O.I. Barkalov

Pressure-induced amorphous phases

Abstract In binary systems many high-pressure phases (HPP) may remain in a metastable state at P = 1 atm and T = T (N 2 ). Under heating an alloy may undergo spontaneous amorphisation. We have observed spontaneous amorphisation of HPP in the CdSb, ZnSb, CdSbZnSb, AlGe, GaSb systems. Amorphisation is accompanied by marked volume and exothermal heat effects. All the amorphous phases obtained from metal HPP are semiconductors. The relationship between the tendency of HPP to amorphisation and T-P-C phase diagrams of the systems examined has been discussed.

Neutron scattering study of bulk amorphous GaSb

The structure of amorphous GaSb, produced by spontaneous transformation to the amorphous state of the metastable high pressure phase, is determined by neutron diffraction and compared to published results on a sample of the same composition but produced by sputtering. The structures are shown to be similar but not in complete agreement. We conclude that the local order is tetrahedral and related to the one in the low pressure crystalline GaSb-I. The packing of the tetrahedra is interpreted as a distorted ZnS blende type of network. A comparative analysis of measured static structure factors for a-GaSb with those for other amorphous materials with tetrahedral bonding indicates that the structure of amorphous GaSb has similarities with the structure of amorphous germanium and silicon.

Neutron diffraction study of bulk amorphous Zn41Sb59

The amorphous Zn41Sb59 alloy was studied by means of neutron diffraction. The sample was prepared by solid-state transformation of the quenched high-pressure phase during heating at atmospheric pressure. The data obtained were treated using the reverse Monte Carlo method. According to the radial distribution function obtained, the short-range order of the amorphous alloy was similar to that found for the equilibrium phase of the crystalline compound ZnSb at normal pressure. The effective coordination numbers for the amorphous alloy and the crystalline ZnSb were found to be 4.7 and 5, respectively.

Bulk amorphous ZnSb under normal and high pressures: phase transformations and relaxation phenomena

Abstract Processes occurring upon heating of the high-pressure δ-Zn41Sb59 phase at atmospheric pressure were studied on bulk samples using scanning calorimetry and dilatometry in the temperature interval 170–573 K which includes the temperatures of amorphization and subsequent crystallization. Irreversible relaxation of the amorphous state followed by a heat release and elongation of the sample of the order of 1 4 of those found upon amorphization were observed at 325–425 K. At room temperature the effect of pressure on the electrical resistance of the amorphous Zn41Sb59 samples (P

Phase transitions and equilibrium hydrogen content of phases in the water–hydrogen system at pressures to 1.8 kbar

Using a volumetric technique, the hydrogen solubility X in liquid water (L), low-pressure hexagonal ice (I h), and high-pressure cubic clathrate ice (sII) is studied at hydrogen pressures up to 1.8 kbar and temperatures from−36 to+20 °C. The triple point of the L+I h+sII equilibrium is located at P=1.07(5) kbar and T=−10(1) °C. The hydrogen concentrations of phases at the triple point are X L =0.17(5), and X sII=2.3(1) wt.% H2. The thermal stability and the process of decomposition of the clathrate phase at ambient pressure are studied by neutron diffraction.

PHASE TRANSFORMATIONS OF AMORPHOUS GaSb-Ge ALLOY AT HIGH PRESSURES

Abstract Phase transformations occuring in the initially amorphous bulk (GaSb)38Ge24 semiconductor at pressures to 7.7 GPa and temperatures to 330°C were studied using the measurement of the electrical resistance supplemented by the X-ray examination of the samples quenched to 100 K after a high pressure treatment. The obtained experimental data and model calculations were then used to construct the T-P diagram of metastable equilibria between the crystalline metallic high-pressure phase (hpp) and two unordered phases, semiconducting (sup) and metallic (mup), which are either amorphous or liquid depending on the temperature. The line of the hpp↔mup equilibrium (the melting curve of the high-pressure phase) was shown to terminate at a critical point where the mup becomes thermodynamically unstable as a phase. The line of the hpp↔sup equilibrium conditions the effect of solid state amorphization of the high-pressure phase at decreasing pressure.

Neutron scattering and specific heat study of AlGe and AlSi alloys quenched under high pressure

Neutron scattering and specific heat measurements on pressure-quenched AlGe and AlSi alloys are reported. The neutron results corroborate earlier findings of a pronounced softening of the transverse zone boundary modes in these polycrystalline nonequilibrium alloys, reminiscent of a similar softening observed in metallic glasses. At lower frequencies, however, no pronounced glassy anomaly is found. The additional excitations seen around 9 meV in AlGe can be explained in terms of resonant modes of the heavier Ge atoms. At still lower frequencies, neutrons see only sound waves. The more sensitive specific heat technique shows the corresponding lattice T3 term and an electronic contribution which follows the classical BCS behaviour at the transition to superconductivity. Below 0.5 K, there remains a linear term about a factor of ten smaller than in typical glasses.

Transport properties of bulk amorphous semiconductor Al32Ge68

Abstract Temperature dependences of the dc conductivity and thermopower of bulk amorphous alloy Al32Ge68 were investigated at 6–420 K and at 80–370 K, respectively. The samples were prepared by solid-state amorphisation of a quenched crystalline high-pressure phase while heating from 77 to 400 K at ambient pressure. Amorphous Al32Ge62 was found to be p-type semiconductor with an unusual combination of transport properties. The change of properties was described semi-quantitatively in terms of a modified Mott–Davis model assuming that the Fermi level lies inside the valence band tail.

Formation and structural evolution of the bulk amorphous Al32Ge68 alloy on heating

Abstract The solid state amorphization of the quenched high pressure of phase Al32Ge68 (at.%) and subsequent crystallization of the amorphous alloy were studied by means of scanning calorimetry, and the microstructure of the bulk amorphous alloy and its evolution on annealing up to crystallization were analyzed by X-ray diffraction and transmission electron microscopy. A crystalline structure was observed at the early stages of crystallization. This structure was unstable and coagulation took place immediately after formation of the nanoscale crystallites.

Electric properties of bulk amorphous semiconductor Zn41Sb59

Abstract The electric conductivity of amorphous Zn 41 Sb 59 was shown to be of a nearly activation type with E σ ≈ 0.28 eV at 125 ≤ T ≤ 360 K. It displayed no frequency dependence at 220 ≤ T ≤ 300 K and f ≤ 5 MHz. The thermoelectric power, S , was positive and increased nearly linearly with reciprocal temperature at 186 ≤ T ≤ 364 K; E s ≈ 0.19 eV. The Hall coefficient measured at 300 K was negative.