Crystal structure of langbeinite-related Rb0.743K0.845Co0.293Ti1.707(PO4)3

Single crystals of the langbeinite-related phosphate Rb0.743K0.845Co0.293Ti1.707(PO4)3 have been prepared by crystallization of high-temperature self-flux K2O–Rb2O–P2O5–TiO2–CoO.


Chemical context
Nowadays, there are a number of reports on the synthesis and investigation of langbeinite-related complex phosphates, which exhibit interesting properties such as magnetic (Ogorodnyk et al., 2006), luminescence (Zhang et al., 2013;Chawla et al., 2013) and phase transitions (Hikita et al., 1977). It should be noted that compounds with a langbeinite-type structure are prospects for use as a matrix for the storage of nuclear waste (Orlova et al., 2011). Zaripov et al. (2009) and Ogorodnyk et al. (2007a) proved that caesium can be introduced into the cavity of a langbeinite framework that can be used for the immobilization of 137 Cs in an inert matrix for safe disposal.
A large number of compounds with a langbeinite framework based on a variety of different valence elements are known. Three major types of substitutions of the elements are known as well as their combinations. They are: metal substitution in octahedra, element substitution in anion tetrahedra, and substitution of ions in cavities. Among these compounds, potassium-containing langbeinites are the most studied (Ogorodnyk et al., 2006(Ogorodnyk et al., , 2007bNorberg, 2002;Orlova et al., 2003). However, several reports concerning phosphate langbeinites with Rb + in the cavities of the framework are known: Rb 2 FeZr(PO 4 ) 3 (Trubach et al., 2004), Rb 2 YbTi(PO 4 ) 3 (Gustafsson et al., 2005) and Rb 2 TiY(PO 4 ) 3 (Gustafsson et al., 2006).

Structural commentary
The asymmetric unit of Rb 0.743 K 0.845 Co 0.293 Ti 1.707 (PO 4 ) 3 consists of two mixed-occupied (Co/Ti IV ), two (Rb/K), one P and four oxygen positions (Fig. 1). The structure of the title ISSN 2056-9890 compound is built up from mixed (Co/Ti IV )O 6 octahedra and PO 4 tetrahedra, which are connected via common O-atom vertices. Each octahedron is linked to six adjacent tetrahedra and reciprocally, each tetrahedron is connected to four neighboring octahedra into a three-dimensional rigid framework (Fig. 2).
The orthophosphate tetrahedra are also slightly distorted with P-O bond lengths ranging from 1.525 (2) to 1.531 (2) Å . These distances are almost identical to the corresponding ones in K 2 Co 0.5 Ti 1.5 (PO 4 ) 3 [d(P-O) =1.525 (2)-1.529 (9) Å ; Ogorodnyk et al., 2006). A comparison of the corresponding interatomic distances for the octahedra and tetrahedra in Rb 0.743 K 0.845 Co 0.293 Ti 1.707 (PO 4 ) 3 and K 2 Co 0.5 Ti 1.5 (PO 4 ) 3 shows that partial substitution of K + by Rb + and decreasing the amount of cobalt slightly influences the distances in the polyhedra for Rb 0.743 K 0.845 Co 0.293 Ti 1.707 (PO 4 ) 3 .
The corresponding K1-O contacts in K 2 Co 0.5 Ti 1.5 (PO 4 ) 3 (Ogorodnyk et al., 2006) are in the range 2.872 (2)-3.231 (3) Å while the K2-O distances in the K2O 12 polyhedra are in the range 2.855 (2)-3.473 (3) Å , slightly longer than those in Rb 0.743 K 0.845 Co 0.293 Ti 1.707 (PO 4 ) 3 . These results indicate that the substitution of K + cations by Rb + cations in Rb 0.743 K 0.845 Co 0.293 Ti 1.707 (PO 4 ) 3 caused a decrease of the (Rb/K)-O bond length. This fact confirms the rigidity of the framework and the suitability of the cavity dimensions to accommodate different sized ions whose size and nature insignificantly influence the framework.

Synthesis and crystallization
The title compound was prepared during crystallization of a self-flux in the Rb 2 O-K 2 O-P 2 O 5 -TiO 2 -CoO system. The starting components RbH 2 PO 4 (4.0 g), KPO 3 (2.4 g), TiO 2 (0.532 g) and CoO (0.50 g) were ground in an agate mortar, placed in a platinum crucible and H 3 PO 4 (85%, 0.42 g) was added. The mixture was heated up to 1273 K. The melt was kept at this temperature for one h. After that, the temperature was decreased to 873 K at a rate of 10 K h À1 . The crystals of Rb 0.743 K 0.845 Co 0.293 Ti 1.707 (PO 4 ) 3 were separated from the rest flux by washing in hot water. The chemical composition of a single crystal was verified using EDX analysis. The asymmetric unit of Rb 0.743 K 0.845 Co 0.293 Ti 1.707 (PO 4 ) 3 , showing displacement ellipsoids at the 50% probability level.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1. The O-atom sites were determined from difference Fourier maps. It was assumed that both types of alkaline ions occupy cavity sites while the transition metals occupy framework sites. The occupancies were refined using linear combinations of free variables taking into account the total charge of the cell.  (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 1999); software used to prepare material for publication: WinGX (Farrugia, 2012) and enCIFer (Allen et al., 2004).

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.