Samarium monochalcogenides are chemical compounds with the composition SmX, where Sm stands for the
lanthanide element
samarium
Samarium is a chemical element with symbol Sm and atomic number 62. It is a moderately hard silvery metal that slowly oxidizes in air. Being a typical member of the lanthanide series, samarium usually has the oxidation state +3. Compounds of samar ...
and X denotes any one of three
chalcogen
The chalcogens (ore forming) ( ) are the chemical elements in group 16 of the periodic table. This group is also known as the oxygen family. Group 16 consists of the elements oxygen (O), sulfur (S), selenium (Se), tellurium (Te), and the radioac ...
elements,
sulfur,
selenium
Selenium is a chemical element with the symbol Se and atomic number 34. It is a nonmetal (more rarely considered a metalloid) with properties that are intermediate between the elements above and below in the periodic table, sulfur and tellurium, ...
or
tellurium
Tellurium is a chemical element with the symbol Te and atomic number 52. It is a brittle, mildly toxic, rare, silver-white metalloid. Tellurium is chemically related to selenium and sulfur, all three of which are chalcogens. It is occasionall ...
, resulting in the compounds SmS, SmSe or SmTe. In these compounds, samarium formally exhibits
oxidation state
In chemistry, the oxidation state, or oxidation number, is the hypothetical charge of an atom if all of its bonds to different atoms were fully ionic. It describes the degree of oxidation (loss of electrons) of an atom in a chemical compound. C ...
+2, whereas it usually assumes the +3 state, resulting in chalcogenides with the chemical formula Sm
2X
3.
Synthesis
Single crystals or polycrystals of samarium monochalcogenides can be obtained by reacting the metal with sulfur, selenium or tellurium vapors at high temperature.
[ Thin films can be obtained by magnetron ]sputtering
In physics, sputtering is a phenomenon in which microscopic particles of a solid material are ejected from its surface, after the material is itself bombarded by energetic particles of a plasma or gas. It occurs naturally in outer space, and ca ...
electron beam physical vapor deposition Electron-beam physical vapor deposition, or EBPVD, is a form of physical vapor deposition in which a target anode is bombarded with an electron beam given off by a charged tungsten filament under high vacuum. The electron beam causes atoms from the ...
, that is bombardment of samarium metal target with electrons in and appropriate gas atmosphere (e.g. hydrogen disulfide for SmS).[
]
Properties
Samarium monochalcogenides are black semiconducting
A semiconductor is a material which has an electrical resistivity and conductivity, electrical conductivity value falling between that of a electrical conductor, conductor, such as copper, and an insulator (electricity), insulator, such as glas ...
solids with rock-salt cubic crystal structure. Application of moderate hydrostatic pressure converts them into metals. Whereas the transition is continuous and occurs at about 45 and 60 kbar in SmSe and SmTe, respectively, it is abrupt in SmS and requires only 6.5 kbar. Similar effect is observed in monochalcogenides of another lanthanide, thulium
Thulium is a chemical element with the symbol Tm and atomic number 69. It is the thirteenth and third-last element in the lanthanide series. Like the other lanthanides, the most common oxidation state is +3, seen in its oxide, halides and other c ...
.[ This results in spectacular change in color from black to golden yellow when scratching or mechanically polishing SmS.][Eric Beaurepaire (Ed.]
Magnetism: a synchrotron radiation approach
Springer, 2006 p. 393 in the crystal volume. A hysteresis is observed, that is when the pressure is released, SmS returns to semiconducting state at much lower pressure of about 0.5 kbar.band gap
In solid-state physics, a band gap, also called an energy gap, is an energy range in a solid where no electronic states can exist. In graphs of the electronic band structure of solids, the band gap generally refers to the energy difference ( ...
, which amounts at zero pressure to 0.15, 0.45 and 0.65 eV in SmS, SmSe and SmTe, respectively.[K. H. J. Buscho]
Concise encyclopedia of magnetic and superconducting materials
Elsevier, 2005 p. 318
At the transition pressure (6.5 kbar in SmS) the gap is still finite and the low resistivity originates from thermally activated generation of carriers across a narrow band gap. The gap collapses at about 20 kbar when SmS becomes a true metal. At this pressure, the material also changes from paramagnetic
Paramagnetism is a form of magnetism whereby some materials are weakly attracted by an externally applied magnetic field, and form internal, induced magnetic fields in the direction of the applied magnetic field. In contrast with this behavior, ...
to a magnetic state.[
The semiconductor-metal transition in samarium monochalcogenides requires application of pressure or presence of intrinsic stress, for example in thin films, and the reverse changes occur upon release of this stress. Such release can be triggered by various means, such as heating to about 200 °C]
Potential applications
The change in electrical resistivity in samarium monochalcogenides can be used in a pressure sensor or in a memory device triggered between a low-resistance and high-resistance state by external pressure, and such devices are being developed commercially. Samarium monosulfide also generates electric voltage upon moderate heating to about 150 °C that can be applied in thermoelectric power converters.other articles on this topic
References
{{Samarium compounds
Samarium compounds
Chalcogenides