The plumbides SrPdPb and SrPtPb

1 Introduction

The ternary systems RE-T-Pb (RE = rare earth element, T = electron-rich transition metal) have intensively been studied in the past 30 years with respect to phase formation and the rare earth magnetism. ^{1 , 2} So far, more than 280 RE _x T _y Pb _z plumbides ³ have been characterized. These plumbides have two pitfalls: (i) Due to the comparatively low boiling temperature of lead (2013 K), the samples need to be synthesized in sealed high-melting metal ampoules. Synthesis by arc-melting would result in too large lead evaporation. (ii) The high heavy metal toxicity of lead hampers any application. Nevertheless, these RE _x T _y Pb _z plumbides are an important family of intermetallic compounds for basic research in order to study structure property relationships. An advantage is the possibility to grow single crystals via a lead self-flux technique. Two striking examples are GdPtPb and Yb₂Pt₂Pb. GdPtPb orders antiferromagnetically at T_N = 15.5 K with a planar, collinear magnetic structure and a pronounced metamagnetic step at a critical field of 20 kOe. ⁴ Ytterbium is trivalent in Yb₂Pt₂Pb and orders antiferromagnetically below T_N = 2.07 K. ^{5 , 6} The Shastry-Sutherland lattice geometry leads to high magnetic anisotropy what is also evident from the multistep metamagnetic behavior in the low-temperature magnetization isotherms. ⁷

Since the europium-based plumbides EuPdPb, ⁸ EuRhPb₂ and EuPdPb₂ ⁹ all exhibit stable divalent ground states, at least related strontium-based alkaline earth (AE) plumbides should exist. A look at the Pearson database readily revealed that only few AE _x T _y Pb _z plumbides have been synthesized. Figure 1 gives a short overview on the ternary alkaline earth plumbides that have been structurally characterized. It is remarkable that no phases have yet been reported for the Mg-T-Pb systems. ³

Figure 1:

Structurally characterized alkaline earth plumbides.

The largest series of compounds comprises the Ca₅TPb₃ (T = V, Cr, Mn, Fe, Co. Ni, Cu, Zn, Ru, Ag, Cd) ¹⁰ phases which crystallize with a stuffed Mn₅Si₃ variant. Most of the equiatomic plumbides AETPb (AE = Ca, Sr, Ba, T = Zn, Pd, Ag, Cd, Pt, Hg) ^{11 , 12 , 13 , 14 , 15 , 16} crystallize with superstructure variants of the aristotype AlB₂. ¹⁷ CaCdPb ¹² adopts the hexagonal ZrNiAl type. Ca₂Au₂Pb ¹⁸ is a singular compound which crystallizes with the Mo₂B₂Fe-type structure. With higher lead content, the AETPb₂ (AE = Ca, Sr, Ba; T = Rh, Pd, Pt) ⁹ phases crystallize with the MgCuAl₂-type structure with a lonsdaleite-related lead substructure. Further lead-rich compounds are BaRh₃Pb₆ (own type) ¹⁹ and the Remeika phases Ca_∼4Rh₆Pb_∼19 and Sr₃Rh₄Pb_13. ^{20 , 21}

Further phase analytical studies in the AE-T-Pb systems now led to the synthesis and structural characterization of the plumbides SrPdPb and SrPtPb. So far, SrPdPb was structurally characterized only on the basis of a Rietveld refinement. ²² Herein we report on single crystal X-ray diffraction data on SrPdPb and SrPtPb with higher resolution.

2 Experimental

3 Structure refinements

Both data sets showed primitive orthorhombic lattices and the systematic extinctions were in agreement with space group Pnma. The starting atomic parameters were then deduced with the charge-flipping algorithm ²⁸ implemented in Superflip ²⁹ and the structures were refined on F ² with the Jana2020 software package, ^{30 , 31} with anisotropic displacement parameters for all sites. Separate refinements of the occupancy parameters gave no hints for deviations from the ideal compositions. The final difference Fourier analysis were contourless. Further details on the refinements, the atomic coordinates, the displacement parameters and the interatomic distances are listed in Tables 2–4.

CCDC–2478225 (SrPdPb) and CCDC–2478227 (SrPtPb) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

4 Crystal chemistry

The plumbides SrPdPb and SrPtPb crystallize with the orthorhombic TiNiSi-type ³² structure, space group Pnma. They are ternary ordered variants of the aristotype AlB₂. This crystal chemical relation is well documented via group-subgroup relations. ¹⁷ The near-neighbor coordination of the strontium atoms in SrPdPb and SrPtPb is presented in Figure 2 along with some closely related tetrelides. Each strontium atom is coordinated by two tilted and orthorhombically distorted Pd₃Pb₃ respectively Pt₃Pb₃ hexagons. The strong tilts lead to inter-layer bonding and this directly affects the strontium atoms. Instead of the regular 6 + 2 coordination in the aristotype AlB₂, the tilting pushes four of the six strontium neighbors out of the first coordination sphere and the strontium atoms get a 2 + 2 strontium coordination.

Figure 2:

Coordination of the calcium, strontium and europium atoms in the structures of SrPdSn, ²⁵ CaPdPb, ²² SrPdPb, EuPdPb ⁸ and SrPtSb. Calcium (strontium, europium), palladium (platinum) and tin (lead) atoms are drawn as medium grey, blue and magenta circles, respectively. Relevant interatomic distances (pm) are indicated.

Figure 2 shows the flexibility of the structure type. In the four near-neighbor coordinations drawn around SrPdPb, always one of the atoms is substituted, keeping the same valence electron count. The structure can adjust changes in the atom size and the electronegativity by small changes in the interatomic distances and by the puckering. The largest changes are observed for the substitutions Ca→Sr and Sn→Pb, accounting for the larger differences in size. In agreement with the course of the radii, the cell volumes increase in the sequence CaPdPb→EuPdPb→SrPdPb. This is similar to the AETPb₂ plumbides. ⁹ SrPtPb has a slightly smaller cell volume than SrPdPb, a consequence of the relativistic contraction of platinum. Again, this trend was also observed for the AEPdPb₂ and AEPtPb₂ plumbides. ⁹

Exemplarily we discuss the interatomic distances in SrPdPb. The palladium atoms have a strongly distorted tetrahedral lead coordination with Pd–Pb distances in the comparatively small range from 283–290 pm, comparable to the sum of the covalent radii ³³ of 282 pm for Pd + Pb. The palladium atoms in MgCuAl₂-type EuPdPb₂ have coordination number 6 with slightly longer Pd–Pb distances of 286–291 pm. ⁹ Within the [Pd₂Pb₂] rhombs that form between the tilted hexagons we observe weak Pb–Pb interactions. The Pb–Pb distances of 370 pm are only slightly longer than in fcc lead (12 × 350 pm). ³⁴

Within the Sr@Pd₆Pb₆Sr₄ coordination, the strontium atom has three shorter Sr–Pd (319–328 pm) and three Sr–Pb (339–342 pm) distances. Both are comparable to the sums of the covalent radii (320 pm for Sr + Pd and 346 pm for Sr + Pb), ³³ indicating substantial Sr–Pd and Sr–Pb bonding. This is in line with the Pauling electronegativities ³³ of 0.95 (Sr), 2.20 (Pd) and 2.33 (Pb).

The four strontium neighbors of each strontium atom have Sr–Sr distances 396 pm, somewhat smaller than the Sr–Sr distances in fcc strontium (12 × 430 pm), ³⁴ but comparable to twice the covalent radius of 384 pm. ³³ This expresses the partial ionization of strontium in SrPdPb, comparable to EuPdPb where ¹⁵¹Eu Mössbauer spectroscopic data revealed a stable divalent ground state. ⁸

For further general crystal chemical trends in the large family of TiNiSi related intermetallic phases we refer to relevant review articles. ^{17 , 35 , 36 , 37 , 38 , 39 , 40}