Abstract
N-dopants in bulk monoclinic ZrO2 and their magnetic interactions have been investigated by DFT calculations, using the B3LYP hybrid functional. The electronic and magnetic properties of the paramagnetic N species, substitutionals and interstitials, are discussed. Their thermodynamic stability has been estimated as a function of the oxygen partial pressure. At 300 K, N prefers interstitial sites at any range of oxygen pressure, while at higher temperatures (700–1000 K), oxygen poor-conditions facilitate substitutional dopants. We have considered the interaction of two N defects in various positions in order to investigate the possible occurrence of ferromagnetic ordering. A very small magnetic coupling constant has been calculated for several 2N-ZrO2 configurations, thus demonstrating that magnetic ordering can be achieved only at very low temperatures, well below liquid nitrogen. Furthermore, when N atoms replace O at different sites, resulting in slightly different positions of the corresponding N 2p levels, a direct charge transfer can occur between the two dopants with consequent quenching of the magnetic moment. Another mechanism that contributes to the quenching of the N magnetic moments is the interplay with oxygen vacancies. These effects contribute to reduce the concentration of magnetic impurities, thus limiting the possibility to establish magnetic ordering.
Highlights
The two electrons associated to an O vacancy, which occupy energy levels lying high in the gap, are transferred to the low-lying N 2plevels, with a consequent net energy gain. This mechanism leads to a complete quenching of the magnetic moment on the N-dopants, and further contributes to the reduced magnetic ordering at high temperature
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Summary
The investigation of N-doped ZrO2 was carried out with periodic DFT calculations employing the Becke-339 and Lee-Yang-Parr[40] (B3LYP) exchange and correlation functional as implemented in the CRYSTAL14 program[41]. The choice to use B3LYP instead of other, theoretically more sophisticated, hybrid functionals is dictated by the fact that using the same approach we have studied in the past other N-doped oxides: N-TiO2, N-SnO2, N-ZnO, N-MgO44,45. This allows us to directly compare the nature of a Nitrogen impurity in oxides with different electronic and geometric structure, providing the basis for a solid comparative study. All electron basis set have been used for O and N atoms: 8–411(d1) and 7–311(d1), respectively. The hyperfine spin-Hamiltonian, Hhfc =S · A · I, is given in terms of the hyperfine matrix A, which described the coupling of the electron with the nuclear spin
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