Flux-growth method for the targeted synthesis of the salt-inclusion copper(II) phosphate Rb₉Na₂Cu₆(P₂O₇)₄Cl₇

There is a great deal of inter-est in the synthesis and structural features of a newly emerged class of salt-inclusion solids (SISs), which are made of a hybrid framework exhibiting integrated covalent (metal oxides) and ionic (metal halides) structural units. In general, we explored the conditions under which desired phase formation occurs and, for this study, we selected one of the com-pounds in the series of salt-templated phosphates and arsenates, A ₂ M ₃(X ₂O₇)₂·(salt) [where A = K, Rb, Cs; M = Mn, Cu; X = P, As] commonly known as CU-2 materials, which crystallize in two different space groups (I4/mcm and P4/nbm) due to the variation of the structural units. In our attempt to incorporate mixed Rb⁺/Na⁺ cations along with Cl^- anions into the negatively charged framework, Cu₃(P₂O₇)₂ ^2-, the title com-pound, Rb₉Na₂Cu₆(P₂O₇)₄Cl₇, nona-rubidium disodium hexa-copper(II) tetra-kis-(pyrophosphate) hepta-chloride, has been synthesized by employing a high-tem-per-a-ture RbCl/NaCl eutectic flux and structurally characterized by single-crystal X-ray diffraction, revealing I4/mcm as the space group. In the title com-pound, the same negatively charged framework, Cu₃(P₂O₇)₂ ^2-, characteristic of the CU-2 materials, is retained, while the extended salt-like part of the structure, com-posed of mixed alkali metal chloride fragments, is different from the known CU-2 materials, and includes rarely seen corner-sharing {Na₄Cl₈} units.