Theoretical studies of nonlinear optical properties of compounds K4Ln2(CO3)3F4 (Ln=Pr, Nd, Sm, Eu, Gd)

K 4 Ln 2 ( CO 3 ) 3 F 4 (Ln=Pr, Nd, Sm, Eu, Gd) is a special type of frequency doubling compound, whose crystal structure exhibits a scarcity of fluorine ions. This leads to two different coordination polyhedrons in the general position of K(2) atoms: [K(2)O6F(1)2F(2)] and [K(21)O6F(1)2] in a 2/1 ratio. The chemical bonding structures of all constituent atoms of the compound K4Gd2(CO3)3F4 (KGCOF) are comprehensively studied; moreover, the relationship between the chemical bonding structure and the nonlinear optical (NLO) properties is investigated from the chemical bond viewpoint. The theoretical prediction of the NLO tensor coefficient d11 of KGCOF is in agreement with experimental observation. Theoretical analyses show that the nonlinearity of this crystal type mainly originates from K–O bonds. In addition, the correlation between the NLO tensor d11 and the refractive index n0 of KGCOF is discussed.