Mixed-metal carbonate fluorides as deep-ultraviolet nonlinear optical materials

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: KMgCO₃F and Cs₉Mg₆(CO₃)₈F₅. Both materials are noncentrosymmetric (NCS). KMgCO₃F crystallizes in the achiral and nonpolar NCS space group P62m, whereas Cs₉Mg₆(CO₃)₈F₅ is found in the polar space group Pmn2₁ The compounds have three-dimensional structures built up from corner-shared magnesium oxyfluoride and magnesium oxide octahedra. KMgCO₃F (Cs₉Mg₆(CO₃)₈F₅) exhibits second-order harmonic generation (SHG) at both 1064 and 532 nm incident radiation with efficiencies of 120 (20) × α-SiO₂ and 0.33 (0.10) X β-BaB₂O₄, respectively. In addition, short absorption edges of <200 and 208 nm for KMgCO₃F and Cs₉Mg₆(CO₃)₈F₅, 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 AMCO₃F (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.