${\mathrm{H}}_3\mathrm{LiI}{\mathrm{r}}_2{\mathrm{O}}_6$ is the first honeycomb-lattice system without any signs of long-range magnetic order down to the lowest temperatures, raising the hope for the realization of an ideal Kitaev quantum spin liquid. Its honeycomb layers are coupled by interlayer hydrogen bonds. Static or dynamic disorder of these hydrogen bonds was proposed to strongly affect the magnetic exchange and to make Kitaev-type interactions dominant. Using dielectric spectroscopy, here we provide experimental evidence for dipolar relaxations in ${\mathrm{H}}_3\mathrm{LiI}{\mathrm{r}}_2{\mathrm{O}}_6$ and deuterated ${\mathrm{D}}_3\mathrm{LiI}{\mathrm{r}}_2{\mathrm{O}}_6$, which mirror the dynamics of protons and deuterons within the double-well potentials of the hydrogen bonds. The detected hydrogen dynamics reveals glassy freezing, characterized by a strong slowing down under cooling, with a crossover from thermally activated hopping to quantum-mechanical tunneling towards low temperatures. Thus, besides being Kitaev quantum-spin-liquid candidates, these materials also are quantum paraelectrics. However, the small relaxation rates in the mHz range, found at low temperatures, practically realize quasistatic hydrogen disorder, as assumed in recent theoretical works to explain the quantum-spin-liquid ground state of both compounds.

• Received 20 February 2020
• Revised 17 April 2020
• Accepted 17 April 2020

DOI:https://doi.org/10.1103/PhysRevB.101.184410