Lithium iridate

Lithium iridate, Li₂IrO₃, is a chemical compound of lithium, iridium and oxygen. It forms black crystals with three slightly different layered atomic structures, α, β, and sometimes γ. Lithium iridate exhibits metal-like, temperature-independent electrical conductivity, and changes its magnetic ordering from paramagnetic to antiferromagnetic upon cooling to 15 K.

Structure

Li₂IrO₃ typically crystallizes in the α or β phase, and a rare γ phase has been reported. The crystal structure of α-Li₂IrO₃ consists of an alternate stacking of hexagonal Li layers and honeycombs of edge-sharing IrO₆ octahedra with Li in the center. The offset in adjacent layers results in a relatively low (monoclinic) crystal symmetry. Li₂IrO₃ crystals have abundant twinning defects where the ab crystal planes are rotated by 120° around the c axis.

Synthesis

Li₂IrO₃ crystals can be grown by direct sintering of Ir and Li metals, which both oxidize during heating in ambient atmosphere. The α phase is formed at 750–1050 °C, while heating to higher temperatures results in the β phase. The use of Li metal instead of more traditional lithium carbonate, which is easier to handle and store, results in larger crystals. The γ phase can be obtained by the calcination of lithium carbonate and iridium(IV) oxide, followed by annealing in molten lithium hydroxide at 700–800 °C.

Properties

Lithium iridate is black in color and has a relatively high, temperature-independent electrical conductivity characteristic of metals. Its both α and β phases exhibit the Kitaev exchange coupling between magnetic spins originating from Ir⁴⁺ ions. These spins form an antiferromagnetic lattice at temperatures below 15 K (Néel temperature, T_N), while the material is paramagnetic above T_N.

Potential applications

Lithium iridate is a potential electrode material for the lithium-ion battery. This application is hindered by the high costs of Ir, as compared to the cheaper Li₂MnO₃ alternative.

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