Structural Principles of A-site ordered double perovskites: ferroelectric CaMnTi2O6 as a model system

Perovskites are a broad and versatile class of crystalline materials. Their unique structure combined with the countless possible ionic combinations makes them highly tunable and gives rise to many possible properties such as piezo-, pyro-, and ferroelectricity as well as ferromagnetism and superconductivity. Double perovskites even enhance the possibilities of tuning the material. Perovskites as functional materials are used in many applications such as sensors and energy storage and the demand for new materials with tailored properties is still rising. Nowadays, many high-performing functional materials, such as the ferroelectric, perovskite-like material lead zirconate titanate (PZT) are made of harmful or rare elements. Using computational methods to find more sustainable materials and experiments to synthesize and analyze their properties is a major task for today's materials scientists. For this thesis, first principles hybrid density functional theory is used to computationally approach A-site double perovskites in general and CaMnTi2O6 in particular. High-pressure/high-temperature synthesis is used to synthesize bulk samples of Ca2-xMnxTi2O6 with x = 0.2 to 1.0 and single crystal samples of CaMnTi2O6 which are analyzed by powder and single crystal X-ray diffraction together with Rietveld refinement, scanning electron microscopy, ferroelectric measurements, and Raman spectroscopy. The predictive power of a new tolerance factor for A-site double perovskites is studied. While it still can be used as a starting point, the new tolerance factor does not perform as well for A-site double perovskites as it does for B-site double perovskites and simple perovskites due to possible large size differences in A-site ions. Computational methods to predict and confirm the cation ordering and tilt system in A-site ordered double perovskites, demonstrated on CaMnTi2O6, prove to be a suitable addition to the new tolerance factor. Structural changes, as well as changes in the dielectric (ferroelectric) behavior, determine the centrosymmetric to non-centrosymmetric symmetry transition in Ca2-xMnxTi2O6 to be around x = 0.3 and x =0.4. A synthesis of CaMnTi2O6 single crystals can be realized at lower temperatures and at lower pressures than previously reported.