The Crystal Structure of Defect KBB’O6 Pyrochlores (B,B’: Nb,W,Sb,Te) Revisited from Neutron Diffraction Data

Sergio Mayer,María Fernández-Díaz,José Alonso,Horacio Falcón

doi:10.3390/cryst8100368

Abstract

Three defect pyrochlores KNbWO6·xH2O, KNbTeO6 and KSbWO6 were synthesized by solid state reaction at 750 °C, from stoichiometric mixtures of K2C2O4, Sb2O3, Nb2O5, WO3 and 20% excess TeO2. A neutron powder diffraction (NPD) data analysis allowed unveiling some structural features. They are all defined in the cubic F d 3 ¯ m space group symmetry, with α = 10.5068(1) Å, 10.2466(1) Å and 10.2377(1) Å, respectively. Difference Fourier synthesis for KNbWO6·xH2O clearly showed the presence of crystallization water, with extra O’ oxygen and H+ atoms that were located from NPD data. These O’ oxygen atoms are placed at 32e Wyckoff sites, conforming a K2O’ sublattice interpenetrated with the covalent framework constituted by (Nb,W)O6 octahedra. The H+ ions coordinate the O’ atoms at partially occupied 96g Wyckoff sites while K+ ions shift also along 32e sites, but closer to the 16c special site (0,0,0). By contrast, extra H2O molecules are absent in the other two pyrochlores: in KNbTeO6 and KSbWO6 K+ ions are shifted along 32e (x,x,x) sites further away from the origin than for the previous material, and the higher covalency of the octahedral network determines more compact structures, with shorter B–O distances and narrower B–O–B angles in the proposed AB2O6 defect pyrochlore structure.

Highlights

Pyrochlore oxides, both natural and synthetic ceramics, have been extensively studied over the years because of their properties of technological interest, associated with their appealing electrical, dielectric, magnetic, optical, or catalytic properties [1], what makes them interesting candidates for catalysts and photocatalysts [2,3], promising solid fuel cell electrodes [4,5], superconductors [6], and nuclear waste treatment materials [7], among others
These patterns were used for the Pattern Matching analysis carried out prior to neutron powder diffraction (NPD) Rietveld refinements
A first Rietveld refinement was performed based on the proposed structural model for KSbTeO6 pyrochlore [8], with the origin choice No 2 of the space group Fd3m (No 227), in which BV (Nb or Sb) and BVI (W or Te) atoms were statistically distributed over the 16d (1/2,1/2,1/2) sites, and the O oxygen atoms were placed at 48f (x,1/8,1/8) positions

Summary

Introduction

Pyrochlore oxides, both natural and synthetic ceramics, have been extensively studied over the years because of their properties of technological interest, associated with their appealing electrical, dielectric, magnetic, optical, or catalytic properties [1], what makes them interesting candidates for catalysts and photocatalysts [2,3], promising solid fuel cell electrodes [4,5], superconductors [6], and nuclear waste treatment materials [7], among others These materials are mostly cubic, defined in the Fd3m (No 227) space group, Z = 8. Fourier syntheses unveiled a more complex formula for the defect pyrochlore KNbWO6 ·1.17(6) H2 O, similar to that previously reported for KNbWO6 ·0.69D2 O [17]

Experimental Section

Results and Discussion

Conclusions