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

ABSTRACT

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.