The X-ray powder diffraction patterns for three bulk Zn1−xMgxSe crystals are reported. The data were obtained with the help of an automated Bragg–Brentano diffractometer using Ni-filtered Cu Kα radiation. One of the samples is of the sphalerite structure type, and it has the magnesium content slightly below the sphalerite–wurtzite phase transition. The two remaining ones are of the wurtzite type with low and high magnesium content. The lattice constant for the sphalerite Zn0.86Mg0.14Se is a0=5.7011(1) Å. For the wurtzite alloys the lattice constants are a0=4.0540(1) Å, c0=6.6270(2) Å (for Zn0.72Mg0.28Se), a0=4.1195(1) Å, c0=6.6941(2) Å (for Zn0.37Mg0.63Se).
