Negative and Positive Anisotropic Thermal Expansions in a

Article pubs.acs.org/cm

Negative and Positive Anisotropic Thermal Expansions in a Hexagonally Packed Columnar Discotic Liquid Crystal Thin Film Hyo-Sik Kim,† Sung-Min Choi,*,† Brian D. Pate,‡ Shin-Hyun Kang,† Jun-Bo Sim,† and Jun-Ki Lee† †

Department of Nuclear and Quantum Engineering, Korea Advanced Institute of Science and Technology, Daejeon 305-701, Republic of Korea ‡ Department of Materials Science and Engineering, University of Maryland, College Park, Maryland 20742-2115, United States S Supporting Information *

ABSTRACT: The negative thermal expansion showing contraction of unit cell dimensions on heating is an unusual phenomenon which is rarely observed in organic materials. Here we report that a highly aligned hexagonal columnar liquid crystalline phase of discotic liquid crystal, cobalt octakis(n-decylthio)porphyrazine (CoS10), in thin films shows negative thermal expansion in the transverse plane and positive thermal expansion along the column axis. This exceptional thermal expansion behavior of CoS10 at the hexagonal columnar liquid crystalline phase was understood as the synergetic effect of the increased vibrational motion of n-alkyl side chains perpendicular to the chain axes and the increased tilting of the cores with temperature.

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INTRODUCTION Most of materials expand on heating due to the anharmonicity of interatomic or intermolecular potentials.1 This phenomenon, called positive thermal expansion, is usually observed along all three crystallographic axes. Some materials, however, exhibit a very unusual thermal property, so-called negative thermal expansion, which entails contraction of the unit cell dimensions upon heating.2−4 These materials have been of great interest for various potential applications as well as for fundamental scientific reasons. In the case of organic materials, a very few materials, such as a series of cyanide-bridged coordination polymers,5,6 metal−organic-frameworks,7−9 and organic molecules with specific packing arrangements,10−14 have been found to show the negative thermal expansion behavior. Although several mechanisms have been proposed to explain such negative thermal expansion behavior in organic materials, these proposed mechanisms are typically specific for each type of material, and they are also highly dependent on the geometrical arrangements of atoms or molecules and the interatomic or intermolecular interactions which restrict the thermal vibrations. Here, we report that an alkyl-substituted discotic liquid crystal, cobalt octakis(n-decylthio)porphyrazine (CoS10; Figure 1a) in the hexagonal columnar liquid crystalline (LC) phase, shows negative thermal expansion in the transverse plane and positive thermal expansion along the columnar axis over the entire range of the LC phase. This exceptional thermal expansion behavior of CoS10 was understood in terms of the vibrational motion of n-alkyl side chains perpendicular to the chain axes and the tilting of π-stacked aromatic cores with temperature. Furthermore, the self-healing of defects in the two-dimensional (2D) hexagonal columnar packing, which is a © XXXX American Chemical Society

Figure 1. (a) Chemical structure of CoS10 and (b) schematic representation of GIXS geometries for the uniaxially aligned columnar superstructure of CoS10.

very important issue for organic electronic applications, has been observed to be strongly enhanced near the transition from Received: February 24, 2015 Revised: April 15, 2015

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DOI: 10.1021/acs.chemmater.5b00720 Chem. Mater. XXXX, XXX, XXX−XXX

Article

Chemistry of Materials

Figure 2. Two-dimensional GIXS patterns of the uniaxially aligned CoS10 thin film (95 nm) at different temperatures, which were measured (a) in the small-angle regime with the incident beam parallel to the columnar axis and (b) in the wide-angle regime with the incident beam perpendicular to the columnar axis.

scales), GIXS patterns were measured for the two orthogonal directions of samples as shown in Figure 1b; one was measured with the incident beam parallel to the columnar director (in the small-angle regime to measure the columnar arrangement), and the other was measured with the incident beam perpendicular to the columnar director (in the wide-angle regime to measure the molecular arrangements within columns).

the LC to the isotropic phase. These behaviors have been investigated for highly aligned thin film samples using the grazing-incidence X-ray scattering (GIXS) technique. Discotic liquid crystal molecules consist of a rigid aromatic core and several alkyl chains peripherally attached to the core moiety, and exist in various phases, such as columnar, nematic, and isotropic phases depending on the temperature, composition, and molecular structure.15 The thermodynamic phase transitions are the result of competition between enthalpic gain from π-stacking of the cores and entropic gain from the flexible nature of the side chains. In the columnar phase, one-dimensional self-assembly of disk-shaped molecules often shows high charge-carrier mobility only along the columnar axis.16 This anisotropic electrical property has gained significant interest for potential applications in organic electronics, such as field-effect transistors, light-emitting diodes, and photovoltaic devices.17−20 Recently, we developed a new method to fabricate a uniaxially oriented and highly ordered columnar superstructure of CoS10 in thin films on a large area of substrate, by simultaneous utilization of an applied magnetic field (ca. 1.0 T) and the interfacial interaction of CoS10 with an octadecyltrichlorosilane (OTS)-functionalized substrate.21 This uniaxially oriented and highly ordered columnar superstructure of CoS10 provides an excellent test bed to perform detailed studies of the dependence of the packing structure on the external conditions. The detailed structural changes of the CoS10 superstructure with temperature have been characterized by the GIXS technique, which is a powerful tool to investigate the internal nanoscale structures in thin films. To observe the structural changes in all three directions (and on two different length

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EXPERIMENTAL SECTION

Materials and Sample Preparation. CoS10 was synthesized according to a previously reported procedure.22 OTS (90+%), anhydrous toluene (99.8%), and chloroform (99.8%) were purchased from Sigma-Aldrich. Si substrates were cut into 20 × 20 mm2 pieces and cleaned by piranha solution to create surface hydroxyl groups. For the OTS functionalization, the substrates were soaked in an OTS/ anhydrous toluene solution (6 g L−1) and then sonicated for 15 min in anhydrous toluene. CoS10 thin films were spin-coated on the OTSfunctionalized substrates using CoS10/chloroform solution. The thicknesses of the CoS10 thin films were controlled by the concentration of the CoS10 solution and measured by atomic force microscopy (AFM). These thin films on the OTS-functionalized Si substrates were abruptly heated to 210 °C and then cooled to 80 °C under an applied magnetic field of 1.0 T (BE25, Bruker) with a cooling rate of 2 °C/min. GIXS Experiment. The GIXS measurements were performed using the 4C2 beamline at the Pohang Accelerator Laboratory, Pohang, Korea. The incident angle α of the X-ray beam (λ = 1.3807 Å) was set to 0.15−0.20°, which is smaller than the critical angle of the silicon wafer (0.203°) and larger than that of CoS10 (0.145°). The sample-todetector distance was calibrated by a silver behenate standard sample. The scattering vectors Qy and Qz were defined by Qy = (2π/λ){cos(β) sin(γ)} and Qz = (2π/λ){sin(α) + sin(β)}, where β and γ are exit angles along the out-of-plane direction and along the in-plane direction, respectively. B


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RESULTS AND DISCUSSION

The GIXS measurements of the uniaxially aligned CoS10 thin film (of 95 nm thickness; Supporting Information, Figure S1) on an OTS-functionalized silicon substrate were performed in the temperature range of 30−226 °C with a heating rate of 1 °C/min. The sample shows three different phases with increasing temperature, crystalline (K), liquid crystalline (LC), and isotropic (I) phases. The representative patterns are shown in Figure 2. The GIXS patterns at 30 °C indicate that CoS10 is in the crystalline phase with a 2D rectangular columnar packing structure with lattice constants of a = 58.4 Å and b = 17.0 Å, respectively, and the aromatic cores of CoS10 (with a cofacial distance of ca. 3.8 Å) are tilted by 40° against the columnar director.21 Upon heating to 65 °C, the rectangular columnar packing was changed to hexagonal columnar packing. The GIXS patterns at 85 °C show that CoS10 is in the LC phase with a 2D hexagonal columnar packing structure possessing a lattice constant a = 25.5 Å, and the cofacial distance of the disks within the columns is 4.1 Å. Interestingly, the GIXS pattern in the wide-angle regime shows broad scattering peaks at around Qy = ±1.1 Å−1 with an azimuth angle of 31.6°, indicating that the cores of the CoS10 molecules are tilted with respect to the columnar axis by 31.6°, even in the hexagonal columnar phase. It is known that the core tilting is dictated by side chain sulfur lone pair donation to the half-occupied dz2 orbital of the central cobalt ion of an adjacent macrocycle, which is maintained in the hexagonal phase.23,24 The formation of the hexagonal columnar phase with columns of tilted cores may indicate that the columns of tilted cores are rotationally uncorrelated, which is facilitated by the liquid-like nature of the alkyl side chains (as indicated by the broad scattering halo).25 Since only a small fraction of the rotationally uncorrelated columns of tilted cores satisfy the Bragg condition, the scattering intensity corresponding to the core tilting is fairly weak. Upon further heating, CoS10 transforms into the isotropic phase at around 215 °C. The GIXS patterns measured at 225 °C show weak broad halos in both the small-angle and the wide-angle regimes, which indicates the isotropization of CoS10 molecules without long-range order. Since the degradation temperature of CoS10 is around 240 °C, the integrity of the sample is expected to be maintained throughout the measurements. This was confirmed by measuring the GIXS patterns of the sample upon cooling after the highest temperature used in this study was reached, which showed the isotropic to LC and the LC to crystalline phase transitions as expected. To understand the temperature-dependent change of the 2D hexagonal columnar packing structure of CoS10 in the LC phase, the GIXS patterns measured in the small-angle regime were analyzed. The intercolumnar distance a in the 2D hexagonal columnar packing, which was extracted from the circularly averaged GIXS patterns in the small-angle regime, decreases with temperature (Figure 3a). This clearly indicates that CoS10 at the LC phase exhibits negative thermal expansion behavior in the transverse plane (the plane perpendicular to the columnar axis). In Figure 3a, it should be noted that there are two regions with different slopes. The negative thermal expansion coefficient along the a axis (αa = [(∂ ln a)/∂T]p) estimated from the lattice constant a in the temperature range from 65 to 207 °C is αa = −5.42 × 10−5 K−1. Near the transition temperature from the LC to the I phase (∼207−211 °C), the negative thermal expansion coefficient

Figure 3. Temperature dependence of (a) lattice parameter a obtained from the circularly averaged GIXS patterns in the small-angle regime, (b) azimuthal FWHM, (c) peak height (inset for a 290 nm thin film), and (d) integrated intensity extracted from the azimuthally averaged GIXS patterns as shown in the inset of (b).

increases abruptly to αa = −6.14 × 10−4 K−1, which is exceptionally large. It should be noted that this value is about 5 times larger than the colossal negative thermal expansion found in Ag3[Co(CN)6] (−1.2 × 10−4 to −1.3 × 10−4 K−1).5 To understand the change of the degree of ordering with temperature, the (10) Bragg reflection was azimuthally averaged (as indicated in the inset of Figure 3b), from which the full width at half-maximum (FWHM) along the azimuth angle and the peak height of the (10) reflection were estimated. As temperature increases in the LC phase region, the FWHM decreases and the peak height increases gradually (Figure 3b,c). This clearly indicates that the degree of ordering of the 2D hexagonal columnar packing structure in the LC phase is enhanced with temperature. It should be noted that the FWHM decreases and the peak height increases more rapidly near the transition temperature from the LC to the I phase. The enhanced increase of ordering near the transition temperature was more clearly observed in a thicker film of CoS10 (thickness of 290 nm), which has more defects in the 2D hexagonal columnar packing structure (Figure 3c, inset; Supporting Information, Figure S2).26 This is often called a self-healing effect, which is one of the most important properties of columnar discotic LC materials for their electronic applications. The integrated intensity of the (10) reflection did not change throughout the LC phase region (Figure 3d), which may indicate that the uniaxial orientation of CoS10 columns was maintained throughout the LC phase (if the uniaxial orientation of CoS10 is not maintained, the integrated intensity would decrease). To understand the temperature-dependent change of molecular arrangements within the columns of CoS10 in the LC phase, the GIXS patterns measured in the wide-angle region were analyzed. The cofacial distance between adjacent CoS10 molecules in columns was extracted from the sectored circular average of the scattering peak corresponding to the core−core correlation, as indicated in the inset of Figure 4a. Since the C


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correlation distance, which was estimated from the sectored circular average of the scattering halo as indicated in the inset of Figure 4d, increases with temperature in the LC phase region. This may indicate that the density of the alkyl side chain region decreases with temperature in the LC phase region. It is interesting to note that, at the transition from the LC to the I phase, the side chain−side chain correlation distance decreases abruptly and then increases with temperature again (Figure 4d). The negative thermal expansion in the transverse plane and the positive thermal expansion along the columnar axis of the CoS10 thin film in the LC phase can be understood in terms of the synergetic effects of the thermal vibrational motion of alkyl side chains and the tilting of aromatic cores to which the side chains are attached (Figure 5). As temperature increases, the

Figure 5. Schematic representation of the negative and positive thermal expansion behavior of CoS10 at the hexagonal columnar LC phase. The circular arrow indicates the rotationally uncorrelated columns of tilted cores.

Figure 4. Temperature dependence of the (a) cofacial distance, (b) tilt angle with respect to the columnar axis, (c) molecular displacement along the columnar axis, and (d) average distance between n-alkyl side chains in the CoS10 thin film (95 nm).

alkyl side chains contract due to an increased vibrational motion perpendicular to the chains (i.e., due to motions in the direction of weak van der Waals interactions),27,28 although the overall volume of the side chain region is increased (as indicated by the increase of the side chain−side chain correlated distance). To accommodate the thermal expansion of the alkyl chain region with temperature while the alkyl chain region is contracted along the covalent chain direction, the columns extend along the columnar axis.29,30 This extension is realized by the increased tilt of the cores with respect to the columnar axis. The increase of the cofacial distance with temperature further contributes to the extension of the columns. The increased contraction of the side chains along the chains and the tilt of the cores with temperature synergetically lead to the negative thermal expansion in the transverse plane. The increase of the cofacial distance and core tilt angle with temperature leads to the positive thermal expansion along the columnar axis. Considering that the thermal expansion coefficient of the cofacial distance (αcofacial = [(∂ ln dcofacial)/∂T]p) is 3.05 × 10−4 K−1 and the thermal expansion coefficient along the columnar axis is 6.17 × 10−4 K−1, the contribution of core tilting to the thermal expansion along the columnar axis is as pronounced as that of the increased cofacial distance. The enhanced contraction of a and tilting of the cores near the transition temperature (from the LC to the I phase) may be attributed to increased molecular mobility, especially the vibrational motion of the side chains. The increased molecular mobility near the transition temperature, which may allow the removal of defects more efficiently, may be responsible for the enhanced ordering near the transition temperature (Figure 3b,c; see also Supporting Information, Figure S2).31,32

scattering intensity in the sectored region has two contributions (one from the core−core correlation and the other from the side chain−side chain correlation), the scattering from the side chain−side chain correlation was subtracted before estimation of the cofacial distance (Supporting Information, Figure S3). The cofacial distance monotonically increases with temperature in the entire LC phase region (Figure 4a). The tilt angle of the cores with respect to the columnar axis was extracted from the azimuthal average of the scattering patterns, as indicated in the inset of Figure 4b. As temperature increases, the tilt angle monotonically increases from 31° to 35° in the entire LC phase region and then increases more rapidly near the transition temperature (Figure 4b). From the measured cofacial distance (dcofacial) and tilt angle (θtilt), the molecular displacement along the columnar axis was calculated (dc = dcofacial/(cos θtilt)), which monotonically increases with temperature (Figure 4c). Unlike the intercolumnar distance a, the molecular displacement dc along the columnar axis shows a positive thermal expansion behavior. The positive thermal expansion coefficient along the columnar axis in the LC phase region (αc = [(∂ ln dc)/∂T]p) was calculated to be 6.17 × 10−4 K−1. This value is very large compared with those of other columnar discotic molecules in the LC phase.13 The αc near the LC to I phase transition was not estimated due to the limited number of data points available, although dc shows a more rapid increase with temperature. From the transverse plane and the columnar axis linear thermal expansion coefficients, the volumetric thermal expansion coefficient (βV = 2αa + αc) in the LC phase was calculated to be 5.10 × 10−4 K−1. The side chain−side chain D



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The thermal expansion behavior of discotic columnar systems occurs by the interplay of thermal motion of the side chains and core−core interaction along the columnar axis.33−36 The negative thermal expansion of CoS10 induced by the synergetic effects of the increased thermal motion of the side chains and the tilting of the aromatic cores is in contrast with previously reported negative thermal expansions of discotic columnar systems in which either the thermal motion of the side chains or the tilting of the aromatic cores is the main driving mechanism.13,14 For example, the transverse negative thermal expansion of hexakis(hexylthio)triphenylene in the hexagonal columnar LC phase was attributed to the increased thermal motion of the side chains with increased cofacial distance without tilting,14 and the transverse negative thermal expansion of the hexa-peri-hexabenzocoronene derivative with monoiodine substitution in the crystalline phase with a herringbone structure was mainly attributed to the tilting of the cores with temperature.13

CONCLUSION In summary, we report that the highly aligned hexagonal columnar LC phase of discotic liquid crystals (CoS10) in a thin film shows negative thermal expansion in the transverse plane and positive thermal expansion along the column axis. This exceptional thermal expansion behavior of CoS10 at the hexagonal columnar LC phase was understood as the coupled effect of the increased vibrational motion of the n-alkyl side chains perpendicular to the chain axes and the increased tilting of the cores with temperature. The self-healing effect with temperature, which is one of the most important properties of columnar discotic LC materials for their electronic applications, was observed to be strongly enhanced near the transition from the LC to the I phase. The systematic understanding of the unique thermal expansion behavior and the self-healing effects in the LC phase of CoS10 may provide new insights for designing organic electronic devices. ASSOCIATED CONTENT

S Supporting Information *

Figures showing the 2D GIXS patterns for the uniaxially aligned CoS10 thin film, temperature dependence of the intercolumnar distance a in the 2D hexagonal columnar packing of CoS10, temperature dependence of FWHM, peak height, and integrated intensity extracted from the azimuthally averaged GIXS patterns, and data handling procedure for the GIXS patterns in the wide-angle regime. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

(1) Ashcroft, N. W.; Mermin, N. D. Solid State Physics; Holt, Rinehart and Winston: New York, 1976; pp 490−495. (2) Fortes, A. D.; Suard, E.; Knight, K. S. Science 2011, 331, 742. (3) Shen, X.; Viney, C.; Johnson, E. R.; Wang, C.; Lu, J. Q. Nat. Chem. 2013, 5, 1035. (4) Panda, M. K.; Runčevski, T.; Sahoo, S. C.; Belik, A. A.; Nath, N. K.; Dinnebier, R. E.; Naumov, P. Nat. Commun. 2014, 5, 4811. (5) Goodwin, A. L.; Calleja, M.; Conterio, M. J.; Dove, M. T.; Evans, J. S. O.; Keen, D. A.; Peters, L.; Tucker, M. G. Science 2008, 319, 794. (6) Korčok, J. L.; Katz, M. J.; Leznoff, D. B. J. Am. Chem. Soc. 2009, 131, 4866. (7) Dubbeldam, D.; Walton, K. S.; Ellis, D. E.; Snurr, R. Q. Angew. Chem., Int. Ed. 2007, 46, 4496. (8) Lock, N.; Wu, Y.; Christensen, M.; Cameron, L. J.; Peterson, V. K.; Bridgeman, A. J.; Kepert, C. J.; Iversen, B. B. J. Phys. Chem. C 2010, 114, 16181. (9) Henke, S.; Schneemann, A.; Fischer, R. A. Adv. Funct. Mater. 2013, 23, 5990. (10) Das, D.; Jacobs, T.; Barbour, L. J. Nat. Mater. 2010, 9, 36. (11) Haas, S.; Batlogg, B.; Besnard, C.; Schiltz, M.; Kloc, C.; Siegrist, T. Phys. Rev. B 2007, 76, 205203. (12) Percec, V.; Rudick, J. G.; Peterca, M.; Heiney, P. A. J. Am. Chem. Soc. 2008, 130, 7503. (13) Grigoriadis, C.; Haase, N.; Butt, H.-J.; Müllen, K.; Floudas, G. Adv. Mater. 2010, 22, 1403. (14) Krishna Prasad, S.; Shankar Rao, D. S.; Chandrasekhar, S.; Kumar, S. Mol. Cryst. Liq. Cryst. 2003, 396, 121. (15) Bushby, R. J.; Lozman, O. R. Curr. Opin. Colloid Interface Sci. 2002, 7, 343. (16) Boden, N.; Bushby, R. J.; Clements, J.; Movaghar, B.; Donovan, K. J.; Kreouzis, T. Phys. Rev. B 1995, 52, 13274. (17) Sergeyev, S.; Pisula, W.; Geerts, Y. H. Chem. Soc. Rev. 2007, 36, 1902. (18) Mas-Torrent, M.; Rovira, C. Chem. Soc. Rev. 2008, 37, 827. (19) Schmidt-Mende, L.; Fechtenkötter, A.; Müllen, K.; Moons, E.; Friend, R. H.; MacKenzie, J. D. Science 2001, 293, 1119. (20) Wendorff, J. H.; Christ, T.; Glüsen, B.; Greiner, A.; Kettner, A.; Sander, R.; Stümpflen, V.; Tsukruk, V. V. Adv. Mater. 1997, 9, 48. (21) Kim, H.-S.; Choi, S.-M.; Lee, J.-H.; Busch, P.; Koza, S. J.; Verploegen, E. A.; Pate, B. D. Adv. Mater. 2008, 20, 1105. (22) Pate, B. D.; Choi, S.-M.; Werner-Zwanziger, U.; Baxter, D. V.; Zaleski, J. M.; Chisholm, M. H. Chem. Mater. 2002, 14, 1930. (23) Ricciardi, G.; Rosa, A.; Ciofini, I.; Bencini, A. Inorg. Chem. 1999, 38, 1422. (24) Dye, D. F.; Raghavachari, K.; Zaleski, J. M. Inorg. Chim. Acta 2008, 361, 1177. (25) Safinya, C. R.; Liang, K. S.; Varady, W. A.; Clark, N. A.; Andersson, G. Phys. Rev. Lett. 1984, 53, 1172. (26) Kim, H.-S.; Choi, S.-M.; Pate, B. D.; Park, P. G. ChemPhysChem 2009, 10, 2642. (27) Tasumi, M.; Shimanouchi, T. J. Chem. Phys. 1965, 43, 1245. (28) Kerch, G. M.; Irgen, L. A. Mech. Compos. Mater. 1975, 11, 468. (29) Ungar, G.; Percec, V.; Holerca, M. N.; Johansson, G.; Heck, J. A. Chem.Eur. J. 2000, 6, 1258. (30) Lehmann, M.; Köhn, C.; Meier, H.; Renker, S.; Oehlhof, A. J. Mater. Chem. 2006, 16, 441. (31) Fischer, E. W. Pure Appl. Chem. 1972, 31, 113. (32) Yeh, G. S. Y.; Hosemann, R.; Loboda-Č ačković, J.; Č ačković, H. Polymer 1976, 17, 309. (33) Möller, M.; Wendorff, J. H.; Werth, M.; Spiess, H. W. J. NonCryst. Solids 1994, 170, 295. (34) Fimmen, W.; Glüsen, B.; Kettner, A.; Wittenberg, M.; Wendorff, J. H. Liq. Cryst. 1997, 23, 569. (35) Glüsen, B.; Kettner, A.; Kopitzke, J.; Wendorff, J. H. J. NonCryst. Solids 1998, 241, 113. (36) Gearba, R. I.; Lehmann, M.; Levin, J.; Ivanov, D. A.; Koch, M. H. J.; Barberá, J.; Debije, M. G.; Piris, J.; Geerts, Y. H. Adv. Mater. 2003, 15, 1614.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Research Foundation grants funded by the Ministry of Education, Science and Technology of the Korean Government (2014R1A2A1A05007109 and 2011-0031931). We acknowledge the Pohang Accelerator Laboratory for providing beamline 4C2 used in this work. E


Negative and Positive Anisotropic Thermal Expansions in a

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