Single Crystals of CaNa[ReO₄]₃: Serendipitous Formation and Systematic Characterization

Abstract

In an attempt to crystallize Ce[ReO4]4·xH2O from aqueous solutions of equimolar amounts of Ce[SO4]2 and Ba[ReO4]2 via salt‐metathesis the serendipitous formation of colorless, transparent, rod‐shaped single crystals of CaNa[ReO4]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[MoO4]3 structure type with the hexagonal space group P63/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 Ca2+ in tricapped trigonal prismatic (d(Ca–O) = 6 × 249 pm + 3 × 254 pm), Na+ in octahedral (d(Na–O) = 6 × 241 pm), and Re7+ 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[ReO4] and Ca[ReO4]2 stemming from aquatically obtained precursors.

On the Thermal Dimorphy of the Strontium Perrhenate Sr[ReO4]2

“…However, the black color of the sample might indicate the presence of a mixed valency between Re(VII) and Re(VI), since it is literature‐known that anhydrous rhenium compounds with rhenium in high oxidation states like +7 tend to be partially reduced under the release of oxygen at high temperatures in atmospheres with a lack of oxygen [1,18] . A similar observation was also made previously for the mixed cationic meta ‐perrhenate CaNa[ReO 4 ] 3 [11] and for La[ReO 4 ] 3 ⋅ x H 2 O turning into La 9 [ReO 6 ] 5 upon dehydration and deoxygenation [19]…”

“…Since no order in the partial occupation of the (Sr2) 2+ sites was observed upon refinement in the non‐centrosymmetric space group P 6 3 , a structure model featuring the space group P 6 3 / m was chosen. With this decision, we stand in good tradition with other CdTh[MoO 4 ] 3 ‐type compounds containing [ReO 4 ] − anions, such as CaNa[ReO 4 ] 3 [11] and AM [ReO 4 ] 3 ( A =Na, M =Pb; A =Na, Ag, K; M =Sr) [12] . Solution and subsequent refinement of the crystal structure was performed using the S helx 1997 program package [23] .…”

“…In this case, owing to the resulting short Sr 2 + • • • Sr 2 + distances of 323 pm, a clearly visible deviation of the (Sr1) 2 + cations from the center of the octahedra would be expected. Therefore, the molecular formula of h-Sr[ReO 4 ] 2 can also be written as (Sr2)(Sr1) 0.5 & 0.5 [ReO 4 ] 3 , emphasizing the relationship with the other already known meta-perrhenates of the CdTh[MoO 4 ] 3 -type structure, such as SE 0.75 Na 0.75 [ReO 4 ] 3 (SE = La and Nd), [10] CaNa[ReO 4 ] 3 [11] or the series of most recently published examples AM[ReO 4 ] 3 (A = Na, M = Pb; A = Na, Ag, K; M = Sr). [12] Due to the rather low quality of the single crystals, the obtained data set is not of high quality as well.…”

Sr[ReO4]2: The First Single Crystals of an Anhydrous Alkaline‐Earth Metal Meta‐Perrhenate

“…Sr[ReO 4 ] 2 , the first anhydrous alkaline-earth metal metaperrhenate investigated by means of single-crystal X-ray diffraction, crystallizes in the monoclinic space group P2 1 /n with the lattice parameters a = 627.31(4) pm, b = 1004.56 (7) pm, c = 1271.25(9) pm and β = 97.118(3)°for Z = 4 ( Table 1 and Table 2) matching well, when compared to the unit-cell dimensions obtained from powder X-ray diffraction data in 1992 (a = 628.8(2) pm, b = 1006.4(3) pm, c = 1272.9(3) pm and β = 97.06(2)°) [10] . Unique Sr 2+ cations on 4e sites are surrounded by eight crystallographically different O 2- A view at the Madelung Part of the Lattice Energy (MA-PLE) [11][12][13] shows just a small difference between Sr[…”

“…Even though there are recent reports on anhydrous alkali metal meta ‐perrhenates A [ReO 4 ] ( A = Na – Cs), [ 6 ] no structural data on anhydrous alkaline‐earth metal meta ‐perrhenates have been reported up to now with the exception of some mixed cationic ones, like for example CaNa[ReO 4 ] 3 . [ 7 ] However, Jeitschko et al did excellent work on ortho ‐perrhenates (e.g. Sr 5 [ReO 6 ] 2 [ 8 ] ) and mixed anionic meso ‐perrhenates, such as Sr 5 X [ReO 5 ] 3 ( X = Cl, Br and I) [ 9 ] in the past.…”