Phase transformations and the nature of the semiconductor-to-metal transition in bulk a-GaSb and a-(Ge2)1-x(GaSb)x semiconductors under high pressure

The pressure-induced transitions in bulk amorphous GaSb and Ge-GaSb solid solutions, prepared by a solid state amorphization of the high-pressure phases after decompression, were studied under pressure up to 9 GPa. According to x-ray diffraction, volume, and resistivity measurements the amorphous semiconductors reversibly transform to crystalline high-pressure metallic phases (GaSb II or solid solutions with GaSb II-like structure). For Ge-GaSb alloys the transition occurs abruptly in the range 4.5-6.5 GPa for various concentrations of the components. The a-GaSb samples gradually transform in a wide pressure range between 3.5 and 8.5 GPa. It is shown that such behavior is due to heterogeneity of microscopic structural characteristics of the network and to partial crystallization of zinc-blende GaSb. A semiconductor-to-metal transition in a-GaSb is observed at 3.5-4 GPa, and is driven by the percolation mechanism. Bulk moduli of amorphous compounds exhibit substantial softening above P∼1-2 GPa, which is accompanied by intensification of the irreversible resistivity relaxation with pressure. At room pressure the amorphous tetrahedral network of a-GaSb (B ≈ 35 GPa) is more compressible than the crystalline lattice of GaSb (B ≈ 55 GPa). Thermodynamics of the structural transformation in a-(Ge₂)_0.27(GaSb)_0.73 was studied by differential thermal analysis, and discussed in the framework of the nonequilibrium phase diagram of an amorphous solid.