A. Tanaka et al., ELECTRONIC-STRUCTURE OF THE QUASI-ONE-DIMENSIONAL ORGANIC CONDUCTORS DCNQI (N,N'-DICYANOQUINONEDIIMINE)-CU SALTS, Physical review. B, Condensed matter, 52(11), 1995, pp. 7951-7959
A comparative study of the electronic structure of in situ synthesized
quasi-one-dimensional organic conductors (DMe-DCNQI)(2)Cu and (MeBr-D
CNQI)(2)Cu has been carried out using various techniques of electron s
pectroscopy, where DMe-DCNQI and MeBr-DCNQI are 2,5-dimethyl-N,N'-dicy
anoquinonediimine and 2,5-methylbromine-N,N'-dicyanoquinonediimine, re
spectively. From the photon-energy dependence of the valence-band phot
oemission spectra obtained using synchrotron radiation, the origins of
each observed feature are unambiguously characterized. While the feat
ure at the Fermi level is primarily derived from pi-bonded C and N 2p
states, the contribution of Cu 3d states at the Fermi level is larger
in the (MeBr-DCNQI)(2)Cu compared to the (DMe-DCNQI)(2)Cu. X-ray photo
emission spectra of the valence band imply extensive hybridization of
the Cu 3d states with C and N 2p states near the Fermi level. Line-sha
pe analyses of the Cu 2p core-level spectra show that the ratio of CU2
+ to Cu+, is higher in (MeBr-DCNQI)(2)Cu compared to (DMe-DCNQI)(2)Cu,
with the ratio being closer to 1:2 for (MeBr-DCNQI)(2)Cu. From a comp
arison of C KW and Cu LW Auger spectra with the self-convolution of th
e valence-band spectra, it is found that the effective on-site Coulomb
correlation energies between the valence electrons are high on C site
s as well as Cu sites in both salts, with U(pp)=6.5 eV and U(dd)=8.0 e
V, respectively. In conjunction with core-level spectra, the spectra i
ndicate that the on-site Coulomb correlation, the hybridization streng
th, and the charge-transfer energy between the Cu 3d and N 2p ligands
are very similar in the two salts. The metal-insulator transition in (
MeBr-DCNQI)(2)Cu at 160 K is then facilitated by the proximity of the
Cu2+-to-Cu+ ratio to 1:2 supporting charge disproportion, while deviat
ion from it stabilizes the metallic phase in (DMe-DCNQI)(2)Cu down to
very low temperatures.