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Zr-based nickel aluminides: Crystal structure and electronic properties
Journal Pre-proof Zr-based nickel aluminides: Crystal structure and electronic properties O. Shved, L.P. Salamakha, S. Mudry, O. Sologub, P.F. Rogl, E. Bauer PII:
S0925-8388(19)34572-4
DOI:
https://doi.org/10.1016/j.jallcom.2019.153326
Reference:
JALCOM 153326
To appear in:
Journal of Alloys and Compounds
Received Date: 6 June 2019 Revised Date:
5 December 2019
Accepted Date: 7 December 2019
Please cite this article as: O. Shved, L.P. Salamakha, S. Mudry, O. Sologub, P.F. Rogl, E. Bauer, Zrbased nickel aluminides: Crystal structure and electronic properties, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/j.jallcom.2019.153326. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Author Contribution Statement. All authors contributed equally to the work.
1 Zr-based nickel aluminides: crystal structure and electronic properties O. Shved1, L.P. Salamakha1,2, S. Mudry1, O. Sologub2,*, P.F. Rogl3, E. Bauer2 1
Department of Physics of Metals, I. Franko L’viv National University, 79005 L’viv, Ukraine 2
Institute of Solid State Physics, TU Wien, A-1040 Vienna, Austria 3
Institute of Materials Chemistry, University of Vienna, A-1090 Vienna, Austria
Abstract In our motive to contribute to a better understanding of phase transitions, structural changes and physical properties of Zr-based alloys, we have studied three zirconium-rich compounds in the Zr-Ni-Al system annealed at 800 °C. For Zr6NiAl2, which was investigated hitherto only by the use of X-ray powder film data, the precise crystal structure has been derived from Rietveld refinement of powder X-ray diffraction data revealing a significant disorder of Al (81.3% Al+18.7% Ni in 2d) and Ni (76.1%Ni+23.9% Al in 1a) atomic sites rendering final composition Zr6Ni1.14Al1.86 in accord with energy dispersive X-ray (EDX) spectroscopy analysis. The crystal structures of ZrNiAl and Zr6NiAl2 have been confirmed; despite the disorder of certain atom sites (94% Al+6% Ni in 3g and 90% Ni+10%Al in 2d for ZrNiAl; 93% Zr+7% Al in 2e and 95% Al+5% Zr in 2f for Zr5Ni4Al; Rietveld refinements of powder X-ray diffraction data), the deviations from the stoichiometry for these compounds are negligible. Cell parameters and atomic coordinates have been also refined in the scope of local density approximation being in good agreement with experimental data. For all compounds the electronic density of states has been calculated. Electrical resistivity measurements characterize ZrNiAl, Zr6NiAl2 and Zr5Ni4Al as metals in concord with electronic band structure calculations. Keywords: A. Zr-based compounds; A. Aluminides; B. crystal chemistry; B. electronic structure; B. electrical properties; E. X-ray diffraction *
Corresponding author.
E-mail address: [email protected]
2
Introduction
Zirconium has a wide range of applications. Due to its low neutron absorption cross section for thermal neutrons, good corrosive resistance and relatively good mechanical properties, zirconium alloys are being used as e.g., a cladding material for fuel rods in nuclear reactors [1]. Moreover, zirconium and its alloys are biocompatible and therefore become promising materials for surgical implants [2,3]. Ni and Al additions are used for promoting the mechanical properties and improving oxidation resistance of Zr-based alloys [4,5]. Besides that, the compounds of zirconium in combination with nickel and aluminium are potential candidates for hydrogen storage materials [6]. On the other hand, scientific and engineering interests in phase equilibria and crystal structures of compounds in the Zr-Ni-Al system were stimulated also by possibilities of development of new glass-type alloys with a wide temperature region of supercooled liquid state and a large temperature interval before crystallization [7,8]. Several research groups have dealt with the Zr-Ni-Al system to elucidate formation of compounds, their crystal structure and phase relations at various temperatures in the range 800 ºC-1050 ºC [9-17]. Seven ternary phases were reported to form, Zr5Ni4Al [18], Zr6NiAl2 [19-22], ZrNi2Al [9], ZrNiAl [23,24], Zr6Ni8Al15 [9,25], ZrNixAl2-x [9,26] and ZrNi2Al5 [27,28]. A large number of publications describe the structure and crystallization behaviour of Zr–Ni–Al glasses focusing on the composition Zr60Ni25Al15, which exhibits the highest stability in the supercooled-liquid state [29-32]. The heat of formation of Zr-Ni-Al liquid alloys and the heat capacity of undercooled liquids have been studied both experimentally and theoretically for the samples within glass-forming composition ranges by different research groups [33-37]; from these studies, a maximum in the heat capacity near the liquidus temperature was assigned to temperature dependent chemical short-range ordering [35]. Furthermore, several reports have been published on hydrogenation behavior of Zr6NiAl2 [6,20-22] revealing a large hydrogen storage capacity for this compound (9.7 H-atoms per formula unit) and relatively high thermal stability of its hydride with respect to decomposition as compared to ZrNi2 hydrides [38]. Despite the Zr-Ni-Al equilibrium phase diagram seems to be well established at/above 850 ºC [17], the isothermal section at 800 ºC is not defined unambiguously. Moreover, the precise structure of Zr6NiAl2 (relevant to the concentration regime where the amorphous alloys can be prepared besides bearing a promising hydrogen storage capacity) is still
3 lacking as well as no information on existence of homogeneity ranges for Zr-rich phases has been reported so far. To the best of our knowledge, the low temperature physical properties of Zr-rich zirconium nickel aluminides have not been explored as well. Band structure and electronic density of states have been calculated previously only for Zr5Ni4Al [39] in terms of generalized gradient approximations, characterizing the material to be a non-magnetic metal in which d-states of Zr and Ni together dominate at the Fermi level, however no electronic structure calculations for the further compounds have been performed. In order to contribute to a better understanding of phase transitions and structural changes in the crystalline and amorphous Zr-Ni-Al alloys, we have initiated a systematic investigation of this system [40]. Considering the lack of detailed investigations involving the structural and physical properties characterization of compounds and in view of the conflicting results on phase equilibria [9-17] particularly at 800 ºC, we present the crystal structure studies of three Zr-rich phases in the Zr-Ni-Al system and provide the validation for experimentally derived parameters by density functional theory as well as we report on their electrical properties and electronic density of states. Further experiments on the elucidation of the crystal structure of Al-rich compounds and their physical properties as well as on the exploration of the isothermal section of the Zr-Ni-Al system at 800 ºC are in progress and will be subjects of our forthcoming paper [41].
Experimental
Synthesis and structural characterization
Alloys (each of a weight of 1.5 g) were prepared from ingots of pure elements (Zr 99.7 %, Ni 99.99 %, aluminium 99.999 %, all supplied by ChemPur, Germany) by repeated arc melting under Ti-gettered 6N argon with a non-consumable tungsten electrode on a watercooled copper plate. The alloys were remelted at least three times in order to achieve complete fusion and homogeneity. Mass-losses were checked to be below 0.5%. The arcmelted buttons were cut into pieces, wherefrom one piece was wrapped in tantalum foil and vacuum-sealed in a quartz tube for annealing at 800 °C for 480-720 hours. The annealed samples were polished applying standard procedures and were examined by energy dispersive X-ray spectroscopy (EDX) in a scanning electron microscope (SEM) using a Philips XL30 ESEM with EDAX XL-30 EDX-detector operated at 20 kV. Lattice parameters and standard deviations were calculated by least-squares fits of room temperature X-ray powder diffraction
4 data obtained from a Guinier-Huber image plate system G-670 employing monochromatic CuKa1 radiation (λ = 1.54056 Å, germanium monochromator, 8° ≤ 2θ ≤ 100°, ∆2θ = 0.005°) to the indexed θ -values employing Ge as internal standard (aGe = 5.657906 Å). Quantitative Rietveld refinements of the X-ray powder diffraction data were performed with the program FULLPROF [42] with use of its internal tables for scattering lengths and atomic form factors.
Electrical resistivity
The temperature dependent electrical resistivity of the compounds described below was studied using a four point probe technique and employing a Lakeshore 370 a.c. resistance bridge in the range from room temperature down to 4.2 K.
Computational method
The calculations were performed within the DFT using Quantum ESPRESSO [43]. The correlation and exchange effects of the electrons were handled by using the local density approximation of Perdew and Zunger (PZ) [44]. Electron-ion interactions were treated with the pseudopotential method [45,46] applying fully relativistic pseudopotentials constructed according to the code supplied by the PSLibrary (version 1.0.0) [47]. For zirconium and aluminum, the s- and p- states were considered as valence electrons. The electron wave functions were expanded into plane waves with a kinetic energy cutoff of 680 eV. For the charge density, a kinetic energy cutoff of 5442 eV was used. For each compound the optimization of both cell parameters and atomic positions had been performed using the Broyden–Fletcher–Goldfarb–Shanno algorithm implemented in the package. The k-point mesh for each compound had been constructed using Monkhorst–Pack method [48] on a grid of the size that guarantees less then 0.05×2π/Å spacing between the k-points for the calculations related to the cell parameters optimization procedure and less then 0.03×2π/Å spacing for the final optimized cell relaxation. The convergence threshold for self-consistentfield iteration was set at 10–8 eV. The calculations were performed taking into account the spin-orbit interactions assuming the total magnetization to be zero. For the structures that have partially or mixed occupied positions, corresponding atomic sites were treated as fully occupied by the dominating atom.
5 Results and Discussion Crystal structure of ZrNiAl and Zr6NiAl2 The structural chemistry of the Fe2P-type derivative structures (space group
, Fe1
in 3g, Fe2 in 3f, P1 in 2d, P2 in 1a) and atom site preferences for transition metals and pelements in ternary and quaternary Fe2P substitution variants in aluminide, germanide and silicide systems have been described earlier in detail [49]. The Zr-Ni-Al system shows the existence of compounds with two different superstructures of the Fe2P-type, the ZrNiAl-type and β1-K2UF6-type phases. The structure of ZrNiAl (a substitution variant of the Fe2P type, space group
, Al
in 3g, Zr in 3f, Ni2 in 2d, Ni1 in 1a) was first obtained by Krypyakevich et al. [23] from Xray single crystal film data. Precise single crystal diffractometer data were reported more recently by Zumdick et al. (space group
, a=6.915(2) Å, c=3.466(1) Å) for the sample
annealed at 700 °C confirming an ordered distribution of Zr, Al and Ni atoms on two Fe and two P atom sites, respectively [24]. Our Rietveld refinement of X-ray powder diffraction data for the ZrNiAl alloy annealed at 800 °C confirmed the hitherto reported positional parameters, however, revealed mixed occupancies of Ni and Al at Ni (2d) and Al (3g) sites (Table 1). Analogously to the results obtained from single crystal samples [24], our structure refinement disclosed a large displacement parameter for one nickel position, indicative of a slight nickel dislocation. No superstructure reflections have been observed, which would require a doubling of the c axis [50] (Fig. 1). The results obtained support the possibility of homogeneity range for the ZrNiAl phase, as has been observed earlier for certain series of intermetallics of ZrNiAl-type [51,52]. The existence of Zr6NiAl2 (β1-K2UF6-type, space group
, Zr1 in 3g, Zr2 in 3f, Al
in 2d, Ni in 1a) was recognized some forty years ago from powder X-ray film data [19]. Since then, only lattice parameters were evaluated for this phase in the course of hydrogen absorption/desorption properties studies [20-22], however no precise positional and displacement parameters have ever been reported. In the current study, the positional parameters of atoms in Zr6FeAl2 [53] were taken as starting values for a Rietveld refinement of powder X-ray diffraction data. The structure was successfully refined to low residuals (Table 1, Fig. 1). The extremely contrasting values of thermal displacement parameters of Ni and Al atoms hinted at Ni/Al mixed occupancies of the corresponding atom positions in Zr6NiAl2. Refinements allowing the occupancies of atoms to vary freely resulted in 0.813(4)Al+0.187(4)Ni and 0.761(5) Ni+0.239(5) Al for Al and Ni atom sites respectively
6 leading to a slightly Ni-rich formulae Zr6Ni1.14(1)Al1.86(1); this composition has been also confirmed from EDX analysis (Table1, Fig. 2). Albeit no considerable homogeneity region was found hitherto for Zr6NiAl2 from phase diagram studies at/above 850, our lattice parameters (a=7.9009(1) Å, c=3.36823(7) Å) differ considerably from those given earlier (a=7.928 Å, c=3.347 Å [21,22] and a=7.933 Å, c=3.368 Å [20]).
Crystal structure of Zr5Ni4Al Two structure modifications, both derived from U3Si2 (space group P4/mbm), were found to exist for Zr5Ni4Al from investigation of the crystallized, initially amorphous Zr–Ni– Al specimens by diffraction methods [18]. A low temperature form occurs right above the crystallization temperature (500 °C) and exhibits the disordered Zr3Al2-type structure (space group P42/mnm, Al in 8j, Zr1 in 4g, Zr2 in 4f, Zr3 in 4d), where Ni occupies the atom position of Al (8j) while Zr and Al randomly fill the 4d site. The high temperature structure of the Zr5Ni4Al-type results from an additional ordering of zirconium and aluminium atoms in 2e and 2f sites, respectively (P42/m space group). Our powder X-ray diffraction pattern recorded from the Zr50Ni40Al10 alloy annealed at 800 °C clearly showed the superstructure reflections (100) and (102) at 2θ=12.3º and 2θ=29.7º, respectively, which are forbidden for the low temperature Zr3Al2-type structure, however no complete Zr/Al ordering could be achieved even after prolonged (30 days) annealing time (Table 1, Figure 1).
7
Figure 1. Rietveld refinement of X-ray powder diffraction intensity data of Zr66.7Ni11.1Al22.2, Zr33.3Ni33.3Al33.4 and Zr50Ni40Al10 samples annealed at 800 °C. In insets the crystal structures of compounds are shown emphasizing the constituent structural fragments arranged in columns which run along the c-axes: i) trigonal prisms [NiZr6] (green) and [NiAl6] (yellow) for the ZrNiAl-type phase (dark grey and light grey respectively in black/white version); ii) trigonal prisms [AlZr6] and [NiZr6] (both green) for the β1-K2UF6-type phase (both light grey in black/white version); iii) cubes [ZrZr8] (violet, dark grey in black/white version), [AlZr8] (yellow, light grey in black/white version) and trigonal prisms [NiZr6] (left open) for the Zr5Ni4Al-type phase. For a better comparison, the unit cell of Zr5Ni4Al is displaced for ½,½,0.
8
Figure 2. SEM image for Zr64Ni12Al22 sample annealed at 800 °C. Light phase: Zr6NiAl2; dark phase: ZrNiAl.
Table 1. Crystal structure data for Zr–Ni–Al compounds annealed at 800 °C as obtained from Rietveld refinement of powder X-ray diffraction data. Nominal composition Formula from refinement Composition from EDX Space group Structure type Cell parameters a (Å) c (Å) Reflections in refinement Reliability factors RF=Σ|F0-Fc|/ΣF0 RI=Σ|I0-Ic|/ΣI0 Rexp=[(N-P+C)/Σwiy0i2]1/2 χ2=(RwP/Re)2 Zr1; Occ., Biso Zr2; Occ., Biso Zr3; Occ., Biso Al1; Occ. Biso Ni1; Occ. Biso Ni2; Occ. Biso
Zr66.7Ni12.7Al20.6 Zr6Ni1.14(1)Al1.86(1) Zr66.6(6)Ni12.5(6)Al20.9(6)
Zr33.3Ni33.6Al33.1 ZrNi0.99(1)Al1.01(1) Zr33.3(5)Ni33.4(5)Al33.3(5)
; No. 189 β1-K2UF6
; No. 189 ZrNiAl
Zr49.8Ni40.0Al10.2 Zr4.98(1)Ni4Al1.02(1) Zr50.0(5)Ni40.0(5)Al10.0(5) P42/m; No. 84 (origin 0,0,0) Zr5Ni4Al
7.9009(1) 3.36823(7) 66
6.91558(5) 3.47024(3) 49
7.18574(8) 6.60849(7) 234
0.0554 0.0397 0.0160 13.5 3g (x,0,½) x=0.25870(6) 1.00 Zr , 0.38(3) 3f (x,0,0) x=0.59853(6) 1.00 Zr, 0.60(2)
0.0580 0.0359 0.0174 19.3 3f (x,0,0) x=0.59354(9) 1.00 Zr, 0.50(3)
-
-
2d (⅓,⅔,½) 0.813(4)Al+0.187(4)Ni 0.98(8) 1a (0,0,0) 0.761(5)Ni+0.239(5)Al 0.46(8)
3g (x,0,½) x=0.2491(3) 0.94(1)Al+0.06(1)Ni 0.72(7) 1a (0,0,0) 1.00 Ni 1.13(7) 2d (⅓,⅔,½) 0.90(1)Ni+0.10(1)Al 0.37(5)
0.0370 0.0556 0.0208 4.19 4j (x,y,0) x=0.3089(7); y=0.1533(8) 1.00 Zr, 0.33 (4) 4j (x,y,0) x=0.1476(8); y=0.6833(7) 1.00 Zr, 0.36(4) 2e (0,0,¼) 0.93(1)Zr+0.07(1)Al, 0.75(3) 2f (½,½,¼) 0.95(1)Al+0.05(1)Zr 0.78(5) 8k (x,y,z) x=0.1285(1) y=0.3743(1), z=0.2581(6) 1.00 Ni 0.47(2)
-
-
-
9 Cell optimization
A DFT optimisation of the unit cell parameters and of the atomic positions was performed employing the procedure described above. The initial cell parameters were chosen to be approximately 3 percent bigger than those obtained experimentally. Results regarding calculated cell parameters optimization are summarized in Table 2 for all three compounds.
Table 2. Crystal structure data for Zr–Ni–Al compounds as obtained from DOS relaxation procedure Zr6NiAl2 ZrNiAl Zr5Ni4Al
acalc (Å) 7.745 6.708 6.979
ccalc (Å) 3.277 3.378 6.403
acalc/aexp (%) 0.98 0.97 0.97
ccalc/cexp (%) 0.97 0.97 0.97
Unit cell parameters for the compounds received as a result of the cell optimization for all compounds are in good agreement and in general 3% smaller than those obtained experimentally, which is a common feature of all first - principal calculations [54]. The atomic coordinates, received as the result of relaxation, are summarized in the Table 3.
Table 3. Optimized atomic coordinates for compounds as obtained from DOS relaxation procedure Zr6NiAl2
ZrNiAl
Zr5Ni4Al
Atom Zr1 Zr2 Al Ni Atom Zr Al Ni1 Ni2 Atom Zr1 Zr2 Zr3 Al Ni
x 0.2544 0.5982 ⅓ 0 x 0.5911 0.2496 0 ⅓ x 0.3034 0.1437 ½ 0 0.1310
y 0 0 ⅔ 0 y 0 0 0 ⅔ y 0.1580 0.6814 ½ 0 0.3789
z ½ 0 ½ 0 z 0 ½ 0 ½ z 0 0 ¼ ¼ 0.2700
For all three compounds the shifts of coordinates of the atoms and changes of the cell parameters due to the relaxation are within the error margin, supporting the stability of the
10 phases at low temperatures. Further density of states calculations for these compounds were performed using the optimized cells.
Density of states (DOS) Electronic density of states of ZrNiAl (Figure 3) had been calculated following the procedure described above. At the Fermi level, the electronic density of states of ZrNiAl is rather low, 3.4 states/eV, suggesting a metallic behavior of the compound. Below the Fermi level (around -3 eV), the density of states of ZrNiAl exhibits a peak formed by d-states of Ni1 and Ni2. As the Fermi level is approached from the left side, the influence of d-levels of Zr increases, until they start to dominate the density of states around 1 eV. At the Fermi level more then 40 percent of the density is produced by the d-levels of Zr (see inset in Figure 3). Above the Fermi level the influence of zirconium on the density of states increases even further leading to a complete dominance. In contrast to ZrNiAl, the electronic density of states of Zr6NiAl2 at the Fermi level is higher and equal to 8.5 states/eV. At the energies below the Fermi level (at around -3 eV), the DOS of Zr6NiAl2 is dominated by the d-states of Ni. Moving along the energy scale towards 0 eV, the effect of Ni d-states gets lower and the p-states of aluminium start to dominate the density of states. Around -1 eV, the contribution of Ni is almost negligible, while the DOS is formed by d-states of Zr, together with the p-states of aluminium in almost equal proportion. Such a behavior of the density of states is also observed at and above the Fermi level. The electronic density of states of Zr5Ni4Al at the Fermi level is even higher and equal to around 15 eV-1 (Figure 3). It is characterized by a large peak around -3 eV influenced by Ni d-state`s that dominate the DOS in the negative energy region up to -1 eV where the density of states of Ni d-states becomes marginally equal to that of Zr atoms. At the Fermi level the contribution of Ni d-, Zr1 d-, Zr2 d- and Zr3 d-states are almost equal with Ni d-states getting suppressed at 1 eV. Our results on DFT calculations obtained for Zr5Ni4Al agree well with the literature data discussed above.
11
Figure 3. Total electronic density of states of ZrNiAl (a), Zr5Ni4Al (b) and Zr6NiAl2 (c) per unit cell along with prevailing partial densities (other states are omitted for better readability).
Electrical resistivity
Electrical resistivity of Zr5Ni4Al was measured down to 4.2 K (Figure 4). It behaves metallically decreasing with temperature and exhibits a slight curvature at temperatures around 100 K. The compound is also characterized by a rather low RRR value of 1.89 ( RRR = ρ300 / ρ 0 ). The resistivity of a material exhibiting a comparable set of features is commonly described in terms of the Bloch-Grüneisen-Mott model
ρ B −G = ρ 0 + C
T5
θD6
θ D /T
∫ 0
x5 dx − AT 3 (e x − 1)(1 − e − x )
where ρ 0 stands for the residual resistivity at 0 K. The second term represents the contribution of scattering of electrons on acoustic phonons, while T 3 term accounts for the corrections due to s-d scattering. A least-squares fit of the latter formula to the experimental data leads to a Debye
temperature
θ D = 191 K ,
ρ0 = 113.9 µ Ωcm
and
a
Mott
coefficient
A = 4,04*10−7 µΩcm / K 3 . Similarly to Zr5Ni4Al, the resistivity of ZrNiAl shows a metallic behavior, decreasing with decreasing temperature. The Bloch-Grüneisen-Mott model described above was applied to obtain a Debye temperature θ D = 120 K , a ρ 0 = 53µΩcm and a Mott coefficient
12 A = 4,13*10−7 µΩcm / K 3 as a result of the fit. The residual resistivity ratio was found to be equal to 2.17. Unlike the aforesaid compounds, Zr6NiAl2 is characterized by a much higher overall resistivity and a higher RRR ratio of 3.9. It also exhibits a much stronger curvature in the medium temperature range. The results of the Bloch-Grüneisen-Mott fit are summarized in Table 4, together with the data for the above mentioned compounds.
Table 4. Residual resistivity, Debye temperature and Mott coefficient together with residual resistivity ratio obtained for Zr6NiAl2, ZrNiAl and Zr5Ni4Al using the fit model described in text. Zr6NiAl2 ZrNiAl Zr5Ni4Al
ρ0 , µΩcm
θD , K
194.7 53 113.9
129 120 191
A, µΩcm / K 3 4.04x10-7 4.13x10-7 4.04x10-7
RRR 3.9 2.17 1.89
Figure 4. The electrical resistivity of the compounds in the Zr-Ni-Al system. b) same data represented as ρ / ρ300 K . Solid lines correspond to models described in text
13 Concluding remarks Three ternary Zr-based aluminides with nickel have been characterized from powder X-ray diffraction and SEM/EDX analysis from arc melted samples, annealed at 800 ºC. Rietveld refinements confirmed isotypism with formerly reported structure types: ZrNi0.99(1)Al1.01(1) with ZrNiAl-type, Zr6Ni1.14(1)Al1.86(1) with β1-K2UF6 and Zr4.98(1)Ni4Al1.02(1) with Zr5Ni4Altype. For both Fe2P-type derivative structures, a slight disorder was observed for Al and Ni atom sites, whereas in the U3Si2-type derivative structure, the Zr and Al atoms mutually substitute each other in two atom positions. All three structures exhibit columns of structural fragments extending along the c-axes. The relaxed structures, obtained as a result of the LDA approach, were found to be in close agreement with the experimentally deduced ones. The electronic density of states was calculated for all three compounds. Electrical resistivity measurements performed on samples annealed at 800 °C demonstrated the absence of any phase transitions for these compounds in the temperature region from 4.2 K to 300 K.
Acknowledgements O. Shved and L.P. Salamakha are thankful to the ÖAD for fellowships. This work was supported by the Austrian Science Fund (FWF project P31979-N36). The authors would like to thank M. Waas for the SEM/EDX measurements.
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Highlights
1. New results on crystal structures of ZrNi0.99(1)Al1.01(1), Zr6Ni1.14(1)Al1.86(1) and Zr4.98(1)Ni4Al1.02(1) compounds show the isotypism with earlier reported structure types and reveal the slight disorder of atom sites. 2. DFT calculations assured lattice parameters, crystal symmetry, atomic coordinates. 3. Electrical resistivity of three compounds demonstrated the absence of any phase transitions in the region from 4 K to 300 K.
Declaration of interests X The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0925-8388(19)34572-4
DOI:
https://doi.org/10.1016/j.jallcom.2019.153326
Reference:
JALCOM 153326
To appear in:
Journal of Alloys and Compounds
Received Date: 6 June 2019 Revised Date:
5 December 2019
Accepted Date: 7 December 2019
Please cite this article as: O. Shved, L.P. Salamakha, S. Mudry, O. Sologub, P.F. Rogl, E. Bauer, Zrbased nickel aluminides: Crystal structure and electronic properties, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/j.jallcom.2019.153326. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Author Contribution Statement. All authors contributed equally to the work.
1 Zr-based nickel aluminides: crystal structure and electronic properties O. Shved1, L.P. Salamakha1,2, S. Mudry1, O. Sologub2,*, P.F. Rogl3, E. Bauer2 1
Department of Physics of Metals, I. Franko L’viv National University, 79005 L’viv, Ukraine 2
Institute of Solid State Physics, TU Wien, A-1040 Vienna, Austria 3
Institute of Materials Chemistry, University of Vienna, A-1090 Vienna, Austria
Abstract In our motive to contribute to a better understanding of phase transitions, structural changes and physical properties of Zr-based alloys, we have studied three zirconium-rich compounds in the Zr-Ni-Al system annealed at 800 °C. For Zr6NiAl2, which was investigated hitherto only by the use of X-ray powder film data, the precise crystal structure has been derived from Rietveld refinement of powder X-ray diffraction data revealing a significant disorder of Al (81.3% Al+18.7% Ni in 2d) and Ni (76.1%Ni+23.9% Al in 1a) atomic sites rendering final composition Zr6Ni1.14Al1.86 in accord with energy dispersive X-ray (EDX) spectroscopy analysis. The crystal structures of ZrNiAl and Zr6NiAl2 have been confirmed; despite the disorder of certain atom sites (94% Al+6% Ni in 3g and 90% Ni+10%Al in 2d for ZrNiAl; 93% Zr+7% Al in 2e and 95% Al+5% Zr in 2f for Zr5Ni4Al; Rietveld refinements of powder X-ray diffraction data), the deviations from the stoichiometry for these compounds are negligible. Cell parameters and atomic coordinates have been also refined in the scope of local density approximation being in good agreement with experimental data. For all compounds the electronic density of states has been calculated. Electrical resistivity measurements characterize ZrNiAl, Zr6NiAl2 and Zr5Ni4Al as metals in concord with electronic band structure calculations. Keywords: A. Zr-based compounds; A. Aluminides; B. crystal chemistry; B. electronic structure; B. electrical properties; E. X-ray diffraction *
Corresponding author.
E-mail address: [email protected]
2
Introduction
Zirconium has a wide range of applications. Due to its low neutron absorption cross section for thermal neutrons, good corrosive resistance and relatively good mechanical properties, zirconium alloys are being used as e.g., a cladding material for fuel rods in nuclear reactors [1]. Moreover, zirconium and its alloys are biocompatible and therefore become promising materials for surgical implants [2,3]. Ni and Al additions are used for promoting the mechanical properties and improving oxidation resistance of Zr-based alloys [4,5]. Besides that, the compounds of zirconium in combination with nickel and aluminium are potential candidates for hydrogen storage materials [6]. On the other hand, scientific and engineering interests in phase equilibria and crystal structures of compounds in the Zr-Ni-Al system were stimulated also by possibilities of development of new glass-type alloys with a wide temperature region of supercooled liquid state and a large temperature interval before crystallization [7,8]. Several research groups have dealt with the Zr-Ni-Al system to elucidate formation of compounds, their crystal structure and phase relations at various temperatures in the range 800 ºC-1050 ºC [9-17]. Seven ternary phases were reported to form, Zr5Ni4Al [18], Zr6NiAl2 [19-22], ZrNi2Al [9], ZrNiAl [23,24], Zr6Ni8Al15 [9,25], ZrNixAl2-x [9,26] and ZrNi2Al5 [27,28]. A large number of publications describe the structure and crystallization behaviour of Zr–Ni–Al glasses focusing on the composition Zr60Ni25Al15, which exhibits the highest stability in the supercooled-liquid state [29-32]. The heat of formation of Zr-Ni-Al liquid alloys and the heat capacity of undercooled liquids have been studied both experimentally and theoretically for the samples within glass-forming composition ranges by different research groups [33-37]; from these studies, a maximum in the heat capacity near the liquidus temperature was assigned to temperature dependent chemical short-range ordering [35]. Furthermore, several reports have been published on hydrogenation behavior of Zr6NiAl2 [6,20-22] revealing a large hydrogen storage capacity for this compound (9.7 H-atoms per formula unit) and relatively high thermal stability of its hydride with respect to decomposition as compared to ZrNi2 hydrides [38]. Despite the Zr-Ni-Al equilibrium phase diagram seems to be well established at/above 850 ºC [17], the isothermal section at 800 ºC is not defined unambiguously. Moreover, the precise structure of Zr6NiAl2 (relevant to the concentration regime where the amorphous alloys can be prepared besides bearing a promising hydrogen storage capacity) is still
3 lacking as well as no information on existence of homogeneity ranges for Zr-rich phases has been reported so far. To the best of our knowledge, the low temperature physical properties of Zr-rich zirconium nickel aluminides have not been explored as well. Band structure and electronic density of states have been calculated previously only for Zr5Ni4Al [39] in terms of generalized gradient approximations, characterizing the material to be a non-magnetic metal in which d-states of Zr and Ni together dominate at the Fermi level, however no electronic structure calculations for the further compounds have been performed. In order to contribute to a better understanding of phase transitions and structural changes in the crystalline and amorphous Zr-Ni-Al alloys, we have initiated a systematic investigation of this system [40]. Considering the lack of detailed investigations involving the structural and physical properties characterization of compounds and in view of the conflicting results on phase equilibria [9-17] particularly at 800 ºC, we present the crystal structure studies of three Zr-rich phases in the Zr-Ni-Al system and provide the validation for experimentally derived parameters by density functional theory as well as we report on their electrical properties and electronic density of states. Further experiments on the elucidation of the crystal structure of Al-rich compounds and their physical properties as well as on the exploration of the isothermal section of the Zr-Ni-Al system at 800 ºC are in progress and will be subjects of our forthcoming paper [41].
Experimental
Synthesis and structural characterization
Alloys (each of a weight of 1.5 g) were prepared from ingots of pure elements (Zr 99.7 %, Ni 99.99 %, aluminium 99.999 %, all supplied by ChemPur, Germany) by repeated arc melting under Ti-gettered 6N argon with a non-consumable tungsten electrode on a watercooled copper plate. The alloys were remelted at least three times in order to achieve complete fusion and homogeneity. Mass-losses were checked to be below 0.5%. The arcmelted buttons were cut into pieces, wherefrom one piece was wrapped in tantalum foil and vacuum-sealed in a quartz tube for annealing at 800 °C for 480-720 hours. The annealed samples were polished applying standard procedures and were examined by energy dispersive X-ray spectroscopy (EDX) in a scanning electron microscope (SEM) using a Philips XL30 ESEM with EDAX XL-30 EDX-detector operated at 20 kV. Lattice parameters and standard deviations were calculated by least-squares fits of room temperature X-ray powder diffraction
4 data obtained from a Guinier-Huber image plate system G-670 employing monochromatic CuKa1 radiation (λ = 1.54056 Å, germanium monochromator, 8° ≤ 2θ ≤ 100°, ∆2θ = 0.005°) to the indexed θ -values employing Ge as internal standard (aGe = 5.657906 Å). Quantitative Rietveld refinements of the X-ray powder diffraction data were performed with the program FULLPROF [42] with use of its internal tables for scattering lengths and atomic form factors.
Electrical resistivity
The temperature dependent electrical resistivity of the compounds described below was studied using a four point probe technique and employing a Lakeshore 370 a.c. resistance bridge in the range from room temperature down to 4.2 K.
Computational method
The calculations were performed within the DFT using Quantum ESPRESSO [43]. The correlation and exchange effects of the electrons were handled by using the local density approximation of Perdew and Zunger (PZ) [44]. Electron-ion interactions were treated with the pseudopotential method [45,46] applying fully relativistic pseudopotentials constructed according to the code supplied by the PSLibrary (version 1.0.0) [47]. For zirconium and aluminum, the s- and p- states were considered as valence electrons. The electron wave functions were expanded into plane waves with a kinetic energy cutoff of 680 eV. For the charge density, a kinetic energy cutoff of 5442 eV was used. For each compound the optimization of both cell parameters and atomic positions had been performed using the Broyden–Fletcher–Goldfarb–Shanno algorithm implemented in the package. The k-point mesh for each compound had been constructed using Monkhorst–Pack method [48] on a grid of the size that guarantees less then 0.05×2π/Å spacing between the k-points for the calculations related to the cell parameters optimization procedure and less then 0.03×2π/Å spacing for the final optimized cell relaxation. The convergence threshold for self-consistentfield iteration was set at 10–8 eV. The calculations were performed taking into account the spin-orbit interactions assuming the total magnetization to be zero. For the structures that have partially or mixed occupied positions, corresponding atomic sites were treated as fully occupied by the dominating atom.
5 Results and Discussion Crystal structure of ZrNiAl and Zr6NiAl2 The structural chemistry of the Fe2P-type derivative structures (space group
, Fe1
in 3g, Fe2 in 3f, P1 in 2d, P2 in 1a) and atom site preferences for transition metals and pelements in ternary and quaternary Fe2P substitution variants in aluminide, germanide and silicide systems have been described earlier in detail [49]. The Zr-Ni-Al system shows the existence of compounds with two different superstructures of the Fe2P-type, the ZrNiAl-type and β1-K2UF6-type phases. The structure of ZrNiAl (a substitution variant of the Fe2P type, space group
, Al
in 3g, Zr in 3f, Ni2 in 2d, Ni1 in 1a) was first obtained by Krypyakevich et al. [23] from Xray single crystal film data. Precise single crystal diffractometer data were reported more recently by Zumdick et al. (space group
, a=6.915(2) Å, c=3.466(1) Å) for the sample
annealed at 700 °C confirming an ordered distribution of Zr, Al and Ni atoms on two Fe and two P atom sites, respectively [24]. Our Rietveld refinement of X-ray powder diffraction data for the ZrNiAl alloy annealed at 800 °C confirmed the hitherto reported positional parameters, however, revealed mixed occupancies of Ni and Al at Ni (2d) and Al (3g) sites (Table 1). Analogously to the results obtained from single crystal samples [24], our structure refinement disclosed a large displacement parameter for one nickel position, indicative of a slight nickel dislocation. No superstructure reflections have been observed, which would require a doubling of the c axis [50] (Fig. 1). The results obtained support the possibility of homogeneity range for the ZrNiAl phase, as has been observed earlier for certain series of intermetallics of ZrNiAl-type [51,52]. The existence of Zr6NiAl2 (β1-K2UF6-type, space group
, Zr1 in 3g, Zr2 in 3f, Al
in 2d, Ni in 1a) was recognized some forty years ago from powder X-ray film data [19]. Since then, only lattice parameters were evaluated for this phase in the course of hydrogen absorption/desorption properties studies [20-22], however no precise positional and displacement parameters have ever been reported. In the current study, the positional parameters of atoms in Zr6FeAl2 [53] were taken as starting values for a Rietveld refinement of powder X-ray diffraction data. The structure was successfully refined to low residuals (Table 1, Fig. 1). The extremely contrasting values of thermal displacement parameters of Ni and Al atoms hinted at Ni/Al mixed occupancies of the corresponding atom positions in Zr6NiAl2. Refinements allowing the occupancies of atoms to vary freely resulted in 0.813(4)Al+0.187(4)Ni and 0.761(5) Ni+0.239(5) Al for Al and Ni atom sites respectively
6 leading to a slightly Ni-rich formulae Zr6Ni1.14(1)Al1.86(1); this composition has been also confirmed from EDX analysis (Table1, Fig. 2). Albeit no considerable homogeneity region was found hitherto for Zr6NiAl2 from phase diagram studies at/above 850, our lattice parameters (a=7.9009(1) Å, c=3.36823(7) Å) differ considerably from those given earlier (a=7.928 Å, c=3.347 Å [21,22] and a=7.933 Å, c=3.368 Å [20]).
Crystal structure of Zr5Ni4Al Two structure modifications, both derived from U3Si2 (space group P4/mbm), were found to exist for Zr5Ni4Al from investigation of the crystallized, initially amorphous Zr–Ni– Al specimens by diffraction methods [18]. A low temperature form occurs right above the crystallization temperature (500 °C) and exhibits the disordered Zr3Al2-type structure (space group P42/mnm, Al in 8j, Zr1 in 4g, Zr2 in 4f, Zr3 in 4d), where Ni occupies the atom position of Al (8j) while Zr and Al randomly fill the 4d site. The high temperature structure of the Zr5Ni4Al-type results from an additional ordering of zirconium and aluminium atoms in 2e and 2f sites, respectively (P42/m space group). Our powder X-ray diffraction pattern recorded from the Zr50Ni40Al10 alloy annealed at 800 °C clearly showed the superstructure reflections (100) and (102) at 2θ=12.3º and 2θ=29.7º, respectively, which are forbidden for the low temperature Zr3Al2-type structure, however no complete Zr/Al ordering could be achieved even after prolonged (30 days) annealing time (Table 1, Figure 1).
7
Figure 1. Rietveld refinement of X-ray powder diffraction intensity data of Zr66.7Ni11.1Al22.2, Zr33.3Ni33.3Al33.4 and Zr50Ni40Al10 samples annealed at 800 °C. In insets the crystal structures of compounds are shown emphasizing the constituent structural fragments arranged in columns which run along the c-axes: i) trigonal prisms [NiZr6] (green) and [NiAl6] (yellow) for the ZrNiAl-type phase (dark grey and light grey respectively in black/white version); ii) trigonal prisms [AlZr6] and [NiZr6] (both green) for the β1-K2UF6-type phase (both light grey in black/white version); iii) cubes [ZrZr8] (violet, dark grey in black/white version), [AlZr8] (yellow, light grey in black/white version) and trigonal prisms [NiZr6] (left open) for the Zr5Ni4Al-type phase. For a better comparison, the unit cell of Zr5Ni4Al is displaced for ½,½,0.
8
Figure 2. SEM image for Zr64Ni12Al22 sample annealed at 800 °C. Light phase: Zr6NiAl2; dark phase: ZrNiAl.
Table 1. Crystal structure data for Zr–Ni–Al compounds annealed at 800 °C as obtained from Rietveld refinement of powder X-ray diffraction data. Nominal composition Formula from refinement Composition from EDX Space group Structure type Cell parameters a (Å) c (Å) Reflections in refinement Reliability factors RF=Σ|F0-Fc|/ΣF0 RI=Σ|I0-Ic|/ΣI0 Rexp=[(N-P+C)/Σwiy0i2]1/2 χ2=(RwP/Re)2 Zr1; Occ., Biso Zr2; Occ., Biso Zr3; Occ., Biso Al1; Occ. Biso Ni1; Occ. Biso Ni2; Occ. Biso
Zr66.7Ni12.7Al20.6 Zr6Ni1.14(1)Al1.86(1) Zr66.6(6)Ni12.5(6)Al20.9(6)
Zr33.3Ni33.6Al33.1 ZrNi0.99(1)Al1.01(1) Zr33.3(5)Ni33.4(5)Al33.3(5)
; No. 189 β1-K2UF6
; No. 189 ZrNiAl
Zr49.8Ni40.0Al10.2 Zr4.98(1)Ni4Al1.02(1) Zr50.0(5)Ni40.0(5)Al10.0(5) P42/m; No. 84 (origin 0,0,0) Zr5Ni4Al
7.9009(1) 3.36823(7) 66
6.91558(5) 3.47024(3) 49
7.18574(8) 6.60849(7) 234
0.0554 0.0397 0.0160 13.5 3g (x,0,½) x=0.25870(6) 1.00 Zr , 0.38(3) 3f (x,0,0) x=0.59853(6) 1.00 Zr, 0.60(2)
0.0580 0.0359 0.0174 19.3 3f (x,0,0) x=0.59354(9) 1.00 Zr, 0.50(3)
-
-
2d (⅓,⅔,½) 0.813(4)Al+0.187(4)Ni 0.98(8) 1a (0,0,0) 0.761(5)Ni+0.239(5)Al 0.46(8)
3g (x,0,½) x=0.2491(3) 0.94(1)Al+0.06(1)Ni 0.72(7) 1a (0,0,0) 1.00 Ni 1.13(7) 2d (⅓,⅔,½) 0.90(1)Ni+0.10(1)Al 0.37(5)
0.0370 0.0556 0.0208 4.19 4j (x,y,0) x=0.3089(7); y=0.1533(8) 1.00 Zr, 0.33 (4) 4j (x,y,0) x=0.1476(8); y=0.6833(7) 1.00 Zr, 0.36(4) 2e (0,0,¼) 0.93(1)Zr+0.07(1)Al, 0.75(3) 2f (½,½,¼) 0.95(1)Al+0.05(1)Zr 0.78(5) 8k (x,y,z) x=0.1285(1) y=0.3743(1), z=0.2581(6) 1.00 Ni 0.47(2)
-
-
-
9 Cell optimization
A DFT optimisation of the unit cell parameters and of the atomic positions was performed employing the procedure described above. The initial cell parameters were chosen to be approximately 3 percent bigger than those obtained experimentally. Results regarding calculated cell parameters optimization are summarized in Table 2 for all three compounds.
Table 2. Crystal structure data for Zr–Ni–Al compounds as obtained from DOS relaxation procedure Zr6NiAl2 ZrNiAl Zr5Ni4Al
acalc (Å) 7.745 6.708 6.979
ccalc (Å) 3.277 3.378 6.403
acalc/aexp (%) 0.98 0.97 0.97
ccalc/cexp (%) 0.97 0.97 0.97
Unit cell parameters for the compounds received as a result of the cell optimization for all compounds are in good agreement and in general 3% smaller than those obtained experimentally, which is a common feature of all first - principal calculations [54]. The atomic coordinates, received as the result of relaxation, are summarized in the Table 3.
Table 3. Optimized atomic coordinates for compounds as obtained from DOS relaxation procedure Zr6NiAl2
ZrNiAl
Zr5Ni4Al
Atom Zr1 Zr2 Al Ni Atom Zr Al Ni1 Ni2 Atom Zr1 Zr2 Zr3 Al Ni
x 0.2544 0.5982 ⅓ 0 x 0.5911 0.2496 0 ⅓ x 0.3034 0.1437 ½ 0 0.1310
y 0 0 ⅔ 0 y 0 0 0 ⅔ y 0.1580 0.6814 ½ 0 0.3789
z ½ 0 ½ 0 z 0 ½ 0 ½ z 0 0 ¼ ¼ 0.2700
For all three compounds the shifts of coordinates of the atoms and changes of the cell parameters due to the relaxation are within the error margin, supporting the stability of the
10 phases at low temperatures. Further density of states calculations for these compounds were performed using the optimized cells.
Density of states (DOS) Electronic density of states of ZrNiAl (Figure 3) had been calculated following the procedure described above. At the Fermi level, the electronic density of states of ZrNiAl is rather low, 3.4 states/eV, suggesting a metallic behavior of the compound. Below the Fermi level (around -3 eV), the density of states of ZrNiAl exhibits a peak formed by d-states of Ni1 and Ni2. As the Fermi level is approached from the left side, the influence of d-levels of Zr increases, until they start to dominate the density of states around 1 eV. At the Fermi level more then 40 percent of the density is produced by the d-levels of Zr (see inset in Figure 3). Above the Fermi level the influence of zirconium on the density of states increases even further leading to a complete dominance. In contrast to ZrNiAl, the electronic density of states of Zr6NiAl2 at the Fermi level is higher and equal to 8.5 states/eV. At the energies below the Fermi level (at around -3 eV), the DOS of Zr6NiAl2 is dominated by the d-states of Ni. Moving along the energy scale towards 0 eV, the effect of Ni d-states gets lower and the p-states of aluminium start to dominate the density of states. Around -1 eV, the contribution of Ni is almost negligible, while the DOS is formed by d-states of Zr, together with the p-states of aluminium in almost equal proportion. Such a behavior of the density of states is also observed at and above the Fermi level. The electronic density of states of Zr5Ni4Al at the Fermi level is even higher and equal to around 15 eV-1 (Figure 3). It is characterized by a large peak around -3 eV influenced by Ni d-state`s that dominate the DOS in the negative energy region up to -1 eV where the density of states of Ni d-states becomes marginally equal to that of Zr atoms. At the Fermi level the contribution of Ni d-, Zr1 d-, Zr2 d- and Zr3 d-states are almost equal with Ni d-states getting suppressed at 1 eV. Our results on DFT calculations obtained for Zr5Ni4Al agree well with the literature data discussed above.
11
Figure 3. Total electronic density of states of ZrNiAl (a), Zr5Ni4Al (b) and Zr6NiAl2 (c) per unit cell along with prevailing partial densities (other states are omitted for better readability).
Electrical resistivity
Electrical resistivity of Zr5Ni4Al was measured down to 4.2 K (Figure 4). It behaves metallically decreasing with temperature and exhibits a slight curvature at temperatures around 100 K. The compound is also characterized by a rather low RRR value of 1.89 ( RRR = ρ300 / ρ 0 ). The resistivity of a material exhibiting a comparable set of features is commonly described in terms of the Bloch-Grüneisen-Mott model
ρ B −G = ρ 0 + C
T5
θD6
θ D /T
∫ 0
x5 dx − AT 3 (e x − 1)(1 − e − x )
where ρ 0 stands for the residual resistivity at 0 K. The second term represents the contribution of scattering of electrons on acoustic phonons, while T 3 term accounts for the corrections due to s-d scattering. A least-squares fit of the latter formula to the experimental data leads to a Debye
temperature
θ D = 191 K ,
ρ0 = 113.9 µ Ωcm
and
a
Mott
coefficient
A = 4,04*10−7 µΩcm / K 3 . Similarly to Zr5Ni4Al, the resistivity of ZrNiAl shows a metallic behavior, decreasing with decreasing temperature. The Bloch-Grüneisen-Mott model described above was applied to obtain a Debye temperature θ D = 120 K , a ρ 0 = 53µΩcm and a Mott coefficient
12 A = 4,13*10−7 µΩcm / K 3 as a result of the fit. The residual resistivity ratio was found to be equal to 2.17. Unlike the aforesaid compounds, Zr6NiAl2 is characterized by a much higher overall resistivity and a higher RRR ratio of 3.9. It also exhibits a much stronger curvature in the medium temperature range. The results of the Bloch-Grüneisen-Mott fit are summarized in Table 4, together with the data for the above mentioned compounds.
Table 4. Residual resistivity, Debye temperature and Mott coefficient together with residual resistivity ratio obtained for Zr6NiAl2, ZrNiAl and Zr5Ni4Al using the fit model described in text. Zr6NiAl2 ZrNiAl Zr5Ni4Al
ρ0 , µΩcm
θD , K
194.7 53 113.9
129 120 191
A, µΩcm / K 3 4.04x10-7 4.13x10-7 4.04x10-7
RRR 3.9 2.17 1.89
Figure 4. The electrical resistivity of the compounds in the Zr-Ni-Al system. b) same data represented as ρ / ρ300 K . Solid lines correspond to models described in text
13 Concluding remarks Three ternary Zr-based aluminides with nickel have been characterized from powder X-ray diffraction and SEM/EDX analysis from arc melted samples, annealed at 800 ºC. Rietveld refinements confirmed isotypism with formerly reported structure types: ZrNi0.99(1)Al1.01(1) with ZrNiAl-type, Zr6Ni1.14(1)Al1.86(1) with β1-K2UF6 and Zr4.98(1)Ni4Al1.02(1) with Zr5Ni4Altype. For both Fe2P-type derivative structures, a slight disorder was observed for Al and Ni atom sites, whereas in the U3Si2-type derivative structure, the Zr and Al atoms mutually substitute each other in two atom positions. All three structures exhibit columns of structural fragments extending along the c-axes. The relaxed structures, obtained as a result of the LDA approach, were found to be in close agreement with the experimentally deduced ones. The electronic density of states was calculated for all three compounds. Electrical resistivity measurements performed on samples annealed at 800 °C demonstrated the absence of any phase transitions for these compounds in the temperature region from 4.2 K to 300 K.
Acknowledgements O. Shved and L.P. Salamakha are thankful to the ÖAD for fellowships. This work was supported by the Austrian Science Fund (FWF project P31979-N36). The authors would like to thank M. Waas for the SEM/EDX measurements.
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Highlights
1. New results on crystal structures of ZrNi0.99(1)Al1.01(1), Zr6Ni1.14(1)Al1.86(1) and Zr4.98(1)Ni4Al1.02(1) compounds show the isotypism with earlier reported structure types and reveal the slight disorder of atom sites. 2. DFT calculations assured lattice parameters, crystal symmetry, atomic coordinates. 3. Electrical resistivity of three compounds demonstrated the absence of any phase transitions in the region from 4 K to 300 K.
Declaration of interests X The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: