Correlation effects in ionic perovskite crystals

Licentiate thesis, 2022

Perovskites are a large family of versatile materials with favourable properties for spintronics and optoelectronics applications. The mineral CaTiO3 defines the archetypal crystal structure of perovskite materials with a general composition ABX3. The structure is ideally cubic, where the largest A cation is found in the centre, a smaller B cation occupies the edge sites and X anions form an octahedron around each B. Various electric and magnetic properties are observed depending on the electronic configurations, the exchange interactions and the crystal geometry formed by substitution of chemical elements or molecules.

Our studies concern two classes of perovskite materials, the double perovskite, transition metal oxides LaSr1-xCaxNiReO6  for x = 0, 1 and the hybrid organometallic perovskite halides (CH3NH3)PbX3 for X = Br, Cl. We have mainly studied these compounds with muon spectroscopy (μ+SR), using muons as a magnetic probe to map the form and evolution of intrinsic electronic and nuclear magnetic fields.

The rhenium-based perovskites points of interest are high magnetic transition temperatures along with a variety of metallic or insulating behaviors. In the case of LaSr1-xCaxNiReO6  , x = 0, 1 we expect the substitution of Sr with Ca to alter the type of exchange interaction between the Ni and Re cations. This incentive drove us to investigate the formation and evolution of magnetic ordering as a function of temperature in both compounds.

The hybrid perovskites (CH3NH3)PbX3 , X = Br, Cl belong to the increasingly researched family of organometallic perovskite solar cell materials with long carrier diffusion lengths and a straightforward implementation into devices. However, intrinsic and extrinsic stability issues hinder the progress of application of these compounds. We aimed to indirectly identify intrinsic stability factors in the kinetics of the CH3NH3 molecule and the halide Br, Cl ion diffusion through their effect on the nuclear magnetic field distribution and dynamics.