CeO 2 -ZrO 2 mixed oxides were synthesized by two mild chemical routes: complexation with citric acid and the sol-gel method. XRD and ESEM investigations revealed the appearance of a solid solution around 400 °C, depending on the ratio between the two oxides and the preparation method. Solids obtained by the citrate method were more homogeneous on the whole range of composition. Solid-state investigations of the prepared materials did not show segregation of cerium or zirconium oxides, except in the high Ce ranges for samples prepared by the sol-gel method. The thermal stability of both oxides was enhanced by mutual substitution. The most stable solid solutions were obtained in the range 50 < Ce < 90% mol.

The crystal structure of Luz(Si04)0:Ce ("LSO") has two trivalent cation sites which may be occupied by Ce3* to form luminescence centers. Previous investigations revealed the existence of two distinct sets of Ce3' excitation and emission spectra and suggested that the differences in the spectra are due to differences in the crystal fields at the two lattice sites that shift the 5d levels of Ce".In the present report, we reexamine this issue and present new evidence which suggests a different interpretation. In particular, spectra measured at 13 K suggest that both lattice sites may give rise to indistinguishable excitation and emission spectra while the second set of observed spectra may arise fi-om Ce" located in interstitial sites. The evidence for the interstitial sites is a disappearance of the doublet structure in the emission spectra and a large variation in the population of the sites as a function of total cerium concentration. In addition, the results indicate that the scintillation properties of LSO are influenced by the relative population of Ce in interstitial sites compared to lattice sites as well as by differences in the transfer efficiencies to different sites.

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A mechanism of osmium reduction during zinc iodide-osmium tetroxide (ZIO) fixation is proposed. X-ray powder microanalyses of ZIO precipitates formed both in the presence or absence of tissues are identical with those of CuOsO4 and CuRuO4. Therefore, and based on indexation methods, ZnOsO4 was found to be the formula of the ZIO mixture reduction; this zinc osmate has an orthorhombic crystalline lattice. In smooth muscle preparations, ZIO electron dense deposits are localized in both cisternae of the sarcoplasmic reticulum and in mitochondria after a short fixation time. According to the microanalysis results, the zinc osmate has been associated to Ca2+ high affinity sites since Zn2+ is either replacing Ca2+ and/or displacing it by having a higher affinity for Ca2+ binding sites. Consequently, the ZIO mixture might be useful in revealing some Ca2+ storage sites in cells. This hypothesis was tested in ABRM preparations by selectively depleting sites which are known to bind Ca2+. In this case, the sarcoplasmic reticulum only retains the staining deposits after a short ZIO fixation. It is likely that OsO4 alone, used as fixative in cytology might be due to the formation of metallic osmates (e.g., divalent osmates like CaOsO4). In addition, of course, reduction of osmium during tissue fixation is accompanied by oxidation of double bonds of lipoproteic complexes or unsaturated lipids, and oxidation of sulfhydryl groups and amino groups.

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