The monoclinic form of trilithium dichromium(III) tris(orthophosphate)

The monoclinic form of trilithium dichromium(III) tris(orthophosphate), Li3Cr2(PO4)3, was prepared by the reactive halide flux method. The structure of the title compound is composed of a three-dimensional anionic framework with composition ∞ 3[Cr2(PO4)3]3− and Li+ ions situated in the empty channels. The rigid framework built up from CrO6 octahedra and PO4 tetrahedra is the same as that found in other monoclinic Li3 M 2(PO4)3 (M = Fe, Sc, V) phases. The three Li+ cations of Li3Cr2(PO4)3 are unequally disordered over six crystallographically different sites. The classical charge balance of the title compound can be represented as [Li+]3[Cr3+]2[P5+]3[O2−]12. Solid-state UV/Vis spectra indicate that the crystal filed splitting (Δ0) of the Cr3+ ion is around 2.22 eV.


Comment
The structures of trilithium dimetal tris(orthophosphates), Li 3 M 2 (PO 4 ) 3 (M = Fe, Sc, Cr, V) have been widely investigated due to their ion transport properties and temperature-dependent phase transitions (d′Yvoire et al., 1983;Verin et al., 1985;Maksimov et al., 1986). For Li 3 Cr 2 (PO 4 ) 3 , the structure of the orthorhombic phase has been studied based on singlecrystal diffraction data (Genkina et al., 1991). The monoclinic and rhombohedral phases have been identified by powder diffraction techniques but no detailed structure determinations have been reported yet (d′Yvoire et al., 1983). In attempts to prepare new mixed alkali metal phosphates by using various alkali metal halides, the monoclinic form of Li 3 Cr 2 (PO 4 ) 3 has been isolated as single crystals and the detailed structural characterization of this phase is reported here.
The Li + ions in the empty channels are surrounded by four O atoms in distorted tetrahedral coordinations. There are six crystallographically independent Li sites for this phase and three Li + ions are unequally disordered over them. It has been reported that the positions of Li + ions in Li 3 V 2 (PO 4 ) 3 can vary depending on the synthetic conditions while those of the V, P, and O atoms comprising the rigid framework remain intact (Yang et al., 2010). The positions of the Li1, Li3, and Li4 sites in this work are very close to those of the ordered Li sites found in Li 3 V 2 (PO 4 ) 3 (Kee & Yun, 2013).
The classical charge balance of the title compound can be represented as [Li + ] 3 [Cr 3+ ] 2 [P 5+ ] 3 [O 2-] 12 . Solid-state UV/Vis spectra indicate that the crystal filed splitting(Δ 0 ) of the Cr 3+ ion is around 2.22 eV, which is in agreement with the green color of the crystals.

Experimental
The title compound, Li 3 Cr 2 (PO 4 ) 3 , was prepared by the reaction of the elements with the use of the reactive halide-flux technique. A combination of the pure elements, Cr powder (Cerac, 99.95%) and P powder (Aldrich, 99%), were mixed in a fused silica tube in a molar ratio of Cr:P = 1:1 and then LiCl (Cerac, 99.8%) and CsCl (Alfa, 99.9%) mixed in molar ratio of LiCl:CsCl = 4:1 were added. The mass ratio of the reactants and the halides was 1:3. The tube was evacuated to 0.133 Pa, sealed, and heated gradually (30 K/h) to 1123 K, where it was kept for 48 h. The tube was cooled to room temperature at a rate of 4 K/h. The excess halide was removed with water and greenish block-shaped crystals were obtained. The crystals are stable in air and water. A qualitative X-ray fluorescence analysis of selected crystals indicated the presence of Cr and P. The final composition of the compound was determined by single-crystal X-ray diffraction.

Refinement
After the positions of heavy elements (Cr, P, and O) had been established, six significant residual peaks suitable for Li + sites were revealed by difference Fourier maps. A model including disorder for these sites was applied. The sum of the Li + occupancies was fixed to 3 to meet the charge balance of the compound; the temperature factors of all Li sites were refined isotropically. The highest peak (0.71 e Å -3 ) and the deepest hole (-0.49 e Å -3 ) are 0.57 Å and 0.81 Å from atom Li4 and Cr1, respectively.   The polyhedral representation of the anionic framework structure built up from [Cr 2 (PO 4 ) 3 ] units. The disordered Li + cations are located in the channels of this framework.