Thiospinel ${\mathrm{Co}}_{1.3}{\mathrm{Ir}}_{1.7}{\mathrm{S}}_4$ is a rare diamond-lattice antiferromagnetic insulator with a relatively high Néel temperature of $T_{\mathrm{N}}\sim 290$ K at ambient pressure. We report here a systematic study of the pressure dependence of the structural, transport, and magnetic properties of ${\mathrm{Co}}_{1.3}{\mathrm{Ir}}_{1.7}{\mathrm{S}}_4$ up to 12 GPa. Below 5 GPa, ${\mathrm{Co}}_{1.3}{\mathrm{Ir}}_{1.7}{\mathrm{S}}_4$ retains its insulating behavior, with $T_{\mathrm{N}}$ initially increasing and then decreasing, accompanied by a gradual reduction in the band gap. At $P\sim 6$ GPa, an insulator-to-metal transition occurs. In the pressure range of $7\text{GPa}\le P\le 10$ GPa, ${\mathrm{Co}}_{1.3}{\mathrm{Ir}}_{1.7}{\mathrm{S}}_4$ exhibits a metallic state, with the resistivity showing a broad hump likely corresponding to short-range magnetic order. Above 10.5 GPa, it further evolves into a normal metal, suggesting the disappearance of magnetism. No structural distortion is detected across the sequential insulator-to-metal and magnetism-suppression transitions associated with the two-step phase evolution of the Mott transition. Density-functional theory calculations capture the key experimental trends, highlighting the crucial role of bandwidth and electronic correlations evolution under pressure in driving the Mott transition. A magnetism-pressure phase diagram for ${\mathrm{Co}}_{1.3}{\mathrm{Ir}}_{1.7}{\mathrm{S}}_4$ has been established, wherein the absence of quantum critical behavior implies that the bandwidth-controlled Mott transition may proceed as a first-order transition. Our results advance the broader understanding of pressure-induced quantum phase transitions in diamond-lattice antiferromagnets.

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• High-pressure studies
• Phase diagrams
• Mott insulators
• Density functional theory
• Resistivity measurements
• X-ray diffraction