Hafnium Salts Burn First-principles Study Of Glauconite Geometry And Electronic Structure

Pyrochlore oxide materials have a general formula A₂B₂O₇, where A and B are metallic cations that can either be trivalent and tetravalent or divalent and pentavalent, such that they have a remarkable range of compositions, more than 500 combinations of A- and B-site chemistries. Pyrochlore oxide materials exhibit many fascinating phenomena, such as ferroelectric, ion-conducting, ferro- and ferri-magnetism, luminescence and giant magnetoresistance properties which make them have extensive applications in catalysis, piezoelectricity, fluorescence, fuel batteries and other similar parts of apparatus. Due to their important technological applications, pyrochlores have been extensively investigated both experimentally and theoretically. Of these compositions, the titanate and zirconate pyrochlores have received considerable attention. So far, a few investigations have been performed on hafnate pyrochlores.Within decade years ascribe to the increasing of computation level and method, the Density Functional Theory (DFT) has gone through a rapid development, and has been used to investigate the transition metal system. Recently, first-principles method has proved to be a useful tool for gaining fundamental understanding of the structural, electronic and energetic properties of materials. In this paper, systematic investigation of electronic properties of pyrochlore oxide materials has been carried out by means of periodic DFT calculations.1. The structural and electronic properties of A₂Hf₂O₇ (A = La and Gd) pyrochlore compounds are investigated by means of first-principles total energy calculations. Also, the formation energies of defects are calculated, and the results can be used to explain the stability of pyrochlores. Hybridizations between A 5p and O 2s and between A 5d and O 2p states are observed. Gd₂Hf₂O₇ compound shows much different density of state distribution from that of La₂Hf₂O₇. Mulliken overlap population analysis shows that the A-O48f and A-O8b bonds in Gd₂Hf₂O₇ are more ionic than the corresponding bonds in La₂Hf₂O₇, while the Hf-O48f bond in Gd₂Hf₂O₇ is more covalent. These calculations suggest that A-O48f and A-O8b bonds may play important roles in their responses to irradiation-induced amorphization observed experimentally.2. First-principles calculations based on DFT/GGA method have been carried out on A₂Hf₂O₇ (A = Dy, Ho, Er) compounds to explore their chemical stabilities and radiation response behavior. Our optimized structural parameters are in reasonable agreement with experimental values. The cation antisite, Frenkel-pair, coupled cation antisite/Frenkel-pair formation energies and split interstitial structure defect formation energies of these three compounds have also been calculated. It turns out that for Er₂Hf₂O₇ the defect fluorite structure is more stable than pyrochlore structure, agreeing well with previous investigations. Also, we noted that the coupled cation antisite/Frenkel-pair defect formation energies are very low for Dy₂Hf₂O₇ and Ho₂Hf₂O₇, indicating that these two compounds are easily to form to defect fluorite structures and will be resistant to amorphization under irradiation environment. That is to say, with the decreasing radii of A-site cations, the hafnium-based pyrochlores are more readily to form the defect fluorite structure which is agreement with previous investigations. Different from Er₂Hf₂O₇ pyrochlore, Dy₂Hf₂O₇ prefers pyrochlore structure, which is consistent with the recent experiment, but disagrees with earlier studies. For Ho₂Hf₂O₇, our finding that the pyrochlore structure is preferred does not support experimental and empirical theoretical results.