The influence of thermal annealing on cation disorder in ABi
 2
Nb
 2
O
 9
 (A=Sr, Ba) has been studied using synchrotron powder X-ray diffraction methods. BaBi
 2
Nb
 2
O
 9
 adopts a tetragonal (I4
 1
/mmm) structure, whilst SrBi
 2
Nb
 2
O
 9
 is orthorhombic, space group A2
 1
am, irrespective of the annealing conditions. In both complexes the A-type cations, Sr and Ba, are disordered over the perovskite and Bi
 2
O
 2
 layers. Quenching the samples from high temperature results in a slight increase in both the cell volume and the degree of disorder, the extent of disorder being greater in the Ba complex. Variable temperature diffraction studies are also reported.

Effect of temperature on cation disorder in ABi2Nb2O9 (A=Sr, Ba)

Structure of ZrV2O7from −263 to 470°C

Recent advances in Bi2MoO6 based Z-scheme heterojunctions for photocatalytic degradation of pollutants

The structure of cubic ZrP2O7 at room temperature has been solved and refined using a combination of modeling and high-resolution neutron powder diffraction data. The cell edge is 24.74 A, the space group is Pa3, and Z is 108. For those P2O7 units not on a 3-fold axis, the P−O−P angles range from 134° to 162°. Two crystallographically distinct P2O7 groups are on three fold axes with P−O−P angles thus constrained to be 180° on average. The structure of cubic ZrP2O7 was also refined from data taken at 227, 290, 371, 435, and 610 °C. The 3 × 3 × 3 superstructure present at room temperature disappears at about 290 °C, and all P−O−P angles of P2O7 are then constrained by symmetry to be 180° on average. The exceptionally low thermal expansion shown by ZrP2O7 above 290 °C is likely related to the unusual P−O−P angle.

Unusual 180° P−O−P Bond Angles in ZrP2O7

In a previous study, we confirmed by in situ combined high-resolution powder diffraction/X-ray absorption spectroscopy and Raman scattering experiments that the crystal formation of the well-known γ-Bi2MoO6 catalyst occurred in two steps through the formation of an intermediate fluorite structure (Kongmark, C.Chem. Commun. 2009, 4850−4852; Catal. Today 2010, 157, 257–262). Here, for the first time, by combining these results with complementary ex situ studies (HTXRD, SEM/EDX, Raman scattering), it was possible to elucidate the nature of the intermediate phase and to propose a complete scenario of the growth of γ-Bi2MoO6 crystals. In addition, we performed a kinetic analysis from the three different in situ characterization techniques and confirmed a 2D-diffusion limited growth process.

U. Wildgruber

Papers

Structural Refinement of the High Temperature Form of Bi2MoO6