On the Synthesis of Co2HfIn and Co2ZrIn Huesler Compounds

Geoffrey Nash1, Philip Nash2 1Chemistry Department, University of Sussex

2Thermal Processing Technology Center, IIT, Chicago, IL 60616, USA

Abstract

Co2HfIn and Co2ZrIn have been predicted as stable Heusler compounds by DFT calculations, but have not been experimentally synthesized. In this work we attempted to synthesize these two compounds using arc melting of elemental powders and from diffusion couples of CoHf-In and Co2Zr-In. The samples were investigated using differential scanning calorimetry, scanning electron microscopy with energy dispersive X-ray spectroscopy, and X-ray powder diffraction. The compounds could not be synthesized contradicting first-principles calculations, which predict the energy of formation of Co2HfIn and Co2ZrIn to be -27.7 kJ/mol and -25.2 kJ/mol respectively. The results of the experiments indicate that neither Huesler phase was present in the samples but rather the ternary alloys consisted of a mixture of CoHf, CoIn, and HfIn, and CoZr, CoIn and ZrIn binary phases.

Introduction

Heusler compounds with the L21 structure (Pearson symbol cF16, space group) are of interest because they can exhibit a number of functional properties, including thermal and magnetic shape memory and magnetocaloric effects. In a recent review of Heusler compounds, Co2HfIn and Co2ZrIn were identified as Heusler compounds that have been predicted by DFT calculations {1} but whose existence had not been experimentally verified {2}. Furthermore there are no ternary phase diagram data available although the following ternary compounds have been reported: Hf8.14 Co7.86In14 {3}, Zr4Co2In5 {4} and Zr9Co7In14 {5}. Based on an examination of the five binary phase diagrams for these two ternary systems it is apparent that the liquid immiscibility between Co and In, could make it very difficult to alloy from the liquid state when trying to synthesize a Heusler alloy composition.

(Add ternary phase diagram from OQMD)Experimental details

Co2HfIn and Co2ZrIn arc melted as-cast/annealed samples

Solid high-purity (~99%) elements of Co, Hf, Zr, and In were prepared with the stoichiometric composition of Heusler alloys (X2YZ) to make 20 g samples. The solid elements were then hermetically sealed in the arc melter and evacuated then backfilled with high purity Ar gas. After melting a Ti getter, both samples were melted attempts to re-melt them were unsuccessful as they would not form a homogeneous alloy. The Indium metal stuck to the copper hearth during arc melting and a small amount was lost in the process.

The experiment was then repeated using solid high-purity powders of the elements Co, Zr, Hf, and In to make a 14 g samples with compositions Co2HfIn and Co2ZrIn. An extra 1 g of In powder was added to the samples to counterbalance the loss of Indium powder in the arc melter. The powders were pressed into pellets and then arc melted under an Ar atmosphere. During this process, however, there was approximately a 1 g loss of In powder for both as indicated by a blue-violet flame appearing while melting the sample.

These samples formed homogeneous ingots that were then cut using a variable speed diamond wheel saw. After metallographic mounting the as-cast Co2HfIn and Co2ZrIn samples in phenolic resin, the samples were ground and then polished using 9, 3, and 1 micron alumina suspensions on a polishing wheel. Part of the remaining ingots were annealed in a furnace at 600°C for 20 days. The final polished and annealed samples were then analyzed using SEM/EDS. The other half of the cut samples were crushed and ground into powder and then analyzed using XRD.

(Need data for Co2ZrIn annealed sample)

CoHf-In and Co2Zr-In diffusion couples

A second synthesis method was used in an attempt to form the Heusler phase by preparing CoHf-In and Co2Zr-In diffusion couples. The binary Cobalt compounds were selected based on their existence in the binary phase diagrams {6 Springer Materials Database}. Solid pieces of Co, Zr, and Hf were prepared and arc melted under Ar atmosphere using the previous procedure to form master alloys CoHf and Co2Zr. The single phase alloys formed were cut with the diamond wheel saw and placed in boron nitride crucibles along with solid In metal. The samples were then placed in a furnace at 600°C for 15 days.

After quenching in water, the samples were mounted in resin and polished using 9, 3, and 1 micron alumina suspensions on a polishing wheel. The samples were then analyzed using SEM/EDS to obtain the composition and to confirm whether the Heusler phases were present.

Results and discussion

Co2HfIn and Co2ZrIn arc melted annealed samples

The arc-melted Co50Hf25In25 sample, annealed at 600ºC for 20 days, formed a homogeneous alloy that was not observed to have a Co2HfIn Heusler phase when analyzed using SEM/EDS. The SEM micrographs show 3 distinctive grey contrast phases (dark, normal, light) in the annealed sample, which

consist of an unknown phase, CoHf phase, Co2Hf phase, and Co8Hf8In14 phase. Composition analysis by EDS shows that the unknown phase, consisting of about 23% Co, 38% In, and 39% Hf (table 1), does not correspond to any ternary phase predicted in the phase diagram from

OQMD {1}. Fig. 1 SEM micrograph #1 of Co2HfIn 600ºC annealed sample, with elemental composition measured using EDS at spectrums 1-9, with mean composition measured with spectrum 10

Fig. 2 SEM micrograph #2 of Co2HfIn 600ºC annealed sample, with elemental composition measured using EDS at spectrums 1-9, with mean composition measured with spectrum 10

Table 1 Results of EDS analysis of the Co2HfIn 600ºC annealed sample in SEM micrograph 1 (fig. 1)Micrograph 1 (Fig 1) At. %Co At. % In At. % Hf Phase

Spectrum 1 22.09 39.78 38.13 ?Spectrum 2 23.46 37.32 39.22 ?Spectrum 3 22.94 37.72 39.34 ?Spectrum 4 46.66 4.62 48.72 CoHfSpectrum 5 48.28 51.72 CoHfSpectrum 6 48.28 51.72 CoHfSpectrum 7 29.20 44.37 26.43 Co8Hf8In14Spectrum 8 46.81 20.65 32.54 ?Spectrum 9 46.09 17.92 35.99 ?Spectrum 10 37.34 18.72 43.94 ?

Micrograph 2 (Fig 2) At. %Co At. % In At. % Hf Phase

Spectrum 1 65.67 34.33 Co2HfSpectrum 2 66.13 33.87 Co2HfSpectrum 3 42.95 20.21 20.97 Co2HfIn (?)Spectrum 4 23.98 38.21 37.81 ?

Spectrum 5 22.69 39.20 38.11 ?Spectrum 6 23.58 38.07 38.35 ?Spectrum 7 21.65 47.18 27.51 Co8Hf8In14Spectrum 8 21.53 47.72 27.45 Co8Hf8In14Spectrum 9 21.58 47.69 27.99 Co8Hf8In14Spectrum 10 38.49 24.49 37.02 ?

The powder XRD analysis of the arc melted Co2HfIn sample, shown in fig. 3, also does not show any conclusive evidence of the presence of the Heusler phase, as the (111), (200) and (220) diffraction peaks were not observed. However, it does show the presence of Co8Hf8In14, Co2Hf, and CoHf phases, which were observed using SEM/EDS.

Fig. 3 XRD Analysis showing the sample’s peak intensities in black and the simulated Heusler phase’s crystal structure peaks in red. Heusler phase peaks are not observed in the sample’s XRD data.

Fig. 4 XRD Analysis showing the sample’s peak intensities in black, Co2Hf peaks in blue and CoHf peaks in dark green.

Fig. 5 XRD analysis showing the sample’s peak intensities in black and the Co8Hf8In14 phase intensity peaks in light green.

CoHf-In and Co2Zr-In diffusion couples

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