Mullite-type dibismuth-nonaoxometallates(III): the effects of strontium doping

Th. M. Gesing, M. Schowalter, C. Weidenthaler, M. M. Murshed, G. Nénert, C. B. Mendive, M. Curti, A. Rosenauer, H. Schneider, R. X. Fischer
2012 Acta Crystallographica Section A Foundations of Crystallography  
Stable (GeTe) n (Sb 2 Te 3 ) (315 exhibit significant deviations from the cubic metrics at RT, but no pronounced diffuse scattering [4]. They resemble GeTe itself but form the cubic HT phase in a first order phase transition after a certain degree of vacancy ordering (formation of finite vacancy layers) has taken place at intermediate temperatures. The qualitative information from the diffuse intensity distribution of the Laue patterns was supplemented by precise monochromatic-beam data
more » ... at ID 11 (ESRF) at various temperatures (cf. Fig. 1 ). They allow one to derive the atomic structure of ordered domains as well as the stacking sequences in disordered ones, where the intensity distribution along diffuse streaks yields reliable information concerning the average defect layer spacing (which depends on the chemical composition), its variance and the structural distortion around the defect layers. For low n, the defect layers resemble the van der Waals gaps in the stable RT phases, whereas for high n the fcc Te atom arrangement remains little distorted. Strontium doped dibismuth-nonaoxometallate-(III) phases were produced at 1023 K. Partial substitution of bismuth by strontium in the (Bi 1-x Sr x ) 2 M 4 O 9-x structure (M = Al, Ga) yields oxygen vacancies for charge balance. Introducing oxygen vacancies into the structure causes the rearrangement of associated M 2 O 7 double-tetrahedra forming "M 3 O 10 " tri-clusters which were identified by multi-quantum MAS NMR. Both STEM-EDX and XPS showed a homogeneous distribution of strontium in the bulk and on the surface, respectively. Moreover, XPS confirms the trivalent state of bismuth after doping. The orientations of bismuth 6s 2 lone electron pairs were calculated using DFT methods. Additionally, while replacing half of the bismuth atoms by strontium (Fig. 1the resulting structural distortions were calculatedtaking different possibilities into account. The amount of strontium in the crystal structure of the aluminum containing phase was further confirmed from the decomposition product SrAl 12 O 19 formed during the temperature-dependent X-ray powder diffraction. Thermal expansion coefficients were calculated out of these evaluated data. The crystal structural proofwas carried out refining (Bi 0.94 Sr 0.06 ) 2 Al 4 O 8.94 combined from powder neutron and X-ray diffraction data. Rietveld refinements clearly showed that partial occupation of one oxygen site and the simultaneous shift of two aluminum atoms from the double-tetrahedra to two tri-cluster sites occurred. Thermal expansion and high-temperature neutron diffraction powder data analysis confirms the thermal stability of the Sr-doped phases up to 1250 K. Fig.1 Al 2 O 7 double-tetrahedra configuration with bridging O3 atom in undoped mullite-type Bi 2 Al 4 O 9 (top left) and the structural effect of three possibilities of strontium doping:1,2 (top right), 3,4(bottom left) and 1,4 (bottom right)in (Bi 1-x Sr x ) 2 M 4 O 9-x for x = 0.5.
doi:10.1107/s0108767312099114 fatcat:3d6gtmy3fzdhtis76rwsog5o2m