Isotropic-compression Bragg-angle shift ($d\theta /dp=\kappa \tan \theta /3$)

For an isotropically compressed cubic crystal, $d\ln a/dp=-\kappa /3$. Differentiating Bragg's law at fixed wavelength gives the displayed derivative. Constant isothermal compressibility gives $a(p)=a_0{e}^{-\kappa p/3}$, not an exactly linear lattice parameter. The integrated prediction presumes no intervening structural phase change.