Molecular dynamics simulations of polarization domains and flexoelectricity in BaTiO3

This thesis is a part of a project started in December 2016 in collaboration with Prof. Jorge Alcalá Cabrelles (UPC, Barcelona) and Dr. Jan Ocenasek (ZCU institute, Czech Republic) aiming to perform piezoelectric and flexoelectric studies in barium titanate by means of molecular dynamics simulations. The specific objective of this work is to gain expertise on molecular dynamics simulations of piezoelectric response, develop several analytical tools and characterize the basic thermodynamic and electric properties through a core-shell model of barium titanate fitted from first principle calculations. With the emergence of increasingly powerful and cheap supercomputing, atomistic simulations are quickly becoming a very attractive and reliable method for testing materials at the nanoscale and mesoscale. This thesis provides a literature review regarding the basics of molecular dynamics simulations performed to investigate dielectric properties of barium titanate, which is a key archetypal material used in sensors and memory storage devices as well as in the development of new supercapacitors. This work comprises a series of 13 simulations of barium titanate aimed to characterize the spontaneous polarization developing in the ferroelectric phases, study the effect of externally applied electric fields, build the temperature versus pressure phase diagram, estimate the piezoelectric coefficients of the tetragonal phase, calculate the hysteresis loops, identify ferroelectric switching and analyze the paraelectricity of the cubic phase. Furthermore, the limitations of simulating barium titanate using periodic boundary conditions are described and alternatives are given in the context of the investigation of flexoelectric response. Phonon dispersion is introduced as a key aspect to elucidate the underlying mechanisms in phase transformations. Finally, considerable amount of software has been developed to facilitate tracking, measuring and analysis of the physical properties in each simulatio