J. M. Gil

Analysis of Mössbauer spectra of silicate glasses using a two-dimensional Gaussian distribution of hyperfine parameters

A new method is proposed to evaluate the hyperfine parameter distribution in Mossbauer spectra of silicate glasses. The method assumes a distribution of isomer shift and quadrupole splitting with a two-dimensional Gaussian shape. Application of the method to Mossbauer spectra of CaOSiO2FeO glasses containing different redox ratios, Fe3+/Fe2+, is discussed. It is shown that the two-dimensional Gaussian distribution method leads to a very good description of the data with a relatively small number of free parameters.

Oxide muonics: II. Modelling the electrical activity of hydrogen in wide-gap and high-permittivity dielectrics

Following the prediction and confirmation that interstitial hydrogen forms shallow donors in zinc oxide, inducing electronic conductivity, the question arises as to whether it could do so in other oxides, not least in those under consideration as thin-film insulators or high-permittivity gate dielectrics. We have screened a wide selection of binary oxides for this behaviour, therefore, using muonium as an accessible experimental model for hydrogen. New examples of the shallow-donor states that are required for n-type doping are inferred from hyperfine broadening or splitting of the muon spin rotation spectra. Electron effective masses are estimated (for several materials where they are not previously reported) although polaronic rather than hydrogenic models appear in some cases to be appropriate. Deep states are characterized by hyperfine decoupling methods, with new examples found of the neutral interstitial atom even in materials where hydrogen is predicted to have negative-U character, as well as a highly anisotropic deep-donor state assigned to a muonium–vacancy complex. Comprehensive data on the thermal stability of the various neutral states are given, with effective ionization temperatures ranging from 10 K for the shallow to over 1000 K for the deep states, and corresponding activation energies between tens of meV and several eV. A striking feature of the systematics, rationalized in a new model, is the preponderance of shallow states in materials with band-gaps less below 5 eV, atomic states above 7 eV, and their coexistence in the intervening threshold range, 5–7 eV.

Shallow versus deep hydrogen states in ZnO and HgO

The muonium states mimicking interstitial hydrogen in ZnO and HgO are compared. Whereas in ZnO a theoretically predicted shallow donor state is confirmed, in HgO we find a considerably deeper state. The respective ionization temperatures are around 40 K and 150 K and the donor ionization energies are 19±1 and 136±3 meV, deduced from the temperature dependence of the µSR (muon spin-rotation) signal amplitudes. The µSR spectra provide a comprehensive characterization of the undissociated paramagnetic states: the hyperfine parameters, which measure the electron spin density on and near the muon, differ by a factor of ~30. These define a hydrogenic radius of 1.1 nm in ZnO but indicate a much more compact electronic wavefunction in HgO, more akin to those of Mu* and the AA9 centre in Si. These data should largely carry over to hydrogen as a guide to its electrical activity in these materials.

The first 25 years of semiconductor muonics at ISIS, modelling the electrical activity of hydrogen in inorganic semiconductors and high-κ dielectrics

Early muonium studies provided the very first atomistic pictures of interstitial hydrogen in semiconductors. By the time ISIS muons came on line, the main crystallographic sites, and the electronic structures for the neutral centres, were established in archetypal materials such as Si and GaAs. The results were quite unanticipated, and raised awareness of this deceptively simple defect system. This paper marks contributions to the subject made using ISIS muon beams, in the first 25 years of their operation since 1987. By this time, hydrogen was understood to be a significant and unavoidable impurity in all electronic grade material, and attention was turning to the interaction with charge carriers, revealing an equally unanticipated interplay of site and charge state. In particular, muonium spectroscopy now provides a model for hydrogen in dozens of materials where hydrogen itself is difficult or impossible to study directly, and is able to predict its effect on the electronic properties of new materials, such as those envisaged for optoeletronic or dielectric applications. Donor, acceptor and so-called pinning levels are known in a good many of these materials, revealing intriguing systematics and providing severe tests and challenges to current theory. Progress and prospects are summarized in this report, addressing the obvious questions such as ‘why, how and what next?’

Study of NbH phases using perturbed angular correlation techniques

Abstract Niobium hydride phases were investigated by the perturbed angular correlation technique. By this method the electric field gradient (EFG) at radioactive probe atoms inside the NbH lattice is measured. We find two distinct EFGs in the region of the ϵ phase of NbH whereas, in the temperature range of the ζ and β phases, only a single EFG with a spread around the mean value is observed. Only a very weak disturbance of the cubic symmetry is found in the α phase. The observed EFGs are discussed in the frame of the point charge model.

A magnetization and neutron powder diffraction study of compounds (R = Y, Dy, Ho, Er)

Neutron powder diffraction experiments and magnetic measurements were performed on compounds of the series (R = Y, Dy, Ho and Er). The influence of the R element on both the structural and the magnetic properties of the different compounds is discussed, as well as the possible correlation between the iron environments and the local moments. Comparison is made with a previous Mossbauer study on the same compounds.

PAC study of O-H and O-D in Ta

A PAC experiment was performed on a181HfTa sample which contained ∼ 0. 1 at. oxygen and ∼1 at. hydrogen. At low temperature a new quadrupole interaction withνQ=470 MHz, η=0.95 was found. This interaction is attributed to an O-H complex. At 55 K this interaction disappears and the well known oxygen frequency shows up, indicating a breakup of the O-H pair at 55 K. With deuterium instead of hydrogen the same frequency but a different (105 K) breakup temperature was found.

Shallow donor versus deep acceptor state in II–VI semiconductor compounds

Abstract Information on the properties of the possible muonium states in II–VI semiconductor compounds is obtained in this study. In these materials, muonium may either form a shallow donor state, which is characterized by a small hyperfine interaction and a level-energy close to the conduction band, or an acceptor state, which corresponds to muonium at an interstitial site with a tightly bound electron and a hyperfine interaction close to that of free muonium. We show here that in CdS, CdSe and ZnO muonium preferentially forms a donor state whereas the acceptor state is preferred in ZnS and ZnSe. In CdTe both states are observed, indicating that the level energies are similar.

Phase transitions in the tantalum-hydrogen system observed by PAC

The perturbation felt by181Hf probes in a181HfTa lattice loaded with 30 at% hydrogen was observed by PAC as a function of temperature. Three different interactions were identified: 1) ΝQ1=433 (6) MHz, η=0.45 2) ΝQ2=142 (9) MHz, η=0.9, and 3) ΝQ≃0, σ=4–14 Μ−1 which are attributed to the Β−, ε− and α-phase in TaH system, respectively.

The Secondary Scintillation of Rare Gases under the Influence of Magnetic Fields

The influence of magnetic fields on the secondary scintillation of xenon and argon is studied. A small gas cell is used with excitation by alpha-particles and magnetic fields perpendicular to the direction of the electric fields. For reduced electric fields normally used in gas proportional scintillation counters the intensity of the secondary scintillation remains constant, within an accuracy of about ± 1 % for xenon and ± 2% for argon, when the magnetic field intensity varied from 0 to 0.4 Tesla. A discussion and interpretation of the experimental data obtained is presented. It is concluded that gas proportional scintillation counters can be used in high magnetic field environments.