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[en] Piezoelectric, pyroelectric, dielectric, elastic and thermoelastic properties of BiB3O6, point group symmetry 2, are reported. The eight independent components of the piezoelectric tensor could be derived from first-order longitudinal and transverse electrostrictive effects, employing a Michelson interferometer. For the determination of the elasticity tensor ultrasonic resonances of plane-parallel plates of different orientations and, in addition, the resonant ultrasound spectra (RUS) of rectangular parallelepipeda were measured. For the evaluation of the elasticity tensor from these resonant frequencies the piezoelectric coupling had to be taken into account. BiB3O6 exhibits a maximum longitudinal piezoelectric effect of 39.5 x 10-12 mV-1 along the twofold axis, a value about 17 timeslarger than that of α-quartz. The longitudinal elastic stiffness attains a minimum of about 5 x 1010 N m-2 along the same direction and a maximum perpendicular to the twofold axis of about 33 x 1010 N m-2, one of the largest elastic anisotropies observed in ionic crystals. This behaviour corresponds to the earlier observed extreme anisotropy of thermal expansion. The phenomena can be qualitatively interpreted by structural details, especially the preferential orientation of the lone electron pair of Bi3+. (orig.)
[en] Series of Bi2MoO6/NiAl-LDH hetero-junction photocatalysts were synthesized by a two-step hydrothermal method with a self-assembled process. It indicated that the in-situ intercalated NiAl-LDH showed a tight contact with Bi2MoO6 layers and thus exhibited the rapid transfer of photogenerated electrons on the interface. The photocatalytic performance was evaluated by the degradation of rhodamine B (RhB) under visible light irradiation. The resulted Bi2MoO6/NiAl-LDH sample showed a higher visible-light photocatalytic activity for the degradation of RhB than any single component. The optimal LDH content in Bi2MoO6/NiAl-LDH is 15% with the degradation efficiency for more than 90% during 60 min. The excessive amount of NiAl-LDH in Bi2MoO6/NiAl-LDH could reduce the light absorption efficiency of Bi2MoO6 and thus affected the generation of photoinduced electron–hole pairs. The enhanced photocatalytic activity and stability of Bi2MoO6/NiAl-LDH was attributed to the synergistic effects of the introduction of narrow band-gap NiAl-LDH and the construction of layered hetero-junction. Our synthetic method of self-assembly sheets has certain guided significance for the synthesis and application of other photocatalysts.
[en] The structure of Bi4V2O11 consists of Bi2O2 layers interleaved with V2O7 sheets. Differential thermal analysis showed three reversible phase transitions at 720, 840 and 1150 K. The cells of the low temperature forms derive from the high temperature tetragonal cell. Conductivity measurements confirm these transitions and reveal two further ones at 920 and 975 K. The conductivity is in the range 0.001-0.01 (ω cm)-1 for the β phase and 0.1-1 (ω cm)-1 for the γ phase. The mobile species are probably O2- ions, at least above 720 K. 9 refs.; 2 figs.; 3 tabs