Here, we report the structure-property relationship in pressurized $4\mathit{Ha}-{\mathrm{TaSe}}_2$ single crystals. Electrical transport and low-temperature Raman spectroscopy measurements indicate a charge density wave (CDW) transition at $T_{\mathrm{CDW}}\sim 120$ K under ambient pressure. High-pressure transport experiments reveal a competitive relationship between superconductivity (SC) and CDW in $4\mathit{Ha}-{\mathrm{TaSe}}_2$. Under compression, the CDW order is progressively suppressed and vanishes abruptly at a critical pressure $P_{\mathrm{c}}$ of ∼19 GPa. In contrast, the superconducting transition temperature is gradually enhanced from $T_{\mathrm{c}}\sim 3.1$ K at ambient pressure to an optimal $T_{\mathrm{c}}\sim 7.8$ K at $P_{\mathrm{c}}$, followed by a slow reduction up to 83.7 GPa. Synchrotron x-ray diffraction experiments show that the pristine hexagonal structure remains stable under pressure, while the dimensionality transforms from quasi-2D to quasi-3D near $P_{\mathrm{c}}$. Simultaneously, Raman vibrational modes exhibit significant anomalies around $P_{\mathrm{c}}$, including changes in Raman intensity and pressure coefficients of normal in-plane modes, as well as in a two-phonon mode. Thus, the collapse of the CDW near $P_{\mathrm{c}}$ and the domelike superconducting phase diagram could be related to a dimensional crossover in the crystal structure. These findings not only establish the pressure-temperature phase diagram of $4\mathit{Ha}-{\mathrm{TaSe}}_2$ polytype but also facilitate the systematic understanding of the interplay between SC and CDW across the transition-metal dichalcogenides family of materials.

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• Charge density waves
• Superconductivity
• Superconductors
• Transition metal dichalcogenides
• Density functional theory
• Electrical transport techniques
• Raman spectroscopy
• X-ray diffraction