Isolated hydrogen configurations in zirconia as seen by muon spin spectroscopy and ab initio calculations


We present a systematic study of isolated hydrogen in diverse forms of ZrO2 (zirconia), both undoped and stabilized in the cubic phase by additions of transition-metal oxides (Y2O3, Sc2O3, MgO, CaO). Hydrogen is modeled by using muonium as a pseudoisotope in muon-spin spectroscopy experiments. The muon study is also supplemented with first-principles calculations of the hydrogen states in scandia-stabilized zirconia by conventional density-functional theory (DFT) as well as a hybrid-functional approach which admixes a portion of exact exchange to the semilocal DFT exchange. The experimentally observable metastable states accessible by means of the muon implantation allowed us to probe two distinct hydrogen configurations predicted theoretically: an oxygen-bound configuration and a quasiatomic interstitial one with a large isotropic hyperfine constant. The neutral-oxygen-bound configuration is characterized by an electron spreading over the neighboring zirconium cations, forming a polaronic state with a vanishingly small hyperfine interaction at the muon. The atom-like interstitial muonium is observed also in all samples but with different fractions. The hyperfine interaction is isotropic in calcia-doped zirconia [A(iso) = 3.02(8) GHz], but slightly anisotropic in the nanograin yttria-doped zirconia [A(iso) = 2.1(1) GHz, D = 0.13(2) GHz] probably due to muons stopping close to the interface regions between the nanograins in the latter case.






Vieira, RBL; Vilao, RC; Marinopoulos, AG; Gordo, PM; Paixao, JA; Alberto, HV; Gil, JM; Weidinger, A; Lichti, RL; Baker, B; Mengyan, PW; Lord, JS

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The technical help of the mu SR team at ISIS is gratefully acknowledged. We also thank INNOVNANO for providing the YSZ sample used in this work and acknowledge helpful discussions with Joao Calado, Elsa Antunes, and Sofia Soares. The computer resources of the Department of Physics of the University of Coimbra were used, including the Milipeia cluster at the Laboratory for Advanced Computing. This work was supported with funds from (i) FEDER (Programa Operacional Factores de Competitividade COMPETE) and from FCT - Fundacao para a Ciencia e Tecnologia under Projects No PEst-OE/FIS/UI0036/2014 and No PTDC/FIS/102722/2008; (ii) European Commission under the 7th Framework Programme through the

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