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Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX
Density Wave State in New α‑Type Organic Conductor, α‑(BEDTTTF)2MHg(XCN)4 (M = NH4, X = Se): A Key Material for Universal Phase Diagram of X = S and X = Se Systems Akihiro Ohnuma,† Hiromi Taniguchi,§ Yukihiro Takahashi,‡ and Atsushi Kawamoto*,† Department of Physics, Faculty of Science and ‡Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo 060-0810, Hokkaido, Japan § Graduate School of Science and Engineering, Saitama University, Saitama 338-8570, Japan Downloaded via WESTERN SYDNEY UNIV on October 16, 2018 at 03:20:09 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
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ABSTRACT: A family of organic conductors, α-(BEDTTTF)2MHg(SCN)4, is known to show the density wave (DW) state for M = Tl, K, and Rb salts, or the superconducting (SC) state for M = NH4 salt at low temperatures. In contrast, α-(BEDT-TTF)2MHg(SeCN)4 shows no phase transition and retains its metallic characteristics down to low temperatures. Since no α-(BEDTTTF)2MHg(SeCN)4 salt shows the DW or SC states, it was unclear whether the system of α-(BEDT-TTF)2MHg(SeCN)4 could be understood by the same phase diagram as that of α(BEDT-TTF)2MHg(SCN)4. Here, we succeeded in synthesizing a key material, α-(BEDT-TTF)2NH4Hg(SeCN)4, and determined its crystal structure. The temperature dependence of its electric conductivity and spin susceptibility showed its DW state at low temperatures, indicating that the α-(BEDT-TTF)2MHg(SeCN)4 system is linked to the α-(BEDTTTF)2MHg(SCN)4 system. From the established phase diagram, we found that the dihedral angle between crystallographically independent A and B molecules, ΘB, and that between A and C molecules, ΘC, are good tuning parameters as in θ-type BEDTTTF salts. In the ΘB−ΘC plot, the SC salt is located in the large ΘB and small ΘC regions, whereas the metallic salts are located in the small ΘB and large ΘC regions. The DW salts are located in the intermediate ΘB and ΘC regions. The relationship between their location in the ΘB−ΘC plot and the ground states supports the prediction that the local density of state between A and B molecules determines the ground states.
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INTRODUCTION Quasi-two-dimensional (2-D) organic conductors based on TTF derivatives show various physical properties. Organic conductors that have +0.5 formal charge per donor molecule fall into two main categories. One is the effectively half-filled band system with strong dimeric structures of donor molecules, for example, the κ(BEDT-TTF)2X system, where BEDT-TTF denotes bis(ethylenedithio) tetrathiafulvalene.1 In this system, as well as in high-TC cuprates, the superconducting (SC) phase is adjacent to the antiferromagnetic phase. The relationship between its antiferromagnetism and superconductivity has been discussed in detail. The other category is the quarterfilled band system in which the dimerization is rather weak, such as α-(BEDT-TTF)2MHg(XCN)4 (X = S, Se, M = NH4, K, Rb, and Tl)2 and θ-(BEDT-TTF)2MZn(SCN)4.3 These materials show metallic or insulating behaviors depending on the degree of the long-range Coulomb interaction. Thus, the electronic properties of molecular conductors may be controlled by varying the arrangement of the molecular components. In the latter category, α-(BEDT-TTF)2MHg(XCN)4 is a well-investigated system. α-(BEDT-TTF)2MHg(XCN)4 has © XXXX American Chemical Society
one-dimensional (1-D) and two-dimensional (2-D) Fermi surface. At low temperatures, the X = S salt with M = NH4 shows superconductivity at around 1 K,4,5 whereas the X = S salts with M = K, Rb, and Tl show anomalies around 10 K.6−8 Below this temperature, reconstruction of the Fermi surface due to nesting instability was observed in the angulardependent magnetoresistance oscillations9−11 and anisotropy of the magnetic susceptibility was observed as in typical magnetic ordering systems, suggesting the anomaly could be due to the spin density wave (SDW) transition.12 13C NMR measurement, however, revealed that there is no internal field at low temperatures.13 In contrast, incommensurate satellite reflections were observed by X-ray diffraction (XRD) measurement, indicating the anomaly could be due to the charge density wave (CDW) transition.14 However, the correlation length of the satellite reflections saturates below 50 K and does not increase below about 10 K, which is not consistent with conventional CDW transition behavior. This anomaly has been discussed, yet a question regarding the ground state of the X = Received: September 12, 2018 Revised: September 30, 2018 Published: October 2, 2018 A
DOI: 10.1021/acs.jpcc.8b08912 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C
states, we cannot confirm whether the electronic state of the X = Se system can be described by the same phase diagram as that of the X = S system. To investigate the phase diagram by chemical substitution, an X = Se salt that shows the DW or SC states is desired as the missing link between the X = S and X = Se systems. From the phase diagram with c/a, substitution of the S atom by the Se atom increases c/a, suggesting that the c/a of α(BEDT-TTF)2NH4Hg(SeCN)4 would increase up that of X = S salts with M = K, Rb, and Tl.20,22,23 Hence, α-(BEDTTTF)2NH4Hg(SeCN)4 salt is a key material to link the X = S system to the X = Se system and useful for a systematic study of the phase diagram without the restriction of the pressure cell. However, as far as the authors are aware, there has been no report for α-(BEDT-TTF)2NH4Hg(SeCN)4. We report here the synthesis, crystal structure, and transport and magnetic properties of α-(BEDT-TTF)2NH4Hg(SeCN)4. Furthermore, by comparing the crystal structure and magnetic properties of α-(BEDT-TTF)2NH4Hg(SeCN)4 with those of other salts, we suggest that dihedral angles between the molecules can be regarded as a tuning parameter for physical properties of α-type salts, including X = Se salts.
S salts with M = Tl, K, and Rb remains unresolved. However, at least the ground state could be the density wave (DW) state. Furthermore, it was reported that α-type salts have charge ordering fluctuations caused by off-site Coulomb repulsion below 200 K,15,16 which may be involved in the superconductivity. To determine the mechanism of the superconductivity in α-type salts, it is essential to establish a general phase diagram of the α-(BEDT-TTF)2MHg(XCN)4 system. Generally, there are two methods to establish a general phase diagram. One method is the application of physical pressure. In organic conductors, application of physical pressure is sufficient to induce a drastic change in their electronic state because they are less resistant to compression than inorganic materials. The other method is application of chemical substitutions. A substitution with counter anions changes the volume of a unit cell, thereby changing the transfer integrals among molecules. In α-(BEDT-TTF)2MHg(SCN)4, resistivity measurement was performed using uniaxial strain carefully plotted, which is a form of physical pressure. Using the uniaxial strain technique, Maesato et al. demonstrated the electronic state of α-(BEDTTTF)2MHg(SCN)4 (M = K, NH4) changed from the SC state to the normal metal via the DW state.17 They proposed a general phase diagram described by c/a, where a and c are lattice parameters in a two-dimensional conducting sheet, of which the ratio could be controlled by the uniaxial strain. Applying c-axial strain decreases c/a inducing the SC state, whereas applying a-axial strain increases c/a inducing the DW state.17 Since there are three nonequivalent BEDT-TTF molecules A, B, and C in a unit cell, site sensitive measurements are needed. To investigate ground states of α-type salts microscopically, site-selective 13C NMR measurements were performed in the salt with X = S and M = NH4 and Rb.18,19 In the X = S salts with M = NH4 and Rb, disproportionation of the spin density among molecules was observed,18 suggesting a weak charge order instability in the high-temperature metallic state. The local density of state (DOS) at each molecule was estimated from the spin-lattice relaxation time T1, revealing that modulation of the DOS between the A and B molecules was enhanced in the X = S salt with M = Rb. The DOSs at A and B molecules were comparable in the X = S salt with M = NH4, suggesting uniform electronic structure was realized. The degree of modulation correlated with the ground states, and a phase diagram with NB/NA was proposed by Ihara et al., where NB and NA are the local DOS at B and A molecules, respectively.19 However, the connection between c/a and NB/ NA remains unclear. Application of uniaxial strain may be unsuitable to investigate the magnetism in α-(BEDT-TTF)2MHg(XCN)4 because such application changes not only c/a but the bandwidth significantly, decreasing the magnitude of the DOS. Moreover, the direction of the magnetic field is restricted by the shape of the pressure cell. Therefore, the chemical approach is more suitable than the physical approach for α-type salts. Chemical pressure can be controlled by substituting the M and X atoms while retaining the magnitude of the DOS. Since X = S salts show DW and SC states but no salt shows metallic state at low temperatures, we cannot scan the phase diagram using the X = S salts completely. In contrast, X = Se salts with M = Tl and K show no phase transition yet retain their metallic characteristics down to extremely low temperatures.20,21 Since no X = Se salt shows the DW or SC
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EXPERIMENTAL SECTION Synthesis. α-(BEDT-TTF)2NH4Hg(SeCN)4 had not been synthesized because NH4SeCN salt is difficult to isolate and very sensitive to air and light in its solid state. The key to its preparation is the use of liq. NH3 as a solvent. KSeCN (0.25 mol) was dissolved in liq.NH3 (ca. 30 ml) at −70 °C and was added to a solution of NH4Cl (0.5 mol) in liq.NH3 (ca. 30 mL) at −70 °C with argon purge. Then, small white KCl crystals were precipitated. When the mixture reached room temperature, the liq.NH3 was removed. The resultant solids were extracted using a small amount of CH3CN (ca. 30 mL). The CH3CN solution was poured into dry ether (ca. 1 L). White precipitates (NH4SeCN, 0.25 mol) were immediately collected and washed in ether. The NH4SeCN salt was then dissolved in EtOH (ca. 100 mL) because of its sensitivity to air. This ethanol mixture of 2.5 × 10−3 mol/mL NH4SeCN was used for the electrochemical oxidation. Single crystals of α(BEDT-TTF)2NH4Hg(SeCN)4 were prepared by electrochemical oxidation of BEDT-TTF, by dissolving in 1,1,2trichloroethane and 10% volume absolute ethanol in the presence of a mixture of NH4SeCN, Hg(SeCN)2, and 18crown-6. Electrical crystallization was conducted with Pt electrode (length: ca. 10−20 mm, diameter: ca. 1 mm). The electrooxidation of BEDT-TTF was performed at a constant current of 8 μA at room temperature, and platelike α-(BEDTTTF)2NH4Hg(SeCN)4 was obtained. Crystal Structure. To compare the crystal structure of α(BEDT-TTF)2NH4Hg(SeCN)4 with that of other α-type salts, we determined the crystal parameters not only of α-(BEDTTTF)2NH4Hg(SeCN)4 but α-(BEDT-TTF)2TlHg(SeCN)4, α-(BEDT-TTF)2KHg(SCN)4, α-(BEDT-TTF)2KHg(SeCN)4, α-(BEDT-TTF)2NH4Hg(SCN)4, and α-(BEDT-TTF)2RbHg(SCN)4 using the same XRD apparatus (Rigaku R-AXIS RAPID). XRD measurements were performed using graphitemonochromated Mo Kα radiation (λ = 0.710 75 Å). The crystal structures of α-(BEDT-TTF)2NH4Hg(SCN)4 and α(BEDT-TTF)2NH4Hg(SeCN)4 were solved by the direct method (SIR2011) and refined on F2 with full-matrix leastsquares (SHELXL97).24 B
DOI: 10.1021/acs.jpcc.8b08912 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C Table 1. Cell Parameters of α-(BEDT-TTF)2MHg(XCN)4 a (Å)
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)
c/a
TlHg(SCN)4 KHg(SCN)4
10.051(2) 10.0279(4)
20.549(4) 20.6095(9)
9.934(2) 9.9318(4)
103.63(1) 103.6104(13)
90.48(1) 90.4660(13)
93.27(1) 93.3304(13)
1990.0(1) 1991.11(15)
0.988(2) 0.9904(4)
KHg(SCN)4 NH4Hg(SCN)4
10.082(10) 10.0934(5)
20.565(4) 20.5877(10)
9.933(2) 9.9675(5)
103.70(2) 103.6366(15)
90.91(4) 90.4884(18)
93.06(4) 93.2897(15)
1997.0(21) 2009.03(18)
0.985(2) 0.9875(5)
NH4Hg(SCN)4 RbHg(SCN)4
10.091(1) 10.0695(6)
20.595(2) 20.5819(9)
9.963(1) 10.0038(5)
103.65(1) 103.6803(12)
90.53(1) 90.4529(19)
93.30(1) 93.2580(17)
2008.1(3) 2010.71(17)
0.987(1) 0.9935(5)
RbHg(SCN)4 TlHg(SeCN)4
10.050(3) 10.1071(5)
20.566(4) 20.8080(8)
9.965(2) 10.0547(4)
103.57(2) 103.5337(9)
90.57(2) 90.5059(12)
93.24(2) 93.2675(12)
1998.5(8) 2052.00(14)
0.992(2) 0.9948(4)
TlHg(SeCN)4 KHg(SeCN)4
10.105(1) 10.0784(4)
20.793(3) 20.8882(8)
10.043(1) 10.0374(4)
103.51(1) 103.5375(11)
90.53(1) 90.5585(10)
93.27(1) 93.3587(10)
2047.9(8) 2050.25(13)
0.994(1) 0.9959(4)
KHg(SeCN)4 NH4Hg(SeCN)4
10.048(2) 10.1169(8)
20.722(4) 20.8524(14)
9.976(3) 10.0757(7)
103.59(2) 103.5939(19)
90.43(2) 90.584(2)
93.26(2) 93.240(2)
2015.2(9) 2062.1(3)
0.993(3) 0.9959(7)
refs 22 this work 23 this work 23 this work 23 this work 26 this work 9 this work
Figure 1. (a) Crystal structure of α-(BEDT-TTF)2NH4Hg(SeCN)4 viewed along the c axis. (b) Donor layer of α-(BEDT-TTF)2NH4Hg(SeCN)4 viewed along the b axis.
Figure 2. S−S contacts are shorter than the sum of the van der waals radii in (a) α-(BEDT-TTF)2NH4Hg(SCN)4 and (b) α-(BEDTTTF)2NH4Hg(SeCN)4. S−S contacts between A and B molecules and those between A and C molecules are indicated as blue and pink broken lines, respectively.
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Resistivity. Electronic resistance was measured in α(BEDT-TTF)2NH4Hg(SeCN)4 by the conventional fourprobe DC method with the current parallel to the conducting layer from 300 to 1.1 K. Spin Susceptibility. The static susceptibility of randomly oriented crystals (20 mg) in α-(BEDT-TTF)2TlHg(SeCN)4, α-(BEDT-TTF)2NH4Hg(SCN)4, α-(BEDT-TTF)2NH4Hg(SeCN)4, and α-(BEDT-TTF)2RbHg(SCN)4 was measured in a cooling and warming process with a commercially available SQUID magnetometer under an applied magnetic field of 10 kOe. The spin susceptibility was evaluated by subtracting the core-diamagnetic contribution from the static susceptibility according to Pascalś law.
RESULTS AND DISCUSSION
Crystal Structure. The cell parameters of α-(BEDTTTF)2NH4Hg(SeCN)4 and c/a at room temperature are listed in Table 1 along with those of other α-type salts. All α-type salts were isostructural. The cell volumes can be described by the ionic radius of the M and X atoms in MHg(XCN)4. Here, we discuss the c/a of α-type salts at room temperature. The c/a at low temperatures could be related with the ground state intrinsically. On the other hand, the c/a at room temperature could also be related with the ground state; in fact, it was reported that the c/a of θ-type and κ-type salts at room temperature could tune ground state systematically.25 Although the c/a of the X = S salts line up from the SC salt to C
DOI: 10.1021/acs.jpcc.8b08912 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Figure 3. N−N distances between the NH+4 and SeCN− in the anion layer of (a) α-(BEDT-TTF)2NH4Hg(SCN)4 and (b) α-(BEDTTTF)2NH4Hg(SeCN)4.
about half that of α-(BEDT-TTF)2NH4Hg(SCN)4. In the anion layer, the Hg(SCN)4 anions are located between A and A′ molecules. Hence, it is considered that substituting an Se for an S atom increases the distance between A and A’ molecules thereby decreasing c1. Figure 5 shows the band structure and the Fermi surface of α-(BEDT-TTF)2NH4Hg(SeCN)4. The Fermi surface of α-
DW salt, we could not find a systematic difference among the X = Se salts. The crystal structure of α-(BEDT-TTF)2NH4Hg(SeCN)4 is depicted in Figure 1. Conducting BEDT-TTF and insulating anion layers are stacked alternately along the b-axis. In the conducting layer, there are three crystallographically nonequivalent BEDT-TTF molecules A, B, and C in a unit cell. The S−S contacts shorter than the sum of the van der Waals radii in α-(BEDT-TTF)2NH4Hg(SCN)4 and α-(BEDTTTF)2NH4Hg(SeCN)4 are shown in Figure 2. The S−S contacts of α-(BEDT-TTF)2NH4Hg(SeCN)4 are longer than those of α-(BEDT-TTF)2NH4Hg(SCN)4 because substituting an Se atom for the S atom increases the unit cell volume. Freezing of the NH+4 rotation was reported by 2D NMR.27 The space around NH+4 affects the NH+4 rotation. The N−N distances between the NH4+ and XCN− in α-(BEDTTTF)2NH4Hg(XCN)4 are shown in Figure 3. Different from S−S contacts, N−N distances in α-(BEDT-TTF)2NH4Hg(SeCN)4 are shorter than in α-(BEDT-TTF)2NH4Hg(SCN)4, suggesting the rotational barrier of α-(BEDT-TTF)2NH4Hg(SeCN)4 is larger than that of α-(BEDT-TTF)2NH4Hg(SCN)4. Band Structure. The transfer integrals, band structure, and Fermi surface of the α-(BEDT-TTF)2NH4Hg(SeCN)4 were calculated by the extended Hückel-tight-binding approximation. The transfer integrals are listed in Figure 4. The A and A′ molecules are connected by inversion center symmetry. The transfer integrals of α-(BEDT-TTF)2NH4Hg(SeCN)4 salt are almost the same as those of other α-type salts. However, the transfer integral c1 of α-(BEDT-TTF)2NH4Hg(SeCN)4 is
Figure 5. (a) Extended Hückel-tight-binding band structure and (b)the Fermi surface of α-(BEDT-TTF)2NH4Hg(SeCN)4.
type salts consists of 1-D and 2-D Fermi surfaces as in other αtype salts; hence, one-dimensional instability would be expected. Resistivity. Temperature dependence of the electric resistance of α-(BEDT-TTF)2NH4Hg(SeCN)4 is shown in Figure 6. From electric resistance measurements, we can determine whether the DW state is realized. X = Se salts with M = Tl and K show no phase transition or metallic behavior down to 0.3 and 0.2 K, respectively,20,21 and X = S salt with M = NH4 shows the SC transition below 1.5 K,4,5 whereas X = S salts with M = Tl, K, and Rb are known to show metallic behavior with kink anomalies at 12, 8, and 12 K, respectively,6−8 which are considered to be reconstructions of the Fermi surface due to the DW transition. The present α-(BEDT-TTF)2NH4Hg(SeCN)4 salt shows metallic behavior above approximately 13 K and a kink change to semiconducting state. Spin Susceptibility. The characteristic feature of the DW transition is a steep decrease in spin susceptibility below the transition temperature due to the formation of a spin singlet state. The temperature dependence of the spin susceptibilities of α-(BEDT-TTF)2NH4Hg(SeCN)4 along with those of other α-(BEDT-TTF)2MHg(XCN)4 is shown in Figure 7. Above around 150 K, all α-type salts display similar behavior and except for α-(BEDT-TTF)2NH4Hg(SCN)4, spin susceptibility decreases monotonically with decreasing temperature. The
Figure 4. Transfer integrals of α-(BEDT-TTF)2NH4Hg(SeCN)4. D
DOI: 10.1021/acs.jpcc.8b08912 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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the electronic properties. To investigate the molecular motion of NH+4 ions in α-(BEDT-TTF)2NH4Hg(SeCN)4, further studies such as 2D NMR measurement in α-(BEDTTTF)2NH4Hg(SeCN)4 are desired. Ground State of α-(BEDT-TTF)2NH4Hg(SeCN)4. α(BEDT-TTF)2NH4Hg(SeCN)4 has 1-D and 2-D Fermi surfaces and is expected to have nesting instability as that in other α-type salts. In α-(BEDT-TTF)2NH4Hg(SeCN)4, the anomaly was revealed below about 13 K by resistivity measurement. Below this temperature, spin susceptibility decreased drastically, similar to the behavior of DW salts. This drastic decrease of spin susceptibility might have been caused by the spin-Peierls transition, SDW transition, or CDW transition. However, α-(BEDT-TTF) 2 NH 4 Hg(SeCN) 4 showed metallic behavior down to just above the transition temperature; hence, we can exclude the possibility of the spinPeierls transition. Remaining possible causes of the ground state could be the SDW or CDW states. As for the SDW state, our preliminary 13C NMR measurement in α-(BEDTTTF)2NH4Hg(SeCN)4 (data not shown) suggested the absence of an internal field at low temperatures, as in other DW salts.13 These magnetic behaviors predict the ground state of α-(BEDT-TTF)2NH4Hg(SeCN)4 could be the same as the DW state and that α-(BEDT-TTF)2NH4Hg(SeCN)4 can connect the X = Se system to the X = S system. Whereas the resistivity of the DW salts shows a small hump around transition temperature and retains metallic behavior in the X = S salts with M = Tl, K, and Rb, that of α-(BEDTTTF)2NH4Hg(SeCN)4 shows a smooth change in semiconducting behavior below the transition temperature. Since resistivity may be sensitive to the nesting condition of the 1-D Fermi surface, the slight difference in Fermi surface may have influenced resistivity at low temperatures. One of the anomalous characteristics in the DW state is the short coherent length below the transition temperature from the satellite reflection in XRD measurement. The short coherence could enhance a disorder and affect the resistivity. The behavior of satellite reflection in α-(BEDT-TTF)2NH4Hg(SeCN)4 is interesting. Tuning Parameter of α-Type Salts. Since we confirmed the DW state of α-(BEDT-TTF)2NH4Hg(SeCN)4, the X = S and X = Se systems could be understood by the same phase diagram and connected with a universal phase diagram. Note that the absolute values of spin susceptibility at room temperatures systematically change among SC salt of α(BEDT-TTF)2NH4Hg(SCN)4 and the metallic salt of α(BEDT-TTF)2TlHg(SeCN)4. At room temperature, SC salt showed larger spin susceptibility than that of metallic salt. The DW salts of α-(BEDT-TTF)2RbHg(SCN)4 and α-(BEDTTTF)2NH4Hg(SeCN)4 showed intermediate spin susceptibility. This systematic change in spin susceptibly suggested the metallic salt of α-(BEDT-TTF)2TlHg(SeCN)4 was not adjacent to SC but rather DW salts. Figure 8 shows a summary of our results. The X = S system covers the SC and DW salts, whereas the X = Se system covers the DW and metallic salts. The metallic salts of the X = Se system could not be adjacent to the SC salt but could be adjacent to DW salts. The tuning parameter in α-type system, however, remains unclear. We found no systematic difference in cell parameters among X = Se salts. We explored the tuning parameter for both X = S and X = Se salts from the crystal structures. To explore the tuning parameter, it is useful to compare α-type salts with θ-
Figure 6. Temperature dependence of electric resistance in α-(BEDTTTF)2NH4Hg(SeCN)4.
Figure 7. Temperature dependence of spin susceptibility of α(BEDT-TTF)2NH4Hg(SeCN)4 compared with that of α-(BEDTTTF)2MHg(XCN)4.
spin susceptibility of α-(BEDT-TTF)2TlHg(SeCN)4 decreases monotonically showing no drastic change down to low temperatures. Similar to that of X = S salts with M = K and Rb, the spin susceptibility of α-(BEDT-TTF)2NH4Hg(SeCN)4 decreases drastically below around 12 K, suggesting that this anomaly of resistance is due to the DW transition. The spin susceptibility of α-(BEDT-TTF)2NH4Hg(SCN)4 was shown to differ from that of other α-type salts around 150 K and showed a slight hump around 60 K accompanied by change in the molecular motion of NH+4 ions measured by 2D NMR in α-(BEDT-TTF)2NH4Hg(SCN)4.27 If the molecular motion of NH+4 ions affects the electronic state, similar behavior should be observed in α-(BEDT-TTF)2NH4Hg(SeCN)4 at almost the same or a relatively higher temperature because the rotation barrier of α-(BEDT-TTF)2NH4Hg(SeCN)4 is expected to be larger than that of α-(BEDTTTF)2NH4Hg(SCN)4. The spin susceptibility of α-(BEDTTTF) 2 NH 4Hg(SeCN) 4, however, does not show such deviation or feature below 150 K. This result suggests that the molecular motion of NH+4 ions could not significantly affect E
DOI: 10.1021/acs.jpcc.8b08912 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Figure 8. Schematic low-temperature phase diagram of α-(BEDTTTF)2MHg(XCN)4.
type salts.3 The donor arrangements of α-type and θ-type salts are shown in Figure 9a,b, respectively. θ-type salts have a
Figure 10. Phase diagram of α-(BEDT-TTF)2MHg(XCN)4. The horizontal and vertical axes are the dihedral angles between A and B molecules and A and C molecules, respectively. The blue circle, orange squares, and red triangles denote the SC, DW, and metallic salts, respectively.
integrals between A and B (tB = p1 or p4) and A and C molecules (tC = p2 or p3), respectively. This suggests SC salt could be located in the large tB and small tC regions and metallic salts could be in the small tB and large tC regions. This scenario is one of the possibilities only from crystal structures. To reveal the electronic structure in α-type salts, systematic and microscopic investigations, i.e., 13C NMR measurement, is desired. In X = S salts with M = Rb and NH4, local spin susceptibility at each molecule was investigated by 13C NMR.18,19 The 13C NMR at each site suggested the charge disproportionation was due to off-site coulomb repulsion. At room temperature, the local DOSs at B and C sites, which are proportional to the charge on the molecules, differ a little, and the local DOS at A site was about 70% larger than that at B and C sites, suggesting a vertical stripe structure, as shown in Figure 11a. With decrease in temperature, an additional disproportionation in B and C column developed. The local DOS at B site increased and that at C site decreased, as shown in Figure 11b, whereas the A and A’ columns are uniform due to crystal symmetry. 13 C NMR measurements revealed the disproportionation between A and B molecules was more enhanced in the DW salt α-(BEDT-TTF)2RbHg(SCN)4 than in SC salt α-(BEDTTTF)2NH4Hg(SCN)4, suggesting NB/NA could be a tuning parameter for α-type salts.19 The difference between DW and SC salts is the disproportionation between A and B molecules. The charge disproportionation between A and B molecules is caused by off-site coulomb repulsion between A and B molecules VA−B. The degree of the charge disproportionation is characterized by VA−B/tB. When tB is significantly larger than VA−B, the degree of charge disproportionation between A and B molecule is suppressed. In the ΘB−ΘC plot, the SC salt is located in the small ΘB region. Decrease in ΘB leads to increase in tB, whereas VA−B does not change significantly, indicating NB became similar to NA. The ΘB−ΘC plot explains the proposed NB/NA phase diagram. Furthermore, the Θ B−ΘC plot brings an additional prediction. The angle of ΘC might be related to NC/NA, which could also be a tuning parameter of α-type salts. In the
Figure 9. Donor arrangements of (a) θ-type and (b) α-type salts. Transfer integrals and dihedral angles are indicated by arrows.
quarter-filled band as do α-type salts. In θ-type salts, it was suggested that the dihedral angle (Θ) tunes the ground states because the transfer integral (p) decreases between adjacent stacks and bandwidth as the dihedral angle increases. On the other hand, α-type salts have four kinds of transfer integrals between adjacent stacks, p1, p2, p3, and p4, as shown in Figure 9b,2 whereas θ-type salts have only p. Both p1 and p4 are dominated by the dihedral angle between A and B molecules (ΘB), and p2 and p3 by that between A and C molecules (ΘC). Hence, the bandwidth of α-type salts may be controlled by the two dihedral angles. In θ-type salts, there is only one dihedral angle, which shows a negative correlation with c/a. Therefore, c/a is also a good tuning parameter for θ-type salts. However, in α-type salts, there are two dihedral angles, ΘB and ΘC; ΘB shows a positive whereas ΘC shows a negative correlation with c/a. We plotted the α-type salts on ΘB−ΘC coordinates, as shown in Figure 10. The SC, DW, and metallic salts are indicated by circles, squares, and triangles, respectively. Most of the salts were plotted along a linear line. Since the A, B, and C molecules cannot tilt independently because of steric repulsion among molecules, some relationship between ΘB and ΘC might be expected. The SC salt was located in the small ΘB and large ΘC regions, whereas the metallic salts were located in the large ΘB and small ΘC regions. The DW salts were located in the intermediate ΘB and ΘC regions. Here, we discuss the relationship between the dihedral angle and electronic state. In θ-type salts, it was suggested that a decrease in the dihedral angle leads to an increase in the transfer integral between adjacent stacks.3 On the basis of the relationship between the dihedral angle and transfer integral, decrease in ΘB and ΘC could lead to an increase in transfer F
DOI: 10.1021/acs.jpcc.8b08912 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Figure 11. Schematic diagram of charge disproportionation at (a) room temperature and (b) 50 K. The scale of the yellow circles represents the degree of the local DOS. Transfer integrals between adjacent stacks are also indicated.
ΘB−ΘC plot, the metallic salts are located in the large ΘB and small ΘC regions. Hence, metallic salts could be realized when NB/NA decreases and NC/NA increases. In our preliminary 13C NMR measurement in α-(BEDTTTF)2NH4Hg(SeCN)4 (data not shown), we observed that the linewidth of A site broadened below 200 K, which is similar to the behavior of α-(BEDT-TTF)2RbHg(SCN)4.18 This result indicated charge disproportionation. To detect charge fluctuation, further studies such as infrared measurements in α-(BEDT-TTF)2NH4Hg(SeCN)4 are desired.
(2) Mori, H.; Tanaka, S.; Oshima, M.; Saito, G.; Mori, T.; Maruyama, Y.; Inokuchi, H. Crystal and electronic structures of (BEDT-TTF)2MHg(SCN)4 (M = K and NH4). Bull. Chem. Soc. Jpn. 1990, 63, 2183−2190. (3) Mori, H.; Tanaka, S.; Mori, T. Systematic study of the electronic state in θ-type BEDT-TTF organic conductors by changing the electronic correlation. Phys. Rev. B 1998, 57, 12023−12029. (4) Wang, H.; Carlson, K.; Geiser, U.; Kwok, W.; Vashon, M.; Thompson, J.; Larsen, N.; McCabe, G.; Hulscher, R.; Williams, J. A new ambient-pressure organic superconductor: (BEDTTTF)2(NH4)Hg(SCN)4. Phys. C 1990, 166, 57−61. (5) Taniguchi, H.; Sato, H.; Nakazawa, Y.; Kanoda, K. Highly anisotropic superconductivity in the organic conductor α-(BEDTTTF)2NH4Hg(SCN)4. Phys. Rev. B 1996, 53, R8879−R8882. (6) Sasaki, T.; Toyota, N.; et al. Transport properties of organic conductor (BEDT-TTF)2KHg(SCN)4: I. Shubnikov-de Haas oscillations and spin-splitting effect. Solid State Commun. 1990, 75, 93−96. (7) Kinoshita, N.; Tokumoto, M.; Anzai, H. Electron spin resonance and electric resistance anomaly of (BEDT-TTF)2RbHg(SCN)4. J. Phys. Soc. Jpn. 1991, 60, 2131−2134. (8) Kushch, N.; Buravov, L.; Kartsovnik, M.; Laukhin, V.; Pesotskii, S.; Shibaeva, R.; Rozenberg, L.; Yagubskii, E.; Zvarikina, A. Resistance and magnetoresistance anomaly in a new stable organic metal (ET)2TlHg(SCN)4. Synth. Met. 1992, 46, 271−276. (9) Sasaki, T.; Toyota, N. Anisotropic galvanomagnetic effect in the quasi-two-dimensional organic conductor α-(BEDT-TTF)2KHg(SCN)4, where BEDT-TTF is bis(ethylenedithio)tetrathiafulvalen. Phys. Rev. B 1994, 49, 10120−10130. (10) Kartsovnik, M. V.; Kovalev, A. E.; Kushch, N. D. Magnetotransport investigation of the low-temperature state of transition (BEDT-TTF)2TIHg(SCN)4: evidence for a Peierls-type transition. J. Phys. I 1993, 3, 1187−1199. (11) Uji, S.; Terashima, T.; Aoki, H.; Brooks, J. S.; Tokumoto, M.; Kinoshita, N.; Kinoshita, T.; Tanaka, Y.; Anzai, H. Fermi-surface studies in the two-dimensional organic conductors (BEDTTTF)2MHg(SCN)4 (M = Tl, K, Rb, NH4). Phys. Rev. B 1996, 54, 9332−9340. (12) Sasaki, T.; Sato, H.; Toyota, N. Spin-splitting Shubnikov-de Haas oscillations in organic conductor (BEDT-TTF)2KHg(SCN)4. Synth. Met. 1991, 42, 2211−2214. (13) Miyagawa, K.; Kawamoto, A.; Kanoda, K. 13C NMR study of nesting instability in α-(BEDT-TTF)2RbHg(SCN)4. Phys. Rev. B 1997, 56, R8487−R8490. (14) Foury-Leylekian, P.; Ravy, S.; Pouget, J. P.; Muller, H. X-Ray study of the density wave instability of α-(BEDT-TTF)2MHg(SCN)4 with M = K and Rb. Synth. Met. 2003, 137, 1271−1272. (15) Drichko, N.; Dressel, M.; Kuntscher, C. A.; Pashkin, A.; Greco, A.; Merino, J.; Schlueter, J. Electronic properties of correlated metals in the vicinity of a charge-order transition: Optical spectroscopy of α(BEDT-TTF)2MHg(SCN)4 (M = NH4, Rb, Tl). Phys. Rev. B 2006, 74, 1−11.
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CONCLUSIONS We synthesized α-(BEDT-TTF)2NH4Hg(SeCN)4 salt, which showed DW state around 12 K as in α-(BEDT-TTF)2MHg(SCN)4 salt (M = Tl, K, and Rb). This salt is the key material that links the X = S system to X = Se system and connects the X = S and X = Se systems to a universal phase diagram. From the universal phase diagram, we found ΘB−ΘC was a possible tuning parameter. The ΘB and ΘC of α-type salts could be plotted on a linear line. In this ΘB−ΘC plot, SC salt is located in the small ΘB and higher ΘC regions, whereas metallic salts are located in the small ΘC and higher ΘB regions. The tuning parameter ΘB−ΘC supports the previous NB/NA phase diagram. Spin susceptibility measurements indicated that the rotation of NH4+ could not affect the electronic state significantly. α-(BEDT-TTF)2NH4Hg(SeCN)4 may facilitate further precise experiments without restrictions inherent in the pressure cell.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Yukihiro Takahashi: 0000-0002-3252-9502 Atsushi Kawamoto: 0000-0001-5296-7123 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank Dr Y. Fukuda and N. Akiko of Ochanomizu University for determination of the crystal structure. This work was partially supported by KAKENHI Grant No. 16K05427.
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REFERENCES
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