Oxypnictide
In chemistry, oxypnictides are a class of materials composed of oxygen, a pnictogen (group-V, especially phosphorus and arsenic) and one or more other elements. Although this group of compounds has been recognized since 1995,[1] interest in these compounds increased dramatically after the publication of the superconducting properties of LaOFeP and LaOFeAs which were discovered in 2006[2] and 2008.[3][4] In these experiments the oxide was partly replaced by fluoride.
These and related compounds (e.g. the 122 iron arsenides) form a new group of iron-based superconductors known as iron pnictides or ferropnictides since the oxygen is not essential but the iron seems to be.
Oxypnictides have been patented as magnetic semiconductors in early 2006.[5]
The different subclasses of oxypnictides are oxynitrides, oxyphosphides, oxyarsenides, oxyantimonides, and oxybismuthides.
Structure
[edit | edit source]Many of the oxypnictides show a layered structure.[6] For example, LaFePO with layers of La3+O2− and Fe2+P3−.[2] This structure is similar to that of ZrCuSiAs, which is now the parent structure for most of the oxypnictide.[7]
Superconductivity
[edit | edit source]The first superconducting iron oxypnictide was discovered in 2006, based on phosphorus.[2] A drastic increase in the critical temperature was achieved when phosphorus was substituted by arsenic.[3] This discovery boosted the search for similar compounds, like the search for cuprate-based superconductors after their discovery in 1986.
The superconductivity of the oxypnictides seems to depend on the iron-pnictogen layers.
Some found in 2008 to be high-temperature superconductors (up to 55 K) of composition ReOTmPn, where Re is a rare earth, Tm is a transition metal and Pn is from group V e.g. As.[8]
| Material | Tc (K) |
|---|---|
| LaO0.89F0.11FeAs | 26[9] |
| LaO0.9F0.2FeAs | 28.5[10] |
| CeFeAsO0.84F0.16 | 41[9] |
| SmFeAsO0.9F0.1 | 43[9] |
| La0.5Y0.5FeAsO0.6 | 43.1[11] |
| NdFeAsO0.89F0.11 | 52[9] |
| PrFeAsO0.89F0.11 | 52[12] |
| GdFeAsO0.85 | 53.5[13] |
| SmFeAsO~0.85 | 55[14] |
Tests in magnetic fields up to 45 teslas[15][16] suggest the upper critical field of LaFeAsO0.89F0.11 may be around 64 T. A different lanthanum-based material tested at 6 K predicts an upper critical field of 122 T in La0.8K0.2FeAsO0.8F0.2.[10]
Practical use
[edit | edit source]Because of the brittleness of the oxypnictides, superconducting wires are formed using the powder-in-tube process (using iron tubes).[17]
See also
[edit | edit source]
- Charge-transfer complex – Association of molecules in which a fraction of electronic charge is transferred between them
- Color superconductivity – Predicted phase in quark matter in quarks
- Kondo effect – Physical phenomenon due to impurities
- Magnetic sail – Proposed spacecraft propulsion method
- National Superconducting Cyclotron Laboratory – Building in Michigan, United States
- Spallation Neutron Source – Accelerator-based neutron source in Oak Ridge, Tennessee, US
- Superconducting radio frequency – Technique used to attain a high quality factor in resonant cavities
- Superfluid film – Thin layer of liquid in a superfluid state
- Timeline of low-temperature technology
References
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- ^ H. Hosono et al. (2006) Magnetic semiconductor material European Patent Application EP1868215
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External links
[edit | edit source]- Hosono at JST Archived 2010-01-06 at the Wayback Machine Has diagram of LaO & FeAs layers in LaOFeAs