The vast majority of materials have a positive coefficient of thermal expansion and their volume increases on heating. There has been considerable recent interest in materials which display the unusual property of contracting in volume on heating; i.e. those with a negative coefficient of thermal expansion. This Perspective gives an overview of some of the physical phenomena that can give rise to this unusual effect. Recent insights into negative thermal expansion materials and some of their unusual structural properties are discussed.

Negative thermal expansion materials

Magnetic properties of Y 2 Fe 14 B and Nd 2 Fe 14 B single crystals

In R-Fe compounds, the magnetic properties are mainly determined by the Fe-Fe interatomic distances and the number of Fe nearest neighbors. Below the ordering temperatures, the R 2 Fe 17 compounds are generally ferromagnetic or ferrimagnetic colinear. The compounds with rare earth ions of small atomic radii, Ce 2 Fe 17 , Tm 2 Fe 17 , and Lu 2 Fe 17 , are exceptions; they are helimagnetic. At low temperatures they exhibit a transition either to a fan spin or to a ferrimagnetic colinear configuration. For all compounds, a negative thermal expansion is observed below their ordering temperatures. Negative interactions occuring mainly in the substitution zones are thus deduced. Varying strongly with distances, they lead to the observed anomalous thermal expansion, and being largest in the compounds with the shortest Fe-Fe distances, their helimagnetic structure is explained.

Magnetic transition and anomalous thermal expansion in R 2 Fe 17 compounds

The authors review available experimental information on the existence, thermodynamic stability and physical properties of hydrides formed by the absorption of hydrogen gas in intermetallic compounds of two transition metals. The emphasis is on stability. It is shown that empirical models for the stability of ternary hydrides can be reconciled with ideas based on the results of band structure calculations of (binary) metallic hydrides. It is concluded that metallic hydrides can be looked upon as alloys of metallic hydrogen. In addition to the thermodynamic properties of ternary metallic hydrides the authors discuss experimental information on electronic properties (magnetic, super-conductivity, band structure features) and on crystallographic and metallurgical properties (neutron scattering, nuclear and electron spin resonance, Mossbauer spectroscopy, diffusion). Applications are briefly reviewed.

https://iopscience.iop.org/article/10.1088/0034-4885/45/9/001/pdf

Hydrides formed from intermetallic compounds of two transition metals: a special class of ternary alloys

Magnetic properties of ternary rare-earth compounds of the type R2Fe14B

The magnetocrystalline anisotropy has been known for a long time to be very large in the RCo5 intermetallics. The recent availability of high D.C. magnetic fields up to 200 kOe has enabled us to carry out an extensive experimental study of the energy and magnetization anisotropies in YCo5 from 4.2 K to 300 K. The magnetization anisotropy reaches 4% at 4.2 K. From a polarized neutron study on NdCo5 we find that this anisotropy of magnetization originates mainly from the 2c site. Previous measurements on YCo5 had shown that the orbital contribution to the 2c moment is large. Hence, we conclude that the Co magnetization anisotropy in RCo5 comes from a large orbital contribution to the magnetic moment. Finally, we account for the observed discontinuity in the magnetization at its reorientation in RCo5 ferrimagnets, and for the absence of a similar discontinuity in NdCo5 which is a ferromagnet.

Co energy and magnetization anisotropies in RCo5 intermetallics between 4.2 K and 300 K

Lattice parameter analysis and studies of thermal expansion show that the orthorhombic CeNi compound is, like CeSn 3 , an intermediate valence compound in which the cerium valence state varies from 3.5 to 3.3 between 4 and 300 K. This compound behaves as an enhanced Pauli paramagnet in which the magnetic susceptibility along the c axis passes through a maximum at around 140 K. Magnetic susceptibility, resistivity and heat capacity measurements are characteristic of an almost magnetic Fermi liquid in which spin fluctuations are present. Unlike the other cerium intermediate valence compounds which are generally cubic, large anisotropic effects due to the local surroundings are observed in CeNi because of its orthorhombic symmetry. All these properties can be understood in the light of the band structure of this type of alloy in which a large hybridization occurs between the Ni 3d electrons and the Ce 5d electrons.

R. Lemaire

Papers

Co energy and magnetization anisotropies in RCo5 intermetallics between 4.2 K and 300 K

Intermediate valence state of cerium in CeNi

4f magnetism in CeNi, PrNi and NdNi single crystals

Onset of magnetism in the yttrium-nickel compounds: II. Very weak itinerant ferromagnetism in YNi3

Magnetic properties of iron compounds with yttrium, lutetium and gadolinium