High-temperature neutron powder

Tao SW, Irvine ITS (2006) Phase transition in perov-skite oxide Lao.75Sro.25Cro.5Mno.503 g observed by in situ high temperature neutron powder diflfaction. Chem Mater 18 5453-5460... 

Aguadero A, Alonso JA, Femandez-Diaz MT, Escudero MJ, Daza L (2007) In situ high temperature neutron powder diffraction study of La2Nio.6Cuo.4O4 -1- delta in air correlation with the electrical behaviour. J Power Sources 169(1) 17-24... 

Skinner SJ (2003) Characterisation of La2Ni04 + delta using in situ high temperature neutron powder diffraction. SoUd Slate Sci 5(3) 419-426... 

Some 4503, 5 perovskite-structured materials, where A and B represent larger and smaller cations, are ionic conductors, while some other 503 s perovskite-type compounds are mixed conductors. Heavy elements such as La and Ba occupy the A site, but because the mobile O anion is a light element, conventional X-ray powder diffractometry is not sensitive to positional and occupational disordering of oxide ions. To investigate the diffusion path of mobile oxide ions, and structural disorder and crystal structure in perovskite-structured ionic and mixed conductors [5, 6, 8, 10-14], we applied a high-temperature neutron powder diffraction method. Our reasons for choosing this method were as follows [24] ... 

Bragg-Brentano geometry. In contrast, structural analysis based on high-temperature neutron powder diffraction data is less subject to the influence of sintering, grain growth, cracking, evaporation, and thermal expansion. 

Mumme, W. G., L. Cranswick and B. Chakoumakos (1996). Rietveld Crystal Structure Refinement from High Temperature Neutron Powder Diffraction Data for the Polymorphs of Dicalcium Silicate . Neues Jahrbuch Fuer Mineralogie - Abhandlungen 170 (2) 171-188. 

Ibberson RM, Moze O, PetriUo C (1992) High resolution neutron powder diffraction studies of the low temperature crystal structure of molecular iodine. Mol Phys 76 395-403... 

Henderson C.M.B., Knight K. S., Redfern S.A.T., and Wood B. J. (1996) High-temperature study of octahedral cation exchange in olivine by neutron powder diffraction. Science 271, 1713-1715. 

Density measurements clearly establish that the system contains a close-packed anion array with cation vacancies. Moreover, neutron-diffraction data on samples quenched from high temperature have identified the presence of tetrahedral-site iron interstitials surrounded by four octahedral-site vacancies, but the nature of the elementary cluster associated with such an interstitial has been more difficult to estab-lish " . Some of the clusters considered are shown in Fig. 16. From powder neutron diffraction at 985 and 1075 °C, Gavarri et al. have found a ratio of octahedral-site vacancies to interstitial iron to be... 

The defect structure of Fei O with the NaCl-type structure had been estimated to be a random distribution of iron vacancies. In 1960, Roth confirmed, by powder X-ray diffraction, that the defect structure of wiistite quenched from high temperatures consists of iron vacancies (Vp ) and interstitial iron (Fcj) (there are about half as many FCj as Vpe). This was a remarkable discovery in the sense that it showed that different types of crystal defects with comparable concentrations are able to exist simultaneously in a substance, Roth also proposed a structure model, named a Roth cluster, shown in Fig. 1.84. Later this model (defect complex = vacancy -F interstitial) was verified by X-ray diffraction on a single crystal and also by in-situ neutron diffraction experiments. Moreover, it has been shown that the defect complex arranges regularly and results in a kind of super-structure, the model structure of which (called a Koch-Cohen model) is shown in Fig. 1.85 together with the basic structures (a) and (b). 

All of these hexafluorides are dimorphic, with a high-temperature, cubic form and an orthorhombic form, stable below the transition temperature (92). The cubic form corresponds to a body-centered arrangement of the spherical units, with very high thermal disorder of the molecules in the lattice, leading to a better approximation to a sphere. Recently, the structures of the cubic forms of molybdenum (93) and tungsten (94) hexafluorides have been studied using neutron powder data, with the profile-refinement method and Kubic Harmonic analysis. In both compounds the fluorine density is nonuniformly distributed in a spherical shell of radius equal to the M—F distance. Thus, rotation is not completely free, and there is some preferential orientation of fluorine atoms along the axial directions. The M—F distances are the same as in the gas phase and in the orthorhombic form. 

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