M2Al2Ti(PO4)4 (M = Rb, Cs): synthesis and optical property investigation of two titanium phosphates with a dissimilar structural configuration

Abstract

Inorganic transition metal phosphates are favored materials in various optoelectronic fields such as photoluminescence, optical catalysis, and nonlinear optics. Therefore, further exploration of new transition metal phosphates and study of the impact of transition metal atoms on the structure and properties of the compound are persistent scientific goals. Herein, two new Ti-containing open framework aluminium phosphates, M₂Al₂Ti(PO₄)₄ (M = Rb, Cs), were synthesized through a high-temperature flux method. Both structures are orthophosphates containing isolated PO₄, AlO₄, and TiO₆, yet they exhibit dissimilar overall structural configurations. Rb₂Al₂Ti(PO₄)₄ features one-dimensional (1D) [Al–P–O]_∞ chains that are further bridged by TiO₆ to form 2D layers, whereas Cs₂Al₂Ti(PO₄)₄ displays 2D [Al–P–O]_∞ layers that are further bridged by TiO₆ to form a 3D network with large channels. In addition, detailed experimental characterizations and theoretical computations were carried out, and it was determined that both compounds exhibit UV cutoff edges down to 300 nm based on the UV-Vis diffuse reflectance test. Based on calculations, the birefringence Δn = 0.033 for Cs₂Al₂Ti(PO₄)₄ at 1064 nm.