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

Abstract

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, Pt₄Sn₆Pr_3–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 Pt₃Sn₅R_2–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 {PtSn_x} 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.

Short abstract

A family of ternary intermetallic compounds has been observed for the light lanthanides from Ce to Eu, with Nd being a transition point. All compounds contain ∼30 at. % Pt but show high variability of the Pt@Sn_x bonding motifs with the lanthanide size, causing minor compositional and soft structural changes along the series.

Introduction

Cluster complex halides such as {PtPr₆}I₁₀ with isolated {PtPr₆} clusters or {PtPr₃}Br₃ 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., Pt₃Pr₄ 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, PbCl₂, 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 Pt₄Sn₆Pr₃ and Pt₄Sn₆Pr_2.91, as well as Pt₁₂Sn₂₅Pr₄,² prompting deeper investigation of the neighboring Pt/Sn/R systems. The first two are members of a prolific family of intermetallics, T₄E₆R₃ (where T is a transition metal; E is a p-block main group metal or metalloid, and R is a rare-earth metal). There is a growing number of structure types known essentially with this general formula, sometimes with under-occupation and/or disorder: monoclinic Pt₄Ge₆Y₃ (P2₁/m)¹⁴ and the disordered variant Pt₄Yb₃Si_5.7 (P2₁/m),¹⁵ as well as five orthorhombic structures, slightly disordered Pt₄Ge₆Ce₃ (Cmcm),¹⁶ Pt₄Ge₆Pr₃ (Pnma, R = Pr–Dy),¹⁷ Pd₄Sn₆Ce₃ (Pnma, R = La–Pr),¹⁸ Pt₄Al₆Ce₃ (Pnma),¹⁹ and Pt₄Sn₆Pr_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 Pt₄Sn₆Pr₃ and Pt₄Sn₆Pr_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.

Experimental Section

Synthesis

Starting materials were Pt beads (99.9%, AlfaAesar); La, Ce, Pr, Nd, Sm, and Sn pieces (99.9%, Ames Laboratory); and NaCl (99.9% purity, AlfaAesar). NaCl was dried in an oven at 80 °C 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.

Pt₄Sn₆R₃, Pt₄Sn₆Pr_3–x, and Pt₃Sn₅R_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 Pt₄Sn₆R_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 Pt₄Sn₆Nd₃ has been observed to form directly after arc melting but transforms eutectoidally after 5 days of annealing at temperatures of <900 °C.

Pt₇Sn₉Sm₅

The starting composition for Pt₄Sn₆Sm₃ 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 Pt₇Sn₉Sm₅ as the main product with further unknown phases. Small crystals of Pt₇Sn₉Sm₅ were selected and characterized by single-crystal XRD.

Pt₃Sn₅Sm_1.9 and Pt₃Sn₅Nd_1.84

The stoichiometric composition (Pt₃Sn₅Ln₂) 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 Pt₃Sn₅Sm_1.9, together with the pseudobinary solid solution Pt_xSn_3–xSm and Pt₃Sn₅Nd_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 SAINT²¹ 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 program²³ and refined using the SHELXL-2014 program²⁴ within the APEX3 software package. All Pt₄Sn₆R₃ show signs of twinning or potential incommensurate modulation (see Figure S2 in the Supporting Information). Pt₃Sn₅R_2–x and Pt₇Sn₉Sm₅ have been refined as inversion twins of which Pt₇Sn₉Sm₅ is enantiomorphically pure while Pt₃Sn₅Nd_2–x was found to be a racemate. This fact correlates well with somewhat higher residual electron density peaks, compared to the isostructural Pt₃Sn₅Sm_2–x. Crystallographic details and refinement parameters for Pt₄Sn₆R₃ (R = La, Ce, Pr), Pt₃Sn₅R_2–x (R = Nd, Sm, Eu), and Pt₇Sn₉Sm₅ are summarized in Table 1; Table 2 contains atomic positions and equivalent thermal parameters of Pt₄Sn₆La₃, Pt₃Sn₅Nd_1.84 and of Pt₇Sn₉Sm₅. Further data have been deposited (see the Supporting Information).

Table 1. Crystallographic Details and Refinement Parameters for Pt_xSn_yR_z (R = La, Ce, Nd, Sm).

parameter	Pt4Sn6La3	Pt4Sn6Ce3	Pt4Sn6Pr3	Pt4Sn6Nd3	Pt3Sn5Nd1.84	Pt3Sn5Sm1.89	Pt3Sn5Eu2a	Pt7Sn9Sm5
CCDC No.	1833492	1833495	1833516		1833494	1833493	1732670	1833491
structure type	Pt4Ge6Pr3	Pt4Ge6Pr3	Pt4Ge6Pr3	Pt4Ge6Pr3	Rh3Sn5Y2	Rh3Sn5Y2	Rh3Sn5Y2	Zr5Pd9P7
formula weight, fw [g/mol]	1909.23	1912.86	1915.23	1925.22	1443.95	1462.51	1482.64	3185.59
space group	Pnma (No. 62)	Pnma (No. 62)	Pnma (No. 62)	Pnma (No. 62)	Cmc21 (No. 36)	Cmc21 (No. 36)	Cmc21 (No. 36)	Amm2 (No. 38)
Z	4	4	4	4	4	4	4	Z
a [Å]	27.787(5)	27.7018(7)	27.623(1)	27.647(3)	4.515(3)	4.4978(5)	4.5330	4.3289(5)
b [Å]	4.6380(9)	4.6149(1)	4.5958(2)	4.5858(9)	26.14(2)	26.298(4)	26.629	28.798(4)
c [Å]	9.399(2)	9.3712(2)	9.3499(5)	9.326(1)	7.291(5)	7.2925(8)	7.318	7.2534(9)
V [Å3]	1211.3(4)	1198.02(5)	1187.0(1)	1182.4(8)	860.4(9)	862.6(2)	883.35	904.2(2)
density (calculated) [g/cm3]	10.47	10.61	10.72	10.80	11.15	11.26	11.15	11.70
μ [mm–1]	68.4	69.8	71.3	72.2	73.6	75.2	75.1	82.0
F (000)	3132	3144	3156	3168	2377	2404	2440	2612
θ range [°]	1.5 to 35.0	2.0 to 29.1	2.3 to 33.2		3.1 to 30.6	3.1 to 30.6		2.8 to 30.0
index ranges	–44 ≤ h ≤ 43	–38 ≤ h ≤ 38	–40 ≤ h ≤ 40		–6 ≤ h ≤ 6	–4 ≤ h ≤ 6		–6 ≤ h ≤ 6
	–7 ≤ k ≤ 7	–6 ≤ k ≤ 6	–7 ≤ k ≤ 7		–37 ≤ k ≤ 32	–36 ≤ k ≤ 37		–40 ≤ k ≤ 35
	–15 ≤ l ≤ 15	–13 ≤ l ≤ 13	–13 ≤ l ≤ 14		–10 ≤ l ≤ 9	–10 ≤ l ≤ 8		–9 ≤ l ≤ 10
intensity data/independent	22068/2942	13338/1956	22092/2380		2952/1418	3221/1364		4663/1956
Rint/Rσ	0.0649/0.0527	0.0714/0.0660	0.0757/0.0492		0.0912/0.0912	0.0319/0.0493		0.0388/0.0454
refinement method	Full-matrix least-squares on F2
data/parameters	2942/80	1956/80	2380/80		1418/60	1364/64		1394/68
goodness of fit, GOF (F2)	1.032	1.025	1.050		1.09	1.02		1.043
Flack parameter	–	–	–		0.48(2)	0.29(1)		0.036(13)
R1; ωR2 [I0 > 2σ(I)]	0.0389; 0.0812	0.0405; 0.0984	0.0373; 0.0545		0.0484; 0.1053	0.0246; 0.0458		0.0224; 0.0438
R1; ωR2 (all data)	0.0610; 0.0875	0.0593; 0.1057	0.0843; 0.0634		0.0615; 0.1098	0.0269; 0.0465		0.0243; 0.0441
largest diff. peak and hole [e Å–3]	4.31 and −4.34	4.38 and −7.15	3.58 and −4.41		5.31 and −4.42	3.06 and −2.49		2.20 and −2.58

^aData taken from ref (27).

Table 2. Atomic Positions and Equivalent Thermal Parameters of Pt₄Sn₆La₃, Pt₃Sn₅Nd_1.84, and Pt₇Sn₉Sm₅.

atom	Wyckoff	x	y	z	Ueq
Pt4Sn6La3
Pt1	4c	0.54367(2)	1/4	0.62889(8)	0.00461(9)
Pt2	4c	0.68685(2)	1/4	0.37954(8)	0.00369(9)
Pt3	4c	0.45061(2)	3/4	0.87904(9)	0.0056(1)
Pt4	4c	0.68668(2)	1/4	0.86909(8)	0.00362(9)
Sn1	4c	0.49845(3)	1/4	0.8777(1)	0.0044(1)
Sn2	4c	0.49713(3)	3/4	0.6281(1)	0.0043(1)
Sn3	4c	0.35691(3)	3/4	0.8749(1)	0.0083(1)
Sn4	4c	0.63739(3)	1/4	0.6244(1)	0.0050(1)
Sn5	4c	0.71562(3)	3/4	0.27941(9)	0.0045(2)
Sn6	4c	0.71571(3)	3/4	0.96892(9)	0.0045(2)
Nd1	4c	0.59312(2)	3/4	0.8666(1)	0.0060(1)
Nd2	4c	0.40707(2)	1/4	0.6171(1)	0.0060(1)
Nd3	4c	0.72051(2)	3/4	0.62433(8)	0.0042(1)
Pt3Sn5Nd1.84
Nd1	4a	1/2	0.47763(8)	–0.5012(4)	0.0028(4)
Nd2a	4a	1	0.32537(10)	–0.0184(5)	0.0028(4)
Pt1	4a	0	0.44899(7)	0.2158(2)	0.0016(4)
Pt2	4a	1/2	0.39387(7)	–0.2428(2)	0.0031(4)
Pt3	4a	0	0.27753(6)	0.4671(3)	0.0100(4)
Sn1	4a	0	0.54906(12)	0.3353(5)	0.0017(6)
Sn2	4a	0	0.37976(11)	0.5024(4)	0.0036(6)
Sn3	4a	1/2	0.40123(12)	0.1306(5)	0.0035(6)
Sn4	4a	1/2	0.28934(13)	0.2586(4)	0.0044(6)
Sn5	4a	1/2	0.29479(15)	–0.3157(5)	0.0121(8)
Pt7Sn9Sm5
Pt1	2a	0	1/2	0.3897(2)	0.0048(2)
Pt2	4e	1/2	0.92479(3)	0.3938(1)	0.0081(2)
Pt3	4e	1/2	0.64679(3)	0.4415(1)	0.0063(2)
Pt4	4d	0	0.79125(3)	0.4858(1)	0.0051(2)
Sn1	4e	1/2	0.55556(6)	0.5255(2)	0.0049(3)
Sn2	2b	1/2	0	0.6327(3)	0.0052(4)
Sn3	4e	1/2	0.83999(6)	0.5624(2)	0.0046(3)
Sn4	4d	0	0.70123(6)	0.3701(2)	0.0049(3)
Sn5	4d	0	0.86855(6)	0.2296(2)	0.0076(3)
Sm1	2a	0	1/2	0.7993(2)	0.0054(3)
Sm2	4d	0	0.41027(4)	0.2013(2)	0.0059(2)
Sm3	4e	1/2	0.72874(4)	0.6957(1)	0.0051(2)

^aSOF = 0.839(8).

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 anti-derivative of Fe₂P, or with the MgSrSi type, which is an anti-derivative of cotunnite (PbCl₂), R = La–Eu.^5,25 In PtSnPr, Pr has 12 nearest neighbors from 3.130 Å to 3.596 Å, as heterometallic {PrPt₆Sn₆} clusters, and Pt, the most electronegative of the three atom types, has four Sn atoms (2.729–2.897 Å) and six Pr atoms (3.130–3.596 Å) as nearest and second-nearest neighbors.

The reaction of binary Pt/Pr alloys, such as Pt₂Pr₃, with an obviously reactive tin flux yielded two new ternary intermetallics in the Pt/Sn/Pr system: Pt₁₂Sn₂₅Pr₄, with 29.2 mol % Pt, and Pt₄Sn₆Pr₃, with 30.8 mol % Pt.² Subsequent stoichiometric loadings of Pt₄Sn₆Pr₃ with and without an unreactive NaCl flux resulted in two different products: stoichiometric Pt₄Sn₆Pr₃ isostructural with the Pt₄Ge₆Pr₃ type of structure,¹⁷ as well as slightly substoichiometric Pt₄Sn₆Pr_2.91, with a new structure type. Utilizing the same flux method, single crystals of Pt₄Sn₆R₃ were grown for R = La, Ce, Pr, and Nd, all of which adopt the reported Pt₄Ge₆Pr₃ type. However, note that an alternative description model in a (3 + 1)D superspace group has been recently proposed for Pt₄Ge₆Ce₃²⁶ that is representative of the same Pt₄Ge₆Pr₃ 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 Pt₄Sn₆R_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 Pt₄Ge₆R₃ type. All attempts to produce the isocompositional Pt₄Sn₆Sm₃ or Pt₄Sn₆Sm_3–x have failed, resulting in a compositional split into Pt₃Sn₅Sm_1.9 and Pt₇Sn₉Sm₅ (51 mol % Sn and 43 mol % Sn, respectively, vs 47 mol % in the hypothetical Pt₄Sn₆Sm₃).

Crystal Structures

Polar intermetallics of the composition Pt₄Sn₆R₃ (1, 23.1 mol % R) exist with R = La–Nd (oP52, Pnma, a = 27.6–27.8 Å, b = 4.59–4.64 Å, c = 9.33–9.40 Å). With R = Pr, a closely related composition, Pt₄Sn₆Pr_3–x, has been uncovered with a previously unreported crystal structure (2, oP52, Pnma, a = 7.2863(3) Å, b = 4.4909(2) Å, c = 35.114(1) Å). Unexpectedly, Nd and Sm, under the same reaction conditions, formed compositionally related Pt₃Sn₅Nd_1.8 (3a, oP52, Cmc2₁, a = 4,515(3) Å, b = 26.14(2) Å, c = 7.291(5) Å), Pt₃Sn₅Sm_1.9 (3b, oS40, Cmc2₁, a = 4.533 Å, b = 26.629 Å, c = 7.318 Å), and Pt₇Sn₉Sm₅ (4, 23.8 mol % Sm, oS42, Amm2, a = 4.3289(5) Å, b = 28.798(4) Å, c = 7.2534(9) Å). The closely related Pt₃Sn₅Eu₂ (20 mol % Eu) had already been reported and is isostructural with Pt₃Sn₅R₂ (R = Nd and Sm; 3c, oS40, Cmc2₁, a = 4.533 Å, b = 26.629 Å, c = 7.318 Å).²⁷Table 1 summarizes crystallographic details for all structures of the compounds just mentioned, except for Pt₄Sn₆Pr_2.91 which has been reported in a preceding article.²Table 2 gives atomic parameters for Pt₄Sn₆La₃, Pt₃Sn₅Nd_1.84, and Pt₇Sn₉Sm₅.

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_1.50R_0.75, 2 = PtSn_1.50Pr_0.73, 3 = PtSn_1.67R_0.61–0.67 (R = Nd, Sm, and Eu²⁷), and 4 = PtSn_1.29Sm_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, Pt₄Sn₆La₃ and Pt₇Sn₉Sm₅ both show some correlation of unit-cell parameters to Pt₃Sn₅Nd_1.84 (Pt₃Sn₅Eu₂). The new ternary intermetallics Pt₄Sn₆R₃ (R = La–Pr, 1) are isostructural with the analogous “germanides”, Pt₄Ge₆R₃, which include R = Pr, Nd, Sm, Gd, Tb, and Dy,¹⁷ while Pt₄Ge₆La₃ has not been reported and those with R = Ce¹⁶ and Y¹⁵ 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 atoms, or second, with Pt-centered Sn clusters that are surrounded, in the second coordination sphere, by R atoms. Either way, these building units, {RPt_xSn_y} or {PtSn_y}R_z (Figures 1 and 2), are connected to 3D structures. In the first description, a 3D heteroatomic network of atoms with high electronegativities, 2.28 (Pt) and 1.96 (Sn) in the Pauling scale,²⁸ encapsulates electropositive lanthanide atoms R (EN = 1.13 for Pr). In the second description, the atom with the highest electron affinity, EA(Pt) = 205.3 kJ/mol, is surrounded by five to seven Sn atoms, EA(Sn) = 116 kJ/mol, and additional R atoms at larger distances. Philosophically, we treat the structures of these polar ternary intermetallics as either Werner-type coordination complexes, with the positive central atom (R) surrounded by negative ligands (Pt, Sn), or we consider them as anti-Werner-type cluster complexes²⁹ with less electron-affine Sn/R clusters with Pt as the central atom. The latter description paves a way to a better understanding of the condensation of clusters from cluster complexes such as {PtPr₆}I₁₂Pr via binary Pt₃Pr₄ (with {PrPr_x} clusters) to ternaries such as Pt₄Sn₆Pr₃.

Figure 1.

Projections of the crystal structures of (a) Pt₄Sn₆R₃ (1), (b) Pt₄Sn₆Pr_3–x (2), (c) Pt₃Sn₅Nd_2–x (3), and (d) Pt₇Sn₉Sm₅ (4) onto equivalent planes. [Color legend for atoms: green, R; orange, Pt; and blue, Sn.]

Figure 2.

R- and Pt-centered clusters in the crystal structures of Pt₄Sn₆R₃ (1), Pt₄Sn₆Pr_3–x (2), Pt₃Sn₅Nd_2–x (3), and Pt₇Sn₉Sm₅ (4).

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 Pt₄Sn₆R₃ (1) with the Pt₄Ge₆R₃-type structure exhibits linear one-side branched channels along the b- and c-axes. The structure is then formed of R-centered heteroatomic clusters {RPt₇Sn₉} forming the stem and {RPt₇Sn₉}, 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 Pt₄Sn₆Pr_3–x (2) has the same space group symmetry, as well as almost identical compositions and unit-cell volumes, the compound shows distinct differences in atomic packing (Figure 1b) and exhibits disorder of both cationic (R) and anionic (Sn) sites. From the cationic point of view, the structure contains three building units: 4–7–4 {RPt₇Sn₈}, 5–7–5 {RPt₇Sn₁₀}, and 5–4–5 {RPt₆Sn₈}. The 4–7–4 units form two-sided branched octagonal tunnels along the b-axis (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_0.55Au₂Ga₂).^31,32

Pt₃Sn₅R_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@Pt₆Sn₈} (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@Pt₆Sn₁₀} is an average version for the two remaining polyhedra in 2. The latter form two sets of tunnels along the a-axis, sharing pentagonal Pt₂Sn₃ faces and smaller trigonal PtSn₂ 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

Pt₇Sn₉Sm₅ (4) crystallizes with a slightly lower symmetric, well-ordered representative of the series, although with slightly different atomic ratios (i.e., PtSn_1.29Sm_0.71 instead of PtSn_1.5R_0.75). The new Pt₇Sn₉Sm₅ belongs to the very rare Zr₅Pd₉P₇ structure type linking polar intermetallics to metal phosphides, in accord with the formulation P₇Pd₉Zr₅ = Pt₇Sn₉Sm₅. Similar to 2, three types of {SmPt_xPr_y} polyhedra of similar architectures (Figure 2g) build the entire structure. They are connected to form pentagonal channels along the a-axis with a different degree of fusion with the neighboring units. 5–3–5 {SmPt₅Sn₈} polyhedra are responsible for the zigzag chains along the b-axis sharing larger pentagonal faces with two identical polyhedra (see Figures 1d and 2g, shown in violet). 5–4–5 {SmPt₆Sn₈} (red) together with 5–3–5 {SmPt₅Sn₈} (green), form fly-shaped trimers formally separated in the bc-plane. Green polyhedra share pentagonal faces with two red ones, while the latter have only a small trigonal face in common. The selected packing results in a limited number of small empty trigonal channels along the a-axis.

In summary, the electropositive rare-earth element atoms R in Pt₄Sn₆R₃ (1, R = La–Nd), Pt₄Sn₆Pr_3–x (2), Pt₃Sn₅R_2–x (3, R = Nd–Eu), and Pt₇Sn₉Sm₅ (4) have high coordination numbers of 15 and 16 (for 1), 14, 15, and 17 (for 2), 14 and 16 (for 3), and 13 (for 4,) with heteroatomic “ligand” spheres of Pt and Sn atoms. Since the atomic radii^36,37 of Sn (1.45 Å) and Pt (1.35 Å) are very similar, a mixed-ligand surroundings of the R atoms seems reasonable, which is actually obvious from the adoption of the anti-types of binary Fe₂P and PbCl₂, respectively, for the equiatomic PtSnR phases. For Pt₄Sn₆Pr₃, 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 {RSn₉Pt_z} clusters, they vary only little with the size of the rare-earth atoms, but there is a small lanthanide-contraction effect through the Pt₄Sn₆R₃ (R = La–Pr) series: 3.489 (La) to 3.467 Å (Pr). A practicallly opposite trend is observed for the Pt₃Sn₅R_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 Pt₇Sn₉Sm₅ 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.

Table 3. Average Distances and Molar Volumes for Ternary Pt/Sn/R Intermetallics.

	Average Distances (Å)
	R@PtxSny	Pt@SnxRy		Molar Volumes, Vma (cm3/mol)
	d(R–(Sn+Pt))	d(Pt–Sn)	d(Pt–R)	PtxSnzRy	PtxGezRy
Pt4T6La3	3.489	2.675	3.536	182.4	–
Pt4T6Ce3	3.477	2.666	3.523	180.4	153.5
Pt4T6Pr3	3.467	2.660	3.489	178.7	152.5
Pt4T6Nd3	3.463	2.658	3.482	178.3	–
Pt4T6Pr2.91	3.445	2.735	3.389	173.0	–
Pt3T5Nd1.84	3.397	2.755	3.383	129.5	–
Pt3T5Sm1.89	3.401	2.737	3.387	129.9	–
Pt7T9Sm5	3.290	2.816	3.122	272.3	–
Pt4T6Sm3	–	–	–	–	149.0
Pt3T5Eu2	3.454	2.781	3.476	133.0	–

^aThe molar volume, V_m, is calculated from the cell volume, V_E, via the expression V_m = (V_E × N_A)/Z or V_m = M/ρ (where N_A is Avogadro’s number, M is the molar weight, and ρ is the density).

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, Pt₄Sn₆R₃ (1, R = La–Nd), Pt₄Sn₆Pr_3–x (2), Pt₃Sn₅R_2–x (3, R = Nd–Eu), and Pt₇Sn₉Sm₅ (4) exhibit {PtSn₅} 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, {PtSn₆} 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 Pt₄Sn₆R₃ stoichiometry. However, all of the structures do exhibit identical building principles, forming chains through the edge and vertex sharing of the common {PtSn₅} pyramids and {PtSn₆} prisms.

The polyhedra are mostly square pyramids but there are also prisms, distorted octahedra, and others. Average Pt–Sn distances in Pt₄Sn₆R₃ 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_1.50R_0.75, 2 = PtSn_1.50R_0.73, 3 = PtSn_1.67R_0.61–0.67, and 4 = PtSn_1.29Sm_0.71, which happens in rather different ways (see Figure 3).

Figure 3.

Packing/connection of {PtSn_x} polyhedra in the crystal structures of Pt₄Sn₆R₃ (1), Pt₄Sn₆R_3–x (2), Pt₃Sn₅R_2–x (3), and Pt₇Sn₉Sm₅ (4). [Color legend: green, R; orange, Pt; and blue, Sn.]

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 Pt₄Sn₆Pr₃.²

Conclusions

The series Pt₄Sn₆R₃ has been observed for the light rare-earth elements (R = La–Nd). They are isostructural with Pt₄Ge₆R₃ (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 Pt₄Sn₆Pr_3–x because of the high-temperature modification with a slight under-occupation of the corresponding R sites (x = 0.09). Pt₄Sn₆Pr_3–x crystallizes in the same space group (Pnma) and has an almost identical unit-cell volume. However, the crystal structures of Pt₄Sn₆Pr₃ and Pt₄Sn₆Pr_3–x are distinctly different and show, besides under-occupation of Pr sites, disorder of both Sn and Pr sites. None of these phases has been observed for the heavier rare-earth elements (i.e., Sm or Eu). Instead, Pt₃Sn₅R_2–x, x = 0.16 (Nd) and 0.11 (Sm) surprisingly crystallize isostructurally with Pt₃Sn₅Eu₂. All attempts to synthesize Pt₄Sn₆R₃ and Pt₄Sn₆R_3–x with Nd or Sm have failed. Instead, compositionally related Pt₇Sn₉Sm₅ was obtained, a new example for the rare Zr₅Pd₉P₇-type structure linking polar intermetallics to metal phosphides, in accordance with the formulation P₇Pd₉Zr₅ = Pt₇Sn₉Sm₅. Bonding in all of these compounds is predominantly heterometallic with Pt–Sn, Sn–R, and Pt–R bonding contributions decreasing in this sequence.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c00884.

• Supplemental graphic information (Figure S1–S3); supplemental tabular information (Table S1) (PDF)

Accession Codes

CCDC 1833491–1833495, contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via ww.ccdc.cam.ac.uk/data_request/cif, or by emailing data_request@ccdc.cam.ac.uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: + 44 1223 336033.

The authors declare no competing financial interest.