First-order magnetostructural transition in single crystals of the honeycomb lattice ruthenate Li_2RuO_3

摘要

Li_2RuO_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_2RuO_3 and Na substituted crystals (Li_(0.95)Na_(0.05))_2RuO_3 crystallizing in the P2/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 Li_2RuO_3 the magnetic and structural transitions most likely occur at slightly different temperatures. The structural transition which is first-order-like occurs first (T_(onset) ≈ 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 Li_2RuO_3 to four short and two long bonds per honeycomb in (Li_(0.95)Na_(0.05))_2RuO_3 which can be viewed as two interpenetrating armchair patterns.