Synthesis, Crystal Structure, and Cooperative 3d-5d Magnetism in Rock Salt Type Li₄NiOsO₆ and Li₃Ni₂OsO₆

Two new transition metal oxides with the nominal chemical compositions of Li₄NiOsO₆ and Li₃Ni₂OsO₆ were successfully synthesized. Both compounds crystallize in an ordered rock salt structure type in the monoclinic C2/m space group. The crystal structures were determined using both synchrotron X-ray and time-of-flight neutron, powder diffraction data. In both phases, Ni²⁺ ions are present while oxidation states of osmium are +6 and +5 in Li₄NiOsO₆ and Li₃Ni₂OsO₆, respectively. Ni²⁺ ions in the hypothetical fully ordered phase form a honeycomb arrangement in the ab crystallographic plane and these hexagons are centered by osmium ions. The magnetic layers are separated along the c axis by the octahedra, which are centered by Li⁺ (or Li⁺/Ni²⁺, depending on the chemical compositions). Crystal structure refinements reveal that there is some degree of mixed occupancy in cationic positions. Temperature dependent magnetic susceptibility data for both phases show ferrimagnetic transitions with predominant antiferromagnetic (AFM) interactions among 3d electrons of nickel and 5d electrons of osmium. Iso-thermal magnetization loops as a function of the applied magnetic field below the transition temperatures confirm the ferrimagnetic nature in magnetic transitions. Temperature dependent heat capacity data, however, did not exhibit any anomaly in either phase, indicating the absence of long-range magnetic ordering. The lack of long-range order for both Os⁵⁺ and Os⁶⁺-based compounds was also confirmed by low temperature neutron diffraction data down to 10 K. Temperature dependent AC magnetic susceptibility data in various frequencies for both samples indicate that Li₄NiOsO₆ exhibits spin-glass-like behavior, while the transition temperature for Li₃Ni₂OsO₆ is nearly frequency independent.