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TWO-PHONON PROGRESSIONS ASSOCIATED WITH VIBRONIC EXCITONS IN LAYERED
3d-METAL COMPOUNDS
G. Benedek, I. Pollini, W. Bauhofer
To cite this version:
G. Benedek, I. Pollini, W. Bauhofer. TWO-PHONON PROGRESSIONS ASSOCIATED WITH VI-
BRONIC EXCITONS IN LAYERED 3d-METAL COMPOUNDS. Journal de Physique Colloques,
1981, 42 (C6), pp.C6-301-C6-304. �10.1051/jphyscol:1981687�. �jpa-00221623�
JOURNAL DE PHYSIQUE
Colloque C6, suppl6ment au n012, Tome 42, d&cmbm 1981 page c6-301
TWO-PHONON PROGRESSIONS ASSOCIATED WITH VIBRONIC EXCITONS IN LAYERED 3d-METAL COMPOUNDS
G. Benedek, I. Pollini and W. ~ a u h o f e r *
I s t i t u t o d i Fisica d e l l 'Universitci, GNSM, M i Z a o , I t a l y
" M U X - P Z ~ ~ ~ I n s t i t u t fitr Festkdrperforschung, S t u t t g a r t 80, F . R . G.
Abstract.
-
We have observed two-phonon structures in some intraconfiguration- a1 crystal field transitions of V-, Mn- and Ni-halides. We also report for the first time the crystal field spectrum of cr13r2 (3d4), where, despite its unfilled eg semishell, exciton-phonon (ep] interaction vanishes to first order yielding a two-phonon sequence in 3A1 (G) + ~A~(F) band.Electron-phonon interaction in layered transition-metal halides yields a complex vibronic structure in crystal field spectra /I/. The quasi-molecular nature of these crystals, due to their low dimensional structure and reduced ionicity, results in a flat dispersion of both d-d excitons and optical phonons (vibronic excitons with small excitation transfer) /2/. This allows for the experimental observation of sharp phonon progressions in the absorption spectra of dn -t dn parity forbidden transitions.
From these structures we can learn much on the dynamics of the excited states. For
3 3 5 3 ' 8 2
intraconfigurationaZ transitions in h a l f - f i l l e d s h e l l s (d (t 2g ), d (t eL), d (ea)) 2g g 0 the orbital ep coupling vanishes to first order /3/: thus a second-order progression is expected to be the basic vibronic structure, provided that also the spin-depend- e n t ep interaction (via the phonon modulation of spin-orbit (SO) coupling) is zero.
Such a selection rule is fulfilled by the transitions listed in TABLE, where 2nd- -order phonon progressions are possible.
I
Electronic 1 Electronic Transitions confi,wrationl predicted
I observed
I I
4 3 3
We have included also the special case of d (tZgeg) in CrBr2 (space group C2h /4/), Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1981687
C6-302 JOURNAL DE PI3YSIQUE
where despite i t s unfilled e semishell a vanishing ep interaction f o r some transi- g
tions between mnoclinally s p l i t levels i s found. Previously /5/ wereporteda strong evidence of pure two-phonon progressions i n NiC12 and N i B r 2 A2(F) 3 + 'E(D) t r a n s i - tion. This i s a rather r a r e case, since configurational mixing can switch on a weak 1 s t order ep coupling and mask two-phonon contribution. Nevertheless the theoretical prediction of a 2nd order structure often provides a key of interpretation of other complicated phonon structures we observed i n other 3d-metal compounds.
WAVENUMBER ( lo3 cm-1)
Fig.1: 'E(G) and T2 (G) bmds i n VC12 2 from
60
5 5 and i n VBr2 a t 5 K.Fig.1 shows the 'E(G) and T2(G) absorption bands of VC12 and VBr2, well separated 2 by a deep minimum. The SO coupling accounts f o r the s p l i t t i n g observed i n T (G), but 2 neither SO nor trigonal f i e l d perturbations can l i f t the degeneracy of E(G). While 2
the small s p l i t t i n g of 84 an-' (!XI2) and 122 odl (VBrZ) could well be due t o opti- c a l magnons (quickly washed out by increasing temperature), the broad sidebands ob- served above the E(G) peaks cannot have but a phonon origin. The spacing of 404 an-' 2 and 260 6' corresponds quite well t o twice the respective Ranan Eg frequencies a t 5 K /6/. The 6 ~ 1 ( ~ ) + 4 ~ ( ~ ) + 4 A1(G) transitions in MnC12, W r 2 /1/ and Mn12, show a puzzling phonon structure, together with the notable common feature t h a t the spacings between the f i r s t two sharp peaks a r e nearly twice the Raman Eg frequencies:they have been already discussed i n /1/.
In N i C I Z and N i B r 2 , besides the sharp two-phonon structure occurring i n E(D)/5/, 1 we found a pure E p g progression also in the Al(G) bands (Fig.2). Unlike N i B r 2 1
1 1 1
where the A1 (G) band occurs below T2 (D)
,
i n NiC12 the A1 (G) band i s j u s t atopf
1 ( ~ ) and T1(P) bands. In t h i s case i t s assignment i s possible thanks t o the 3 dis-WAVENUMBER ( l ~ ~ c r n - ' )
NiBr 5 K
'A, ( G 1
Fig. 2: The A (G) band in and N ~ B $ ~ a t 5 K.
WAVENUMBER (lo3 cm-1)
play of the two-phonon progression(spacing 316 f 6 a-l): no such a structure could be brought by l T 2 ( ~ ) o r T (P) transitions. 3
While i n d3, d5 and d configurations a vanishing f i r s t order ep coupling de- scends from group thepry f o r the l i s t e d transitions, i n d t h i s occurs accidentally. 4 For t h i s reason we have grown CrBr2 crystals by the flow system method and recorded i t s c r y s t a l f i e l d s p e c t m (Fig.3). F i t t i n g observed transitions t o octahedral crys- t a l f i e l d diagram including spin-orbit coupling /7/ gives Dq 2 1200 cn-l. Only one vibronic structure was found i n the whole spectrum. This i s associated with the near-
3 3
l y degenerate transitions A2(F)+ A (G) and has a spacing of 306
6'.
This frequen- 1cy scales q u i t e well with twice the Faman A frequency of VBr2 (316 6 ' ) and M r 2 l g
(302 and therefore corresponds t o a second-order vibronic structure.
JOURNAL DE PHYSIQUE
WAVENUMBER (
lo3 cm-'1
Fig.3: The c r y s t a l f i e l d spectrum of a C r B r Z crystal a t
m.
In the i n s e r t a two-phonon progression i n the 'Al (G) r3AZ (F) electronic band i s shown.References
/l/ P o l l i n i I., Spinolo G. and Benedek G., Phys. Rev.
E,
(1980) 6369 and references quoted therein./ 2 / Suni H., J. Phys. Soc. Japan
36
(1974) 770;2
(1975) 825./3/ Sugano S., Tanabe Y. and Kamimura H., bhiltiplets of Transition Metal Ions i n Crystals pag. 153, Academic Press, New York, 1970.
/4/ Tracey J.W., Gregory N.W. and Lindgafelter E.C., Acta Cryst.
9
(1962) 672 /5/ Benedek G., P o l l i n i I., P i s e r i L. and Tubino R., Phys. Rev.z,
4303 (1979)./ 6 / Bauhofer W., Giintherhodt G., Anastassakis E., Frey A. and Benedek G., Phys. Rev.
B
22
(1980) 5873./7/ Konig E. and Kremer S.
