Structural phase transition in (BEDT-TTF)3CuBr4 at 60 K

Solid State Communications, Vol. 100, No. 11, pp. 755-758, 1996 Copyright @ 1996 Elsevier Science Ltd Printed in Great Britain. All rights re.serwd 0038-1098i96 $12.00+.00

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STRUCTURAL PHASE TRANSITION IN (BEDT-TTF)3CuBr4 AT 60 K M. Watanabe,’ Y. Nogami,” K. Oshima,” J.-I. Yamaura,b T. Enokib and G. Saito’ “Department of Physics, Faculty of Science, Okayama University, Tsushimanaka 3-1-1, Okayama 700, Japan department of Chemistry, Faculty of Science, Tokyo Institute of Technology, Ookayama 2-12-1, Megro 152, Tokyo, Japan ‘Department of Chemistry, Faculty of Science, Kyoto University, Sakyo 606-01, Kyoto, Japan (Received 4 December

1995; in revised form 7 August 1996; accepted 22 August 1996 by H. Kamimura)

Low temperature X-ray diffraction study for organic semiconductor (BEDT-TTF)3CuBr4 revealed structural phase transition from p21/c to PC by losing inversion and screw symmetries at 60.2 K where the magnetic susceptibility shows a steep decrease. Abrupt changes in (Ok 0) X-ray reflection intensity and lattice parameters proved that this phase transition has first-order nature. Structure analysis at 65.9 K verified that the average coordination number of Br around Cu is four. Copyright @ 1996 Elsevier Science Ltd

Keywords: A. organic crystals, C. crystal structure and symmetry, D. phase transitions.

1. INTRODUCTION Recently, organic compounds possessing ps-d electrons such as @MeDCNQI)2Cu [l, 21 and metal-phthalocyanine [3], have attracted attentions of physicists and chemists, owing to their possible electronic and magnetic interactions among pr and d electrons and their potentiality of producing organic superconductors containing magnetic interactions. Among them, organic semiconductor (BEDT-T’TF)3CuBr4 (where BEDT-TTF is bis-ethylenedithiotetrathia-fulvalene) shows a CurieWeiss type magnetic property above 60K with a large negative Weiss temperature of 8 = -110 K reflecting strong interaction among constituent magnetic moments of d-electrons on Cu2+ cations and of unpaired P’Relectrons on BEDT-TlF molecules [4, 51. In contrast to widely-known (BEDT-TTF)2X (where X is a monovalent anion group) compounds such as superconductors /3-(BEDT-TTF)213 [6] and K-(BEDT-TTF)2Cu(NCS), [7], the present compound (BEDT-TTF)3CuBr4 has 3 : 1 stoichiometry with a divalent counter anion of CUBI‘,. There exists contrary reports on room temperature structure for the present compound [8,9]. The difference is on the average coordination number of Br- surrounding Cu: the number has been 3 after Mori et al. [8], while

4 after Guionneau et al. [9]. Since the coordination number of Br- can affect the magnetic property of Cu seriously through both a ligand field and an oxidation number of Cu, the determination of the number is important for understanding (BEDT-TTF)3CuBr4. In this sense, although better R-factor in Ref. [9] than in Ref. [8] suggests that the coordination number is 4, we assert the necessity to confirm the number through a structure analysis more sensitive to atomic population. For this reason, we carried out a structure analysis at 65.9K with use of temperature factors; around this temperature, the factors become highly sensitive for atomic population, though they are almost insensitive at room temperature. As a result, we verified that the coordination number is 4. Below Tc of 59-60 K, static and spin susceptibilities of (BEDT-‘lTF)3CuBr4 decrease abruptly [4, 51, owing to disappearance of contribution of magnetic moments on BEDT-TI’F molecules [5]. The mechanism of the magnetic anomaly remains unclear. However, recently observed peak shift in visible light reflectivity at 30 K has been interpreted as an indication of Jahn-Teller distortion of CuBr4 [5] which is planar at room temperature [8, 91. Furthermore around Tc, a large X-type specific heat anomaly reaching 15% of the total amount [4] leads us to an idea of a possible structural change. However,

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STRUCTURAL PHASE TRANSITION IN (BEDT-‘ITF)sCuBr4 AT 60 K

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there exists no direct measurement about low temperature structure. To detect a possible structural phase transition, we checked space group symmetry and measured temperature dependence of lattice parameters around Tc. As a result, we observed a first order structural phase transition from F2t/c to PC at 60.2K associated with the magnetic anomaly. 2. EXPERIMENTAL A glossy black single crystal of (BEDT-TI’F)$uBr4 of 0.03 x 0.17 x 0.70mm3 was used for the X-ray measurement. Its lattice parameters at room temperature were a = 17.00& b = 10.13 A, c = 14.20& (Y= 89.99”, /3 = 102.74”, y = 90.00”, V = 2358.5A3. A small deviation of CYfrom 90” is due to an experimental error. The Huber 5042 four-circle diffractmeter equipped with Air Product 201 cryocooler able to cool the sample down to 10 K was used. The crystal was mounted using GE varnish 7031 on a single-crystal sapphire sample holder attached to a Cu block cold finger of the cryocooler. The sample chamber was evacuated to avoid X-ray scattering by air. For the measurement to detect a weak X-ray intensity by extinction rule breaking, a monochromatized MoKa X-ray beam (50 kV, 236 mA) was used. This is because an electron has a larger scattering power for a MOKCYX-ray 6” = 0.7107A) than for an AgKa X-ray (X = 0.5608A) owing to X2 dependence. On the other hand, for the structure analysis, a monochromatized AgKol X-ray beam (58 kV, 166 mA) was used, because of smaller absorption and fluorescence yield than those of a MOKCY X-ray. In addition, a psi scan absorption correction was applied for the structure analysis. For the determination of the lattice parameters, computer-centered positions of 24 Bragg reflections were used with a MoKa! X-ray beam. 3. RESULTS AND DISCUSSION At first, we present briefly crystallographic properties of (BEDT-TTF)3CuBr4 above Tc (see Fig. 1). Lattice paramtters at 65.9K are a = 16.94& b = 10.05& c = 13.95A, (Y= 89.99”, 6 = 102.52”, y = 90.00”, V = 2317.7 A3. There is no significant difference among room temperature structure analyses [8, 91 and our structure analysis at 65.9K besides Br population (see below). This compound contains two independent BEDT-ITF molecules: molecule A on the inversion center and molecule B on general position. The unit cell contains two equivalent columns which have the series of molecules B-A-B-B-A-B-B.. . Divalent anion CuBr, is located on the inversion center. At room temperature, molecular ionicity distribution among A and B molecules has been proposed from the difference in BEDT-TIF molecular structure [5, 8,9] and from the

a

a Fig. 1. Crystal structure of (BEDT-TTF)3CuBr4 at 65.9 K. molecular vibration frequency [5]: molecule A is quasi neutral and molecule B quasi ionic. From BEDT-TTF molecular structure observed by an X-ray, molecular ionicity distribution seems to survive similarly at least at 65.9 K. First we determine Br- coordination number around Cu2+. As mentioned above, at low temperature, temperature factors are highly sensitive for atomic population. That is: incorrectly assumed small (large) atomic population reduces (enlarges) temperature factors to compensate atomic scattering powers. This modification to temperature factors by the wrong population will be marked at low temperature, compared with small magnitude of original temperature factors. On the other hand, this modification can be masked by large magnitude of temperature factors at room temperature. Consequently, we can use temperature factors as a criterion for atomic population analysis at only low temperature. We checked two cases: (1) average Br- number around CL? is 3 (occupancy is 0.75) after Mori et al. [8] and (2) average Br- number is 4 (occupancy is unity) after Guionneau et al. [9]. We found that case 1 leads to unreasonable negative values of isotropic temperature factors of -0.059 (for one crystallographic independent Br-) and -0.072 (for the other Br-). On the other hand, case 2 gives moderate temperature factors of 0.669 (for one Br-) and 0.653 (for the other Br-). Thereby coordination number of Br- is considered to be 4. Furthermore we found that case 1 leads to better R-factor (3.5%) than case 2 does (6.9%). As quoted above, similar tendency in R-factor has been observed at also room temperature [8, 91.

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STBUCTUR4L PHASE TRANSITION IN (BEDT-TTF)&uBr,

Concerning the symmetry of (BEDT-TTF)sCuBr4, X-ray extinction rules are used to study space group symmetry. The space group of (BEDT-TTF)&uBq at room temperature has been reported monoclinic F2t/c [8, 91 without descriptions of systematic evaluation of X-ray extinction rules. In P2i/c crystal, there should be 3 extinction rules: (h 0 1; odd 1 reflections disappear), (0 k 0; odd k ones disappear), (0 0 1; odd 1 disappear). First we investigated (0 k 0) extinction rule. Figure 2 shows the temperature dependence of (0 5 0) reflection profile along b*. Below 60.2 K, one can find (05 0) reflection peak. On the other hand, the peak disappears above 60.9 K, except very weak scattering probably due to crystal imperfection of trifling multiple reflection. Asymmetry peak broadening around Tc is discussed later. Figure 3 shows temperature variation of (05 0) integrated intensity where background X-ray scattering intensity is subtracted. The integrated intensity increases abruptly between 60.9 and 60.2K without any further increase below 60.2 K. This change in (0 k 0) reflection intensity suggests an extinction rule breaking and a symmetry change. To confirm the symmetry change, X-ray intensities are investigated systematically in two ways. (1) Psi scan of (0 5 0) reflection at 21 K and 65.9 K were carried out to distinguish an actual symmetry change from accidental X-ray multiple reflection effect. A multiple reflection is observed at a particular psi angle when (0 5 0) point and

Fig. 2. Temperature dependence of (05 0) reflection profile.

k

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AT 60K

50

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Temperature(K) Fig. 3. Temperature dependence of (05 0) integrated intensity. Note a jump between 60.2 and 60.9K. another reciprocal point satisfy Bragg condition simultaneously. As an experimental result, we aZways observed finite (05 0) reflection intensity for any psi angles at 21 K. On the contrary, at 65.9 K we observed nearly zero intensity for (05 0) reflection except a few psi angles. These results show that (0 k0) extinction rule existing above Tc is broken below Tc. (2) The disappearance of a lot of (h 0 1: odd 1) and (0 0 1: odd 1) reflections with long time X-ray intensity detection no less than 50s at 26.1, 51.1 and 66.2K were checked. This systematic disappearance shows conservation of (h 0 1) and (0 0 1) extinction rules independent of temperature. To summarize, we firstly observed an evidence for a structural phase transition at Tc. Space group symmetry below Tc satisfying observed (h 0 1) and (0 0 1) extinction rules for a monoclinic crystal is PC [lo]. In addition we confirmed P2ilc space group symmetry above Tc. Comparing low temperature phase (L-phase) of PC with high temperature phase (H-phase) of Q/c, L-phase keeps glide plane symmetry, but loses the screw and inversion symmetry originally existing in H-phase. The loss of inversion symmetry should be noted, because BEDT-TT’F molecule A and CuBq anion on the inversion center in H-phase can shift in L-phase. Furthermore two molecules B combined with each other by inversion symmetry in H-phase become independent in L-phase. Generally speaking, a structural phase transition is accompanied with displacements of atoms (molecules). In the present case, we assert that b components of displacements are less than 1% of b length from the small increment of (0 5 0) reflection intensity (see Fig. 3); (0 k0) reflections are insensitive for a or c components. Here, a is interlayer, b is side by side and c is stacking directions (see Fig. 1). Now we discuss the order of the phase transition. In addition to the (0 k0) intensity, an abrupt change in unit cell volume below Tc was observed by cooling (see Figs 3 and 4). The jump in the (0 k 0) intensity related with the order parameter and jump in unit cell volume related

STRUCTURAL PHASE TRANSITION IN (BEDT-TIF)aCuBr4 AT 60 K

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Temperature(K) Fig. 4. Temperature dependence of unit cell volume. with lattice energy reveal that the structural phase transition is first order. This corresponds to the large specific heat anomaly around Tc [4]. Then we treat (Ok 0) reflection width. As shown in Fig. 2, the width of the reflection is temperature independent below 59.0K and almost equal to those of general (hk 1) Bragg reflections. This result supports the first order nature of the transition and indicates a long structural coherence length more than a few thousand A for molecular displacements in L-phase. On the other hand, asymmetric broadening of peak profile observed at 60.2K is attributable to local strain induced by coexistence of H- and L-phases which have different volumes (see Fig. 4). In this sense, Tc for the structural phase transition is found to be 60.2K. This agrees with the Tc values for the magnetic and specific heat anomalies within experimental error. Finally, let us discuss a possible correlation between the structural phase transition and magnetic anomaly. As mentioned above, a decrease in magnetic susceptibility at Z’c has been interpreted as disappearance of unpaired electrons on BEDT-‘ITF molecules [5]. This reminds us of the spin-Peierls transition found in one-dimensional compounds such as MEM-(TCNQ)* [ll] and CuGeOa [12] where unpaired electrons on the molecules (atoms) condense nonmagnetic spin singlet state by molecular (atomic) dimerization accompanied with superlattice structures. In the present compound, it must be noted that a superstructure is not inevitable for molecular dimerization and for spin singlet state, since there are 3 crystallographic independent BEDT-TIF molecules in the L-phase (PC). From this viewpoint, magnetic anomaly results in possible molecular dimerization associated with the present structural phase transition. if so, the steep decrease in magnetic susceptibility below Tc is

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related closely with abrupt molecular displacements reflected in (0 5 0) integrated intensity. As mentioned above, I.R. Marsden et al. observed temperature variation in d-d transition peak of Cu2+ and proposed occurrence of Jahn-Teller distortion of CuBr, [5]. Since Jahn-Teller distortion requires disappearance of inversion symmetry around Cu2+, the present structural change from P21/c (with inversion symmetry) to PC (without inversion symmetry) can be related with it. However, any substantial static Jahn-Teller distortion more than one degree around Cu2+ was not found in our preliminary structural analysis for L-phase at 20 K. Acknowledgment-N. Fujimura and M. Nishikawa are thanked for their help in X-ray measurement. The authors thank the committee of X-ray Laboratory of Okayama University for the use of the Huber 5042 four-circle diffractometer. This work was supported in part by a Grant-in-Aid from the Ministry of Education, Science and Culture. REFERENCES 1. 2. 3. 4. 5.

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