Very low frequency light scattering in glass matrix

Journul of Molecdar Structure, 267 ( 1992 ) 241-246 Elsevier Science Publishers B.V., Amsterdam

241

Very low frequency light scattering in glass matrix K.E.Lipinska-Kalita”

1 and G.Mariottob

“International

Centre for Theoretical

Physics, 34100 Trieste, Italy

‘Dipartimento

di Fisica, Universita di Trento, 38050 Povo (Trento),

Italy

Abstract Room-temperature

Raman

scattering

spectra

of glasses of the types: (SiOz) have been collected. The low frequency band at about 5007~~1 appears to be a common property of all analysed glasses. Changes in the optical Raman modes region were accompanied by the modification of the spectral shape in the acoustic Raman modes region. The differencies in the spectral distribution of the light scattering excess in the acoustic Raman modes region ( the so called boson peak ) have been interpreted in terms of

(Al;03), (SiO2)(Gu203), (GeOz) (A&03), and (GeOs)(GasOs)

changes in the microstructure

of the amorphous matrix.

1. INTRODUCTION Raman scattering bands observed in the spectra of oxide glasses can be characterized as beeing of two types: (1) acoustic Raman modes which occur at the lowest values of frequency, usually in the region below 200cm-’ and (2) optical Raman modes in the frequency region 200 - 1200n-’ . Recently, interest has developed in the low frequency or acoustic Raman modes region. A unique aspect of the Raman scattering spectra of glasses is the presence of a broad, asymmetric, low frequency band usually called the boson peak. This band at about 50n~’ appears to be a common property of oxide glasses and arises because of the limited structural correlation length of the glass network [ 1,2 1. It should be pointed out that the main focus in the following discussion is on the low frequency ( acoustic Raman modes ) region of the light scattering spectra. In our opinion, acoustic modes region of Raman scattering in glasses could be an important source of additional ( to the optical modes region ) informations on the structure of amorphous matrix. ion leave from Regional Laboratory Jagiellonian University, Krakow, Poland

0022-28fN/92/$05.00

of Physicochemical Analysis and Structural

63 1992 Elsevier Science publishers B.V. All ri’ghts reserved

Research,

242 2. METHODS Glasses of the same basic composition:

80 SiOz(GeOz), 15 A1203(Gaz03), 5 equivalent to KAlGe308 crystal were prepared with high purity oxides by melting in platinum crucibles and quenching in air. Raman Stokes measurements were performed using an Art ion laser operating in 514.5 nm with the power of 300 mW at the sample surface. The light scattered at 90” passed through double grating monochromator ( Jobin-Yvon Ramanor HGZS, holographic gratings, 1800 lines/mm ) interfaced with a multichannel analyser. Signal from the photomultiplier detector was stored and processed in a personal computer.

KZO ( mol% ) as well as glass having the composition

3. RESULTS 3.1 Optical

and DISCUSSION modes region of the Raman scattering

(200 - 1300cm-*) .

Raman scattering spectra of studied glasses in this region exhibit bands which are characteristic for the fundamental network-forming units ( i.e. germanate or silicate tetrahedra ). It should be stressed that the features of the spectra of SiOs - and GeOz - based glasses are similar - an intense and polarized low frequency band and a pair of weak, depolarized high frequency bands. These bands have been asigned to similar vibrational motions. The observed differencies in the positions and halfwidths of the Raman bands may be attributed to the tighter configurational restraints in the germanate glasses caused by the larger size of the germanium atom. Intense, symmetric stretch anion bands for GeOs - based glasses ( fig.1 c-e ) are much narrower than theirs counterparts in the SiO2 - based glasses (fig.1 ab), indicating that Ge - 0 - Ge bond angle has smaller fluctuations in germanate than Si - 0 - Si bond angle in silicate glasses. The position of the symmetric stretch anion band in all studied glasses also indicate that their networks are mainly composed of six-membered rings of tetrahedra (SiOd, A104, GeOd, Ga04) connected in a random three-dimensional structure [3].

3.2 Acoustic

modes region of Raman scattering

(below 2OOcm-’ ).

Because of disorder which destroys the translation symmetry, it is possible to observe in non-crystalline materials like glasses an inelastic light scattering from acoustical phonons. The intensity I(w) of light scattered with a frequency shift w/27r is given by the following expression [l]: I(w) = C(w)s(w)Mw)

+

II/w

The Bose - Einstein population factor [n(w) + l] for Stokes scattering has to be replaced by n(w) for antistokes scattering, g(w) is the phonon density of states associated with band of vibrations, and C(w) describes the coupling of the vibrational

248

440 I

600

a 800

tO

11oo

>I--"

a

b

Z LLI I--Z I.--I

Z ,=:[

C

rr" ._J

d

I-Z LLI

.~-

r~ I.U rL X W

e 840 930

0

400

WAVENUMBER

800

:1200

[cm -1 ]

Fig.1 Room-temperature, unpolarized Raman spectra of glasses: (a) Si02 - AlsO (b) S i 0 2 Ga2 3, (c) KAIGeaOs, (d) GeO~ - Al2Oa, (e) Ge02 Ga~Oa

244 modes of frequency w/27r to the light. From the Martin and Brenig [ 2 ] model for Brillouin scattering by an amorphous solid, coupling constant C(w) is proportional to the square of frequency w when:

where vt is the transverse sound velocity, and 26 the structural correlation length in the glass. Above mentioned model provides a direct explanation for the universal presence of the low frequency band ( boson peak ) in the Raman scattering spectra of oxide glasses. Coupling factor C(w) exhibits a maximum for: w =

vt/6,

and for: w > Q/6 coupling factor C(w) decreases to zero. It seems to be clear now that the boson band can be attributed to a maximum in the Raman coupling factor C(w) which in its turn depends on the disorder or more precisely on the structural correlation length of the glass network (26). We will now discuss the influence of the two parameters: sound velocity (wt) and the structural correlation length (26) on the low frequency region of our glass spectra. The substitution of A13+by Ga3+ cations in the SiOz glass network leads to a strong decrease of the boson band intensity which can be explained by the decrease of the electronic polarizability ( fig.2 a,b ). Th is effect is less significant for the GeOz - based glasses ( Fig.2 d,e ). Additionally, the maximum of the boson band shifts toward lower frequency. It should be pointed out that large Ga3+ ions are not easily accomodated in the SiOz glass. They tend to break out network, and give rise to increased number of non-bridging oxygens as well as can reduce the structural correlation length in the glass matrix. The low frequency band (boson) consequently should shift toward higher frequencies. Unfortunately, no data on sound velocity are available, but we suggest that the observed low frequency shift of the boson peak is mainly caused by the difference in sound velocities (vt). As was mentioned above, at this moment no data are available on sound velocity in analysed glasses. For this reason we cannot estimate the real value of the second parameter determining the position of the low frequency boson band - structural correlation length of the glass network (26) . We tentatively propose that the well distinguished difference in the position of the boson band in SiOz - and GeOz based glasses ( fig.2 a,d ) is also caused mainly by differencies in sound velocities. At the end we will discuss another, interesting feature - the presence of very weak bands, located on the low frequency side of the boson peak, near the Rayleigh line, in the spectra of (SiOz)(GazOs) and (GeOz)(GazOs) glasses ( fig.2 b,e ). It was shown [4] that this maxima comes from the scattering from so called surface vibrational modes of structural units ( spherical particles or crystallites ), the energies of which fall just in the acoustic region of the glass spectra.

245

60

-

I

I

100

WAVENUMBER

I

I

200

[cm-’

I

Fig.2 Low frequency, room temperature, unpolarized Raman spectra SiOz - A1203, (h) SiOz - Gaz03, (c) KAlGesOe, (d) GeOz - Al203,

Ge02 - Ga203

of glasses: (e)

(a)

246

4. CONCLUSIONS A more precise and detailed Raman scattering investigations are needed to provide a better understanding of the nature and origin of the low frequency bands (bosons) in the glass spectra. To compare the different amorphous structures it could be interesting to consider the temperature-reduced spectra in the acoustic Raman modes region with a different frequency dependence of coupling coefficient C(w).

5. REFERENCES

1. RShuker and R.Gamon, Raman-scattering selection-rule breaking and the density of states in amorphous materials, Phys.Rev.Lett. 25 (1970) 222-225 2. A.J.Martin and W.Brenig, Model1 for Brillouin scattering in amorphous solids, PhysStatus Solidi (b) 64 (1974) 163-172 3. K.E.Lipinska - Kalita and D.Mowbray, The structure of Al,Fe,K silica - germanate glasses investigated by Raman and infrared spectroscopy, J.Non-Cryst. Solids 122 (1990) l-9 4. K.E.Lipinska - Kalita and G.Mariotto, Low frequency inelastic light scattering from GeOz based glasses: early steps of crystallization, J.Non-Cryst. Solids 128 (1991) 285-293