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, interest in these compounds increased dramatically after the publication of the superconducting properties of LaOFeP and LaOFeAs which were discovered in 2006 and 2008. 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.

The different subclasses of oxypnictides are oxynitrides, oxyphosphides, oxyarsenides, oxyantimonides, and oxybismuthides.

Structure
Many of the oxypnictides show a layered structure. For example, LaFePO with layers of La3+O2− and Fe2+P3−. This structure is similar to that of ZrCuSiAs, which is now the parent structure for most of the oxypnictide.

Superconductivity
The first superconducting iron oxypnictide was discovered in 2006, based on phosphorus. A drastic increase in the critical temperature was achieved when phosphorus was substituted by arsenic. 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.

Tests in magnetic fields up to 45 teslas 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.

Practical use
Because of the brittleness of the oxypnictides, superconducting wires are formed using the powder-in-tube process (using iron tubes).