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 P6̅2m, 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) × 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.