New X-ray diffraction and scanning electron microscopy data are given for the incorporation of Np and Pu in zirconolite, at levels of tens of percent. The actinide valences and the cations they replace are deduced from the microanalysis of the zirconolite compositions, and X-ray absorption data are used to obtain more direct information on the valences of Ce and Nd, which are used as simulants of Pu and trivalent actinides respectively. Trivalent rare earths and actinides have extensive solid solubility in zirconolite, mainly but not exclusively in the Ca site. Tetravalent rare earths and actinides have considerable solid solubility in the Zr site of zirconolite, and some solubility in the Ca site, but the strong tendency of zirconolite with ions substituted in the Zr site to undergo phase separation complicates structural interpretation. In zirconolite-rich Synroc-type ceramics designed to immobilise waste actinides, the target actinide waste loading has been set at 20 wt% and early leach results indicate the durability is at least as good as that of Synroc-C.

Trivalent Pu can be incorporated in the silicate apatite structure to form Ca2Pu8(SiO4)6O2 by sintering under reducing conditions, while the incorporation of tetravalent Pu in the Ca/rare earth sites in oxidizing or neutral conditions is limited to only 0.6 formula units (f.u.). The d‐spacings and intensities of the X‐ray pattern of hexagonally structured Ca2Pu8(SiO4)6O2 after firing at 1250°C are given, and the a and c lattice parameters are 0.95611 and 0.70281 nm, respectively. The respective solid solubility limits of U and Hf in Ca2Gd8–x(U/Hf)x(SiO4)6O2 apatite samples were 0.3 and 0.2 f.u.

Up to 0.15 formula unit (f.u.) of U4+ incorporated in the Zr site of zirconolite by firing CaUxZr(1−x)Ti2O7 compositions in argon at 1400°C allows retention of the 2M polytype. Further U4+ substitution for Zr, up to 0.4 f.u., produces the 4M polytype (containing ∼0.4 f.u. of U) plus the 2M polytype containing 0.15 f.u. of U. The pyrochlore structure (containing 0.6 f.u. of U) forms in conjunction with the 4M polytype at U contents of 0.4 f.u. up to 0.7 f.u. Higher U contents give the pyrochlore structure, but the solid‐state reactivity of even alkoxide‐based preparations becomes increasingly poor for x > ∼0.7 and hot pressing in graphite dies at ∼1250°C is necessary to achieve near single‐phase pyrochlore structures for x= 1. When samples of CaUxZr1−xTi2O7 stoichiometry (x= 0.1 and 0.2) are oxidized at 1400°C in air, diffuse reflectance spectroscopy (DRS) shows evidence for U5+ formation at the expense of U4+ via enhanced absorption bands, ∼50 nm in half‐width, near 970 and 1500 nm and correspondingly weakened absorption at 1150 and 1660 nm. Weight gains consistent with complete oxidation of U4+ to U5+ are observed when finely powdered argon‐fired samples with x= 0.1 and 0.4 are heated in air to 1200°C. Evidence for U valence states higher than +4 in both argon‐ and air‐heated materials containing charge compensators to encourage U5+ or U6+ formation was also derived from DRS (showing U5+ in particular), and XANES. DRS shows weak absorption bands attributable to U4+ in zirconolites containing ∼0.2 f.u. of U incorporated in the Ca site via Mg or Al substitutions in the Ti sites, with the spectrum being closely but not exactly similar to that attributed to U4+ in the Zr site. Zirconolite and pyrochlore compositions which are melted at 1500° or 1550°C in argon and furnace‐cooled yield broadly similar phase assemblages to the corresponding sintered materials, but there is evidence of incongruent melting in all materials.

The lower limit of the size of the octahedral A4+ ion in the ATi2O6 brannerite structure is just smaller than that of Ce/Pu. Attempts to expand the A ion size beyond that of Th by (a) substituting a Ba ion plus two U5+ ions for three A ions or (b) substituting one Ba plus one hexavalent ion for two A ions did not succeed. Ge, Sn and Zr substitutions in the Ti site of ThTi2O6 do not exceed 0.2 formula unit in ceramic preparations. These and other coupled substitutions in the B site of ThTi2O6 showed that the average B site size could tolerate deviations of < 1%. Ce4+ is unusually stabilised in air atmospheres at temperatures close to the melting point of 1400°C in the A site of brannerite. Lattice parameter data on different endmember ATi2O6 brannerites are given. The lower and upper size limits for the eightfold A ions in the pyrochlore structure are around 0.100 and 0.117 nm respectively. A BaUTi2O7 stoichiometry did not produce a pyrochlore structure, and when fired in either argon or air yielded a mixture of BaUTiO6, whose structure is still uncertain, plus brannerite and rutile.

At room temperature the sequence of phases with increasing amounts of strontium in the system CaTiO3/SrTiO3is orthorhombic (Pnma), orthorhombic (Bmmb), tetragonal (I4/mcm), and cubic (Pm3m). All phase boundaries shift toward smaller strontium contents with increase of temperature. Volume changes resulting from phase transformations are small for all compositions. Shape changes are greatest (∼0.3%) for theBmmb/I4/mcmtransition, but would probably be accommodated by microtwinning and so are unlikely to affect the mechanical integrity of a specimen.

This study reports on the use of zirconolite-rich Synroc to demonstrate the safe immobilisation of ‘high-fired’ PuO 2 . The zirconolite-rich Synroc used in this study was prepared by adding 13 wt% Pu with equimolar amounts of Gd and Hf, relative to Pu, as neutron absorbers. The incorporation of the Pu and neutron absorbers has been studied microstructurally as well as by longer-term leach testing. This work has shown that the sintered ceramic can immobilise 13 wt% of Pu with almost complete incorporation of the Pu (≈ 98%) into the zirconolite phase. Durability studies have shown that under a wide range of leaching conditions there is no major separation of the Pu and neutron absorbers, with the majority of these elements either remaining in the matrix or leaching at low (<10 -4 g m -2 d -1 ) and comparable rates from the waste form.

Very low or negative thermal expansion apparently can result from three different mechanisms.In the case of anisotropic materials such as cordierite (Mg2Al4Sis01s).thermal expansion in certain directions is coupled with thermal contraction in other directions.The net result can be a very small volume thermal expansion.This behavior can be easily modeled as caused by the thermal expansion of Mg-0 bonds.In .the thermal expansion of Li-0 bonds is very important, but the slight negative volume expansion requires that the Li distribution over tetrahedral and octahedral sites change as temperature changes.A third mechanism for very low or negative thermal expansion is based on anisotropic thermal vibration of oxygen in open network structures.This is the apparent mechanism for negative thermal expansion of certain compounds with the cubic ZrP207 structure such as ZrV 2 0 7 .A remarkable example of this behavior is found in cubic ZrW20s.where negative thermal expansion is observed from 0.3 K to 1050 K. Materials with negative thermal expansion are finding applications in composites to lower the overal thermal expansion of the composite.

Ca(1−x)Gd(x)TiO3samples prepared by sintering in air at 1550°C exhibit a primary perovskite-structured solid solution forxup to about 0.2. It is concluded from quantitative X-ray microanalysis using a scanning electron microscope that charge compensation in these solid solutions takes place via the formation of one formula unit of Ti3+per formula unit of Gd. High-resolution transmission electron microscopy showed that thex= 0.15 sample consisted of a pure perovskite structure, with no observable stacking faults and only a few dislocations. Microanalysis showed that Gd3+substituted for Ca2+can be charge compensated by Al3+substituted for Ti4+, as expected. Micro_analysis also indicated that charge compensation can take place in air-fired Ca(1−3x/2)Gd(x)TiO3byx/2 formula units of Ca vacan_cies per formula unit of Gd for compositions in whichx< 0.3.

Several aspects of Synroc which fall into the broad class of interface phenomena are discussed. These are radiation damage processes which give rise to interfaces between damage tracks and neighbouring unirradiated material, intergranular films which have deleterious effects on chemical durability, and aqueous leaching of Synroc which takes place primarily at the interface between the solid and groundwater.

Nd3+ and Ce3+ can substitute for about 65% of the Ca in monoclinic zirconolite, using AI as a charge compensator in a Ti site. Further substitution up to 85% of Nd3+ and Ce3+ produces an orthorhombic structure, while more than 85% substitution produces additional phases. Substitution of Ce4+ in the Zr site appeared to be quite limited. Incorporation of U4+ into the Ca and Zr sites in zirconolite gave results which were similar to those observed by others. Both trivalent and tetravalent Np and Pu can be substituted in the Ca and Zr sites, respectively, under oxygen partial pressures of 0.2−1 × 10−5 atm, provided appropriate charge compensators are present. The implications of these results for formulating actinide-bearing zirconolite-rich ceramics are discussed briefly.

Previously reported anomalous thermal expansion effects in the 1200-1500-degrees-C range for hot-pressed CaZrTi2O7 were shown to be due to irreversible bloating effects, from occluded gases. X-ray diffraction and differential thermal analysis of ordered CaZrTi2O7 did not reveal evidence of a solid-state transformation at temperatures up to 1450-degrees-C.

The thermal expansion coefficients of zirconolite, both ordered and disordered, and perovskite, were measured by X-ray powder diffraction analysis in the temperature range 25-1200-degrees-C. The principal results are: (1) The mean thermal expansion coefficients (K-1) of ordered zirconolite are: alpha(a) = (11.35 +/- 0.25) x 10(-6); alpha(b) = (8.72 +/- 0.22) x 10(-6) ; alpha(c) = (10.0 +/- 0.5) x 10(-6). (2) The mean thermal expansion coefficients (K-1) of disordered zirconolite are alpha(parallel-to) = (9.89 +/- 0.15) x 10(-6) in the (001) plane and alpha(perpendicular-to) = (9.37 +/- 0.14) x 10(-6) normal to (001). (3) The mean thermal expansion coefficients (K-1) of perovskite are: alpha(a) = (7.86 +/- 0.30) x 10(-6); alpha(b) = (13.46 +/- 0.17) x 10(-6); alpha(c) = (16.55 +/- 0.26) x 10(-6).

Perovskite is the least durable of the resistate minerals comprising Synroc-C and it is desirable to reduce its abundance in Synroc. Kinetic limitations and competition with Csapparently affect the incorporation of Sr into hollandite during hot-pressing at 1200°C/20 MPa so that ~ 10% of perovskite (a value below the percolation limit) is probably an optimum target. Zirconolite-rich Synroc formulations have been prepared for actinide-rich wastes. Background XRD and TEM studies have also been performed to study the crystal-chemical behaviour of Nd (a simulant of trivalent actinides) in zirconolite. Either rare-earth compensated perovskite or freudenbergite in Synroc can evidently be used to immobilise Na-bearing HLW.

Crystallisation of zirconolite (CaZrTi2O7) from a stoichiometric alkoxide precursor was studied by X-ray diffraction, differential thermal analysis and high resolution electron microscopy. The X-ray amorphous oxide formed by drying mixed ethanolic solutions crystallised to a disordered fluorite structure on heating to ~ 700 °C. Zirconolite with a pseudo-trigonal structure formed at ~ 900 °C, and at higher temperatures gradually transformed to fully ordered monoclinic zirconolite. Activation energies for the amorphous to fluorite and fluorite to pseudo-trigonal zirconolite transformations were obtained from the DTA measurements by the Ozawa method, but were not consistent with the rates of transformation observed by X-ray diffraction in isothermal measurements. The pseudo-trigonal zirconolite was found by high-resolution electron microscopy to be highly disordered monoclinic zirconolite.

Specimens of perovskite (CaTiO3) irradiated with fast neutrons have been examined by X-ray diffraction. Expansion of the unit cell was anisotropic. The volume expansion increased with dose and approached a saturation value of 3.8%. X-ray diffraction lines showed both broadening and attenuation. The attenuation could be explained by postulating random atomic displacements, akin to thermal disorder, having an r.m.s. magnitude of ~ 0.02 nm. Recovery of the lattice parameters takes place in the temperature range 300–900°C, with a spectrum of activation energies. Effective activation energies of 2.52 ± 0.08 and 3.73 ± 0.13 eV were observed during the early and middle stages of recovery.

Re-examination of the X-ray powder diffraction data obtained from a compound in the CaOTiO2 system, reported to be Ca2Ti5O12, suggests that the compound is a hitherto unknown defect pyrochlore of composition Ca2Ti2O6. The lattice parameter is 9.953 ± 0.002 Å.

Fast neutron irradiation of barium hollandite to a dose of 6.5 × 1023n/m2 (⩾1 MeV) eliminates all high-angle X-ray reflections (2θ>70°) and selectively broadens and attenuates the low-angle reflections. It produces lattice parameter changes of + 0.10% in the a direction and −0.36% in the c direction. These changes are attributed to isolated vacancies and planar clusters of interstitials.