<p>Orthopyroxene is a major mineral in the Earth’s upper mantle. In this study, we conducted in-situ high pressure and temperature Raman spectroscopy measurements on single-crystal orthopyroxenes with varying (Fe, Al) concentrations using externally heated diamond anvil cells. The α-β transition was identified by systematic changes in characteristic Raman modes, and transition boundaries were determined in the range of 400–700&#xa0;K up to ~ 20 GPa. All samples exhibit the same structural transition pathway, while the transition pressure and Clapeyron slope vary systematically with composition. Our results demonstrate that increasing Fe content from 3 to 9&#xa0;mol% raises the Clapeyron slope from 0.6(3) to 2.1(2) MPa/K. In contrast, incorporation of Al yields a negative Clapeyron slope of -0.8(2) MPa/K, insensitive to Fe variations up to 21&#xa0;mol%. By combining our experimental constraints with literature pressure-volume data, we modeled density-depth profiles of orthopyroxene with varying compositions along a representative cold slab geotherm. The modeling shows that the α-β transition from the En₉₇Fs₃ to a mantle-typical composition (Fs₉) occurs at nearly the same depths of ~ 420–440&#xa0;km, with density jumps of ~ 0.8-1.0%. This phase transition in cold subductions labs can enhance the slab pull and promote slab penetration. In contrast, for Fe contents up to 21&#xa0;mol%, incorporation of 2–7&#xa0;mol% Al can delay the transition to a slightly greater depth of ~ 470–520&#xa0;km and yields density jumps of ~ 0.5–0.8%. This means that Al-bearing low-density α-phase can persist to a deeper depth and may contribute to slab stagnation in the transition zone.</p>