First principle studies of electronic and magnetic properties of Lanthanide-Gold (RAu) binary intermetallics

Abstract

In this article we explore the electronic and magnetic properties of RAu intermetallics (R=Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu) for the first time. These properties are calculated by using GGA, GGA+U and hybrid density functional theory (HF) approaches. Our calculations show that HF provides superior results, consistent to the experimentally reported data. The chemical bonding between rare-earth and gold atoms within these compounds are explained on the basis of spin dependent electronic clouds in different planes, which shows predominantly ionic and metallic nature between Au and R atoms. The Cohesive energies of RAu compounds show direct relation with the melting points. Spin-dependent electronic band structure demonstrates that all these compounds are metallic in nature. The magnetic studies show that HoAu and LuAu are stable in non-magnetic structure, PrAu is stable in ferromagnetic phase and CeAu, NdAu, SmAu, GdAu, TbAu, DyAu, ErAu, TmAu, YbAu are anti-ferromagnetic materials.

Introduction

Noble metals (Cu, Ag, Au) based compounds are attractive for material scientists for their high oxidation and corrosion resistance, high stability, good strength, ductile and magnetic nature, very high melting points, good conductance and wide-range applications in high-temperature structural materials [1]. Generally the intermetallics with B2 or CsCI structure that contains one transition and a simple metal atom exhibit diverse physical phenomena. They are best for the systematic study of magnetic properties, electronic structure, cohesive properties, charge transfer and chemical bonding [2]. This class of compounds is antiferromagnetic [3] and metallic in nature with no band gap. The binary alloys of gold with the rare-earth elements are characterized by the cubic CsCl-type crystal structure B2 with space group Pm3m (No. 221), having the Wyckoff positions: R atom at (0,0,0) and Au atom at (0.5,0.5,0.5) [4], [5], [6] except CeAu [5] which has CrB type crystal structure [7], [8]. However experiments also show the cubic CsCl-type structure for CeAu compound [9]. Lanthanides in these compounds are trivalent [7], [10], [11] except Yb which is divalent [12]. Therefore, some of the physical properties of Yb such as the metallic radius, electronegativity etc are quite different than those of the normal trivalent rare-earth metals.

The compounds of lanthanides is the first group of atoms containing f-orbital [13]. One of the interesting subgroups of lanthanide compounds is the noble metals (Cu, Ag and Au) based stable lanthanides. Among this group the most stable compounds are those of Au with the rare-earth elements [14]. The bond stabilities are in the order of Rau≫RCu>RAg (R=Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu) [15].

Ionic as well as metallic bonds are present between gold and lanthanide atoms [7], [16], [17]. Electron transfer from the rare-earth metal to gold occurs as the electronegativity difference of these atoms are favorable for ionic bond formation [18]. The ionic and metallic bonding in these intermetallics can be confirmed from the fact that these compounds have high melting points [16], [19], [20], [21] high dissociation energies [7], [16], [22] and high bond energies [23]. The stable nature of these compounds is due to the large electronegativity difference (>1.13) and equiatomic stoichiometry of gold and lanthanides [10], [15], [19]. There exists a correlation between the stability trend and the relative melting behavior of these compounds [17]. Their melting points increase steadily as a function of atomic number from 1372 °C for CeAu to 1780 °C for LuAu respectively, with the exception of YbAu whose melting point is 1292 °C [6]. The low melting point of YbAu is attributed to the divalency of Yb in YbAu [24].

The RAu compounds were synthesized by reacting the reactants in a sealed boron nitride container at 1372K temperature [25]. The reactions were highly exothermic and the heat evolved was used to sustain the reaction [17], [23] and stable compounds were formed [26]. The CsCl structure for these compounds was confirmed by X-ray diffraction [26], [27].

Although these compounds are very important due to their interesting physical properties, but even then limited experimental as well as theoretical studies are reported on them, that make their applications limited. To the best of our knowledge no experimental or theoretical work has been reported on the electronic structure of these compounds, and similarly almost no theoretical work is available on the magnetic properties of these compounds. In this paper we explore the electronic structure and magnetic properties of these compounds, using the full potential linearized augmented plane waves (FP-LAPW) method within the framework of density functional theory.