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

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

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]

oxypnictides

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 LaFeAsO_0.89F_0.11 may be around 64 T. A different lanthanum-based material tested at 6 K predicts an upper critical field of 122 T in La_0.8K_0.2FeAsO_0.8F_0.2.^[10]

Practical use

Because of the brittleness of the oxypnictides, superconducting wires are formed using the powder-in-tube process (using iron tubes).^[17]

See also

• 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