Response of lanthanide-bearing ternary oxides to extreme environments
Keywords: Geological and Environmental Sciences.

This dissertation reports phase modifications induced in A2B2O7 and A2BO5 structure-types by swift heavy ion irradiation, high temperature, and high pressure. Swift heavy ions are particles with specific energies of ~1 MeV/u or greater, which interact with matter primarily through electronic excitation. The dense energy deposition in the vicinity of a swift ion induces diverse structural and chemical modifications, including ion tracks, which are cylindrical damage zones along the ion trajectories. Temperature and pressure are critical thermodynamic parameters that control material properties, with the latter reducing interatomic distances and modifying electronic orbitals and bonding. Ion irradiation, annealing, and pressure treatment are versatile tools that can tailor intrinsic and extrinsic material parameters and synthesize novel phases not accessible at ambient conditions. In this dissertation, trends in pressure tolerance, radiation susceptibility, and subsequent annealing behavior are described in terms of compositional variation, crystal chemical characteristics, and topological degrees of freedom. The materials selected for this dissertation are well-suited for a wide range of engineering applications. For example, A2B2O7 pyrochlore-type materials find use as actinide host phases, inert matrix nuclear fuels, electrolytes in solid oxide fuel cells, and thermal barrier coatings for gas turbine engines. A2BO5-type materials are frequently used as neutron absorbers in control rods, charge-trapping flash memory devices, biosensors, and as a gate for electrolyte-insulator-semiconductor devices. These materials are particularly interesting because their structural and physical properties are highly dependent on their compositional variations, bonding, coordination environments, and structural degrees of freedom. Therefore, they provide an ideal system in which to predict and evaluate how complex structures respond to extreme conditions of swift heavy ion irradiation, high pressure, and high temperature. Furthermore, these extreme variables provide a unique means by which to create non-equilibrium states of matter, and as such, they enable possibilities for creating materials with novel properties that are tailored to performance requirements. Both A2Ti2O7 (A= Gd and La) and A2TiO5 (A = Nd, Gd, and Yb) structure-types become amorphous in response to swift heavy ion irradiation (2.0 GeV 181Ta and 2.2 GeV 179Au ions), with their susceptibility to structural modification strongly dependent on their composition. Compounds with large cation size mismatch (large rA/rB) are more susceptible to radiation damage and amorphize more easily than those with small mismatches. The composition dependence of the response of A2B2O7 and A2BO5-type materials to dense electronic excitation is attributed to the influence of the cation size mismatch on the energetics of cation site exchange. The morphology and diameter of ion tracks in these types of materials depend on the ability of displaced atoms to return to their initial sites during cooling from a thermal spike. A concentric, core-shell morphology surrounding an amorphous core is evident in the ion tracks of the pyrochlore Gd2Ti2O7 (Fd-3m), whereas complete amorphization within ion tracks is observed for the perovskite-type La2Ti2O7 (P21). In Yb2TiO5 (Fm-3m), a heterogeneous track morphology is found, in which the individual ion tracks consist of both amorphous and disordered crystalline phases. Upon isochronal annealing, irradiation-induced amorphous A2Ti2O7 and A2TiO5 phases recover their original structures. The recrystallization temperature depends on the energy difference between the amorphous state and accessible crystalline structures, as well as the kinetics of the atomic rearrangements necessary to recover these structures. Materials with a high degree of structural freedom, or fewer structural constraints, have lower recrystallization temperatures from lower energetic barriers for atomic rearrangement. When the B-site is substituted with Zr (A2Zr2O7), such that the rA/rB is reduced and bond type becomes more ionic, susceptibility to radiation damage decreases. Rather than amorphizing in response to ion irradiation, zirconate materials (A = Nd, Sm, and Er) were shown to disorder from a pyrochlore (Fd-3m) to a defect-fluorite (Fm-3m) structure. The disordering of zirconate pyrochlore is favored over amorphization due to the cation and anion site exchange that readily occurs, owing to the smaller mismatch of cation radii. An exception is found when the A-site is Nd, such that rA/rB is still relatively large (rA/rB = 1.56). In this case, rapid quenching during the thermal spike process inhibits recrystallization at the interface between the melted core and the crystalline matrix along the ion path, resulting in amorphization. Upon compression, all studied lanthanide complex oxides transform to a defect cotunnite-like (Pnma) structure. In A2Ti2O5-type materials, the stages of the phase transformation process and the onset pressure of the high pressure phase differ depending on the initial structure. While all compounds undergo a sluggish transformation to a defect cotunnite-like phase for a certain range of pressures, an intermediate metastable phase (P21am) is found when A2TiO5 has an initial orthorhombic structure (Pnma). Decompression of various A2TiO5 materials produce different metastable recovered phases. Most interestingly, a high pressure cubic X-type phase (Im-3m) is confirmed, for the first time, using high-resolution transmission electron microscopy on recovered pyrochlore-type Er2TiO5. In A2Zr2O7- type materials, high pressure formation of the defect cotunnite-like phase can be inhibited by pre-irradiating the materials, such that the increased concentration of anion defects from ion irradiation increases the enthalpy of formation of the cotunnite-like phase. Thus, the amorphous phase is made energetically favorable. The irradiation-induced modification of the phase stability field of pyrochlore compounds at high pressures is attributed to the interplay between the introduction of atomic defects by dense electronic excitation, changes in bonding that accompanies electron orbital overlap at high pressure, and collective atomic motions related to the high pressure phase transformations. The following conclusions can be drawn from the research described in this dissertation: (i) Increased susceptibility of a complex oxide in the systems A2B2O7 and A2BO5 to irradiation-induced damage is attributed to the increased enthalpy of formation from increased rA/rB, decreased cation electronegativity, decreased bond ionicity and decreased bond lengths; (ii) Recrystallization of amorphous material in A2Ti2O7 and A2TiO5 depends on the topological freedom of the materials and the energy difference between the amorphous and crystalline states, as well as the possible crystalline structures; (iii) A2TiO5 and A2Ti2O7 structure-types undergo high-pressure phase transformations to a defect-cotunnite-like phase (Pnma) with phase transformation pressures varying based on the A-site composition; (iv) Ion irradiation can alter the high pressure stability fields of these materials by introducing antisite defects that cause kinetic frustration of the system.