Heat capacity of LiMn₂O_4-δ: Effect of oxygen content on charge and magnetic ordering

We present the heat capacity of spinel LiMn₂O_4-δ (0<δ<0.09) over a wide temperature range of 2 to 420 K. Both the charge-ordering transition temperature T_co and the entropy of transition ΔS_co increase with increasing δ. The transition is accompanied by a large change in the effective Debye temperature Θ_D(T), signifying strong charge-lattice coupling due to the concomitant cooperative Jahn-Teller distortion. In the low-temperature region, where antiferromagnetically interacting spins are geometrically frustrated, peaks due to long-range spin ordering are observed between 50 and 60 K. With increasing δ, both T_N and the heat-capacity jump ΔC_p at T_N increase, while the excess heat capacity at lower temperatures due to the short-range-ordered spin component is reduced progressively. The change in magnetic behavior with varying δ is conjectured to arise from different degrees of charge ordering on the pyrochlore lattice.