 Indexing a diamond-cubic powder pattern

For cubic diffraction, $4{\sin }^2\theta /{\lambda }^2=(h^2+k^2+l^2)/a^2$. Ratios identify the allowed integer sequence after applying both FCC centring and diamond-basis selection rules. A common sequence starts $3,8,11,16,19$ for111,220,311,400,331. The absolute parameter remains proportional to the explicitly selected radiation wavelength.