallographically independent iodine atoms, which are placed in the centre of distorted octahedra formed by oxygen atoms, also confirmed by our room-temperature single-crystal XRD (SCXRD) data. Perpendicular to the a axis, the crystal structure of Ba 5 (IO 6 ) 2 is made up of alternating layers formed by I(6)O 6 and I(7)O 6 octahedra, respectively, as shown in Fig. 1. These IO 6 octahedra from two consecutive layers do not share any direct connection to each other. The octahedra are symmetrically distributed in three dimensions over the crystal structure between the five different barium atoms of the Ba 5 (IO 6 ) 2 structure.
In order to detect any possible structural transitions in Ba 5 (IO 6 ) 2 , SCXRD measurements were also performed at 100 K and 80 K. By comparing the results obtained at 80 K with those of 298 K, average thermal dilation coefficients of 4.8 Â 10 À6 K À1 , 17.56 Â 10 À6 K À1 , 5.23 Â 10 À6 K À1 were found along a, b and c axes, respectively.
The temperature evolution of the IO 6 octahedra are quite different from the barium atom 'matrix' within which they are distributed. As can be seen in Fig. 2, the IO 6 octahedra at 80 K are almost identical with respect to interatomic distances and angles to the ones refined at 298 K. However, the interatomic distances between Ba atoms are very sensitive to the  Bond lengths (in Å ) and equatorial plane angles (in ) for the two IO 6 octahedra of the Ba 5 (IO 6 ) 2 structure. The values obtained at 80 K and 298 K are indicated in blue and red, respectively.

Figure 3
Dilation coefficient of the Ba-Ba interatomic distances from 80 K to 298 K.

Figure 1
The crystal structure of Ba 5 (IO 6 ) 2 visualized along different crystallographic axes. Green, violet and red balls represents barium, iodine and oxygen atoms, respectively. temperature in another way. They dramatically expand when Ba 5 (IO 6 ) 2 is heated from 80 K to 298 K (Fig. 3). Except for the Ba2-Ba4 distance, all the interatomic distances show a dilation coefficient that is up to one order of magnitude higher than those of the lattice constant.
In conclusion, single crystals of Ba 5 (IO 6 ) 2 have been grown using a flux method. The crystal structures at two different temperatures, from 298 K down to 80 K, have been refined using high-quality SCXRD data. We confirm the assignment of the structure to the centrosymmetric space group Pnma (No. 62), which cannot be distinguished via extinction rules from the Pna2 1 (No. 33) space group by powder XRD. From the low-temperature XRD data, evolution of the lattice constants was found to be inhomogeneous. While IO 6 octahedra size and distortion do not change drastically between 80 K and 298 K, the packing of the barium atoms around the octahedra grows upon cooling. This leads to an increase of the density from 6.097 (8) g cm À3 at 298 K to 6.134 (7) g cm À3 at 80 K.

Synthesis and crystallization
Iron oxalate dihydrate (FeC 2 O 4 Á2H 2 O), barium carbonate (BaCO 3 ) and barium iodide dihydrate (BaI 2 Á2H 2 O) were mixed in a molar ratio 2:2:10 and placed in an alumina crucible. The mixture was heated to 1273 K at a rate of 100 K per hour. The furnace was maintained at this temperature for 24 h, followed by a slow cooling to 1173 K at a rate of 1 K per hour, and was finally quenched to room temperature. The whole synthesis process took place under atmospheric conditions. Crystals were collected from the wall of the crucible.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1.
On considering the large value of beam attenuation coefficient of the title compound, a nice crystal with small size and Mo K radiation was selected for structure determination.
Data sets were collected up to the resolution of 0.6 Å at two different temperatures. No restraints were applied for the refinement at 298 K, but one O atom in the structure at 80 K had abnormal ADPs, and an EADP constraint was applied on it to eliminate the level B checkCIF alert. Then both models were modified to follow the same bonding and labelling style as the one in the database, and the EADP constraint was removed. For both structures, data collection: CrysAlis PRO (Rigaku OD, 2020); cell refinement: CrysAlis PRO (Rigaku OD, 2020); data reduction: CrysAlis PRO (Rigaku OD, 2020); program(s) used to solve structure: SHELXT2018/3 (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2018/3 (Sheldrick, 2015b); molecular graphics: OLEX2 (Dolomanov et al., 2009); software used to prepare material for publication: OLEX2 (Dolomanov et al., 2009).

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.