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

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.

A comprehensive examination of the local- and long-range structure of Sb6O13 pyrochlore oxide

“…This final refined structure for the Sb 6 O 13 oxide is strongly endorsed by one of our recent reports, where some of our group revisited the general structural data of a pyrochlore family with a similar ( B 2 O 6 ) framework 24 . Certainly, the shift of the O′ atoms to the 32 e ( x , x , x ) sites, concerning the lower-multiplicity 8 a (1/8,1/8,1/8) position, has been confirmed as a recurrent peculiarity throughout the whole pyrochlore family 24 .…”

“…This final refined structure for the Sb 6 O 13 oxide is strongly endorsed by one of our recent reports, where some of our group revisited the general structural data of a pyrochlore family with a similar ( B 2 O 6 ) framework 24 . Certainly, the shift of the O′ atoms to the 32 e ( x , x , x ) sites, concerning the lower-multiplicity 8 a (1/8,1/8,1/8) position, has been confirmed as a recurrent peculiarity throughout the whole pyrochlore family 24 . Indeed, in Sb 6 O 13 , the distribution of Sb 3+ ions at 96 g instead of 16 c Wyckoff positions is presumably due to the lone electron pair and its stereochemical effect, that would displace, by repulsion forces, the Sb′ atom about 0.353(9) Å from the (0,0,0) position to an adjacent 96 g ( x , x , z ) site, with six-times greater multiplicity.…”

Unveiling the crystal structure of ‘antimonic acid’: short- and long-range perspectives

“…It comes to no surprise that AA presents some major water affinity, as a small amount of moisture was adsorbed on the dry sample during the TGA/DSC experiment setup, and later degassed in the heating process up to 150 °C. Seamlessly, this extraordinary water affinity was recently identified and reported by our group for a potassium pyrochlore-like niobate and tungstate33 , probably bound to many series of pyrochlore-like materials as a common behaviour pattern. For the assessment of H content in the dry sample, this last moisture-related contribution was considered.…”

“…Recently, we have found a similar shift along the 32e (x,x,x) direction due to the increasing occupancy of H3O + groups on a related family of Sb and Te pyrochlore-like acid oxides 31 . Certainly, it was confirmed that the shift of the O' atoms along this path, regarding the lower-multiplicity 8a (⅛,⅛,⅛) Wyckoff site, is frequent within the pyrochlore family 33 .…”

New KNbTeO6 transparent tellurate ceramics

“…The XRD pattern looks completely similar to the one collected on starting powders (either raw or attrited), therefore testifying that the SPS process does not affect the crystal structure. Then, a Rietveld refinement was performed using the Fd-3 m cubic defect pyrochlore structural model previously reported [38] octahedra sharing corners to form a three dimensional framework possessing tunnels running down the axes in which the K cations are located, is embedded (Fig. 6).…”