ACdCO3F (A = K and Rb): new noncentrosymmetric materials with remarkably strong second-harmonic generation (SHG) responses enhanced via π-interaction

Abstract

Two new noncentrosymmetric (NCS) materials, namely, ACdCO₃F (A = K and Rb) containing both a d¹⁰ cation (Cd²⁺) and a π-conjugated parallel carbonate anion (CO₃²⁻), were synthesized through conventional solid state reactions. ACdCO₃F exhibits a 3-dimensional structure that is composed of the stacked layers of [Cd(CO₃)]_∞. Each [Cd(CO₃)]_∞ layer is connected by infinite Cd–F–Cd chains and the [CO₃] triangles are oriented in the same direction with a coplanar alignment. KCdCO₃F and RbCdCO₃F reveal remarkably strong second-harmonic generation (SHG) responses of approximately 9.0 and 7.2 times that of potassium dihydrogen phosphate (KDP), respectively, and both materials are phase-matchable. ACdCO₃F exhibit wide transparent regions ranging from far UV to mid IR. Theoretical calculations confirm that the large SHG efficiencies indeed originate from enhancement via interatomic interactions between the s and p states of Cd²⁺ and the π-conjugated groups of the [CO₃]²⁻ unit within the [Cd(CO₃)]_∞ layers.