First-order magneto-structural transition in single crystals of the honeycomb lattice Ruthenate Li₂RuO₃

Li$_2$RuO$_3$ is known to crystallize in either $C2/m$ or $P2_1/m$ structures at room temperature. We report the first single crystal growth of Li$_2$RuO$_3$ and Na substituted crystals (Li$_{0.95}$Na$_{0.05}$)$_{2}$RuO$_3$ crystallizing in the $P2_1/m$ structure where a magneto-structural transition is observed at high temperatures. Using high temperature ($T \leq 1000$~K) magnetic susceptibility $\chi$ measurements we study the magnetic anisotropy across the magneto-structural transition. Our results show for the first time that for Li$_2$RuO$_3$ the magnetic and structural transitions most likely occur at slightly different temperatures. The structural transition which is first order-like occurs first ($T \approx 570$~K) and drives the magnetic transition ($T \approx 540$~K). Rather surprisingly, just $5\%$ Na substitution for Li affects the magneto-structural transition in an interesting way. The first order transition temperature stays $\approx 540$~K, the magnetic anisotropy is reversed, and the Ru-Ru dimerization pattern changes from two short and four long Ru-Ru bonds per honeycomb in an armchair pattern for Li$_2$RuO$_3$ to four short and two long bonds per honeycomb in (Li$_{0.95}$Na$_{0.05}$)$_{2}$RuO$_3$ which can be viewed as two inter-penetrating armchair patterns.