Mössbauer Spectrometry

Mössbauer spectrometry gives electronic, magnetic, and structural information from within materials. A Mössbauer spectrum is an intensity of γ-ray absorption versus energy for a specific resonant nucleus such as ^(57)Fe or ^(119)Sn. For one nucleus to emit a γ-ray and a second nucleus to absorb it with efficiency, both nuclei must be embedded in solids, a phenomenon known as the “Mössbauer effect.” Mössbauer spectrometry looks at materials from the “inside out,” where “inside” refers to the resonant nucleus. Mössbauer spectra give quantitative information on “hyperfine interactions,” which are small energies from the interaction between the nucleus and its neighboring electrons. The three hyperfine interactions originate from the electron density at the nucleus (the isomer shift), the gradient of the electric field (the nuclear quadrupole splitting), and the unpaired electron density at the nucleus (the hyperfine magnetic field). Over the years, methods have been refined for using these three hyperfine interactions to determine valence and spin at the resonant atom. Even when the hyperfine interactions are not easily interpreted, they can often be used reliably as “fingerprints” to identify the different local chemical environments of the resonant atom, usually with a good estimate of their fractional abundances. Mössbauer spectrometry is useful for quantitative phase analyses or determinations of the concentrations of resonant element in different phases, even when the phases are nanostructured or amorphous. Most Mössbauer spectra are acquired with simple laboratory equipment and a radioisotope source, but the recent development of synchrotron instrumentation now allow for measurements on small 10 µm samples, which may be exposed to extreme environments of pressure and temperature. Other capabilities include measurements of the vibrational spectra of the resonant atoms, and coherent scattering and diffraction of nuclear radiation. This article is not a review of the field, but an instructional reference that explains principles and practices, and gives the working materials scientist a basis for evaluating whether or not Mössbauer spectrometry may be useful for a research problem. A few representative materials studies are presented

Self-organized carbon nanostrips with a new LiC10 structure derived from carbon nanotubes

Single walled carbon nanotubes (SWNTs) were reacted with molten lithium at 220 °C for two weeks. This induced dramatic changes in their structure as shown by x-ray and electron diffractometry and Raman spectroscopy. A significant fraction of the initial SWNTs transformed into flat nanostrips having intercalated lithium in between them. Lithium forms a superlattice commensurate with that of the graphitelike nanostrips with [square root of]7×[square root of]3 in-plane distribution. This new structure corresponds to the LiC10 composition

An XRD Study of Chemical Self-Discharge in Delithiated Cobalt Oxide

Changes in samples of Li1–xCoO2 were measured by X-ray diffractometry (XRD) after thermal aging treatments that cause capacity losses in electrochemical cells. Changes in lattice parameters were used to identify lithium re-intercalation into Li1–xCoO2 when it was aged in the presence of LiClO4, LiPF6, and LiAsF6 in propylene carbonate (PC). Li+ re-intercalation could account for the reversible capacity loss. Thermal aging at 75°C in pure PC or pure argon gas resulted in other changes that are attributed to the formation of spinel phase. The rate of the lithium re-intercalation increases in the following sequence: LiPF6<LiClO4<LiAsF6

Phonon anharmonicity of rutile TiO_2 studied by Raman spectrometry and molecular dynamics simulations

Raman spectra of rutile titanium dioxide (TiO_2) were measured at temperatures from 100 to 1150 K. Each Raman mode showed unique changes with temperature. Beyond the volume-dependent quasiharmonicity, the explicit anharmonicity was large. A new method was developed to fit the thermal broadenings and shifts of Raman peaks with a full calculation of the kinematics of three-phonon and four-phonon processes, allowing the cubic and quartic components of the anharmonicity to be identified for each Raman mode. A dominant role of phonon-phonon kinematics on phonon shifts and broadenings is reported. Force-field molecular dynamics calculations with the Fourier-transformed velocity autocorrelation method were also used to perform a quantitative study of anharmonic effects, successfully accounting for the anomalous phonon anharmonicity of the B_1_(g) mode

Hexagonal to Cubic Spinel Transformation in Lithiated Cobalt Oxide

A transmission electron microscopy (TEM) investigation was performed on LiCoO2 before and after it had been subjected to charge/discharge cycling in electrochemical cells, as well as on delithiated Li(1–x)CoO2 before and after thermal aging. Turbostratic disorder involving small rotations of the O-Co-O slabs was found in as-received material, and in material subjected to a few cycles. In LiCoO2 subjected to extensive charge/discharge cycling, it was found that increasing amounts of the trigonal O3 phase had transformed to H1-3 phase and to the cubic spinel phase. The transformation appears to initiate on the surfaces of trigonal crystals. The orientation relationship between the trigonal and spinel phases was determined from diffraction patterns to be {0001}trigonal parallel {111}cubic and trigonal parallel cubic. The difference in unit cell dimensions leads to transformation stresses when spinel crystals are formed, and spallation of surface layers was observed. The formation of a spinel phase could suppress electrochemical performance of LiCoO2 cathodes in heavily cycled cells. Aging in the charged state also can alter particle surfaces and therefore the performance

Metallic Hydrides I: Hydrogen Storage and Other Gas-Phase Applications

A brief survey is given of the various classes of metal alloys and compounds that are suitable for hydrogen-storage and energy-conversion applications. Comparisons are made of relevant properties including hydrogen absorption and desorption pressures, total and reversible hydrogen-storage capacity, reaction-rate kinetics, initial activation requirements, susceptibility to contamination, and durability during long-term thermal cycling. Selected applications are hydrogen storage as a fuel, gas separation and purification, thermal switches, and sorption cryocoolers

Ab initio free energy of vacancy formation and mass-action kinetics in vis-active TiO2

Recent reports have identified bulk defects such as oxygen vacancies as key players in visible-light photoactive TiO2. This would imply greater visible light absorption rates may be possible provided effective defect engineering can be achieved. To further this we have developed methods to simulate vacancy formation in bulk TiO2 using ab initio techniques. Initial results of these methods show an entropic reduction in the free energy of vacancy formation of 2.3 eV over a range of 266 K. The use of this result is illustrated by a 'toy' mass-action kinetics model which offers insight into vacancy concentration, rate constants, and enthalpy of reaction

Miscibility gap and phonon thermodynamics of Fe-Au alloys studied by inelastic neutron scattering and nuclear-resonant inelastic x-ray scattering

Recent measurements of the phonon spectra of several Au-rich alloys of face-centered-cubic Fe-Au using inelastic neutron scattering and nuclear-resonant inelastic x-ray scattering are summarized. The Wills-Harrison model, accounting for charge transfer upon alloying, is used to explain the observed negative excess vibrational entropy of mixing, which increases the miscibility gap temperature in the system by an estimated maximum of 550 K and we adjudicate to a charge transfer from the Fe to the Au atoms that results in an increase in the electron density in the free-electron-like states and in stronger sd-hybridization. When Au is the solvent, this softens the Fe–Fe bonds but stiffens the Au–Au and Au–Fe bonds which results in a net stiffening relative to the elemental components

Phonon thermodynamics and elastic behavior of GaAs at high temperatures and pressures

The phonons of wurtzite and zinc blende GaAs were calculated at simultaneously elevated temperature and pressure, and elastic constants were calculated as functions of pressure. Pressure caused instabilities of shorter-wavelength transverse acoustic modes in both wurtzite and zinc blende GaAs, causing them to fall to zero at 18 and 20 GPa, respectively. The Born stability criteria, which depend on elastic constants and only long wavelength phonons, therefore overestimated the pressure needed to induce instability at 0 K. At elevated temperatures, explicit anharmonicity pushes the onset of instability to higher pressures in both wurtzite and zinc blende GaAs. Phonon linewidth and densities of states data showed that the quasiharmonic approximation failed to account for temperature-induced phonon frequency shifts, and the quasiharmonic approximation became less reliable at elevated pressure. In general, the number of three-phonon processes increased with pressure, thereby increasing the temperature-driven broadening of phonon spectral lineshapes

Spatial periodicities of defect environments in ^(57)Fe_3Al studied by Mössbauer powder diffractometry

Mössbauer powder diffractometry was used to study partially-ordered ^(57)Fe_3Al. Multiple diffraction patterns were measured at Doppler velocities across all nuclear resonances in the sample. The superlattice diffractions were analyzed to provide data on the long-range order of Fe atoms having different numbers of Al neighbors. Comparing experimental data to calculations showed that Fe atoms having three Al atoms as first-nearest neighbors (1nn) have simple cubic long-range order, similar to that of Fe atoms with four Al 1nn. The simple cubic periodicity of Fe atoms with three Al 1nn was significantly lower than expected for homogeneous antisite disorder, however. Monte-Carlo simulations and transmission electron microscopy suggest that a significant fraction of aperiodic Fe atoms with three Al 1nn are near antiphase domain boundaries