The ternary system Tm–Ni–In at 870 K

2.1 Synthesis

The samples were prepared by arc-melting of high purity metals (99.5 wt.% purity for Tm; 99.92 wt.% for Ni and 99.999 wt.% for In) under a purified argon atmosphere with a total mass of about 1 g per sample. The arc-melted buttons were sealed in evacuated silica tubes and annealed at 870 K for 720 h and subsequently quenched in cold water without breaking the tubes. More than 80 ternary and binary samples were synthesized. The weight losses were smaller than 1 % for all samples. Chemical and phase compositions of the samples are shown in Fig. 1.

Fig. 1:

Chemical and phase composition of studied samples and the isothermal section of the Tm–Ni–In system at T = 870 K. Circles 1, 2, and 3 mean single-, two-, three-phase samples.

2.2 X-ray diffraction

Investigation of the isothermal section of the Tm–Ni–In ternary system was carried out by X-ray powder diffraction techniques. The X-ray phase analysis was performed using film data (Debye–Scherrer technique, RKD-57.3 camera, CrK radiation) and measurements using automatic diffractometers: DRON-2.0 (FeK_α radiation, Ge as an internal standard), HZG-4a (CuK_α), Philips X’PERT (CuK_α) and STOE STADI P (CuK_α1).

The indexing of the obtained diffraction data of the ternary samples was performed by comparison with calculated data using the program Powder Cell [11]. The lattice parameters of some phases were calculated using the program Latcon [12]. The microstructure of the samples was studied visually on polished and etched surfaces, by using a microscope “NEOPHOT 30” in reflected light. In selected cases the composition of samples was analyzed using a scanning electron microscope–microanalyser REMMA-102-02 (SEM and EDX modes) by means of energy-dispersive X-ray spectroscopy analyses (EDX analyses).

The crystal structure studies were made by X-ray powder diffraction patterns using the Fullprof [13] and WinCSD [14] packages for the Rietveld refinements.

3.1 Phase analysis and crystal structure of the compounds

The isothermal section of the ternary system Tm–Ni–In at T = 870 K was constructed and the result is shown in Fig. 1. It was confirmed that the following binary compounds exist at this temperature: Tm₂In (Ni₂In type), Tm₅In₃ (Mn₅Si₃ type), TmIn (CsCl type), Tm₃In₅ (Tm₃Ga₅ type), TmIn₃ (AuCu₃ type), Tm₃Ni (Fe₃C type), TmNi (FeB type), TmNi₂ (MgCu₂ type), TmNi₃ (PuNi₃ type), TmNi₅ (CaCu₅ type), Tm₂Ni₁₇ (Th₂Ni₁₇ type), Ni₃In (Ni₃Sn type), Ni₂In (Ni₂In type), ε-phase (Ni_xIn_1–x, NiAs type), Ni₁₃In₉ (Ni₁₃Ga₉ type), NiIn (CoSn type) and Ni₂In₃ (Ni₂Al₃ type). Almost none of the binary compounds dissolves significant amounts of the third component. An exception is the existence of solid solutions based on the binary compounds NiIn and TmNi₂, which are described below. The phase equilibrium of the system Tm–Ni–In at 870 K is characterized by the formation of nine ternary compounds of which the crystallographic parameters are summarized in Table 1.

A solid solution based on NiIn (CoSn-type structure, space group P6/mmm) was found in the Tm–Ni–In system. The Tm atoms are included in the hexagonal channels of the parent NiIn structure with partial reordering of indium atoms as it was described for the systems with Eu, Tb, Dy, and Er [4, 5, 30, 31]. The lattice parameters and the unit cell volume of the Tm_xNiIn solid solution increase with the concentration up to ~8 at.% Tm (x = 0.17) (Fig. 2, Table 1). An attempt to describe the resulting structure of the solution as incommensurately modulated Tm_xNiIn (0 ≤ x ≤ 0.22) was reported in [32].

Fig. 2:

Lattice parameters and unit cell volume of the Tm_xNiIn phases vs. the concentration of Tm.

The solid solution Tm_1−xNi₂In_x was investigated on the basis of X-ray powder data (results of this work and literature data [15]) and single crystal data for the composition TmNi₄In [15, 16]. The closely related crystal structures of the Laves phases TmNi₂ (MgCu₂ type) and TmNi₄In (MgCu₄Sn type; ordered Laves phase) allow the formation of a solid solution with Tm/In substitution because the difference in the atomic radii between Tm and In is small [33]. For the solid solution the lattice parameter a decreases rather linearly according to the Vegard rule up to the composition TmNi₄In (Fig. 2 of Ref. [17], Table 1) but no two-phase region was noticed, as it was also observed for the ErNi₂–ErNi₄In section [5]. Thus, Tm_1–xNi₂In_x behaves similar to the solid solutions described for Tb [4] and Dy [31].

The crystal structure of Tm₁₀Ni₉In₂₀ was investigated in Refs. [17] and [18]. According to single crystal data [18], all atomic sites are completely filled for Tm₁₀Ni₉In₂₀, but some disorder is found for compounds with other rare earths. The cell parameters, obtained in this work, differ slightly from literature data and indicate the existence of a narrow homogeneity region for this compound.

TmNiIn belongs to the Fe₂P structure type or its ordered ternary derivative ZrNiAl [19]. The homogeneity region was determined for the compound along the section of 33.3 at.% Tm. Variations of the lattice parameters within this region (Fig. 3) reveal its extension from 33.3 to 46.6 at.% In resulting in the composition TmNi_1–0.60In_1–1.40. The cell volume and the lattice parameter a increase, whereas c decreases, due to Ni/In substitution on the 2d position of Ni. The homogeneity region was investigated also by single crystal data, and similar results were described in [20]. The homogeneity range of TmNi_1–xIn_1–x extends up to x = 0.65 [21]. Besides Ni/In mixing, single crystal data indicate also a small deficiency on the Ni 1a position (up to ~10 %) resulting in the general composition TmNi_1–x–yIn_1+x.

Fig. 3:

Lattice parameters and unit cell volume of the solid solution TmNi_1–0.60In_1.00–1.40 vs. the concentration of In.

The compounds Tm₂Ni₂In and Tm₂Ni_2–xIn [22, 23] are isotypic with Y₂Ni₂In (Mn₂B₂Al type) and Gd₂Ni_1.78In (Mo₂B₂Fe type). At 870 K these compounds exist at very close compositions: Tm₄₀Ni₄₀In₂₀ (for Tm₂Ni₂In) and Tm_41.8Ni_37.2In_21.0 (for Tm₂Ni_2–xIn, x = 0.22) [24]. For the latter compound precise single crystal data [21] revealed defects (up to 10 %) for the Ni 4d site and In/Tm mixing up to 22 % Tm for the 2a In position. Some differences in the cell parameters for samples of different composition could be explained by homogeneity ranges of Tm₂Ni_2–xIn.

The Mo₅SiB₂ type was observed in the RE–Ni–In systems for RE = Er and Tm [24]. According to this work, the structure can be considered as an intergrowth of distorted CuAl₂ and U₃Si₂ (binary aristotype of Mo₂FeB₂) related slabs of compositions Tm₂In and (Tm/In)₃Ni₂. Unlike Er₅Ni₂In, the phase Tm_4.83Ni₂In_1.17 is characterized by a Tm/In mixture on the 4c position.

The indide Tm₅Ni₂In₄ was reported in [25] and its magnetic properties were presented in [26, 34]. This investigation also revealed the existence of the thulium-rich phase Tm₁₁Ni₄In₉ which extends the RE₁₁Ni₄In₉ series [35] with respect to the heavy rare earth elements. The existence of Tm₁₁Ni₄In₉ was recently confirmed [27]. Tm₁₁Ni₄In₉ (≡ Tm_45.8Ni_16.7In_37.5) and Tm₅Ni₂In₄ (≡ Tm_45.5Ni_18.2In_36.3) have very similar compositions and similar structures, which are built up from distorted CsCl and AlB₂ related slabs. These structures, together with Tm₂Ni₂In and Tm₂Ni_2–xIn, represent a homologous series RE_m+nT_2nX_m where m and n are the numbers of CsCl (TmIn) and AlB₂ (TmNi₂) related slabs, respectively. For Tm₅Ni₂In₄m = 1 and n = 4, for Tm₁₁Ni₄In₉m = 2 and n = 9, and for Tm₂Ni₂In and Tm₂Ni_2–x In m = n = 1.

The two phases Tm₆Ni_2.37In_0.63 [28] and Tm₁₄Ni₃In₃ [29] were found in the thulium-rich corner of the Tm–Ni–In system. The structures were determined from single crystal X-ray diffraction data. Both compounds show some deviation from the ideal composition and, obviously, have some homogeneity ranges. Thus, simultaneous Ni/In and Tm/In mixing in two 4c positions was observed for Tm_13.47Ni_3.28In_3.25 [29].

3.2 Magnetic properties of the Tm–Ni–In compounds

Magnetic properties have been determined for selected (X-ray pure) compounds only. Magnetic, specific heat and powder diffraction data indicate that TmNiIn is an antiferromagnet with T_N = 2.5 K. The magnetic structure is an incommensurate one and may be described by the propagation vector k = [k_x, k_x, 1/2], where k_x = 0.281(2) [36]. Tm₂Ni₂In orders also antiferromagnetically with T_N = 5.0 K which has been confirmed by dc and ac magnetic measurements and specific heat data [37].

The results of magnetic dc and ac as well as calorimetric measurements of Tm₅Ni₂In₄ indicate antiferromagnetic behavior below T_N = 4.2 K [26]. Neutron diffraction measurements have confirmed the antiferromagnetic ordering described by a propagation vector k = [0, 1/2, 1/2]. The magnetic order has a complex character with different values for the magnetic moments at different sites. Neutron diffraction data support the complex modulated magnetic structure [34].

The data on the magnetic properties (Néel temperature T_N, paramagnetic Curie temperature θ_p, effective magnetic moment μ_eff, moment in the ordered state μ_S at H = 90 kOe, moment from neutron diffraction data μ_ND) are summarized in Table 2. In all compounds the magnetic moments of the Tm³⁺ ions order at low temperatures. The difference between the magnetic moment derived from the magnetization and neutron diffraction indicate a strong influence of the crystal electric field.