Interplay of magnetism and dimerization in the pressurized Kitaev material β−Li2IrO3

We present magnetization measurements on polycrystalline $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Li}}₂{\mathrm{IrO}}₃$ under hydrostatic pressures up to 3 GPa and construct the temperature-pressure phase diagram of this material. Our data confirm that magnetic order breaks down in a first-order phase transition at ${p}_{\mathrm{c}}\phantom{\rule{4pt}{0ex}}\ensuremath{\approx}1.4$ GPa and additionally reveal a steplike feature---magnetic signature of structural dimerization---that appears at ${p}_{\mathrm{c}}$ and shifts to higher temperatures upon further compression. Following the structural study by L. S. I. Veiga et al. [Phys. Rev. B 100, 064104 (2019)], we suggest that a partially dimerized phase with a mixture of magnetic and nonmagnetic ${\mathrm{Ir}}⁴⁺$ sites develops above ${p}_{\mathrm{c}}$. This phase is thermodynamically stable between 1.7 and 2.7 GPa according to our ab initio calculations. It confines the magnetic ${\mathrm{Ir}}⁴⁺$ sites to weakly coupled tetramers with a singlet ground state and no long-range magnetic order. Our results rule out the formation of a pressure-induced spin-liquid phase in $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Li}}₂{\mathrm{IrO}}₃$ and reveal peculiarities of the magnetism collapse transition in a Kitaev material. We also show that a compressive strain imposed by the pressure treatment of $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Li}}₂{\mathrm{IrO}}₃$ enhances signatures of the 100 K magnetic anomaly at ambient pressure.