Structural characterisation and optical properties of new alkaline earth phosphate CaMg(P 4 O 12 ) and alkaline alumophosphate Cs 3 Al 4 (PO 4 ) 5 .

Two new family members of mixed alkali-earth metal phosphate and aluminophosphate CaMg(P 4 O 12 ) and Cs 3 Al 4 (PO 4 ) 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 C 2/ c and P 2 1 / n and feature a three-dimensional (3D) network. The 3D structure of CaMg(P 4 O 12 ) consists of [CaO 6 ], [MgO 6 ] octahedra and [P 4 O 12 ] rings, in which the [MgO 6 ] and [P 4 O 12 ] rings link to form a 3D structure and Ca 2+ cations are filled within the structure. Interestingly, for compound Cs 3 Al 4 (PO 4 ) 5 , its structure features 4, 8, and 12-ring channels with [Al 2 O 4 O 4 P 2 O 4 ], [Al 4 O 8 O 8 P 4 O 8 ] and [Al 6 O 12 O 12 P 6 O 12 ] 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(P 4 O 12 ) 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(P 4 O 12 ) has an indirect band gap with the value of 5.86 eV, and Cs 3 Al 4 (PO 4 ) 5 has a direct band gap of 5.21 eV.