Synthesis and crystal structure of NaCuIn(PO4)2

In the crystal, edge-sharing [CuO5] trigonal bipyramids form dimers that are linked to PO4 tetrahedra via a common edge. The obtained [Cu2P2O12] units are interconnected through vertices to form sheets that are sandwiched between undulating layers resulting from the junction of PO4 tetrahedra and [InO6] octahedra. The two types of layers are alternately stacked along [101] and are joined into a three-dimensional framework through vertex- and edge-sharing, leaving channels parallel to the stacking direction in which the Na+ ions are located.


Chemical context
Transition-metal phosphates have been the subject of intensive research as a result of their interesting physical properties and potential applications in wide-ranging fields such as catalysis, electrochemistry, luminescence (Tie et al., 1995;Pan et al., 2006;Yang et al., 2016) and ion exchange (Cheetham et al., 1999;Han et al., 2015;Manos et al., 2005Manos et al., , 2007Plabst et al., 2009;Stadie et al., 2017). In these materials, the anionic framework is built up from PO 4 tetrahedra linked to different kinds of transition metal (TM) coordination polyhedra of the form [TMO n ] (n = 4, 5 and 6), leading to a large variety of crystal structure families. This structural diversity is mainly associated with the ability of TM cations to adopt different oxidation states in various coordination polyhedra. Based on previous hydrothermal investigations aimed at orthophosphates of general formula (M,M 00 ) 3 (PO 4 ) 2 ÁnH 2 O (M and M 00 = bivalent cations), we have reported on synthesis and characterization of the phosphates Ni 2 Sr(PO 4 ) 2 Á2H 2 O (Assani et al., 2010), Mg 1.65 Cu 1.35 (PO 4 ) 2 ÁH 2 O (Khmiyas et al., 2015) and Mn 2 Zn(PO 4 ) 2 ÁH 2 O (Alhakmi et al., 2015). In this context, the aim of the present study was to develop new phases belonging to the series AM 00 M 000 (PO 4 ) 2 where A, M 00 and M 000 are mono-, bi-and trivalent cations, respectively. As a result, we report here on synthesis and crystal structure of the new compound NaCuIn(PO 4 ) 2 .  (Fig. 1). The P-O bond lengths in the two phosphate tetrahedra are similar and comparable with those of similar phosphates. However, the P1-O distances, varying between 1.5035 (10) and 1.5729 (9) Å , indicate a somewhat higher distortion of this tetrahedron than the P2-O distances [between 1.5297 (9) and 1.5488 (9) Å ] of the other tetrahedron.

Structural commentary
In this phosphate, two [CuO 5 ] triangular bipyramids share one edge to form a [Cu 2 O 8 ] dimer, the ends of which are linked to two P1O 4 tetrahedra by edge-sharing. The obtained [Cu 2 P 2 O 12 ] groups are linked together via the vertices to form sheets extending parallel to (101), as shown in Fig. 2. On the other hand, the [InO 6 ] octahedra and the P2O 4 tetrahedra are interconnected through common vertices to build up an undulating layer extending in the same direction (Fig. 3). The copper phosphate layers are sandwiched between the undulating indium phosphate layers. By sharing corners and edges, an alternating stacking of the layers along [101] leads to a three-dimensional framework structure with channels in which the Na + cations are located (Fig. 4). The four nearest oxygen atoms around the alkali metal cation form a distorted disphenoid with Na-O distances between 2.3213 (12) and 2.4275 (11) Å (Fig. 1).
NaCuIn(PO 4 ) 2 is isotypic with KCuFe(PO 4 ) 2 (Badri et al., 2011), whereby potassium is substituted by sodium and iron by indium. However, we note a significant difference in the coordination number of sodium and potassium in the two structures. Whereas sodium has a fourfold coordination in NaCuIn(PO 4 ) 2 , potassium is surrounded by nine oxygen atoms in KCuFe(PO 4 ) 2 because of its greater ionic radius.
Bond-valence-sum calculations (Brown & Altermatt, 1985) are in good agreement with the expected values (in valence units) for sodium(I), copper(II), indium(III) and the phosphorus(V) cations, viz. Na I = 0.845 (2)    The sodium cations located in channels running parallel to [101] in the crystal structure of NaCuIn(PO 4 ) 2 .

Figure 1
The principal building units in the crystal structure of NaCuIn(PO 4 ) 2 . Displacement ellipsoids are drawn at the 50% probability level.

Database survey
Phosphate-based materials with general formula AM II M 0III (PO 4 ) 2 commonly show crystal structures where channels or, more rarely, layers are formed by the [M II M 0II (PO 4 ) 2 ] À framework to delimit suitable environments to accommodate the A + cations. A recent survey given by Yakubovich et al. (2019) revealed that all compounds of the morphotropic series AM II M 0III (PO 4 ) 2 , where A = Na, K, Rb or NH 4 , M 00 = Cu, Ni, Co, Fe, Zn or Mg and M 000 = Fe, Al or Ga, crystallize in the monoclinic crystal system and can be classified into seven subgroups according to their structure types, viz. In addition, the structures of certain members of this phosphate family are similar to those of the zeolite-ABW structural type (Badri et al., 2014). When the trivalent cation is lanthanum or yttrium, the crystal structures KM II La(PO 4 ) 2 (M II = Mg or Zn) are isotypes of the monazite monoclinic structure of LaPO 4 with space-group type P2 1 /n (Pan et al., 2006;Tie et al., 1995), while KMgY(PO 4 ) 2 turns out to be an isotype of the xenotime structure YPO 4 adopting a tetragonal symmetry with space-group type I4 1 /amd (Tie et al., 1996).

Synthesis and crystallization
Stoichiometric amounts of NaNO 3 , CuO, In 2 O 3 and NH 4 H 2 PO 4 as precursors in the molar ratio 1:1:0.5:2 were ground in an agate mortar and pre-heated at 473 and 673 K in a platinum crucible to eliminate gaseous products. The resulting powder was subsequently heated to a temperature of 1473 K. The product was then cooled to room temperature at a rate of 5 K h À1 . The obtained product contained green single crystals corresponding to the title phosphate.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1.
Labelling of atoms and their coordinates were adapted from isotypic KCuFe(PO 4 ) 2 (Badri et al., 2011). Since not all atoms in the latter description are part of one unit cell, a translation by (z + 1) relative to the original coordinates brings all corresponding atoms inside one unit cell. Moreover, oxygen atoms O11 and O14 were translated by (x À 1 2 , Ày + 1 2 , z À 1 2 ) and (x, y, z À 1), respectively, to be linked directly to P1.

Crystal data
NaCuIn ( 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.