Lattice Dynamics, Photophysical Properties and Electronic Structure of Perovskites

Abstract: Perovskites are a family of materials with a wide conformational as well as compositional flexibility, giving them unique physical, optical and electrical characteristics useful for diverse applications — from high-K capacitors and spintronics to solar cells and solid oxide fuel cells. The perovskite family of materials can therefore serve as a platform to not only investigate intriguing physical properties from a fundamental point of view but also design futuristic technologies. 
Lattice dynamics refer to the vibrations of the atoms in a crystal. Whilst traditional crystallography often leads to the image of atoms being held in static positions through stiff chemical bonds, they still tell us that atoms can be vibrating with an amplitude. Thus, we need to understand lattice dynamics in order to have a complete picture of crystalline materials. In this regard, we consider two model perovskite oxide series A2SmTaO6 (A = Ba, Sr, Ca) and Ln2AlMnO6 (Ln = La, Pr, Nd) with different crystallographic phases and investigate their carrier-lattice interactions using a combined Raman spectroscopy and density functional theory (DFT) calculations. Density functional perturbation theory (DFPT) calculations extract the theoretical phonon properties in these materials. The computational results provide insights into the chemical bonding, electronic structure, linear optical properties and the experimental  X-ray photoemission (XPS) spectra of these materials, which is pivotal to address the  fundamental physics and the complete physical property landscape in these de-novo modelled systems. 
Understanding the lattice dynamics of materials is also important to address a number of key physical phenomena for e.g., light-matter interactions, phase transitions and thermal conductivity. To demonstrate this, we test the thermal sensitivity of CH3NH3PbI3 (MAPbI) perovskite metasurfaces with the aid of conventional lattice dynamics techniques. In order to observe the local changes in heat transport induced by nanopatterning, Raman spectroscopy is employed with peak shifts acting as markers for the thermal sensitivity. The spectral peaks are assigned and compared to theoretical results obtained from DFT and molecular dynamics simulations available in literature. The lattice dynamics suggests a comparatively higher thermal fluctuation for the patterned thin films with different geometries as compared to the pristine thin films, confirming the existence of a thermal tolerance for the metasurfaces within which optical efficiencies can be maximized. 
Thus, the strong “synergetic interplay” of lattice, phonons and carriers can be actively probed  using the lattice dynamics and electronic structure of the crystals in both theoretical and  experimental domains, providing a platform for modelling new materials as well extracting the  underlying physics related to their intriguing physical properties.