Structure and magnetic properties of Sm₂Rh₃Sn₅ – an intergrowth of TiNiSi- and NdRh₂Sn₄-related slabs

1 Introduction

The rare earth-based intermetallic compounds RE₂T₃X₅ (RE=rare earth metal; T=electron-rich transition metal; X=Si, Ge, In, Sn) show a rich structural variety. Depending on the valence electron count and the radii, these compounds crystallize in six different structure types: U₂Mn₃Si₅ type (tP40, P4/mnc) [1], U₂Co₃Si₅ type (oI40, Ibam) [2], Lu₂Co₃Si₅ (mS40, C2/c) [3], Y₂Rh₃Sn₅ (oS40, Cmc2₁) [4], Y₂Pt₃Sn₅ (oP40, Pnma) [5] or Ce₂Au₃In₅ (oP80, Pmn2₁) [6]. So far more than 300 entries are listed in the Pearson data base [7]. The RE₂T₃X₅ phases have intensively been investigated with respect to their greatly varying magnetic and electrical properties. Striking examples are the 1.8 K superconductor La₂Rh₃Sn₅ [8], valence fluctuating Ce₂Rh₃Sn₅ [9], the 1.5 K antiferromagnet Ce₂Rh₃(Sn,Bi)₅ [10], mixed-valent Yb₂Au₃In₅ [11], the non-magnetic heavy-fermion system U₂Co₃Si₅ [12] or the 58 K ferromagnet Pu₂Pt₃Si₅ which shows remarkable square-loop hysteresis [13].

The series of RE₂Rh₃Sn₅ stannides crystallizes with the Y₂Rh₃Sn₅ type (oS40, Cmc2₁) and representatives are known with RE=Y, Ce–Nd and Gd–Tm [4], [8], [9]. The highest magnetic ordering temperature of T_N=10 K was observed for Gd₂Rh₃Sn₅ [4]. An exception is the superconducting stannide La₂Rh₃Sn₅ [4] which adopts the orthorhombic U₂Co₃Si₅ structure. So far, no representatives with samarium, ytterbium and lutetium have been reported. Herein we present the synthesis, crystal chemistry and magnetic properties of Sm₂Rh₃Sn₅.

2 Experimental

3 Crystal chemistry

Sm₂Rh₃Sn₅ fills the gap within the series of RE₂Rh₃Sn₅ stannides [4], [8]. The cell volume fits in between the values for Nd₂Rh₃Sn₅ and Gd₂Rh₃Sn₅ [8]. The Sm₂Rh₃Sn₅ structure contains two samarium, three rhodium, and five tin sites. In Fig. 1 we present a projection of the Sm₂Rh₃Sn₅ structure along the a axis. Sm₂Rh₃Sn₅ is a so-called two-layer structure. All atoms lie on mirror planes at x=0 and x=1/2. From a geometrical point of view, all rhodium atoms have slightly distorted trigonal prismatic coordination by samarium and tin atoms. All of these trigonal prisms are capped on the rectangular sites leading to coordination number nine (tricapped trigonal prism), often observed in metal-rich rare earth intermetallics [17], [18].

Fig. 1:

Projections of the Sm₂Rh₃Sn₅, NP-CePtSn and NdRh₂Sn₄ structures along the short unit cell axis. Rare earth, transition metal and tin atoms are shown as light gray, blue and red circles, respectively. The trigonal prismatic coordination of the transition metal atoms is emphasized. Crystal chemically related slabs are shaded.

Although the description of a structure by trigonal prismatic building units is a purely geometrical one, it facilitates comparison of different structure types. The prisms around the Rh1 atoms are built up from four samarium and two tin atoms. These prisms are condensed to chains running along the c axis via joint samarium edges. This structural motif (shaded in light red) is well known from the TiNiSi type structure [19]. Neighboring chains are shifted with respect to each other by half of the a axis. Unfortunately, the equiatomic RERhSn stannides [20], [21] do not crystallize with the TiNiSi type. They indeed show trigonal prismatic coordination of the rhodium atoms, but in a ZrNiAl type arrangement. In Fig. 1 we therefore present a projection of the normal-pressure phase of CePtSn [22].

The space between the chains is filled by double-prisms, coordinating the Rh2 and Rh3 atoms. Again, neighboring double prisms are shifted by half of the a axis. These double prisms are interlocked with the TiNiSi related chains, leading to a dense packing within the Sm₂Rh₃Sn₅ structure. A similar motif of double prisms (shaded in light greenish-blue) occurs in the NdRh₂Sn₄ type [23]. We can thus consider Sm₂Rh₃Sn₅ as a 1:1 intergrowth structure or chemical twinning [24], [25], [26], [27], [28] of TiNiSi and NdRh₂Sn₄ related slabs of compositions SmRhSn and SmRh₂Sn₄. Both ternaries are known as proper phases, however, SmRhSn adopts the ZrNiAl type. In Fig. 1 we have drawn the NdRh₂Sn₄ structure, as in the series RERh₂Sn₄ (RE=La–Nd, Sm) [23] only the positional parameters of the neodymium compound had originally been reported.

Condensed double prisms have also been observed in the structures of ZrFe₄Si₂ [29] and the recently reported germanide YRh₄Ge₂ [30]. As a consequence of the different atom sizes, the double prisms can either be linear or show different tilts (puckering effect). With the intergrowth concept presented for Sm₂Rh₃Sn₅ in Fig. 1, one can also describe the more complex structures of Yb₂Pt₃Sn₅ [5] and Ce₂Au₃In₅ [6], with double prism sequences deriving from both the YRh₄Ge₂ and NdRh₂Sn₄ type. Again, YbPtSn and CeAuIn [7] adopt the ZrNiAl type, while these compositions show a TiNiSi-like arrangement in the intergrowth structures.

We now turn to the polyanionic substructure of Sm₂Rh₃Sn₅ (Fig. 2). The shortest interatomic distances in the Sm₂Rh₃Sn₅ structure occur for Rh–Sn. Each rhodium atom has between five and seven tin neighbors at Rh–Sn distances covering the broader range from 261 to 291 pm. Especially the shorter ones compare well with the sum of the covalent radii [31] for Rh+Sn of 265 pm, and we can assume a substantial degree of covalent Rh–Sn bonding, similar to structurally related stannides like NdRh₂Sn₄ (268–292 pm Rh–Sn) [23], CeRhSn₂ (262–277 pm Rh–Sn) [32] or Ce₃Rh₄Sn₁₃ (266 pm Rh–Sn) [33].

Fig. 2:

Perspective view of the Sm₂Rh₃Sn₅ structure approximately along the a axis. Samarium, rhodium, and tin atoms are drawn as light gray, blue and red circles, respectively. The three-dimensional [Rh₃Sn₅] polyanionic network is emphasized. The two crystallographically independent samarium sites are highlighted.

Due to the comparatively high tin content we observe a variety of shorter Sn–Sn distances in the range of 304–340 pm. Again, the shorter ones compare well with the structure of β-Sn (4×302 and 2×318 pm) [34]. Thus, the [Rh₃Sn₅]^δ− polyanionic network is stabilized by Sn–Sn interactions as well. The two crystallographically independent samarium atoms fill two kinds of cavities within the [Rh₃Sn₅]^δ− polyanion: Sm1@Rh₇Sn₁₀ and Sm2@Rh₅Sn₈. Besides the change in coordination number (CN17 for Sm1 vs. CN13 for Sm2), it is evident that the Sm2 atoms have the closer Sm2–Rh contacts, while the Sm–Sn distances are similar. Somehow, the shorter Sm2–Rh contacts compensate the lower coordination number.

Polyanionic networks with two different cavities are suitable matrices for intermediate-valent or static mixed-valent compounds. The present structure type shows some remarkable examples: Yb₂Au₃In₅ with intermediate ytterbium valence [11], the antiferromagnet Eu₂Pt₃Sn₅ [35] with stable divalent europium and the valence fluctuating stannide Ce₂Rh₃Sn₅ [8], [9]. In the following paragraph we focus on the magnetic behavior of Sm₂Rh₃Sn₅.

4 Magnetic properties

The temperature dependence of the magnetic susceptibility of Sm₂Rh₃Sn₅ was investigated in the temperature range of 2.5–300 K. Measurements in zero field cooled (ZFC) mode were performed with a magnetic field of 10 kOe. Sm₂Rh₃Sn₅ (Fig. 3) exhibits van Vleck type behavior, caused by the small difference in energy of the excited ⁶H_7/2 multiplet and the ⁶H_5/2 ground multiplet of the Sm³⁺ ions [36].

Fig. 3:

Magnetic properties of Sm₂Rh₃Sm₅: (top) Temperature dependence of the magnetic susceptibility measured in ZFC mode with a magnetic field of 10 kOe from 3 to 300 K. (Inset) ZFC/FC measurement at 100 Oe in the temperature range of 2.5–75 K; (bottom) magnetization isotherms measured at 3, 10 and 50 K with magnetic fields up to 80 kOe.

The ZFC graph of Sm₂Rh₃Sn₅ detected at 10 kOe shown in the top panel of Fig. 3 revealed a nonlinear dependence of the susceptibility in the temperature range of 3–300 K. The inverse magnetic susceptibility was fitted using the following equation developed by Hamaker et al. [37]:

χ(T)=NAkB[μeff23(T−θP)+μB2δ]

In this equation N_A is the Avogadro number, k_B is the Boltzmann constant, μ_eff is the effective magnetic moment, θ_P is the paramagnetic Curie temperature, μ_B is the Bohr magneton and δ is an energy scale defined as δ=ΔE 7/20. The first term represents the Curie Weiss susceptibility of the ground state, while the second term describes the van Vleck susceptibility caused by the population of the ⁶H_7/2 multiplet [36], [37]. The data obtained by the ZFC measurement could be described well with the fit parameters of μ_eff=0.85(1) μ_B, θ_P=–10.6(5) K and δ=246(5) K. The obtained effective moment is in good agreement with the free ion value of the J=5/2 ground state of Sm³⁺ [38]. The energy difference between the ground and excited state derived from δ (ΔE=703 K) is much lower than the 1550 K predicted by Stewart [39] and lower than the 1346 K detected for SmPdGa₃ [40] but comparable to Sm₃Pt₄Ge₆ (ΔE=454 K) [41].

The inset of the top panel of Fig. 3 shows the ZFC/FC measurement of Sm₂Rh₃Sn₅ at 100 Oe depicted from 2.5 to 50 K. The sample reveals a transition to an antiferromagnetically ordered state with a Neél temperature of T_N=3.5(5) K. Magnetization isotherms measured at 3, 10 and 50 K are given in the bottom panel of Fig. 3. All three isotherms exhibit an almost linear dependence of the magnetization on the magnetic field in the range of 0–80 kOe and give no hint for field induced magnetic transitions.