Cadmium arsenide
| Cadmium arsenide | ||
|---|---|---|
| Other names Tricadmium diarsenide | ||
| Identifiers | ||
| CAS number | 12006-15-4 | |
| ChemSpider | 10678197 | |
| EC number | 234-484-1 | |
| Jmol-3D images | {{#if:[Cd+2].[Cd+2].[Cd+2].[AsH6-3].[AsH6-3]|Image 1 | |
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| Properties | ||
| Molecular formula | Cd3As2 | |
| Molar mass | 487.08 g/mol | |
| Appearance | solid, dark grey | |
| Density | 3.031 | |
| Melting point | 716 °C | |
| Solubility in water | decomposes in water | |
| Hazards | ||
| NFPA 704 |
 1
4
0
| |
| U.S. Permissible exposure limit (PEL) |
5 micrograms (Cd)/m3 | |
| LD50 | no data | |
| Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa) | ||
| Infobox references | ||
Cadmium arsenide (Cd3As2) is an inorganic, crystalline semiconductor with a tetragonal structure in the II-V family. Cadmium arsenide shows the Nernst effect.
Properties
Thermal
Cd3As2 is proven to dissociate thermally between 220° to 280° according to the reaction
Cd 3 As 2 (s) 3Cd(g) + 0.5 As4 (g) [1]
An energy barrier was found for the nonstoichiometric vaporization of arsenic due to the irregularity of the partial pressures with temperature. The range of the energy gap is from 0.5 to 0.6 eV. Cd3As2 melts at 716 °C and changes phase at 615 °C/[2]
Phase Transition
Pure cadmium arsenide undergoes several phase transitions at high temperatures, making phases labeled α (stable phase), α’, α” (metastable phase), β.[3] At 593° the polymorphic transition α → β happens.
α-Cd3As2 <-> α’-Cd3As2 happens at ~500K.
α’-Cd3As2 <-> α’’-Cd3As2 happens at ~742 and is a regular first order phase transition with marked hysteresis loop.
α”-Cd3As2 <-> β-Cd3As2 happens at 868 K.
Singly crystal x-ray diffraction was used to determine the lattice parameters of Cd3As2 between 23 and 700°C. Transition α → α′ happens slowly and there is most likely an intermediate phase. The transition α′ → α″ occurs much faster than α → α′ and has very small thermal hysteresis. This transition results in a change in the fourfould axis of the tetragonal cell, causing Crystal twinning. The width of the loop is independent of the rate of heating although it becomes narrower after several temperature cycles.[4]
Electronic
The compound cadmium arsenide has a lower vapor pressure (0.8 atm) than both cadmium and arsenic separately. Cadmium arsenide does not decompose when it is vaporized and re-condensed. Carrier Concentration in Cd3As2 are usually between 1 and 4 x 1018 electrons/cm3. Despite having high carrier concentrations, the electron mobilities are also very high (up to 100,000 cm2/Vs). Cd3As2 has the highest mobility out of the semiconductors.[5]
Conducting
Cadmium Arsenide is a II-V semiconductor showing degenerate N-type semiconductor intrinsic conductivity with a large mobility, low effective mass and highly non parabolic conduction band, or a Narrow-gap semiconductor. It displays an inverted band structure, and the optical energy gap, eg, is less than 0. When deposited by thermal evaporation (deposition), cadmium arsenide displayed the Schottky (Thermionic emission) and Poole–Frenkel effect at high electric fields.[6]
Preparation
Cadmium arsenide can be prepared as amorphous semiconductive glass. According to Hiscocks and Elliot,[7] the preparation of cadmium arsenide was made from cadmium metal, which had a purity of 6 N from Kock-Light Laboratories Limited. Hoboken supplied β-arsenic with a purity of 99.999%. Stoichiometric proportions of the elements cadmium and arsenic were heated together. Separation was difficult and lengthy due to the ingots sticking to the silica and breaking. Liquid encapsulated Stockbarger growth was created. Crystals are pulled from volatile melts in liquid encapsulation. The melt is covered by a layer of inert liquid, usually B2O3, and an inert gas pressure greater than the equilibrium vapor pressure is applied. This eliminates the evaporation from the melt which allows seeding and pulling to occur through the B2O3 layer.
Crystal Structure
The unit cell of Cd3As2 is tetragonal. The arsenic ions are cubic close packed and the cadmium ions are tetrahedrally coordinated. The vacant tetrahedral sites provoked research by von Stackelberg and Paulus (1935), who determined the primary structure. Each arsenic ion was surrounded by cadmium ions at six of the eight corners of a distorted cube and the two vacant sites were at the diagonals.[8]
Nernst Effect
Cadmium arsenide is used in infrared detectors using the Nernst effect, and in thin-film dynamic pressure sensors. It can be also used to make magnetoresistors, and in photodetectors.
Cadmium arsenide can be used as a dopant for HgCdTe.
References
- ↑ Mass Spectrometric Study Of The Nonstoichiometric Vaporization Of Cadmium Arsenide J. B.Westmore - K. H.Mann - A. W.Tickner - Journal Of Physical Chemistry - 1964
- ↑ On The Preparation, Growth And Properties Of Cd3As2 S. E. R.Hiscocks - C. T.Elliott - Journal Of Materials Science - 1969">
- ↑ Lukaszewicz, K., and A. Pietraszko. "A Refinement of the Crystal Structure of 'a'-Cd3As2." Acta Crystallographica Section B (1969).
- ↑ Thermal Expansion And Phase Transitions Of Cd3As2 And Zn3As2 A. Pietraszko - K. Łukaszewicz - Physica Status Solid (a) - 1973
- ↑ Inverted Band Structure Of Cd3As2 B. Dowgiałło-Plenkiewicz - P. Plenkiewicz - Physica Status Solid (b) - 1979
- ↑ Van Der Pauw Resistivity Measurements On Evaporated Thin Films Of Cadmium Arsenide, Cd3As2 M Din - R Gould - Applied Surface Science - 2006
- ↑ "On The Preparation, Growth And Properties Of Cd3As2 S. E. R.Hiscocks - C. T.Elliott - Journal Of Materials Science - 1969
- ↑ Lukaszewicz, K., and A. Pietraszko. "A Refinement of the Crystal Structure of 'a'-Cd3As2." Acta Crystallographica Section B (1969).
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