Isolated hydrogen configurations in zirconia as seen by muon spin spectroscopy andab initiocalculations

Abstract

We present a systematic study of isolated hydrogen in diverse forms of ZrO 2 (zirconia), both undoped and stabilized in the cubic phase by additions of transition-metal oxides (Y 2 O 3 , Sc 2 O 3 , 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.

Slow-muon study of quaternary solar-cell materials: Single layers and p−n junctions

“…There are slight variations for the low temperature value of the depolarization rate, both for different materials and for the same material (CIGS) at different muon implantation energies, suggesting that there is an additional contribution to the diamagnetic signal. In recent experiments on zirconia [15] a slowly relaxing paramagnetic signal was observed. This signal is attributed to a weakly bound muonium state with an extremely small average hyperfine interaction.…”

“…In this configuration, the muon may exist as positive state (Mu + ) or as muonium (Mu 0 bound ). This latter state is paramagnetic but its hyperfine interaction is orders of magnitude smaller than the vacuum value, and may be barely distinguishable from the diamagnetic state [15,16].…”

“…(iii) Negative muonium, Mu − ; this state is predicted in theoretical calculations [15,[17][18][19][20] but its formation probability is small compared with other charged states since the formation of Mu − is a two-stepped process requiring the capture of two electrons in the implantation, an unlikely event.…”

Barrier model in muon implantation and application to Lu2O3

“…In the past few years we have used muon spin spectroscopy in order to model the behavior of the isolated hydrogen impurity in dielectric oxides [13,[18][19][20][21][22][23]. These materials find increasing interest and application in modern electronic devices [24][25][26].…”

“…This work has been performed often in conjunction with ab initio DFT calculations on the same systems [27][28][29][30][31]. Many similarities have been found, and the typical signature of the acceptor configuration shows up as the almost free muonium in an interstitial site [5,6,13,18,20,28,[31][32][33], whereas the donor configuration typically appears as a shallow-donor state bound to oxygen [5][6][7][8]20,23]. However, significant unexplained differences have been found in particular with respect to the formation probabilities of the different states.…”