Developments in Mössbauer Spectrometry: From Instrumentation to High Pressure Studies on Spins and Phonons

Citation

Guzman, Pedro (2024) Developments in Mössbauer Spectrometry: From Instrumentation to High Pressure Studies on Spins and Phonons. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/hyry-q484. https://resolver.caltech.edu/CaltechTHESIS:05302024-193052659

Abstract

The well-established technique of ⁵⁷Fe Mössbauer spectrometry is used to investigate the local chemical environment in iron-containing materials. This technique relies on the recoil-free emission and absorption of γ-rays by resonant nuclei within a solid. The key component of a Mössbauer spectrometer is the velocity Doppler drive, which modulates the energy of the incident γ-rays to detect the hyperfine structure of resonant nuclei. Since the 1970s, the conventional velocity Doppler drive has been constructed using a pair of electromagnetic coils, one for power and the second for feedback. An alternative Mössbauer spectrometer was developed, utilizing an amplified piezoelectric actuator as the Doppler velocity drive under feedback control. The actuator, driven with a quadratic displacement waveform, produced a linear velocity profile and was optimized using measurements from a laser Doppler vibrometer (LDV). In transmission geometry, ⁵⁷Fe Mössbauer spectra of α-iron display minimal peak distortions, enabling Mössbauer spectrometry in applications requiring compact size and low mass, such as geochemical studies on the Moon, Mars, or asteroids.

Synchrotron radiation is used for numerous experimental techniques, including X-ray diffraction (XRD), nuclear resonant inelastic X-ray scattering (NRIXS), and nuclear forward scattering (NFS), also known as synchrotron Mössbauer spectrometry. Diamond-anvil cells, capable of reaching high pressures at various temperatures, combined with synchrotron experimental methods, provide the means to investigate the vibrational, magnetic, and thermophysical properties of materials. Measurements on ⁵⁷Fe₅₅Ni₄₅ were conducted using synchrotron XRD, NRIXS, and NFS under various pressures and temperatures. XRD measurements at 298 K and 392 K under pressures up to 20 GPa confirmed a pressure-induced Invar effect between 7 GPa and 13 GPa, where the coefficient of thermal expansion is nearly zero. NFS measurements revealed a decrease in the magnetic moment of ⁵⁷Fe under pressure, indicating an increase in magnetic entropy. The ⁵⁷Fe phonon density of states (DOS) was measured with NRIXS from which a phonon entropy was extracted. Using thermodynamic Maxwell relations, magnetic and phonon contributions to thermal expansion were determined, demonstrating that the low thermal expansion in the pressure-induced Invar region stems from a competition between the thermal expansion from spins and from phonons.

Thesis Files