Structural and magnetic properties of the superconductors Mo₃Sb₇, Mo₃Al₂C and KₓFe₂₋ySe₂ under high pressure

This thesis is based on high pressure structural and superconducting phase diagrams of the three compounds Mo3Sb7, Mo3Al2C and KxFe2−ySe2, which have all been reported to exhibit unconventional superconductivity. A new technique for measuring magnetic susceptibility at high pressure in a Bridgman type pressure cell has been developed which places counterwound detection and compensation coils within the high pressure sample space, which allows for improved signal-to-noise ratios compared to other methods. X-ray powder diffraction using diamond anvil cells was utilised to study the structural evolution of these compounds with increasing pressure. The pressure-temperature phase diagram of Mo3Sb7 has been extended to higher pressures than reported previously and shows that the superconducting transition temperature increases. Structural measurements reveal that the cubic structure remains stable up to 13.6 GPa. Similar measurements on Mo3Al2C show a slight pressure dependence of Tc. A jump in the unit cell volume of Mo3Al2C indicates a structural transition occuring near 13.8 GPa. Finally high pressure X-ray powder diffraction measurements confirm a structural transition in both superconducting and insulating samples of K0.8Fe2−ySe2 (y = 0, 0.4). Low temperature structural measurements have been carried out to build up the pressure-temperature phase diagram of superconducting K0.8Fe2Se2. The strucutral transition occurs at a pressure between 13.2 and 15 GPa at low temperatures and appears to be strongly linked to the emergence of a second superconducting phase.