Willis defects

The parent structure of the anion-deficient fluorite structure phases is the cubic fluorite structure (Fig. 4.7). As in the case of the anion-excess fluorite-related phases, diffraction patterns from typical samples reveals that the defect structure is complex, and the true defect structure is still far from resolved for even the most studied materials. For example, in one of the best known of these, yttria-stabilized zirconia, early studies were interpreted as suggesting that the anions around vacancies were displaced along < 111 > to form local clusters, rather as in the Willis 2 2 2 cluster described in the previous section, Recently, the structure has been described in terms of anion modulation (Section 4.10). In addition, simulations indicate that oxygen vacancies prefer to be located as second nearest neighbors to Y3+ dopant ions, to form triangular clusters (Fig. 4.11). Note that these suggestions are not... 

The precise biochemical defect for Prader-Willi syndrome is unknown, but this region of chromosome 15 encompasses an imprinting control center and at least 6 maternally imprinted genes. 

In fluorite-structure oxygen-deficient oxides MO2-X there is general agreement that the oxygen vacancy is the point-defect responsible for non-stoichiometry. Unfortunately, no direct observation is available of a basic cluster species for MO2-1 such as the Willis cluster in UO2+X. 

Figure 5.16 Willis clusters in UOj+y Open circles, oxygens small filled circles, uranium atoms in the UO2 structure. A 2 2 2 defect cluster consists of two interstitial oxygen atoms (large filled circles) and two normal oxygen atoms displaced from their ideal positions (open squares) to new interstitial positions (hatched circles). A 2 1 2 defect cluster would consist of one interstitial oxygen and two normal oxygens displaced from their ideal positions. (After Anderson, 19726.)...

Ran AK, Willis J, Hassell B, Dangdmaier C, Holmsen H, Smith JB. Platdet-sictivating factor is a weak platelet atgomsb evidence fiomnonnal human platelets and platelets with congenital secretic defects. AmerJHematol 1984 17 153-165... 

Acquired platelet function defects have been associated with several canine diseases. Decreased platelet aggregation responses have been reported for dog with liver disease, pancreatitis, myeloproliferative disease, and ehrlichiosis (Harms et al. 1996 Willis et at. 1989). 

Rao AK, Willis J, Kowalska MA, WaditfogelY, Colman RW. Differential requirements for epinephrine induced platelet aggregation and inhibition of adenylate cydase. Studies in fimilial alpha2-adrenergic receptor defect Blood 1988 71 494-501. 

Presence of these interstices provides to the fluorite stmcture extremely specific features. In UO2 particularly, it allows for placement of some radioactive decay products, these sites are responsible for existence of hyperstoichiometric UO2+X phase, where the extra oxygen ions fill the empty interstitial sites in the fluorite lattice etc. First case is extremely important in radiation damaged UO2. Second one is cmcial in oxidation of pure UO2 in atmospheric conditions. Diffusion of atmospheric oxygen into the bulk of crystal brings excess oxygens into empty interstices. These become filled more or less randomly only at low x, at higher concentration of extra anions they form different types of clusters, including so-called 2 2 2 Willis dimers Willis), tetra- and pentameric defects clusters of cuboctahedral symmetry Allen and Tempest). Last defects appear due to interaction of extra anions with intrinsic crystal FP defects (anion Frenkel pairs, i.e. anion vacancies and anion interstitials). 

Li, Y., Moir, R. D., Sethy-Coraci, I. K., Warner, J. R., and Willis, I. M. (2000). Repression of ribosome and tRNA synthesis in secretion-defective cells is signaled by a novel branch of the cell integrity pathway. Mol. Cell. Biol. 20, 3843-3851. 

At high temperatures in this region of oxygen pressure, the same complex defect (0i 0 Fq = 2 2 2) as proposed by Willis (188) is predominant ... 

Gerdanian (245) has developed a statistical-mechanical model for UO2 + X with small departure from stoichiometry to interpret the experimental values of AHq (244). His model is based on three types of defects—(a) f + 2Uy [Kroger and Vink notation (262)] in U02 [denoted by (a)], (b) 2 1 2 Willis cluster and two [denoted by (/)] in U02+ ,., and (c) 2 2 2 Willis clusters [denoted by (ii)] in U02+ (. The semi-grand partition function (SGPF) related to the defects... 

Wil3 Willis, B.T.M., in The Chemistry of Extended Defects in Non-metallie Solids Editors L. 

Willis B. T. M. The defect structure of hyper-stoichiometric uranium dioxide. Acta crystallogr., A34, 1978, 88-90. 

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