Quadrupole Interaction at Surfaces -Nuclear Methods

An Isotope Separator On-Line (ISOL), dedicated to on-line perturbed angular correlation (PAC) and positron annihilation spectroscopy (PAS), has been constructed at CIAE based on the HI-13 tandem accelerator, and a 30 keV radioactive beam of 62Zn with intensity of 2 ×107/s produced by it. Using the 62Zn radioactive beam the investigation of the nano-crystalline soft magnetic Fe73.5Cu1Nb3Si13.5B9 materials has been fulfiled for the first time by the on-line time differential PAC technique. Two quadrupole interaction frequencies ω01 ( = 440 Mrad/s) and ω02 ( = 90 Mrad/s) were gained, indicating there are two implantation sites of 62Zn. The frequency ω02 shows a fluctuation of the crystal lattice constants and proves the characteristic distribution model of nano-crystalline grain boundary. The frequency ω01 demonstrates that the implanted 62Zn occupies the site of the ordered grain core. The fractions of ω01 and ω02 are f01 = 38 % and f02 = 62 %, respectively.

PAC study of martensitic transformations in RCu compounds

Rare-earth-deficient R_(1-x)Ni_2 Laves phases, which reportedly crystallize in a C15 superstructure with ordered R vacancies, have been investigated by perturbed angular correlation (PAC) measurements of electric quadrupole interactions at the site of the probe nucleus ^111Cd. Although ^111Cd resides on the cubic R site, a strong axially symmetric quadrupole interaction (QI) with frequencies ν(q)approximate to265-275 MHz has been found in the paramagnetic phases of R_(1-x)Ni_2 with R=Pr,Nd,Sm,Gd. This interaction is not observed for the heavy R constituents R=Tb,Dy,Ho,Er. The fraction of probe nuclei subject to the QI in R_(1-x)Ni_2, R=Pr,Nd,Sm,Gd, decreases from 100% at low temperatures to zero at T>300 K and 500 K for R=Sm,Gd and R=Pr,Nd, respectively. At T=100 K the QI is static within the PAC time window, but at T=200 K fluctuations with correlation times τ_(C) 500 K nuclear spin relaxation related to vacancy hopping is observed in nearly all R_(1-x)N_i2. Auxiliary ^111Cd PAC measurements have been carried in Sc_0.95Ni_2, ScNi_2, ScNi_0.97, Gd_2Ni_(17), GdNi_5, GdNi_3, and GdNi.

The theory of the electronic structure of clean metal and semiconductor surfaces is reviewed, starting from an effective one-electron Schrodinger equation. Methods for solving the Schrodinger equation at surfaces are briefly described, and the effects of the surface on the electronic wavefunctions are discussed using simple models. The results of detailed calculations of the surface electronic structure of s-p bonded metals, transition metals and semiconductors are reviewed, with an emphasis on the effect of the local environment on the density of states. Properties like the work function and surface energy depend on the surface electronic structure, and their variation with material and surface is discussed; the surface energy contains an important contribution from the interaction between electrons, and this will be considered in some detail. The change in electronic structure compared with the bulk leads to changes in atomic structure, with surface reconstruction on semiconductor and some metal surfaces, and this is also discussed. The interplay between theory and experiment is very important in surface studies, and the theoretical surface energy bands reviewed compare well with experimental photoemission results; this comparison has proved particularly useful for understanding surface reconstructions.

The possible presence of a large magnetic field due to spin polarization of a Cd nucleus (decay product of 111In) at an Al substitutional site in AlN is investigated with perturbed angular correlation (PAC) spectroscopy. The PAC spectra of 111In/111Cd in AlN show two probe environments: a weak quadrupole interaction (quadrupole interaction constant, \(\nu _{\rm Q}^{\,\,\,\rm lattice} = 30\) MHz) due to 111In probes at a defect free Al substitutional site and an unknown large interaction (\(\nu _{\rm Q}^{\,\,\,\rm complex} = 300\) MHz) tentatively attributed to a nearest neighbour pair between 111In and a nitrogen vacancy (VN) aligned along the c-axis. Surprisingly, in density functional theory (DFT) calculations, such a large electric field gradient (EFG) could not be reproduced. However, an inclusion of spin polarization in the calculations indicates a strong magnetic field at ~50 % of the 111In/111Cd site. An attempt to verify the presence of the strong magnetic field and to explain the origin of the strong interaction is made. Orientation measurements show, the large interaction is not characterised by a magnetic interaction and is predominantly due to the EFG. However, in the presence of an external magnetic field, the strong interaction probe environment becomes more uniform and the EFG increases by ~10 %. This definitely hints towards some sort of magnetic interaction at the strong interaction probe site.

Theory-Guided Machine Learning Finds Geometric Structure-Property Relationships for Chemisorption on Subsurface Alloys

In the present work, a simplified model of the Fe(111) surface’s promoter-oxide system was investigated in order to experimentally verify the previously proposed and known models concerning the structure and chemical composition of the surfaces of iron nanocrystallites in the ammonia-synthesis catalyst. It was shown that efficient oxygen diffusion from metal oxides to the clean Fe(111) iron surface took place even at temperatures lower than 100 °C. The effective wetting of the iron surface by potassium oxide is possible when the surface is covered with oxygen at temperatures above 250 °C. In the TOF-SIMS spectra of the surface of iron wetted with potassium, an emission of secondary FeOK+ ions was observed that implies that potassium atoms are bound to the iron surface atoms through oxygen. As a result of further wetting the iron surface with potassium ions, a heterogeneous surface structure was formed consisting of a thin K2O layer, next to which there was an iron-oxide phase covered with potassium ions. Only a limited increase in calcium concentration was observed on the Fe(111) iron surface upon sample annealing at up to 350 °C. As a result of wetting the iron surface with calcium ions, an oxide solution of CaO-FexOy was formed. In the annealing process of the sample containing alumina, only traces of this promoter diffusing to the iron surface were observed. Alumina formed a solution with a passive layer on the iron surface and under the process conditions (350 °C) it did not wet the pure iron (111) surface. The decrease in Fe+-ion emission from the Fe-Ca and Fe-Al samples at 350 °C implies a reduction in the oxygen concentration on the sample surface at this temperature.

Perturbed angular correlation (PAC) spectroscopy has been used to characterize several structural aspects of a high-temperature, ionic conducting ceramic, ${\mathrm{CaZr}}_{3.95}$${\mathrm{Hf}}_{0.05}$${\mathrm{P}}_{6}$${\mathrm{O}}_{24}$. Hafnium was introduced into the material to provide the PAC probe nuclei, $^{181}\mathrm{Hf}$${/}^{181}$Ta, which were located primarily at Zr sites. PAC measurements were made over a range of temperatures from 77 to 1180 K, and they have been analyzed and interpreted using several simple models. The distorted octahedral crystal field at the Zr site produced a (low-frequency) static electric quadrupole interaction which can be accurately described by the point-charge model. But, the temperature dependence of the associated electric field gradient (EFG) cannot be described accurately by purely static considerations via the point-charge model and high-temperature x-ray diffraction data. Although a high-frequency static interaction was also observed, the measurements were not sufficiently accurate to identify its origin unambiguously. Some of the high-temperature measurements show evidence of a time-varying interaction, which may result from ${\mathrm{Ca}}^{2+}$-ion jumping. But, jump frequencies derived classically from high-temperature electrical dc conductivity measurements are too low to agree with those indicated by the PAC data. However, the dc conductivity measurements support a simple model of thermally activated ${\mathrm{Ca}}^{2+}$-ion transport. The temperature dependence of the EFG (corresponding to the low-frequency interaction) was used to determine an effective Debye-Waller factor. As a result of using this approach to analyze this type of PAC data, this factor was shown also to agree qualitatively with the predictions of the Debye crystal model, although significant theoretical limitations were encountered. These particular results suggest that the PAC technique may provide new insights into understanding advanced ceramic materials.

Recent developments in both methods, Perturbed Angular Correlation (PAC) and Perturbed Angular Distribution (PAD) concern improvements in the instrumentation for various applications and sophistications in detail rather than methodology. These will be illustrated by examples from PAC and PAD studies which investigate local structural properties of semiconductors and metallic surfaces, and local magnetic contributions in ferromagnetic metals, nonmagnetic materials, and high-temperature superconductors and related oxides.

Studies are presented of the static quadrupole interaction of the 482 keV level of 181Ta in a polycrystalline HfO2 source, made using a differential perturbed angular correlation (PAC) method. The experimental procedure and method of analysis are described and the results obtained for the following interaction parameters are: the transition frequencies ω0 = 819 ± 10, ω1 ± 1483 ± 25, ω2 = 2350 ± 50 Mrad/s, the quadrupole interaction frequency ωQ = 125.8 ± 1.9 Mrad/s, the asymmetry parameter η = 0.283 ± 0.027, and the electric field gradient at active lattice sites Vzz = 13.1 ± 0.6 × 1017 V/cm2. These results are compared with those recently obtained by other workers in this field.

Thermal stability of a supersaturated solution of hafnium in aluminium

The bonding of alkali metals to semiconductor surfaces: a direct and inverse photoemission study

The local atomic order around very dilute indium impurities in c-axis-oriented YBa2Cu3O7 and YBa2Cu3O6.25 films at room temperature and in YBa2Cu3Ox bulk powders held in air or oxygen over a wide temperature range were investigated using 111In/Cd \gamma--\gamma perturbed angular correlation (PAC) spectroscopy. The probe atoms occupy a single substitutional lattice position in YBa2Cu3Ox, and evidence reported here strongly supports previous claims that this is the yttrium position. In YBa2Cu3Ox powders, the local atomic order changes continuously with temperature. At room temperature the electric field gradient measured by PAC in bulk YBa2Cu3Ox powder is indistinguishable from that in the fully oxygenated YBa2Cu3O7 film. Near the decomposition temperature, the bulk YBa2Cu3Ox powder data are semiquantitatively similar to data in the room temperature YBa2Cu3O6.25 film, with quantitative differences that we attribute to temperature averaging in the former. Other sites usually found in PAC spectra of YBa2Cu3Ox bulk samples arise from 111In dissolved in Y2BaCuO5 and Y2Cu2O5 impurity phases. The latter phase apparently arises because of significant barium loss during processing; the barium deficiency is clearly demonstrated by comparison of PAC data with the alloy phase diagram above the decomposition temperature. PAC data on pure Y2Cu2O5 are reported here also.

Study of atomic relaxations on clean and oxygen covered (100), (410) and (510) copper surfaces by channeling

The phase transformations of nanoscaled (n-) ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$-coated and noncoated ${\mathrm{ZrO}}_{2}$ particles synthesized in a microwave plasma have been investigated by perturbed-angular-correlation (PAC) measurements of the electric quadrupole interaction (QI) of ${}^{181}\mathrm{Ta}$ on Zr sites between 290 and 1600 K. For the phase identification and structural characterization the QI parameters of the nanoscaled particles are compared to those of coarse-grained ${\mathrm{ZrO}}_{2}$ which were measured between 290 and 2160 K. The PAC spectra of the nanoscaled particles in the as-prepared state are characterized by a broad distribution of strong, axially asymmetric QI's, which reflects a highly disordered oxygen environment of the Zr sites. Upon annealing, the tetragonal phase is the first well-crystallized structure to emerge at about 500 K, both in coated and noncoated $n\ensuremath{-}{\mathrm{ZrO}}_{2},$ in contrast to the previously reported annealing reaction of $n\ensuremath{-}{\mathrm{ZrO}}_{2}$ synthesized by gas-phase condensation in which the tetragonal phase has not been observed. This disorder-order transformation is partially reversible upon cooling. In $n\ensuremath{-}{\mathrm{ZrO}}_{2}{/\mathrm{A}\mathrm{l}}_{2}{\mathrm{O}}_{3}$ synthesized in a microwave plasma the monoclinic phase can be completely suppressed up to 1600 K. In the noncoated particles monoclinic ${\mathrm{ZrO}}_{2}$ starts to appear at 600 K. At 1400 K the transformation from the monoclinic to the tetragonal phase of noncoated $n\ensuremath{-}{\mathrm{ZrO}}_{2}$ was observed to occur with a transformation rate of $\ensuremath{\lambda}=6.9(1.2)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}{\mathrm{s}}^{\mathrm{\ensuremath{-}}1}.$ In nanoscaled coated and noncoated tetragonal ${\mathrm{ZrO}}_{2}$ the ${T}^{3/2}$-temperature dependence of the quadrupole frequency ${v}_{q}$ is weaker than in the same phase of the coarse-grained compound, suggesting a decrease of the mean-square vibrational amplitudes with decreasing particle size.