Role of Acentric Displacements on the Crystal Structure
and Second-Harmonic Generating Properties of RbPbCO<sub>3</sub>F and
CsPbCO<sub>3</sub>F
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Abstract
Two lead fluorocarbonates, RbPbCO<sub>3</sub>F and CsPbCO<sub>3</sub>F, were synthesized and characterized.
The materials were synthesized through solvothermal and conventional
solid-state techniques. RbPbCO<sub>3</sub>F and CsPbCO<sub>3</sub>F were structurally characterized by single-crystal X-ray diffraction
and exhibit three-dimensional (3D) crystal structures consisting of
corner-shared PbO<sub>6</sub>F<sub>2</sub> polyhedra. For RbPbCO<sub>3</sub>F, infrared and ultraviolet–visible spectroscopy and
thermogravimetric and differential thermal analysis measurements were
performed. RbPbCO<sub>3</sub>F 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 RbPbCO<sub>3</sub>F and CsPbCO<sub>3</sub>F
using 1064 nm radiation revealed an SHG efficiency of approximately
250 and 300 × α-SiO<sub>2</sub>, respectively. Charge constants <i>d</i><sub>33</sub> of approximately 72 and 94 pm/V were obtained
for RbPbCO<sub>3</sub>F and CsPbCO<sub>3</sub>F, respectively, through
converse piezoelectric measurements. Electronic structure calculations
indicate that the nonlinear optical response originates from the distorted
PbO<sub>6</sub>F<sub>2</sub> 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 Pb<sup>2+</sup>. 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