Quaternary nickel orthophosphates, ANi 4 (PO 4 ) 3 (A=Alkali metals) have been extensively investigated because they show interesting properties such as ionic conductivities, optical, and magnetic behaviors.1-9 They are built up from NiO 6 octahedra and PO 4 tetrahedra. These basic building blocks are connected via common oxygen atoms to form the three-dimensional frameworks and the alkali metal ions reside in the empty space.For NaNi 4 (PO 4 ) 3 , two different structures have been reported and this can be caused by the different reaction conditions. While NaNi 4 (PO 4 ) 3 prepared as single crystals with the use of NaCl flux at 1373 K crystallizes in the space group Amam and show a partial disorder of one of the PO 4 tetrahedra, 3 NaNi 4 (PO 4 ) 3 and KNi 4 (PO 4 ) 3 synthesized with alkali metal nitrate at 1073 K as powders adopt an ordered structure with the space group Pnnm.4 As far as we know, no structural studies of KNi 4 (PO 4 ) 3 from single crystal diffraction data have been reported yet and RbNi 4 (PO 4 ) 3 is a new compound. In this paper we report on the synthesis and structural investigations using single crystals of two nickel orthophosphates, KNi 4 (PO 4 ) 3 and RbNi 4 (PO 4 ) 3 . ExperimentalSynthesis. KNi 4 (PO 4 ) 3 : KNi 4 (PO 4 ) 3 was prepared by the reaction of elements with the use of the reactive halide-flux technique. A combination of the pure elements, Ni powder (Alfa Aesar 99.8%), S powder(Sigma-Aldrich) and P powder (SigmaAldrich 99%) were mixed in a fused silica tube in molar ratio of Ni:P:S=4:5:6 and then KCl (Alfa Aesar 99%) was added. The mass ratio of the reactants and the halide was 1:3. The tube was evacuated to 0.133 Pa, sealed, and heated gradually (30 K/h) to 1023 K, where it was kept for 72 h. The tube was cooled to room temperature at the rate of 6 K/h.RbNi 4 (PO 4 ) 3 : RbNi 4 (PO 4 ) 3 was prepared by the reaction of elements with the use of the reactive halide-flux technique. A combination of the pure elements, Ni powder (Alfa Aesar 99.8%), Se powder (Sigma-Aldrich) and P powder (Sigma-Aldrich 99%) were mixed in a fused silica tube in molar ratio of Ni:P:Se=3:4:8 and then RbCl (Alfa Aesar 99%) was added. The mass ratio of the reactants and the halide was 1:2. The tube was evacuated to 0.133 Pa, sealed, and heated gradually (20 K/h) to 923 K, where it was kept for 72 h. The tube was cooled to room temperature at the rate of 12 K/h.In both cases, the excess halide was removed with distilled water and yellow needle-shaped crystals were obtained. The role of chalcogens in the reactions is not clear but it is helpful to obtain the product as single crystals. The crystals are stable in air and water. A qualitative X-ray fluorescence analysis of the crystals indicated the presence of K or Rb, Ni, and P. The compositions of the compounds were determined by single-crystal X-ray diffraction.Crystallographic Studies. The structures of ANi 4 (PO 4 ) 3 (A=K, Rb) were determined by single crystal X-ray diffraction methods. Preliminary examination and data collection were performed ...

KNi4(PO4)3 (I) and Rb Ni4(PO4)3 (II) are prepared from the elements in a reactive halide flux.