Browsing by Author "Delaire, Olivier"

Advancements in inelastic X-ray and neutron scattering techniques nowadays allow for the nearly complete measurement of anharmonic phonon behaviors near phase transitions. It is difficult to explain the observed effects of phonon anharmonicity without using first-principles calculations, for instance to obtain renormalized second- order and higher-order force-constants. Moreover, pump-probe experiments can now resolved femtosecond atomic dynamics by using an ultrafast laser pulse to excite materials out of equilibrium and then track the Bragg or diffuse intensities with a time-resolved pulse of X-rays. However, the response of materials to the laser pulse is complex, necessitating ab initio calculations to achieve a physical understanding of the processes underlying pump-probe experiments. Using ab initio calculations, this thesis investigates the significance of phonon anharmonicity in the thermal phase transition, as well as the lattice dynamics of photo-excited crystals, specifically in VO2 and SnS/SnSe single crystals.

The thermal transport behavior and thermodynamics of VO2, which exhibits a well-studied metal-insulator transition (MIT), are both heavily influenced by its anharmonic lattice dynamics. However, the precise evolution of its phonon dispersions over the MIT has remained unknown. Strong phonon softening and damping in rutile VO2 were revealed by our inelastic X-ray scattering (IXS) measurements. We reproduced the phonon softening and phonon damping in rutile VO2, as well as their absence in M1 VO2 and rutile TiO2, using first-principles simulations. Our simulations reveal that the anti-ferroelectric distortions are important to understand the phonon softening in rutile VO2, and the large mean square displacement contributes to the extensive damping of its low-energy phonon branches.

In addition to the thermally-induced transition, photo-excitation can also drive the insulator-to-metal transition (IMT) of VO2 on an ultrafast time scale. Using X-ray pump-probe (XPP) total scattering and Megaelectron-volt ultrafast electron diffraction (MeV-UED), we investigated the lattice dynamics upon photo-excitation. We used MeV-UED to track the Bragg intensity and structural response of M1 VO2 at laser fluences below saturation. Below the saturation laser fluence, our ab initio calculations reproduce the initial structural deformation towards the rutile structure. We also use ab initio calculations to show the dominant factor that causes the structural deformation below the saturation fluence. Moreover, we used XPP total scattering to track both the Bragg and diffuse scattering intensities of VO2 across the IMT at laser fluences above saturation. Above the saturation fluence, our ab initio calculations reproduce the evolvement of Bragg intensity and diffuse intensity. The photo-induced phase transition was found to proceed as an ultrafast disordering phase transition. The disordering IMT is enabled by the flattening of the potential energy surface following photo-excitation, which quickly enlarges the phase space for atomic motions.

Using inelastic neutron scattering (INS) and high-resolution Raman spectroscopy, we investigated the lattice dynamics across the high-temperature Pnma-Cmcm phase transition of SnS (and SnSe). We performed first-principles simulations to understand the strong phonon anharmonicity and its impact on lattice thermal conductivity. INS detected a drastic softening of the TA phonon branch in the Cmcm phase and a broad reconstruction of the low frequency TA and TO phonon modes in the Pnma phase, revealing high directionality of the bonding strength and anharmonicity. Our first-principles simulations use renormalized force-constants to reproduce the phonon reconstruction and explain the phonon anharmonicity, demonstrating that the substantial phonon softening has a direct impact on the lattice thermal conductivity, beyond perturbative scattering, primarily by decreasing phonon lifetime, through the reconstruction of scattering phase space.

Free-electron based laser pump-probe measurements of Pnma SnSe investigated the atomic displacement directions after photo-excitation. This thesis investigates the photo-excitation mechanism of SnSe using different methods and emphasizes the significance of selecting and comparing different simulation methods when interpreting experimental results. The distribution of electrons after photo-excitation inspires different schemes to approximate the complex photo-excitation process. Following photo-excitation, hole doping and constrained-DFT methods predict a structural distortion from Pnma to Immm, whereas electronic heating predicts a structural distortion from Pnma to Cmcm. In addition to the well-known electronic structure instability in the Pnma-Cmcm phase transition, the electronic structure changes from Pnma to Immm are also examined. This study further demonstrates the importance of first-principles modeling to understand the atomic dynamics following photo-excitation.

The discovery and design of emergent energy materials require a detailed understanding of their underlying transport mechanisms, strongly associated with their intrinsic anharmonic lattice dynamics. By combining neutron/x-ray scattering spectroscopy with first-principles calculations, the intrinsic phonon properties enabling the high performance can be revealed in these complex systems. In this thesis, I present how phonons affect the transport properties, i.e. thermal and ionic conduction in materials, with increasing anharmonicity, from the nearly-instable system Mg3Sb2 to superionic conductors with sublattice melting AgCrSe2 and Li6PS5Cl.

By using a combination time-of-flight inelastic neutron scattering (INS), inelastic x-ray scattering (IXS), and ab initio molecular dynamics (AIMD), we showed that the low thermal conductivity (kl) in Mg3X2 (X = Sb, Bi) compared with isostructural ternaries CaMg2X2 and YbMg2X2 with heavier elements originates from abnormally soft phonons and extra low energy shoulders in the phonon density of states (DOS) of the binary compounds. This reflects a near-instability due to the ionic size mismatch and results in a weakened Mg-X bond. The phonon propagation in Mg3X2 was strongly disturbed by the enhanced scattering phase space.

In the superionic thermoelectric AgCrSe2, we investigated the complex atomic dynamics and the thermal conduction mechanism with combined INS, IXS measurements and theoretical calculations, providing both thermal transport and superionic diffusion mechanism. The transverse acoustic (TA) phonons breakdown selectively depending on the involvement of the mobile ions, and the spectral weight from overdamped modes transferred from vibrational to diffusive dynamics in the superionic regime. Our IXS measurements showed the persistence of long wavelength TA phonons in the superionic state, contrary to the traditional phonon-liquid picture, and we show that the ultralow kl originates from the intrinsic anharmonicity that exists even at low temperature.

In the solid-state electrolyte Li6PS5Cl, we measured both vibrational and diffusive dynamics with neutron scattering, using 7Li enriched samples to minimize neutron absorption. We combined quasiharmonic neutron scattering measurements with machine-learned molecular dynamics (MLMD) calculations to investigate the diffusion mechanism and found good agreements in diffusion constant and activation energy with reported experimental values. Our constrained MLMD showed stronger effects from the translation of PS4 to the diffusion than rotation, and the low-energy Li partial DOS below 10meV strongly couples with the diffusion. INS measurements revealed strong softening of the low energy modes and we associated the finite value in DOS at energy zero to the Li ion diffusion.

Lattice dynamics and phonon quasi-particles are commonly used to describe atomic vibrations in crystalline materials, typically within a harmonic approximation. However, anharmonicity is crucial to understand the thermal transport and thermal dynamics properties, as well as critical phenomena such as phase transitions. Using inelastic neutron scattering (INS) and inelastic x-ray scattering (IXS) techniques, one can probe the phonon spectra functions, providing information about frequencies and linewidths at chosen momentum transfers, and as a function of temperature. Further, modern first-principles simulations and machine learning accelerated molecular dynamics simulations, enable the computation of anharmonicity and temperature dependent effects explicitly. By combining these experimental and computational approaches, this thesis investigates phonon anharmonicities in different perovskite systems. Anomalies in measured INS intensities of phonon are observed in SrTiO3 , upon cooling to approach this material’s ferroelectric quantum critical point. First-principles simulations including anharmonic effects enable us to quantitatively reproduce the experimental phonon S(Q, E), and its temperature dependence. It is found that changes in the eigenvectors of transverse acoustic and transverse optic modes lead to the anomalous INS intensity behaviors. Moreover, these changes were associated to the temperature dependent force-constants of the nearest Ti-O bond. A striking diffuse scattering signal of rods along the cubic Brillouin zone edges was found in CsPbBr3 in neutron/x-ray diffuse scattering experiments, forming a complex network in reciprocal space. Those diffuse rods reveal dynamic two-dimensional fluctuations of the system in real space. Using ab-initio molecular dynamics and temperature-dependent effective force-constants extracted up to third-order, we showed the strong diffuse signal arises from overdamped zone boundary acoustic modes with wave vectors connecting M and R points. These low-energy modes correspond to large amplitude PbBr6 octahedra rotations and dynamically modulate optoelectronic properties, through their coupling with Pb and Br derived band edge electronic states. In Cs2AgBiBr6 , diffuse signals are observed in neutron/x-ray diffuse scattering experiment, showing similarity to those observed in CsPbBr3 , implying that two-dimensional halide octahedra fluctuations likely exist in the broader halide perovskite family. Owing to the Brillouin zone folding as the unit cell doubles, the diffuse signals in Cs2AgBiBr6 is derived from damped low-energy optic modes connecting Brillouin zone-centers, as revealed by our inelastic neutron experiments and confirmed by our machine-learning accelerated molecular dynamics simulation. Further, a previously unreported low temperature phase transition was observed around 38 K, corresponding to the appearance of multiple superlattice peaks around wave vector (0.2,0.2,0). Our density functional theory (DFT) simulations on the tetragonal phase predict a soft phonon mode around this wavevector, providing insight into the phase transition mechanism. The long wavelength modulation corresponding to the superlattice indicates that the ground state structure likely contains hundreds of atoms. In the oxide perovskite KTa(1−x)NbxO3 (KTN, x = 0.35), correlated disorder from Nb and Ta off-centering is revealed via sheets of diffuse scattering intensity in reciprocal space, similar to previous observation in pure KNbO3 and BaTiO3. Our neutron scattering measurements of these diffuse sheets in KTN and a cross-correlation technique allow us to identify that they come from slow motions (energy ≤ 1 meV). Pure KTaO3 did not exhibit such quasi-elastic diffuse sheets on the other hand. Because of the correlated atomic disorder, selective phonon broadening is observed in inelastic neutron scattering experiments, with the zone-center ferroelectric mode and transverse acoustic modes polarized along h100i becoming damped. Our first-principles simulations and machine-learning molecular dynamics confirm that the disorder comes from correlated B-site (in a ABO3 system) off-centering in KTN. NbNiTe 2 was proposed to be a Weyl topological semimetal, while its crystal structure remains debated. Our inelastic x-ray scattering experiments reveal a zone-center soft mode in this compound, suggesting a second-order or weakly first-order phase transition at ∼373 K. Furthermore, our DFT simulations predicted a monoclinic ground state structure of this compound, which was later confirmed by single-crystal structure refinement.

Measured Inelastic Neutron Scattering Data (INSD) is obtained in energy-momentum space by neutron spectroscopy methods. With recent advances in time-of-flight chopper spectrometers (i.e. ARCS, CNCS, HYSPEC), the entire 4-dimensional (Q_x,Q_y,Q_z,E) space can be efficiently collected. With that said, the spatial dimensions of the detectors limit the span of the crystal’s reciprocal space {H_hkl} covered. Depending on the crystallographic space group of the measured crystal, the tiling of INSD in reciprocal space must display a corresponding set of symmetry relations. In this thesis, we detail the construction of a post-processing algorithm that overlaps the dataset of a given crystal sample to its Primitive Brillouin Zone (PBZ). Furthermore, to ensure crystal symmetry is preserved, an average of the overlapped dataset over the crystal’s symmetry equivalent q_l points inside the PBZ is then obtained by applying the rotational symmetry operations to the folded dataset. The method we apply is consistent with band-unfolding methods for supercell calculations where the primitive cell’s translational symmetry is preserved (Ikeda & Popescu & Allen). We fold INSD of Germanium and Niobium, both crystals whose supercells preserve the translational symmetry of the primitive lattice, and present the improvement of the statistical quality of the data through the evolution of the folding.

Advances in neutron scattering instrumentation and first-principles calculations (due to increasing computational power) now allow for detailed studies of phonon anharmonicity, even in relatively complex systems. The work in this thesis leverages such improvements to gain unprecedented insights into the lattice dynamics of ultra-low thermal conductivity materials, structural phase transitions, phonon renormalization with temperature, and the impacts of anharmonic lattice vibrations on electrical properties. These phenomena are studied in a number of important materials for energy conversion technologies and the resultant fundamental insights will prove valuable to engineering advanced materials for improved performance.

A spectacular and highly unusual softening of a whole manifold of acoustic and optical phonon modes was uncovered in thermoelectric materials SnSe and SnS as they approach structural phase transitions. This was achieved though detailed phonon measurements which will be invaluable to advancing anharmonic phonon theories. These measurements resolved a debate in the literature regarding the mechanism of transition. The experimentally observed drastic phonon renormalization with temperature, which is neglected in most calculations, has significant implications on thermal conductivity. Such effects were probed with first-principles simulations benchmarked against experimental data and demonstrated that phonon renormalization is vitally important to understanding thermal conductivity in highly anharmonic materials, such as those close to lattice instabilities.

Highly unusual two-dimensional lattice dynamics was uncovered in cubic CsPbBr3. Through detailed neutron and x-ray measurements alongside first-principles simulations we were able to specifically identify how these fluctuations arise from overdamped zone-boundary phonons associated with lead-halide octahedral rotations. We showed that they significantly alter the electronic structure which has been largely ignored in the literature. These results are highly relevant to optimizing thermal and electronic properties, and their coupling, in these technologically-pertinent materials.

There is ongoing debate surrounding the vibrational properties of fillers in skutterudites and how they impact thermal conductivity. From studying the skutterudite Yb-filled CoSb3, I have shown that the thermal conductivity is reduced through a complex interplay of phonon-disorder and phonon-phonon scattering which is highly-temperature dependent. This was demonstrated through the benchmarking of advanced machine learned molecular dynamics against detailed experimental measurements. I further found that filling leads to a broadening of the entire phonon density of states (PDOS) and a softening of higher energy modes. The shape of the Yb modes in the PDOS depends significantly on filler concentration and leads to an effective stiffening with additional filling.