Role of intervalley scattering in the radiative recombination in Pb$_{1-x}$Eu$_x$Te alloys (0 $\le x\le$ 1)

Measurements of the photoluminescence from epitaxial layers of Pb$_{1-x}$Eu$_x$Te alloys with 0 $\le x\le$ 0.32 are carried out. It is found that the luminescence intensity decreases with increasing Eu content and, already for $x$ as low as about 0.1, drops by more than one order of magnitude. No luminescence is observed for 0.2 $< x\le$ 0.32. This behavior is explained by the fact that, for $x\approx$ 0.1, the absolute minimum in the conduction band changes from the $L\to X$ point, which results in the scattering of nonequilibrium electrons to the $X$ valley and, thus, causes a decrease in the quantum efficiency of the emission. According to published data, for $x >$ 0.85, optical transitions also take place with the participation of the $X$ valley; in this case, the emission is governed by the formation of magnetic polarons. The temperature dependences of the band gap are determined for 0 $\le x\le$ 0.11. These dependences have a wide linear region characterized by a positive $dE_g/dT$ coefficient, which decreases with the Eu content to become negative in pure EuTe.