Intrinsic and radiation-induced defects in monoclinic ZrO2: polaron mechanism of defect formation

Mechanisms of radiation-induced defects formation in m-ZrO2 powders were investigated using DFT+U calculations. In unirradiated crystal, oxygen vacancies exist in doubly charged state (VO2+) in threefold coordinated lattice sites together with two remote electron polarons. As a consequence, additional configurational entropy leads to considerable increase in vacancy and polaron concentrations. After crystal irradiation, the Frenkel pairs VO2+ - Oi2− and electron-hole polaron pairs are formed. Interaction of such quasiparticles with Frenkel pairs leads to formation of the long-lived metastable centers observed in EPR experiment. Various charge states of oxygen vacancies were represented as complexes of doubly charged vacancy and one, two or three electron polarons. The most stable is complex VO+ with one trapped polaron, while concentration of another paramagnetic state of vacancy VO− is negligibly small at room temperature. Concentration of equilibrium electron polarons (Zr3+ centers) is sufficiently decreased after irradiation because such polarons are captured by nonequilibrium VO2+ vacancies of Frenkel pairs. O− centers were identified as the interstitial oxygen which traps the hole polaron. Electron-hole polaron pairs in irradiated m-ZrO2 itself are short-lived defects and cannot be observed in experiment. However, interaction of polarons with native defects and impurities is important for formation of long-lived defects. In particular, we elucidate the nature of radiation-induced activation of phosphorus impurity. We show that in the ZrO2 doped with phosphorus the self compensation occurs. After irradiation, polaron is captured by PZr+ that converts this defect into paramagnetic state PZr0.