zro2- and hfo2-based materials
RE Free Zr02- and Hf02-Based Materials and Their Solid Solutions
As it was mentioned above, Zr02 and НЮ2 are considered as twin oxides. Both of them have the bandgap of 5.8-6.5 eV, calculated conduction band offset on Si of about 1.5 eV and experimental one is in the range 1.4-2.2 eV that is large enough for microelectronic application. Zr02 and НЮ2 being doped with silicon can form different silicates. Among them, ZrSi04 and HfSi04 are glassy oxides with bandgaps of -6.5 eV. ZrSi04 (HfSi04) consists of chains of alternate edge-sharing Zr04 (НЮ4) and Si02 tetrahedra, with additional Zr-O (Hf-O) bonds between the chains, leading to an overall six-fold Zr(Hf) coordination. The bandgap of both silicates was estimated to be about 6.5 eV with the conduction band offset of 1.5 eV and sometimes a bit higher than that for Zr02 (НЮ2).
Most of the properties of these materials are determined by the native defects that are different types of oxygen vacancies as it is shown for oxides in Figure 9.7. Similar defects are observed in their silicates. In most cases, intense cathode- and photoluminescence from oxygen vacancies can be found (Korsunska, & Khomenkova, 2019). However, to achieve intense light emission in specific spectral range, the doping with other ions can be performed similar to the case described for A1203.
Considering the nature of these defects as well as their charge states, Yang et al. (2018) reported on the first-principles calculations of energetically favorable defect type with the structural stability in ZrSi04 as well as on the formation energy, electronic property, and the structural deformation of point defects in ZrSi04. Similar conclusions can be made for Hf silicates also.
It was found that isolated neutral point defects are unlikely to be frequently observed. Besides, the positive charged vacancy has the lowest formation energy under five equilibrium conditions. However, when locate close to the conduction band minimum, they become more easily observed than in the О-rich environment. The charge density differences show that the charges are mainly localized on cation or anion vacant sites. Because of the Coulomb interaction, the nearest О atom of cation and anion vacant sites exhibits an opposite relaxation behavior. For the interstitial case, and with 4+ charge state are energetically favorable, no matter the environment is О-rich or О-poor. In addition, the charge is concentrated at the interstitial sites and it is also spread between the ions along the atomic bonds. The Frenkel-pair and Schottky defects with charge states have much lower formation energy than their neutral counterpart. This finding allowed understanding of another role of Zr02-based materials such as catalysts or markers of different reaction. For instance, to use them for the storage state and disposal of excess weapons-grade Pu and high- actinide wastes.
One other interesting case is the doping of Hf02 with Zr and vice versa. Such materials demonstrate not only the unique magnetic properties (see for instance, Lee et al., 2016; Shi et al., 2010; Starschichet al., 2017), but also the stability of chemical composition of the film and no Hf and Zr diffusion at 750°C in Si substrate. This fact gives a hand for the application of such films in negative capacitance FinFET transistors. One more advantage of such films is their amorphous nature contrary to single oxide counterparts that minimizes leakage current. The transformation of their structure stimulated by annealing starts after annealing at 950°C and, depending on the film composition, a different crystalline phase is formed ranging from monoclinic to tetragonal with increasing Zr content.
It should be noted that both НЮ2 and Zr02 have high refractive index (n - 1.9-2.2 at 1.95 eV, depending on preparation method), high optical transparency in the ultraviolet- infrared spectral range (bandgap is about 5.8-6.5 eV), and compactness and hardness offer optical applications as well. The phonon cutoff energy (-about 780 cm-1) reduces the probability of nonradiative phonon-assisted relaxation that is attractive for doping these materials in particular with the RE elements (Khomenkova, Korsunska, Labbe, Portier, & Gourbilleau, 2019). However, in spite of the mentioned advantages, these oxides are not often addressed as a host for the RE ions.
RE-Doped Zr02 and Hf02 and Their Silicates
Among different RE ions, trivalent erbium (Er3+) is considered aiming at the application in optical communication. The trivalent neodymium (Nd3+) was used in the inorganic laser materials attracting great attention of scientific researches and industrial applications. Nd3+ ions exhibit broad and strong absorption band around 800 nm and a very intense emission in the near-infrared luminescence range from 800 to 1430 nm associated with the 4F3/2—>4Ij (7 = 9/2, 11/2, 13/2) optical transitions in the 4/inner electronic shell of Nd3+ ions, following “four-level” scheme. The 4F3/2 emitting state can be populated conveniently by the emission of low-cost commercially available laser diodes. The doping of Hf02 and/or Zr02 and their silicates with RE ions allows not only to obtain specific light emission, but also to develop different types of catalysis and markers of chemical reactions.
Nd-Doped Zr Silicates
In the case of trivalent RE ions such doping results in the formation of oxygen vacancies required for RE charge compensation. These vacancies can play positive role that is the stabilization of tetragonal or cubic phases demonstrating higher dielectric permittivity and better catalytic activity. However, the appearance of numerous oxygen vacancies can result in their clustering and formation of cavities or voids causing the mechanical instability of the material. For instance, this was demonstrated for the ZrSi04 doped with Nd3+ ions by Li et al. (2019). It was shown that such materials are promising candidates for actinide immobilization. The series of Zr1.vNdvSi04.Jv/2 (x = 0, 0.02, 0.20, and 1.0) ceramics was studied in terms of phase evolution and acidity on the chemical durability of ZrSi04- based nuclear waste forms. The single-phase ZrSi04 sample was found to have better chemical stability than that of the biphasic Zr0.8Ndo.2Si03.9 sample due to the existence of a secondary highly soluble phase (Nd2Si207), which increases the contact area with leachate (Figure 9.8.)
The degradation of Nd2Si207 ceramics after leaching into different leachates for 42 days at room temperature is clearly seen. Relatively slight corrosion at the grain boundaries was found for the microstructure of the samples with x = 0.20 leached into 1 M HN03 leachate (Figure 9.8h). The increase of HN03 concentration up to 4 M HN03 results in significant surface corrosion of the samples with x = 0.2 (Figure 9.8i). Besides, a small number of pores caused by corrosion are observed from the microstructure of the Nd2Si207 ceramics after leaching into the 0.1 and 1 M HN03 leachates (Figure 9.8j, k). The Nd2Si207 ceramic leached into the 4M HN03 leachate became highly porous, increasing the area of the contact between the sample and the leachate (Figure 9.81). At the same time, no significant changes on the surfaces of ZrSi04 and Zro.98Ndo.02SiO3.99 ceramics are observed in all leachates (Figure 9.8a-f). This observation showed that doping of ZrSi04 with Nd ions of small quantity (x = 0.02) results in smaller grain size and higher compactness of the ceramic along with high enough stability of the surface toward HN03 treatment. The increase of Nd content as well as acid concentration results in the increase of both normalized release rates of Zr and Nd in the Zri_vNd vSi04.v/2 ceramic waste forms. The lowest values for Zr and Nd were detected in ZrSi04 and Zr0.98Nd0.02SiO3 99 ceramics, respectively, whereas the highest ones were reported for the Zr0.8Nd0.2SiO3 9 and Nd2Si207 ceramics. The difference in the normalized release rates of Zr and Nd was explained by the difference
FIGURE 9.8 SEM images of Zri.xNdxSi04.x/2 (x = 0, 0.02, 0.20. and 1) ceramics after 42 days of leaching in 0.1, 1, and 4 M HNOj at room temperature.
in the energies of their bonds with oxygen atoms as well as by the changes in the surfaces of the leached ceramics. Specifically, the formation of oxygen vacancies due to Nd3+ incorporation in ZrSi04, required by the charge compensation, results in the weakening of the lattice and degradation of ceramic surface.