Standard enthalpies of formation of selected XYZ half-Heusler compounds

Accepted Manuscript Standard enthalpies of formation of selected XYZ half-Heusler compounds Ming Yin, Philip Nash PII: DOI: Reference:

S0021-9614(15)00240-2 http://dx.doi.org/10.1016/j.jct.2015.07.016 YJCHT 4313

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J. Chem. Thermodynamics

Received Date: Revised Date: Accepted Date:

26 February 2015 4 July 2015 15 July 2015

Please cite this article as: M. Yin, P. Nash, Standard enthalpies of formation of selected XYZ half-Heusler compounds, J. Chem. Thermodynamics (2015), doi: http://dx.doi.org/10.1016/j.jct.2015.07.016

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Standard enthalpies of formation of selected XYZ half-Heusler compounds Ming Yin*, Philip Nash Thermal Processing Technology Center, Illinois Institute of Technology (IIT), 10 West 32nd Street, Chicago, IL 60616, USA Abstract The standard enthalpies of formation of selected ternary half-Heusler type compositions XYZ (X = Au, Co, Fe, Ir, Ni, Pd, Pt, Rh, Ru; Y = Hf, Mn, Ti, Zr; Z = Ga, Sn) were measured using high temperature direct reaction calorimetry. The measured standard enthalpies of formation (in kJ/mole of atoms) of the half-Heusler compounds (prototype MgAgAs, Pearson symbol cF12, space group F-43m) are, IrMnSn (-29.4 ± 1.8); NiTiSn (-52.6 ± 2.4); PtHfSn (-98.8 ± 3.4); PtMnSn (-55.8 ± 2.6); PtTiSn (-93.6 ± 3.3); PtZrSn (-104.9 ± 3.8); for the B2 compound (prototype CsCl, Pearson symbol cP2, space group Pm-3m), RuMnGa (-26.9 ± 1.7); for the C1 structured (prototype CaF2, Pearson symbol cF12, space group Pm-3m) or the C1b structured compound IrMnGa (-40.9 ± 1.7). Indicative standard enthalpies of formation of the following compounds were obtained, half-Heusler compounds AuMnSn, CoTiSn, IrZrSn, NiHfSn, NiZrSn, PdHfSn, PdZrSn, RhTiSn; Heusler compound (prototype Cu2MnAl, Pearson symbol cF16, space group Fm-3m) RhMnSn; hexagonal compound (prototype BeZrSi, Pearson symbol hP6, space group P63/mmc) PtMnGa and another type of hexagonal compound (prototype RhHfSn, Pearson symbol hP18, space group P-62c) RhHfSn, IrZrsn, RhZrSn. Values were compared with ab initio calculations from AFLOW and OQMD. Lattice parameters of these compounds were determined using X-ray diffraction (XRD) analysis. Microstructures were characterized using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). Selected alloys were further annealed to investigate phase transformations and phase relationships. Keywords: Half-Heusler compound; Standard enthalpy of formation; Calorimetry *Corresponding author:

Ming Yin Phone: 1+(312) 567 3203 fax: 1+(312) 567 8875 e-mail: [email protected]

1.

Introduction

Half-Heusler compounds XYZ have a C1b structure (prototype AgAsMg, space group F-43m, Pearson symbol cF12) with three Wyckoff positions, 4a (0, 0, 0), 4b (1/2, 1/2, 1/2) and 4c (1/4, 1/4, 1/4) as shown in Fig. 1(a). X is usually a late transition element and preferentially occupies the 4c site. Y is an early transition element while Z is in group III-V in the periodic table and they occupy the 4a and 4b sites which are equivalent. The C1b half-Heusler structure can be derived from the C1 structure (prototype CaF2, space group Fm-3m, Pearson symbol cF12) and it is also closely related to the X2YZ L21 Heusler structure (prototype Cu 2MnAl, space group Fm-3m, Pearson symbol cF16) since it can be obtained when 4 X atoms in X2YZ are substituted by vacancies, as shown in Fig. 1 (c).

When investigating the half-Heusler structure, several other structures at the equiatomic composition are often observed. PtMnGa compound is reported to have a hexagonal structure (prototype BeZrSi, Pearson symbol hP6, space group P63/mmc). Another hexagonal structure (prototype HfRhSn, Pearson symbol hP18, space group P-62c) is also encountered, such as RhHfSn and RhZrSn, which is an ordered structure of AlNiZr (prototype AlNiZr, Pearson symbol hP9, space group P-62m) and they both belong to the Fe2P family (prototype Fe2P, Pearson symbol hP9, space group P-62m).

The open structure bestows the half-Heusler compounds with many fascinating properties. A large polar magneto-optic Kerr effect at room temperature was observed in PtMnSb by van Engen et al [1], which is important for optical read-out of magnetically stored information in erasable videos and audio discs. NiXSn (X = Ti, Zr, Hf) compounds are well known as excellent n-type thermoelectric materials composed of nontoxic elements with the dimensionless figure of merit ZT up to 1.5, which can be used at around 1000 K to directly convert heat into electrical power [2]. With a large composition space to explore for potential applications, it is important to have a

thorough understanding of the phase equilibria and thermodynamics in the alloy systems in which these compounds are found.

In this work, selected half-Heusler composition samples were prepared using powder metallurgy in a high temperature direct reaction calorimeter to verify their stability and obtain the standard enthalpies of formation for use in future alloy design programs.

2. Materials and methods All elemental materials were purchased from Alfa Aesar® except Au which is from Cerac Inc.. Purity and size of the powders are listed in Table 1. Ga and Zr powders were filed from ingots and the Zr powders were sieved (< 149 µm). Co, Fe and Ni powders were reduced in hydrogen at 873 K for half an hour to remove the surface oxide and cooled in the furnace. Then the reduced powders were ground and sieved (<149 µm).

The detailed experimental procedure for measuring the standard enthalpy of formation using the high temperature direct reaction calorimeter (also known as the Kleppa calorimeter) was described previously [3, 4]. NIST SRM 720 sapphire was used for calibration of the enthalpy value. The temperature of the calorimeter was maintained at 1373 K with a flowing argon atmosphere purified using a Titanium gettering furnace. Stoichiometric amounts of elemental powders were mixed together and compressed to make 7 pellets which were then individually dropped into a boron nitride crucible in the calorimeter to measure the heat of reaction (∆Hr), as illustrated by Eqn. (1). X (298 K) + Y (298 K) + Sn (298 K) = XYSn (1373 K)

∆Hr

(1)

The reaction time was around 20 min for each sample to reach thermal equilibrium. The obtained samples were cooled to room temperature and dropped into the calorimeter again to measure the heat content (H1373- H298), as illustrated by Eqn. (2). XYSn (298 K) = XYSn (1373 K)

H1373- H298

(2)

The standard enthalpy of formation (∆fH°)

is obtained from the heat of reaction minus the heat

content, (1)-(2). X (298 K) + Y (298 K) + Sn (298 K) = XYSn (298 K)

∆fH°

(3)

The weight loss for each sample was less than 2 %. After the heat of reaction measurement, if a single half-Heusler phase was not obtained, the samples were sealed in quartz tubes under vacuum after flushing with argon and further annealed in a furnace. Once a reasonable amount (>90 vol. %) of half-Heusler phase was obtained, the samples were used to measure the heat content. Microstructures of the reacted samples, annealed samples and samples after the heat content measurement were examined after standard metallographic preparation using scanning electron microscopy (JEOL, JSM-5900LV) with an energy dispersive spectrometer (EDS) to determine the impurity phases as well as to verify if the two measurements, heat of reaction and heat content, have the same phases at high temperature which can be inferred if the room temperature structures are the same. The overall composition was determined at a minimum magnification of 90× and a maximum area measurement. The estimated instrument error is 5 atm. %. XRD analysis (Bruker, D2 PHASER) was used to determine lattice parameters and identify additional phases as well as their relative amounts through fitting the experimental intensity using the simulated ones from commercial software CrystalMaker® and CrystalDiffract®. The difference of XRD patterns between the L21 Heusler structure and the C1b half-Heusler structure are shown in Fig. 2 using Ni2TiSn and NiTiSn for illustration. The experimental XRD pattern of Ni2TiSn (a = 0.6091 nm) is from the author’s unpublished result while that of NiTiSn (a = 0.5946 nm) is from this work. In the C1b structure, the intensity ratios of (111) and (200) to the (220) peak are bigger than those in the corresponding Heusler structure.

Melting points were determined using a Setsys 1700 DSC with a heating rate of 10 K/min in argon connected with the atmosphere (1 atm.). NIST SRM 720 sapphire was used as the reference material. Heat contents at 1373 K for each compound were calculated using the empirical

Neumann-Kopp rule [5]. The elemental data are tabulated in Dinsdale [6].

3. Results In the following text, phase compositions, in atomic percentage, measured by EDS are provided in parentheses after the phase designation and the amount of impurity phases are presented in volume fraction.

a) XHfSn XHfSn (X = Ni, Pd, Pt, Rh) compounds were studied. The experimental XRD results are shown in Fig. 6. 0.05 Ni2HfSn and 0.01 Hf5Sn4 were observed in the NiHfSn alloy. 0.04 Hf5Sn4 and 0.04 Pd2HfSn were found in the PdHfSn alloy. A small amount of unknown impurity (Pt41Hf42Sn17) and 0.03 HfO2 were found in PtHfSn. Due to the very strong diffraction ability, small peaks caused by Kβ X-rays were observed. The same phenomenon was observed in other Pt containing half-Heusler compounds, PtMnSn, PtTiSn and PtZrSn. RhHfSn (Rh33Hf35Sn32) crystallizes in a hexagonal structure (prototype HfRhSn, Pearson symbol hP18, space group P-62c) which is the same as that found by Zumdick and Pottgen [7] with around 0.10 of unknown compound (composition Rh41 Hf30Sn29) and 0.01 Sn in the Rh33.3Hf33.3Sn33.3 alloy.

b) XMnSn XMnSn (X = Au, Ir, Pd, Pt, Rh, Ru) were investigated. Three phases, AuMnSn (Au35Mn31Sn34), AuMn (Au50Mn46Sn4) and Sn were observed in the as cooled structure after the heat of reaction measurement. The AuMnSn phase seems to result from a peritectic transformation involving the primary phase AuMn.

After annealing at 673 K for 50 days, the amount of AuMn and Sn phases was reduced greatly but reappeared after re-dropping into the calorimeter for the heat content measurement, as shown in Fig.

7.

The result indicates that the half-Heusler compound AuMnSn decomposed at high temperature which is consistent with the peritectic temperature (AuMnSn = AuMn + L (Sn), 743 K) measured by L. Offernes et al [8]. On solidification, the peritectic reaction does not go to completion, resulting in a significant amount of AuMn and Sn. For the compound IrMnSn, a minor amount of Sn was found. The Heusler compound Pd2MnSn (Pd43Mn31Sn26) was found coexisting with MnSn2 (Pd 18Mn18Sn64) and a minor amount of Sn in the Pd 33.3Mn33.3Sn33.3 alloy. The microstructure did not change after annealing at 673 K for 50 days or 1073 K for 14 days. PtMnSn alloy is a single phase half-Heusler structure. Impurity phases Rh3Sn2 (Rh27Mn35Sn38) and β-RhSn2 (Rh24Mn14Sn62) were observed in Rh33.3Mn33.3Sn33.3 alloys with the Heusler compound (Rh42Mn30Sn28). After annealing at 1073 K for 5 days or 973 K for 30 days, the observed phases did not change. Ru33.3Mn33.3Sn33.3 is composed of Ru2MnSn (Ru42Mn31Sn27), RuSn2 (Ru22Mn13Sn65) and Mn2Sn (Ru25Mn36Sn39). The backscattered electron micrographs of Pd33.3Mn33.3Sn33.3, Rh33.3Mn33.3Sn33.3 and Ru33.3Mn33.3Sn33.3 alloys are shown in Fig. 8. The primary phase in all three alloys appears to be the Heusler composition X2MnSn.

c). XTiSn Alloys of the type XTiSn (X = Co, Ir, Ni, Pd, Pt, Rh, Ru) were investigated. Relatively large amounts of impurity Sn5Ti6 (Co 3Sn44Ti53) and Sn were found with CoTiSn (Co 43Ti28Sn29) and this phase relationship is in agreement with the high temperature phase diagram (1070 K) provided by Yin et al [9]. The amount of Sn5Ti6 and Sn reduced after annealing at 973 K for 30 days. The annealed samples were used for the heat content measurement.

IrSn2 (Ir34Ti12Sn54) and γ-IrTi (Ir54Ti38Sn8) were found in Ir33.3Ti33.3Sn33.3, together with a small amount of Ir3Sn7 (Ir20Ti8Sn72). Annealing at 1073 K for 5 days or 973 K for 30 days did not change

the structure. The prototype of IrSn2 is CaF2 (space group Fm-3m, Pearson symbol cF12) which has a very similar XRD pattern with the half-Heusler compound. This may explain why Stadnyk et al [10] believed they obtained a single half-Heusler phase of ‘IrTiSn’. In addition to the NiTiSn halfHeusler phase, 0.07 Sn, 0.04 Sn5Ti6 (Ni4Sn43Ti53) and a minor amount of Ni2TiSn was found. After annealing at 1073 K for 12 days, only 0.02 Sn5Ti6 and 0.02 Ni2TiSn was observed. The annealed samples were used for the heat content measurement. The first principles calculated enthalpy of formation of NiTiSn by Colinet et al [11] was -53.0 kJ/mole of atoms, which was greatly affected by the choice of atomic occupation. No C1b-structured compound was observed in Pd33.3Ti33.3Sn33.3. Instead, Pd 2TiSn (Pd50Ti25Sn25) was found with PdSn (Pd 50Sn50) and an unknown compound (Pd 22Ti35Sn43). 0.03 PtSn (Pt52Sn48) was found with the PtTiSn half-Heusler phase. There was 0.04 RhSn2 and 0.03 Sn in Rh33.3Ti33.3Sn33.3 besides the half-Heusler phase (Rh39Ti31Sn30). Ru33.3Ti33.3Sn33.3 alloy was composed of RuTi with a B2 structure (Ru50Ti35Sn15), Sn and Sn5Ti6 (Ru1Sn46Ti53).

c) XZrSn XZrSn (X = Ir, Ni, Pd, Pt, Rh) was investigated. Similar to RhHfSn, IrZrSn has an ordered Fe2P structure. 0.04 Ir2Zr ((Ir63Zr31Sn6) and 0.03 Ir3Zr5 (Ir44Zr54Sn2) were also observed. 0.03 Sn3Zr5 (Ni9Zr56Sn35) and 0.07 Ni2SnZr2 (Ni36Sn24Zr40) were found in NiZrSn. In the Pd33.3Zr33.3Sn33.3 alloy, 0.06 Pd2ZrSn coexisted with PdZrSn. 0.03 PtSn (Pt52Zr48) was observed together with PtZrSn. Rh33.3Zr33.3Sn33.3 is composed of two phases, RhZrSn with an ordered Fe2P structure and 0.08 of Sn3Zr5 (Rh13Zr56Sn31).

d) XVSn XVSn (X = Co, Fe) was investigated. Co 2VSn (Co40V40Sn20) was found together with CoSn2 (Co44V4Sn52), Sn and a minor amount of possibly unreacted V in the Co 33.3V33.3Sn33.3 alloys. The samples were annealed at different temperatures, 673 K, 823 K, 973 K and 1073 K. However, half-

Heusler-structured phase was not found. Fe2VSn (Fe43V45Sn12), Fe3Sn2 (Fe36V22Sn42), Sn and a minor amount of unreacted V were found in the reacted Fe33.3V33.3Sn33.3 alloys, while Fe2VSn (Fe48V38Sn16) was found with Fe3Sn2 (Fe31V27Sn42) and Sn3V2 (Fe5V30Sn65) after annealing at 823 K for 50 days or 973 K for 30 days. Lue et al [12] also encountered serious segregation problems when producing the half-Heusler compounds CoVSn and FeVSn, but they succeeded using an RF induction furnace.

e) Ga-based systems IrMnGa, PtMnGa, RhTiGa and RuMnGa alloys were studied. The measured XRD pattern of the Ir33.3Mn33.3Ga33.3 alloy matches the simulated C1 structure or C1b structure in regards of the position of the peaks but not the relative intensity. Further investigation is needed to determine the crystal structure. PtMnGa was a hexagonal compound (prototype BeZrSi, space group P63/mmc, Pearson symbol hP6,) with 0.05 Ga3Pt2 (Pt41Mn9Ga50). RhTi (Rh49Ti27Ga24) of a tP2 structure was found with the intermetallic GaTi3 (Rh26Ti38Ga36) (prototype Al3Ti, space group I4/mmm, Pearson symbol tI8) in the Rh33.3Ti33.3Ga33.3 alloy. The microstructure did not change after annealing at 1073 K for 12 days or 873 K for 28 days. Dwight [13] reported synthesizing RhTiGa half-Heusler alloy from XRD but no additional characterization evidence was provided. 0.10 Ga2Ru was observed in the reacted Ru33.3Mn33.3Ga33.3 alloy which disappeared after annealing at 1073 K for 5 days leaving a single phase of either a B2 or an L21 structure.

The measured lattice parameters of the reacted samples are compiled in Table 2, together with data from the literature [8, 13-28]. The measured overall compositions and Chemical Abstracts Service (CAS) registry numbers for each compound are provided as well.

The measured standard enthalpies of formation are listed in Table 3. Data from at initio calculations from the AFLOW [29] and the OQMD [30] database are included for comparison. The identified

structure and relative amount of additional phases before the heat content measurement are also given.

Table 4 summarizes the measured heats of reaction and heat contents, together with the values calculated from the Neumann-Kopp rule.

The information of the phases presented in the multiphase alloys is summarized in Table 5.

4. Discussion The measured lattice parameters are consistent with the literature values. The differences are within experimental error and can be explained by the variation of composition.

The AFLOW database provides enthalpies of formation from ab initio calculations of all three configurations of the C1 b half-Heusler structure, and the most negative one is included in Table 3 for comparison with the experimental value. So far, calculated enthalpies of formation of other structures at the equiatomic composition such as the hexagonal structure are not available from the AFLOW. Thus, all data from the AFLOW in Table 3 correspond to the half-Heusler structure even for PtMnGa etc with a hexagonal structure. The OQMD does not include all three configurations of the half-Heusler structure and this is why the enthalpy of formation is not reported in Table 3 in some cases. However, enthalpy of formation of hexagonal structure is provided for those with reported experimental information such as RhHfSn. Therefore, the data from OQMD correspond to the structure listed in Table 3. The two databases are complementary to each other in some sense. It can be seen from Table 3 that when the experimentally determined samples do not have the halfHeusler structure, such as IrZrSn and RhMnSn, the measured standard enthalpy of formations are more negative when compared with that of the half-Heusler structure from the AFLOW, as one would expect. Also, the OQMD has a reasonable prediction of the equiatomic composition with a

hexagonal structure, such as RhHfSn and RhZrSn. For the half-Heusler structured compounds, it is obvious that when proper configuration is adopted, the ab initio calculations have very good predictions except for the Mn containing compounds, which are ferromagnetic in most of the cases, as shown in Fig. 3. Also, the reason for the difference could be that some of the compounds are not stable at the stoichiometric composition, as for example observed by Masumoto et al [31] who could only obtain a single half-Heusler phase at the off-stoichiometric composition Ir1.07Mn1.07Sn0.86. The other explanation could be that defects, such as substitutional atoms or vacancies, are not considered in the first principles calculations. The presence of impurity phases will also affect the experiment result. No obvious relationship is observed between the enthalpy values of half-Heusler compounds and their Heusler analogues.

It could also be seen from Table 3 that the standard enthalpy of formation becomes more negative as X element goes from period 4 to 6, such as Ni, Pd and Pt or when the amount of valence electrons increases such as from Co to Ni. For the Y element, Zr containing half-Heusler compounds have the most negative standard enthalpy of formation. Those containing Hf are more negative than their Ti analogues as shown in Fig. 4 while the Mn containing half-Heusler compounds have the least negative standard enthalpy of formation.

Basically, the Neumann-Kopp rule can predict reasonably well the heat content of the samples which do not melt at the Kleppa calorimeter temperature (1373 K). The difference is in the range of 5 kJ/mole of atoms. But it is noticeable that when the samples contain the element Zr or Hf, the measured heat contents are often smaller than the predicted ones as shown in Fig. 5. The reason for this phenomenon is not known but is also observed in the Heusler compound systems [32].

4. Conclusions A series of Ga or Sn containing half-Heusler type compositions were investigated. Standard

enthalpies of formation were obtained using high temperature direct reaction calorimetry and lattice parameters of 28 compositions were measured. The half-Heusler structure was identified in NiHfSn, PdHfSn, PtHfSn, AuMnSn, IrMnSn, PtMnSn, CoTiSn, NiTiSn, PtTiSn, RhTiSn, NiZrSn, PdZrSn and PtZrSn compounds. RhMnSn has a Heusler structure and RuMnGa has a B2 structure. IrMnGa has either a C1 or a C1 b structure. PtMnGa, RhHfSn, IrZrSn and RhZrSn have a hexagonal structure. No half-Heusler structure was observed in Co33.3V33.3Sn33.3, Fe33.3V33.3Sn33.3, Ir33.3Ti33.3Sn33.3, Pd33.3Mn33.3Sn33.3, Pd 33.3Ti33.3Sn33.3, Rh33.3Ti33.3Ga33.3, Ru33.3Mn33.3Sn33.3 and Ru33.3Ti33.3Sn33.3 alloys.

Acknowledgements This research is supported by NSF Grant #DMR1307631.

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Table 1. Purity and particle size of the elemental powders used in this work. Element Al Au Co Fe Ga Hf Ir Mn Ni Pd Pt Rh Ru Sn Ti V Zr

Purity, (wt. %) 99.97 99.95 99.8 99.9+ 99.99 99.6 99.9 99.95 99.9+ 99.9 99.9+ 99.95 99.95 99.999 99.9 99.5 99

Particle size, (µm) 44 44 1.6 10 Filed from ingot 44 44 44 30 <1 44 44 44 44 149 44 Filed from bulk and sieved, <149

Table 2. Measured overall compositions and lattice parameters (a) of the reacted samples of XYZ compounds at room temperature. a/nm This work a Literature AuMnSn 211101-71-2 C1b 0.6332 0.6338 [8] CoTiSn 105110-44-9 C1b 0.6012 0.5997 [14], 6.003 [15] IrMnGa 12592-60-8 C1 or C1 b 0.6029 0.602 [16], 0.60236 [17] IrMnSn 958458-67-8 C1b 0.6231 0.62015 [18], 0.6199 [19] a 0.7329 IrZrSn 117728-15-1 hP18 Ir29.3Zr38.3Sn32.4 c 0.7318 a 0.7321, c 0.7322 [7] NiHfSn 12532-16-0 C1b Ni33.1Hf32.7Sn34.2 0.6079 0.6083 [20] NiTiSn 12534-03-1 C1b Ni31.1Ti36.4Sn32.4 0.5946 0.5941 [20], 0.5937 [21] NiZrSn 12534-04-2 C1b Ni32.6Zr36.0Sn31.5 0.6111 0.61161 [22], 0.6098 [23] PdHfSn 57594-98-6 C1b Pd34.8Hf34.3Sn30.9 0.6285 PdZrSn 12535-02-3 C1b Pd34.9Zr32.8Sn32.2 0.6319 0.6321 [20], 0.6299 [24] PtHfSn 51913-14-5 C1b Pt32.6Hf33.9Sn33.5 0.6304 0.6315 [25], 0.631 [13] a .4332 PtMnGa 12592-64-2 hP6 Pt34.0Mn32.5Ga33.5 c .5563 a 4.336, c 5.590 [26] PtMnSn 12502-75-9 C1b Pd32.9Mn32.1Sn35.1 0.6260 0.6263 [27], 0.6261 [28] PtTiSn 51890-41-6 C1b Pt32.5Ti33.5Sn34.0 0.6165 0.6170 [25], 0.616 [13] PtZrSn 51890-43-8 C1b Pt36.2Zr28.3Sn35.5 0.6332 0.6337 [25], 0.6339 [13] a 0.7318 RhHfSn 51913-16-7 hP18 Rh34.5Hf34.3Sn31.2 c 0.7134 a 0.7320, c 0.7148 [7] RhMnSn 161357-26-2 L21 Rh34.2Mn29.8Sn36.0 0.6219 RhTiSn 51890-49-4 C1b Rh32.7Ti33.1Sn34.2 0.6187 0.62 [13] a 0.7332 RhZrSn 51890-51-8 hP18 Rh33.8Zr35.7Sn30.5 c 0.7205 RuMnGa 12592-66-4 B2 Ru34.8Mn29.9Ga35.3 0.3019 0.6150 [16] Standard uncertainties u are u(x) = 5 atm. %, u(a) = 0.001 nm, u(p) = 10 kPa, u(T) = 2 K. Compound

CAS NO.

Structure

Composition, (atm. %) a Au30.5Mn34.0Sn35.6 Co38.0Ti27.7Sn34.3 Ir35.4Mn37.2Ga27.4 Ir33.8Mn33.9Sn32.3

Table 3. Measured standard enthalpies of formation (∆fH°) compared with data from the AFLOW and OQMD and the relative amount of impurity phases in the experimental samples. ∆fH°/(kJ·mole of atoms-1) Compound Structure This work AFLOW OQMD ▲ AuMnSn C1b -48.4 ± 4.0 -6.4 -7.7 ▲ CoTiSn C1b -42.6 ± 1.6 -32.3 IrMnSn C1b -29.4 ± 1.8 -9.9 NiTiSn C1b -52.6 ± 2.4 -52.7 -55.0 NiZrSn▲ C1b -69.6 ± 2.8 -67.1 NiHfSn▲ C1b -62.6 ± 2.5 -61.8 ▲ PdZrSn C1b -86.6 ± 1.6 -76.7 PdHfSn▲ C1b -68.4 ± 3.1 -70.4 -72.6 PtMnSn C1b -55.8 ± 2.6 -34.7 -36.3 PtTiSn C1b -93.6 ± 3.3 -82.2 -84.2 PtZrSn C1b -104.9 ± 3.8 -100.2 PtHfSn C1b -98.8 ± 3.4 -96.5 ▲ RhTiSn C1b -66.0 ± 3.1 -55.8 -57.5 IrMnGa C1 or C1b -40.9 ± 1.7 -7.4 ▲ IrZrSn hP18 -89.1 ± 2.6 -69.5 -74.6 ▲ RhHfSn hP18 -80.9 ± 3.0 -68.1 -74.0 ▲ RhZrSn hP18 -78.2 ± 4.1 -69.5 -78.1 ▲ PtMnGa hP6 -58.7 ± 2.8 -28.8 ▲ RhMnSn L2 1 -39.8 ± 2.6 -16.1 RuMnGa B2 -26.9 ± 1.7 12.8 Standard uncertainties u are u(p) = 10 kPa, u(T) = 2 K. a

a

▲

Comment b

0.02 AuMn, 0.06 Sn 0.03 Sn5Ti6, 0.07 Sn 0.03 Sn 0.02 Sn5Ti6, 0.02 Ni2TiSn 0.03 Sn3Zr5, 0.07 Ni2SnZr2 0.05 Ni2HfSn, 0.01 Hf5Sn4 0.06 Pd2ZrSn 0.04 Hf5Sn4, 0.04 Pd2HfSn Single phase 0.03 PtSn 0.03 PtSn 0.03 HfO2 0.04 RhSn2, 0.03 Sn 0.01 Mn 0.04 Ir2Zr, 0.03 Ir3Zr5 0.10 Rh41Hf30Sn29, 0.01 Sn 0.08 Sn3Zr5 0.05 Ga3Pt2 0.03 Rh3Sn2, 0.08 βRhSn2 Single phase

indicates experimental values are only indicative of the standard enthalpy of formation of the

compound due to significant amounts of impurity phases; b the amount of impurity phases are presented in volume fraction.

Table 4. Measured heats of reaction (∆Hr), heat contents (H1373- H298) at the atmospheric pressure compared with calculated data from Neumann-Kopp rule and the melting point (Tm). (H1373- H298)/ (kJ·mole of atoms-1) Tm/K This work a Neumann-Kopp ▲ 733 48.6 ± 4.0 AuMnSn 0.2 ± 0.1 41.4 1597 CoTiSn -13.4 ± 0.5 29.3 ± 1.5 33.9 1563 IrMnGa -8.7 ± 1.0 32.2 ± 1.4 35.8 1449 IrMnSn 0.8 ± 1.0 30.2 ± 1.5 37.2 >1700 IrZrSn -60.1 ± 2.1 29.0 ± 1.4 35.7 >1700 NiHfSn -36.0 ± 2.0 26.6 ± 1.6 34.8 1438 NiTiSn -23.7 ± 2.2 29.0 ± 0.8 32.6 >1700 NiZrSn -41.7 ± 1.1 27.9 ± 2.6 37.1 >1700 PdHfSn -40.7 ± 2.0 27.7 ± 2.4 33.8 1688 PdZrSn -51.7 ± 1.4 35.0 ± 0.8 36.1 >1700 PtHfSn -76.3 ± 3.3 22.5 ± 1.0 33.7 ▲ 1325 55.7 ± 2.7 PtMnGa -3.0 ± 0.6 36.0 ▲ 1283 56.0 ± 2.6 PtMnSn 0.2 ± 0.1 37.4 1635 PtTiSn -60.2 ± 2.4 33.4 ± 2.2 31.5 >1700 PtZrSn -77.1 ± 3.7 27.8 ± 1.2 36.0 >1700 RhHfSn -56.4 ± 2.7 24.6 ± 1.1 34.2 ▲ >1700 45.3 ± 2.5 RhMnSn 5.5 ± 0.8 37.9 >1700 RhTiSn -36.6 ± 2.5 29.4 ± 1.8 32.0 >1700 RhZrSn -52.2 ± 3.1 26.0 ± 2.6 36.5 >1700 RuMnGa 3.7 ± 1.4 30.6 ± 1.0 36.8 Standard uncertainties u are u(x) = 5 atm. %, u(a) = 0.001 nm, u(p) = 10 kPa, u(T) = 2 K, u(Tm) = 5 Compound

∆Hr/(kJ·mole of atoms-1)

K. a

▲

indicates samples melted in the calorimeter (1373 K).

Table 5. Phases observed in multiphase alloys. Composition Nominal Real Co33.3V33.3Sn33.3 Co32.2V33.3Sn34.6

Phase 1

Phase 2

CoSn2

Co 2VSn Sn

Fe33.3V33.3Sn33.3

Fe32.7V33.2Sn34.1

Fe2VSn

Fe3Sn2

Sn3 V2

Ir33.3Ti33.3Sn33.3

Ir34.2Ti29.1Sn36.7

Ir3Sn7

IrSn2

γ-IrTi

Phase 3

Pd33.3Mn33.3Sn33.3

Pd34.2Mn30.7Sn35.1 Pd2MnSn MnSn2

Pd33.3Ti33.3Sn33.3

Pd30.8Ti30.7Sn38.6

Sn5Ti6

Pd2TiSn PdSn

Rh33.3Ti33.3Ga33.3

Rh34.1Ti35.3Ga30.6

GaTi3

RhTi

Ru33.3Mn33.3Sn33.3 Ru33.5Mn31.4Sn35.1 Ru2MnSn RuSn2

Sn

Mn2Sn

Ru 33.3Ti33.3Sn33.3 Ru 29.9Ti33.1Sn36.9 RuTi Sn5Ti6 Sn Standard uncertainties u are u(x) = 5 atm. %, u(p) = 10 kPa, u(T) = 2 K.

Figure captions Fig. 1. Crystal structures of (a) AgAsMg of the C1 b half-Heusler structure; (b) CaF2 of the C1 structure; (c) Cu2MnAl of the L21 Heusler structure. Fig. 2. Comparison of XRD patterns in relative intensity (10 2I/Imax) of the Heusler structure (Ni2TiSn) and the half-Heusler structure (NiTiSn). E indicates experimentally measured result and C indicates simulated result. The highest intensity peak (RI = 100) corresponds to the (220) diffraction peak in each pattern. Fig. 3. Comparison of enthalpy of formation between experimental results ( ∆ f H Eo ) and ab initio calculations ( ∆ f H Co ) with data in kJ/mole of atoms. Fig. 4. Comparison of standard enthalpy of formation (∆fH°) according to periodic classification. Fig. 5. Comparison of the experimentally measured heat contents (H1373- H298)E and the predictions using Neumann-Kopp rule (H1373- H298)P. Fig. 6. Experimentally measured XRD results for XHfSn (X = Ni, Pd, Pt, Rh) alloys in relative intensity (10 2I/Im). Fig. 7. Experimentally measured XRD pattern of AuMnSn alloy, (a) annealed sample; (b) heat content sample in relative intensity (102I/Im). Fig. 8. Backscattered electron micrographs of (a) Pd 33.3Mn33.3Sn33.3; (b) Rh33.3Mn33.3Sn33.3; (c) Ru33.3Mn33.3Sn33.3.

Fig. 1. Crystal structures of (a) AgAsMg of the C1 b half-Heusler structure; (b) CaF2 of the C1 structure; (c) Cu2MnAl of the L21 Heusler structure.

Fig. 2. Comparison of XRD patterns in relative intensity (10 2I/Imax) of the Heusler structure (Ni2TiSn) and the half-Heusler structure (NiTiSn). E indicates experimentally measured result and C indicates simulated result. The highest intensity peak (RI = 100) corresponds to the (220) diffraction peak in each pattern.

Fig. 3. Comparison of enthalpy of formation between experimental results ( ∆ f H Eo ) and ab initio calculations ( ∆ f H Co ) with data in kJ/mole of atoms.

Fig. 4. Comparison of standard enthalpy of formation (∆fH°) according to periodic classification.

Fig. 5. Comparison of the experimentally measured heat contents (H1373- H298)E and the predictions using Neumann-Kopp rule (H1373- H298)P.

Fig. 6. Experimentally measured XRD results for XHfSn (X = Ni, Pd, Pt, Rh) alloys in relative intensity (10 2I/Imax).

Fig. 7. Experimentally measured XRD pattern of AuMnSn alloy, (a) annealed sample; (b) heat content sample in relative intensity (102I/Imax).

Fig. 8. Backscattered electron micrographs of (a) Pd 33.3Mn33.3Sn33.3; (b) Rh33.3Mn33.3Sn33.3; (c) Ru33.3Mn33.3Sn33.3.

Research highlights


Standard enthalpies of formation of XYZ were measured using the Kleppa calorimeter;




First principles calculations generally agree with measured data;




Lattice parameters and related phase relationships were consistent with literature data;




Lattice parameters of PdHfSn and RhZrSn were reported for the first time.