NEW FREQUENCY-DOUBLING COMPOUNDS - K(4)LN(2)(CO3)(3)F-4 (LN=PR, ND, SM, GD, EU) - CRYSTAL-STRUCTURE AND CHARACTERIZATION

The structural, physical and chemical characterization of a new struct ural family K(4)Ln(2)(CO3)(3)F-4 (Ln = Pr, Nd, Eu, Gd), obtained by hy drothermal growth (T = 740 degrees C, P = 220 MPa, t = 24 h) is presen ted. The crystal structure, determined by single crystal X-ray diffrac tion (MoK alpha, lambda = 0.7109 Angstrom), is rhombohedral, R32, Z = 3, with a(h) = 9.027(1) Angstrom, c(h) = 13.684(6) Angstrom, R-1 = 0.0 18, wR(2) = 0.046, 2329 unique reflections for K4Gd2(CO3)(3)F-4. The s tructure results from the stacking of carbonate planes and fluoride sh eets. Their formulation, KLn(2)(CO3)(3) and K3F4, when cations are tak en into account, constitutes an original feature as compared to severa l other fluoride carbonates. The LnO(6)F(3) polyhedra are connected on e to each other by oxygen vertices in the (a,b) plane and by fluorine edges or vertices along the c axis, in order to build the 3D network. The acentric nature of the structure is confirmed by SHG tests: a powd er of K4Eu2(CO3)(3)F-4 presents a frequency doubling efficiency simila r to that of KTiOPO4.