A new oxyborate in the ternary phase diagrams Li2O-Ln(2)O(3)-B2O3 : Li(2)Ln(5)O(4)(BO3)(3) (Ln=Yb, Lu): Crystal structure of the ytterbium phase

A new type of lithium rare earth oxyborate of formula Li(2)Ln(5)O(4)(BO3)(3 ) (Ln = Yb, Lu) has been discovered in the ternary phase diagrams Li2O-Ln(2 )O(3)B(2)O(3). Single crystals of Li2Yb5O4(BO3)(3) were grown by the flux m ethod. The structure of this oxyborate was solved from a data collection wi th a four circle automatic diffractometer and with MoK alpha radiation. The cell is monoclinic (space group P2(1)/m, Z = 2) with a = 10.095(2) Angstro m, b = 3.519(2) Angstrom ,c = 15.647(11) Angstrom, beta = 105.45(3)degrees, V = 535.7(5) Angstrom (-3). Refinement of 86 parameters using 3298 indepen dent reflections having intensity I>2 sigma (I) led to R = 0.037 (wR = 0.09 1). The structure of Li2Yb5O4(BO3)(3) is made up of a bidimensional framewo rk of edge or corner-sharing YbO7, polyhedra parallel to the (101) plane an d connected by three-coordinated boron atoms and lithium ions. It can also be described as a bidimensional assembly of OYb4 tetrahedra, ((Yb10O8)(14+) )(n), separated by borate groups and lithium coordination polyhedra forming ribbons, ((LI4B6O18)(14-))(n), along the b direction. The presence of oxyg en atoms sharing only Yb polyhedra justifies the oxyborate label. (C) 2001 Academic Press.