Structural characterisation and optical properties of new alkaline earth phosphate CaMg(P₄O₁₂) and alkaline alumophosphate Cs₃Al₄(PO₄)₅

Two new family members of mixed alkali-earth metal phosphate and aluminophosphate CaMg(P₄O₁₂) and Cs₃Al₄(PO₄)₅ were prepared from a phosphate system using a high-temperature solution method. The structural analysis results show that two compounds crystallize in the monoclinic space group C2/c and P2₁/n and feature a three-dimensional (3D) network. The 3D structure of CaMg(P₄O₁₂) consists of [CaO₆], [MgO₆] octahedra and [P₄O₁₂] rings, in which the [MgO₆] and [P₄O₁₂] rings link to form a 3D structure and Ca²⁺ cations are filled within the structure. Interestingly, for compound Cs₃Al₄(PO₄)₅, its structure features 4, 8, and 12-ring channels with [Al₂O₄O₄P₂O₄], [Al₄O₈O₈P₄O₈] and [Al₆O₁₂O₁₂P₆O₁₂] units as BBUs, respectively; the Cs⁺ cations are located in the cavities. Furthermore, IR spectral analysis and thermal properties are discussed. UV-vis-NIR diffuse reflectance spectroscopy data show that the UV cutoff edges of CaMg(P₄O₁₂) are below 200 nm. Remarkably, in order to determine optical properties and the structure-properties relationship, theoretical calculations were adopted. Electronic structure calculations demonstrate that CaMg(P₄O₁₂) has an indirect band gap with the value of 5.86 eV, and Cs₃Al₄(PO₄)₅ has a direct band gap of 5.21 eV.