Crystal structures of the solid solutions Na3Zn0.912Cd0.088B5O10 and Na3Zn0.845Mg0.155B5O10

The solid solutions Na3Zn0.912Cd0.088B5O10 and Na3Zn0.845Mg0.155B5O10 adopt the orthorhombic form of the parent compound Na3ZnB5O10 where parts of the zinc cations are replaced by cadmium and magnesium cations, respectively.


Chemical context
Over the past few decades, borate materials have attracted increasing interest owing to their promising applications in non-linear optical materials, birefringent materials, ferroelectric and piezoelectric materials, and host materials for luminescence (Becker, 1998;Chen et al., 1999). In general, boron atoms can be coordinated by either three or four oxygen atoms forming BO 3 or BO 4 groups, respectively. These groups may interconnect with each other via common oxygen atoms to produce polyborate anionic groups that can adopt different coordination modes to bind to metal cations. The crystal chemistry of the resultant borates is rich, including infinite chains, sheets or networks for the anionic groups. For instance, in a series of pentaborates with general composition A 3 MB 5 O 10 (A = Na, K; M = Mg, Zn, Cd, Co, and Fe), at least three kinds of structure types have been reported, including K 2 NaZnB 5 O 10 in space group C2/c (Chen et al., 2010), -Na 3 ZnB 5 O 10 , Na 3 CoB 5 O 10 and K 3 MB 5 O 10 (M = Zn, Cd) in space group P2 1 /n (Chen et al., 2007a;Strauss et al., 2016;Yu et al., 2011), and -Na 3 ZnB 5 O 10 as well as Na 3 MB 5 O 10 (M = Mg, Fe) in space group Pbca (Chen et al., 2007bStrauss et al., 2016). All of the structures contain polyborate anionic groups [B 5 O 10 ] 5À , which combine with different A + and M 2+ cations. During our exploratory syntheses of novel borate materials to study their structureproperty relationships, we have obtained two new members of this family of compounds, viz. the solid solutions The asymmetric unit of Na 3 Zn 0.912 Cd 0.088 B 5 O 10 supplemented by additional oxygen atoms to show the full coordination around the disordered M site (M = Zn 0.912 (4) Cd 0.088 (4) ). Displacement ellipsoids are drawn at the 50% probability level. [Symmetry codes: (i) 1 2 À x, 1 2 + y, z; (ii) 3 2 À x, 1 2 + y, z; (iii) 1 À x, 1 2 + y, 1 2 À z.]
The asymmetric unit of Na 3 Zn 0.912 Cd 0.088 B 5 O 10 comprises 19 independent sites, i.e. three Na, one disordered (Zn/Cd), five B, and ten O sites, all occupying general positions. Of the three unique Na sites, Na1 is surrounded by seven O atoms with Na-O distances divided into two sets: a set of five short ones is in the range 2.310 (3)-2.700 (3) Å , while another set includes two longer separations [3.054 (3)-3.059 (3) Å , Table 1]. Bond-valence-sum (BVS) calculations using Brown's formula (Brown & Altermatt, 1985) gave a BVS value of 0.89 valence units (v.u.) for the seven-coordinated Na1 cation, confirming that the long bonds participate in the overall metal coordination sphere. The coordination environment can be described as an irregular polyhedron. Similarly, Na2 and Na3 atoms have also adopted the seven-coordinated irregular polyhedral arrangement. This is different from the situation in monoclinic -Na 3 ZnB 5 O 10 , where three distinct Na sites have coordination numbers of six, seven, and eight, respectively (Chen et al., 2007a). In the Na 3 Zn 0.912 Cd 0.088 B 5 O 10 structure, the Na-O distances fall in the range 2.273 (3)-3.059 (3) Å (average range for the three sites 2.553-2.657 Å ), which is similar to the value reported for the seven-coordinated Na + cation in -Na 3 ZnB 5 O 10 [2.318 (2)-2.859 (3) Å , average 2.531 Å ] (Chen et al., 2007a), and in agreement with the value of 2.50 Å computed from crystal radii sums for seven-coordinated Na + and four-coordinated O 2À ions (Shannon, 1976 (Yuan et al., 2005). Of the boron sites, B3 has a tetrahedral configuration, while other B sites are in triangular configurations. The BO 4 and BO 3 groups are rather regular, with average O-B-O angles being close to 109.5 or 120 , respectively. The B-O bond lengths in the tetrahedron cover the range between 1.467 (4) and 1.472 (4) Å , and those in the triangles between 1.305 (5) and 1.407 (4) Å . The average B-O bond lengths (1.469 Å and 1.366-1.373 Å , respectively) are in good agreement with the data reviewed by Hawthorne et al. (1996) (Shannon, 1976).
Symmetry codes: (i) x þ 1 2 ; Ày þ 1 2 ; Àz; (ii) x þ 1 2 ; y; Àz þ 1 2 ; (iii) Àx þ 3 2 ; y þ 1 2 ; z; (iv) Àx þ 1; y þ 1 2 ; Àz þ 1 2 ; (v) x; Ày þ 1 2 ; z þ 1 2 ; (vi) Àx þ 1 2 ; y þ 1 2 ; z. For Na 3 ZnB 5 O 10 , the present study indicates that a partial replacement of Zn 2+ by Cd 2+ or Mg 2+ is favourable for the formation of the orthorhombic Pbca phase. However, keeping the Na + ions unchanged, the complete replacement of Zn 2+ by larger Cd 2+ ions does not result in the isotypic cadmium analogue. We have attempted to prepare a hypothetical compound with nominal composition 'Na 3 CdB 5 O 10 ' via a standard solid-state synthetic route by mixing stoichiometric amounts of Na 2 CO 3 , CdO, and H 3 BO 3 powders followed by annealing the mixture at a temperature of 873 K in air for several weeks. No 'Na 3 CdB 5 O 10 ' has been obtained, only a mixture of known phases, viz. NaBO 2 and Cd 2 B 2 O 5 , was formed instead, according to powder X-ray diffraction analyses. This indicates that the structural variants in the family of compounds A 3 MB 5 O 10 depend strongly on sizes of A + and M 2+ cations.

Synthesis and crystallization
In a typical synthesis of the cadmium-containing compound, a powder mixture of the starting materials Na 2 B 4 O 7 Á10H 2 O, ZnO, CdO, H 3 BO 3 in the molar ratio Na:Zn:Cd:B = 3:2:1:7 was transferred to a platinum crucible of 40 mm in diameter and 40 mm in height. The sample was melted at 1023 K for one week, then cooled down to 773 K at a rate of 0.5 K h À1 , to 573 K at 1.0 K h À1 , followed by cooling to room temperature at 20 K h À1 . Colourless prismatic crystals were isolated from the solidified melt. Energy-dispersive X-ray analyses (EDX) in a scanning electron microscope confirmed the existence of the heavy elements zinc and cadmium with an approximate atomic ratio of 8.2:1.5, close to the refined composition of the crystal (9.12: 0.88) (see Figs. S1-S2 and Table S1 in the Supporting information). The magnesium-containing compound was prepared in the same way, except that the starting materials were Na 2 B 4 O 7 Á10H 2 O, ZnO, MgO, H 3 BO 3 in the molar ratio Na:Zn:Mg:B = 2:2:1:6. EDX measurements for the Na 3 Zn 0.845 Mg 0.155 B 5 O 10 crystal gave an approximate atomic ratio of Zn:Mg = 4.9:3.8, deviating significantly from the refined composition (8.45:1.55) (see Figs. S3-S4 and Table  S2). This may be due to the fact that the Mg-peak in the EDX spectrum is very close to the main peak of Zn, which leads the calculations of the integrated intensities of the Zn and Mg peaks to be inaccurate, consequently producing an inaccurate Zn/Mg atomic ratio. The powder X-ray diffraction pattern of the ground crystals are in good agreement with those calculated from the single-crystal data.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. Based on the EDX measurements, cadmium and magnesium, respectively, was incorporated in the crystals. In fact, refinements of the occupancies of the zinc sites in the two structures revealed a small incorporation of cadmium and a somewhat higher incorporation of magnesium, respectively. For the final models, the occupancies of the disordered M sites (M = Zn, Cd and Zn, Mg, respectively) were constrained to 1.0, with the same coordinates and displacement parameters for the two types of metals. The refined ratios were Zn 0.912 (4) :Cd 0.088 (4) and Zn 0.845 (5) (Sheldrick, 2008); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); molecular graphics: ATOMS (Dowty, 1999); software used to prepare material for publication: publCIF (Westrip, 2010).

Crystal data
Na 3  Extinction correction: SHELXL2014 (Sheldrick, 2015), Fc * =kFc[1+0.001xFc 2 λ 3 /sin(2θ)] -1/4 Extinction coefficient: 0.00127 (16) 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.