We study the effect of isoelectronic doping and external pressure in tuning the ground state of the honeycomb iridate ${\mathrm{Na}}_2{\mathrm{IrO}}_3$ by combining optical spectroscopy with synchrotron x-ray diffraction measurements on single crystals. The obtained optical conductivity of ${\mathrm{Na}}_2{\mathrm{IrO}}_3$ is discussed in terms of a Mott-insulating picture versus the formation of quasimolecular orbitals and in terms of Kitaev interactions. With increasing Li content $x$, $\left({\mathrm{Na}}_{1-x}{\mathrm{Li}}_x\right){}_2{\mathrm{IrO}}_3$ moves deeper into the Mott-insulating regime, and there are indications that up to a doping level of 24% the compound comes closer to the Kitaev limit. The optical conductivity spectrum of single-crystalline $\alpha \text{−}{\mathrm{Li}}_2{\mathrm{IrO}}_3$ does not follow the trends observed for the series up to $x=0.24$. There are strong indications that $\alpha \text{−}{\mathrm{Li}}_2{\mathrm{IrO}}_3$ is not as close to the Kitaev limit as ${\mathrm{Na}}_2{\mathrm{IrO}}_3$ and lies closer to the quasimolecular orbital picture instead. Except for the pressure-induced hardening of the phonon modes, the optical properties of ${\mathrm{Na}}_2{\mathrm{IrO}}_3$ seem to be robust against external pressure. Possible explanations of the unexpected evolution of the optical conductivity with isolectronic doping and the drastic change between $x=0.24$ and $x=1$ are given by comparing the pressure-induced changes of lattice parameters and the optical conductivity with the corresponding changes induced by doping.

• Received 6 July 2017
• Revised 9 October 2017

DOI:https://doi.org/10.1103/PhysRevB.96.195137