Zr/Hf aqueous-acid clusters are relevant to inorganic nanolithography, metal−organic frameworks (MOFs), catalysis, and nuclear fuel reprocessing, but only two topologies have been identified. The (Zr 4 ) polyoxocation is the ubiquitous square aqueous Zr/Hfoxysalt of all halides (except fluoride), and prior-debated for perchlorate. Simply adding peroxide to a Zr oxyperchlorate solution leads to a striking modification of Zr 4 , yielding two structures identified by single-crystal X-ray diffraction. Zr 25 , isolated from a reaction solution of 1:1 peroxide/Zr, is fully formulatedZr 25 is a pentagonal assembly of 25 Zr-oxy/peroxo/ hydroxyl polyhedra and is the largest Zr/Hf cluster topology identified to date. Yet it is completely soluble in common organic solvents. ZrT d , an oxo-centered tetrahedron fully formulated [Zr 4 (OH) is isolated from a 10:1 peroxide/Zr reaction solution. The formation pathways of ZrT d and Zr 25 in water were described by small-angle X-ray scattering (SAXS), pair distribution function (PDF), and electrospray ionization mass spectrometry (ESI-MS). Zr 4 undergoes disassembly by 1 equiv of peroxide (per Zr) to yield small oligomers of Zr 25 that assemble predominantly in the solid state, an unusual crystal growth mechanism. The self-buffering acidity of the Zr-center prevents Zr 25 from remaining intact in water. Identical species distribution and cluster fragments are observed in the assembly of Zr 25 and upon redissolution of Zr 25 . On the other hand, the 10:1 peroxide/Zr ratio of the ZrT d reaction solution yields larger prenucleation clusters before undergoing peroxide-promote disassembly into smaller fragments. Neither these larger cluster intermediates of ZrT d nor the smaller intermediates of Zr 25 have yet been isolated and structurally characterized, and they represent an opportunity to expand this new class of group IV polycations, obtained by peroxide reactivity and ligation.

This work complements our recent discovery of new phases derived from zirconium perchlorate by addition of hydrogen peroxide. Here, we investigate analogous reactions with hafnium perchlorate, which is found to have modifications of the Clearfield–Vaughan tetramer (CVT). For hafnium perchlorate derivatives, we find distorted versions of CVT by X-ray diffraction and study the reaction solutions by SAXS, Raman spectroscopy, and ESI-MS. Furthermore, we investigate mixed Hf–Zr solution and solid phases and find the latter resemble the zirconium family at low Hf concentrations and the hafnium family at higher hafnium contents.

Four defect complexes with indium tracer atoms in undoped and lightly doped Ce02 have been observed by In time-difFerential perturbed-angular-correlation spectroscopy. One complex, identified as an indium-0-vacancy pair, is always present unless the material is doped with Nb or Ta to eliminate oxygen vacancies. The binding energy of the vacancy to indium is greater than 0.35 eV. In materials doped or annealed to introduce as little as 0.05% oxygen vacancies, two different double-vacancy complexes are formed. One is apparently a complex in which the vacancies are trapped on opposite sides of the indium along (ill) directions. The other is likely a complex with one oxygen vacancy near the indium and one in a more distant trap. In material suKciently doped with Nb, the majority of dilute indium dopants are uncomplexed, but a small fraction is strongly bound to a presently unidentified impurity. At low temperatures, an electronic excitation trapped at the time of the parent In decay to Cd clouds interpretation of some details. We speculate that this "aftereffect" affects only Cd nuclei having no vacancies in their nearest-neighbor oxygen shell. Some complexes involving only more distantly trapped vacancies may be unobserved.

Zirconium and hafnium in the tetravalent oxidation state are considered the two most similar elements on the periodic table, based on their coexistence in nature and their identical solid-state chemistry. However, differentiating solution phase chemistry is crucial for their separation for nuclear applications that exploit the neutron capture of Hf and neutron transparency of Zr. Here we provide molecular level detail of the multiple factors that influence Zr/Hf separation in a long-exploited, empirically designed industrial solvent-extraction process that favors Hf extraction into an organic phase. In the aqueous solution, both Hf and Zr form an oxo-centered tetramer cluster with a core formula of [OM4(OH)6(NCS)12]4– (OM 4 -NCS, M = Hf, Zr). This was identified by single-crystal X-ray diffraction, as well as small-angle X-ray scattering (SAXS), of both the aqueous and organic phase. In addition to this phase, Zr also forms (1) a large oxo-cluster formulated [Zr48O30(OH)92(NCS)40(H2O)40] (Zr 48 ) and (2) NCS adducts of OZr 4 -NCS. Zr 48 was identified first by SAXS and then crystallized by exploiting favorable soft-metal bonding to the sulfur of NCS. While the large Zr 48 likely cannot be extracted due to its larger size, the NCS adducts of OZr 4 -NCS are also less favorable to extraction due to the extra negative charge, which necessitates coextraction of an additional countercation (NH4 +) per extra NCS ligand. Differentiating Zr and Hf coordination and hydrolysis chemistry adds to our growing understanding that these two elements, beyond simple solid-state chemistry, have notable differences in chemical reactivity.

We have used several experimental methods to study how a large extrinsic oxygen vacancy density in pure tetragonal ZrO 2 powders depends on details of how those powders are made. Samples were made from oxychloride and nitrate precursor solutions. We used perturbed angular correlation spectroscopy to determine in situ phase structure and the density of oxygen vacancies at 1200°C, XRD and SEM to determine the grain size and morphology of samples annealed at temperatures ranging from 200°-1200°C, and neutron activation analysis (NAA) to investigate purity of samples. NAA results showed that samples contain cation impurities at levels <<100 ppm. The XRD and SEM measurements showed that grains were nanometer-size, had a broad distribution, and grew from ϳ10 nm at 200°C to ϳ1 m at 1200°C. The most striking process dependence is on presence of chlorine during processing. The grain size and phase above 600°C, and both the morphology and the density of oxygen vacancies at 1200°C were strongly affected by presence of chlorine-containing vapor during annealing. Samples processed in a chlorine-free atmosphere had large well-sintered grains and large (>500 ppm) oxygen vacancy concentrations at 1200°C, whereas samples processed in flowing H 2 O/HCl vapor had smaller grains, porous morphology, and small (<100 ppm) vacancy density. All samples were loose powders consisting of single grain particles at <1000°C and multiple-grain particles at 1200°C.