The chemical instability of Na2IrO3 in air

• Na2IrO3 decomposes rapidly in laboratory air.
• The decomposition requires the simultaneous presence of CO2 and H2O.
• Decomposition results in a dramatic change in the magnetic properties.
• Second 5 K feature in magnetic susceptibility not previously reported.

We report that Na2IrO3, which has a layered honeycomb iridium oxide sublattice interleaved by Na planes, decomposes in laboratory air while maintaining the same basic crystal structure. The decomposition reaction was monitored by time-dependent powder X-ray diffraction under different ambient atmospheres, through which it was determined that it occurs only in the simultaneous presence of both CO2 and H2O. A hydrated sodium carbonate is the primary decomposition product along with altered Na2IrO3. The diffraction signature of the altered Na2IrO3 is quite similar to that of the pristine material, which makes the detection of decomposition difficult in a sample handled under ordinary laboratory conditions. The decomposed samples show a significantly decreased magnetic susceptibility and the disappearance of the low temperature antiferromagnetic transition considered to be characteristic of the phase. Samples that have never been exposed to air after synthesis display a previously unreported magnetic transition at 5 K.

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