Monoclinic distortion in hyperhoneycomb Kitaev material $\beta$-ZnIrO$_3$ revealed by improved sample quality

The sample quality of the hyperhoneycomb lattice Kitaev magnet $\beta$-ZnIrO$_3$ was successfully improved by removing the maximum amount of moisture from the reaction ampoule. The X-ray diffraction structural analysis of the high-quality sample confirmed the presence of $P$2$_1$/$c$ superlattice peaks of the Fddd structure in the original structural model. These peaks could not be distinguished due to the presence of impurities in the low-quality sample in a previous study. The structural analysis based on this monoclinic crystal structure model showed no chemical disorder, suggesting that the observed spin liquid type behavior is an intrinsic property unrelated to bond randomness. The details of the $\beta$-ZnIrO$_3$ structure revealed in this study will stimulate the further investigation of Kitaev physics.