  The structural and magnetic properties of a new series of iridium-based
honeycomb lattices with the formula Na3-delta(Na1-xMgx)Ir2O6 (x = 0 to 1) are
reported. As x and delta are increased, the honeycomb lattice contracts and the
strength of the antiferromagnetic interactions decreases systematically due to
a reduction in Ir-O-Ir bond angles. Samples with imperfect stoichiometry
exhibit disordered magnetic freezing at temperatures Tf between 3.4 K and 5 K.
This glassy magnetism likely arises due to the presence of non-magnetic Ir3+,
which are distributed randomly throughout the lattice, with a possible
additional contribution from stacking faults. Together, these results
demonstrate that chemical defects and non-stoichiometry have a significant
effect on the magnetism of compounds in the A2IrO3 materials family.
