First-order magnetostructural transition in single crystals of the honeycomb lattice ruthenate Li2RuO3

Li2RuO3 is known to crystallize in either C2/m or P21/m structures at room temperature. We report the first single crystal growth of Li2RuO3 and Na substituted crystals (Li0.95Na0.05)2RuO3 crystallizing in the P21/m structure where a first order magnetostructural transition is observed at high temperatures. Using high temperature (T≤1000 K) magnetic susceptibility χ measurements we study the magnetic anisotropy across the magnetostructural transition. Our results show that for Li2RuO3 the magnetic and structural transitions most likely occur at slightly different temperatures. The structural transition which is first-order-like occurs first (Tonset≈570 K) and drives the magnetic transition (T≈540 K). Rather surprisingly, just 5% Na substitution for Li affects the magnetostructural transition in an interesting way. The first order transition temperature remains at ≈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 Li2RuO3 to four short and two long bonds per honeycomb in (Li0.95Na0.05)2RuO3 which can be viewed as two interpenetrating armchair patterns.