Pressure coefficient of the direct band gap of ${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ from optical absorption measurements

The shift with hydrostatic pressure of the absorption edge in ${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ compound semiconductors has been measured. The pressure coefficient of the direct-conduction-band minima $\frac{d{E}_g^{\Gamma }}{\mathrm{dP}}$ was obtained as a function of composition in the range $x=0\mathrm{} \mathrm{to} 0.5$. The pressure coefficient, when plotted againt the compositional parameter $x$, is found to increase up to $x=0.25\mathrm{}$ and then to decrease nonlinearly.