Single Crystals of CaNa[ReO 4 ] 3 : Serendipitous Formation and Systematic Characterization

In an attempt to crystallize Ce[ReO 4 ] 4 · x H 2 O from aqueous solutions of equimolar amounts of Ce[SO 4 ] 2 and Ba[ReO 4 ] 2 via salt‐metathesis the serendipitous formation of colorless, transparent, rod‐shaped single crystals of CaNa[ReO 4 ] 3 was observed as a result of calcium and sodium impurities within the improperly deionized water used. Structure analysis by X‐ray diffraction lead to the conclusion that the title compound crystallizes in the ThCd[MoO 4 ] 3 structure type with the hexagonal space group P 6 3 / m and the lattice parameters a = 991.74(6) pm, c = 636.53(4) pm, c/a = 0.642 for Z = 2. The crystal structure contains purely oxygen surrounded and crystallographically unique cations, namely Ca 2+ in tricapped trigonal prismatic ( d (Ca–O) = 6 × 249 pm + 3 × 254 pm), Na + in octahedral ( d (Na–O) = 6 × 241 pm), and Re 7+ in tetrahedral coordination ( d (Re–O) = 171–173 pm). Furthermore, it was possible to yield an almost phase‐pure microcrystalline powder of the title compound from a melt of equimolar amounts of Na[ReO 4 ] and Ca[ReO 4 ] 2 stemming from aquatically obtained precursors.