Isothermal Section of the Ho–Cu–Sn Ternary System at 670 K

The interaction of the components in the Ho - Cu - Sn ternary system was investigated at 670 K over the whole concentration range using X-ray diffraction and EPM analyses. Four ternary compounds were formed in the Ho–Cu–Sn system at 670 K: HoCuSn (LiGaGe type, space group P 6 3 mc ), Ho 3 Cu 4 Sn 4 (Gd 3 Cu 4 Ge 4 -type, space group Immm ), HoCu 5 Sn (CeCu 5 Au-type, space group Pnma ), and Ho 1.9 Cu 9.2 Sn 2.8 (Dy 1.9 Cu 9.2 Sn 2.8 -type, space group P 6 3 /mmc ). The formation of the interstitial solid solution based on HoSn 2 (ZrSi 2 -type) binary compound up to 5 at. % Cu was found.

In the present paper the results of X-ray and EPM analyses of the phase equilibria in the Ho-Cu-Sn system at 670 K and crystallographic data of the ternary compounds are reported. The data concerning the Ho-Sn binary system were taken from Refs. [23][24][25][26][27], those of the Ho-Cu and Cu-Sn systems were found in Refs [28,29].

I. Experimental details
The samples were prepared by a direct twofold arc melting of the constituent metals (holmium, purity of 99.9 wt.%; copper, purity of 99.99 wt.%; and tin, purity of 99.999 wt.%) under high purity Ti-gettered argon atmosphere on a water-cooled copper crucible. The weight losses of the initial total mass were lower than 1 wt.%. The pieces of the as-cast buttons were annealed for one month at 670 K in evacuated silica tubes and then water quenched. The temperature of annealing was chosen taking into account the low melting temperature of Sn (232 0 C) and of the R-Sn binaries at high Sn content. Phase analysis was performed using X-ray powder patterns of the synthesized and annealed samples (DRON-2.0, Fe K α radiation). The observed diffraction   HoCu 2 a = 0.4277 (4)  R ЕММА-102-02 scanning microscope. Quantitative electron probe microanalysis (EPMA) of the samples was carried out by using an energy-dispersive X-ray analyzer with the pure elements as standards (an acceleration voltage was 20 kV; K-and L-lines were used). XRPD data were collected in the transmission mode on a STOE STADI P diffractometer (linear PSD detector, 2 θ / ω -scan; Cu Kα 1 radiation, curved germanium (1 1 1) monochromator). Calculations of the crystallographic parameters were performed using WinCSD and WinPLOTR program packages [30,31].
The DSC analysis was performed on the Ho 3 Cu 4 Sn 4 compound (LINSEIS STA PT 1600 device, argon atmosphere). The Ho 3 Cu 4 Sn 4 sample was heated up to 1023 K and cooled down to room temperature at a rate of 10 K/min. The weight losses during heating (TG) were negligible (less than 0.3%).

Isothermal section of the Ho-Cu-Sn system.
Phase equilibria in the Ho-Cu-Sn system have been studied using X-ray analysis and scanning electron microscopy of 15 binary and 29 ternary alloys annealed at 670 K (Fig. 1). The phase compositions of the selected samples are listed in Table 1, the SEM-pictures of some alloys are shown in Fig. 2.
The presence of the all binary compounds in the Ho-Cu and Cu-Sn systems corresponding to the reference data was confirmed at 670 K. Taking into account the reported data on Ho-Sn system including experimental study and thermodynamic optimization, and previously known binaries [23][24][25][26][27], the samples with compositions corresponding to the reference data were synthesized and analyzed by X-ray powder diffraction. The performed analysis confirmed the formation of Ho 5 Sn 3 (Mn 5 Si 3type), Ho 5 Sn 4 (Sm 5 Ge 4 -type), Ho 11 Sn 10 (Ho 11 Ge 10 -type), HoSn 2 (ZrSi 2 -type), Ho 2 Sn 5 (Er 2 Ge 5 -type) and HoSn 3 (GdSn 2.75 -type) binaries. However, two binaries Ho 4 Sn 5 and Ho 3 Sn 7 were not identified at temperature of annealing, corresponding samples contained only    [27]. Crystallographic characteristic of the Ho-Sn binary compounds are given in Table 2. The formation of the substitutional solid solution based on the HoCu 5 binary compound (AuBe 5type) up to 5 at.% Sn was found (a = 0.7028(2) nm for HoCu 5 , a = 0.7048(2) nm for Ho 17 Cu 78 Sn 5 ). The limiting composition was confirmed by EPMA data (Ho 17.62 Cu 77.55 Sn 4.83 , Fig. 2a). The interstitial solid solution HoCu x Sn 2 (up to 6 at.% Cu) based on the HoSn 2 (ZrSi 2 -type) binary compound was observed similarly to Ref. [32]  According to performed X-ray and microprobe analyses the phase relations in the Ho-Cu-Sn system at 670 K are characterized by the formation of four ternary compounds listed in Table 3. All ternary compounds are characterized by narrow homogeneity regions at investigated temperature. Among formed ternary compounds Ho 3 Cu 4 Sn 4 contains the higher Sn content (36 at. %), and we checked this phase with the differential scanning calorimetric analysis (DSC). No thermal peak corresponding to decomposition of Ho 3 Cu 4 Sn 4 compound was observed on DSC curve up to 1023 K.

Crystal structure.
The existence of the HoCu 5 Sn compound with CeCu 6 structure type and its lattice parameters were reported earlier [21]. In our work the crystal structure of HoCu 5 Sn stannide was refined by X-ray powder diffraction method (STOE STADI P diffractometer, WinCSD program package). After Rietveld refinement it was deduced that this compound belongs to the CeCu 5 Au type structure (ordered variant of CeCu 6 -type, space group Pnma, a=0.81889(7), b=0.49599(4), c=0.50652(8)   Table 4. The observed, calculated and difference X-ray patterns of HoCu 5 Sn compound are shown in Fig.  3. The interatomic distances in the HoCu 5 Sn structure are close to the sum of the atomic radii of the components.  Crystal structure model of the HoCu 5 Sn compound is shown in Fig. 4. CeCu 5 Au structure type similarly to CeNi 5 Sn in which RNi 5 Sn stannides with light rare earths crystallize [17], is related to CaCu 5 structure. Both structures contain the fragments of CaCu 5 type [33,34]. Package of polyhydra for rare earth atoms in CeCu 5 Au (a) and CeNi 5 Sn (b) structures is shown in Fig. 5. During present work, the crystal structure of Ho 3 Cu 4 Sn 4 stannide was refined by X-ray powder diffraction method (STOE STADI P diffractometer, WinPLOTR program package). Performed calculation confirmed that Ho 3 Cu 4 Sn 4 belongs to the Gd 3 Cu 4 Ge 4 structure type (space group Immm, a = 0.44197(1) nm, b = 0.69065(1) nm, с = 1.45799(3) nm). Refined atomic coordinates and displacement parameters are listed in Table 5. The observed, calculated and difference X-ray patterns of Ho 3 Cu 4 Sn 4 compound are shown in Fig. 6.

Final remarks
Comparing investigated Ho-Cu-Sn and previously studied R-Cu-Sn systems with heavy rare earths, it should be note a close analogy in stoichiometry and crystal structure of the most formed compounds. Similarity in the interaction of the elements in all investigated systems is demonstrated by the formation of the compounds RCuSn, R 3 Cu 4 Sn 4 , R 1.9 Cu 9.2 Sn 2.8 and RCu 5 Sn (except Lu). Crystal structure of the studied HoCu 5 Sn compound is characterized by ordered distribution of the all atoms corresponding to CeCu 5 Autype in comparing to reported previously isotypic compound with Er (CeCu 6 -type), the stoichiometry of which weakly deviates from ErCu 5 Sn to ErCu 4.5 Sn 1.5 [21]. The equiatomic RCuSn compounds exist with all rare earths, but depending on the valence state and size of rare earth element they crystallize in different structure types -LiGaGe-type (or CaIn 2 -type) (Y, La-Sm, Gd-Er, Lu) [35][36][37][38], CeCu 2 -type (Eu) [39], TiNiSi-type (Yb) [40] and ZrBeSi-type (La, Ce) [35,41]. The structure type Sm 2 Cu 4 Sn 5 realizes in the systems with Gd, Tb and Dy. The stannides with MgCu 4 Sn-type are typical for Y-Cu-Sn and Yb-Cu-Sn systems.