Immobilization of high-level waste calcine (radwaste) in perovskites

In the field of applied mineralogy, the technical applications of perovskites are of great interest due to their extraordinary ferroelectric, dielectric, pyroelectric, and piezoelectric properties. Therefore, they are applied for different purposes like automobile exhaust purification, fuel cells, N₂O decomposition, solar cells (photovoltaics), and in many other fields of technology. The broad fields of application are based on the variety of perovskite structures, due to their structural modularity and the great chemical variability. Those skills are presented in the outstanding compilation of the perovskite family, Perovskites: Modern and Ancient, by Mitchell. Usually, perovskites with the general formula ABO₃ are well known, and it is assumed that perovskites should have cubic symmetry sg. Pm3m (aristotype), which is true for many perovskite phases, but dependent on temperature, pressure, and bulk chemistry; perovskites exhibit complicated tilt systems, which are responsible for the stabilization of hettotypes related to the aristotype by group-subgroup relations. Different types and degrees of disorder for the coordination polyhedra of A- or B-site are a result of the decrease in symmetry. Aside from ABX₃-perovskites with a cubic close packing, a large group of layered perovskites with a hexagonal close packing also exists. Their unit cells can be transformed easily and hexagonal perovskites exhibit polytypes with large stacking layer sequences. We have to deal with ordered perovskites, the so-called double perovskites with 1:1, 1:2, and 1:3 ordering either on A- or B-site. Ion ordering on both A- and B-sites is also possible. Quadruple-layered defect perovskites belong to this species. They are related closely to layered superconducting perovskites of the 1212C-type, where most of them are derived from Ruddlesdon-Popper and Arivillius compounds. Layered phases of the Dion-Jacobson series and A_nB_nX_3n+2 compounds also belong to the large perovskite family. Another important property of perovskite phases is the high degree of A- or X-site deficiency comprising bronzes like ReO₃ or the framework defect perovskites, such as the brownmillerite group A₂B₂X₅, whose members of the solid solution system Ca₂Fe₂O₅-Ca₂Al_1.33Fe_0.66O₅ are important in the field of cement chemistry. Not only artificial perovskites are of interest for mineralogists, because up to 93% of the lower mantle consists of silicate perovskites. At high pressures and temperatures, comparable to the conditions at Earth's core-mantle boundary, MgSiO₃ perovskite transforms to a new high-pressure form, the so-called post perovskite. The majority of titanates and niobates in the earth crust and certain halides and hydroxide (soehngenite, schoenfliesite, and stottite groups) are minerals with a perovskite-type structure. In this work, the application of CaTiO₃, a member of the synroc (synthetic rock) phases, should be proofed to act as a host for the fixation of radwaste (radioactive waste) ions. Of special interest is the crystal chemistry of very different perovskite phases and their solid solutions series, which are formed under the heat treatment of synroc ceramics together with different types of radwastes at non-ambient temperatures. The close relation between natural and technical phases should be demonstrated.