Bi17Yb7O36 and BiYbO3 crystal structures. Characterization of thulium and lutetium homologous compounds

Two new compounds Bi17Yb7O36 and BiYbO3 which were recently evidenced in the Bi2O3Yb2O3 system, have been characterized and their structure determined from X-ray diffraction powder data using the Rietveld method in an orthorhombic and a triclinic lattice, respectively. The thulium and lutetium homologous compounds have been prepared for the first time. They are isostructural to the ytterbium homologous. The six compounds have been characterized versus temperature by DTA, X-ray thermodiffractometry, and they systematically show a transformation from their low temperature form, to a δ-Bi2O3-type phase, occurring on heating. For Bi17Ln7O36, the δ-type phase is obtained, directly with Ln = Lu (870 °C) and Yb (855 °C), and only after an intermediate biphasic orthorhombic-cubic mixture in the range 780-825 °C for Ln = Tm. For BiLnO3, there is a large two-phased domain (δ-Bi2O3 + Ln2O3 related phases) from 955 to 1100 °C for Lu, 970 to 1200 °C for Yb and 980 to 1270 °C for Tm. The lattice constants thermal expansion coefficients have been determined from the cell parameters evolutions versus temperature. The conductivity properties of the Bi17Ln7O36 series, which could be obtained as ceramic pellets after sintering in the stability range of their low-temperature forms, have been investigated.