Fig. 2

Cubic crystal structure of \(\hbox {Sn}_\mathrm{x} \hbox {Pb}_{1-\mathrm{x}}\hbox {Te}\)  [60]. a original unit cell with space group \(\hbox {Fm}\bar{3}\hbox {m}\) (225), and \(a = b = c = 6.32\text {-}6.46\) Å. The structure consists of mixed lead and tin (in dark gray and dark red, respectively) sites and tellurium sites (brass-colour) occupying 4a (0, 0, 0) and 4b \(\left(\frac{1}{2},\frac{1}{2},\frac{1}{2}\right)\) positions with the same point symmetry \(\hbox {m}\bar{3}\hbox {m}\). b, c Two out of 8 possible unique structures for \(x=\frac{1}{2}\) and cell \(1\times 2\times 1\) (see Table 2). b The highest-symmetry structure corresponds to tetragonal \({\text{P}}4/{\text{mmm}}\) space group (123), whose corresponding primitive cell is highlighted with black lines. c Lowest-symmetry structure with orthorhombic \({\text{Pmm}}2\) space group (25)