The superionic conducting properties of fluorite structured compounds are discussed in this paper, starting with a brief historical overview of the topic. The current state of knowledge concerning the dynamic lattice disorder associated with the high values of ionic conductivity is summarized and illustrated using three examples taken from the author’s research program. These are chosen to highlight different aspects of the structure-property relationships within fluorite-structured superionics and, in particular, demonstrate the role of the neutron diffraction technique in investigations of compounds showing high levels of ionic conductivity. The paper concludes with an overview of some current developments within this research field.

Temperature dependent Photoluminescence (PL) and Raman spectra have been used to study the defect structure of untreated and thermochemically reduced calcia stabilized zirconia (CaSZ) and yttria stabilized zirconia (YSZ) single crystals. Over the whole temperature range studied the emission (EM) spectrum of the untreated crystals can be decompose into three broad bands, indicating the presence of several radiative electron transitions which can be associated with anion vacancies centers. Results points out to intrinsic F type centers and extrinsic F type centers (FA, FAA) as the main defect generated in the stabilization process. Effect of thermal reduction on the Raman activity caused a decreased in the acoustic mode region and a shift in the maximum of the excitation (EX) spectra. These variations can be related to a broad absorption band centered at ∼365 nm. This is consistent with a nonrandom arrangement of vacancies, which produces the superposition of periodic sequences of vacancies within domains. Concentration of the color centers was determined by optical absorption in the UV-VIS region. PL temperature dependence study in colored samples suggest that no significant new defect was generated in the reduction process, but a recombination of the charges states of the initial intrinsic and extrinsic F centers.

Y2O3 thin films deposited by Ion Beam Sputtering (IBS) deposition technique exhibit a particular disordered microstructure. In order to obtain a better knowledge on phase transition mechanisms occurring during irradiation, thin films with different microstructures have been implanted with xenon ions at different energies and different doses. This work established two types of transition (cubic-amorphous or cubic-monoclinic) depending mainly on the ion energy with a possibility to control the damaging kinetic via the pre-existing oxygen disorder.

New experimental results on pyrochlore and defect fluorite phases in HfO2-La2O3 and HfO2-Gd2O3 systems are summarized. Fluorite Hf0.5Gd0.5O1.75 was formed by containerless melting and quenching. Melts with 25-65 mol% La2O3 did not produce any fluorite-type phases, but pyrochlores with cell parameters 10.74 to 10.86 Å. The fluorite phase of Hf0.5La0.5O1.75 can be formed on crystallization of an amorphous precursor from aqueous precipitation. Both La- and Gd- fluorite phases transform to ordered pyrochlore on annealing at 1450 °C. The enthalpies of formation from oxides are −107 ±5 kJ/mol for Hf2La2O7 and −49 ±5 kJ/mol for Hf2Gd2O7 as measured by high-temperature solution calorimetry. Further experiments are needed to elucidate the nature of stabilization of fluorite phase in thin films and powders. Occurrence of disordered phases in thin films, nanoparticles and radiation damaged materials is discussed.

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.

We studied oxygen migration in calcia-stabilized cubic zirconia (CSZ) using density functional theory. A Ca atom was substituted for a Zr atom in a 2×2×2 ZrO2 cubic supercell, and an oxygen vacancy was produced to satisfy the charge neutrality condition. We found that the formation energies of an oxygen vacancy, as a function of its location with respect to the Ca atom, were varied. The relative formation energies of the oxygen vacancies located at the first-, second-, third-, and fourth-nearest-neighbors were 0.0, −0.07, 0.19, and 0.19 eV, respectively. Therefore, the oxygen vacancy located at the second-nearest-neighbor site of the Ca atom was the most favorable, the oxygen vacancy located at the first-nearest-neighbor site was the second most favorable, and the oxygen vacancies at the third- and fourth-nearest-neighbor sites were the least favorable. We also calculated the energy barriers for the oxygen vacancy migration between oxygen sites. The energy barriers between the first and the second nearest sites, the second and third nearest sites, and the third and fourth nearest sites were 0.11, 0.46, and 0.23 eV, respectively. Therefore, the oxygen vacancies favored the first- and second-nearest-neighbor oxygen sites when they drifted under an electric field.

Nanocrystalline 8 and 12 mol % yttria stabilized zirconia (YSZ) powders with fluorite-type structure were synthesized by a precipitation method. Powders were characterized by X-ray diffraction, differential scanning calorimetry, thermogravimetry, and nitrogen adsorption analyses. Precipitation produced amorphous powder which crystallized at approximately 450-470 °C into a cubic phase with a crystallization enthalpy ranging from 13.7 ± 0.6 kJ/mol for 8YSZ to 11.7 ± 0.5 kJ/mol for 12YSZ. Integral heat of water adsorption at room temperature measured on 8YSZ was -63 ± 2 kJ/mol for coverage 4.1 ± 0.1H2O/nm2. Drop solution calorimetry experiments were performed in a custom made Calvet twin calorimeter using sodium molybdate 3Na2O 4MoO3 solvent. The preliminary values for surface enthalpies of hydrated surfaces are 0.8 ± 0.1 J/m2 for 8YSZ and 1.3 ± 0.1 J/m2 for 12YSZ.

Selected area electron diffraction patterns are routinely used to determine the effects of irradiation damage in nuclear materials. Using zone axis orientations, the intensities of Bragg beams change from a dynamical to kinematic-like state due to the presence of amorphous domains in the material. Such changes in beam intensities, together with the increased diffuse scattering from the increasing amorphous fraction, present a major obstacle to the determination of cation or anion disorder in the crystalline fraction.

We have synthesized Gd-doped ceria polycrystalline samples (5, 10, 15 %mol), having relative densities exceeding 95% and grain sizes between 30 and 160 nm after axial hot pressing (750 °C, 250 MPa). The samples were prepared by sintering nanopowders obtained by sol-gel chemistry methods having a very narrow size distribution centered at about 16 nm. SEM and X-ray diffraction were performed to characterize the sample microstructures and to assess their structures. We report ionic conductivity measurements using impedance spectroscopy. It is important to investigate the properties of these systems with sub-micrometric grains and as a function of their composition. Therefore, samples having micrometric and nanometric grain sizes (and different Gd content) were studied. Evidence of Gd segregation near the grain boundaries is given and the impact on the ionic conductivity, as a function of the grain size and Gd composition, is discussed and compared to microcrystalline samples.

Synthetic pyrochlore samples Y2Ti2-xSnxO7 (x=0.4, 0.8, 1.2, 1.6), Nd2Zr2O7, Nd2Zr1.2Ti0.8O7, and La1.6Y0.4Hf2O7, were irradiated in-situ using the IVEM-TANDEM microscope facility at the Argonne National Laboratory. The critical temperatures for amorphisation have revealed a dramatic increase in tolerance with increasing Sn content for the Y2Ti2-xSnxO7 series. This change has also found to be linear with increasing Sn content. Nd2Zr1.2Ti0.8O7 and La1.6Y0.4Hf2O7 were both found to amorphise, while Nd2Zr2O7 was found to be stable to high doses (2.5×10^15 ions cm-2). The observed results are presented with respect to previously published results for irradiation stability predictions and structural disorder.

This work considers effect of type and content of dopant on the real structure, state of surface Pt species and oxygen mobility of nanocrystalline Lnx(Ce0.5Zr0.5)1-xO2-δ (Ln=La3+, Gd3+, Pr3+/4+) solid solutions prepared by Pechini route. For the reactions of methane selective oxidation and dry reforming into syngas, catalytic activity correlates rather well with either surface (in diluted feeds) or bulk (in realistic feeds) oxygen mobility as well as Pt dispersion controlled by the type and content of a dopant.

After an introduction to the rare earth – hydrogen phase diagram, stressing the often broad existence range of the solid solution (α), dihydride (β) and trihydride (γ) phases, we are describing in detail the fluorite-type dihydride and its superstoichiometric composition, RH2+x, where the x atoms occupy the available octahedral interstitial sites. It is shown how these additional x atoms interact with each other to form ordered H superlattices (sometimes distorting the cubic CaF2 structure) and how the latter influences the electronic structure of the systems modifying the magnetic properties and/or leading to metal-semiconductor transitions.

SSZ-based ceramics were obtained by sintering of nanopowders derived at room temperature by mechanochemical synthesis from refined technical grade ZrO2 nano-precursors. RT-treatment by 2.5 MeV electrons up to 1563 K was used for the modification of ceramics. Powders and ceramics were characterized by XRD, Raman, SEM and EDS, TEM, SIMS techniques. The phase composition of Zr0.89Sc0.1Ce0.01O1.95 ceramics was very close to cubic structure but better fitting of XRD patterns was obtained for rhombohedral lattice. Conductivity of solid electrolytes for IT SOFC was studied by complex impedance method. To stabilize cubic structure and increase conductivity at operation temperature of To ∼ 1000 K, the composition of SSZ solid electrolyte was optimized by addition of yttria and sintering aids. The interaction of admixtures with minor dopants leading to intergrain phase was revealed. During fast sintering, ceramics keep a memory about inhomogeneous disordered solid solutions in a form of nanostructuring. Conductivity data indicate nanostructuring of ceramics too: activation energies of bulk and grain boundary conductivities are close (Eb ∼ 0.9 eV, Egb ∼ 1.05 eV). Annealing of ceramics at high temperatures increases conductivity at To and promotes grain growth.