We report the structural and magnetic properties of the 4}\textit{ \textcolor{ black}{ d }}\textcolor{ black}{ (M = Rh) based and 5}\textit{ \textcolor{ black}{ d }}\textcolor{ black}{ (M = Ir) based systems Ba$_{ 3}$M$_{ x}$Ti$_{ 3-x}$O$_{ 9}$ (nominally $x$ = 0.5, 1). The studied compositions were found to crystallize in a hexagonal structure with the centrosymmetric space group }$P6_{ 3}/mmc$.\textcolor{ black}{ { } The structures comprise of A$_{ 2}$O$_{ 9}$ polyhedra (with the A site (possibly) statistically occupied by M and Ti) in which pairs of transition metal ions are stacked along the crystallographic }\textit{ \textcolor{ black}{ c}}\textcolor{ black}{ -axis. These pairs form triangular bilayers in the $ab$-plane. The magnetic Rh and Ir ions occupy these bilayers, diluted by Ti ions even for $x$ = 1. These bilayers are separated by a triangular layer which is dominantly occupied by Ti ions. From magnetization measurements we infer strong antiferromagnetic couplings for all of the materials but the absence of any spin-freezing or spin-ordering down to 2~K. Further, specific heat measurements down to 0.35~K show no sign of a phase transition for any of the compounds. Based on these thermodynamic measurements we propose the emergence of a quantum spin liquid ground state for Ba$_{ 3}$Rh$_{ 0.5}$Ti$_{ 2.5}$O$_{ 9}$, and Ba$_{ 3}$Ir$_{ 0.5}$Ti$_{ 2.5}$O$_{ 9}$, in addition to the already reported Ba$_{ 3}$IrTi$_{ 2}$O$_{ 9}$. }
Categories: cond-mat.str-el