Phase transitions and anomalous compressibility in 1-2-2 Iron-based superconductors

Interplay of pressure and chemistry play an important role in discovery of novel properties such as high temperature superconductivity (high-Tc). In 2008, Hosono et al discovered superconductivity at 26 K in iron-based layered LaFeAsO (1-x)Fx. This observation was a surprise since iron based compounds are generally known to be magnetic and non-superconducting. This was quickly followed by the discovery of high temperature superconductivity in a second class of Fe-based layered pnictides AFe2As2 (1-2-2) (A= Ba, Sr, Ca, Eu). The undoped (parent) 1-2-2 compounds are non-superconducting at ambient pressure but become superconducting when their structures are tuned by the application of high pressure or through chemical doping. Application of high pressures is advantageous over chemical doping in that it provides a clean method to tune the electronic properties that determine the superconducting and magnetic states of the novel materials. Interplay of pressure induced structural transitions, magnetic and superconducting properties of Fe-based materials is not well understood and this may form a foundation for testing present theories, discovering materials with higher-Tc for wide industrial applications close to room temperature, and may lead to better theories for solving the long standing problem of high-Tc superconductivity. In my work, I have used designer diamond anvil cell (DAC) to study the electrical transport properties of 1-2-2 parents of iron based superconductors under extreme conditions of pressure and low temperatures. I have used high pressure and low temperature time of flight neutron diffraction technique at the Spallation Neutron Source and synchrotron x-ray diffraction techniques at the Advance Photon Source to determine and refine the crystallographic parameters of the 1-2-2 materials under extreme conditions, and to relate their transport and structural properties under high pressures. My works have resulted into a discovery of anomalous compressibility and a concurrent tetragonal to collapsed tetragonal (T-cT) isostructural phase transition in 1-2-2 parents of iron-based superconductors. We showed that the anomaly is a common phenomenon for pure and doped ThCr2Si2 type pnictides of the type AT2As2 (A= divalent alkaline earth or rare earth element Ba, Ca, Sr, Eu; T=transition metal). We determine a general relation for predicting isostructural T-cT phase transition pressure for any 1-2-2 pnictide given its ambient pressure volume. Our work suggests that the collapsed tetragonal phase is non-superconducting. We have establish how onset of pressure induced superconductivity (TConset) in 1-2-2 parent materials depends on the compression behavior of As-Fe-As tetrahedral bond angles, tetragonal lattice parameter c, Fe2As2 layer thickness and As-As inter-atomic bond distance for parent 1-2-2 materials. The evolution of pressure driven tetragonal distortion, quantitative crystallographic parameters and pressure-volume equation of state for 1-2-2 materials under high pressures are presented up to 70 GPa and low temperatures down to 4 K.