The platinum-rich scandium silicide Sc₂Pt₉Si₃

1 Introduction

Of the many rare earth-based intermetallic compounds those with scandium often show anomalies in their crystal chemical behavior. This is a consequence of the small size of the scandium atoms which are distinctly smaller than the lutetium ones [1]. That is why isotypic series of rare earth intermetallics often end with the lutetium member. In some cases the scandium compounds show small structural distortions (superstructure formation) to reach a crystal structure similar to the respective lutetium compound. Overviews on the crystal chemical behavior of scandium intermetallics have been published [2], [3], [4].

⁴⁵Sc solid state NMR spectroscopy is a complementary tool for structure determination [5], [6], [7], [8], [9], especially where X-ray diffraction attains its limits. Particularly in the case of superstructure formation, multiple scandium sites can be resolved in ⁴⁵Sc solid state NMR spectra. Striking examples are the stannide ScAgSn [10] and the complex carbides Sc₃TC₄ (T=Co, Ni, Ru, Rh, Os, Ir) [11] where two respectively three crystallographically independent scandium sites could be resolved.

Since Sc³⁺ is diamagnetic, most ternary scandium intermetallics simply show Pauli paramagnetism of the conduction electrons. Some of these compounds exhibit transitions to a superconducting state at very low temperature. Detailed studies have been performed, e.g., ScRu₄B₄ (T_C=6.28–7.70 K) [12], [13], Sc₂Fe₃Si₅ (T_C=4.25–4.52 K) [14], [15], Sc₅Co₄Si₁₀ (T_C=4.9 K), Sc₅Rh₄Si₁₀ (T_C=8.5 K), Sc₅Ir₄Si₁₀ (T_C=8.4 K) [16], or ScIrP (T_C=3.4 K) [17], [18].

Detailed phase analytical work on ternary scandium intermetallic compounds is scarce. For several phase diagrams just the equiatomic phases have been reported so far, e.g., ScPdSi in the Sc-Pd-Si system [4], [19], [20]. In contrast, a series of silicides is known for the corresponding system Sc-Pt-Si, i.e., ScPtSi [19], ScPt₂Si [21], Sc₄Pt₇Si₂ [22], Sc₅Pt₉Si₇ [21], Sc₃PtSi₃ [23] and Sc₂Pt₃Si₂ [24].

In a recent study on rhodium- and iridium-rich silicides we obtained ScRh₄Si₂ and ScIr₄Si₂ [25], crystallizing with the orthorhombic YRh₄Ge₂ type structure. When searching for an isotypic platinum compound we obtained the new platinum-rich silicide Sc₂Pt₉Si₃, the seventh silicide in this ternary system. The crystal chemical details of Sc₂Pt₉Si₃ are reported herein.

2 Experimental

3 Crystal chemistry

Sc₂Pt₉Si₃ crystallizes with the monoclinic Tb₂Pt₉Ge₃ type structure [33], space group C2/c. The series of germanides has been reported for the rare earth elements Y and Gd–Lu. The aluminum-rich phase Eu₂Pd₃Al₉ [34] is also isotypic with Tb₂Pt₉Ge₃, however, with site inversions Pt↔Al and Ge↔Pd (anti-type formation).

A projection of the Sc₂Pt₉Si₃ structure along the monoclinic axis is presented in Fig. 1. At first sight the structure seems rather complex, but it can geometrically be explained by a stacking of two different layers A and B approximately along the c axis. The stacking sequence of these layers is ABA′B′, where the layers A′ and B′ are inverted with respect to A and B. Separate drawings for the layers A and B are shown in Fig. 2. Layer A consists of slightly distorted scandium hexagons which are centered by Pt₃ triangles. The brighter shading in Fig. 2 (left) indicates the inverted layer. Layer B is built up exclusively from platinum and silicon atoms and can be considered as a condensed double layer of puckered hexagons Pt₃Si₃. Therefore, it is thicker than layer A.

Fig. 1:

Projection of the Sc₂Pt₉Si₃ structure onto the ac plane. Scandium, platinum and silicon atoms are shown as light gray, blue and red circles, respectively. The monoclinic unit cell is indicated by a dashed green line. The crystallographically independent platinum and silicon sites are emphasized and the stacking sequence of layers A, B, A′ and B′ is given at the right-hand part.

Fig. 2:

Slabs A and B of Sc₂Pt₉Si₃. Left: Projection of two layers A′ and A onto the ab plane with the viewing direction along the c axis. The A slab at about z=1/4 is shown in brighter shading and with dashed lines. Right: The B slab at z≈1/2. Platinum and silicon atoms are shown in blue and red color, respectively. The Pt atoms and bonds located below the paper plane are shown in brighter shading or as dashed lines, respectively. Parts of the monoclinic unit cell are indicated by a dashed green line as a guide to the eye.

The monoclinic Sc₂Pt₉Si₃ structure is closely related to the family of the orthorhombic Y₂Co₃Ga₉ type [35], space group Cmcm. This structure type is adopted by more than 90 aluminides and gallides RE₂T₃Al(Ga)₉ with T=Co, Ru, Rh, Ir [32], [35], [36], [37], [38], [39], [40], [41], [42]. The only inverted example is the palladium-rich antimonide Ce₂Pd₉Sb₃ [43]. The orthorhombic compounds contain similar layers A and B and can be considered as another stacking variant. The crystal chemical details are discussed in [33], [40], [41], [42]. Also, the complex structure of Yb₂Pd₃Ga₉ [44], [45] with space group symmetry P6₁22 belongs to this family of stacking variants.

The shortest interatomic distances in the Sc₂Pt₉Si₃ structure occur for Pt–Si, covering the broad range from 236 to 277 pm. In particular, the shorter ones agree well with the sum of the covalent radii [1] of 246 pm for Pt+Si and thus one can certainly assume substantial covalent Pt–Si bonding, similar to the other scandium platinum silicides. The platinum atoms as the majority component in Sc₂Pt₉Si₃ show a pronounced platinum substructure. Each platinum atom has between four and six platinum neighbors at Pt–Pt distances ranging from 270 to 294 pm. These Pt–Pt distances compare well with the Pt–Pt distance of 277 pm in fcc platinum [46]. The platinum-rich silicide Sc₄Pt₇Si₂ shows slightly longer Pt–Pt distances of 282–303 pm [22].

Since scandium and silicon are clearly the minority components in Sc₂Pt₉Si₃, one consequently observes no Sc–Si and Si–Si bonding. Thus, we can construct the Sc₂Pt₉Si₃ structure by three basic coordination polyhedra: Sc@Pt₁₁, Si1@Pt₈ and Si2@Pt₈ (Fig. 3). The layers of condensed Sc@Pt₁₁ polyhedra (Sc–Pt distances ranging from 284 to 301 pm) are at z≈1/4 and z≈3/4, whereby they are inverted with respect to each other. The Sc@Pt₁₁ polyhedra can be considered as trigonal prisms with five additional platinum atoms capping the rectangular faces. Both crystallographically independent silicon atoms have slightly distorted cubic platinum coordination. The Si1@Pt₈ (light gray) and Si2@Pt₈ (green) cubes are condensed via common edges. Again we observe two layers around z≈0 and z≈1/2 that are inverted. The kind of stacking sequence adopted by the 2-9-3 and 2-3-9 compounds might depend on radii criteria.

Fig. 3:

The basic layers of the Sc₂Pt₉Si₃ structure built up from Sc@Pt₁₁ (magenta), Si1@Pt₈ (light gray) and Si2@Pt₈ (green) polyhedra. The viewing direction for the distorted Si1@Pt₈ and Si2@Pt₈ cubes is approximately the space diagonal of the cubes.