Highsymmetry transitionmetal sites in Ti5 6Ni28−x Fe x Si1 6 quasicrystalsR. A. Dunlap, M. E. McHenry, R. C. O’Handley, D. Bahadur, and V. Srinivas Citation: Journal of Applied Physics 64, 5956 (1988); doi: 10.1063/1.342162 View online: http://dx.doi.org/10.1063/1.342162 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/64/10?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Transition-metal and metalloid substitutions in L10-ordered FeNi J. Appl. Phys. 115, 17A710 (2014); 10.1063/1.4862722 Effects of quasicrystal formation on the crystallization of (Ti36.1Zr33.2Ni5.8Be24.9)100−xCux (x=5, 7, 9, 11, 13,15, 17) metallic glasses J. Appl. Phys. 113, 033508 (2013); 10.1063/1.4775836 Valence states of transition-metal ions in cubic perovskites Sr Mn 1 − x Fe x O 3 J. Appl. Phys. 101, 09G523 (2007); 10.1063/1.2713206 Approximate Molecular Orbital Calculations for the TransitionMetal Carbonyls, Ni(CO)4, Co(CO)4−, Fe(CO)4=,Fe(CO)5, and Cr(CO)6 J. Chem. Phys. 52, 1948 (1970); 10.1063/1.1673238 Unimolecular Decomposition of Negative Ions Formed from the TransitionMetal Carbonyls of Ni, Fe, Cr, Mo, andW J. Chem. Phys. 44, 1964 (1966); 10.1063/1.1726969

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High .. symmetry transition .. metal sites in TissNi2B_xFexSi16 quasicrystals R. A Dunlap, a) M. E. McHenry, b) and R. C. O'Hsndley Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge. Massachusetts 02139

D. Bahadur") and V. Srinivas Department of Physics. Dalhousie University, Halifax, Nova Scotia B3H 3J5, Canada

Rapidly solidified Ti-Ni-Fe-Si alloys are found to form a metastable phase with x-ray diffraction patterns which can be indexed to a structure with icosahedral symmetry. 57Fe Mossbauer effect spectra show a negligible quadrupole splitting for the icosahedral phase. This observation, along with a comparison of the measured x-ray diffraction line intensities with calculated values. suggests a structure based on a decoration of a three-dimensional Penrose tiling, with transition-metal atoms in sites with local icosahedral symmetry. Magnetic susceptibility measurements show that a local moment of ~ 0.2 It B forms on the Fe atom. These results are discussed in terms of possible local Fe environments.

t INTRODUCTION

Since the discovery I of icosahedral symmetry in rapidly quenched Al-Mn alloys, the properties of quasicrystals have attracted considerable interest. While early work concen­trated on Al transition-metal systems/,3 similar phases have now been observed in a number of other systems.4

-8 Zhang,

Ye, and Kuos have reported an icosahedral phase in rapidly quenched Ti-Ni-V alloys and Chatterjee and O'Handley9

have shown that the addition ofSi promotes the formation of large quasicrystallites. Dunlap et al. 10 have suggested an in­dexing of the x-ray diffraction pattern on the basis of that established by Bance! etaU Tis6N28 xFexSil6 quasicrystals are the first icosahedral phases to show strong, direct evi­dence (essentially zero quadrupole splitting in a single­phase icosahedral structure) of transition-metal species Fe(Ni) occupying sites of icosahedral symmetry.1l In the present work, we report on structural, thermal, and magnet­ic measurements on quasicrystalline Tis6 Ni28 _ xFcxSil6 al­loys O<:x<:20 at. %. These results are compared with those of crystalline and amorphous alloys of similar compositions.

II. EXPERIMENTAL METHODS

Alloys of the series Ti56Ni28 _xFexSil/j (O<:x<:20) were prepared by arc melting together the high-purity elemental components, followed by roller quenching at a roller surface velocity of 18-60 m/s. X-ray diffraction measurements were made using either a Rigaku rotating anode diffractometer, or a Siemens scanning diffractometer using CuKa radiation. Room-temperature 57Fe Mossbauer effect measurements were made using a Pd 57 Co source and a Wissel system II spectrometer with an intrinsic 57Fe linewidth of 0.24 mm/s. Magnetic susceptibility measurements were made in an ap­plied field of 10 kOe on a SHE VTS905 SQUID magnetome­ter at the Francis Bitter National Magnet Laboratory. Dif-

alOn leave from Department of Physics, Dalhousie University, Halifax, Nova Scotia B3H 3J5, Canada.

b) Present address, MST-5, Los Alamos National Laboratory, Los Alamos, NM87545.

clOn leave from Advanced Centre for Materials Science, Indian Institute of Technology, Kanpur 208016, India.

ferendal thermal analysis (DT A) measurements were made on a Fisher 260F thermal analyzer.

III. RESULTS

As-cast (slow cooled) alloys are found to be a mixture of two crystalline phases, NiTi and Tis Si3 • Presumably, in the Fe-containing alloys, Fe substitutes primarily for Ni in NiTL Alloys which were roller quenched at 18 mls all show a preference for an icosahedral phase. In the slow roller quenching, the Tis6 Ni28 Silo alloy shows a small quantity of additional crystalline phase, possibly NiTi, while aU alloys with 2.S<:x<:20 are single phase. Figure 1 shows the x-ray diffraction pattern of Tis6NizsSi16 solidified by roller quenching at 1 g m/s. Alloys with x> 20 could not be pre­pared in this metastable phase by this method. Dunlap et al.1O have shown this x-ray diffraction pattern to be indexed to a quasicrystalline phase with icosahedral symmetry. The figure shows indices for the quasicrysta11ine peaks as indexed according to the scheme of Bancel et ai. 2 The quasilattice constant a, as defined by Elser, 12 is obtained from the Miller indices (n j n2 n3 n4 nS 1t6 ) of a reflection at a scattering vector q as

a = {[51T"(1 + r)I!2J1q} I~ni€il, (1)

where r is the golden ratio and the basis vectors are cyclic

FIG. l. CuKa x-ray diffraction patterns of rapid quenched Tis6Ni2.Si'6 us­ing a (a) roller surface velocity of 18 mls and (b) roller surface velocity of 60 m/s. For (a), icosahedral indices are given according to the scheme of Hancel et al. (Ref. 2).

5956 J. Appl. Phys. 64 (10), 15 November 1988 0021-8979/88/225956-03$02.40 (~) 1968 American Institute of Physics 5956

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TABLE I. Crystallographic properties of some icosahedral alloys. d is the average interatomic spacing of the elements in crystalline alloys.

a d System CA) CA) aid Reference

AI-Mn-Si 4.60 2.80 1.64 11,12 Al-Mg-Zn 5.14 2.57 2.00 12 Pd-U-Si 5.13 2.90 1.76 8,12 Ti-Ni·Si 4.76 2.62 1.82 10

permutations of (1 + -;2) - 112 ( 1,7,0) ,2.10 Values of a were obtained from a linear regression for measured q values as a function of i ~ni E I. No significant variation in a is found as a function of x for the alloys in the series studied here. This, presumably, indicates that Fe and Ni, which have atomic radii which are very nearly the same, substitute for each other on the same sites. From the present data we find a = 4.76 A. A comparison of the crystallographic properties of icosahedral Ti-Ni-Si and other weU-known icosahedral systems8

•i3 is given in Table I. Quenching at a higher roUer

surface velocity, and hence at a higher cooling rate, yielded an amorphous phase as illustrated in Fig, 1 (b). This phase shows a crystallization exotherm in the DT A measurements between 800 and 950 K with an increase in the crystalliza­tion temperature Tx with increasing x, X-ray diffraction studies of crystallized samples indicate that the principal crystallization product is the quasicrystalline phase. An­nealing near 1200 K. was found to cause a transition from the quasicrystalline phase to a crystalline Tiz Ni phase.

Typical room-temperature s7Fe Mossbauer effect spec­tra are illustrated in Fig. 2. Spectra of all quasicrystalline alloys show a single line, while spectra of amorphous alloys aU show a well-resolved doublet. Mean values of the isomer shift and quadrupole splitting are given in Table II. Quasi­crystalline spectra were fit to one singlet and to two singlets. The relative improvement in MISFIT!4 for the two-singlet fit is taken to be an indication of the validity of fitting to more than one site. For x<7.5 a single Fe site seems to describe the data well, while for larger x a two-site model seems to be preferable. In all cases there is no evidence of any measurable quadrupole splitting, i.e., A :s; 0.1 mm/s, The amorphous al­loy spectra were all fitted to a symmetric quadrupole split doublet. Although some of these spectra show some asym­metry, it is very small, indicating a weak correlation between

FIG. 2. Room-temperature 57Fe Miissbauer effect spec­ira for (a) quasicrys­talline Ti,,,Ni2C.5 Fe7 .S ~>i'M (b) amorphous TisoNi,o.5 Fe7., Si'6' (1:) quasicrystalline Tis6Ni-8Fe,oSi'6' and (d) amor­phous Ti%NigFe2"Si",.

the quadrupole splitting distribution and the isomer shift distribution.

The temperature dependence of the magnetic suscepti­bility X of some of the quasicrystaUine alloys is illustrated in Fig. 3. The aHoy with x = 0 shows a small temperature-inde­pendent component Xo and a weak temperature dependence below - 20 K. The Fe-containing alloys show a considerable increase in Xo and a more strongly temperature-dependent component at low temperatures. The measured susceptibili­ties were fitted to a Curie behavior of the form

(2)

where fleff is the effective paramagnetic moment in units of J.t B, and (} is the paramagnetic Curie temperature. Table HI gives the parameters from Eq, (2) which are obtained from a least-squares fit of the data in Fig. 3. These resu.lts may be compared with those obtained for Al transition-metal alloys where Fe carries no measurable moment, 15.16 while Mn car­ries a moment of up to 1.3 J.t B (Ref. 17) depending on the stoichiometry of the alloy. The present measurements cer­tainly indicate a measurable Fe moment in the Ti-Ni-Fe-Si quasicrystals. The small nickel moment 0.078 J.t 8 taken from the data for x = 0 is slightly larger than that reported earlier? for Tis6 Nizs Si 16 •

TABLE II. 57Fe Mossbauer effect parameter for quasicrystalline and amorphous Tis6NizB _. x Fex Si '6 alloys. The values of {) are given relative to a-Fe. The linewidth r is given as the FWHM. A is the ratio of MISFIT for a one-singlet fit to that for a two-singlet fit. Spectra were not obtained for amorphous alloys of all compositions. A suitable fit to the x = 2.5 quasi crystalline alloy could not be obtained while using two singlets,

Quasicrystalline Amorphous

bt 52 r, r 2 {j r A x (mm/s) (mm/s) (mm/s) (mmls) A (mm/s) (mmls)

2.5 - 0.180 0.254 - 0.105 0.456 0.43 5 - 0.167 0.388 1.8 7.5 - 0,175 0.346 1.3 - 0.112 0.384 0.45

10 - 0,251 - 0.118 0.248 0.328 5.5 15 -0.239 -0.097 0.304 0.3i4 21.0 20 - 0.218 -o.on 0.342 0.294 8.5 - 0.124 0,377 0.42

5957 J. App!. Phys., Vol. 64, No. 10, i5 November 1988 Dunlap et al. 5957

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1O'---_..L....1 _-:-<1 __ ...,..; o 100 200 300

T (K)

IV. DISCUSSION

FIG. 3. Magnetic sus­ceptibility of Ti56Nin _ .• Fex Silo alloys in the icosahedral phase.

X-ray diffraction patterns of the Ti-Ni-Fe-Si alloys studied here, which have been roller quenched using a sur­face velocity of 18 mis, can be indexed according to the pattern proposed by Bance! et ai.2 and Elserl2 for icosahe­dral Al-Mn alloys. The relative line intensities observed here are quite different, as would be expected, from those mea­sured on AIMnSi quasicrystals. However, they are in good agreement with calculated values l8

•19 based on a structural

model of a three-dimensional Penrose tiling (3DPT) com­prised of i.nterconnected rhombohedra where atoms are lo­cated on rhombohedral vertices and edges. Elser and Hen­ley 12, 13 have suggested that the quantity aid given in Table I is an important indicator of the details of the decoration of the 30PT in icosahedral structures. The present data indi­cate a decoration intermediate between that proposed for A!~ Mn-Si and that proposed for Al~Mg-Zn.13 The present pro­posal for a decorated 3DPT is consistent with the previous models of Henley and Elser. 13 The high symmetry of the Fe sites, as indicated by the negligible quadrupole splitting of the 57Fe Mossbauer spectra of those icosahedral alloys, is consistent with the above model where high-symmetry Fe sites are located at rhombohedral vertices which correspond to the center of icosahedral clusters. The present Mossbauer measurements suggest that for x -7.5, Fe may as well go to a second site.

The magnetic moments which exist on Fe atoms ob­served in the present work are much larger than those which have been observed previously on Fe in other quasicrystal~ line alloys. Calculations on Mn containing icosahedral a1-!oys20.21 have shown that the high symmetry found at the center of icosahedral clusters is conducive to magnetic mo­ment fonnation. This is, as well, a possible explanation for the existence of Mn moments in AI-Mn-Si quasicrystals which are much larger than in analogous crystalline AI-Mn­Si aUoys.21

In conclusion, we observe a quasicrystaHine phase in rapidly quenched Ti-Ni-Fe-Si alloys. Quenching at a higher rate produces an amorphous phase. There is evidence pro­vided by the present results that these alloys form an icosahe­dral phase with the following properties:

(1) a single highly symmetric Fe site at least for x S 7.5 based on Mossbauer measurements;

(2) a value of aid suggesting an icosahedral phase inter-

5958 J. Appl. Phys., Vol. 64, No. 10, 15 November 1988

TABLE III. Magnetic susceptibility parameters obtained from Ti56Ni'8 _ x Fex Si16 from Eq. (2). For x = 0, f.teff is given per Ni atom and for x > 10, f.teff is given per Fe atom.

%0 jJ-err e x CjJ-emu/gOe) (JLJj ) (K)

0 1.5 0.G78 3.5 10 14.2 0.144 2.4 15 14.4 0.224 7.9

mediate between that observed in AI-Mn-Si and that ob­served in AI-Mg-Zn;

(3) an x-ray diffraction pattern with line intensities con­sistent with point (2); and

( 4) the fonnation of a significant Fe magnetic moment. All of the above observations point to a new icosahedral

structure with at least some transition-metal sites having ico­sahedral symmetry.

ACKNOWLEDGMENTS

Portions of this work conducted at M.l. T. were support­ed by a grant from the U.S. Army Research Office, contract No. DAAL-03-87-K-0099. Portions of this work conducted at Dalhousie University were funded by grants from the Natural Sciences and Engineering Research Council of Can­ada.

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