Motional-Narrowing of the Electron Spin Resonance Absorption in the

Subscriber access provided by UNIV OF LOUISIANA

C: Physical Processes in Nanomaterials and Nanostructures

Motional-Narrowing of the Electron Spin Resonance Absorption in the Plastic-Crystal Phase of [(CH)N]FeCl 3

4

4

Jorge Salgado-Beceiro, Socorro Castro-García, Manuel Sánchez-Andujar, and Francisco Rivadulla J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b09367 • Publication Date (Web): 09 Nov 2018 Downloaded from http://pubs.acs.org on November 13, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 17 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Motional-Narrowing of the Electron Spin Resonance Absorption in the Plastic-Crystal Phase of [(CH3)4N]FeCl4 Jorge Salgado-Beceiro,† Socorro Castro-Garc´ıa,† Manuel Sánchez-Andújar,∗,† and Francisco Rivadulla∗,‡ †QuiMolMat group, Department of Chemistry, Faculty of Science and CICA, University of A Coru˜ na, Campus A Coru˜ na, 15071 A Coru˜ na, Spain ‡Centro Singular de Investigación en Qu´ımica Biolóxica e Materiais Moleculares (CiQUS), Departamento de Qu´ımica-F´ısica, Universidade de Santiago de Compostela. 17582 Santiago de Compostela (Spain) E-mail: [email protected]; [email protected]

Abstract We report the temperature dependence of the Electron Spin Resonance (ESR) absorption and magnetic susceptibility of plastic crystal [(CH3 )4 N]FeCl4 . We demonstrate the extreme sensitivity of ESR to the static and dynamic structural transitions characteristic of this material. We observed a narrowing of the ESR line in the high temperature plastic-crystal phase, which is due to fast rotation (τ ≈10−9 s) of the [FeCl4 ]− anion. This is a rare example of the effect of fast motional narrowing in crystalline solids, in which ions occupy fixed positions at the crystal lattice.

1

ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Keywords plastic crystals, electron spin resonance, motional narrowing

INTRODUCTION During the last decade, organic-inorganic hybrid materials have attracted increasing attention due to their high potential to obtain novel functionalities linked to unusual crystal structures. 1–5 Hybrid systems include a wide family of materials, such as metal-organic frameworks (MOFs), 6 hybrid inorganic-organic perovskites (HIOP), 7–9 and plastic crystals (PC). 10–12 A plastic crystal is a crystalline compound that possess some orientational or conformational degree of freedom. 13,14 In the PC phase, the center of mass of the molecules or ionic species are fixed on a crystalline lattice; however, they are dynamically disordered with respect to their orientational degrees of freedom. Most plastic crystals are formed by molecules or ionic species of more or less globular shape, providing little steric hindrance for reorientational processes. Typical examples are simple organic molecules (such as solid ethanol, 15 adamantane, 16 cyclo-hexanol, 17 etc), inorganic salts (such as NaCN, 18 KNO2 , 19 etc), or more complex salts which combine organic cations (such as imidazolium, pyrrolidinium and tetraalkylammonium cations) 20 and inorganic anions, such as halometalates. Hybrid PC are an emerging family of materials, due to the richness of their properties, such as ferroelectricity, 11 different types of magnetic ordering, 21,22 or high ionic conductivity. 23 Cooling a PC freezes the rotational motion, resulting in a low temperature phase in which the ionic species are completely ordered (crystalline), or keep some quasistatic random orientation. This partial disorder reduces the translational symmetry and makes a complete structural characterization of the low temperature phases of the PC challenging. Due to the relevance of these materials, numerous experimental methods have been employed to monitor the degree of static disorder and the rotational dynamics in PC. For exam2


Page 2 of 17

Page 3 of 17 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ple, evidence for rotational motion was obtained from calorimetric studies, X-ray diffraction, infrared and Raman spectroscopy, neutron and X-ray scattering, dielectric studies and nuclear magnetic resonance (NMR). 14,15,18–25 The presence of transition-metal ions in many hybrid PC, make many of them perfectly suitable for Electron Spin Resonance (ESR) studies. Due to the large electron magnetic moment, ESR is much more sensitive than NMR, and may provide accurate information about tiny changes in the local symmetry of these complex materials. 26,27 Moreover, because of the frequencies used in a ESR experiment (GHz) compared to NMR (MHz), the former is more suitable for the study of faster dynamics (≈10−9 s) than the latter (≈10−6 s). However, in spite of these obvious advantages, ESR has not been routinely applied to the study of the dynamics of hybrid PC. Here we report the analysis of the temperature dependence of the ESR absorption of polycrystalline tetramethylammonium tetrachloroferrate (III), [(CH3 )4 N][FeCl4 ]. This is a hybrid PC, 28–31 recently reported to exhibit ferroelectricity and piezoelectricity. 32 As we show in this work, ESR is a very valuable tool for identifying the different structural distortions of these type of materials, as well as the dynamics of the cation/anion rotation characteristic of plastic crystals.

EXPERIMENTAL Polycristalline powder samples of [(CH3 )4 N]FeCl4 were synthesized from commercially available iron (III) chloride hexahydrate (98%, Aldrich), tetramethylammonium chloride (98%, Aldrich), hydrochloric acid (36.5%, Aldrich), and absolute ethanol (Panreac), following a method previously reported by Harada et al. 32 The obtained yellow polycrystalline product was filtered and recrystallized in ethanol, leading to small single crystals of [(CH3 )4 N]FeCl4 . Powder X-ray diffraction patterns were collected at room temperature with a D5000 XRD SIEMENS diffractometer, using CuKα radiation (λ = 1.5418 ˚ A). The as obtained

3



pattern was compared with the generated from the single crystal structure by the Mercury 3.8 software, 33 confirming the purity of the phase (see Figure S1 and S2 in the supporting information). Differential scanning calorimetry (DSC) measurements were performed in a TA-Instruments Q200. The sample, with a mass ≈5 mg, was heated and cooled with a rate of 10 K/min, from 150 K up to 420 K, under nitrogen atmosphere. Magnetic properties were measured in a SQUID magnetometer. Electron Spin Resonance (ESR) experiments were performed on polycrystalline powders, between 100 K and 450 K, in a Bruker EMX spectrometer operating at 9.4 GHz (X-band), equipped with an ER 4102ST resonant cavity (TE102 mode). The linewidth, ∆H, was measured as the peak-topeak distance in the first derivative of the absorption line, ∆Hpp . For a Lorentzian line, √ ∆H= 3∆Hpp . 27 The fittings were done with EasySpin software package. 34

RESULTS AND DISCUSSION Differential Scanning Calorimetry (DSC) analysis shown in Figure 1 reveals a series of solidsolid phase transitions. Previous DSC and X-ray diffraction analysis by Harada et al. 32 identified these as structural transitions between closely related orthorhombic phases (from II to V in Figure 1), plus a cubic phase I, above ≈380 K. The crystal structures of III and IV are non-centrosymmetric, supporting a ferroelectric (FE) distortion. 32 However, their observation is highly dependent on the thermal history of the sample: after repeated thermal cycling only a single intermediate phase from V and I can be defined. This suggests that these transformations actually reflect the interconversion towards a more stable orthorhombic phase between V and I. The other two transitions, between phase V to IV, and II to I remain robust to repeated thermal cycling, and shown an appreciable thermal hysteresis, reflecting their structural nature. The high temperature cubic phase I corresponds to the plastic crystal structure. In this case, the [FeCl4 ]− anions and [(CH3 )4 N]+ cations undergo an isotropic rotational motion, giving rise to a locally disordered molecular structure, which

4


Page 4 of 17

Page 5 of 17

still retains the long-range translational symmetry.

0.3

Heat Flow (mW)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.2 0.1 0.0

VI

V IV

III

II

I

-0.1 -0.2 -0.3 200

250

300

350

400

Temperature (K) Figure 1: Heating and cooling DSC curves of [(CH3 )4 N]FeCl4 . The Roman numerals and the different colors identify and delimit the extension of the different phases reported in Reference [ 32 ], and in this work; see text.

The temperature dependence of the magnetization of [(CH3 )4 N]FeCl4 is shown in Figure 2. The dc susceptibiliy fits perfectly to a Curie-Weiss law:

M C ≈χ= H T − ΘCW 2 2 N µB g C= S(S + 1) 3kB 2zJS(S + 1) ΘCW = 3kB

(1)

In these equations C, kB , µB , ΘCW , J and z are the Curie Constant, the Boltzmann constant, the Bohr magneton, the Curie-Weiss temperature, the magnetic exchange energy, and the number of nearest neighbors exchange-coupled to the magnetic species, respectively. Apart from a small deviation from linearity between ≈236-310 K, the inverse magnetic susceptibility follows equation (1), with ΘCW ≈0 K, and C=0.21 emu K mol−1 Oe−1 , in the whole temperature range. The magnetic moment µ ≈4.7 (1)µB , is ≈ 20% smaller than ≈5.9 5



µB expected for high-spin Fe3+ (d5 : S=5/2). Therefore, the system behaves as an ensemble of independent magnetic moments (J=0); the different structural phases observed in [(CH3 )4 N]FeCl4 do not produce any appreciable variation in the Fe3+ -Fe3+ exchange interaction J, which remains negligible in the whole temperature range. On the other hand, the phase transitions reported by DSC can be also identified from small variations in the magnetic moment (Figure 2c), and from the temperature derivative of the dc magnetic susceptibility (Figure 2d). 1.0

100

H = 100 Oe

0.8 0.6 0.4 0.2 0.0 0

100

200

300

b) -1 (emu-1 mol Oe)

 (emu mol-1 Oe-1)

a)

60 40 20 0 0

400

  4.7(1) B/Fe3+

80

100

Temperature (K)

c) VII

VI

d-1/dT (emu-1 mol Oe K-1)

4.80

V IV III II I

4.75 4.70 4.65 4.60 100

200

300

200

300

400

Temperature (K)

4.85

B/Fe3+

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 17

400

0.3

d)

0.2

VII

V

IV III II

I

0.0 100

Temperature (K)

VI

0.1

200

300

400

Temperature (K)

Figure 2: Temperature dependence of the dc susceptibility (a) and of its inverse (b), measured at H= 100 Oe. c) Temperature dependence of the magnetic moment obtained from the the Curie constant; see equation (1) . d) Derivative of the inverse susceptibility curve shown in b). The Roman numerals and the different colors in c) and d), identify and delimit the extension of the different phases observed by DSC, magnetization, and ESR; see text.

Additionally to previous reports, two new phase transitions are observed in Figure 2c) and d), at ≈ 190 K and ≈ 240 K, respectively. These define phase VI (190 K < T < 240 K) and phase VII (T < 190 K). Their nature was clarified by the temperature dependence of different parameters of the ESR absorption, as discussed below. Figure 3 shows the temperature dependence of the ESR linewidth, ∆Hpp . All the ESR

6


Page 7 of 17 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


lines measured at different temperatures for this work are shown in the supporting information, Figure S3 and S4. This behavior can be summarized in: i) a temperature independent ∆Hpp ≈175(5) G characteristic of phase VII and II-IV; ii) a rapidly changing and wider ESR line, with hysteretic behavior in phase V-VI; iii) a temperature-dependent narrowing at high temperature, above the transition to the plastic-crystal phase I. A simple structure like that of [(CH3 )4 N]FeCl4 can be approximated to a cubic lattice with a volume ≈ 303(8)˚ A, 32 and a density of independent spins ρ≈ 3.30(1)×1021 cm−3 . In this case, given the negligible J, dipolar interactions should be the main responsible of broadening the ESR absorption line, producing a temperature independent linewidth: 35,36

∆H =

√ 5.1(gµB ρ)[S(S + 1)]1/2

(2)

Taking into account the 20% of vacancies in the spin density determined from the dc magnetization, and g ≈2.035 (Figure S4), this equation predicts a homogeneous dipolar broadening of ≈335(10) G, which for a Lorentzian line 27 ∆Hpp =3−1/2 ∆H≈194(5) G, only ≈ 10% larger than the experimental value in phase VII, IV-II (Figure 3). In the intermediate temperature range (phase VI-IV) ∆Hpp changes rapidly with temperature, and shows an appreciable hysteresis, consistent with DSC. Several structural transformations were previously reported in this region, some of the phases supporting a FE distortion. 32 These are produced by cooperative lattice distortions, which are reflected in tiny but perfectly appreciable reductions of the magnetic moment (Figure 2c). Local variations of the ligand-Fe3+ distance result in random variations of the local field, and therefore act as a source of inhomogeneous broadening of the ESR lines. 27 At high temperature, in centrosymmetric phase II and I the magnetic moment recovers a constant value, characteristic of dipolar interaction. But surprisingly, the ESR linewidth decreases continuously with temperature in phase I. Activation of electron-phonon relaxation cannot be responsible of this behavior, as it should result in an increase of ∆Hpp (T) at high temperature. Moreover, the ESR susceptibility agrees fairly well the value of the dc 7



susceptibility at high temperature, showing that all spins contribute to the resonance in this phase. Therefore, no variation in the spin density can be either responsible of this narrowing. Other sources of narrowing are magnetic exchange and motional narrowing. Exchange narrowing can be also discarded in this case, as J=0 in the whole temperature range, and µ ≈constant at high temperature (Figure 2c). Therefore, motional narrowing is the most probable source of ESR line narrowing observed in phase I. Perturbations (homogeneous and inhomogeneous) blur the otherwise sharply defined spin levels splitted by a magnetic field. However, if spins move (precessing, rotating, diffusing, etc) at a rate (ωr ) fast enough compared to the amplitude of the perturbations (ωp ), this movement averages out the effect of local-field inhomogeneities. In other words, if spins move fast, the time-average of the magnetic field sensed by them show less variation than the site-to-site magnetic field, and the net effect of movement is to narrow the ESR resonance line.

350 300

Hpp (G)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 17

250 200 150 VII

100

VI

V

200

IV III II I

300

400

Temperature (K) Figure 3: Temperature dependence of the ESR peak-to-peak linewidth, during heating (closed squares) and cooling runs (open squares). The error bars take into account the uncertainty in the field-sweep, and the variability in the linewidth from multiple measurements with different samples.

In the present case, dipolar broadening constitute the main contribution to ωp (∆Hdip ≈194(5) G), an the resulting linewidth can be calculated from: 26,36 8


Page 9 of 17

450 K 300 K Int. (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


240 K 190 K 100 K

2

3

4

5

H (kG)

Figure 4: First derivative of the experimental ESR absorption lines (open symbols) at different temperatures. The continuous red lines represent the fittings to Lorentzian functions. The fittings below ≈240 K (blue lines) takes into account the contribution of two different spin systems, one of them (20-30%) giving a pure Lorentzian function, and another one giving a convolution of Lorentzian:Gaussian (≈1:2.5) signal; see text.

2 ωdip ωr 2 ∆Hdip ∆H = ∆Hr

∆ω =

(3)

expressed in frequency or magnetic field units. Given that J=0 in this case, ∆Hr contains the contribution from the motional terms which narrow the ESR line. From this equation, the order of magnitude of the rotational time of [FeCl4 ]− can be estimated: τ ≈ (γ∆Hr )−1 ≈10−9 s (see also Figure S5 in the supporting information). Therefore, thermal activation of the fast rotation of [FeCl4 ]− anion, characteristic of the plastic-crystal phase, is responsible of the ESR line narrowing observed in phase I. It is also important to realize that dipolar broadening between like spins in solids produce characteristic Gaussian shapes of the absorption lines. However, in the case of fast rotational movement, the fourth moment of the absorption spectral line increases relative to the square of its second moment, producing a characteristic Lorentzian shape. 27,35,37 Fitting the ESR lines in lower temperature phase VII, requires the inclusion of two populations of spins, one 9



Page 10 of 17

showing a Lorentzian line, and the other one a convolution of a Gaussian plus Lorentzian functions (Figure 4). This reflects the presence of different molecular clusters (or local anisotropy), contributing to the inhomogeneous broadening of the low temperature spectra. Unfortunately, we cannot draw more concrete conclusions from our polycrystalline samples. A deeper analysis of the origin of the inhomogeneous broadening observed at low temperature would require the measurement and fitting of single crystals. On the other hand, above ≈240 K the lines show a pure Lorentzian shape, suggesting overall reorientation and orientation disorder. VII

[(CH3)4N]+: ordered [FeCl4]-: ordered

I

VI - II

[(CH3)4N]+: disordered [FeCl4]-: ordered

[(CH3)4N]+: disordered [FeCl4]-: disordered

TEMPERATURE Figure 5: Evolution of the crystal structure of [(CH3 )4 N][FeCl4 ] at different temperatures. To better visualize the degree of disorder, only one cation and anion are shown. The crystal structures have been reported in reference [ 32 ] (see also the supporting information).

Activation of [(CH3 )4 N]+ cation rotation above 200 K is also reflected in the small narrowing that can be clearly observed in the heating run of Figure 3) between 200 K and 225 K. The sudden increase of ∆Hpp above this temperature due to inhomogeneous broadening precludes the accurate determination of the rotational time of [(CH3 )4 N]+ in this case. Figure 5 summarizes the evolution of the crystal structure at different temperatures, where there is a gradual increase in disorder of the ions upon heating. The mechanism of rotational/diffusional narrowing is usual in liquids and gels/polymers, were atomic diffusion is a certain possibility. However, it is unusual in crystalline solids, were translational symmetry requires the spins to remain at well-defined positions. An example 10


Page 11 of 17 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


of this behavior occurs for instance in magnetically diluted salts of 3d ions in intermediate ligand fields, and at high temperatures. 26 As we demonstrated in this paper, in the case of plastic crystals the fast-rotational movement around the center of mass of the cations and anions averages out the local inhomogeneities of the magnetic field. The fact that different plastic crystals can be designed, with different spins (even including more than one type of spins in the same crystal) and different spin-spin distances (even making the exchange interaction significant), opens enormous possibilities to study the effect of motional narrowing in a more controllable environment, in which theoretical calculations are easier to implement and interpret.

CONCLUSIONS In summary, we have demonstrated the extreme sensitivity of ESR to the different structural transitions, both static and dynamic, characteristic of [(CH3 )4 N]FeCl4 . Fast rotation of the [(CH3 )4 N]+ cation and [FeCl4 ]− anion result in a narrowing of the ESR line, which becomes Lorentzian in shape. The strong narrowing observed in the plastic-crystal phase of this compound is a nice example of the effect of fast motional narrowing in a paramagnetic solid (J=0), in which ions normally occupy fixed positions at the crystal lattice. The high sensitivity of ESR to dynamic phenomena in the ≈10−9 s range, makes this technique ideal for the study of fast rotational movement characteristic of hybrid plastic crystals.

Acknowledgement This work has received financial support from Ministerio de Econom´ıa y Competitividad (Spain) under project No. MAT2016-80762-R, MAT2017-86453-R, Xunta de Galicia (Centro singular de investigación de Galicia accreditation 2016-2019, ED431G/09) and the European Union (European Regional Development Fund-ERDF).

11



Supporting Information Available The following files are available free of charge. Supporting Information. Structural details and the ESR lines recorded at different temperatures are shown in the supporting information. The temperature dependent rotational time in the plastic crystal phase, τ , obtained from the ESR fittings, is also included.

References (1) Cheetham, A. K.; Rao, C. N. R.; Feller, R. K. Structural diversity and chemical trends in hybrid inorganic–organic framework materials. Chem. Commun. 2006, 4780–4795. (2) Cheetham, A. K.; Rao, C. N. R. Materials science. There’s room in the middle. Science (New York, N.Y.) 2007, 318, 58–9. (3) Mitzi, D. B. Synthesis, Structure, and Properties of Organic-Inorganic Perovskites and Related Materials; Wiley-Blackwell, 2007; pp 1–121. (4) Rao, C. N. R.; Cheetham, A. K.; Thirumurugan, A. Hybrid inorganic–organic materials: a new family in condensed matter physics. Journal of Physics: Condensed Matter 2008, 20, 083202. (5) Mercier, N.; Louvain, N.; Bi, W. Structural diversity and retro-crystal engineering analysis of iodometalate hybrids. CrystEngComm 2009, 11, 720. (6) Furukawa, H.; Cordova, K. E.; O’Keeffe, M.; Yaghi, O. M. The chemistry and applications of metal-organic frameworks. Science (New York, N.Y.) 2013, 341, 1230444. (7) Saparov, B.; Mitzi, D. B. Organic–Inorganic perovskites: structural versatility for functional materials design. Chemical Reviews 2016, 116, 4558–4596.

12


Page 12 of 17

Page 13 of 17 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(8) Li, W.; Wang, Z.; Deschler, F.; Gao, S.; Friend, R. H.; Cheetham, A. K. Chemically diverse and multifunctional hybrid organic–inorganic perovskites. Nature Reviews Materials 2017, 2, 16099. (9) Berm´ udez-Garc´ıa, J. M.; Sánchez-And´ ujar, M.; Se˜ nar´ıs-Rodr´ıguez, M. A. A new playground for organic–inorganic hybrids: barocaloric materials for pressure-induced solidstate cooling. The Journal of Physical Chemistry Letters 2017, 8, 4419–4423. (10) Butler, K. T.; Walsh, A.; Cheetham, A. K.; Kieslich, G. Organised chaos: entropy in hybrid inorganic–organic systems and other materials. Chem. Sci. 2016, 7, 6316–6324. (11) Harada, J.; Shimojo, T.; Oyamaguchi, H.; Hasegawa, H.; Takahashi, Y.; Satomi, K.; Suzuki, Y.; Kawamata, J.; Inabe, T. Directionally tunable and mechanically deformable ferroelectric crystals from rotating polar globular ionic molecules. Nature Chemistry 2016, 8, 946–952. (12) Sun, Y. Z.; Huang, B.; Xu, W. J.; Zhou, D. D.; Chen, S. L.; Zhang, S. Y.; Du, Z. Y.; Xie, Y. R.; He, C. T.; Zhang, W. X.; Chen, X. M. Plastic crystals with polar halochromate anion: thermosensitive dielectrics based upon plastic transition and dipole rotation. Inorganic Chemistry 2016, 55, 11418–11425. (13) Timmermans, J. Plastic crystals: A historical review. Journal of Physics and Chemistry of Solids 1961, 18, 1–8. (14) Chandra, D.; Ding, W.; Lynch, R. A.; Tomilinson, J. J. Phase transitions in ”plastic crystals”. Journal of The Less-Common Metals 1991, 168, 159–167. (15) Srinivasan, A.; Bermejo, F. J.; de Andrés, A.; Dawidowski, J.; Z´ un ˜iga, J.; Criado, A. Evidence for a supercooled plastic-crystal phase in solid ethanol. Physical Review B 1996, 53, 8172–8175.

13



(16) Chang, S.-S.; Westrum, E. F. Heat capacities and thermodynamic properties of globular molecules. I. Adamantane and hexamethylenetetramine. The Journal of Physical Chemistry 1960, 64, 1547–1551. (17) Adachi, K.; Suga, H.; Seki, S. Phase changes in crystalline and glassy-crystalline cyclohexanol. Bulletin of the Chemical Society of Japan 1968, 41, 1073–1087. (18) Verweel, . J.; Bijvoet, J. M. Die Kristallstruktur von NaCN. Zeitschrift f¨ ur Kristallographie - Crystalline Materials 1939, 100, 201–207. (19) Solbakk, J. K.; Strømme, K. O.; Rasmussen, S. E.; Smidsrød, O.; Lindberg, A. A.; Jansen, G.; Lamm, B.; Samuelsson, B. On the crystal structure of the trigonal (and cubic) phase of potassium nitrite. Acta Chemica Scandinavica 1969, 23, 300–313. (20) Pringle, J. M.; Howlett, P. C.; MacFarlane, D. R.; Forsyth, M. Organic ionic plastic crystals: recent advances. Journal of Materials Chemistry 2010, 20, 2056. (21) Coronado, E.; Day, P. Magnetic molecular conductors. Chemical Reviews 2004, 104, 5419–5448. (22) de Pedro, I.; Garc´ıa-Saiz, A.; González, J.; Ruiz de Larramendi, I.; Rojo, T.; Afonso, C. A. M.; Simeonov, S. P.; Waerenborgh, J. C.; Blanco, J. A.; Ramajo, B.; Fernández, J. R. Magnetic ionic plastic crystal: choline[FeCl4]. Physical Chemistry Chemical Physics 2013, 15, 12724. (23) Yoshizawa-Fujita, M.; Fujita, K.; Forsyth, M.; MacFarlane, D. R. A new class of protonconducting ionic plastic crystals based on organic cations and dihydrogen phosphate. Electrochemistry Communications 2007, 9, 1202–1205. (24) Smith, G. W. Plastic crystals, liquid crystals, and the melting phenomenon. The importance of order. Advances in Liquid Crystals 1975, 1, 189–266.

14


Page 14 of 17

Page 15 of 17 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(25) Brand, R.; Lunkenheimer, P.; Loidl, A. Relaxation dynamics in plastic crystals. Journal of Chemical Physics 2002, 116, 10386–10401. (26) Abragam, A.; Bleaney, B. Electron Paramagnetic Resonance of Transition Ions; 1970. (27) Poole, C. P. J. Electron Spin Resonance. A Comprehensive Treatise on Experimental Techniques; 1983. (28) Czapla, Z.; Czupi´ nski, O.; Galewski, Z.; Sobczyk, L.; Wa´skowska, A. Successive phase transitions in tetramethylammonium tetrachloroferrate (III). Solid State Communications 1985, 56, 741–742. (29) Ruiz-Larrea, I.;

López-Echarri, A.;

Tello, M. Phase transition sequence in

N(CH3)4FeCl4. Solid State Communications 1987, 64, 1099–1101. (30) Kosturek, B.; Podsiadla, D.; Czapla, Z. Optical investigations of phase transitions in N(CH 3 )4 FeCl4 . Ferroelectrics 1999, 223, 57–61. (31) Wyrzykowski, D.; Kruszynski, R.; Klak, J.; Mrozinski, J.; Warnke, Z. Structural and magnetic characteristics of tetramethylammonium tetrahalogenoferrates(III). Inorganica Chimica Acta 2008, 361, 262 – 268. (32) Harada, J.; Yoneyama, N.; Yokokura, S.; Takahashi, Y.; Miura, A.; Kitamura, N.; Inabe, T. Ferroelectricity and piezoelectricity in free-standing polycrystalline films of plastic crystals. Journal of the American Chemical Society 2018, 140, 346–354. (33) Macrae, C. F.; Bruno, I. J.; Chisholm, J. A.; Edgington, P. R.; McCabe, P.; Pidcock, E.; Rodriguez-Monge, L.; Taylor, R.; van de Streek, J.; Wood, P. A.; IUCr, Mercury CSD 2.0 – new features for the visualization and investigation of crystal structures. Journal of Applied Crystallography 2008, 41, 466–470. (34) Stoll, S.; Schweiger, A. EasySpin, a comprehensive software package for spectral simulation and analysis in EPR. Journal of Magnetic Resonance 2006, 178, 42 – 55. 15



(35) Van Vleck, J. H. The dipolar broadening of magnetic resonance lines in crystals. Phys. Rev. 1948, 74, 1168–1183. (36) Anderson, P. W.; Weiss, P. R. Exchange narrowing in paramagnetic resonance. Rev. Mod. Phys. 1953, 25, 269–276. (37) Kittel, C.; Abrahams, E. Dipolar broadening of magnetic resonance lines in magnetically diluted crystals. Phys. Rev. 1953, 90, 238–239.

16


Page 16 of 17

Page 17 of 17

Heat Flow (mW)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40

0.3


0.2 0.1 0.0

V IV

VI

III

II

I

-0.1 -0.2 -0.3 200

250

300


350

Temperature (K)

400

Motional-Narrowing of the Electron Spin Resonance Absorption in the

Recommend Documents