We report the structural and magnetic properties of the $4d$ ($M$ = Rh) based and $5d$ ($M$ = Ir) based systems ${\mathrm{Ba}}_3{M}_x{\mathrm{Ti}}_{3-x}{\mathrm{O}}_9$ (nominally $x=0.5$, 1). The studied compositions were found to crystallize in a hexagonal structure with the centrosymmetric space group $P{6}_3/mmc$. The structures comprise of $A_2{\mathrm{O}}_9$ polyhedra [with the $A$ site (possibly) statistically occupied by $M$ and Ti] in which pairs of transition metal ions are stacked along the crystallographic $c$ 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 ${\mathrm{Ba}}_3{\mathrm{Rh}}_{0.5}{\mathrm{Ti}}_{2.5}{\mathrm{O}}_9$, and ${\mathrm{Ba}}_3{\mathrm{Ir}}_{0.5}{\mathrm{Ti}}_{2.5}{\mathrm{O}}_9$, in addition to the already reported ${\mathrm{Ba}}_3{\mathrm{IrTi}}_2{\mathrm{O}}_9$.

• Received 14 April 2016
• Revised 11 September 2016

DOI:https://doi.org/10.1103/PhysRevB.94.174410