Superconductivity and Magnetism in Skutterudites

Ctirad Uher

• Publisher: CRC Press
• Year: 2022
• ISBN:
  9781000513806
• Edition: 1
• Topic: Ciencias físicas
• Subtopic: Materia condensada

Superconductivity and Magnetism in Skutterudites discusses superconducting and magnetic properties of a class of materials called skutterudites. With a brief introduction of the fundamental structural features of skutterudites, the book then provides a detailed assessment of the superconducting and magnetic properties, focusing particularly on the rare earth-filled skutterudites where a plethora of fascinating properties and ground states is realized due to interactions of the filler species with the framework ions. Such interactions underpin the exciting forms of superconductivity and magnetism, most notably realized in the exotic heavy fermion superconductor of composition PrOs4Sb12. The two main topics of superconductivity and magnetism are provided with a concise introduction of superconducting and magnetic properties so that a reader can appreciate and understand the main arguments in the text. This book would appeal to graduate students, postdoctoral students, and anyone interested in superconducting and magnetic properties of a large family of minerals called skutterudites.

Key Features:
• Gives a thorough account of the superconducting and magnetic properties of skutterudites.
• Each topic is accompanied by introductory sections to assist in the understanding of the text.
• Supported by numerous figures and all key references.

1 Brief Review of the Structure and Electronic Bands in Skutterudites

DOI: 10.1201/9781003225898-1

1.1 Introduction

Skutterudites have a long history in the annals of science. The word skutterudite was first used by Wilhelm Karl von Haidinger (1845) to describe minerals of composition (Co, Ni, and Fe)As₃ that were mined in a small town called Skuterud in Norway. However, this class of minerals was known well before the time of von Haidinger under various names spanning from cobaltum eineraceum, the name given to it in 1529 by Georgius Agricola, fondly known as the father of mineralogy, to a simple Arsenikkobalt, the name coined by Gustav Rose (1852). Other synonyms occasionally encountered in the literature are modumite, smaltite, kieftite (referring specifically to CoSb₃), and chloanthite (for Ni-rich forms of the structure).

As accessory minerals of hydrothermal origin, they are found in many regions of the world usually accompanied by other Ni-Co minerals. The current description given by the International Mineralogical Association distinguishes four main groups of the mineral: skutterudites (CoAs_3-x), nickelskutterudites (NiAs_3-x), ferroskutterudites ((Fe-Co)As₃), and kieftite (CoSb₃). As recently pointed out by Schumer et al. (2017), because neither the mineral nor synthetic form of NiAs₃ exists, the nickelskutterudite group should properly be classified as (Ni, Co, and Fe)As₃.

One of the first synthetic forms of skutterudites was prepared by Jolibois (1910), who made single crystals from Sn flux; variants of this technique (Sb instead of Sn) have been used even today. Considering that the crystal structure and the phase diagram were not known at that time, Jolibois had to be a quite skilled researcher.

Appropriate structural characterization of synthetic skutterudites was performed by Ivar Oftedal (1928), who identified skutterudites to possess a body-centered cubic lattice belonging to the space group $Im\overline{3}$.

The scientific interest in skutterudites has been driven primarily by two phenomena: the fascinating and exotic superconducting properties of filled skutterudites and the prospect that by filling the structural void of the skutterudite lattice, the thermal conductivity can be dramatically reduced, making such filled skutterudites outstanding thermoelectric materials for power generation. The latter aspect, the development and properties of skutterudites as viable thermoelectric materials, has been extensively described in my recent book titled Thermoelectric Skutterudites, published by Taylor & Francis, Uher (2021). In the present monograph, the focus is on a plethora of fascinating magnetic properties and on the truly exotic nature of superconductivity observed in several filled skutterudites. However, to aid in an understanding of magnetic and superconducting properties of skutterudites, it is necessary to consider structural aspects and electronic bands of skutterudites. Thus, here the essential features of the crystalline lattice of skutterudites and the structure of their electronic bands are included. A detailed account of the above two aspects in given in the aforementioned monograph Thermoelectric Skutterudites.

In general, skutterudites can be divided into three main categories: binary skutterudites, ternary skutterudites, and filled skutterudites, and they are briefly reviewed in turn.

1.2 Binary Skutterudites

As the term binary implies, the structure consists of two elements, and its general formula is MX₃, where M stands for an element of the column-9 transition metals Co, Rh, or Ir, and X represents a pnicogen (also pnictogen or pnictide) element P, As, or Sb. Thus, there are nine possible combinations constituting structurally stable bulk binary skutterudites. Occasionally, one finds reports of a stable form of NiP₃, Jolibois (1910), Biltz and Heimbrecht (1938), and Rundqvist and Larsson (1959). Because it is a thin film, it is possible to synthesize also metastable FeSb₃ (Daniel et al. 2015).

The arrangement of atoms in the crystalline lattice is depicted in Figure 1.1, where the left-hand side panel shows the unit cell with the transition metal atoms M occupying 8c (¼, ¼, ¼) sites (Wyckoff notation) that are octahedrally coordinated by pnicogen atoms at 24 g (0, y, z) sites, where y and z are the positional parameters that specify the exact location of a pnicogen atom. Typical of the skutterudite structure is the tilt of the MX₆ octahedrons, which gives rise to two large structural voids in the unit cell at position 2a (0, 0, 0) and leads to the formation of near-square pnicogen rings. The panel on the right-hand side of Figure 1.1 shows the unit cell shifted by one-quarter distance along the body diagonal. In this view, the transition metal atoms M form a simple cubic sublattice, and the near-square planar pnicogen rings are clearly depicted. There are six such pnicogen rings in the unit cell. Two of the eight small cubes do not contain the pnicogen rings and are locations of the two structural voids mentioned above. The chemical formula describing the unit cell of binary skutterudites is □₂M₈[X₄]₆ = 2(□M₄[X₄]₃) = 2(□M₄X₁₂), where the empty square □ stands for a structural void. It is customary to take just one-half of the unit cell, i.e., □M₄X₁₂, the structure that has a valence electron count (VEC) of 72. The complete specification of binary skutterudites is given by the lattice parameter a accompanied by the positional parameters y and z.

Figure 1.1 (a) Atomic arrangement in the unit cell of a binary skutterudite centered over a void at (0, 0, 0). The picture highlights the octahedral coordination of transition metals by pnicogen atoms and the tilt of the MX₆ octahedrons leading to the formation of two structural voids and four-membered pnicogen rings. (b) The unit cell of a binary skutterudite shifted by one-quarter distance along the body diagonal depicting the four-membered pnicogen rings that occupy six of the eight small cubes. The front upper left cube and the back bottom right cube do not contain pnicogen rings and are sites of two structural voids, often called cages.

The original structural analysis of skutterudites by Oftedal (1928) assumed a square planar configuration of the pnicogen rings, resulting in the so-called Oftedal relation

(1.1)$$2\left(y+z\right)=1\text{.}$$

More precise structural measurements performed by Kjekshus and Rakke (1974) revealed that the sides of the pnicogen rings are not equal, and all binary skutterudites possess rectangular rather than square planar ring configurations.

Binary skutterudites (except for NiP₃ and FeSb₃) are diamagnetic semiconductors. This follows from the mostly covalent nature of bonding and the fact that there are no unpaired electrons in the structure. Pnicogen atoms with their ns²np³ valence electron configuration use two electrons to form bonds with their two nearest neighbors on the pnicogen ring and three electrons to bond with two nearest transition metal atoms. Of the nine valence electrons of a transition metal atom of column 9, three electrons are used to establish bonds with the six neighboring pnicogen atoms, giving rise to octahedral d²sp³ hybrid orbitals, and the remaining six non-bonding electrons adopt a maximum spin-pairing d⁶ configuration with zero spin. Structural parameters of binary skutterudites, including the void radius, are given in the monograph by Uher (2021).

While no bulk binary skutterudites form with the transition elements of columns 8 and 10 of the periodic table, (except for the already noted NiP₃), it is possible to partly replace Co, Rh, and Ir with their immediate transition metal neighbors Ni and Fe, Ru and Pd, and Os and Pt, respectively. Of course, such partial replacements alter the VEC and lead to a metallic behavior and paramagnetism. The solid solution limits in antimony skutterudites were established...