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.

Lithium iridate

α-Li2IrO3 (scale bar 0.3 mm) β-Li2IrO3 (scale bar 0.2 mm)
Crystal structure of α-Li2IrO3 with Ir shown in yellow, Li in purple and O in red
Names
Preferred IUPAC name Lithium iridate
Identifiers
CAS Number	61232-88-0 Y
3D model (JSmol)	Interactive image
InChI InChI=1S/Ir.2Li.3O/q;2*+1;;2*-1 Y Key: QPXMDGCGKRSYMN-UHFFFAOYSA-N Y
SMILES [Li+].[Li+].[O-][Ir](=O)[O-]
Properties
Chemical formula	Li2IrO3
Appearance	Black crystals
Structure
Crystal structure	Monoclinic, C2/m
Lattice constant	a = 5.1633(2) Å, b = 8.9294(3) Å, c = 5.1219(2) Å α = 90°, β = 109.759(3)°, γ = 90°
Formula units (Z)	4
Related compounds
Other anions	Lithium ruthenate, lithium platinate
Other cations	Sodium iridate
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). N (what is YN ?) Infobox references

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

Synthesis of Li₂IrO₃ from Li and Ir metals, which are oxidized during heating. The spiral staircase-like growth chamber allows for nucleation of different crystals at different staircase steps.

Time-temperature diagram of Li₂IrO₃ 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.