A new oxyborate in the ternary phase diagrams Li2O-Ln2O3-B2O3: Li2Ln5O4(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 method. The structure of this oxyborate was solved from a data collection with 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) Angstrom, 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 independent reflections having intensity I>2 sigma (I) led to R = 0.037 (wR = 0.091). The structure of Li2Yb5O4(BO3)(3) is made up of a bidimensional framework of edge or corner-sharing YbO7, polyhedra parallel to the (101) plane and 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 oxygen atoms sharing only Yb polyhedra justifies the oxyborate label. (C) 2001 Academic Press.