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κ dielectrics
Published in Michel Houssa, κ Gate Dielectrics, 2003
Gian-Marco Rignanese, Xavier Gonze, Alfredo Pasquarello
The cubic and tetragonal crystalline structures of HfO2 and ZrO2 are illustrated in figure 4.5.1. The cubic phase takes the fluorite structure (space group Fm3¯m), which is fully characterized by a single lattice constant a. The M = (Hf, Zr) atoms are in a face-centred-cubic structure and the O atoms occupy the tetrahedral interstitial sites associated with this fee lattice. The unit cell contains one formula unit of MO2 with M = (Hf, Zr). The tetragonal phase (space group P42/nmc) can be viewed as a distortion of the cubic structure obtained by displacing alternating pairs of O atoms up and down by an amount Δz along the z direction, as shown in figure 4.5.1, and by applying a tetragonal strain. The resulting primitive cell is doubled compared to the cubic phase, including two formula units of MO2. The tetragonal structure is completely specified by two lattice constants (a and c) and the dimensionless ratio dz = Δz/c describing the displacement of the O atoms. The cubic phase can be considered as a special case of the tetragonal structure with dz = 0 and c/a= 1 (if the primitive cell is used for the tetragonal phase, c/a=2).
Nuclear Fuel Materials
Published in C. K. Gupta, Materials in Nuclear Energy Applications, 1989
Regarding alternative fuel cycles, the attraction for the oxide is that it takes its chemically rather similar fissile and fertile relative — PUO2 and ThO2 — into solid solution across the whole range of compositions. All possess the cubic fluorite structure. With so many attractive properties rolled into one, it is no wonder that UO2 has made a name for itself in the service as nuclear fuel.
Effect of radiation and substitution of Ce4+ at Zr site in Y4Zr3O12 using collision cascades: a molecular dynamics simulation study
Published in Journal of Nuclear Science and Technology, 2023
Mohammed Ado, Qingyu Wang, Shehu Adam Ibrahim, Simon Ochieng Adede
δ-phase oxide materials with fluorite-related structures demonstrate exceptional properties that make them appropriate in immobilisation of high-level radioactive nuclear wastes [1–3] and inert matrices (IMF) for plutonium (Pu) and minor actinides (MAs) [4,5]. δ-phase (A4B3O12) belongs to space group R-3 (148) of the fluorite structure, with an orderly arranged trivalent and tetravalent cations (A43+ and B34+) and oxygen anion sublattices [6–12]. Where A and B are metal cations, and O represents oxygen anions. Figure 1 depicts one of the six different cation distributions of the 19 atoms rhombohedral primitive cell [13]. δ-phase has three Wyckoff ions position in the crystal structure. The sites are 3a which is completely occupied by B4+ cations (denoted as B3a), the 18 f site randomly occupied by A3+ cation (denoted as A3+), B4+ cation (denoted as B18f), and the oxygen anions in two sets of 18 f locations, one at 3a cation (denoted as OI) and the other at the body centre (denoted as OII) [13–15]. The third position is 6c, usually occupied by two ordered vacancies with an inversion triad along the [111] direction [13,16]. δ-phase are known for outstanding radiation-induced amorphization resistance and phase stability at elevated temperature [4,5,17–19].
First-principles determination of intergranular atomic arrangements and magnetic properties in rare-earth permanent magnets
Published in Science and Technology of Advanced Materials, 2021
As a candidate for the crystalline intergranular phase in Nd-Fe-B permanent magnets, various crystal structures for Nd-Fe alloys are considered as fcc-type intergranular phases. For stoichiometric NdFe, i.e., NdFe, first-principles calculations identify the fluorite structure as the most stable among tested crystal structures. Then, the variation in the composition as NdFe is examined by the atom substitution as well as the vacancy formation. Furthermore, the addition of a third element is also examined as NdFe alloys, where stands for Al, Co, Cu, and Ga. Randomness in the atomic configuration of antisites, vacancies, and third-element atoms at the Fe sites are considered by the SQS method [45]. The formation energy for is calculated as
Effects of CeO2 geometry on corrosion resistance of epoxy coatings
Published in Surface Engineering, 2020
Wenbo Zhang, Huaiyuan Wang, Chongjiang Lv, Xixi Chen, Zhiqiang Zhao, Yongquan Qu, Yanji Zhu
To further probe into the crystal structures of the nanofillers, the XRD patterns of CeO2 nanospheres, CeO2 nanorods, FCNS, FCNR and EP composite coatings (EP, PVDF, FCNS/PVDF/EP and FCNR/PVDF/EP) are typically depicted in Fig. S3. CeO2 nanospheres and CeO2 nanorods both exhibit a strong sharp crystalline peak of the (111) plane at 2θ =28.4° and the peak of CeO2 nanospheres is higher, suggesting CeO2 nanospheres are more crystallised than CeO2 nanorods [27]. Moreover, the characteristic planes of (200), (220), (311), (222), (400), (331), (420) and (422) are attributed to the cubic fluorite structure according to CDD (PDF2.DAT) (CeO2/Cerianite, syn, DB card number 00-043-1002). The XRD patterns of FCNS and FCNR both remain almost the same after functionalisation, indicating the chemical modification did not change the crystal structure. For pure EP, a broad peak at 2θ of 21° resulted from the scattering of the cross-linking network of EP matrix, which indicates the amorphism of EP. Moreover, FCNS/PVDF/EP and FCNR/PVDF/EP show all characteristic peaks of PVDF and CeO2, which may be attributed to the homogenous dispersion of nanofillers in the entire EP matrix [28].