Mixed-Metal Carbonate Fluorides as Deep-Ultraviolet Nonlinear Optical Materials

Abstract

Noncentrosymmetric mixed-metal carbonate fluorides are promising materials for deep-ultraviolet (DUV) nonlinear optical (NLO) applications. We report on the synthesis, characterization, structure–property relationships, and electronic structure calculations on two new DUV NLO materials: KMgCO 3 F and Cs 9 Mg 6 (CO 3 ) 8 F 5 . Both materials are noncentrosymmetric (NCS). KMgCO 3 F crystallizes in the achiral and nonpolar NCS space group P 6̅2 m, whereas Cs 9 Mg 6 (CO 3 ) 8 F 5 is found in the polar space group Pmn 2 1 . The compounds have three-dimensional structures built up from corner-shared magnesium oxyfluoride and magnesium oxide octahedra. KMgCO 3 F (Cs 9 Mg 6 (CO 3 ) 8 F 5 ) exhibits second-order harmonic generation (SHG) at both 1064 and 532 nm incident radiation with efficiencies of 120 (20) × α-SiO 2 and 0.33 (0.10) × β-BaB 2 O 4, respectively. In addition, short absorption edges of <200 and 208 nm for KMgCO 3 F and Cs 9 Mg 6 (CO 3 ) 8 F 5, respectively, are observed. We compute the electron localization function and density of states of these two compounds using first-principles density functional theory, and show that the different NLO responses arise from differences in the denticity and alignment of the anionic carbonate units. Finally, an examination of the known SHG active AMCO 3 F (A = alkali metal, M = alkaline earth metal, Zn, Cd, or Pb) materials indicates that, on average, smaller A cations and larger M cations result in increased SHG efficiencies.