Browsing by Author "Cranswick, LMD"

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    Lanthanum pyrochlores and the effect of yttrium addition in the systems La2-xYxZr2O7 and La2-xYxHf2O7
    (Elsevier, 2009-03) Whittle, KR; Cranswick, LMD; Redfern, SAT; Swainson, IP; Lumpkin, GR
    The crystal structures of the compounds La2−xYxZr2O7 and La2−xYxHf2O7 with x=0.0, 0.4, 0.8, 1.2, 1.6, and 2.0 have been studied using neutron powder diffraction and electron microscopy to determine the stability fields of the pyrochlore and fluorite solid solutions. The limits of pyrochlore stability in these solid solutions are found to be close to La0.8Y1.2Zr2O7 and La0.4Y1.6Hf2O7, respectively. In both systems the unit cell parameter is found to vary linearly with Y content across those compositions where the pyrochlore phase is stable, as does the x-coordinate of the oxygen atoms on the 48f (x,3/8,3/8) sites. In both systems, linear extrapolations of the pyrochlore data suggest that the disordering is accompanied by a small decrease in the lattice parameter of approximately 0.4%. After the pyrochlore solid solution limit is reached, a sharp change is observed from x~0.41 to 0.375 as the disordered defect fluorite structure is favoured. Electron diffraction patterns illustrate that some short-range order remains in the disordered defect fluorite phases. © 2009, Elsevier Ltd.
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    Pyrochlore to fluorite transitions - ordering in fluorites?
    (Materials Research Society, 2008-12-01) Whittle, KR; Cranswick, LMD; Redfern, SAT; Swainson, IP; Lumpkin, GR
    Two systems have been studied La2-xYxZr2O7 and La2-xYxHf2O7, as part of an on-going study of radiation tolerance in nuclear waste forms and related oxide materials. The structural effects of increasing Y content in La based zirconate and hafnate pyrochlores have been studied with neutron diffraction and electron microscopy. Results have shown a difference in structural stability for both the pyrochlore and fluorite phases within each system, including the presence of two-phase regions in both systems. In the zirconate, the two-phase region lies in the range x = 0.9-1.6. This is shifted to higher Y content in the hafnate system and lies in the range of x = 1.5 to approximately 1.8-1.9. In addition to the differences in phase stability, electron diffraction, predominantly down the [110] zone axis, has shown evidence for ordering in the form of structured diffuse scattering within the fluorite phases.
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    Third structure determination by powder diffractometry round robin (SDPDRR-3)
    (Cambridge University Press, 2009-09) Le Bail, A; Cranswick, LMD; Adil, K; Altomare, A; Avdeev, M; Cerny, R; Cuocci, C; Giacovazzo, C; Halasz, I; Lapidus, SH; Louwen, JN; Moliterni, A; Palatinus, L; Rizzi, R; Schilder, EC; Stephens, PW; Stone, KH; van Mechelen, J
    The results from a third structure determination by powder diffractometry (SDPD) round robin are discussed. From the 175 potential participants having downloaded the powder data, nine sent a total of 12 solutions (8 and 4 for samples I and 2, respectively, a tetrahydrated calcium tartrate and a lanthanum tungstate). Participants used seven different computer programs for structure solution (ESPOIR, EXPO, FOX, PSSP, SHELXS, SUPERFLIP, and TOPAS), applying Patterson, direct methods, direct space methods, and charge flipping approach. It is concluded that solving a structure from powder data remains a challenge, at least one order of magnitude more difficult than solving a problem with similar complexity from single-crystal data. Nevertheless, a few more steps in the direction of increasing the SDPD rate of success were accomplished since the two previous round robins: this time, not only the computer program developers were successful but also some users. No result was obtained from crystal structure prediction experts. © 2009, JCPDS - International Centre for Diffraction Data