Interplay of frustration, strain and phase co-existence in the mixed valent hexagonal iridate Ba₃NaIr₂O₉

Geometrically frustrated magnets- where triangular lattice antiferromagnets (TLAFs) are considered to be an archetype- remain at the forefront of contemporary condensed matter. Of particular interest in recent years have been a number of Ruthenium and Iridium based perovskite. In this talk, we will discuss the structure-property relationship in the mixed valent geometrically frustrated triple perovskite iridate Ba₃NaIr₂O₉ using detailed synchrotron diffraction, magnetization, thermodynamic and transport measurements. In contrast to what is expected from purely structural considerations, this system stabilizes in a high symmetry hexagonal symmetry at room temperatures. On reducing the temperature, the lattice prefers to be strained rather than distort to a low symmetry phase, as is the norm in this family of materials. Though a low symmetry orthorhombic phase is nucleated below 50 K, this conversion is only partial and the high symmetry hexagonal structure remains the dominant one down to the lowest measured temperatures. Magnetic measurements indicate an extended co-operative paramagnetic regime, which finally freezes to a cluster glass-like phase at very low temperatures. This makes an interesting addition to the family of triple perovskite iridates where complex interplay between lattice strain and structural phase co-existence arises as a consequence of a number of competing energy scales.