High-pressure Raman study of the charge ordering in α−(BEDT−TTF)2I3

Abstract

Charge distributions in the donor layer for the paramagnetic semimetal-like and nonmagnetic insulating phases of the $\alpha -(\mathrm{B}\mathrm{E}\mathrm{D}\mathrm{T}-\mathrm{T}\mathrm{T}\mathrm{F}{)}_2{\mathrm{I}}_3$ single crystal [BEDT-TTF: bis(ethylenedithiolo)tetrathiafulvalene] were investigated under quasihydrostatic pressures up to 3.6 GPa using Raman spectroscopy. The results confirmed the appearance of charge disproportionation below the metal-insulator phase transition temperature ${(T}_{\mathrm{MI}}).\mathrm{}$ The average charge-disproportionation ratio, 0.2–0.8, at ambient pressure was quite pressure sensitive and decreased by ca. $0.1e/\mathrm{GPa}$ with increasing pressure. This result is ascribed to the faster increase of the transfer integrals than the intersite Coulomb repulsions under pressure. The loss of the inversion centers in the nonmagnetic insulating phase, concluded by the selection rule, suggests that the charge is ordered such that it forms a so-called horizontal stripe. Nonuniform charge distributions were also found at temperatures above $T_{\mathrm{MI}}$ for the paramagnetic semimetal-like phase. The experimentally determined small charge difference among BEDT-TTF at crystallographically nonequivalent sites is consistent with the band calculations.