Na1.85Mg1.85In1.15(PO4)3 and Ag1.69Mg1.69In1.31(PO4)3 with alluaudite-type structures

The two orthophosphates, Na1.85Mg1.85In1.15(PO4)3 and Ag1.69Mg1.69In1.31(PO4)3 adopt alluaudite-type structures. Edge-sharing [(In,Mg)O6] octahedra are linked together by PO4 tetrahedra, leaving channels in which the Na+ or Ag+ cations are situated..


Chemical context
The crystal structure of the mineral alluaudite was determined by Moore (1971). Since then, many new members of this structure type, including phosphates, arsenates, molybdates, sulfates and, more recently, vanadates have been synthesized and structurally characterized. The growing interest in these kinds of materials is related to their interesting physical properties, in particular in electrochemistry and battery research. For example, the phosphate Na 2 Ni 2 Fe(PO 4 ) 3 (Essehli et al., 2015) is a promising cathode in sodium batteries since its electrochemical behaviour is comparable to that of LiFePO 4 . In this context, alluaudite-type phosphates such as Na 1.67 Zn 1.67 Fe 1.33 (PO 4 ) 3 (Khmiyas et al., 2015), Ag 1.655 Co 1.647-Fe 1.352 (PO 4 ) 3 (Bouraima et al., 2017) and the vanadate (Na 0.7 )(Na 0.70 , Mn 0.30 ) (Fe 3+ , Fe 2+ ) 2 Fe 2+ (VO 4 ) 3 (Benhsina et al., 2016) have been investigated by our group.
In the present work, the synthesis and structure determination of two new magnesium-based alluaudite-type phosphates with composition Na 1.85 Mg 1.85 In 1.15 (PO 4 ) 3 (I) and Ag 1.69 Mg 1.69 In 1.31 (PO 4 ) 3 (II) are reported.

Structural commentary
In the crystal structures of the two isotypic phosphates (I) and (II), site Na1 (Ag1) shows full occupancy and is located on an inversion centre (Wyckoff position 4b), and one mixed-occupied (Mg/In)2 site [occupancy ratio Mg:In = 0.51:0.49 for (I) and 0.314:0.686 for (II)], the second partially occupied Na2 (Ag2) site [occupancy 0.848 (9) for (I) and 0.6988 for (II)] and the P1 site are located on twofold rotation axes (4e) of space group type C2/c. There is another mixed-occupancy (Mg,In)1 site in a general position (8f) with occupancy ratios Mg:In = 0.68:0.32 for (I) and 0.687 (2) (Fig. 3). Adjacent chains are linked together by P1O 4 and P2O 4 tetrahedra into (010) sheets, as shown in Fig. 4. Neighbouring sheets are finally fused into a three-dimensional framework structure by P1O 4 tetrahedra. This framework delimits two types of hexagonal channels oriented parallel to [001], in which the Na + (for (I) or Ag + (for (II) cations are located (Fig. 5). The Na-O distances fall in the range 2.307 (2)-2.960 (2) Å with coordination numbers of six for Na1 and eight for Na2, while those for Ag-O vary between 2.345 (2) and 2.963 (2) Å , with coordination numbers of six for both Ag + cations.

Database Survey
The presence of disordered alkali metal or other cations in the channels of alluaudite-type structures is a concomitant feature  Edge-sharing [(Mg/In)2O 6 ] octahedra and [(Mg/In)1) 2 O 10 ] dimers forming an infinite chain extending parallel to [001]. Data taken from (I).

Figure 1
The principal building units in the structure of Na 1.85 Mg 1.85 In 1.15 (PO 4 ) 3 , (I). Displacement ellipsoids are drawn at the 50% probability level.

Figure 2
The principal building units in the structure of Ag 1.69 Mg 1.69 In 1.31 (PO 4 ) 3 , (II). Displacement ellipsoids are drawn at the 50% probability level. Symmetry codes are as in Fig. 1.

Synthesis and crystallization
Single crystals of (I) and (II) were grown by solid-state reactions. The starting mixtures comprising of Mg(NO 3 ) 2 Á6H 2 O (Sigma-Aldrich, 97%), InI 3 (Ventron, 99%), NH 4 H 2 PO 4 (Alfa Aesar, 98%), ANO 3 (A = Na or Ag) (NaNO 3 : Acros Organics, 99%; AgNO 3 : Sigma-Aldrich, 99%) were weighted in molar ratios A:Zn:In:P = 2:2:1:3 and placed in a platinum cruicible. After intermediate grinding and temperature treatments at 573, 673, 773 and 873 K in a platinum crucible, both mixtures were heated at 1373 K above the melting temperatures. The cruicibles were then cooled slowly to 1093 K at a rate of 5 K h À1 , followed by cooling to room temperature after switching off the furnace. Transparent, colourless crystals with a blocky form were isolated from the two final products. The bulk products were not checked for phase purity.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1. In the initial stages of the refinements the occupancies of the disordered sodium (Na2) or silver (Ag2) sites were refined freely and the mixed-occupancy (Mg/In) sites were refined under consideration of full occu-  (Farrugia, 2012), DIAMOND (Brandenburg, 2006) and publCIF (Westrip, 2010).

Figure 5
Polyhedral representation of the crystal structure of (I) showing Na + cations situated in the two types of channels parallel to [001].
pancy for each of these sites. The obtained occupancy rates of Mg:In were rounded and subsequently fixed for chargeneutrality of the compounds. The maximum and minimum electron densities are located 0.55 Å from Mg2 and 0.38 Å from P1 for (I) and 0.78 and 0.59 Å , respectively, from Ag2 for (II).  (Farrugia, 2012) and DIAMOND (Brandenburg, 2006); software used to prepare material for publication: publCIF (Westrip, 2010).

Sodium magnesium indium(III) tris(orthophosphate) (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.

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.