Hyperfine Interactions 94(1994)2249-2254 2249

EFG studies of the AICuFe quasicrystal and related structures

R.A. Brand a, J. Pelloth a, F. Hippert b and Y. Calvayrac c a Laboratorium fiir Angewandte Physik, Universittit Duisburg,

D-47048 Duisburg, Germany bLaboratoire de Physique des Solides, U.A. 2, CNRS, Universitd de Paris Sud,

Bat. 510, F-91405, Orsay Cedex, France cCECM/CNRS, 15 rue G. Urbain, F-94407 Vitry Cedex, France

It is known that the structure and thermal stability of the AICuFe icosahedral quasicrystalline (i-QC) phase is extremely sensitive to the exact composition. A stable, ideally quasiperiodic phase exists only along a line around A162.0Cu25.sFe12.~. The other i-QC samples show structural changes when annealed around 700 ~ The 57Fe M6ssbauer EFG properties and isomer shifts of these various i-QC phases and of the new rhombohedral approximant phase are presented, and compared with the properties of the A1 site investigated by NMR. Certain new systematic trends will be presented.

The structural and electronic properties of the stable icosahedral phases have recently attracted much attention. Quasicrystalline (QC) alloys are found in the complex region of the ternary phase diagrams. Among the many crystalline (C) phases with compositions close to the QC ones, approximant phases are especially interesting. These have similar local order [1] as in the neighboring icosahedral (i-) and other quasiperiodic phases, and are important in establishing to what extent physical properties of i-QC phases are affected by the quasiperiodicity and which features can then play a role in stabilizing these new QC phases. A well-established feature of all thermodynamically stable i-QC is a strongly reduced density of state at the Fermi level N(EF) compared to estimated free-electron values [2, 3, and references therein]. This has provoked the suggestion that a pseudo-gap at EF can explain the thermal stability of these i-QC as Hume-Rothery (H-R) phase [4-6]. However, the electronic properties of the C approximant phases have recently been found to be very similar to the QC ones [7-9].

The stable i-QC phase is located in a narrow strip between (22.5, 12.5) and (24.4, 13.0) [10, 11] (here and in the following, the Cu and Fe concentrations in at.% are given). In the sense of an H - R phase [4-6], this i-QC line corresponds to a line of constant electron/atom ratio (e/a = 1.862) with effective "valencies" of 3, 1 and - 2 for AI, Cu and Fe, respectively [11]. This surprising result for Fe has

�9 J.C. Baltzer AG, Science Publishers

2250 R.A. Brand et al. / EFG studies of AICuFe

been shown [6] to result from resonance scattering of the d-states at EF, and to be general for H - R phases with transition elements. Thus, no abnormal charge transfer is necessary to account for these results. A stable rhombohedral (r-) phase [11] was recently found to exist along a parallel (r-C) line between approximately (25, 11.7) and (28, 10.4) corresponding to e/a = 1.92 [12]. Along the r-C line, the i-QC phase exists at temperatures higher than 730 ~ but transforms to the r-C phase by annealing at 710 ~ Thus, we can separate chemical and structural influences on the changes in the electronic structure. By studying samples of i-QC from along these two lines, we detect compositional effects. By studying i-QC and r-C samples taken along the rhombohedral lines, we detect structural effects alone. Here, we present some preliminary results of room temperature (RT) 57Fe M6ssbauer spectroscopic (MS) studies of the central line shift (isomer shift) and electric field gradient (EFG), and on NMR spin-echo results of 27A1 for the resonance frequency and shape for samples along these two lines, in the icosahedral and rhombohedral structural states.

The samples used in this study were prepared at CECM by planar flow casting. The flakes were annealed at 800 ~ for 2 hours to produce a perfect a-QC state. This procedure eliminates the small amount of ~-phase present in the as- quenched samples and suppresses phason disorder [10]. The i-QC samples were then quenched from 800 ~ to impede any structural transformation. The r-C samples were produced by annealing the as-quenched flakes at 700 ~ for 3 days. The exact compositions of the ideal i-QC and r-C phases have only recently become clear [10, 11]. Almost all previously reported M6ssbauer spectra are from samples with a nominal composition of A165Cu20Fels, which was previously belived to be the perfect i-QC. Thus, most of these samples were probably in a metastable state (if quenched), or not phase pure (if annealed). For the samples reported here, there can be no more than a few percent of secondary phases present (X-ray positions are to at least within A20 = 0.002 ~ of the perfect icosahedral structure).

Figure 1 shows several RT Mtissbauer spectra. The typical broadened quadrupole doublets indicate a continuous distribution of local environments, with no phases having strongly different hyperfine properties being present. The spectra have been evaluated with a Gaussian quadrupole distribution (not strictly the case, but sufficiently accurate to obtain an idea of the distribution width). The resulting parameters for the average central line shift (t~) (isomer shift with respect to bcc Fe at RT), average quadrupole splitting (AEQ), Ganssian standard deviation tr, linewidth asymmetry F21F1 and line area asymmetry A21AI are given in table 1. The latter two parameters indicate the ratios between left (1 st) and right (2nd) lines of the quadrupole doublet. The results are given first for the i-QC samples on the ideal i-QC line (a-g) , then for the i-QC samples along the r-C line (h- j ) , and finally for the r-C samples along the r-C line (k-m) .

The most striking results are the relatively constant values for (t~) and (AEQ) for the samples along the i-QC line, and a change of about (0.025 + 0.009) mm/s in (t~) and (-0.046 + 0.01) mm/s in (AEQ) for the i-QC samples from the r-C line

R.A. Brand et al. / EFG studies of AlCuFe 2251

Z ~ I I I I I I I

(/3

7

Or" I-- W >

._.1 kl_l Or"

I I I I I I I I -1 0 +1 VELOCITY ( m m / s )

Fig. 1. RT MOssbauer spectra for some samples fitted with a transmission integral. The letters indicate the samples given in table 1.

Table 1

Results from fitting the Mtissbauer spectra with a Gaussian distribution of asymmetric quadrupole spectra. Samples a - g are along (or very close to) the i-QC line, and h - m are along the r-C line. From the latter, h - j are in the i-QC state and k - m in the r-C state. Random errors are in the range of 0.002 mm/s for (r (AEQ) and or, and 0.05 for F2/FI and AJA I.

Sample %Cu, %Fe (8) (AEQ) r F21F I AJA|

a 24.4, 13.0 0.251 0.368 0.095 0.77 0.79

b 24.6, 12.9 0.237 0.397 0.096 0.84 0.86

c 24.9, 12.8 0.248 0.373 0.093 0.83 0.84

d 25.3, 12.6 0.256 0.375 0.090 0.74 0.78

e 25.5, 12.5 0.248 0.380 0.098 0.80 0.82

f 25.5, 12.5 0.244 0.380 0.097 0.83 0.84

g 26.0, 12.2 0.243 0.382 0.093 0.82 0.84

h 24.5, I 1.9 0.221 0.415 0.096 0.94 0.94

i 26.0, I 1.2 0.230 0.424 0.099 0.71 0.82

j 28.0, 10.4 0.215 0.435 0.089 0.89 0.90

k 24.5, 11.9 0.225 0.411 0.089 0.79 0.86

1 26.0, 11.2 0.221 0.436 0.099 0.69 0.84

m 28.0, 10.4 0.215 0.436 0.091 0.93 0.91

2252 R.A. Brand et al. /EFG studies of AICuFe

to those on the i-QC line (with a 1% change in Cu), but no significant change in either (t~) ((0.002 + 0.009) mm/s) or in (AEQ) ((-0.003 + 0.01) mm/s) when going from the r-C samples to the i-QC samples along the r-C line. The residual composition dependence along the i-QC line is essentially zero ((0.005 + 0.01) mm/s pro %Cu). Changes in lattice volume are small enough to be neglected here. Notice that the change in (tS) pro %Cu is several times faster away from than along the perfect i- QC line. These results indicate that there is a decrease in the total s-electron density going from the r-C line to the i-QC line, but little or no change along the lines, or along the r-C line between the two structures (stable rhombohedral or metastable quasicrystalline). Although these changes are small, they have been found in many samples.

The standard deviation tr of the distribution indicates a distribution of quadrupole effects and central line shifts (isomer shifts) found in the sample (which lead to a line broadening), while the ratio of (Lorentzian) linewidths F2/F1 indicates a correlation between the local values of t5 and AEQ (this broadens one line of the doublet at the expense of the other). The ratio of areas is very surprising in these samples. Although this has often been reported for i-QC systems [13,14], there often remains the question of different phases. All texture effects have been eliminated in these spectra by mixing with a fine BN powder, and checked by using magic angle geometry (an angle of 54.7 ~ between sample normal and )'-ray). The remaining explanation is that there is an intrinsic anisotropy of the Lamb-M/Sssbauer (Debye-Waller) effect f (O) [15]. This could be expected to be the case in certain models of the i-QC structure of this system [16]. Alternatives for the assumed Gaussian distribution include the )~2 distribution for n = 5 degrees of freedom (A/o')n-lexp(-0.5(A/o') 2) which results from the Czjzek shell model [17], but is actually more general [18]. The distribution assumed by Lawther and Dunlap [14] with variable n is neither the shell distribution nor is it clear where such a distribution could come from [18].

The spin-echo signal of 27A1 was recorded using pulsed NMR techniques in a fixed external magnetic field of 6.996 T at RT on the same samples used in the Mt~ssbauer studied. Typical spectra are shown in fig. 2. The AI spectra show narrow lines associated with the m = + � 8 9 1 8 9 nuclear spin transition, superimposed on a broad line of about 3 MHz due to the distribution of the quadrupole splitting on the remaining four nuclear transitions (which does not affect the + 1/2 transition to first order) [17, 19]. Thus, there exists a broad distribution of local environments at the AI site as well. The shape of the central line comes mainly from second-order quadrupolar effects and is found to be sample independent within experimental accuracy. However, the sensitivity of NMR to small changes in the EFG distribution is much poorer than that of MS. In contrast, the positions of the NMR lines are sample dependent. It is striking to note that the NMR lines are found to be very similar for all the perfect i-QC samples along the i-QC line. The same is true for all the samples, i-QC and r-C, along the r-C line. No clear differences have been detected for samples with the same composition but different metallurgical state.

R.A. Brand et al. /EFG studies of AICuFe 2 2 5 3

~ 0 . 8

o .E: u

~ 0,/-. n,- >-- z

O. 0 i

i i i i i i

o (b)

I I I I I I

77.55 77.50 77.0& 77.68 Frequency (MHz)

Fig. 2. NMR pulse echo spectra for some samples. The letters indicate the samples given in table 1. Only the +I/2 transition is shown.

The frequency Vm for the maximum intensity along the i-QC line is 77.607 MHz, and is about 5 kHz higher for samples on the r-C line. The shift of the resonance from the reference position V0 (77.6043 MHz, deduced from the AI resonance in fcc A1 in the same field) is very small. K = ( V m - Vo)/V 0 is of the order of 0 < K < 10 -4, to be compared to 1.64 x 10 -3 in A1 metal. Such small values are difficult to analyze quantitatively since they result from several terms such as chemical shift, shift due to second-order quadrupolar effects, demagnetization effect and coupling with the conduction electrons. In metals the last term, the Knight shift Ks, is usually dominant. Ks = (8r~/3)Zp(I l/t(0)12)FS, where Zp = #2 x N(EF) is the Pauli susceptibility and ( I ~t(0)12)~s is the density of s-electrons at the nucleus on the Fermi surface. The small K measured in i- and r-A1CuFe alloys (and the associated long spin lattice relaxation times [7, 19]) are thus in agreement with the observed reduced density of states at the Fermi level. The small changes in K observed from sample to sample cannot be ascribed to changes in the EFG distribution because the spectral line remains unaffected. Changes in either the magnetic susceptibility, the chemical shift or the Knight shift must be invoked. When comparing the NMR and the MS results, we see a striking correlation between the constant isomer shift along the i-QC and r-C lines and the constant NMR frequency along the same line. Any difference between the i-QC and r-C samples on the r-C line is beyond the accuracy of the NMR experiment.

In conclusion, we have presented certain correlations between the MS and NMR hyperfine properties in the A1CuFe system for i-QC and r-C phases. The electronic properties are approximately constant along the i-QC and r-C lines, and confirm the interpretation that these can be taken as H - R phases. The small changes observed in the hyperfine properties confirm that the r-C phase is an approximant of the i-QC phase. These small changes have been separated into composition- and structure-dependent effects. These results help to confirm that the rhombohedral phase is also stabilized by the electronic properties, as was concluded from the

2254 R.A. Brand et al. / EFG studies of AICuFe

NMR TIT [7], as well as from the EXAFS studies [20]. It should be noted that other known nearby phases (the cubic 13 phase, monoclinic X phase, and the tetragonal to phase) have very different EFG properties [21], showing that they have very different local structure.

Acknowledgement

We would like to thank D. Gratias (CECM, Vitry) for fruitful discussions.

References

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[14] D.W. Lawther and R.A. Dunlap, J. Non-Cryst. Solids 153/154(1993)45. [15] See, for example, E.R. Bauminger and I. Nowick, in: M~ssbauer Spectroscopy, eds. D.P.E. Dickson

and F.J. Berry (Cambridge University Press, London, 1986) p. 219. [16] D. Gratias, private communication. [17] G. Czjzek et al., Phys. Rev. B23(1981)2513. [18] G. Le Ca~r and R.A. Brand, Hyp. Int. 71(1992)1507. [19] A. Shastri et al., J. Non-Cryst. Solids 153(1993)347. [20] A. Sadoc et al., Phil. Mag. B68(1993)475. [21] R.A. Brand et al., to be published.