Ternary Polar Intermetallics within the Pt/Sn/R Systems (R = La–Sm): Stannides or Platinides?

Starting generally with a 4:6:3 molar ratio of Pt, Sn, and R (where R = La–Sm), with or without the application of a NaCl flux, seven ternary compounds were obtained as single crystals. The platinides Pt4Sn6R3 (R = La–Nd) crystallize with the Pt4Ge6Pr3 type of structure (oP52, Pnma, a = 27.6–27.8 Å, b = 4.59–4.64 Å, c = 9.33–9.40 Å). With R = Pr, Pt4Sn6Pr3–x (oP52, Pnma, a = 7.2863(3) Å, b = 4.4909(2) Å, c = 35.114(1) Å) is also obtained, which might be considered a high-temperature polymorph with disorder on the Sn- and Pr-sites. For R = Nd and Sm, a structurally related isostructural series with a slightly different composition Pt3Sn5R2–x (oP52, Cmc21, a = 4.50–4.51 Å, b = 26.14–26.30 Å, c ≈ 7.29 Å) has been observed, together with Pt7Sn9Sm5 (oS42, Amm2, a = 4.3289(5) Å, b = 28.798(4) Å, c = 7.2534(9) Å) under the same conditions. The latter exhibits the rare Zr5Pd9P7-type structure, linking polar intermetallics to metal phosphides, in accord with P7Pd9Zr5≡Pt7Sn9Sm5. All structures may be described in terms of either negative Pt/Sn networks encapsulating positive R atoms, or {PtSnx} clusters (x = 5, 6, or rarely 7) sharing vertices and edges with R in the second coordination sphere and with considerable heterometallic Pt–R bonding contributions.


■ INTRODUCTION
Cluster complex halides such as {PtPr 6 }I 10 with isolated {PtPr 6 } clusters or {PtPr 3 }Br 3 with cluster chains constitute a symbiosis between intermetallic and salt. 1 Subsequent elimination of the halide ligands, i.e., successive cluster condensation, results in polar intermetallics, e.g., Pt 3 Pr 4 with predominant heterometallic bonding features. 2 The addition of a reactive metal, e.g., as a tin melt, introduces a competition between the more-electropositive metals Pr and Sn for the first coordination sphere of the most electron affine metal, Pt. Surprisingly, Sn wins, although it has a larger electron affinity than Pr.
There was only one ternary phase in the Pt/Sn/Pr system: the equiatomic PtSnPr with the MgSrSi type of structure, a derivative of cotunnite, PbCl 2 , first reported in 1973. 3 The stoichiometric PtSnR phases are still the most represented for all of the rare earths, 4−10 and thorough studies of magnetic and electronic properties have been performed. 10−13 We have recently added Pt 4 Sn 6 Pr 3 and Pt 4 Sn 6 Pr 2.91 , as well as Pt 12 Sn 25 Pr 4 , 2 prompting deeper investigation of the neighboring Pt/Sn/R systems. The first two are members of a prolific family of intermetallics, T 4 E 6 R 3 (where T is a transition metal; E is a p-block main group metal or metalloid, and R is a rareearth metal). There is a growing number of structure types known essentially with this general formula, sometimes with under-occupation and/or disorder: monoclinic Pt 4 Ge 6 Y 3 (P2 1 / m) 14 and the disordered variant Pt 4 Yb 3 Si 5.7 (P2 1 /m), 15 as well as five orthorhombic structures, slightly disordered Pt 4 Ge 6 Ce 3 (Cmcm), 16 Pt 4 Ge 6 Pr 3 (Pnma, R = Pr−Dy), 17 Pd 4 Sn 6 Ce 3 (Pnma, R = La−Pr), 18 Pt 4 Al 6 Ce 3 (Pnma), 19 and Pt 4 Sn 6 Pr 3−x (Pnma). 2 The structures of this family are usually described as stacked pentagonal and hexagonal nets of mixed Sn and Pt encapsulating the R atoms with high coordination numbers (14−16). Since Pt 4 Sn 6 Pr 3 and Pt 4 Sn 6 Pr 2.91 are closely related, the aim of this research was to determine what the substitution of Pr by other lanthanides (R = La−Sm) would do to the existence and the crystal chemistry of these ternary intermetallics.  Inorganic Chemistry pubs.acs.org/IC Article overnight before being placed inside an argon-filled glovebox. All samples, masses of which were between 250 mg and 500 mg, were weighed and loaded into tantalum ampules inside an argon-filled glovebox. Ampules were sealed under argon, followed by sealing in evacuated silica tubes. Samples were placed in a furnace at 1000°C for 24 h, followed by slow cooling (−20°C h −1 ) to 850°C or 700°C and annealed for 72 h. The NaCl flux was removed with water after the end of the reaction. Pt 4 Sn 6 R 3 , Pt 4 Sn 6 Pr 3−x , and Pt 3 Sn 5 R 2−x . Loadings of rare-earth metals (R = La−Sm) with Pt and Sn pieces in Pt:Sn:R molar ratios of 4:6:3 were weighed and placed inside tantalum tubes, along with ∼250 mg of NaCl. Samples were sealed under the same conditions and placed in a tube furnace, following the heating profile described above. The somewhat disordered Pt 4 Sn 6 R 3−x has been detected in the samples annealed at higher temperatures. No disordered variants have been detected in the samples with R = La and Ce. Isostructural Pt 4 Sn 6 Nd 3 has been observed to form directly after arc melting but transforms eutectoidally after 5 days of annealing at temperatures of <900°C.
Pt 7 Sn 9 Sm 5 . The starting composition for Pt 4 Sn 6 Sm 3 was weighed and loaded according to the above indicated method, with NaCl as a flux. The sample was sealed and heated according to the same scheme.
The resulting product was identified via powder X-ray diffraction (XRD) to be multiphase, containing Pt 7 Sn 9 Sm 5 as the main product with further unknown phases. Small crystals of Pt 7 Sn 9 Sm 5 were selected and characterized by single-crystal XRD.
Pt 3 Sn 5 Sm 1.9 and Pt 3 Sn 5 Nd 1.84 . The stoichiometric composition (Pt 3 Sn 5 Ln 2 ) has been loaded inside a tantalum tube and sealed under the same conditions. Following the same initial heating, the sample was slowly cooled and annealed for 3 days at 600°C. The resulting products contained Pt 3 Sn 5 Sm 1.9 , together with the pseudobinary solid solution Pt x Sn 3−x Sm and Pt 3 Sn 5 Nd 1.84 in multiphase samples with unidentified phases (most likely, ternaries).
Structure Analysis. Powder and single-crystal XRD were used to characterize products. Samples were crushed in air and a portion was ground to a fine powder for phase analysis. Powders were sandwiched between greased Mylar sheets housed by an aluminum holder. Data were gathered on a STOE STADI P image plate diffractometer (Cu Kα1 radiation, λ = 0.71073 Å; Si external standard, a = 5.4308(1) Å) and analyzed using WinXPow software. Single-crystal XRD was performed on a Bruker APEX CCD and Bruker VENTURE diffractometer (both Mo Kα radiation, λ = 0.71073 Å), respectively. The raw frame data were collected using the Bruker APEX3 program, 20 while the frames were integrated with the Bruker Inorganic Chemistry pubs.acs.org/IC Article SAINT 21 software package, using a narrow-frame algorithm integration of the data and were corrected for absorption effects using the multiscan method (SADABS). 22 All positions were refined anisotropically. Initial models of the crystal structures were obtained with the SHELXT-2014 program 23 and refined using the SHELXL-2014 program 24 within the APEX3 software package. All Pt 4 Sn 6 R 3 show signs of twinning or potential incommensurate modulation (see Figure S2 in the Supporting Information). Pt 3 Sn 5 R 2−x and Pt 7 Sn 9 Sm 5 have been refined as inversion twins of which Pt 7 Sn 9 Sm 5 is enantiomorphically pure while Pt 3 Sn 5 Nd 2−x was found to be a racemate. This fact correlates well with somewhat higher residual electron density peaks, compared to the isostructural Pt 3 Sn 5 Sm 2−x . Crystallographic details and refinement parameters for Pt 4 Sn 6 R 3 (R = La, Ce, Pr), Pt 3 Sn 5 R 2−x (R = Nd, Sm, Eu), and Pt 7 Sn 9 Sm 5 are summarized in Table 1; Table 2 contains atomic positions and equivalent thermal parameters of Pt 4 Sn 6 La 3 , Pt 3 Sn 5 Nd 1.84 and of Pt 7 Sn 9 Sm 5 . Further data have been deposited (see the Supporting Information).

■ RESULTS AND DISCUSSION
It is surprising that, until recently, the only ternary intermetallic compounds known in the Pt/Sn/R systems have been the isocompositional PtSnR with R throughout the entire lanthanide series, including yttrium. 3,5,25 The ambient pressure forms of PtSnR with R = Tb−Lu and Y, 3 crystallize with the HoPtSn/ZrNiAl type, which is an antiderivative of Fe 2 P, or with the MgSrSi type, which is an antiderivative of cotunnite (PbCl 2 ), R = La−Eu. 5 However, note that an alternative description model in a (3 + 1)D superspace group has been recently proposed for Pt 4 Ge 6 Ce 3 26 that is representative of the same Pt 4 Ge 6 Pr 3 structure type. Although the same model might be applied to other Ge and Sn representatives, the initial structural description in the Pnma space group can be considered as a commensurately modulated three-dimensional (3D) approximant. The new Pt 4 Sn 6 R 3−x type is obtained only with R = Pr (x = 0.09). These slightly substoichiometric ternary intermetallics appear to be a high-temperature "modification" of the Pt 4 Ge 6 R 3 type. All attempts to produce the isocompositional Pt 4 Table 1 summarizes crystallographic details for all structures of the compounds just mentioned, except for Pt 4 Sn 6 Pr 2.91 which has been reported in a preceding article. 2 Table 2 gives atomic parameters for Pt 4 Sn 6 La 3 , Pt 3 Sn 5 Nd 1.84 , and Pt 7 Sn 9 Sm 5 .
Although the full picture of all phases that might exist in the ternary systems Pt/Sn/R is certainly not known to date, the close compositions of 1 = PtSn 1.50 R 0.75 , 2 = PtSn 1.50 Pr 0.73 , 3 = PtSn 1.67 R 0.61−0.67 (R = Nd, Sm, and Eu 27 ), and 4 = PtSn 1.29 Sm 0.71 , and strong structural similarities may make a point for the strong influence of geometric factors in the variation within the greater structural family. Although being not directly related, Pt 4 Sn 6 La 3 and Pt 7 Sn 9 Sm 5 both show some correlation of unit-cell parameters to Pt 3 Sn 5 Nd 1.84 (Pt 3 Sn 5 Eu 2 ). The new ternary intermetallics Pt 4 Sn 6 R 3 (R = La−Pr, 1) are isostructural with the analogous "germanides", Pt 4 Ge 6 R 3 , which include R = Pr, Nd, Sm, Gd, Tb, and Dy, 17 while Pt 4 Ge 6 La 3 has not been reported and those with R = Ce 16 and Y 15 belong to closely related structural derivatives.
There are usually alternative ways to describe crystal structures. In the present case, one can either start with, first, heteroatomic Pt and Sn clusters encapsulating endohedral R  Let us start with the perhaps more classical description. The crystal structures of all compounds in this Article may be described in terms of network structures where the electronegative Pt and Sn atoms form tunnels along certain directions, including the large R atoms which, viewed alone, form straight or zigzag chains (Figure 1). Thus, the stoichiometric Pt 4 Sn 6 R 3 (1) with the Pt 4 Ge 6 R 3 -type structure exhibits linear one-side branched channels along the b-and c-axes. The structure is then formed of R-centered heteroatomic clusters {RPt 7 Sn 9 } forming the stem and {RPt 7 Sn 9 }, which is responsible for the branches (Figure 1). The latter consist of three parallel 6−4−6 and 5−5−5 membered rings, respectively (see Figure 2a). On the other hand, these clusters can be represented as randomly equatorially capped hexagonal and pentagonal prismatic polyhedra. Each branch polyhedron has common pentagonal faces with two stem polyhedra and shares pentagonal faces with identical units along the b-axis, forming a parallel tunnel.
Although Pt 4 Sn 6 Pr 3−x (2) has the same space group symmetry, as well as almost identical compositions and unitcell volumes, the compound shows distinct differences in atomic packing (Figure 1b) (Figure 1b), having large hexagonal faces shared with the 5−7−5 units forming the branches. Similar tunnels were frequently observed for the A/Au/Tr intermetallics (A = active metal, Tr = triel), 30 including cationic zigzag chains and large positional disorders. A separate set of pentagonal tunnels along the b-direction is observed in between, forming cationic zigzag chains along the a-axis through bigger shared hexagonal faces (see Figures 1b and 2c, shown in violet). The packing of green, yellow, and violet polyhedra (Figure 2c) results in smaller voids in the form of tetrahedral stars, which are, again, reminiscent of the active metal polar intermetallics (e.g., A 0.55 Au 2 Ga 2 ). 31,32 Pt 3 Sn 5 R 2−x (R = Nd, Sm, and Eu (3)) exhibits its own set of coordination polyhedra and their packing (Figure 1c) and, because of multiple structural aspects, can be considered as a more ordered replacement variant for 2 with Nd and also a transition structure from 2 to 4. The compound contains only two symmetrically unequivalent Nd sites with a minor occupational disorder in one of them. One coordination polyhedron, {Nd@Pt 6 Sn 8 } (Figure 2e), is common for 2, 3, and also 4 (see below) forming a set of pentagonal tunnels along the a-axis through the shared distorted hexagonal faces, while {Nd@Pt 6 Sn 10 } is an average version for the two remaining polyhedra in 2. The latter form two sets of tunnels along the a-axis, sharing pentagonal Pt 2 Sn 3 faces and smaller trigonal PtSn 2 faces, forming zigzags. The packing of these polyhedra is not dense and similar to 2, which leads to the formation of distorted cubic voids. Interestingly, unit-cell volumes in the row increase from Nd to Eu, pointing toward at least partial change of the oxidation state of those elements being consistent with the change of stoichiometry as a compensation mechanism. For the latter, this is not something extraordinary, since +2 is the most common oxidation number of Eu in intermetallics, particularly with group 11 metals, 33,34 while a mixed valent state is also not rare. 35 Pt 7 Sn 9 Sm 5 (4) crystallizes with a slightly lower symmetric, well-ordered representative of the series, although with slightly surroundings of the R atoms seems reasonable, which is actually obvious from the adoption of the anti-types of binary Fe 2 P and PbCl 2 , respectively, for the equiatomic PtSnR phases. For Pt 4 Sn 6 Pr 3 , for example, with three crystallographically independent Pr positions, the average Pr−Pt/Sn distance is 3.467 Å (see Table 3), with Pr−Sn distances ranging between 3.224 Å and 3.780 Å, as well as Pr−Pt distances ranging from 3.370 Å to 3.961 Å. Therefore, the shortest distances are close to the sum of the atomic radii of Pr (1.85 Å) and the average of Pt and Sn (1.40 Å) (1.85 Å + 1.40 Å = 3.25 Å). With the large coordination numbers of R (16 and 15), the average distances must be considerably longer. For the {RSn 9 Pt z } clusters, they vary only little with the size of the rare-earth atoms, but there is a small lanthanide-contraction effect through the Pt 4 Sn 6 R 3 (R = La−Pr) series: 3.489 (La) to 3.467 Å (Pr). A practicallly opposite trend is observed for the Pt 3 Sn 5 R 2−x (3, R = Nd−Eu) series, because of multiple factors: the lanthanide-contraction effect is neglected by the change of composition and perhaps oxidation state of Sm and Eu, leading to practically identical values for the Nd and Sm compounds and a more significant increase for the Eu one. The average Sm−Sn/Pt distance in Pt 7 Sn 9 Sm 5 is much smaller (3.290 Å), which might be attributed to a coordination number of only 13 for all three Sm positions in the structure.
In the second, the anti-Werner way to describe the crystal structures of these ternary phases, we take the atom with the highest electronegativity, or electron affinity (Pt) as the central atom. Then, in all of the structures discussed in this Article, Pt is the central atom of a Sn polyhedron/cluster, {PtSn x } with x = 5, 6, 7 (see Figures 2b, 2d, 2f, and 2h). All structures, Pt 4 Sn 6 R 3 (1, R = La−Nd), Pt 4 Sn 6 Pr 3−x (2), Pt 3 Sn 5 R 2−x (3, R = Nd−Eu), and Pt 7 Sn 9 Sm 5 (4) exhibit {PtSn 5 } square pyramids, whereas their proportion is changing from 100% in 1, to 50% in 2 and 4, to 33% in 3. 2, 3, and 4 exhibit polyhedra close to trigonal prisms but have slopes of up to 30°between the horizontal faces. Finally, each of the latter contains one {PtSn x } polyhedron atypical for any other structure, {PtSn 6 } octahedra in 2, regular trigonal prisms in 4, and monocapped trigonal prisms in 3. From this point of view, it becomes clear that the structure of 2 is at a transition point between those with R = La−Pr and R = Sm−Eu, the latter of which have not yet been obtained with Pt 4 Sn 6 R 3 stoichiometry. However, all of the structures do exhibit identical building principles, forming chains through the edge and vertex sharing of the common {PtSn 5 } pyramids and {PtSn 6 } prisms.
The polyhedra are mostly square pyramids but there are also prisms, distorted octahedra, and others. Average Pt−Sn distances in Pt 4 Sn 6 R 3 are close to 2.66 Å (Table 3) and reflect somewhat the lanthanide contraction, which seems surprising. Since Pt has a higher electronegativity/electron affinity than Sn, we are, strictly speaking, dealing with platinides, not stannides, and thus, may remove the question mark from the title! The {PtSn 5−7 } clusters must be connected via common Sn atoms, in accord with the compositions of 1 = PtSn 1.50 R 0.75 , 2 = PtSn 1.50 R 0.73 , 3 = PtSn 1.67 R 0.61−0.67 , and 4 = PtSn 1.29 Sm 0.71 , which happens in rather different ways (see Figure 3).
In the second coordination sphere, Pt is surrounded by R atoms with average distances of ∼3.5 Å with a stronger reflection of the lanthanide contraction. This is only surprising when one considers whether there is Pt−R bonding. The sum of the atomic radii of Pt (1.35 Å) and Pr (1.85 Å) (i.e., 3.20 Å) suggests that there are no significant bonding interactions. The obvious influence of the lanthanide contraction on the Pt−R distances then would simply be a packing effect. However, integrated crystal orbital Hamilton populations show a value of −0.80 eV/bond for Pt−Pr bonding, which is much less than the 2.28 eV/bond for Pt−Sn bonding, but it adds up to 18% of the overall bonding for Pt 4 Sn 6 Pr 3 . 2

■ CONCLUSIONS
The series Pt 4 Sn 6 R 3 has been observed for the light rare-earth elements (R = La−Nd). They are isostructural with Pt 4 Ge 6 R 3 (R = Pr−Dy), the so-called "germanides". These, which are, in fact, platinides, similar to the corresponding "stannides", are subject to the higher electronegativity/electron affinity of Pt than Sn. The Pr compound could be considered to be dimorphic with Pt 4 Sn 6 Pr 3−x because of the high-temperature modification with a slight under-occupation of the correspond-  Inorganic Chemistry pubs.acs.org/IC Article