PHONON AND ELASTIC ANOMALIES IN INTERMEDIATE VALENT TmxSe AND TmSe1-yTey

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PHONON AND ELASTIC ANOMALIES IN

INTERMEDIATE VALENT TmxSe AND TmSe1-yTey

M. Celio, R. Monnier, P. Wachter

To cite this version:

JOURNAL DE P H Y S I Q U E

CoZZoque C6, suppZ6ment au n012, Tome 42, de'cembre 1981 page C6- I I

PHONON AND ELASTIC ANOMALIES IN INTERMEDIATE VALENT TmxSe AND TmSel-yTey

M. Celio, R. Monnier and P. Wachter

Abstract.- In TmxSe and TmSel-yTey the degree of valence mixing can be adjusted between nearly 3+ for Tm0.87Se and 2 . 5 5 + for TmSeo-7Te0.3. The sound velocity v~ decreases with increasing degree of valence mixing and the elastic constant c12 becomes negative. The first order Raman spectrum shows an additional peak

at 60 cm-l for strong valence mixing and a softening of the LO (L)

frequency. The experiments are compared with the calculation of an 8 parameter shell model resulting in projected density of phonon states.

TmSe is one of the most interesting intermediate valent (IV) materials

inasmuchas it is IV at ambient pressure and its degree of valence mix-

ing can be adjusted by composition (TmXse1)) or by alloying with other ions2). Thus the valence as determined by the lattice constant is nearly 3+ for Tm0.87Se and 2.55+ for TmSe0-7Teg.~ 3 ) . On these composi- tions we have performed Raman scattering, ultrasound velocity and elas- tic measurements 4 ) . In these rocksalt type structures the Raman effect measures a defect induced weighted one phonon density of states 3)which

is displayed in Fig. la for

:

7

4 different compositions of Tm,Se.The curve d i f f e r e n c e s p e c t r a

Trn, S e

-

Tm.@,Se

Fig.la: Raman spectra of trivalent Tmo.87Se and intermediate valent TmxSe.

Fig. lb: Contribution of the an malous peak at 60 cm-' to the scattering intensity, computed by substrac- tion of the trivalent spectra.

F R E O U E N C Y S H I F T Ccm-13

C6- 12 JOURNAL DE PHYSIQUE

for the trivalent x=0.87 at the bottom of Fig-la is identical to other

trivalent selenides like GdSe or LaSe and corresponds to TA and LA phonon densities in the low energy range and TO and LO densities for the higher energy peaks 5). As one increases the degree of valence mixing, x + 1.05, a significant peak at 60 cm-I is emerging within the

acoustic region which is shown in more detail in Fig-lb. Also with in-

creasing valence mixing the high energy edge of the LO band softens

and merges with the TO band. This softening of the LO band between a

Tmif87~e and a hypothetical T m 2 + ~ e , but a real yb2+se, is shown in more detail in Fig. 2, where for comparison also the much smaller softening

3+

of the trivalent series Tm0.87Se

-

LaSe due to the lanthanide contrac-

tion is displayed.

The sound velocities v L , ~ T 1 and vT2have also been measured and the elastic constants cll, c12 and c4* have been derived 4). With increas- ing valence mixing vL reduces from 3 . 4 8 ~ 1 0 ~ cm/sec for Tmo a7Se to

,-

2-56-10' cm/sec for Tm0.99Se. At the sane time c becomes negative,

3

going from 2.1.10'~ erg/cm3 to

-

5 . 7 ~ 1 0 ~ ~ erg/cm

,

respectively.

As a consequence the compressibility rises from 1.46.10-~bar-~ for

Tm Se to 4.61.10-~bar-l for Tm0-g9Se. These elastic anomalies per-

0.87

sist down to 4.2 K.

A softening of LO(L)- and LA(A)phonons with increasing degree of va- lence mixing has been predicted already 6, and is well born out by the experiment (see above). The negative c12 for IV TmSe has a dominant effect for v Ill11 which becomes even less than vTI1ll(, forcing the LA

L

branch below the TA branch, Since no 6 )

Softening of the lA(L) is expected

,

a qualitative ~~llllldispersion has

been constructed which indicates for

-1

the anomalous Raman peak at 60 cm 3 )

mainly a density of states effect

.

A different point of view has been taken 7)by assuming that the elec-

+

rl

breathing mode enhance the Raman

intensity of those parts of the pho- non dispersion curves which soften for IV.

2

I

Fig.2: Peak position of the LO We have calculated the phonon spec-

bapd of Tm Se, GdxSe and LaSe.

Stars: w ~ ~ in shell model cal- ~ L ) trum of IV TmSe by using an 8 para-

il-

Tm.,,Se 3 0 0 K

A p r o l e c t e d Cf

WRVENUMBER Ccm-'1

0 5 0 I 0 0 150 2 0 0 _{2 5 0} meter shell model with

'a I'a 2 0 FREQUENCY 6 0 A namical matrix D(q) is a(q)= D RI

(9)

+'she 11

(3

+D

(q)

with DRI the

de f

' 3 0

1,

def ormable ions. The dy-

point charge rigid ion matrix, D

shell the ma- trix for a simple shell

node1 and D accoun-

def

ting for the breathing

+

(rl) of the Tm ion and

+

Fig. 3: Phonon density (B) and projected I.1 a quadrupolar deforma-

₊

density of states (A) of IV TmSe. bility (r25) of the Se

ion. The parameters were adjusted using the lattice constant, cll, c12 and c~~ 4)and uTO and uLO at the I. and I points 'I. The continuum cha- racter of the 5d electrons is reflected in a high polarizability of the Tm ions. Xowever

,

density functional results for the charge distribution around a trivalent impurity in an electron gas at the corresponding density indicate that the induced charge is too localized to effective- ly screen out the long range Coulomb interaction between the ions, thereby justifying the use of a shell model for the description of that systern. The measured dispersion curves 8)are reasonably well reprodu- ced by our model in the 1100) and 11101 direction, but essential diffe- rences appear in the 11111 direction, where the life time broadening

of the longitudinal modes is especially large. In Fig. 3 we show the

one phonon density of states (B) which should be compared with the

experimental curve of Fig. la for Tml*OSe. Very good agreement is ob-

+

tained. In Fig. 3 (A) we also display the I.; projected density of states which is not very pronounced at the experimentally determined 60 cm-l peak, whioh thus is mainly due to a density of states effect from flat regions in the phonon dispersion along 11001 and 1110( near X.

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-

A. Treindl and P. Wachter, Phys. Lett.

64,

147 (1977)

K.H. Bennemann and M. Avignon, Solid State Commun.

31,

645 (1979) H. Bilz, G. Guntherodt, W. Kleppmann and W. Kress, Phys. Rev. Lett. 43, 1998 (1979)