Synthesis and crystal structure of a new magnesium phosphate Na3RbMg7(PO4)6

Trisodium rubidium heptamagnesium hexakis(orthophosphate) exhibits a new structure type, with MgOx (x = 5 and 6) polyhedra linked directly to each other through common corners or edges and reinforced by corner-sharing with PO4 tetrahedra. The resulting anionic three-dimensional framework leads to the formation of channels in which the Na+ cations are located, while the Rb+ cations are located in large interstitial cavities.


Chemical context
Magnesium phosphates are of increasing interest because of their potential applications as host materials for optically active rare earth ions (Seo, 2013;Kim et al., 2013;Boukhris et al., 2015). Moreover, these materials are very attractive in terms of basic research because they exhibit a rich structural chemistry due to their polymorphism (Ait Benhamou et al., 2010;Orlova et al., 2015).
Among the variety of magnesium monophosphates synthesized and characterized up to now, only four compounds belong to the system Na 3 PO 4 -Mg 3 (PO 4 ) 2 , namely NaMgPO 4 , NaMg 4 (PO 4 ) 3 , Na 2 Mg 5 (PO 4 ) 4 and Na 4 Mg(PO 4 ) 2 (Imura & Kawahara, 1997;Ben Amara et al., 1983;Yamakawa et al., 1994;Ghorbel et al., 1974). NaMgPO 4 compound crystallizes in the orthorhombic system with space group P2 1 2 1 2 1 . Its structure involves MgO 6 and MgO 5 polyhedra linked by the monophosphate groups that form a three-dimensional framework. NaMg 4 (PO 4 ) 3 is also orthorhombic, space group Pnma. Its structure is built up from three kinds of MgO 5 units sharing edges and corners and linked to each other by the PO 4 tetrahedra, leading to a three-dimensional framework. Na 2 Mg 5 (PO 4 ) 4 , synthesized under pressure, crystallizes in the triclinic system. Its structure results from a three-dimensional framework of MgO 6 and MgO 5 polyhedra connected either directly via common corners or by means of the phosphate groups. Na 4 Mg(PO 4 ) 2 exhibits two polymorphs, which were only identified by their powder diffraction patterns.
Starting from these compounds, suitable replacements of magnesium and/or sodium by large cations induces their transformation into several structural types for different Mg/P atomic ratios. NaMMg(PO 4 ) 2 (M = Ca, Sr and Ba) compounds are related to the glaserite-type structure (Alkemper & Fuess, 1998;Boukhris et al., 2012Boukhris et al., , 2013. They adopt an anionic twodimensional network with different symmetries as a function of the size of the M 2+ cation. For an atomic ratio M:P of 7:6, ISSN 2056-9890 magnesium phosphate compounds adopt a three-dimensional network related to the fillowite-type structure, as observed in Na 4 Ca 4 Mg 21 (PO 4 ) 18 , Na 2 CaMg 7 (PO 4 ) 6 and Na 2.5 Y 0.5 Mg 7 -(PO 4 ) 6 (Domanskii et al., 1982;McCoy et al., 1994;Jerbi et al., 2010a). All of them crystallize with trigonal symmetry (space group R3) and differ only by their cationic distributions. Three-dimensional anionic networks includes also original structures such as those observed in Na 18 Ca 13 Mg 5 (PO 4 ) 18 , NaCa 9 Mg(PO 4 ) 7 , Na 7 LnMg 13 (PO 4 ) 12 (Ln = La, Eu, Nd) (Yamakawa et al., 1994;Morozov et al., 1997;Jerbi et al., 2010bJerbi et al., , 2012. As a contribution to the investigation of the abovementioned systems, we report here the structural characterization of a new magnesium phosphate Na 3 RbMg 7 (PO 4 ) 6 , which is, to our knowledge, the first magnesium phosphate revealing an original structure for an atomic ratio Mg/P equal to 7/6.

Structural commentary
To the best of our knowledge, Na 3 RbMg 7 (PO 4 ) 6 exhibits a new structure type. A projection along the [010] direction of its structure ( Fig. 1) clearly evidences the three-dimensional character of its anionic framework, which is built up from five different polyhedra MgO x (x = 5, 6) and three kinds of PO 4 tetrahedra connected together by sharing edges and corners. The Na + cations are located within channels running along the [010] direction while the Rb + cations are found in the large interstitial cavities.
A projection of the structure on the (012) plane ( Fig. 2) shows that it can also be described on the basis of three kinds of rows (A, B and C) running parallel to the [100] direction. The first row (A; Fig. 3  A view of parallel rows of ABC polyhedra.
There are five distinct Mg sites. The Mg1 atom is displaced slightly from the inversion center, statistically occupying two symmetry-related positions. As a consequence, the Mg1O 6 polyhedron exhibits two distances that are long [2.241 (5) Å ] compared to the other Mg1-O distances, which vary from 1.969 (10) to 2.030 (10) Å . Thus, this environment can be considered as [4 + 1]. The average value of 2.005 (10) Å calculated from the four short distances is slightly higher but consistent with that of 1.930 (2) Å reported for the tetracoordinated Mg 2+ cation in KMgPO 4 (Wallez et al.,1998). Sites Mg2 and Mg3 are located on twofold rotation axes and have slightly distorted octahedral environments with Mg-O distances varying from 2.052 (3) to 2.202 (2) Å for Mg2 and from 2.042 (2) to 2.169 (2) Å for Mg3. The corresponding average values of 2.123 and 2.103 Å , respectively, are in a good agreement with that of 2.14 Å observed for hexa-coordinated Mg 2+ ions in Mg 3 (PO 4 ) 2 (Jaulmes et al., 1997). Site Mg4 is [5 + 1]-coordinated, with five short distances varying from 1.981 (3) to 2.050 (3) Å and a sixth longer distance of 2.5734 (3) Å . A similar environment has already been observed in Mg 3 (PO 4 ) 2 (Jaulmes et al., 1997). Site Mg5 is fivecoordinated with Mg-O distances ranging from 2.020 (3) to 2.148 (3) Å . The corresponding mean distance of 2.07 Å is close to that of 2.08 Å observed for Mg 2+ with the same coordination in NaMg 4 (PO 4 ) 3 (Ben Amara et al., 1983). The P-O distances within the PO 4 tetrahedra are in the range of 1.518 (2)-1.552 (2) Å with an overall mean value of 1.539 Å , very close to that of 1.537 Å predicted by Baur (1974) for monophosphate groups.
The environments of the alkali cations are shown in Fig. 4. Those of the two crystallographic distinct Na sites were determined assuming a maximum sodium-oxygen distance L max of 3.13 Å , as suggested by Donnay & Allmann (1970). As in the case of the Mg1 atom, the sodium atom Na1 is also moved slightly away from the inversion center and statistically occupying two symmetry-related positions. This moving probably occurs to accommodate the environment of the Na1 site, which then consists of eight oxygen atoms with Na-O distances varying from 2.303 (7) to 2.963 (6) Å . Na2 is bound to only six oxygen atoms, with Na2-O distances in the range 2.246 (3)-2.962 (3) Å . The Rb + ion is located on a twofold rotation axis and occupies a single site whose environment was determined assuming all Rb-O distances to be shorter than the shortest distance between Rb + and its nearest cation. This environment then consists of twelve oxygen atoms with Rb-O distances ranging from 2.923 (3) to 3.517 (2) Å .

Synthesis and crystallization
Single crystals of Na 3 RbMg 7 (PO 4 ) 6 were grown in a flux of sodium molybdate, Na 2 MoO 4 , with a P:Mo atomic ratio of 2:1. Appropriate amounts of the starting reactants (NH 4 )H 2 PO 4 , Na 2 CO 3 , Rb 2 CO 3 , (MgCO 3 ) 4 Mg(OH) 2 Á5H 2 O and Na 2 MoO 4 Á-2H 2 O were dissolved in nitric acid and the obtained solution was evaporated to dryness. The residue was homogenized by grinding in an agate mortar, and subsequently heated in a platinum crucible for 24 h at 673 K and then for 12 h at 873 K. After being reground, the sample was melted for 2 h at 1273 K The environment of the (a) Na1 + , (b) Na2 + and (c) Rb + cations, showing displacement ellipsoids drawn at the 50% probability level. and then cooled slowly down to room temperature at a rate of 10 K h À1 . The solidified melt was washed with boiling water to dissolve the flux. Colourless, irregularly shaped crystals were extracted from the final product.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1 program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); molecular graphics: DIAMOND (Brandenburg et al., 1999); software used to prepare material for publication: WinGX (Farrugia, 2012).

Trisodium rubidium heptamagnesium hexakis(orthophosphate)
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.