Modeling of compositionally graded barium strontium titanate from first principles

Barium Strontium Titanate (BaxSr1-xTiO3 or BST) is a Perovskite alloy of interest for both technological and intellectual reasons. Its ferroelectric and piezoelectric properties make it useful in a variety of electric components such as transducers and actuators, and BST in particular is a material of interest for the development of a ferroelectric RAM for computers. (1) The inclusion of SrTiO3, an incipient ferroelectric, and the fact that the properties of a BST system depend strongly on its relative composition of BaTiO3 (BT) and SrTiO3 (ST), make also this a material of high interest. (2) Compositionally graded systems are of further interest (see e.g., Refs. (3), (4), (5) and references therein), partly because their compositional grading leads to a built-in polarization gradient. Due to this, these systems could act as transcapacitors, devices which act as charge amplifiers in much the same way that transistors act as current amplifiers. (3), (4) Here, compositionally graded BST systems were modeled using a first-principles derived effective Hamiltonian method within Monte-Carlo simulation. (6) The graded systems under consideration had an average Ba composition of 70%. These systems were modeled under stress-free conditions, as well as, under epitaxial strain due to a SrTiO3 substrate. Both the degree of grading and the thickness of the layers were varied. The investigation revealed that graded BST systems behaved differently from bulk BST systems in several ways. First, some graded BST systems possessed both monodomain states qualitatively similar to those found in bulk systems (except that the polarization exhibited a “wave’’ behavior inside the graded systems), and also states with domain striping. Where this occurred, the monodomain state was lower in energy, and was therefore the ground-state, but the striped domain state was found to be metastable, representing a local energy minimum. Analyzing unstrained compositionally graded systems layer by layer revealed that, for small layer thicknesses, the material responded rather homogeneously. However, for large layer thicknesses, each compositionally distinct block responded quite independently, and responded like its equivalent bulk system. This led to some overall systems possessing phases that do not exist in BST bulks (such as monoclinic phases), and to the apparent merging of two phase transitions in unstrained systems. When compositionally graded BST systems were modeled under a compressive epitaxial strain, only the z-component of the polarization (that is the component along the growth direction) was found to increase from zero below a single critical temperature. As in unstrained systems, some strained systems were found to have monodomain and striped domain states, with the monodomain representing the ground state and the striped domains representing a local minimum. Development of a BST thin-film code was also undertaken. Initial simulations of BST thin-films were performed using this code, on both disordered and graded systems. These results verified that the new BST thin-film code was functional, as well as revealed interesting phenomena related to compositional grading in two-dimensional materials. This dissertation is approved for recommendation to the Graduate Council