RE ₁₀Co₃In₁₀ (RE = Y, Gd, Tb, Dy, Ho, Er, Tm) – a new intergrowth structure with CsCl- and AlB₂-type slabs

1 Introduction

Ternary intermetallic compounds with a general formula RE _x T _y In _z (RE = rare earth, T = d-metals) ¹ have been the subject of intensive investigations during the last decades because of their interesting crystal structures and intriguing physical properties. For example, the ternary compounds CeTIn₅ and Ce₂ TIn₈ (T = Co, Ir, Rh) are unconventional heavy-fermion superconductors. ^{2 , 3 , 4 , 5} Compounds with cobalt as perspective magnetic materials are of particular interest in this regard. ^{1 , 6} An isothermal section of the phase diagram was constructed for Ce–Co–In and the existence of only two compounds CeCoIn₅ and Ce₂CoIn₈ were found. ⁷ In other systems, a larger number of compounds were discovered: in particular, 12 ternary compounds in the system Er–Co–In, ⁸ and eight in the system Sc–Co–In. ⁹ In each of the systems, with the exception of those including Pm, Eu, and Yb, the existence of about 10 ternary compounds was reported. ^{1 , 6}

Among the many examples, one can single out a group of compounds that are double-layered along the shortest unit cell direction (∼3.5 Å) and are based on an intergrowth of AlB₂- and CsCl-type related slabs. ^{10 , 11 , 12} They crystallize in the Nd₁₁Pd₄In₉ and Er₂₃Co_6.7In_20.3 structural types with similar compositions (in at%): 45.8 RE, 16.7 Co, 37.5 In and 46 RE, 13.4 Co, 40.6 In, respectively. Continuing our systematic studies of the RE-Co-In systems, in particular Tb–Co–In, in this part of the concentration triangle, we discovered the existence of Tb₁₀Co₃In₁₀, which is also a combination of AlB₂- and CsCl-related fragments.

2 Experimental

The samples were prepared by arc-melting the metallic components under a purified argon atmosphere (p = 50 kPa, Ti sponge was used as a getter). The certified purities were 99.8 wt% for RE, 99.91 wt% for Co and 99.99 wt% for In. All buttons were re-melted twice to ensure homogeneity. The overall weight losses after melting were generally less than 1 % of the starting weight of near 1 g. The samples were annealed at T = 870 K for 1,500 h in evacuated quartz tubes and were finally quenched in cold water without breaking the tubes. To obtain high-quality single crystals of Tb₁₀Co₃In₁₀, longer annealing times and slow cooling to room temperature were used.

For X-ray phase analysis and structure refinement, the samples were measured using a STOE Stadi P (CuKα₁ radiation; λ = 1.54056 Å, Bragg-Brentano geometry, measured angle interval 2θ = 5–110°, step scan mode, step size in 2θ = 0.015°, 700 s per step) and DRON-2.0 M (FeKα radiation; λ = 1.93604 Å) diffractometers. The diffraction data was indexed by comparison with calculated data using the programs PowderCell ¹³ and STOE WinXPOW. ¹⁴ Rietveld refinements of powder data were made using the Fullprof program package. ¹⁵

A complete data set of a single crystal was collected at room temperature by using an Oxford Diffraction Xcalibur system (graphite-monochromatized MoKα radiation; λ = 0.71073 Å) with an Atlas CCD camera. A numerical absorption correction was applied to the data set. Details on the crystallographic data are given in Table 1. The crystal structure was solved using Direct Methods with the program Shelx-2014/7. ¹⁶

CCDC 22429597 contains the supplementary crystallographic data for Tb₁₀Co₃In₁₀. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

Metallographic and quantitative phase analyses were performed using a Tescan Vega 3 LMU scanning electronic microscope equipped with an energy-dispersive X-ray analyser (EDX).

3 Results and discussion

In an attempt to obtain a compound with an equiatomic composition in the Tb–Co–In system, a suitable, at first glance single crystal with an irregular shape was selected from a sample with a starting composition Tb_33.3Co_33.3In_33.4. The analysis of the X-ray dataset from the single crystal showed that the main array consisted of reflections of TbCo₂In, ¹⁷ onto which a fragment of a single crystal of another, unknown compound had grown. Most of the reflections belong to the crystallite of TbCo₂In, but there are also about 10 % of the reflections that are described by a cell with the approximate dimensions of ∼ 3.5 × 24 × 25 ų. None of the known compounds in the RE-Co-In systems has a unit cell with similar metrics, confirming the presence of a new compound. The X-ray powder pattern of a sample of composition Tb_33.3Co_33.3In_33.4 also confirmed the presence of both phases.

Using the data obtained from the single crystal, the structure of TbCo₂In was refined in an anisotropic approximation according to the model of the structure type PrCo₂Ga (orthorhombic space group Pmma) by 243 hkl reflections to the discrepancy factors R ₁ = 0.0332 and wR ₂ = 0.0794. The refined values of the unit cell parameters are: a = 5.028(3), b = 4.041(3), c = 7.117(4) Å and the positions of the atoms: Tb – 2f (1/4 1/2 z, z = 0.71199(9)), Co1 – 2a (0 0 0), Co2 – 2f (z = 0.0937(3)), In – 2e (1/4 0 z, z = 0.34960(3)). The results of the refinement of the TbCo₂In crystal structure are in good agreement with literature data. ¹⁷

Using the dataset belonging to the unknown compound an attempt to determine its crystal structure was made. A satisfactory structure model was constructed within the space group Pbam. The solution of the structure by Direct Methods and refinement with anisotropic atomic displacement parameters for all atoms (Tables 2 and 3) led to the composition Tb₁₀Co₃In₁₀ and showed the typical features of a two-layer structure. All the Tb atoms are in the 4h x y 1/2 sites and form one layer, and the Co and In atoms are in the 4g x y 0 sites and form the second layer. There are four Tb₁₀Co₃In₁₀ formula units per unit cell. The studied crystal was a twin and the new phase was a minor component. This is the reason why the single-crystal data is characterized by high R values.

At the next stage, to obtain the compound in its pure form and to produce higher-quality single crystals, we produced a sample of the composition Tb_43.5Co₁₃In_43.5, which corresponds to that of Tb₁₀Co₃In₁₀. The powder X-ray diffraction pattern collected at room temperature (STOE Stadi P diffractometer, CuKα₁ radiation) from a sample annealed for two months at T = 870 K (Figure 1a) and an as-cast sample revealed complete similarity, indicating the formation of this compound directly from the melt. Microstructural studies (Figure 1b) show that the sample practically contains one phase. According to EDX analysis, its composition is Tb_43.9Co_12.7In_44.4, which is consistent with the composition Tb₁₀Co₃In₁₀. The impurity phase in the sample is Tb₁₁Co₄In₉. ^{10 , 11}

Figure 1:

Powder X-ray diffraction pattern collected at room temperature (CuKα₁ radiation) of the Tb_43.5Co₁₃In_43.5 sample (a) with the constituent phases (from upper to lower bars): Tb₁₀Co₃In₁₀ (own type, 94 wt%) and Tb₁₁Co₄In₉ (Nd₁₁Pd₄In₉ type, Cmmm, 6 wt%). Solid circles stand for the experimental data, lines show the result of Rietveld refinement, vertical bars mark the Bragg reflection positions of the phases, and the difference between the experimental data and the refinement is plotted at the bottom of the figure (blue line). SEM image of the Tb_43.5Co₁₃In_43.5 sample (b) annealed at T = 870 K: (light grey phase – Tb₁₀Co₃In₁₀; dark grey phase – Tb₁₁Co₄In₉), and black holes between crystals.

Our attempts to obtain high-quality single crystals by increasing the duration of annealing to half a year and changing the temperature regime of annealing were unsuccessful. The structure of the compound Tb₁₀Co₃In₁₀ was refined from the powder X-ray diffraction pattern collected at room temperature (CuKα₁ radiation) using the Fullprof program package. ¹⁵ The atomic parameters obtained from the single crystal structure determination were taken as starting values. The results are in good agreement with the single-crystal data: orthorhombic space group Pbam, a = 25.462(13); b = 24.026(12); c = 3.605(2) Å; V = 2,205.0(2) Å³, R _Bragg = 0.084; R _F = 0.052; R _p = 0.024; R _wp = 0.032. The phase content in the sample is 94 wt%. The second phase (6 wt%) is Tb₁₁Co₄In₉ (Nd₁₁Pd₄In₉-type structure) (Figure 1b).

The ternary compound Tb₁₀Co₃In₁₀ crystallizes with a new structure type, Pearson code oP92. The projection of the unit cell on the xy and the coordination polyhedra of the atoms in Tb₁₀Co₃In₁₀ are presented in Figure 2. Interatomic distances are well correlated with the size of the atoms, r _Tb = 1.782, r _Co = 1.253, r _In = 1.623 Å ¹⁸ (Table 4). The largest reductions of the interatomic distances are observed for atom pairs Tb10–In9 (11.1 %), Tb3–Co2 (9.1 %), Tb3–Tb3 (3.5 %), In9–In5 and In10–In4 (9.5 %), In9–Co1 (8.2 %), which is acceptable for intermetallic compounds. Cobalt atoms are not in direct contact. All cobalt atoms are located in the middle of trigonal prisms formed by terbium atoms. The trigonal prisms are additionally capped by three indium atoms in equatorial positions. For the Co1 atoms, the distance to the In atoms (2.64–2.69 Å) is smaller than to the Tb atoms (2.92–2.99 Å). On the contrary, in the case of the Co2 and Co3 atoms, the Co–Tb distances (2.76–2.86 Å) are smaller than the Co–In distances (2.94–3.10 Å). The coordination numbers of the terbium atoms range from 13 to 15. They correspond to coordination polyhedra in the form of pentagonal (atoms Tb1, Tb2, Tb4, Tb6, Tb9, Tb10) and tetragonal (atoms Tb3, Tb5, Tb7, Tb8) prisms with centred bases and some lateral faces. The lateral faces of the pentagonal prisms are centered by three atoms (for Tb9 – two atoms), and those of the tetragonal prisms by four atoms (for Tb8 – three atoms). The coordination numbers of the indium atoms range from 11 to 12. They correspond to coordination polyhedra in the form of tetragonal (atoms In1–In5, In7, In8) and trigonal (atoms In6, In9, In10) prisms with centred bases and some lateral faces. Here, the lateral faces of all tetragonal prisms are centered by two atoms, and those of the trigonal prisms are centered by three atoms.

Figure 2:

Projection of the unit cell of Tb₁₀Co₃In₁₀ onto the xy plane and coordination polyhedra of the atoms.

As mentioned above, the compound Tb₁₀Co₃In₁₀ belongs to a large family of two-layer structures with a layer stacking in the shortest unit cell direction. Considering the RE-transition metal-indium systems, the compounds RE ₂ T ₂In (types Mo₂FeB₂, Mn₂AlB₂, o-La₂Ni₂In), RE ₅ T ₂In₄ (Lu₅Ni₂In₄ type), and RE ₁₁ T ₄In₉ (Nd₁₁Pd₄In₉ type) can be included here. ^{1 , 19 , 20 , 21 , 22 , 23 , 24} In the planes perpendicular to this direction, some layers are built exclusively from RE atoms, and the others are formed by transition metal and indium atoms. These structures together with the Cr₃AlB₄ and W₂CoB₂ types ²⁵ form a homologous series based on CsCl- and AlB₂-related slabs. The composition of the first fragment is REIn, and of the second one it is RET ₂. The general formula of this homologous series is RE _m+n T _2n X _m , where RE, T, and X represent the atoms of the rare earth elements, transition metals, and boron/aluminium/indium, respectively, while m and n indicate the number of CsCl- and AlB₂-related fragments, respectively. The number of fragments per unit cell for the types Mo₂FeB₂ and Mn₂AlB₂ are: m = n = 2, for the type o-La₂Ni₂In – m = n = 4, for the type Lu₅Ni₂In₄ – m = 8, n = 2 and for the type Nd₁₁Pd₄In₉ – m = 18, n = 4. The new structural type, Tb₁₀Co₃In₁₀, belongs to a different homologous series, as it contains, in addition to fragments of TbIn and TbCo₂, a further AlB₂-related fragment, specifically with a TbIn₂ composition (q). The formula of this homologous series will therefore be (TbIn)m + (TbCo₂)n + (TbIn₂)q = Tb _m+n+q Co_2n In _m+2q (=RE _m+n+q T _2n X _m+2q ). For the Tb₁₀Co₃In₁₀ m = 28, n = 6, q = 6. This homologous series also includes structural types Tb₃Ru_0.97In₃ (m = 8, n = 2, q = 2) ²⁶ and Nd₃₉Ir_10.98In_36.02 (Nd₃₉Ir₁₁In₃₆) (m = 62, n = 12, q = 4) ²⁷ (Figure 3). It is worth noting that there is another homologous series, whose members consist of two structural types, Ho₁₆Ru₅In₁₄ ²⁶ and Er₂₃Co_6.7In_20.3 (= Er₂₃Co₇In₂₀). ¹² In addition to the three fragments indicated for the previous homologous series, this series also contains a CsCl-related fragment with the composition RET (p). In this case, the formula of the homologous series, using the compound Ho₁₆Ru₅In₁₄ as an example, can be expressed as (HoIn)m + (HoRu₂)n + (HoIn₂)q + (HoRu)p = Ho _m+n+q+p Ru_2n+p In _m+2q (=RE _m+n+q+p T _2n+p X _m+2q ) (where m = 12, n = 2, q = 1, p = 1). ²⁶ For the structure type Er₂₃Co_6.7In_20.3, m = 36, n = 6, q = 2, and p = 2 a detailed analysis of structures built from fragments of the CsCl and AlB₂ type has been provided in published work. ^{1 , 12 , 22 , 26 , 27}

Figure 3:

Combination of fragments: CsCl (REIn, in blue) and AlB₂ (RET ₂, in yellow and REIn₂, in white) in the Tb₃Ru_0.97In₃ (а), Tb₁₀Co₃In₁₀ (b) and Nd₃₉Ir_10.98In_36.02 (c) structures of the RE _m+n+q T _2n X _m+2q homologous series.

Another two-layer indide structure, namely Gd₈Ru₅In₇, which is a redistribution structure of the Y₅Cu₅Mg₈ type, ²⁸ contains both tetragonal and trigonal prisms, similar to the previous examples. However, it does not belong to the specified homologous series because it contains mixed layers of [Gd12Ru3] and [Gd2Ru4In12] atoms. ²⁶

It should be noted that the considered structures are extremely similar in chemical composition. On the concentration triangle, the compositions of these compounds are limited by straight lines that connect the compositions RET ₂-REIn and T-REIn, or they are located on these lines. According to their rare earth metal content,^[1] they are arranged in the following order (Figure 4):

Figure 4:

Compositions of compounds in the systems RE-T-In belonging to homologous series based on fragments of the CsCl and AlB₂ type (compounds realized in ternary systems RE-Co-In are indicated by black circles).

The Tb₁₀Co₃In₁₀-, Nd₁₁Pd₄In₉-, and Er₂₃Co_6.7In_20.3-type structures are implemented in the cobalt systems.

Isostructural compounds are obtained in the systems with Y, Gd, Dy, Ho, Er, and Tm. Samples contain between 2 and 8 wt% of other phases (Figure 5). Their crystal structures were refined using powder X-ray data (DRON-2.0, FeKα radiation), and the results are listed in Table 5. Figure 6 illustrates the variation of the (V/N) parameter (where V is the volume of the unit cell, and N is the number of atoms per unit cell) for RE ₁₀Co₃In₁₀ compounds, based on the size of the RE ³⁺ ions. ²⁹ This variation is consistent with the effect of the lanthanoid contraction.

Figure 5:

BSE image of RE _43.5Co₁₃In_43.5 sample annealed at T = 870 K (RE = Y (a), Gd (b), Dy (c), Ho (d), Er (e), Tm (f)).

Figure 6:

Plot of (V/N) for RE ₁₀Co₃In₁₀ compounds as a function of the effective ionic radii of the RE atoms.

4 Conclusions

The intermetallic compounds RE ₁₀Co₃In₁₀ (RE = Y, Gd, Tb, Dy, Ho, Er, Tm) were obtained by arc-melting of the elements in an argon atmosphere followed by annealing at T = 870 K for 1,500 h in evacuated and sealed quartz ampoules. The structure model of Tb₁₀Co₃In₁₀ was obtained by Direct Methods using a data set from an intergrown single crystal of known TbCo₂In and the new phase Tb₁₀Co₃In₁₀. The compound crystallizes in its own structure type (orthorhombic space group Pbam, Z = 4).

The structure of the RE ₁₀Co₃In₁₀ compounds was refined from powder X-ray diffraction patterns and belongs to a large family of two-layer structures (with layers perpendicular to the short unit cell axes) based on the intergrowth of AlB₂- and CsCl-type related slabs. Tb₁₀Co₃In₁₀ - together with the structure types Tb₃Ru_0.97In₃ and Nd₃₉Ir_10.98In_36.02 – forms a homologous series RE _m+n+q T _2n X _m+2q, where m is the number of REX slabs (CsCl type), n the number of RET ₂ and q the number of REX ₂ slabs (both AlB₂ type) (T = Co, Ru, Ir; X = In). For Tb₁₀Co₃In₁₀ the values are m = 28, n = 6, q = 6.