ON ELECTRONIC PROPERTIES OF Al-Mn QUASICRYSTALS

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ON ELECTRONIC PROPERTIES OF Al-Mn QUASICRYSTALS

C. Berger, D. Pavuna, F. Cyrot-Lackmann

To cite this version:

C. Berger, D. Pavuna, F. Cyrot-Lackmann. ON ELECTRONIC PROPERTIES OF Al- Mn QUASICRYSTALS. Journal de Physique Colloques, 1986, 47 (C3), pp.C3-489-C3-493.

�10.1051/jphyscol:1986350�. �jpa-00225762�

JOURNAL DE PHYSIQUE

Colloque C3, supplement au n o 7, Tome 47, juillet 1986

ON ELECTRONIC PROPERTIES OF A1-Mn QUASICRYSTALS

C. BERGER, D. P A W N A and F. CYROT-LACKMANN

Laboratoire d'Etudes d e s Propri&t6s Electroniques d e s S o l i d e s , Centre National de l a Recherche S c i e n t i f i q u e , F-38042 Grenoble, France

We present the results on structural and electronic properties of the melt-spun h - M n alloys (10 to 18 at% Mn) which contain more than 50 vol% of the quasicrystalline (qX) phase. The analysis of our data enables us to estimate the resistivity of the qX phase t o be -150-200p.Rcm. High resistivity and high (meta)stability of quasicrystals can be understood within Friedel model of virtual bound states.

While most of the recent research efforts on the studies of quasicrystals were directed toward the explanation of their unusual/unexpected structural character ist icsf we concentrated our attention to the problem of electronic properties.

In this short article we present a brief summary of the structural studies and the results of resistivity measurements (4.2 t o 900 K) of the melt-spun A190Mn10 and A186Mn14 alloys that contain more than - 5 0 vol% of the quasicrystalline ( q X ) phase. Using Landauer two-phese model we estimate the magnitude of electrical resistivity of the qX phase to be of the order 150-200pficm. We proceed with a discussion of the electronic structure. The central point which we emphasize is the idea that the electronic structure as well as transport properties and high (meta)stability of quasicrystals can be at least qualitatively understood within a framework of the Friedel model of virtual bound states?

2. EXPERIMENTAL

We have prepared more than a dozen metallic ribbons of a - M n a1 loys by improved melt-spinning techniques. The samples that we studied contained 3, 10, 14 and 18 at% Mn. We also analysed and measured the samples of A186Mn14 produced at the Bell Laboratories and CEGEDUR-Pechiney. They exibit esentially the same properties as our samples so won't be separately discussed in this article. All the samples were characterised in great detail by x-ray diffraction, electron diffraction microscopy and differential scanning calorimetry (Perkin-Elmer DSC-2). Electrical resistivity was measured by using standard DC four probe technique.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986350

JOURNAL DE PHYSIQUE

Figure 1: a) Transmission electron micrograph of melt-spun A186Mn14. The intergranular zone consists of fcc A 1 with up to 3 at% Mn in solid solution.

Figure 1: b) Electron diffraction pattern of the central dark grain of Figure la.

We have observed the quasicrystalline phase in alloys containing 10, 14 and 18 at% Mn. A190Mn10 and A186Mn14 samples, which we discuss in this article, contain &.m phases :

A190Mn10 consists of -50 vol% of qX phase (typical grain size of 1000 A) imbeded into fcc A1 containing up to 3% Mn

in sol-ia solution.

A 1 8 6 b 1 4 contains *70 vol% of qX phase with a typical grain diambter of 1 p m ; the remainder consists of fcc A1 with varying degree of Mn in solid solution (see Figure 1).

In /addition to the q X phase and the solid solution, A182,Mn18 contains also the decagonal "T" phase; because of this additional third phase and inherent structural complexity we do not discuss the electronic properties of this sample here.

4. RESISTIVITY AND FRIEDEL VBS MODEL

In Figures 2 and 3 we present the temperature dependence of resistivity of A190Mn10 and A186Mn14. We note that the electrical resistivity of our samples varies little with temperature; the temperature coefficient of resistivity (between 4 and 300 K) is small and slightly positive: 2% and 4% in A190Mn10 and A186Mn14, respectively. Both alloys exhibit shallow resistivity minimum at 40 K. This effect as well as field dependence of resistivity will be discussed at

length elsewhere.

Above 600 K the resistivity clearly reveals subsequent stuctural transformations. The exact transformation temperature depends critically on the heating rate. A190Mn10 transforms into mixture of Al, A16Mn and a mixed third phase, while A186Mn14 ends up as a mono phase A16Mn but via complex mixture of phases. The resulting A16 Mn phase exhibits typical metallic behaviour with Q4.,= 5 p R c m and large positive phonon contribution between 4 and 300 K.

We emphasize that the residual resistivity, g,,,of all the samples that we measured (including those from different laboratories) is directly proportional t o the overal concentration of Mn in the sample: Q4,r = 25, 70 and 100 p n c m for samples containing 3, 10 and 14 at% Mn, respectively. This corresponds to - 8 pclRcm/at% Mn which is a typical impurity contribution usually observed in dilute a1 loys. Furthermore, using Landauer formulae and the measured residual reistivity of A197Mn3 (which is roughly the composition of the second phase in our samples) we estimate the resistivity of the qX phase t o be of the order -150-200 ~ R c m . As the approximate composition of the qX phase is A180Mn20 it follows again that each at% of Mn contributes - 8 flcm.

I'

The above analysis of resistivity data and the fact that according to presently available structural models the Mn-s are not to be found at the position of the first nearest neighbours, lead us to believe ttat in the qX phase the Mn-s behave very much like dispersed impurities'(with an average Mn-Mn distance of 4.8 A ) in a nonperiodic A1 matrix?

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MELT-SPUN At9

50 -

*mjxture

0 ^{4 I ,}

I

0 500 1000

T

( K )

Figure 2: The temperature variation of resistivity of A190Mn10.

Figure 3: The temperature variation of resistivity of A186Mn14.

Figure 4: Sohematic presentation of expected electronic structure of 81-Mn quasicrystals.

The overall behaviour of resistivity resembles very much the electron transport in glassy alloys where the resistivity is of the order -100-200 ,uficm and the temperature coefficient is only few percent. The fact that transport electrons loose their coherence is not in contradiction with the fact that electrons involved in diffraction electron microscopy scatter coherently; this is mainly due t o the difference in energy of electrons involved in the phenomenon. The latter cover wide range of enegies while only the electrons with Fermi energy contribute to the electron transport?

Following the above arguments we present in Figure 4 the most likely form of electronic structure of a quasicrystal and note that it is clearly dominated by the presence of virtual bound states. Note that the position of the Fermi level is in the middle of the resonant state. This clearly implies high electrical resistivity: the estimates within the Friedel VBS modelr give a resistivity contribution of -7-8

C

We conjecture that it may well be that the presence of the resonant states is one of the necessary conditions for the formation of the quasicrystalline state.

5 . CONCLUSIONS

In conclusion we note that all presently available data on structure and electrical properties of L-Mn quasicrystals indicate that the high resistivity, which we estimate t o be 450-200,&cm, and high (metalstability of these solids can be

understood within Friedel model of virtual bound states.

Acknowledgements. We greatfully acknowledge stimulating discussions with Professors J. Friedel, M. Cyrot and E. ~ a b i d and structural investigations carried out by Dr P. Germi. We especially thank Dr P. Sainfort (CEGEDUR-Pechiney) and Drs K.V. Rao (RIT, Stockholm) and H.S. Chen (Bell Labs) for kindly giving us their samples of A186Mn14.

1. See other contributions in this Proceedings

2. J. Friedel, Nuoro Cimento, ser. 10, suppl. 7 (1958), p . 287 3. D. Pavuna, C. Berger, E Cyrot-Lackmann, P Germi, A.Pasture1

Solid State Communications, vol. 59, no. 1 (1986) p . 11