Multicomponent Phases with CeAl2Ga2- and Y0.5Co3Ge3-Type Structures in the Gd–Ca–Fe–Co–Ge System

New quinary phases with the CeAl2Ga2 (tI10, I4/mmm) and Y0.5Co3Ge3 (hP8-2, P6/mmm) structure types were found at 500 oC in the Gd–Ca–Fe–Co–Ge system based on X-ray powder diffraction data. They are Gd1-xCaxFe2-yCoyGe2 (x = 0.085(7)-0.551(6), y = 0.25-0.75, a = 3.99468(6)-4.00003(8), c = 10.1279(2)10.3981(5) Å) and Ca0.5-xGdxFe3-yCoyGe3 (x = 0.031(1)-0.314(8), y = 0.75-2.25, a = 5.1081(1)-5.1218(1), c = 3.9751(1)-4.0451(2) Å). The c-parameter of the tetragonal CeAl2Ga2-type (122) phase cell depends much more on the Fe/Co and Gd/Ca ratios, than the a-parameter (which remains nearly the same). The volume of the 122 cell increases with increasing Fe and Ca content. The c-parameter of the hexagonal cell of the Y0.5Co3Ge3-type (0.533) phase also depends more strongly on the Fe/Co content than the a-parameter, but Gd/Ca substitutions have little effect on the cell parameters. The following new quaternary and ternary phases were also discovered: GdFe2-yCoyGe2 (y = 0.5-1.5, a = 3.99419(5)-3.99750(7), c = 10.3271(2)-10.1173(3) Å) with CeAl2Ga2-type structure and Gd0.5Fe3-yCoyGe3 (y = 0.75-1.5, a = 5.1247(8)-5.1225(7), c = 4.052(1)-4.010(1) Å), Ca0.5Fe3-yCoyGe3 (y = 0.75-2.25, a = 5.1153(2)-5.1066(2), c = 4.0451(2)-3.9839(3) Å), Ca0.5Fe3Ge3 (a = 5.10167(9), c = 4.06565(7) Å), and Ca0.5Co3Ge3 (a = 5.0899(2), c = 3.9199(1) Å) with Y0.5Co3Ge3-type structure. The latter two phases, together with the already known compounds Gd0.5Fe3Ge3 and Gd0.5Co3Ge3, are the parent compounds for the probably complete solid solution Ca0.5-xGdxFe3-yCoyGe3, just as the corresponding ternary compounds (except in the Ca–Fe–Ge system) with CeAl2Ga2-type structures open access to the Gd1-xCaxFe2-yCoyGe2 solid solution.


Introduction
The discovery of superconductivity in Ba 0.6 K 0.4 Fe 2 As 2 [1] has drawn the attention to compounds crystallizing with the CeAl 2 Ga 2 (122) structure type (Pearson symbol tI10, space group I4/mmm) [1]. Some 700 compounds with 122-type structure are known in different R-T-X (A = alkaline-earth, rare-earth metal, T = transition metal, X = element of the main group) systems [2], leading to a large number of substitution possibilities.
The aim of this work was to search for new multicomponent phases based on Gd, Ca, Fe, Co, and Ge, that adopt the CeAl 2 Ga 2 (122) structure type.

I. Experiment
Starting materials for the synthesis were ingots of gadolinium, calcium, iron, cobalt, and germanium with purities better than 99.85 %. Quinary alloys with a mass of 0.5 g were synthesized in an arc furnace with a copper water-cooled hearth, using a tungsten electrode under argon atmosphere. The alloys were homogenized in evacuated quartz ampoules at 500ºC for 1440 h in a Vulcan A-550 furnace with an automatic temperature control of ± 1 -2 °C. The annealed alloys were quenched in cold water without breaking the ampoules. X-ray phase and structural analyses were performed using diffraction data obtained from DRON-2.0M and DRON-4.07 powder diffractometers (Fe Kα radiation). For the indexation of the experimental diffraction patterns, theoretical patterns were calculated using the program POWDER CELL-2.4 [3] and the databases TYPIX [4] and PEARSON'S CRYSTAL DATA [2]. Crystal structure refinements by the Rietveld method were performed using the FullProf program [5].

II. Results
At the first stage of the investigation, the crystal structures of the five-component phases Gd 1-x Ca x FeCoGe 2 and Ca 0.5-y Gd y Fe 1.5 Co 1.5 Ge 3 were refined [6] on X-ray powder diffraction data (Figs 1 and 2) from an alloy of composition Gd 1.5 Ca 0.5 FeCoGe 2 (homogenized at 500ºС for two months). The unit-cell parameters of the phase Gd 1-x Ca x FeCoGe 2 (structure type CeAl 2 Ga 2 (122), tI10, I4/mmm, a = 4.00126(9), c = 10.1922(3) Å, x = 0.152 (8)) are of the same magnitude as those of the isotypic ternary compounds GdFe 2 Ge 2 (a = 3.9867, c = 10.4798 Å) [7,8] and GdCo 2 Ge 2 (a = 3.996, c = 10.066 Å) [8]. Refinement of the structure of the phase Ca 0.5-x Gd x Fe 1.5 Co 1.5 Ge 3 (structure type Y 0.5 Co 3 Ge 3 (0.533), hP8-2, P6/mmm, a = 5.1154(2), c = 4.0142(3) Å, x = 0.045 (6)) showed mixed occupation Ca/Gd of site 1a (45.5/4.5 %), while a refinement on diffraction data from an as-cast alloy revealed occupation of site 1a by Ca Because of the closeness of the atomic scattering factors of Fe and Co, their content ratio cannot be accurately refined from X-ray diffraction data and was constrained in this and the following refinements to its value in the nominal composition of the alloy. Relevant crystallographic parameters of the refined structures are listed in Table 1 and Table 2. Models of the 122 and 0.533 structures are presented in Fig. 3. The result, indicating the coexistence of two phases (122 and 0.533) in the alloy with 122 overall composition, motivated more detailed investigations, and additional alloys were synthesized with the compositions given in Table 3.
The diffraction patterns of all of the samples contained 122 and 0.533 phases and small amounts (less than 5 %) of additional ternary and binary phases (among Table 1 Crystallographic parameters of the Gd 1-x Ca x FeCoGe 2 and Ca 0.5-x Gd x Fe 1.5 Co 1.5 Ge 3 phases (homogenized alloy) (8) The calculation of the composition of the alloys for each case is shown in Table 3.

III. Discussion
The refinements carried out on the samples listed in Table 3, showed that the alloy compositions are located in a concentration region between the 122 and 0.533 phases (Fig. 4), obviously because of losses of Ca during arc-melting (unfortunately the weight losses were always in the range 3-5 %).
Considering the values of the cell parameters of the 122 and 0.533 phases (see Table 3), the following conclusions can be drawn: the c-parameter of the tetragonal 122 cell depends more on the Fe/Co and Gd/Ca ratios than the a-parameter (the latter remaining nearly the same). The volume of the 122 cell increases with increasing Fe and Ca content. The c-parameter of the hexagonal 0.533 cell is also more dependent on the Fe/Co content than the a-parameter, but Gd/Ca substitutions have no strong effect on the cell parameters. Contrary to what was observed for the 122 phase, the cell volume of the 0.533 phase increases with decreasing Ca content. The results are shown in Figs 5 and 6, which also take into consideration information (Table 4) about quaternary and ternary phases obtained for other alloys: GdFe 1.5 Co 0.5 Ge 2 (122 phase), GdFeCoGe 2 (122), GdFe 0.5 Co 1.5 Ge 2 (122), CaCo 2 Ge 2 (122), Gd 0.5 Fe 2.25 Co 0.75 Ge 3 (0.533), Gd 0.5 Fe 1.5 Co 1.5 Ge 3 (0.533), Ca 0.5 Co 3 Ge 3 (0.533) [9], Ca 0.5 Fe 3 Ge 3 (0.533), and from [7,8] -GdFe 2 Ge 2 (122), Gd 0.5 Fe 3 Ge 3 (0.533), and GdCo 2 Ge 2 (122), and Gd 0.5 Co 3 Ge 3 (0.533) from [10].

Conclusions
The new quinary phases Gd 1-x Ca x Fe 2-y Co y Ge 2 (x = 0.085 (7) , with Y 0.5 Co 3 Ge 3 -type structure (hP8-2, P 6/mmm), were found at 500ºC in the Gd-Ca-Fe-Co-Ge system. The regions of solid solutions indicated above were deduced from structural refinements of various samples, however, the existence of complete solid solutions based on the ternary compounds cannot be ruled out. The new quaternary 122 phase GdFe 2-y Co y Ge 2 (y = 0.5-1.5, a = 3.99419 (5) (9), c = 4.06565(7) Å), and Ca 0.5 Co 3 Ge 3 (a = 5.0899(2), c = 3.9199(1) Å), were also found during the investigation. The observations raise the question concerning the possible existence of complete multicomponent solid solutions Gd 1-x Ca x Fe 2-y Co y Ge 2 and Ca 0.5-x Gd x Fe 3-y Co y Ge 3 between the boundary ternary compounds.