PHONON SOFTENING IN INTERMEDIATE VALENT SmB6

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Submitted on 1 Jan 1981

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PHONON SOFTENING IN INTERMEDIATE

VALENT SmB6

I. Mörke, P. Wachter

To cite this version:

JOURNAL DE PHYSIQUE

CoZZoque C6, suppZ&ment au n012, Tome 42, de'cembre 1981 page c6-14

PHONON SOFTENING IN INTERMEDIATE VALENT SmB6

I. Mijrke and P. Wachter

lkboratoriwn fiir Pestkiirpewhysik, ETfl Zi&ich, 8093 Ziirich, Switzerland

Abstract.- W e have measured the Raman spectrum of a SmB6 single crystal and compared it to LaB6 and EuBa: Beside the three high energy Raman active phonons we found ad ltlonal excifations in these c mpounds. Most promine t is a peak at 172 cm for SmB6,

214 cm-' for LaB and 220 crn4 for EuB6. The spectra are ana-

lysed in terms o!? defect induced phonon scattering. The soften- ing of the line in intermediate valent (IV) SmB is explained in analogy with the phonon anomalies found in other IV com- pounds.

SmB6 is of special interest among the rare earth hexaborides because the Sm ion has a non integer valence of 2,6. To examine the influence of intermediate valence (IV) on the first order, forbidden Raman scat- tering, we have remeasured the spectra of a SmB6 single crystal and compared it to LaB6 and EuB6. The hexaborides crystallize in the

1

CaB6 structure of space group Oh

_I

1

(Pm3m). For this structure there

I

1

are three Raman active phonon mo-

des of A E and F symmetry.

19' g 2g

Beside these three modes our meas- urements show additianal shoulders

in both spectra. The most promi-

-

nent structure is a peak at

172 cm-l for SmB6, and 214 cm-I

t

I-

for LaB6 (Fig.1). The absence of

-

magnetic ordering in SmB6 and LaB6 and of electronic excitations, in LaB6 (the ground state of la3+ is

) is the reason why we are 8 JEB 6 8 8 9 8 8 1 2 8 8 1 5 8 8

0 FREQUENCY SHIFT ( c m - 1 )

going to interpret this part of

Fig.1 Room temperature Raman

the spectra in terms of phonon _{Spectrum of SmB6 and LaB6 single}

scattering only. The selection crystals.

rules for Raman scattering may be lifted by crystal defects. This effect yields a weighted phonon densityof states in the spectra (1).

-1

The peak at 172 cm in IV SmBs is of special interest because it is

about 40 cm-' softer than the corresponding excitation in metallic

LaB6. The frequencies of the Raman active modes of the other MB6 are in semiconducting compounds always higher than in the metallic sam-

ples (2). This is confirmed on a EU~'B~ powder sample which shows a

-1

weak broad excitation at 220 cm

.

bulk modulus has been observed in SmB6. B=139 GPa, 191 GPa and

157 GPa for SmB6, LaB6 and EuB6, respectively (3)

.

Softening of zone boundary phonons and of the bulk modulus are the striking effects caused by the electron phonon interaction in

I V compounds (4). In I V compounds with NaCl structure the phonons

mostly affected by the coupling to the RE ion are those where planes

of anions move against planes containing only the IV ions ( 5 ) . Such

a vibration is expected at the X-point in the CaB6 structure. The symmetry of normal modes at this point is (we put the origin of the coordinate system in the center of the boron octahedron):

3A

+

+

+

+

+

+

+

19 2g lg 2g g

The compatibility relations are given in Fig. 2. The two modes of A2U

symmetry are those where planes of B ions move in

[loo]

against planes of Sm ions (Fig. 3).

Although it is not possible on the basis of the Raman scattering experiment alone to be sure that

the observed low frequency line

r p o i n t A p o i n t X p o i n t

coincides with either of these A,.-

L U modes, there are some important arguments which favour such an in- terpretation. One is by taking a

suitable linear combination of both

A2u modes, one can deal with the movement of the center of gravity of the whole B6 octahedron. Com-

paring the mass of B6 with that of

a single boron atom, one may expect this vibration to have a low

frequency. A further argument is deduced directly from the compati-

Fig. 2 Compatibility relations for

JOURNAL DE PHYSIQUE

Fiq.3. Normal modes of the two A modes of SmB6 at the X-point. _{2 u} of A symmetry at the X-point may be either of A or E or Flu

2 u lg g

symmetry at the r-point. The T-A has the highest frequency in the lg

hexaborides of about 1300 cm-l. One would not expect this mode to be of the order of 200 cm-I at the zone boundary. The same argument holds for the r-E mode. If one looks at the Flu modes, one of which is the

4

acoustical, one finds two vibrational modes showing exactly the same atom displacements as for the two A2U modes. The only difference is that at the r-point all unit cells move in phase whereas at the X-point neighboring planes of unit cells in X direction move in antiphase. The

Flu mode is expected to be of lower frequency than the A _lg mode. The softening of the low frequency line is thus tentatively explained in terms of a Raman inactive LO (X) mode which couples most strongly to the volume fluctuations.

1. A.Treind1, P.Wachter, Solid State Communications

2,

5,

1075 (1976)

3. H.E. King, Jr.,S.J.La Placa, T.Penney, Z.Fisk, Int. Conf. on Val. Fluctuations in Solids St.Barbara, Jan. 1981

4. K.H. Bennemann, M-Avignon, Solid State Comrnun.

31,

43,