Evidence of a structural phase transition in the triangular-lattice compound CuIr2Te4

Synthesized $\mathrm{Cu}{\mathrm{Ir}}₂{({\mathrm{Te}}_{1\ensuremath{-}x}{\mathrm{Se}}_x)}₄$ samples were comprehensively characterized using various techniques. At ambient pressure, $\mathrm{Cu}{\mathrm{Ir}}₂{\mathrm{Te}}₄$ undergoes an anomalous phase transition at ${T}_s\ensuremath{\sim}250\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ with a large thermal hysteresis of $\mathrm{\ensuremath{\Delta}}{T}_s \ensuremath{\sim}40\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and a superconducting transition at ${T}_c\ensuremath{\sim}3\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ during magnetization and electrical-resistivity measurements. We determined this transition to be a first-order structural phase transition from high-temperature hexagonal ($P\overline{3}m1$) to low-temperature triclinic ($P\overline{1}$) symmetry. Both external physical pressure and chemical doping largely enhance ${T}_s$ and $\mathrm{\ensuremath{\Delta}}{T}_s$ and suppress ${T}_c$, thereby indicating a strong correlation between ${T}_s$ and ${T}_c$. Critically, no superlattice is observed below ${T}_s$ as per the electron-diffraction examinations. Thus, the anomalous phase transition at ${T}_s$, which exhibits a large thermal hysteresis in resistivity and magnetization measurements, is structural rather than a charge-density-wave formation.