Search

The technetium perovskite CaTcO3 has been synthesized. Combining synchrotron X-ray and neutron diffraction, we found that CaTcO3 is an antiferromagnetic with a surprisingly high Neel temperature of ∼800 K. The transition to the magnetic state does not involve a structural change, but there is obvious magnetostriction. Electronic structure calculations confirm the experimental results. [ABSTRACT FROM AUTHOR]

Abstract: Ceramics based on LnTiTaO6 (Ln =La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu) were prepared using a conventional solid-state ceramic route. The structure and microstructure of the samples were analyzed using scanning electron microscopy, synchrotron X-ray powder diffraction and neutron diffraction techniques. The unit cell volume of the ceramics decreased with a decrease in ionic radius of the rare-earth ions until a phase change was seen at Ho where the space group changed from Pnma to Pbcn. The results confirmed that the boundary of the aeschynite to euxenite phase change that lies between DyTiTaO6 and HoTiTaO6 and demonstrate that this is a result of distortion of the channels in the aeschynite structure. [Copyright &y& Elsevier]

Abstract: The structures of the defect pyrochlores AAl0.33W1.67O6 where A=K, Rb or Cs have been investigated using X-ray and neutron powder diffraction methods as well as the ab initio modelling program VASP. The three cubic pyrochlores exhibit a non-linear increase in lattice parameter with respect to ionic radius of the A cation as a consequence of displacive disorder of the A-type cations. Solid state 27Al MAS NMR studies of this pyrochlore system reveal shifts in the δ∼21–22ppm range that are indicative of pseudo-5 coordinate Al environments and emanate from distorted Al octahedral with one abnormally long Al–O bond. Solid state 39K, 85Rb, 87Rb and 133Cs MAS and static NMR studies reflect the local cation disorder demonstrated in the structural studies. [Copyright &y& Elsevier]

Abstract: The structure of the defect pyrochlore CsTi0.5W1.5O6 has been investigated using electron, synchrotron X-ray and neutron diffraction methods. The material is cubic a =10.2773Å with displacive disorder of the Cs cations along the 〈111〉 direction. The local structure, revealed by the diffuse structure in the electron diffraction patterns shows there is correlated displacement of the heavy Cs atoms along the 〈110〉 directions. The thermal expansion of the material is also described. [Copyright &y& Elsevier]

Abstract: We show that the pyrochlore described as “(BiNa)(NbCr)O6” is in fact a non-stoichiometric pyrochlore with an approximate composition of (Bi1.33Na0.67)(Nb1.33Cr0.67)O7− x . Refinement of this structure using constant wavelength powder neutron diffraction data reveals the presence of vacancies in the anion sites coupled with displacive disorder of the Bi and Na cations. This is necessary to achieve a satisfactory bonding arrangement for both the Bi and Na cations that occupy the pyrochlore A-type sites. Attempts to prepare other pyrochlores in the series Bi2− x Na x (NbCr)O6 were unsuccessful and it appears that the pyrochlore is only stable only over a very narrow composition range. The structure of the pure Bi pyrochlore Bi2(NbCr)O7 is also described. [Copyright &y& Elsevier]

The structure of TcCo2O4has been determined using a combination of synchrotron X-ray and neutron powder diffraction methods. It has an inverse spinel structure where the Tc occupies the octahedral sites. Both the refined Tc–O distance and X-ray absorption spectra suggest the Tc is predominantly trivalent. [ABSTRACT FROM AUTHOR]

This work presents the results for identification of chemical phases obtained by several laboratories as a part of an international nuclear forensic round-robin exercise. In this work powder X-ray diffraction (p-XRD) is regarded as the reference technique. Neutron diffraction produced a superior high-angle diffraction pattern relative to p-XRD. Requiring only small amounts of sample, µ-Raman spectroscopy was used for the first time in this context as a potentially complementary technique to p-XRD. The chemical phases were identified as pure UO2 in two materials, and as a mixture of UO2, U3O8 and an intermediate species U3O7 in the third material. [ABSTRACT FROM AUTHOR]

The structures of some A UO 4 ( A =Ca, Sr, or Ba) oxides have been determined using a combination of neutron and synchrotron X-ray diffraction, supported by X-ray absorption spectroscopic measurements at the U L 3 -edge. The smaller Ca cation favours a rhombohedral A UO 4 structure with 8-coordinate UO 8 moieties whilst an orthorhombic structure based on UO 6 groups is found for BaUO 4 . Both the rhombohedral and orthorhombic structures can be stabilised for SrUO 4 . The structural studies suggest that the bonding requirements of the A site cation play a significant role in determining which structure is favoured. In the rhombohedral structure, Bond Valence Sums demonstrate the A site is invariably overbonded, which, in the case of rhombohedral α-SrUO 4 , is compensated for by the formation of vacancies in the oxygen sub-lattice. The uranium cation, with its flexible oxidation state, is able to accommodate this by inducing vacancies along its equatorial coordination site as demonstrated by neutron powder diffraction. [ABSTRACT FROM AUTHOR]

The β -phase of Au 7 Cu 5 Al 4 undergoes a reversible shape-memory phase transformation for which several conflicting martensite phases have been reported. Here we show the significance of the cooling temperature used to obtain the martensite. If Au 7 Cu 5 Al 4 is cooled from the parent phase condition to cryogenic temperatures, e.g. below 200 K, the martensitic phase is orthorhombic (space group Pcmn , a = 4.4841 Å, b = 5.8996 Å, c = 17.8130 Å); however, when this composition is cooled to only ∼260 K it will in general consist of a mixture of orthorhombic and monoclinic phase (the latter has space group P 2 1 / m , a = 4.4742 Å, b = 5.9265 Å, c = 13.3370 Å, β = 91.425°). In contrast, a sample with decreased Al content (Au 7 Cu 5.7 Al 3.3 ) transforms fully to monoclinic phase if cooled to ∼260 K. [ABSTRACT FROM AUTHOR]

The structural basis of selectivity for cesium and strontium of microcrystalline hexagonal tungsten bronze (HTB) phase Na[subx]WO[sub3+x/2]∙zF[sub2]O has been studied using X-ray and neutron diffraction techniques, 1D and 2D [sup23]Na magic angle spinning (MAS) nuclear magnetic resonance (NMR) spetroscopy, and radiochemical ion exchange investigations. For the HTB system, this study has shown that scattering techniques alone provide an incomplete description of the disorder and rapid exchange of water (with tunnel cations) occuring in this system. However, 1D and 2D [sup23]Na MAS NMR has indentified three sodium species within the HTB tunnels—species A, which is located at the center of the hexagonal window and is devoid of coordinated water, and species B and C, which are the di and monohydrated variants, respectively, of species A. Although species B accords with the traditional crystallographic model of the HTB phase, this work is the first to propose and identify the anhydrous species A and monohydrate species .C. The population (total) of species B and C decreases in comparison to that of species A with increasing exchange of either cesium or strontium; that is, species B and C appear more exchangeable than species A. Moreover, a significant proportion of tunnel water is redistributed by these cations. Multiple ion exchange inversitgations with radiotracers [sup137]Cs and [sup85]Sr have shown that for strontium there is a definite advantage in ensuring that any easily exchanged sodium is removed from the HTB tunnels prior to exchange. The decrease in selectivity (wrt cesium) is most probably due to the slightly smaller effective size of Sr[sup2+]; namely, it is less of a good fit for the hexagonal window, ion exchange site. The selectivity of the HTB framework for cesium has been shown unequivovally to be defined by the structure of the hexagonal window, ion exchange site. Compromising the geometry of this window even in the slightest way by either (1) varying the cell volume through changes to hydration or sodium content or (2) introducing disorder in the a—b plane through isomorphous substitution of molybdenum is sufficient to reduce the selectivity. Indeed, it is our hypothesis that this applies for all cations which are strongly bound by the HTB framework. [ABSTRACT FROM AUTHOR]