Role of Acentric Displacements on the Crystal Structure and Second-Harmonic Generating Properties of RbPbCO₃F and CsPbCO₃F

Two lead fluorocarbonates, RbPbCO₃F and CsPbCO₃F, were synthesized and characterized. The materials were synthesized through solvothermal and conventional solid-state techniques. RbPbCO₃F and CsPbCO₃F were structurally characterized by single-crystal X-ray diffraction and exhibit three-dimensional (3D) crystal structures consisting of corner-shared PbO₆F₂ polyhedra. For RbPbCO₃F, infrared and ultraviolet–visible spectroscopy and thermogravimetric and differential thermal analysis measurements were performed. RbPbCO₃F is a new noncentrosymmetric material and crystallizes in the <i>achiral</i> and <i>nonpolar</i> space group <i>P</i>6̅<i>m</i>2 (crystal class 6̅<i>m</i>2). Powder second-harmonic generation (SHG) measurements on RbPbCO₃F and CsPbCO₃F using 1064 nm radiation revealed an SHG efficiency of approximately 250 and 300 × α-SiO₂, respectively. Charge constants <i>d</i>₃₃ of approximately 72 and 94 pm/V were obtained for RbPbCO₃F and CsPbCO₃F, respectively, through converse piezoelectric measurements. Electronic structure calculations indicate that the nonlinear optical response originates from the distorted PbO₆F₂ polyhedra, because of the even–odd parity mixing of the O 2<i>p</i> states with the nearly spherically symmetric 6<i>s</i> electrons of Pb²⁺. 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