PHONON DENSITY OF STATES OF MOLYBDENUM CHALCOGENIDES

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PHONON DENSITY OF STATES OF MOLYBDENUM

CHALCOGENIDES

P. Schweiss, B. Renker, J.-B. Suck

To cite this version:

JOURNAL DE PHYSIQUE _{CoIIoque C6, supplgment au no 8, Tome 39, aoiit 1978, page C6-356}

PHONON D E N S I T Y OF S T A T E S OF MOLYBDENUM CHALCOGENIDES

++

P. Schweiss+, B. ~enker* and J.-B. Suck

+ Kernforschungszentrmm KarZsruhe GmbH, I n s t i t u t fiir Angewandte Kemphysik,

++ Postfuch 3640, 0-7500 ICarZsmhe, R. F. A

I n s t i t u t Laue Langevin, F-38042 Grenobte, Franee

Resume.- Nous presentons des experiences sur la densitd d'6tats de chalcog6nides du molybdene qui montrent une corr6lation qualitative entre un "softening" de phonons et les qualites supraconductri- ces

.

Abstract.- Phonon density of states measurements are reported for a series of Mo-chalcogenides sho- wing mode softening in qualitative correlation with the superconducting transition temperature Tc.

/ I / are of special interest due to their exceptional

Glhwll

-

INTRODUCTION.- The superconducting Mo-chalcogenides

high critical fields /2/ at comparatively high Tc- k&vj values 1 3 1 . Indications for lattice instabilities

and soft modes / 4 / have been reported for several 0.03

I

1

-

-

.-

4

compounds. In this paper we report inelastic neu- tron scattering results which show up characteristic

0.02 vibrational frequencies for the single components

which, when combiyed with the observation of an ano- malous temperature behaviour, may help to find modes _0.01 relevant for superconducting properties. Our samples were prepared from the elements as usual with excep-

tion of Mo6S8 which was produced from NiMo6S8. Neu- 0 tron powder patterns were obtained for all samples and did not show indications of other phases. Gene-

0.01 ralized phonon densities of states G(Hw) were obtai- ned from the inelastic neutron spectra of polycrys- talline samples in the usual way / 5 / . All spectra ₀ were corrected for multi-phonon scattering contri- butions. In order to get the weights of the partial G(Hw) as accurate as possible a larger Debye-Waller O.O1 coefficient had to be considered for the single

sharp line around 4 meV attributed to vibrations of 0 the metal cations.

RESULTS.- The 297 K results for the sulfides Pb1.08M06S8.11~ S ~ I , I ~ M O ~ S S , + ~ , C~i,561106S6,83 and Mo6Se,07are plotted in Figure 1 and those of the

selenides Pb1,2.1M06Se7,8 and M0~Se7.62 in figure 2 . 0 10 2 0 3 0 LO 50 h w l m e V I Spectra which also show 5 K results had to be mea-

sured,with a higher neutron incident energy at the

Fig. 1 : Comparison of G(h) for some Mo-sulfides. expense of resolution in order to enable scattering The actual chem. composition of these defect struc- in the energy loss mode. This affects the height of tures is given in the text

the single sharp line around 4 meV in some of the

spectra. By comparison some general features become apparent : the spectra may be divided in a lower

Fig. 2 :Conparison of G($u) for MoeSeBand PbMo6Ses.

part containing the external modes and a high fre- quency part containing the internal modes of the Mo6X8 units /6/. In agreement with the spectral weights this cut is placed at 18 meV for the sulfi- des and at 14 meV for the selenides respectively. One common feature in the external spectrum is a peak at 15 meV in figure 1 and at 12 meV in figure 2 which in a simplified description is attributed to torsional oscillations of the Mo6Xe units around their axis of inertia. The ratio of the different moments of inertia (~2.0) is somewhat larger than the value found experimentally (1.56). It can be seen that the appearance and the position of the two lowest frequency peaks are correlated with the presence and the mass of the metal cation. The lo- west and most pronounced peak is attributed to an Einstein-like optic mode associated with oscilla- tions of the weakly bound metal atoms in the open channels between neighbouring Mo6Xs units. The sharpness of this peak in frequency, its absence in the binary compounds, its anomalous large Debye- Waller factor, and the fact that the frequency is

not affected when going from PbMoBSes to PbMo6Se favours this hypothesis. The second broader structu- re around 10 meV is attributed to acoustic vibrati- ons of the unit cells. The internal part exhibits three broader maxima, the highest of which in the sulfide spectra is clearly associated with vibrati- ons of the light sulfur atoms. In view of the lar- ge number of internal modes (36 for a Mo6Xs unit) and the low structural symmetry a more detailed at- tribution to eigen-vibrations of a certain species of atoms appears meaningless.

The results of 5 K measurements are shown together with the room temperature results. An in- crease of the spectral weight at low frequencies is observed as a common tendency for the superconduc- ting compounds. The higher frequency modes could be measured up to the second broader maximum in the spectrum of internal vibrations. No frequency softe- ning is observed within this region. Therefore it appears likely that this behaviour is also true for the rest of the internal spectrum.

DISCUSSION.- The interpretation of our G(Mw) spectra is in accordance with the molecular-crystal model/6/ which seems to provide a reasonable basis for the

understanding of the lattice dynamics of these com- plicated compounds. Here we shall focus on possible relations to superconducting properties. Evidently we find the same temperature dependence for the Debye temperature de/dT > 0 as reported for SnMosSs /7/ and characteristic for high T A15 superconduc- tors connected with the occurance of soft modes / 5 / .

The increase in intensity and the small frequency shift which is observed for the S 4 meV peak in the

ternary compounds is mainly attributed to changes in the underlying acoustical spectrum in agreement with the results obtained for Mo6Ses. Almost no

A complementary study of phonon models on sin- References gle crystals of Mo6See and Cu2M06S8 is presently un-

derway. /I/ Chevrel,R., Sergent,M. and Pringent,J., Solid

State Chem.

2

/2/ Odermatt,R., Fixher,@., Jones,H. and Bongi,G.

J. Phys. C l (1973) L13

/3/ Matthias,B.T., Marezio,M., Corenzwit,E., Cooper,

A.S. and Bartz,H.E., Science

175

electrics

E,z

/5/ Scweiss,P., Renker,B., Schneider,E. and Rei-

chardt,W., "Superconductivity in d- and f-band metals" 2nd. Rochester Conf. (1 976) 189. /6/ Bader.S.D., Knapp,G.S., Sinha,S.K., Schweiss,P.

and Renker,B., Phys. Rev. Lett.

37

F.Y., Dunlap,B.D. and Shenoi,G.K., Phys. Rev. Lett.

36

/ 8 / Anderson,O.k., Klose,W. and Nohl,H., Phys. Rev.