ThCr2Si2type

The ThCr2Si2-type, also written as ThCr2Si2-type structure, is a tetragonal crystal structure family common in intermetallic compounds with the general formula AT2X2. In this scheme A is an electropositive element (often an alkaline earth, rare earth, or actinide), T is a transition metal, and X is a p-block element such as Si, Ge, P, or As. The prototype compound is ThCr2Si2.

Structure and key features: The ideal structure crystallizes in the body-centered tetragonal system with space group I4/mmm. It comprises alternating layers along the c axis: a T2X2 layer sandwiched between A layers. In the T2X2 layer, each T atom is coordinated by four X atoms in a tetrahedral geometry, and the X atoms form a connected network within the layer. The A layers separate adjacent T2X2 layers, yielding a characteristic layered, quasi-two-dimensional motif that underpins many physical properties of compounds in this family.

Chemistry and scope: The ThCr2Si2-type accommodates a wide range of chemistries, including many AT2X2 combinations. Notable members include the prototype ThCr2Si2, the 122 iron-pnictide family such as BaFe2As2, SrFe2As2, and CaFe2As2, and CeCu2Si2, among others. The structure tolerates substantial variation in ionic radii, enabling extensive chemical substitution and tuning of properties.

Relevance and phenomena: This structure is important in solid-state chemistry and condensed matter physics. In several members, especially the 122 family, superconductivity can be induced by chemical doping or applied pressure. The framework also exhibits magnetic ordering and, in some compounds, structural transitions such as a collapsed tetragonal phase under pressure.

Relation to other types: The ThCr2Si2-type is part of a broader class of layered tetragonal intermetallics and relates to other 122- and CaBe2Ge2-type structures through similar stacking and coordination motifs.