Samarium monochalcogenides
Encyclopedia
Samarium monochalcogenides are chemical compounds with the composition SmX, where Sm stands for the lanthanide
element samarium
and X denotes any one of three chalcogen
elements, sulfur
, selenium
or tellurium, resulting in the compounds SmS, SmSe or SmTe. In these compounds, samarium formally exhibits oxidation state
+2, whereas it usually assumes the +3 state, resulting in chalcogenides with the chemical formula Sm2X3.
or electron beam physical vapor deposition
, that is bombardment of samarium metal target with electrons in and appropriate gas atmosphere (e.g. hydrogen disulfide
for SmS).
Samarium monochalcogenides are black semiconducting
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
. This results in spectacular change in color from black to golden yellow when scratching or mechanically polishing SmS. The transition does not change the crystal structure, but there is a sharp decrease (about 15%) 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.
Not only color and electrical conductivity, but also other properties change in samarium monochalcogenides with increasing pressure. Their metallic behavior results from the decreasing band gap
, which amounts at zero pressure to 0.15, 0.45 and 0.65 eV in SmS, SmSe and SmTe, respectively.
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
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 or irradiation with a pulsed, high-intensity laser beam.
Lanthanide
The lanthanide or lanthanoid series comprises the fifteen metallic chemical elements with atomic numbers 57 through 71, from lanthanum through lutetium...
element samarium
Samarium
Samarium is a chemical element with the symbol Sm, atomic number 62 and atomic weight 150.36. It is a moderately hard silvery metal which readily oxidizes in air. Being a typical member of the lanthanide series, samarium usually assumes the oxidation state +3...
and X denotes any one of three chalcogen
Chalcogen
The chalcogens are the chemical elements in group 16 of the periodic table. This group is also known as the oxygen family...
elements, sulfur
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
, selenium
Selenium
Selenium is a chemical element with atomic number 34, chemical symbol Se, and an atomic mass of 78.96. It is a nonmetal, whose properties are intermediate between those of adjacent chalcogen elements sulfur and tellurium...
or tellurium, resulting in the compounds SmS, SmSe or SmTe. In these compounds, samarium formally exhibits oxidation state
Oxidation state
In chemistry, the oxidation state is an indicator of the degree of oxidation of an atom in a chemical compound. The formal oxidation state is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic. Oxidation states are typically represented by...
+2, whereas it usually assumes the +3 state, resulting in chalcogenides with the chemical formula Sm2X3.
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 sputteringSputtering
Sputtering is a process whereby atoms are ejected from a solid target material due to bombardment of the target by energetic particles. It is commonly used for thin-film deposition, etching and analytical techniques .-Physics of sputtering:...
or electron beam physical vapor deposition
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 target to transform into the gaseous phase...
, that is bombardment of samarium metal target with electrons in and appropriate gas atmosphere (e.g. hydrogen disulfide
Hydrogen disulfide
Hydrogen disulfide is an inorganic compound. This mal-odorous oil decomposes readily to hydrogen sulfide .-Structure:The structure of hydrogen disulfide is similar to that of hydrogen peroxide, with two central sulfur atoms and two outer hydrogen atoms...
for SmS).
Properties
| Formula | Lattice constant nm |
Resistivity Resistivity Electrical resistivity is a measure of how strongly a material opposes the flow of electric current. A low resistivity indicates a material that readily allows the movement of electric charge. The SI unit of electrical resistivity is the ohm metre... Ohm·cm |
Band gap Band gap In solid state physics, a band gap, also called an energy gap or bandgap, is an energy range in a solid where no electron states can exist. In graphs of the electronic band structure of solids, the band gap generally refers to the energy difference between the top of the valence band and the... eV |
|---|---|---|---|
| SmS | 0.597 | 0.001–0.01 | 0.15 |
| SmSe | 0.620 | ~3000 | 0.45 |
| SmTe | 0.6594 | ~1000 | 0.65 |
Samarium monochalcogenides are black semiconducting
Semiconductor
A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...
solids with rock-salt
Cubic crystal system
In crystallography, the cubic crystal system is a crystal system where the unit cell is in the shape of a cube. This is one of the most common and simplest shapes found in crystals and minerals....
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
Bar (unit)
The bar is a unit of pressure equal to 100 kilopascals, and roughly equal to the atmospheric pressure on Earth at sea level. Other units derived from the bar are the megabar , kilobar , decibar , centibar , and millibar...
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
Thulium is a chemical element that has the symbol Tm and atomic number 69. Thulium is the second least abundant of the lanthanides . It is an easily workable metal with a bright silvery-gray luster...
. This results in spectacular change in color from black to golden yellow when scratching or mechanically polishing SmS. The transition does not change the crystal structure, but there is a sharp decrease (about 15%) in the crystal volume. A hysteresis
Hysteresis
Hysteresis is the dependence of a system not just on its current environment but also on its past. This dependence arises because the system can be in more than one internal state. To predict its future evolution, either its internal state or its history must be known. If a given input alternately...
is observed, that is when the pressure is released, SmS returns to semiconducting state at much lower pressure of about 0.5 kbar.
Not only color and electrical conductivity, but also other properties change in samarium monochalcogenides with increasing pressure. Their metallic behavior results from the decreasing band gap
Band gap
In solid state physics, a band gap, also called an energy gap or bandgap, is an energy range in a solid where no electron states can exist. In graphs of the electronic band structure of solids, the band gap generally refers to the energy difference between the top of the valence band and the...
, which amounts at zero pressure to 0.15, 0.45 and 0.65 eV in SmS, SmSe and SmTe, respectively.
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
Paramagnetism is a form of magnetism whereby the paramagnetic material is only attracted when in the presence of an externally applied magnetic field. In contrast with this, diamagnetic materials are repulsive when placed in a magnetic field...
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 or irradiation with a pulsed, high-intensity laser beam.