Exploring structure property relationships in layered perovskite iridates

Abstract

Geometrically frustrated perovskites where the symmetry of the underlying crystalstructure inhibits long range magnetic ordering is the most prominent structural class where the search for non trivial spin structures appears to bear fruit The physics ofthese systems is especially rich when they comprise of 4d and 5d transition metals where the large spin orbit coupling is known to be responsible for the stabilization of anumber of novel magnetic and electronic ground states During the course of this thesis we have investigated a number of Iridium based systems crystallizing in the tripleperovskite structure of the form Ba3MIr2O9 M Co Ni or Na with the aim of understanding their structure property relationships Our results show that the ground state ofthese systems are remarkably diverse with factors like multiple valence states of M andIr Jahn Teller distortion anti symmetric Dzyaloshinskii Moriya interaction multiplesuper exchange pathways crystal field splitting spin orbit coupling and hybridizationbeing the driving forces For instance Ba3CoIr2O9 and Ba3NiIr2O9 both stabilize in ahexagonal symmetry at room temperature with Ir in the 5 oxidation state Ba3CoIr2O9exhibits a magneto structural transition to a monoclinic phase at 107K the highestknown amongst all the triple perovskite iridates and transforms to a monoclinic phasewith even lower symmetry at 70K with both these phases coexisting down to the lowest measured temperatures Interestingly Ba3NiIr2O9 retains the hexagonal symmetrydown to the lowest measured temperature and exhibits two magnetic transitions at 106Kand 6K that are attributed to antiferromagnetic ordering Ba3NaIr2O9 with a mixed valent state of Ir 5 and Ir 6 is a geometrically frustrated antiferromagnet exhibiting aco operative paramagnetism starting at around 200K On reducing the temperature one observes a strain induced structural transition to an orthorhombic phase nucleati newline newline