Thorium-Based Iron Arsenide with Fe-Fe Bonding

Abstract

This work presents a study on a new thorium iron arsenide, Th₂Fe₁₂As₇, a second discovered compound in the ternary system. It crystallizes in the noncentrosymmetric hexagonal Zr₂Fe₁₂P₇ structure type (space group P6, a = 9.5506(4) Å and c = 3.8645(2) Å). The bonding analysis of Th₂Fe₁₂As₇ reveals that the structure can be represented by a trigonal prismatic environment of the As atoms, which are the geometric locations of anionic components of the structure. The cationic components of the structure are represented by the Th species. Furthermore, the bonding analysis revealed a new feature─a three-dimensional framework of two-atomic bonds between the Fe atoms, which─in the sense of effective charges─is essentially neutral. It plays the role of a bonding mediator within the [Fe-As] framework. Th₂Fe₁₂As₇ is a metallic paramagnet down to the lowest measured temperature, 0.4 K.

1

Crystal structure of Th₂Fe₁₂As₇ represented as a network of interconnected Fe and As atoms (top panel) and condensed trigonal prisms around As atoms, which form a “shamrock” trefoil with a Th atom in the center (bottom panel).

2

Atomic decoration around the [001] line in two models of the Th₂Fe₁₂As₇ crystal structure (left and middle panels) and the distribution of the experimental difference electron density along the [001] line ((010) plane) (right panel). Iron atoms are magenta, whereas arsenic atoms are green. Isolines are drawn with a step of 1 e/A^–3. Zero level is shown as a dashed lines.

3

Electronic density of states (DOS) for Th₂Fe₁₂As₇ together with the contributions of the essential atomic states.

4

Electrical resistivity of Th₂Fe₁₂As₇ in H = 0. Inset: Single crystals of Th₂Fe₁₂As₇ have needle-like morphology with the long axis being the [001] direction.

5

QTAIM atomic shapes and effective charges in Th₂Fe₁₂As₇. Black lines show the borders of the calculated region (orthohexagonal unit cell).

6

Superbasins of the As anions in Th₂Fe₁₂As₇. Red lines represent the shortest interatomic contacts within the Fe–As framework, blue lines show the distances within the coordination sphere of each As species, the black lines show the borders of the calculated region (orthohexagonal unit cell), and the coordination spheres for each As atom are also given.

7

Bond basins of the 2a-Fe–Fe bonds in Th₂Fe₁₂As₇. Blue lines are the shortest interatomic contacts, and black lines show the borders of the calculated region (orthohexagonal unit cell).

8

Bonding picture in Th₂Fe₁₂As₇: As anions, visualized by the bond superbasins (green), interacting ionically with the cationic Th species (blue), mediated by the neutral three-dimensional Fe framework (red lines).