Experimental characterization and first principles calculations of linear and nonlinear optical properties of two orthophosphates A3Al2(PO4)3 (A = Rb, K)

Abstract

Two alkali metal orthophosphates, A₃Al₂(PO₄)₃ (A = Rb, K), have been synthesized through a high temperature solution method. Rb₃Al₂(PO₄)₃ and K₃Al₂(PO₄)₃ crystallize in different space groups Cmc2₁ and Pna2₁, respectively, and they both feature an [Al₂P₃O₁₂]³⁻ anionic framework composed of isolated PO₄ and AlO₄ tetrahedra. Their ultraviolet (UV) cut-off edges are both below 240 nm. The electron densities of states (DOS) calculations suggest that the interaction between oxygen and rubidium/potassium determines their band gaps and the nonbonding O 2p orbitals limit their UV cut-off edges towards the deep UV region. Rb₃Al₂(PO₄)₃ and K₃Al₂(PO₄)₃ exhibit second harmonic generation (SHG) responses of 0.6× and 0.4× KH₂PO₄ (KDP), respectively. The origin of the SHG effect was studied by SHG density calculations. From the dipole moment calculations and structure comparison, it is indicated that the arrangements of P–O and Rb–O/K–O groups play significant roles in the SHG responses of A₃Al₂(PO₄)₃ (A = Rb, K). Our results indicate that suitable arrangements of the units in phosphates may be essential to achieve large SHG responses.