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The search for improved energy-storage materials has revealed Li-and Na-rich intercalation compounds as promising high-capacity cathodes. They exhibit capacities in excess of what would be expected from alkali-ion removal/reinsertion and charge compensation by transition-metal (TM) ions. The additional capacity is provided through charge compensation by oxygen redox chemistry and some oxygen loss. It has been reported previously that oxygen redox occurs in O 2p orbitals that interact with alkali ions in the TM and alkali-ion layers (that is, oxygen redox occurs in compounds containing Li+-O(2p)-Li+ interactions). Na2/3[Mg0.28Mn0.72]O2 exhibits an excess capacity and here we show that this is caused by oxygen redox, even though Mg2+ resides in the TM layers rather than alkali-metal (AM) ions, which demonstrates that excess AM ions are not required to activate oxygen redox. We also show that, unlike the alkali-rich compounds, Na2/3[Mg0.28Mn0.72]O2 does not lose oxygen. The extraction of alkali ions from the alkali and TM layers in the alkalirich compounds results in severely underbonded oxygen, which promotes oxygen loss, whereas Mg2+ remains in Na2/3[Mg0.28Mn0.72]O2, which stabilizes oxygen.

Using first-principles calculations, we perform a comprehensive and systematic analysis to establish the role of van der Waals (vdW) interactions and anharmonicity in the vibrational properties of the low-temperature orthorhombic phase of the hybrid perovskite CH3NH3PbI3. To this end, we consider the most common approaches for including vdW effects in our phonon calculations: the semiempirical Grimme approximations, the Tkatchenko-Scheffler dispersion corrections, and the vdW density-functional method. The vibrational normal modes are first calculated within the harmonic approximation. We consider the LDA and GGA approximations to the exchange-correlation functional and include spin−orbit coupling (SOC) effects. On top of the harmonic calculations, we also evaluate the anharmonicity of the normal modes and the phonon−phonon coupling by solving one-dimensional and two-dimensional nuclear Schrödinger equations, respectively, via the finite-displacement method. We observe that both the LDA and GGA approximations work remarkably well in describing the vibrational properties of CH3NH3PbI3. We find that vdW effects and relativistic effects do not have any significant impact on the vibrational properties of CH3NH3PbI3. Our study also reveals that the spinning modes of the organic CH3NH3+ cations carry considerable anharmonicity but that the anharmonic coupling between different modes is relatively small.

El objetivo principal de esta tesis fue la implementación dentro del código SIESTA de dos métodos de primeros principios para el estudio de excitaciones electrónicas en moléculas y sólidos extendidos. Los métodos son la aproximación GW y el 'constrained' DFT. Los métodos fueron implementados usando bases de orbitales extrictamente localizados generadas por SIESTA. El uso de este tipo de orbitales abre la posibilidad de estudiar sistemas que contienen miles de átomos. La aproximación GW es un método derivado de la teoría de perturbaciones de muchos cuerpos para la funcin de Green de una partícula, que se usa para el cálculo de las energías de excitación de quasipart ículas. En este trabajo, se implementó el método propuesto por F. Giustino et al. [3], que solo requiere el uso de los estados electrónicos ocupados para calcular los ingredientes de esta aproximación: la función de Green y el potencial de Coulomb apantallado. En método de 'constrained' DFT se impone una ligadura sobre la densidad electrnica del sistema a través de un potencial apropiado. En esta tesis se implementó una versión modificada de la aproximación propuesta por Q. Wu and T. Van Voorhis [4] para encontrar el potencial de 'constraining' de manera eficiente. El método se extendió para el caso de sistemas periódicos. Como una aplicación del método, se estudiaron procesos de transferencia de carga en el conductor oránico TTF-TCNQ. Una aplicación interesante de los métodos implementados es el estudio de los procesos electrónicos que tienen lugar en las celdas solares y dispositivos fotovoltaicos. El material ms ampliamente usado en la fabriacin de estos dispositivos es el TiO2 en fase anatase dopada con impurezas de nitrógeno o sensibilizada con colorantes orgánicos. Como un paso previo al estudio de las excitaciones electrnicas de este material, se caracterizaron sus propiedades electrónicas, estructurales, t y vibracionales usando la teoría DFT. Además, se estudiaroon las energías 'binding' de los n, The main aim of this thesis is the implementation in the SIESTA code (a widely used academic computational program based on the density functional theory DFT for the simulation of materials) [1, 2] of two powerful methods for the study of electronic excitations in molecules and extended solids. The methods are the so-called constrained density functional theory (CDFT) and the GW approximation and they are implemented within the strictly localized orbital formalism as defined in SIESTA. The use of local bases opens the possibility of calculating electronic excitations in systems containing thousands of atoms. The GW approximation is a powerful method derived from manybody perturbation theory for the one-particle Green function to calculate quasiparticle excitation energies. We implement the approach proposed by F. Giustino et al. [3] that only requires the use of the occupied electronic states to compute the Green function and screened Coulomb interaction, and from which one constructs the self-energy operator, . In this approach, the Green function is approximated to that of a non-interacting electron system and calculated using a inhomogeneous linear system, while the dynamically screened potential is calculated through the self-consistent linear-response Sternheimer equation. Since the original equations were stated within the plane waves formalism, here we rewrite these equations for basis sets of strictly localized orbitals. As an application of our approach for the dynamically screened potential, we calculate the dielectric function of several insulating and semiconducting materials: LiCl, diamond, silicon and germanium, comparing our results with plane-waves calculations. In the constrained density functional theory, we impose a constraint upon the electronic density of a system through an appropriate potential (called constraining potential) to obtain a neutral excitation. We implement a modified version of the approach proposed by Q.Wu and T. Van Voorhis [4