Rhodium-rich silicides RERh₆Si₄ (RE=La, Nd, Tb, Dy, Er, Yb)

1 Introduction

Combination of an alkaline earth (or rare earth) element with a transition metal (T) and Si, P, Ge or As (generally called X) leads to a manifold of metal-rich compounds with a ratio of metal to X close or equal to 2:1 [1], [2], [3], [4], [5], [6], [7], [8], [9]. The main geometrical motifs of these structures are tricapped trigonal prisms which show various connectivity patterns via common edges and/or faces. In many phase diagrams several of such intermetallics occur, mostly very close in composition, e.g. La₆Ni₂Si₃, La₅Ni₂Si₃, La₁₅Ni₇Si₁₀ and La₂₁Ni₁₁Si₁₅, or Th₂Ni₁₂P₇, Th₆Ni₂₀P₁₃ and Th₁₂Ni₃₀P₂₁ [4], [9]. The growth of adequate single crystals is often a challenging task. In the field of metal-rich phosphides, tin and lead have successfully been used as fluxing agent [10], [11], [12], [13], [14]. Targeted synthesis of a desired compound requires the right starting composition, an adequate amount of the fluxing agent and a suitable temperature program for the annealing sequence.

LiCo₆P₄ [15] is a hexagonal structure type in this family of metal-rich compounds, so far with 29 representatives [9]. Besides the phosphides MgCo₆P₄ [16], MgRh₆P₄ [17], ScRu₆P₄, ErRu₆P₄, ZrCr₆P₄ [18] and α-UCr₆P₄ [19] and the arsenides ARh₆As₄ (A=Mg, Ca, Sr, Yb) [17], CeRh₆Si₄ [20] was the first tetrelide with this structure type. It was discovered during a detailed study of the isothermal section (1070 K) of the ternary system Ce-Rh-Si. With the higher congener germanium, the complete series of RERh₆Ge₄ (RE=Y, La–Nd, Sm–Lu) germanides [21], [22] has been characterized. Several single crystals of these phases were grown from bismuth fluxes serving as non-reactive flux medium.

The chemical bonding characteristics of the different LiCo₆P₄ type phases have been studied by ab initio calculations, especially emphasizing the covalently bonded [T₆X₄]^δ− polyanionic networks, the de-lithiation behavior of LiCo₆P₄ and the magnetic ground states of α-UCr₆P₄, CeRh₆Si₄ and CeRh₆Ge₄ [23], [24]. So far, only few property studies have been reported. YRh₆Ge₄, LaRh₆Ge₄ and LuRh₆Ge₄ are Pauli paramagnetic [22] while the RERh₆Ge₄ phases with RE=Gd–Yb are Curie-Weiss paramagnets. The highest magnetic ordering temperature of T_N=13.6 K has been observed for TbRh₆Ge₄ [22]. Temperature-dependent susceptibility and specific heat measurements reveal ferromagnetic ordering at T_C=2.5 K for CeRh₆Ge₄ [21], [25] and characterize this germanide as a Kondo-lattice compound. In contrast, an intermediate-valent state is observed for cerium in CeRh₆Si₄ [25], [26]. These magnetic ground states had been predicted on the basis of ab initio calculations [24].

In continuation of our systematic crystal growth experiments in bismuth fluxes [21], [22], [27], [28], [29] we have now obtained the remaining series of RERh₆Si₄ (RE=La, Nd, Tb, Dy, Er, Yb) silicides with LiCo₆P₄ type structure. The synthesis conditions and crystal chemical details are reported herein.

2 Experimental

3 Crystal chemistry

Six new silicides RERh₆Si₄ (RE=La, Nd, Tb, Dy, Er, Yb) complement the family of LiCo₆P₄ type intermetallics (non-centrosymmetric space group P6̅m2). The lattice parameters decrease from LaRh₆Si₄ to YbRh₆Si₄ as expected from the lanthanide contraction; however, an anomaly occurs for the cerium compound. The CeRh₆Si₄ cell volume is similar to that of NdRh₆Si₄, a clear crystal chemical hint for at least partially tetravalent cerium. This is in line with magnetic susceptibility data [25], L_III X-ray absorption spectra [26] and ab initio calculations [24] which give evidence for intermediate cerium valence with a value close to 3.2 [26], similar to the large series of intermediate-valent equiatomic CeTX intermetallics [36], [37]. The cell volumes of the RERh₆Si₄ series are smaller than those of the isotypic RERh₆Ge₄ series [21], [22], induced by the larger size of germanium.

Important parameters in the field of metal-rich silicides, more precisely rhodium-rich silicides in the present case, are the synthesis conditions. Detailed phase analytical studies in the Ce-Rh-Si [20] and Sm-Rh-Si [38] system revealed a variety of rhodium-rich silicides which form in a very narrow composition range, similar to other RE-Rh-Si systems [39]. The proximity of these phases often hampers the synthesis of phase-pure samples and crystal growth experiments often lead to agglomerates of crystals with different habit, i.e. different compounds. For the RERh₆Si₄ silicides discussed herein, the main impurity phases were the RE₁₂Rh₆₀Si₃₉ silicides [39]. For RE=Nd, Tb, Dy and Er the samples contained approximately equal amounts of the RE₁₂Rh₆₀Si₃₉ and RERh₆Si₄ phases while LaRh₆Si₄ and YbRh₆Si₄ were only available as isolated crystals.

The family of LiCo₆P₄ type compounds has been discussed in detail with respect to crystal chemistry [15], [16], [17], [18], [19], [20], [21], [22] and chemical bonding [23], [24]. Herein we exemplarily discuss the structural details of LaRh₆Si₄. A projection of the LaRh₆Si₄ structure onto the xy plane is presented in Fig. 1. The striking geometrical motifs are trigonal prisms around the two crystallographically independent silicon sites: Si1@Rh₆ and Si2@La₂Rh₄. All three rectangular faces of the Si1@Rh₆ prisms are capped by additional rhodium atoms, leading to the typical coordination number 9, frequently observed in structurally related metal-rich compounds [1]. Only two of the rectangular faces of the Si2@La₂Rh₄ prisms are capped by additional rhodium atoms. This is a direct consequence of the condensation pattern. Each Si2@La₂Rh₄ prism shares the La₂ respectively Rh₂ edges of four neighboring prisms, leading to a propeller-like motif (Fig. 1). Always four of such propellers condense to a ring in which the Si1@Rh₆ prisms are embedded. Both prism types are shifted by half a translation period c with respect to each other. The crystal chemical description through condensation of trigonal prisms is certainly a purely geometrical one; however, it is simple and effectively allows distinction of the many different condensation patterns, especially in the families of metal-rich phosphides and silicides [1], [2], [3], [4], [5], [6], [7, and references cited therein]. Examples for the rhodium-rich silicides are the structures of Ce₂Rh₁₂Si₇ and Ce₆Rh₃₀Si₁₉ [40].

Fig. 1:

Projection of the LaRh₆Si₄ structure onto the xy plane. Lanthanum, rhodium and silicon atoms are drawn as medium gray, blue and red circles, respectively. All atoms lie on mirror planes at z=0 (thin lines) and z=1/2 (thick lines). The crystallographically independent rhodium and silicon sites and the trigonal prismatic coordination of the silicon atoms are emphasized.

The shortest interatomic distances in the LaRh₆Si₄ structure occur for Rh–Si, ranging from 242 to 249 pm, close to the sum of the covalent radii [41] for Rh+Si of 242 pm. We can therefore expect strong covalent Rh–Si bonding. The crystal orbital overlap population analysis of isotypic CeRh₆Ge₄ [24] nicely underlines this picture (charge transfer from cerium and formation of a [Rh₆Ge₄]^δ− polyanion). The two crystallographically independent rhodium atoms have different silicon coordination, i.e. slightly distorted Rh1@Si₅ square pyramids and Rh2@Si₄ tetrahedra. The condensation patterns of both types of polyhedra is presented in Fig. 2. Always three Rh1@Si₅ square pyramids (grey polyhedra in Fig. 2) are condensed via a common silicon corner, and these trimeric units are further condensed via common edges. Also the Rh2@Si₄ tetrahedra (green polyhedra in Fig. 2) form trimeric units through condensation via a common edge, and they are directly condensed on top of the trimeric units of the condensed Rh1@Si₅ square pyramids.

Fig. 2:

The crystal structure of LaRh₆Si₄. Lanthanum, rhodium and silicon atoms are drawn as medium grey, blue and red circles, respectively. The Rh1@Si₅ square pyramids (grey), Rh2@Si₄ tetrahedra (green) and La@Rh₆Si₆ hexagonal prisms (blue) are emphasized.

The high degree of condensation of the Rh1@Si₅ square pyramids and Rh2@Si₄ tetrahedra via common corners and edges leads to a variety of close Rh–Rh contacts. The Rh–Rh distances range from 278 to 286 pm and they are somewhat longer than in fcc rhodium (269 pm) [42]. Nevertheless, crystal orbital overlap population (COOP) data on CeRh₆Ge₄ [24] revealed that bonding and anti-bonding contributions almost compensate each other and the Rh–Rh bonding does not significantly contribute to the stability of CeRh₆Ge₄. One can safely assume a rigid band model and postulate almost similar bonding characteristics for LaRh₆Si₄.

The network of condensed Rh1@Si₅ square pyramids and Rh2@Si₄ tetrahedra leaves larger cavities which are filled by the lanthanum atoms. The latter have hexagonal prismatic coordination by rhodium (309 pm) and silicon (311 pm) atoms and these prisms perfectly fit into the polyhedral network (Fig. 2), leading to a dense packing. Again, the rectangular faces of these La@Rh₆Si₆ prisms are capped by Rh1 atoms, however, at much longer La–Rh1 distances of 352 pm and this can only be considered as a geometrical feature. The lanthanum atoms within the La@Rh₆Si₆ prisms are well separated from each other. The La–La distance between two face-sharing La@Rh₆Si₆ prisms is 381 pm and corresponds to the lattice parameter c.