PLASMON ELECTRON LOSS SPECTROSCOPY AND ELECTRICAL CONDUCTIVITY AT 300 K OF CRYSTALS AND QUASICRYSTALS IN AlMn AND AlMnSi

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PLASMON ELECTRON LOSS SPECTROSCOPY AND ELECTRICAL CONDUCTIVITY AT 300 K OF

CRYSTALS AND QUASICRYSTALS IN AlMn AND AlMnSi

J.-L. Verger-Gaugry, P. Guyot

To cite this version:

J.-L. Verger-Gaugry, P. Guyot. PLASMON ELECTRON LOSS SPECTROSCOPY AND ELECTRI- CAL CONDUCTIVITY AT 300 K OF CRYSTALS AND QUASICRYSTALS IN AlMn AND AlMnSi.

Journal de Physique Colloques, 1986, 47 (C3), pp.C3-477-C3-483. �10.1051/jphyscol:1986348�. �jpa- 00225760�

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JOURNAL DE PHYSIQUE

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

PLASMON ELECTRON LOSS SPECTROSCOPY AND ELECTRICAL CONDUCTIVITY AT 300 K OF CRYSTALS AND QUASICRYSTALS IN AlMn AND AlMnSi

J.-L. VERGER-GAUGRY and P. GUYOT

L.T.P.C.M. ( U . A . CNRS n o 29)

-

E . N . S . E . E . G . . Domaine

Universitaire, BP n o 75, F-38402 Saint-Martin-d'Heres Cedex, France

Resume - L'excitation des olasmons dans les phases quasi-cristallines et cris- m n e s de AlMn et AlMnSi est etudiee par spectroscopie des pertes d'energie electronique r6alis6e dans un microscooe ?I balayage transmission (STEM). On en d6duit une estimation de la resistivite 6lectrique "microscopique" a 300 K.

Le faible ou negligeable effet de 11ap6riodicit6 compar6e a la periodicit6 ou au d6sordre aleatoire dans la position des atomes Mn semble ttre coherent avec une descri~tion en 6tats li6s virtuels d6coupl6s.

Abstract - The plasmon excitation in quasi-crystals and crystals of AlMn and AlMnSi is studied by electron energy loss spectroscopy in a scanning transmission electron microscope (STEM). One deduce an estimate of the "microscopic"

electrical resistivity at 300 K. The weak or negligible effect of the aperiodicity, as compared with periodicity orrardomness of the Mn atoms distribution seems to be consistent with a description in uncoupled virtual bound states.

I

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INTRODUCTION

The role ~ l a y e d by aperiodicity and 5 fold symetry long range orientat ional order in the physical properties of the quasicrystals is evidently an exciting problem. Ho- wever the experimental determination of these properties,particularly those related to propagating waves, suffers from the absence of macroscopic and homogeneous single quasi crystal 1 ine specimens : the melt-spun ribbons are aggregates of quasicrystalline grains exceeding rarely a few microns in size, eventually embedded in a crystalline matrix when the melt chemical composition is off the quasicrystalline stoi- chiometry.

For example, the analysis of electrical resistivity measurements made by the classi- cal four point method on a ribbon A16Mn containing quasicrystals such as those shown in Fig. 1 is evidently tricky : such a specimen is composite ; it contains a volume fraction of c 40 % of aluminum matrix supersaturated in Mn with a large area of irre- gular interface with the icosahedral (i) phase, and whose contribution to the resistivity is probably farfrombeingneqliaible ; furthermore the i-phase contains defects and probably microcraks, the i-ohase being brittle ; finally the geometry of the ribbons is badly defined and presents macroscopic irregularities inherent to the melt- spinning process.

In this extent, transmission electron microscopy (TEM) appear well suited to the investigation of quasicrystals as well as the associated analytical techniques. For example, X-qay-EDS analysis has allowed to determine the composition of the icosahedral phase in AlMn, % A14Mn, and the Mn content of the matrix /1/ / 2 / .

In this work we present an investigation of electron energy loss spectroscopy (EELS), performed in a 100 kV scanning transmission electron microscope (STEM). Ye study

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986348

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C3-478 JOURNAL DE PHYSIQUE

F i g . 1

-

Transmission e l e c t r o n micrograph o f icosahedral phase p a r t i c l e s ( i n b l a c k ) . The A1 m a t r i x has been removed d u r i n g t h e e l e c t r o l v t i c t h i n n i n g Drocess.

t h e plasmon l o s s domain, i n b o t h q u a s i c r y s t a l s ( i - and decagonal o r T-phase) and c r y s t a l s i n AlMn and AlMnSi. From these r e s u l t s we make an estimate o f t h e i r e l e c t r i - c a l r e s i s t i v i t y , as i t has been done b e f o r e f o r small A1 p a r t i c l e s /3/ and g l a s s y metals /4/. Due t o t h e v e r y small diameter of t h e e l e c t r o n probe, we g e t an i n s i g h t

i n t o a "microscopic" r e s i s t i v i t y , f r e e o f t h e d e f e c t c o n t r i b u t i o n s p r e v i o u s l y men- t ionned.

I 1

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EXPERIMENTAL

I n t h r e e melt-spun a l l o y s (A186Mn14, Al80Mn20, A174MnipSi6) t h e i-phase, t h e decago- n a l o r T phase, t h e f.c.c. a s o l i d s o l u t i o n o f Mn i n have been analyzed, as w e l l as t h e c r y s t a l 1 iz a t i o n products a f t e r heating, i .e. t h e orthorhombic AlsMn and hexagonal A14Mn.

The EELS was performed i n t h i n r e g i o n s o f t h e f o i l s i n o r d e r t o avoid m u l t i p l e scat- t e r i n g . Mn and S i local concentrationswere measured by X-EDS i n t h e analyzed r e g i o n s . The VG H B 501 STEM was operated i n c o n d i t i o n s m i n i m i z i n g t h e plasmon peak broadening i.e. w i t h minimum specimen i l l u m i n a t i o n angle a i and c o l l e c t i o n angle Bc o f t h e i n e l a s t i c e l e c t r o n s through t h e energy analyzer. I n these c o n d i t i o n s t h e e l e c t r o n beam focussed on t h soecimen has a diameter o f 4 nm, and t h e maximum s c a t t e r i n g vec- t o r i s (hnax = .311

1-1'

The l o s s f u n c t i o n has t o be c o r r e c t e d f o r i t s s h i f t and broadening due t o t h e i n t e g r a - t i o n over q < qmpx. and t h e f i n i t e energy w i d t h o f t h e i n c i d e n t e l e c t r o n s (% .5 eV, f o r t h e f i e l d emission gun). A f i r s t - o r d e r deconvolution procedure o f t h e l o s s func- t i o n f o r t h e p o s i t i o n o f t h e nlasmon loss E = f i w and i t s h a l f - h e i g h t w i d t h AE1/2 i s given i n r e f e r e n c e /5/, which f i r s t necessieates ?he determination o f t h e plasmon d i s p e r s i o n r e l a t i o n Ep o r AElI2 V S . ~ ~ .

I 1 1

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RESULTS : PLASWON LOSSES, RESISTIVITY

Examples o f t h e l o s s f u n c t i o n obtained i n A l g f l n l q f o r t h e i-phase and t h e a-f.c.c.

m a t r i x are shown i n F i g . 2. The b u l k plasmons a r e c l e a r l y separated, w i t h an energy s h i f t s u f f i c i e n t l y l a r g e t o be a b l e t o make f i l t e r e d images o f each phase.

F i g . 3 shows t h e v a r i a t i o n s o f E and Y 12, a f t e r energy and angular c o r r e c t i o n s , as a f u n c t i o n o f t h e Mn content-detgrmined

by

X-EDS- f o r each phase i n v e s t i g a t e d . The increase o f Ep has a l s o been found by XPS /6/ on coevaporated A1-Hn a l l o y s , without,

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fcc matrix

A l ~ n Lphase

Fig. 2

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Loss function vs. energy loss in AlggMnlq, the probe being focussed on the i-phase and on the f.c.c. matrix.

however, specifying the amorphous or crystalline state of the specimens. Similarly, a linear increase of AE1/2 with Mn content, as determined by optical measurements also on coevaporated alloys /7/ fits reasonably well our data for Mn concentrations lower than 0.05.

In a free electron model, the bulk olasmon loss is

where w is the plasmon frequency, n the electron density and EO the permitivity of free spgce. The half-height width of the Zorentzian loss function can be written /8/

T~ being the plasmon life time, provided than u+rp >> 1 ; such a condition is here pretty well fullf illed (9 5 2.5 x 1016 s-1, T~ varying between .3 and .95 x 10-35s).

If we make the assumption than the plasmon life time is necessarily limited by the relaxation time T, we get for the electrical conductivity, combining (1) and (2) with

T = T - :

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JOURNAL DE PHYSIQUE

Fig. 3 - Bulk plasmon value E, (a) and half-height width A E l I 2 (b) for the various phases investigated.

The resistivity I,= dl, calculated from the measured values of Ep and A E l / z , is gi- ven in Table 1 and plotted in Fig. 4, as a function of the Mn concentration. The un- certainty is particularly high for the reference A1 whichhscsa small half-heigttwidth close to the limit of energy resolution. These measurements have been made at 300 K ,

which is probably the specimen temperature, if one considers that a local heating by the electron beam should be small for such a metallic specimen.

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d - A I orth

AI~~M~,~['

A hex

Fig. 4

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³⁰⁰^Kelectrical resistivity for crystals and quasicrystals, cal.culated from the plasmon loss data, vs. the Mn content.

Table I - Quasicrystals and crystals resistivity. T % 300 K. The extremal bounds are determined by the first-order deconvolution

~rocedure.

IV

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DISCUSSIGN AND CONCLUSION

P ( ~ 6 3 cm) 24 + ²⁰

51 + 21 66 t 29 58 -1 26 66

+

³⁰

68 + 25 23 ? 20 24

+

²⁰

The Matthiessen rule, p = PAT + p~ cyn (1

-

^CM^),would lead for fully disordered Mn solid solutions, on the basis o?

iA1

(300

KT

⁼2.77pQcm and a specific residual resistivity p ~ , = 612pncm /9/, to the following 300 K resistivities : 21.1 ~ Q c m for a-A1 ( C M ~ = 3.1 at .%), 76,4 m c m for Alg6Vnlq, 100,7 ~9 cm for A180Mn20. On the other

CMn (at.%) 3.1 14 t 4

20 20 20 19.5 1.8 -4

0 A1 1 oy

8gMn14

A'80Mn20 A1 74Mn20Si pure A1

Phase a-A1

orthorhombic i

T

hexagonal i

a-A1

A 1

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C3-482 JOURNAL DE PHYSIQUE

hand,Dunlop et al. /lo/ report for the 300 K resistivity of the orthorhombic compound A1 6Mn 80 @ cm.

Discarding the pure A1 resistivity, which is too small to be measured by E.E.L.S.

for energy resolution mainly, the comparison with our data shows :

i/ the E.E.L.S. resistivity of a-A1 is in good agreement with the Matthiessen estimate.

ii/ for orthorhombic A16Mn the E.E.L.S. resistivity is 37 % lower than the value measured by Dunlop et al. /lo/, itself close to the Matthiessen estimate. Such ef- fects already noted by Reda et al. for amorphous Cu-Ag films /4/, are understandable in terms of the "microscopic" nature of the EELS resistivity.

iii/ the most interesting point is certainly the weak, if any, EELS resistivity change between crystal and quasicrystal with the same Yn content (1).

Two facts may help in understanding this result :

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the crystallization of glassy metals gives a drop of resistivity generally of

a few to 20 % ; the i-phase which is long range orientationnally ordered, should give still a smaller decrease.

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a special reference has to be made to the dilute 41-T3 (T3 = transition metal of the third row) compounds investigated by, Dunlop et al. /lo/. The main result of their investigation, analyzed in terms ofMn virtual bound states array, was the almost equivalence between solid solution and crystalline compound of same composition, suggesting the importance of the T3 local environment by A1 atoms. A periodical stacking of local units in the compound, or at random in the solid solution, besides charge transfer in the crystal, are not very different, principally for the 300 K resistivity level : at this temperature the conduction electron coherent scat- tering by a regular stacking of Mn virtual bound states is lost because of thermal motion. If we further notice that local order is probably identical in i-phase and crystal /12/, with no Mn first-neighbours and consequently negligible d-d orbitals overlap , the virtual bound states description should be similar whatever their distribution is random, quasi-periodical or periodical, at 1 east for high temperature transport properties-our results support this view.

ACKNOWLEDGMENTS

This work has been supported by an ATP-CNRS no 90-429. PASTUREL A. is acknowledged for pointing out the work of Dunlop and co-workers.

REFERENCES

/I/ Guyot, P., J. Microsc. Spectres. Electron., 10 (1985) 333.

/2/ Krishnan, K.M., Gronsky, R. and Tanner, L.E.TScripta Met. 20 (1985) 239.

/3/ Batson, P.E., Solid State Comn., 34 (1980) 477.

/4/ Reda, I.M., Schattschneider, P., m e d l , K., Wagendristel, A., Bangert, H. and Gautier, F., Thin Solid Films, 116 (1984) 269.

/5/ Verger-Gaugry, J.L., Guyot, P . and Audier, M., submitted to Physics Lett.

/6/ Steiner, P., Hoechst, H., Steffen, W. and Huefner, S., Z. Phys. B (Cond. Matt.) 38 (1980) 191.

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(1) This result is strongly at variance with those of Parthasarathy et al. /11/, who observed a very large drop o f resistivity upon crystallization under pressure of the i-phase in A178Mn22 and AlggMnl4 (however extrapolation of their results at atmos- pheric pressure glve much toosmall values for the crystals, with furthermore a decrease of resistivity with increasing Mn content ! ) .

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/7/ Beaglehole, 0. and Wihl, M., J. Phys. F. (Met. Phys.), 2 (1972) 43.

/8/ Raether, H., E x c i t a t i o n o f Plasmons and I n t e r b a n d T r a n s i t i o n s by Electrons, Springer-Verlag, B e r l i n (1980).

/9/ Mondolfo, L.F., Aluminium A l l o y s : s t r u c t u r e and p r o p e r t i e s , Butterworths, London (1976).

/ l o / Dunlop, J.B., Gruner, G. and Caplin, A.D., J. Phys. F : Metal. Phys. 4 (1974) 2205.

/11/ Parthasarathy, G., Subbana, G.N., Sekhar, I.A., Rama Rao, P. and Gopal, E.S.R. : Proceedings o f I n t . Conf. San Diego, ASM (1986) p. 373.

/12/ Audier, M. and Guyot, P., P h i l . Mag. B

53

(1986) L43 ; t h i s conference.