C. N. Saunders, V. V. Ladygin, D. S. Kim, C. M. Bernal-Choban, S. H. Lohaus, G. E. Granroth, D. L. Abernathy, and B. Fultz, Phys. Rev. Mater., in press.
Atomic vibrational dynamics in cuprite, Cu_2O, was studied by inelastic neutron scattering and
molecular dynamics (MD) simulations from 10 K to 900 K. At 300 K, a diffuse inelastic intensity (DII)
appeared in the phonon dispersions, and dominated the spectral intensity at higher temperatures.
Classical MD simulations with a machine learning interatomic potential reproduced general features
of the DII. Better agreement with experiment was obtained with the addition of a stiffer potential at
close approaches of the Cu and O-atoms. The DII originates from random phase shifts of vibrating
O-atoms that have brief (10 fs) anharmonic interactions with neighboring Cu-atoms. The spectrum
of DII gives information about the interaction time of anharmonic interactions between atoms, and
its intensity gives a strength of coupling between vibrating atoms and a thermal bath.
Bernal-Choban, CM; Ladygin, V; Granroth, GE; Saunders, CN; Lohaus, SH; Abernathy, DL; Lin, JYY; Fultz, B, Commm. Mater. 51, 271 (2024). DOI10.1038/s43246-024-00695-x
The latent heat, L, is central to melting, but its atomic origin remains elusive. It is proportional to the entropy of fusion, Delta S_fus = L/T_m (T_m is the melting temperature), which depends on changes of atom configurations, atom vibrations, and thermal electron excitations. Here, we combine inelastic neutron scattering and machine-learned molecular dynamics to separate Delta(S_fus) into these components for Ge, Si, Bi, Sn, Pb, and Li. When the vibrational entropy of melting, Delta(S_vib), is zero, Delta(S_fus) is 1.2 k_B per atom. This result provides a baseline for Delta S_config and nearly coincides with "Richard's Rule" of melting. The Delta S_fus deviates from this value for most elements, however, and we show that this deviation originates with both extra Delta S_vib and extra Delta S_config. These two components are correlated for positive and negative deviations from Richard's rule - the extra Delta S_config is consistently close to 80% of Delta S_vib. Our results, interpreted with potential energy landscape theory, imply a correlation between the change in the number of basins and the change in the inverse of their curvature for the melting of pure elements.
Lohaus, SH; Heine, M; Guzman, P; Bernal-Choban, CM; Saunders, CN; Shen, G; Hellman, O; Broido, D; Fultz, B; Nature Physics 19, 1642 (2023) DOI10.1038/s41567-023-02142-z
The anomalously low thermal expansion of Fe-Ni Invar has long been associated with magnetism, but to date, the microscopic underpinnings of the Invar behaviour have eluded both theory and experiment. Here we present nuclear resonant X-ray scattering measurements of the phonon and magnetic entropies under pressure. By applying a thermodynamic Maxwell relation to these data, we obtain the separate phonon and magnetic contributions to thermal expansion. We find that the Invar behaviour stems from a competition between phonons and spins. In particular, the phonon contribution to thermal expansion cancels the magnetic contribution over the 0-3 GPa pressure range of Invar behaviour. At pressures above 3 GPa, the cancellation is lost, but our analysis reproduces the positive thermal expansion measured separately by synchrotron X-ray diffractometry. Ab initio calculations informed by experimental data show that spin-phonon interactions improve the accuracy of this cancellation over the range of Invar behaviour. Spin-phonon interactions also explain how different phonon modes have different energy shifts with pressure.
The iron-nickel alloy Invar has an extremely small coefficient of thermal expansion that has been difficult to explain theoretically. A study of Invar under pressure now suggests that there is a cancellation of phonon and spin contributions to expansion.
Guzman, P; Quine, CM; Lohaus, SH; Schul, L; Toda, R; Scott, VJ; Fultz, B; Rev. Sci. Instr. 94, 115107 (2023). DOI10.1063/5.0157651
A Mossbauer spectrometer was built and evaluated using an amplified piezoelectric actuator under feedback control for the Doppler velocity drive. The actuator was driven with a quadratic displacement waveform, giving a linear velocity profile. The optimization of the piezoelectric Doppler drive under feedback control was performed with measurements from a laser Doppler vibrometer. 57Fe Mossbauer spectra of alpha-iron in transmission geometry show minimal peak distortions. The performance of this piezoelectric Doppler drive makes Mossbauer spectrometry possible in applications requiring small size, mass, and low cost.
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