The technique of hydrogen/deuterium isotopic substitution has been used to extract detailed information concerning the solvent structure in pure ammonia and metallic lithium-ammonia solutions. In pure ammonia we find evidence for approximately 2.0 hydrogen bonds around each central nitrogen atom, with an average N-H distance of 2.4 A. On addition of alkali metal, we observe directly significant disruption of this hydrogen bonding. At 8 mol % metal there remains only around 0.7 hydrogen bond per nitrogen atom. This value decreases to 0.0 for the saturated solution of 21 mol % metal, as all ammonia molecules have then become incorporated into the tetrahedral first solvation spheres of the lithium cations. In conjunction with a classical three-dimensional computer modeling technique, we are now able to identify a well-defined second cationic solvation shell. In this secondary shell the nitrogen atoms tend to reside above the faces and edges of the primary tetrahedral shell. Furthermore, the computer-generated models reveal that on addition of alkali metal the solvent molecules form voids of approximate radius 2.5-3.0 A. Our data therefore provide new insight into the structure of the polaronic cavities and tunnels, which have been theoretically predicted for lithium-ammonia solutions.

Photoluminescence, infrared absorption, positron annihilation, and deep-level transient spectroscopy ͑DLTS͒ have been used to investigate the radiation damage produced by 24 GeV/ c protons in crystalline silicon. The irradiation doses and the concentrations of carbon and oxygen in the samples have been chosen to monitor the mobility of the damage products. Single vacancies ͑and self-interstitials͒ are introduced at the rate of ϳ1 cm −1 , and divacancies at 0.5 cm −1 . Stable di-interstitials are formed when two self-interstitials are displaced in one damage event, and they are mobile at room temperature. In the initial stages of annealing the evolution of the point defects can be understood mainly in terms of trapping at the impurities. However, the positron signal shows that about two orders of magnitude more vacancies are produced by the protons than are detected in the point defects. Damage clusters exist, and are largely removed by annealing at 700 to 800 K, when there is an associated loss of broad band emission between 850 and 1000 meV. The well-known W center is generated by restructuring within clusters, with a range of activation energies of about 1.3 to 1.6 eV, reflecting the disordered nature of the clusters. Comparison of the formation of the X centers in oxygenated and oxygen-lean samples suggests that the J defect may be interstitial related rather than vacancy related. To a large extent, the damage and annealing behavior may be factorized into point defects ͑monitored by sharp-line optical spectra and DLTS͒ and cluster defects ͑monitored by positron annihilation and broadband luminescence͒. Taking this view to the limit, the generation rates for the point defects are as predicted by simply taking the damage generated by the Coulomb interaction of the protons and Si nuclei.

The technique of isotopic substitution in neutron diffraction has been used to measure the structure of saturated solutions of lithium and potassium in ammonia. Isotopic substitution of *N by N15, combined with difference analysis, shows that K+ acts as a “structure breaking” ion while Li+ acts as a “structure making” ion. As a consequence, it is only for the latter that we observe intermediate range order, in the form of a pre-peak at ∼1 Å−1 in the k-space data. From our analysis, we also find that lithium is tetrahedrally coordinated by ammonia molecules, at both 230 and 100 K. Potassium, on the other hand, is octahedrally coordinated at 160 K.

No abstract

Phase-pure magnesium ferrite (MgFe2O4) spinel nanocrystals are synthesized by a fast microwave-assisted route. The elemental composition is optimized via the ratio of the precursor mixture and controlled by energy-dispersive X-ray spectroscopy. Fine-tuning of the magnetic properties without changing the overall elemental composition is demonstrated by superconducting quantum interference device (SQUID) magnetometry and Mössbauer spectroscopy. Together with X-ray absorption spectroscopy and X-ray emission spectroscopy, we confirm that the degree of cation inversion is altered by thermal annealing. We can correlate the magnetic properties with both the nanosize influence and the degree of inversion. The resulting nonlinear course of saturation magnetization (M s) in correlation with the particle diameter allows to decouple crystallite size and saturation magnetization, by this providing a parameter for the production of very small nanoparticles with high M s with great potential for magnetic applications like ferrofluids or targeted drug delivery. Our results also suggest that the optical band gap of MgFe2O4 is considerably larger than the fundamental electronic band gap because of the d5 electronic configuration of the iron centers. The presented different electronic transitions contributing to the absorption of visible light are the explanation for the large dissent among the band gaps and band potentials found in the literature.

In situ X-ray absorption and emission spectroscopies (XAS and XES) are used to provide details regarding the role of the accessibility and extent of redox activity of the Mn ions in determining the oxygen reduction activity of LaMnO3 and CaMnO3, with X-ray absorption near-edge structure (XANES) providing the average oxidation state, extended X-ray absorption fine structure (EXAFS) providing the local coordination environment, and XES providing the population ratios of the Mn2+, Mn3+, and Mn4+ sites as a function of the applied potential. For LaMnO3, XANES and XES show that Mn3+ is formed, but Mn4+ ions are retained, which leads to the 4e– reduction between 0.85 and 0.6 V. At more negative potentials, down to 0.2 V, EXAFS confirms an increase in oxygen vacancies as evidenced by changes in the Mn–O coordination distance and number, while XES shows that the Mn3+ to Mn4+ ratio increases. For CaMnO3, XANES and XES show the formation of both Mn3+ and Mn2+ as the potential is made more negative, with little retention of Mn4+ at 0.2 V. The EXAFS for CaMnO3 also indicates the formation of oxygen vacancies, but in contrast to LaMnO3, this is accompanied by loss of the perovskite structure leading to structural collapse. The results presented have implications in terms of understanding of both the pseudocapacitive response of Mn oxide electrocatalysts and the processes behind degradation of the activity of the materials.

We report the changes with lattice isotope of the energies of the zero-phonon lines (ZPLs) and some of the local vibrational modes (LVMs) of commonly encountered radiation-damage centers in silicon. On changing from nat Si to 30 Si, ZPLs of the different centers shift by +0.8 to +1.8 meV. The carbon-oxygen "C" center is taken as the primary example. For this center, the measured changes in the frequencies of the LVMs in the electronic ground state of the center agree closely with the results of density functional theory (DFT). We suggest that the LVM frequencies in the excited state can be obtained from DFT calculations of the positive charge state. The effect on the ZPL is broken into three parts. The dependence of the ZPL on the isotopically induced change in the LVMs and in the lattice volume is shown to be small compared to the effects of the electron-phonon coupling to the continuum of lattice modes. This dominant effect can be found, in principle, from the temperature dependence of the energy of the ZPL, but data can only be measured over too small a temperature range. We suggest that an estimate of the isotope effects can be derived by rescaling the appropriate data for the indirect energy gap. This simple empirical approach reproduces the measured isotope shifts of the ZPLs of the "C" and "P" centers within ϳ10%, of the "I" and "T" centers within ϳ30%, and within a factor of 2 for the "G" center.

The structure of solutions of lithium in ammonia has been studied at 0, 2, 8, and 22 mol % metal (MPM) and 200 K by wide-angle x-ray diffraction. The principal diffraction peak shifts from 2.14(2) Å−1 at 0 MPM to 1.93(3) Å−1 at 22 MPM, reflecting the 30% decrease in overall density as the solution expands to accommodate the excess electrons. We find that the solvent is significantly perturbed over both the short- and intermediate-length scales. The nearest neighbor (N–N) coordination number decreases from 11.8(10) at 0 MPM to 7.6(10) at 22 MPM. In addition, electrostriction around the fourfold coordinated lithium ions causes N–N correlations to become progressively shorter as concentration is increased. At 22 MPM a strong diffraction prepeak is located at 1.05(3) Å−1. Upon dilution to 2 MPM, our experiments find that this feature shifts to 1.29(5) Å−1. We conclude that the prepeak observed in our experiments is a signature of polaronic solvent cavities of approximate radius 2.6 Å. The first solvation shell of an excess electron then contains about 7 ammonia molecules, the second shell about 30 ammonia molecules. This picture is in excellent agreement with interpretation of magnetic resonance data.