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Optical properties of organic conductor (ET)4Hg3I8 : alternative type of electron-vibrational interaction
M. Kaplunov, R. Lyubovskaya, R. Lyubovskii
To cite this version:
M. Kaplunov, R. Lyubovskaya, R. Lyubovskii. Optical properties of organic conductor (ET)4Hg3I8 : alternative type of electron-vibrational interaction. Journal de Physique I, EDP Sciences, 1994, 4 (10), pp.1461-1467. �10.1051/jp1:1994200�. �jpa-00247005�
Classification Phj>.çic..ç Absn.aci.ç
78.20 63.20K
Optical properties of organic conductor (ET )~Hg~I~ :
alternative type of electron-vibrational interaction
M. G. Kaplunov, R. N. Lyubovskaya and R. B. Lyubovskii
Institute of Chemical Phy~ic~ in Chernogolovka, Russian Academy of Sciences. Chernogolovka, 142432. Russia
jRe<.eived 6 De<.etlibei J993, ie>,i.çed J Jwie J994, a<.<.epied 7 July 1994)
Abstract. The reflectivity spectra of single cry~tal~ of (ET )iHg,Ix and jd~-ET)iHgilx were
studied. In contrast with earlier studied clorine- and brominemercur~lte compounds of the series
(ET jjHg, ~-Xz IX Cl, Br, I), the spectra of iodinemercurate compounds exhibit low reflectivity without well-defined plasma edge and without electron-vibrational bonds in the region 1000- 400 cm '. The latter fact may be due to electron-vibrational interaction with non-totally symmetric modes which m~ly originale from peculiaritie; of crystal uructure of iodinemercurate
compound,. A phase transition of the fir;t type i~ observed bath for (ET)iHg,Ix and
(d~-ET)jHgilz. At the transition temperature a jump in conductivity and reflectivity i~ ob;erved.
Introduction,
Salts of bisjethilenedithio)tetrathiafulvalene (BEDT-TTF or ET) with halide-mercurate anions (in particular the compounds of the serres (ET )iHg~ ôX~ IX
= Cl, Br, I)) form a wide class of
organic conductors with a variety of electrical properties from dielectrics and semiconductors to metals and superconductors Il -7]. Previously, optical properties of two compounds of this
series were studied : (ET )~Hgi ~clg[8] and (ET )~Hg~g~Brg [9, 10], which are organic metals
and pass into supérconducting state correspondingly at Tjj = 8 K and P
=
12 kbar and at Tjj = 4.3 K and ambient pressure il -3]. In the present work, we have investigated the optical properties of another compound from this serres, (ET )iHgil~, which exhibit de-conductivity of semiconducting type characterized by the phase transition ;emiconductor-dielectric at 260 K (4, 5] and of its isostructural il Il deuterated analog, (d~-ET)jHg~Ç. We have shown that
optical properties of semiconducting iodine sait differ from that oi metallic chlorine and bromine salts in the electron-vibrational region which may be explained by different types of
electron-vibrational interaction. For the deuterated Salt we hâve also measured the temperature dependence of de-conductivity.
Experiment.
We have measured the reflectivity spectra from conducting ah plane in the range 300 to
5000cm~' at
room temperature for both compounds and in the interval 300-230 K for
1462 JOURNAL DE PHYSIQUE I N° 10
(d~-ET)jHg~Ç. The sample of (ET )jHg~[ was made of five ;ingle cry;tais of characteri~tic
dimen~ions x 0? x 0.02 mm oriented parallel to each other. The simple of (d~-ET)jHg)g
was made of one single crystal with the dimensions about 5 x 15x 0.02 mm~ for
room-
temperature measurements and of two ;uch crystals for low temperature mea;urement;.
During the low-temperature measurements, the sample was kept in vacuum and attached to a cold copper blook. Directions of the crystallographic axes were determined according to the externat ~hape of the crystals. DC-conductivity of (dx-ET )jHg~[ single crystals was measured by a standard four-probe method. Platinum contacts (20 mkm diameter) were attached to the
crystal with a graphite paste.
Results and discussion.
Room TEMPERATURE OPTICAL PROPERTIES. Our mea;ureiuents have shown thai ilie
reflectivity ;pectra of (ET)jHg~[ in unpolarized light practically do not differ from that of
(d~-ET )jHgJ~. Therefore, further in this work we mainly discuss the spectra of (d~-ET )jHg~I~.
In figure la, polarized reflectivity spectra of (d~-ET)jHg~I~ are ;hown for the polarization;
parallel and perpendicular to the crystallographic b axis. For comp~irison, the ;pectra of earlier
o 6
(c)
o.5
0.4
o.3
0 2
______~_
0
"'
""
~ O.O
) (b)
" 3
j
Î 0 2
c£
o.
O O
(°J
o.3
02
O.i
°~O IOOO 2000 3000 4000 SOOO 6000
Frequency, cm~~
Fig. l. la) Reflectivity spectra of (dx-ET)~Hg,Ix single crystal for E Î b (solid fine) and E i b (dashed fine) polarizations jthis work). (b) Reflectivity ;pectra of (ET )jHg~z~~Br~ ;ingle cry;tal for unpolarized light jRef. [9]). jc) Reflectivity ,pectra of (ET)iHg, ~Cl~ ~ingle cry~tal polarized in two main optical
direction, (Ref. [8])
studied chlorine- and brominemercurate compounds (ET )iHg~ _,~C[ and (ET )iHg~~,~Br~ [8- l0] are given (Figs. lc, 16). Two main distinguishing features are characteristic for iodine-
containing compound. First is the difference in electronic excitations spectrum. Instead of relatively high retlectivity with a well-defined plasma edge at 4 000-5 000 cm-', the spectra of the iodinemercurate compound in both polarizations exhibit low retlectivity without an edge
which merely increases in the region of low frequencies. Such a spectrum was observed earlier for ET compounds with p-phase structure (12-15] for the polarization perpendicular to ET
stacks (£-;pectra), that is in the direction where the interaction between adjacent molecules is due exclusively to shortened S S contacts between side sulphur atoms of ET molecules. In
analogy with £-spectrum of p-phase (12], the retlectivity of j BT jHg~[ may be approximately
described by Drude-Lorentz model with very high electron damping about 10 000 cm-' and
plasma frequency about 3 500-3 700 cm-'. Weakly-defined plasma edge (large damping) gives evidence of difficult electron transfer which does not contradict to low de-conductivity
and semiconducting temperature dependence of conductivity (4].
The mo~t considerable difference from the (ET)iHg~_ôX~ IX = Cl, Br) ~pectra is that
observed in the region of molecular vibrational frequencie; 000-1 400 cm- ' For chlorine- and bromine-mercurate compounds (Figs. lb, c), a characteristic group of electron-vibrational
bands with maxima about 250 and 350 cm ' is observed (8-10]. These bands are observed
also for many other conducting ET salts, such as p- (ET )~I~ il 2, 13], K -(ET )~Cu (NCS )~ (9, 15-17], K-(ET)~Cu IN (CN)~]Br il 8] and are usually attributed to electron-vibrational inter- action involving totally symmetric intramolecular vibrational modes (8-18]. On the contrary,
only a weak broad hump is found in this region for iodinemercurate compounds (Fig. la).
Absence of contribution of totally symmetric modes to the spectra of iodinemercurate
compounds could be simply accounted for by low values of corresponding electron-vibrational
coupling constants, which, m tum may be a consequence of low density of ;rates on Fermi level (14, 19]. This, however, does net explain the presence of a hump in electron-vibrational
region.
On the other hand, this spectral dissimilarity may be connected with differences in the
crystal structures. The structures of chlorine- and brominemercurate compounds belong to the K-phase type which is characterized by the presence of ET molecular pairs with good
overlapping of « face-to-face
» type within a pair (1, 2(. For the iodinemercurate compounds, the direction of ET molecular stacks could be formally specified in this structure (along b axis), but really the « face-to-face
» molecular overlapping within the stacks is very small in view of large interplanar distances and large shift of molecules along their ;hort axes (4[. The intermolecular interaction is accessed only to shortened S S contacts between side sulphur
atoms of ET molecules (« side-by-side » mode of interaction) (4].
The effect of the mode of intermolecul~ir interaction on electron-vibr~ltional spectra was
considered in our previous works devoted to ET-based family of conducting compounds
(ET)~(Hg~X~,(C~,H,Y)]~ (X, Y
= Cl, Br) (20, 21]
It ii known that the electron-vibrational bands in the IR spectra of conducting organic
compounds are connected with molecular vibrations arising due to the relaxation of molecules to the equilibrium configurations after the electron charge transfer. If the molecular ;ymmetry
con;erves during the tran;ition, these vibrations will be totally symmetric. Probably, this case
is realized for those ET salts, which contain molecules overlapping in
« face-to-face
» manner
in their crystal structures (for example, in the K-phase of for p-pha;e in the direction of ET molecular stacks).
However, if the neighboring molecule~ interact in
« ;ide-by-side » manner, violation of the molecular symmetry is possible during the process of charge transfer ;ince the transferred
charge may be concentrated at one end of the molecule during the tran;ition. In this case, non-
1464 JOURNAL DE PHYSIQUE I N° 10
totally ~ymmetric vibration; can arise as the result of the configurational relaxation. The
change of symmetry of vibrational modes involved in the electron-vibrational interaction
evidently should lead to sufficient changes in the infrared spectra.
Thi~ conclusion is confirmed by theoretical analysis of IR spectra. Calculations of
reflectivity spectra for K-phase conductors (ET )iHg~ ôX~(X
=
Cl, Br with the use of « phase phonons » model including totally ~ymmetric modes (8- loi give a good coincidence with the experiment. In particular, the electron-vibrational bands in the region of 000-1 400 cm- ' are
present. On the contrary, the model calculations of reflectivity spectra for
(ET )~(Hg~X~, (Cj,H,Y ]~ including non-totally symmetric mode~ give the spectra without well- defined bonds in this region (only a broad hump is pre~ent) (20] which qualitatively remind the spectra of (ET)jHgi[ (Fig. lc).
Thus, the qualitative differences in the ~pectra of (ET )jHg~ ôX~ IX
= Cl, Br on one hand
and X
=
I on the other hand may be explained by different symmetry type; of vibrational modes involved in the electron-vibrational interaction.
One more peculiarity of (ET )jHg~[ spectra is that there is no significant difference between the spectra of (ET)jHg~Ç and (d~-ET)jHg~[ in the electron-vibrational region 1000- l400 cm-'. Thi; is in contrast with the compounds of K-phase structure such as
(ET)~Cu(NCS)~ where changes in the shape of electron-vibrational band due to deuterium substitution were observed and attributed to isotopic shift of CH-deformation mode il 6, 17].
TEMPERATURE DEPENDENCE oF OPTICAL PROPERTIES. It is known that optical properties of
organic conductors may be sensitive to temperature especially in the regions of pha~e
transitions. For example, the spectra of (ET )x(Hg~Brj,(CôH~Br)]~ below the point of metal-
in;ulator transition 160 K show some relatively narrow bands in the electron-vibrational region which were ab;ent above this point (?0]. Fine structure of electron-vibrational bond aise
appears for a-phase of (ET bI~ below M-I transition point 135 K il 2].
ùc
T, K
Fig. 2. The temperature dependence of de-resistivity for (d~-ET)iHg,lz along b-axi,.
(ET )jHg~I~ is known to exhibit a semiconductor-dielectric phase transition of the first type at
265 K (4]. For this reason, the search of correlations between the temperature dependence of
it; optical and de properties is of great interest. We have studied such correlation for (dg-ET )jHg~[.
The temperature dependence of (d~-ET)jHg~I~ resistivity along the b axis is shown in
figure 2. Room iemperaiure conductivity measured along the b axis for various single crystals
is in the range of (0.5-2 Ohm~ ' cm~ '. The conductivity measured in the direction tran;ver;e to the conducting ah plane corresponds to the ani;otropy about 10~. A semiconductor-dielectric
phase transition occurs at T
=
262 K. At this temperature, the conductivity sharply decreases
by a factor of about 15. The conductivity activation energy above the transition is 600 K and below it is 3 200 K. The observed phase transition is of the first type one with a hysteresis
about 6 K. It should be noted that the electrical characteristics are very similar to that reported
for (ET)~Hg3Ig [4].
In figure 3a, the unpolarized reflectivity spectra of (d~-ET)iHg~Ç at room temperature and T
=
230 K are shown. Qualitatively, the two spectra differ but slightly : there is only some
lowering of retlectivity in the low frequency region and a very small increase at high frequencies. These changes may be accounted for by a slight growth of damping in Drude-
Lorentz model. The temperature dependence of reflectivity is in accordance with the phase
transition observed in electrical properties the reflectivity changes occur abruptly near the
transition temperature, and slight hysteresis is observed with a temperature cycling (Fig. 4).
It must be noted that there is no sharpening or splitting of reflectivity band in the molecular vibrations region at low temperatures as it takes place in case of (ET)~(Hg~Brô(Cj,H~Br)[
(20]. This may be explained by the assumption that the structure transition practically does not
concern the ET layer (in particular, the overlapping of
« face-to-face » type do not appear) and
is due to changes in radine-mercurate antan layer. This is in accordance with suppositions of [4].
O.3 (a)
§O.2~ w
~(Dl
O.O
~E 60 (b)
i~
)
40
,,,,
À ;""'
à 20
(
~0 1000 2000 3000 4000 5000 Frequency, cm~~
Fig. 3. la) Reflectivity ~pectra of (dx-ET)jHg,Iz single cryual in unpolarized light at temperatures 300 K (solid lme) and 230 K jdashed fine). lb) Optical conductivity spectra of jdz-ET)jHg,Iz single crystal in unpolarized light at temperatures 300 K jsolid fine) and 230 K (da~hed [[ne).
1466 JOURNAL DE PHYSIQUE I N° 10
.1
~ lb)
ù
70.9 à
, t la)
~
" 0.8
~
0 7
~'§50 260 270 280
T, K
Fig. 4.-Temperaiure depcndencic, of'reflecti,<iiy of' (d~-ET)jHg,1, <it (<1) 91)0cm ~lnd (b) 321)1) cm ~.
In figure 3b, the optical conductivity ;pectra at T
=
300 K ~ind T
=
230 K obtained from the
reflectivity data by Kramer~-Kronig analysi; are shown for unpolarized light. For bath femperatures, a maximum of optic~il conductivity is ob;erved ~ibout 3 000 cm ' which may be
an evidence of a gap or a pseuudogap in the electron energy spectrum il 2-15]. The second
maximum about 1000cm~' originates from vibrational hump and is due to electron-
vibration~il interaction.
Acknowledgments.
Thi; work 1; supported by the Russian Foundation of Fundamental Investigations. project
N 93-03-4531.
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