A series of ternary molybdenum chalcogenide compounds. They were reported by R. Chevrel, M. Sergent, and J. Prigent in 1971. The compounds have the general formula MxMo6X8, where M represents any one of a large number (nearly 40) of metallic elements throughout the periodic table; x has values between 1 and 4, depending on the M element; and X is a chalcogen (sulfur, selenium or tellurium). The Chevrel phases are of great interest, largely because of their striking superconducting properties.
Most of the ternary molybdenum chalcogenides crystallize in a structure in which the unit cell, that is, the repeating unit of the crystal structure, has the overall shape of a rhombohedron with a rhombohedral angle close to 90°. Some of the ternary molybdenum chalcogenides display a slight distortion of the rhombohedral crystal structure at, or below, room temperature to a triclinic structure in which the three axes of the unit cell, and the three angles between them, are unequal.
The building blocks of the Chevrel-phase crystal structure are the M elements and Mo6X8 molecular units or clusters. Each Mo6X8 unit is a slightly deformed cube with X atoms at the corners, and Mo atoms at the face centers. One of these structures, that of PbMo6S8, is shown in the illustration. See also: Crystal structure; Crystallography
Many of the Chevrel-phase compounds exhibit superconductivity, a phenomenon in which a metal, when cooled below its superconducting transition temperature Tc, loses all resistance to flow of an electric current. Several of the Chevrel-phase compounds have relatively high values of Tc, the maximum being about 15 K (−433°F) for PbMo6S8. Prior to 1986, the highest Tc of any known material was 23 K (−418°F) for the A15 compound Nb3Ge. See also: A15 phases
Since 1987, superconductivity at temperatures above the boiling point of liquid nitrogen (77 K or −321°F) has been discovered in several families of copper oxide compounds. A value of Tc of approximately 122 K (−240°F) has been observed in a compound comprising the elements thallium, barium, calcium, copper, and oxygen. As in conventional superconductors, the superconductivity is associated with electron pairs, called Cooper pairs; but the attractive mechanism responsible for the formation of the Cooper pairs may be different in the high-Tc superconducting oxides than in conventional superconductors, like the Chevrel phases and the A15 phases, where it involves the interaction between the negatively charged electrons and the positively charged ions of the crystal lattice.
The superconductivity of any superconducting compound can be destroyed with a sufficiently high magnetic field, called the upper critical magnetic field Hc2(T), which depends on the temperature T and varies from material to material. As the temperature is lowered, Hc2(T) generally increases from zero at Tc to a maximum value, Hc2(0), as the temperature approaches absolute zero. The Chevrel-phase PbMo6S8 has a value of Hc2(0) of about 60 teslas (600 kilogauss), the largest value observed for any material prior to 1986. The highest Hc2(0) that had been previously reported for a non-Chevrel-phase material was 40 teslas (400 kilogauss) for the A15 compound Nb3Ge. In comparison, the values of Hc2(0) for the highest Tc copper oxide superconductors may reach several hundred teslas, values so large that they cannot readily be measured by using the magnetic fields and techniques that are presently available in the laboratory.
A number of Chevrel-phase compounds of the form RMo6X8, where R is a rare-earth element with a partially filled 4f electron shell and X is S or Se, display magnetic order at low temperatures in addition to superconductivity. The superconductivity is primarily associated with the mobile 4d electrons of Mo, while the magnetic order involves the localized 4f electrons of the R atoms which occupy regular positions throughout the lattice. Superconductivity has been found to coexist with antiferromagnetic order, but to be destroyed by the onset of ferromagnetic order. See also: Antiferromagnetism; Ferromagnetism; Superconductivity