Interaction of the components in the Gd-Mn-Sn ternary system at 873 and 673 K

The interaction of the components in the Gd-Mn-Sn ternary system was studied using the methods of X-ray and microstructure analyses, in the whole concentration range. The phase diagrams of the Gd-Mn-Sn system were constructed at 873 and 673 K. At both temperature of investigation the Gd-Mn-Sn system is characterized by existence of two ternary compounds: GdMn6Sn6 (MgFe6Ge6 structure type, space group P6/mmm) and Gd4Mn4Sn7 (Zr4Co4Ge7 structure type, space group I4/mmm). The formation of the interstitial solid solution GdMnхSn2 based on GdSn2 (ZrSi2-type) binary compound was found up to 10 at. % Mn at 873 K and 673 K. The existence of the substitutional solid solution based on GdMn2 (MgCu2-type) was observed up to 5 at.% Sn and 3 at. % Sn at 873 K and 673 K, respectively.


Introduction
Intermetallics of rare earth metals (R) cause the special attention of the scientists as a basis for searching and creation of new perspective magnetic materials. Partially, widely studied the magnetic properties of compounds contained magnetic rare earths and magnetic d-elements (iron, cobalt, nickel). The basic step for searching of new materials is investigation of component interaction in metallic systems which give the possibility to determine the temperature and concentration range of existence of intermediate phases, influence of some factors on their stability, as important characteristics for next study of physical properties. During investigation of the ternary stannides with rare earths and manganese (R = Y, Pr, Nd, Sm, Gd-Lu) the existence of the ternary compounds with stoichiometry 1:6:6 was found and it was established that they crystallize with MgFe 6 Ge 6, HoFe 6 Sn 6, and SmMn 6 Sn 6 structure types [1][2][3][4][5][6][7]. Among RMn 6 Sn 6 stannides the SmMn 6 Sn 6 compound is characterized by three structural modifications with MgFe 6 Ge 6 , YCo 6 Ge 6 and SmMn 6 Sn 6 (disordered variant of HfFe 6 Ge 6 -type) structure types depending on temperature of annealing. Magnetic property measurements of RMn 6 Sn 6 compounds indicated considerable influence of manganese atoms on magnetic behavior of these compounds which order ferro-or ferrimagnetically with temperature of magnetic ordering higher that room temperature (including the compounds with nonmagnetic rare earth metals Y, Lu) [3][4][5][6]8]. In the most R-Mn-Sn systems also the ternary compounds with Zr 4 Co 4 Ge 7 structure type were found [9]. It worth to note that except the compounds with Zr 4 Co 4 Ge 7 and MgFe 6 Ge 6 structure types for stannides with manganese, Tm and Lu the formation of compounds with Hf 3 Cr 2 Si 4type was found [10], and Mg 5 Si 6 -type is realized only for Yb 4 Mn 2 Sn 5 stannide [11].
Taking into account the low melting temperature of tin (505.05 K), the most of the studied R-M-Sn ternary systems (М -d-element) were investigated at 670 K [12]. The higher temperature of annealing used during investigation of some R-{Cu,Ag}-Sn and R-Ni-Sn systems indicated an influence of temperature factor on stability of ternary compounds with high Sn content [13,15]. With the aim to study an influence of annealing temperature on interaction of the components, formation of solid solutions and stability of intermediate phases we studied the Gd-Mn-Sn system at 873 and 673 K. Experimental results of this investigation are given in the presented work.

I. Experimantal
To study the phase equilibria in the Gd-Mn-Sn system 31 ternary and 13 binary samples were prepared by arc melting of the constituent elements (content of the basic component not lower than 99.9 wt. %). The homogenizing annealing of separate particles of the synthesized alloys was performed in the evacuated up to 0.1 Pа quartz tubes for 720 hours at temperatures 873 K and 673 K with subsequently quenching in ice water. Xray phase analysis of the samples was carried out using the powder patterns obtained on DRON-2.0 (FeKα radiation) diffractometer. The observed diffraction intensities were compared with reference powder patterns of binary, known ternary phases and pure elements. The chemical and phase compositions of the obtained samples were examined by Scanning Electron Microscopy (SEM) using REMMA-102-02 scanning electron microscope. The data for the crystal structure refinements were collected at room temperature using STOE STADI P diffractometer (graphite monochromator, CuK α1 radiation, 20 -100° 2 θ range with scanning step 0.02°). Calculations of the crystallographic parameters and theoretical patterns were performed using the WinPLOTR [16] program packages.

II. Results and discussion
For construction of the phase diagrams of Gd-Mn-Sn ternary system at 873 K and 673 K we prepared and analyzed 31 ternary and 13 binary alloys. After annealing all synthesized samples were examined by X-ray phase and microstructure analyses. The isothermal sections of the Gd-Mn-Sn system at 873 K and 673 K are presented in Figs. 1, 2, respectively. In the Mn-Sn and Gd-Mn binary systems the presence of the all binary compounds corresponding to the reference data [17][18][19][20] was confirmed at both temperature of investigation. At 873 K only two compounds -Mn 3 Sn, Mn 2 Sn, were formed in Mn-Sn system that corresponds to the reported phase diagram according to which MnSn 2 binary exists up to ~820 K. According to EDX data the homogeneity range of Mn 2 Sn compound is limited by Mn 67.81 Sn 32.11 and Mn 63.87 Sn 36.73 compositions. GdMn 2 binary at stoichiometric composition crystallizes in MgCu 2 structure type. To check the formation of GdMn 2 binary with MgZn 2 -type [21] in course of our investigation two alloys with Gd 37 Mn 63 and Gd 40 Mn 60 compositions were prepared and annealed at 673 and 873 K. Performed phase analysis of the Gd 37 Mn 63 and Gd 40 Mn 60 samples indicated the presence of the main cubic phase GdMn 2 with MgCu 2 -type and Gd. In the Gd-Sn binary system at both temperatures of annealing the existence of binaries Gd 5 Sn 3 (Mn 5 Si 3 -type), Gd 5 Sn 4 (Sm 5 Ge 4 -type), Gd 11 Sn 10 (Ho 11 Ge 10 -type), GdSn 2 (ZrSi 2 -type), Gd 3 Sn 7 (Gd 3 Sn 7type) and GdSn 3 was confirmed. GdSn 3 binary is characterized by polymorphic transformation at ~ 665 K, thus at 873 K belongs to Cu 3 Au structure type [19], and at 673 K is characterized by orthorhombic structure of GdSn 2.75 -type [20].
A formation of interstitial solid solutions based on the RSn 2 (R-rare earths of Yttrium group) series of binary compounds with ZrSi 2 structure was studied and reported in Ref. [22]. During investigation of Gd-Mn-Sn system the existence of interstitial solid solution GdMn х Sn 2 based on the GdSn 2 binary (ZrSi 2 -type) was observed. The solubility of Mn atoms was found to be up to 10 at. % (GdMn 0.33 Sn 2 ) at both temperatures 873 K and 673 K. The lattice parameters change from a = 0.4431(2), b = 1.6408(8), с = 0.4325(4) nm (for   homogeneity ranges at investigated temperatures. Crystallographic characteristics of ternary compounds are given in Table 1. The SEM pictures and phase compositions of some alloys are shown in Figs. 3, 4. The existence of the GdMn 6 Sn 6 compound with MgFe 6 Ge 6 structure type and its lattice parameters were reported earlier [23]. During present work, the crystal structure of this stannide was refined by X-ray powder diffraction method. Performed structure refinement confirmed that GdMn 6 Sn 6 belongs to MgFe 6 Ge 6 structure type (space group P6/mmm, a = 0.55369(2) nm, с = 0.90270(4) nm, R p = 0.0378, R wp = 0.0484, R Brag = 0.0455). Refined atomic parameters are listed in Table 2. The observed, calculated and difference X-ray patterns of the GdMn 6 Sn 6 compound are shown in Fig. 5.
Comparing the investigated in our work Gd-Mn-Sn  system with studied earlier {Y, Ce, Dy}-Mn-Sn and known in the literature ternary compounds [12] we may note that the R-Mn-Sn ternary systems contain a relatively small number of ternary phases. Series of stannides with CeNiSi 2 and Gd 3 Cu 4 Ge 4 structure types are formed only in the systems with rare earths of Cerium group. R-Mn-Sn systems here R is a rare earth element of Yttrium group are characterized by formation of the ternary compounds with Zr 4 Co 4 Ge 7 -type and MgFe 6 Ge 6 structure types, which are realized in studied Gd-Mn-Sn system. The structural analysis of ternary compounds formed in the Gd-Mn-Sn system with regard on the coordination polyhydra for smallest and bigger atoms showed that they are related to corresponding structures of the binary compounds (Fig. 6). The structure of Gd 4 Mn 4 Sn 7 is a derivative from the Ho 11 Ge 10 structure type in which crystallizes Gd 11 Sn 10 binary compound. Main structural fragment is octahedron with additional atoms formed by Sn atoms around Gd atoms. Structure of GdMn 6 Sn 6 compound is derivative from the structure of GdMn 12 binary [24] which contains fragments of hexagonal CaCu 5 -type. Main structural fragment is hexagonal prism with six additional atoms formed by Sn atoms around Gd atoms and its deformed derivatives.

Conclusions
Performed in our work investigation of component interaction in the Gd-Mn-Sn system at 673 and 873 K indicated the stability of GdMn 6 Sn 6 and Gd 4 Mn 4 Sn 7 formed ternary compounds at both temperature of investigation. An influence of the temperature of annealing takes place in character of the phase equilibria in the part of Gd-Mn-Sn system closed to Mn-Sn binary system, above 50 аt. % Sn. It is caused by increasing of the liquidus range with increasing of temperature up to 873 K and subsequently absence of MnSn 2 binary. Increasing of annealing temperature from 673 tо 873 K results in increasing of solubility of Sn in the GdMn 2 binary (from 3 to 5 at. %).