Advanced functional materials play a vital role in modern industry and human society. Therefore, accelerating the discovery and exploration of novel functional materials is critical for us as a society to tackle energy issues and further developments. In this regard, computational materials science based on quantum mechanics is now well established as a crucial pillar in condensed matter physics, chemistry, and materials science research, in addition to experiments and phenomenological theories.
In this thesis, the strategy of designing new functional materials driven by the lattice degree of freedom is explored, where "lattice" refers to (1) the ground state crystal structures, (2) elementary excitations as represented by phonons, (3) coupling within themselves (i.e., anharmonicity) and the other degrees of freedom (i.e., electron-phonon interaction). We systematically studied several classes of physical phenomena and the resulting properties, such as magneto-structural coupling and magnetocalorics, anharmonicity and thermal conductivity, electron-phonon interaction and superconductivity. Additionally, an integrated computational paradigm that combines high-throughput (HTP) calculations, phonon theory, and CALPHAD methods is established and applied to design metastable functional materials, extending the applicability of DFT beyond 0 K.
Considering lattice as crystal structures, we selected MAB phases with nanolaminated crystal structure as a test case, and performed an HTP screening for stable magnetic MAB compounds and predicted potential candidate magnets for permanent magnets and magnetocaloric applications. After a comprehensive validation, 21 novel compounds are predicted to be stable based on the systematic evaluation of thermodynamic, mechanical, and dynamical stabilities, and the number of stable compounds is increased to 434 taking the tolerance of convex hull being 100 meV/atom. The detailed evaluation of the magnetocrystalline anisotropy energy (MAE) and the magnetic deformations leads to 23 compounds with significant uniaxial anisotropy (MAE > 0.4 MJ/m3) and 99 systems with reasonable magnetic deformation (> 1.5 %).
For those compounds containing no expensive, toxic, or critical elements, it is observed that Fe3Zn2B2 is a reasonable candidate as gap permanent magnet, and Fe4AlB4, Fe3AlB4, Fe3ZnB4, and Fe5B2 as potential magnetocaloric materials. This work paves the way for designing novel magnetic materials for energy applications based on the combinatorial sampling of the chemical space with specific crystal structure prototypes.
Moreover, considering the elementary excitations of lattice vibrations, i.e., phonons, the anharmonicity caused by phonon-phonon interaction leads to many intriguing properties, such as the lattice thermal conductivity. We performed DFT calculations to evaluate the thermal transport properties of novel 2D MoSi2N4 and WSi2N4, and found their thermal conductivities being 162 W/mK and 88 W/mK at room temperature, respectively, which are 7 and 4 times the one for monolayer MoS2, 16 and 9 times the one for silicone. These results show that, MoSi2N4 and WSi2N4 have promising potential being thermal management materials. Additionally, to gain insight into the low thermal conductivity of 2D materials, we investigated the mechanism of anharmonicity from the fundamental phonon mode and electronic structure level for GaX (X= N, P, As). The thermal conductivity of GaP is calculated to be 1.52 W/mK, which is unexpectedly ultra-low and in sharp contrast to GaN and GaAs. The reason for the low thermal conductivity of the GaP can be attributed to the fact that the FA phonon dominates the thermal conductivity of GaN but contributes less to that of GaP, which is due to the symmetry-based selection rule and difference in atomic structure. The phonon anharmonicity quantified by the Grüneisen parameter is further analyzed to understand the phonon–phonon scattering, indicating the strong phonon-phonon scattering of GaP and the strongest phonon anharmonicity of GaP. The buckling structure has a strong influence on the anharmonicity, leading to low thermal conductivity. The non-bonding lone pair electrons of P and As atoms are stronger, which induces nonlinear electrostatic forces upon thermal agitation, leading to increased phonon anharmonicity in the lattice and thus reducing the thermal conductivity. Furthermore, high order phonon anharmonicity could have a significant effect on the thermal transport properties in materials within strong anharmonicity. Hence, we calculated the thermal conductivity of pristine EuTiO3. And the role of the quartic anharmonicity in the lattice dynamics and thermal transport of the cubic EuTiO3 was elucidated by combining ab initio self-consistent phonon theory with compressive sensing techniques. The anti-ferromagnetic G-type magnetic structure is used to mimic the para-magnetic EuTiO3. We find that the strong quartic anharmonicity of oxygen atoms plays an important role in the phonon quasiparticles without imaginary frequencies and causes the hardening of the vibrational frequencies of soft modes.
Furthermore, in terms of electron-phonon interaction, we derived from DFT calculations the formation energies of a newly synthesized orthorhombic compound GeNCr3, which is a metastable phase. In accordance with the experimentally discovered superconductivity in antiperovskite MgCNi3, we performed calculations to evaluate the electron-phonon interaction and the resulting superconducting critical temperature of GeNCr3. It is observed that its superconducting temperature is about 8.2 K, driven by the electron-phonon interaction. Correspondingly, it is suspected that superconductivity may exist in the other MAX, MAB, and APV compounds, which will be investigated in the future based on the established workflow to evaluate the electron-phonon coupling. Such a workflow allows us to obtain the T-dependence of electric conductivities and also the lattice thermal conductivities.
Last but not least, considering the thermodynamic properties where the lattice free energy plays a dominant role at the finite temperatures, we combined DFT calculations and CALPHAD modeling to optimize the phase diagrams, which can be validated with experiments and be bridged to phase field simulations to map out the processing-microstructure-property relationships. For instance, the thermodynamic properties of the Fe-Sn system are studied. First-principles phonon calculations with the quasi-harmonic approximation (QHA) approach were performed to compute the thermodynamic properties at finite temperatures. Thermodynamic properties, phonon dispersions of pure elements, and intermetallics were predicted to make up for the shortage of experimental data.
A set of self-consistent thermodynamic parameters of the Fe-Sn system are obtained by the CALPHAD approach. Thermodynamic modeling of the Fe-Sn phase diagram has been re-established. The metastable phase Fe3Sn was first introduced into the current metastable phase diagram and corrected phase locations of Fe5Sn3 and Fe3Sn2 under the newly measured corrected temperature ranges.
In summary, in my thesis, a systematic computational paradigm has been established based on DFT to tackle both the thermodynamic and non-equilibrium transport properties associated with the lattice degree of freedom. Such a paradigm allows us to design and optimize functional materials with physical properties driven by magneto-structural coupling, phonon-phonon coupling, and electron-phonon interaction, and also to bridge to large-scale simulations
