Microreflectance Infrared Study of Bis(ethylenedithio)- tetrathiafulvalene (BEDT-TTF or ET) Salts JOHN R. FERRARO,* H. HAU WANG, MYUNG-HWAN WHANGBO,* and PHIL STOUT Chemistry and Materials Science Divisions, Argonne National Laboratory, Argonne, Illinois 60439 (J.R.F., H.H. W.) ; Department o[ Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204 (M.-H. W.); and Bio-Rad, Digilab Division, Cambridge, Massachusetts 02137 (P.S.) For several B- and ~-phase salts of bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF or simply ET) and its deuterium analog ds-ET, microre- flectance infrared spectra were obtained by employing polarized and unpolarized light, and their vibronic regions were examined. These salts exhibit a strong vibronic absorption under polarized light. The vibronie absorption of the B-phase salts has a much stronger polarization-depen- dency than that found for the ~-phase salts. For the B-phase salts, the optimum vibronic absorption occurs when the polarized light vector is parallel to their donor-molecule stacking direction. Among the ET salts without structural disorder, the highest C-C-H bending frequency for the superconductors is lower than ~1320 cm ' while that for the non- superconductors is higher than ~ 1320 em-l. Index Headings: Polarized infrared; Microreflectanee; Organic con- ducting salts. INTRODUCTION Reflection infrared spectroscopy is an important tool for characterizing organic conducting salts. 1-7 The sam- ples of such salts are small, for the most part, and opaque. Therefore, infrared properties of organic salts are best obtained by use of a microreflectance infrared (MR-IR) technique, in which a microscope is interfaced with an FT-IR spectrometer. In the salts of bis(ethylene- dithio)tetrathiafulvalene (BEDT-TTF, CloHsS8, or ET), the C-H bonds of ET form numerous C-H...anion con- tacts, as shown in Fig. 1 for ~-(ET)2IBr2. The C-H... anion interactions of ET salts have been studied with the MR-IR technique. 8-1°The MR-IR spectra of ET salts provide "fingerprints" of their C-H...anion contact en- vironments, and have been employed to distinguish be- tween a- and ~-phase salts of ET 9 and to follow the thermal conversion of a-(ET)213 to at-(ET)213 .10 In the present work we obtain room-temperature MR-IR spec- tra of several ~- and K-phase ET salts by employing po- larized and unpolarized light. On the basis of the MR- IR spectra we discuss the electron-molecular vibration coupling u-2° and the lattice softness 21-25of these salts. EXPERIMENTAL Room-temperature MR-IR measurements on ET salts were obtained with a Digilab FTS-40 purged spectrom- eter interfaced (typically 256 scans per spectrum) with a UMA-300-A microscope and a cadmium-mercury-tel- luride detector (resolution at 4 cm-1). The reflectance mode was used because the single crystals are opaque. A Kramers-Kronig transformation was applied to all re- flectance spectra, and the results are reported in arbi- Received 15 May 1992. * Authors to whom correspondence should be sent. trary absorbance units. Care is taken to use a crystal that demonstrates predominately reflectance characteristics and little, if any, transmission. The polarized light is reflected off smooth surfaces of the crystals, and striated surfaces were avoided. Measurements made on two ad- jacent smooth surfaces gave the same data, thus obvi- ating any artifacts occurring in the measurement. STRUCTURE OF DONOR-MOLECULE LAYER Figure 2(a-c) shows the packing patterns of ET mol- ecules in the donor molecule layers of B- and K-phase salts. In both phases, the 7r-framework of each ET mol- ecule is not perpendicular but inclined to the donor layer. In/3-(ET)2X (X = Is, AuI2, IBr2) the ET molecules cant along the donor-stack direction (i.e., approximately along the a + c direction of Fig. 2a). 26In K-(ET)2Cu[N(CN)2]X (X = C1, Br, I) 25,27,28 the donor layer defines the ac-plane (Fig. 2b), and in K-(ET)2Cu(NCS)2 the bc-plane (Fig. 2c) 29 The donor molecules of K-(ET)2Cu[N(CN)2]X cant along the a-direction, which is parallel to the Cu[N(CN)2]X- anion chain direction, 23,24,27,28 while those of K-(ET)2Cu- (NCS) 2 cant along the c-direction, which is perpendicular to the Cu(NCS)2- chain directionY ,29 As far as donor- molecule layers are concerned, the b- and c-axes of K-(ET)2Cu(NCS) 2 correspond to the c- anda-axes of K-(ET)2Cu[N(CN)2]X, respectively. In our MR-IR study of ET salts with polarized light, it is necessary to specify the alignment of the light po- larization vector with respect to the crystallographic axes of the salts. For ~-(ET)2X, K-(ET)2Cu[N(CN)2]X, and the K-(ET)2Cu(NCS)2 we define the alignment angle 0 to be zero when the light polarization vector is parallel to the a-, c-, and b-axis directions, respectively. Therefore, the donor molecules cant approximately along the 0 = 90° direction in ~-(ET)2X, K-(ET)2Cu[N(CN)2]X, and K-(ET)2Cu(NCS)2. RESULTS AND DISCUSSION Vibronic Envelope. As representative examples, Figs. 3a and 3b show the MR-IR spectra of K-(ET)2Cu(NCS)2 and K-(d8-ET)2Cu(NCS)2, respectively. The MR-IR spec- tra of ET salts have two characteristic features. One is a broad band extending from the infrared to ~ 3000 cm-1 region (i.e., plasma region), and is attributed to inter- and intramolecular electronic transitions 2°,3°-~2 associated with the ET molecules. The other feature is an envelope in the 1200-1450 cm -1 region (i.e., vibronic region), which is attributed to the coupling of conduction electrons to totally symmetrical vibrations of ET molecules, u-2° The 1520 Volume 46, Number 10, 1992 0003-7028/92/4610-152052.00/0 APPLIED SPECTROSCOPY © 1992Societyfor Applied Spectroscopy