Charge-carrier density collapse in layered crystals of the family [(Ge,Sn,Pb)(Te,Se)]$_{m}$[(Bi,Sb)$_{2}$(Te,Se)$_{3}$]$_{n}$ ($m,n$ = 0,1,2 $\dots$)

The causes of the “charge-carrier density collapse”, i.e., a sharp increase in the equilibrium charge-carrier density $n,p$ = 1 $\cdot$ 10$^{19}$ $\to$ (2–5) $\cdot$ 10$^{20}$ cm$^{-3}$ in going from binary alloys such as GeTe and Bi$_{2}$Te$_{3}$ to the family of ternary alloys [(Ge,Sn,Pb)(Te,Se)]$_{m}$[(Bi,Sb)$_{2}$(Te,Se)$_{3}$]$_{n}$ ($m,n$ = 0,1,2 $\dots$) are discussed. The phenomenon is associated with the positional disordering of heterovalent cations (Ge$^{+2}$, Sn$^{+2}$, Pb$^{+2}$ $\leftrightarrow$ Bi$^{+3}$, Sb$^{+3}$) in the cation sublattice of ternary alloys. The phenomenon is not observed during the disordering of isovalent cations (Bi$^{+3}$ $\leftrightarrow$ Sb$^{+3}$) or anions (Te$^{-2}$ $\leftrightarrow$ Se$^{-2}$).