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A pressure-induced collapse of magnetic ordering in $\beta$-Li$_2$IrO$_3$ at$P_m\sim1.5- 2$ GPa has previously been interpreted as evidence for possibleemergence of spin liquid states in this hyperhoneycomb iridate, raisingprospects for experimental realizations of the Kitaev model. Based onstructural data obtained at \emph{room temperature}, this magnetic transitionis believed to originate in small lattice perturbations that preserve crystalsymmetry, and related changes in bond-directional anisotropic exchangeinteractions. Here we report on the evolution of the crystal structure of$\beta$-Li$_2$IrO$_3$ under pressure at low temperatures ($T\leq50$ K) and showthat the suppression of magnetism coincides with a change in lattice symmetryinvolving Ir-Ir dimerization. The critical pressure for dimerization shiftsfrom 4.4(2) GPa at room temperature to $\sim1.5-2$ GPa below 50 K. While adirect $Fddd \rightarrow C2/c$ transition is observed at room temperature, thelow temperature transitions involve new as well as coexisting dimerized phases.Further investigation of the Ir ($L_3$/$L_2$) isotropic branching ratio inx-ray absorption spectra indicates that the previously reported departure ofthe electronic ground state from a $J_{\rm{eff}}=1/2$ state is closely relatedto the onset of dimerized phases. In essence, our results suggest that thepredominant mechanism driving the collapse of magnetism in$\beta$-Li$_2$IrO$_3$ is the pressure-induced formation of Ir$_2$ dimers in thehyperhoneycomb network. The results further confirm the instability of the$J_{\rm{eff}}=1/2$ moments and related non-collinear spiral magnetic orderingagainst formation of dimers in the low-temperature phase of compressed$\beta$-Li$_2$IrO$_3$.

Pressure-induced structural dimerization in the hyperhoneycomb iridate $\beta$-Li$_2$IrO$_3$ at low temperatures