Role of Acentric Displacements on the Crystal Structure and Second-Harmonic Generating Properties of RbPbCO(3)F and CsPbCO(3)F

[Image: see text] Two lead fluorocarbonates, RbPbCO(3)F and CsPbCO(3)F, were synthesized and characterized. The materials were synthesized through solvothermal and conventional solid-state techniques. RbPbCO(3)F and CsPbCO(3)F were structurally characterized by single-crystal X-ray diffraction and exhibit three-dimensional (3D) crystal structures consisting of corner-shared PbO(6)F(2) polyhedra. For RbPbCO(3)F, infrared and ultraviolet–visible spectroscopy and thermogravimetric and differential thermal analysis measurements were performed. RbPbCO(3)F is a new noncentrosymmetric material and crystallizes in the achiral and nonpolar space group P6̅m2 (crystal class 6̅m2). Powder second-harmonic generation (SHG) measurements on RbPbCO(3)F and CsPbCO(3)F using 1064 nm radiation revealed an SHG efficiency of approximately 250 and 300 × α-SiO(2), respectively. Charge constants d(33) of approximately 72 and 94 pm/V were obtained for RbPbCO(3)F and CsPbCO(3)F, respectively, through converse piezoelectric measurements. Electronic structure calculations indicate that the nonlinear optical response originates from the distorted PbO(6)F(2) polyhedra, because of the even–odd parity mixing of the O 2p states with the nearly spherically symmetric 6s electrons of Pb(2+). The degree of inversion symmetry breaking is quantified using a mode-polarization vector analysis and is correlated with cation size mismatch, from which it is possible to deduce the acentric properties of 3D alkali-metal fluorocarbonates.