Unique 3D framework formed by adding M${}^{II}$O${}_4$ groups into high Sb/P ratio phosphatoantimonates

Explorations on a mixed metal phosphatoantimonate system led to the discovery of two new anhydrous phosphatoantimonates, namely, Cs${}_4$MSb${}_6$P${}_4$O${}_{28}$ (M=Mg, Zn), which represent the first examples of quinary A${}^I$ – M${}^{II}$ –Sb${}^V$ – P${}^V$– O systems. Single-crystal structure analysis reveals that the two title compounds crystallize in the tetragonal space group I4${}_1$/a (No. 88). Their structures feature complicated 3D frameworks with interesting tunnel structures comprised of corner sharing MO${}_4$ tetrahedra, SbO${}_6$ octahedra and PO${}_4$ tetrahedra, with the Cs${}^{+}$ cations sitting in the tunnels to balance the valence. Optical reflectance spectrum measurements show that these two compounds are insulators with band gaps of about 4.5 eV.