Intermetallic Phases

Jürgen Nuss, Ulrich Wedig, Sabine Prill-Diemer, Claus Mühle, Fei Wang

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Among the classes of inorganic solids, the intermetallics are the ones that are least understood with respect to their bonding properties. Unlike ionically or covalently bonded compounds, it is virtually impossible to apply simple heuristic concepts to reliably deduce compositions or structural features from the elements involved and from the valence electron concentration per atom (VEC) provided. Also, the simple Fermi liquid picture usually fails to properly explain even basic physical properties such as electrical conductivity and magnetic susceptibility. This is mainly due to the fact that some of the features that primarily determine the chemical bond, such as the degrees of valence electron transfer and localization, vary strongly and almost continuously with composition and nature of the elements involved.

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Platinides

Introduction

The importance of relativistic effects grows with increasing atomic number and becomes rather prominent when reaching the 6^th period. The effects are particularly noticeable for gold, which exhibits the highest electron affinity among all metals (2.31 eV). The auride anion Au^– is stable in the gas phase, in solution and in the solid state. Platinum is the metal with the second highest electron affinity (2.13 eV) after gold, exceeding that of any chalcogen (highest value 2.08 eV; sulphur) which readily form, as typical non-metals, negatively charged dianions. Due to this fact it appeared promising to attempt the synthesis of platinides containing platinum with a twofold negative ionic charge realizing the closed-shell valence state.

Cs2Pt Cs2Pt is an ionic insulator crystallizing in the Ni2In structure type, which is currently regarded as the end member of the sequence of polymorph modifications adopted by alkali-metal monochalcogenides under increased pressure. In this sense Pt2– is considered as a higher homologue of the chalcogenides. In full accord with the red colour and the transparency of the crystals band structure calculations show the existence of a remarkable band gap, independently of the method of calculation used.

 

BaPt BaPt can be formulated, in terms of the Zintl-Klemm concept, as (Ba2+)(1∞[Pt]-)·e–. It crystallizes in the NiAs structure type with a c/a ratio = 1.07, the smallest value ever found for this structure type. Pt is coordinated linearly by two other Pt atoms at a distance as short as 2.71 Å. The band structure reveals 2D metallic conductivity parallel to the ab plane.

 

Ba2Pt Ba2Pt is the first intermetallic phase that adopts the CdCl2 structure, which is otherwise preferably found for ionic compounds with either a polarizable cation or anion. In agreement with the metallic type of conductivity, the compound can be formulated as (Ba2+)2(Pt2–)·2e–, and shows striking similarities to the alkaline earth subnitrides, in particular to Ba2N.

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Geometric variations and electron localizations in PbFCl type compounds

Introduction

The geometric structures of intermetallic phases are determined by a complex interplay of various bonding types. Ionic and covalent interactions caused by charge transfer among the atoms and by local electronic structures compete with the forces that itinerant electrons exert on the atomic cores. Compounds with the general formula A₂B, crystallizing in the anti-PbFCl type of structure, were investigated exemplarily by experimental and theoretical means to demonstrate the complexity of chemical bonding in intermetallic phases. The PbFCl structure is adopted by about 540 known compounds. All of them are isopointal, i.e. they crystallize in the same space group (P4/nmm) and the atoms occupy the same sites c2a (tP6): F in 2a, Cl in 2c and Pb in 2c. In the A₂B intermetallic compounds regarded here, a transition metal A takes up the positions of F (A1) and Cl (A2) and a pnictogen atom B (As or Sb) that of Pb.

Results and Discussion

The variations in the structural details are reflected by different c/a ratios and the spread of the two free atomic parameters z_A2 and z_B. The ratio of the latter ones q = z_A2/(1–z_B) is a measure of the unevenness of the (A2)B layer and may be plotted against p = c/a in order to generate a structure field diagram (fig.). Compounds with q larger than 1.5 are generally ionic or very polar. The low q value together with a rather low c/a ratio is characteristic for the intermetallic representatives and corresponds to a moderate or in the case of Cu₂Sb even reversed buckling of the (A2)B layers (fig.).

Because of these geometrical features, the description of the structures in terms of coordination polyhedra, which is suitable for the more ionic compounds, is less clear. The intermetallic compounds may be regarded more appropriately as arrangements of stacked layers or as variants of the NaCl structure, where half of the (A2)₄B₄ heterocubane units are filled by A1. Cu₂Sb is unique among the compounds investigated here, being the only representative with q < 1. The Cu1—Cu2 distance (260 pm) is much shorter than d(Cu1—Cu1) = 283 pm, suggesting at first sight Cu—Cu attractive interactions in the nonplanar 4⁴ nets of Cu1 and Cu2, as highlighted by red planes in the left part of the figure. However, d(Cu1—Cu2) is still larger than the atomic distance in elemental copper (256 pm). The relation of the geometrically different A—A distances is inverted when going from Cu₂Sb over Mn₂Sb to Fe₂Sb, and finally increases via Sc₂Sb continuously to ZrSiS. This trend is associated with an increase of p and q. The distance between two neighboring Sc1 atoms is 298 pm, 45 pm shorter than d(Sc1—Sc2) and even 24 pm shorter than the atomic distance in metallic scandium. This indicates metal-metal interactions in the planar 4⁴ nets formed by Sc1 only, which are highlighted by a red plane in the right part of the figure. Even stronger interactions between the A1 components (Si) were expected in ZrSiS, according to its position in the structure map.

To investigate how local bonding patterns influence the geometric structure, the corresponding scalar fields of the Electron Localization Function (ELF) were analyzed. Selected domains of the ELF are shown in the figure below. Two center Si—Si bonds with a bond order of ½ were deduced from the ELF of ZrSiS. In addition, the charge distribution as obtained from the topological analysis of the ELF and of the electron density justifies the formal description of ZrSiS within the Zintl-Klemm concept as [Zr⁴⁺S^2–]²⁺[Si]^2–. Due to the strong interaction in the Si (A1) planes, ZrSiS holds one of the extreme positions within the structure map. The other extreme position is occupied by Cu₂Sb, where no ELF attractors are found besides the domains that are attributed to the atoms. ELF domains in the valence region between the transition metal atoms appear in Mn₂Sb. The same ELF topology is observed in Cr₂As, Mn₂As and Fe₂As. Compounds like Sc₂Sb, containing early transition metal elements, exhibit large ELF basins in the pyramidal voids enclosed by the A1 and A2 atoms. The integrated valence electron density within this basin amounts to 1.3 electrons in the case of Sc₂Sb. This localization pattern, which also appears in YTiSi and YTiGe, is according to our detailed analysis an intrinsic property of the ELF topology of this family of compounds and is directly related to the bonding interaction within the planar 4⁴ nets, formed by the A1 atoms.

When going from left to right in the first row of transition metals, the d electrons become more localized. For the compounds A₂B under discussion, this leads, on one hand, to a continuous change of the charge transfer from the transition metal to the pnictogen atoms which in Cu₂Sb is even inverted. On the other hand, rather peculiar bonding conditions occur, which are reflected by the structural differences. We find multicenter bonding among the early transition metals and direct metal-metal interactions in the Cr, Mn and Fe representatives, whereas Cu₂Sb may be described as Cu atoms being embedded in metallic antimony.

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J. Nuss, U. Wedig, M. Jansen: Geometric Variations and Electron Localizations in Intermetallics: The Case of La₂Sb Type Compounds
Z. Anorg. Allg. Chem. 637 (2011) 1975-1981. doi:10.1002/zaac.201100331

V. Saltykov, J. Nuss, M. Konuma, M. Jansen: SrAu_0.5Pt_0.5 and CaAu_0.5Pt_0.5, analogues to the respective Ba compound, but featuring purely intermetallic behaviour
Solid State Sciences 12 (2010) 1615-1619. doi:10.1016/j.solidstatesciences.2010.07.011

J. Nuss, U. Wedig, M. Jansen: Synthesis, Structure and Properties of Scandium Dysprosium Antimonide, ScDySb
Z. Anorg. Allg. Chem. 636 (2010) 913-918. doi:10.1002/zaac.201000095

J. Nuss, U. Wedig, A. Kirfel, M. Jansen: The Structural Anomaly of Zinc: Evolution of Lattice Constants and Parameters of Thermal Motion in the Temperature Range of 40 to 500 K
Z. Anorg. Allg. Chem. 636 (2010) 309-313. doi:10.1002/zaac.200900460

V. Saltykov, J. Nuss, M. Konuma, M. Jansen: Investigation of the Quasi Binary System BaAu-BaPt
Z. Anorg. Allg. Chem. 635 (2009) 70-75. doi:10.1002/zaac.200800300

D. Errandonea, D. Santamaría-Perez, A. Vegas, J. Nuss, M. Jansen, P. Rodríguez-Hernandez, A. Muñoz: Structural stability of Fe₅Si₃ and Ni₂Si studied by high-pressure x-ray diffraction and ab initio total-energy calculations
Phys. Rev. B 77 (2008) 094113-1-12. doi:10.1103/PhysRevB.77.094113

J. Nuss, U. Wedig, M. Jansen: Geometric variations and electron localizations in intermetallics: PbFCl type compounds,
Z. Kristallogr. 221 (2006) 554-562. doi:10.1524/zkri.2006.221.5-7.554

A. S. Karpov, M. Konuma, M. Jansen: An experimental proof for negative oxidation states of platinum: ESCA-measurements on barium platinides,
Chem. Commun. 8 (2006)838-40. doi:10.1039/b514631c

A. S. Karpov, U. Wedig, R. E. Dinnebier, M. Jansen: Dibariumplatinide: (Ba²⁺)₂Pt^2-·2e-, and Its Relation to the Alkaline-Earth-Metal Subnitrides,
Angew. Chem. 117 (2005) 780-783. doi:10.1002/ange.200462055; Angew. Chem. Int. Ed. Engl. 44 (2005) 770-773. doi:10.1002/anie.200462055

A. Karpov, J. Nuss, U. Wedig, M. Jansen: Covalently Bonded 1∞[Pt]^–-Chains in BaPt — Extension of the Zintl-Klemm Concept to Anionic Transition Metals?
J. Am. Chem. Soc. 126 (2004) 14123-14128. doi:10.1021/ja0401186

A. Karpov, J. Nuss, U. Wedig, M. Jansen: Cs₂Pt, ein Platinid(-II) mit vollständiger Ladungstrennung, Angew. Chem. 115 (2003) 4966-4969 doi:10.1002/ange.200352314 ; Cs₂Pt a Platinide(-II) Exhibiting Complete Charge Separation, Angew. Chem. Int. Ed. Engl. 42 (2003) 4818-4821. doi:10.1002/anie.200352314

J. Nuss, M. Jansen: Demarcation of the PbFCl and Cu₂Sb Structure Families: Crystal Structure Re-Determinations and Refinements of CuMgSb, Cu₂Sb and CuMgAs,
Z. Anorg. Allg. Chem. 628 (2002) 1152-1157. doi:10.1002/1521-3749(200206)628:5<1152::AID-ZAAC1152>3.0.CO;2-1