Metal/semiconductor crossover

While there is no doubt that a crossover occurs, there exists some contradictory evidence regarding the exact placement of the metal/semiconductor boundary. Data taken on polycrystalline samples prepared under a strictly controlled oxygen partial pressure by the use of a buffer gas place the boundary between Gd and Tb. ° Reports based on single crystals show that the A = Gd phase is nonmetallic. Further studies have demonstrated that the single crystals are oxygen deficient... 

The pressure-induced semiconductor-metal transitions of SmSei xSx solid solutions are discontinuous for x>0.2 and continuous forx<0.2 as detected by electrical resistivity q versus pressure measurements. The strength of the first-order phase transition (expressed by Ag/g p where Ag is the resistivity jump and Qtr s the resistivity at the transition pressure Ptr) decreases smoothly to zero at x = 0.2. This composition is characterized by a sharp break in slope at 34.6 kbar when q versus p is plotted but no hysteresis is noticed as pressure is released. Transition pressure p r and Ag/ptr in comparison with the theoretical phase transition strength (calculated with a modified Falicov-Kimball model) as a function of composition are shown in Fig. 81, p. 170, Bucher, Maines [2], also see Bucher et al. [3]. The change from continuous to discontinuous transition is interpolated to be at x = 0.28 the experimental value is 0.25 (determination technique not given in the paper), Narayan, Ramaseshan [4]. The course of the configuration crossover f d°- f d for Sm(Se,S) under pressure is illustrated by Wilson [5]. 

---