Na 8 [Cr 4 B 12 P 8 O 44 (OH) 4 ][P 2 O 7 ]⋅ n H 2 O: A 3D Borophosphate Framework with Spherical Cages

A chromium borophosphate–phosphate (Na 8 [Cr 4 B 12 P 8 O 44 (OH) 4 ][P 2 O 7 ] ⋅ n H 2 O, 1 ), which has an unusual 3D framework structure, was synthesized under hydrothermal conditions. The framework consists of spherical cages composed of CrO 6 , PO 4 , BO 4 , and BO 3 polyhedra. The cages are located at the vertices and the body center of the cubic cell and are interconnected through 12‐membered‐ring windows along the {111} direction. The actual framework structure is very complex, but the description can be simplified by using the 5‐connected fundamental building cluster [CrP 5 B 3 O 24 ] 11− . In addition, 1 represents the first borate‐rich borophosphate that contains a 3D borophosphate partial framework ( ${{{\hfill 3\atop \hfill \infty }}}$ [B 3 P 2 O 11 (OH)]) in which the fundamental building unit is an oB dreier single ring (Δ4□:□<Δ2□>□). The cavities of the framework are filled with disengaged water molecules and Na + counterions. The former can be reversibly desorbed and reabsorbed and the latter can be exchanged by Li + ions, which results in significant shrinkages of the cell volume.