A Wide Magnetic Thermal Memory Effect (~55 K) Above Room

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A Wide Magnetic Thermal Memory Effect (~55 K) Above Room Temperature Coupled to a Structure Phase Transition of Lattice Symmetry Reduction in High-Temperature Phase in an S = ½ Spin Chain Molecule Crystal Xuan-Rong Chen, Shao-Xian Liu, Qiu Ren, Zheng-Fang Tian, Xing-Cai Huang, Lifeng Wang, and Xiaoming Ren J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.8b10492 • Publication Date (Web): 05 Dec 2018 Downloaded from http://pubs.acs.org on December 5, 2018

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The Journal of Physical Chemistry

A Wide Magnetic Thermal Memory Effect (~55 K) Above Room Temperature Coupled to a Structure Phase Transition of Lattice Symmetry Reduction in High-Temperature Phase in an S = ½ Spin Chain Molecule Crystal Xuan-Rong Chen,†, Shao-Xian Liu,† Qiu Ren,† Zheng-Fang Tian,‡ Xing-Cai Huang,  Lifeng Wang§ and Xiao-Ming Ren*† †

State Key Laboratory of Materials-Oriented Chemical Engineering and College of Chemistry &

Molecular Engineering, Nanjing Tech University, Nanjing 210009, P. R. China 

School of Chemistry & Environmental Engineering and Instrumental Analysis Center,

Yancheng Teachers University, Yancheng, 224051, P. R. China ‡

Hubei Key Laboratory for Processing and Application of Catalytic Materials, Huanggang

Normal University, Huanggang 438000, P. R. China §

Institute for Frontier Materials (IFM), Deakin University, 75 Pigdons Road, Waurn Ponds,

Victoria 3216, Australia

ACS Paragon Plus Environment

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ABSTRACT: One-dimensional (1D) S = ½ Heisenberg antiferromagnetic (AFM) chain system shows frequently a spin-Peierls-type transition owing to strong spin-lattice coupling. From hightemperature phase (HTP) to low-temperature phase (LTP), the spin chain distortion leads to the reduction of lattice symmetry in LTP, so-called symmetry breaking (SB) phase transition. Herein, we report the first example that a 1D S = ½ AFM molecular crystal, [Et3(nPr)N][Ni(dmit)2] (Et3(n-Pr)N+ = triethylpropylammonium, dmit2- = 2-thioxo-1, 3-dithiole-4,5dithiolate), shows a structural phase transition with lattice symmetry increase in LTP, which is contrary to the SB phase transition. Particularly, the structure phase transition leads to magnetically bistable state with TC  375 K, TC  320 K and a surprisingly large thermal hysteresis (~55 K). Additionally, LTP and HTP coexist in a temperature region near TC but not at TC in this 1D spin system. The large hysteresis is related to the huge deformation of anion stack, which needs high activation energy for the structure transformation and magnetic transition between LTP and HTP. This study would not only provide new insight into understanding the relationship of spin-Peierls-type transition and structure phase transition, but also will offer a roadmap for searching molecular scale magnetic bistable materials, which are in large demand in future electronic, magnetic and photonic technologies.


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Introduction Bistable molecular functional materials are in large demand in electronic, magnetic and photonic technologies.1-7 Among them, the thermally induced magnetic-bistable material is an important subclass, normally including transition metal spin-crossover (SCO) complexes,8-15 cyanide-bridged heterometallic valence tautomeric complexes 16-19 and 1D S = ½ spin-Peierlstype compounds etc.20,21 In the nickel-bis-1, 2-dithiolene monoanion complex, the central nickel ion is surrounded by four S atoms and -C=C- units to form planar and electronically delocalized architecture. With face-to-face stacking of this architecture,22-24, it forms a 1D S = ½ spin chain with strong spinlattice coupling property.2320,21,23 Therefore, the nickel-bis-1,2-dithiolene monoanion complex is one of excellent building blocks for constructing spin-Peierls-type materials. Recently, spinPeierls-type transition has been reported in a series of 1D nickel-bis-maleonitriledithiolate monoanion compounds.21,23 In these complexes, the magnetic transition is always associated with the SB structural transformation, and below the transition temperature, the uniform spin chain is dimerized and the lattice symmetry is also lower in LTP than that in HTP. In contrary to the SB structural transition, the inverse symmetry breaking (ISB) structural transformation shows lower lattice symmetry in HTP but higher lattice symmetry in LTP. The ISB structure transformation which undergoes at specific temperature, according to quantum field theory at finite temperature, was first noted by Weinberg.25 He predicted that the symmetry breaking at zero Kelvin in a crystal may not be restored at all at high temperature. Hence, such a phenomenon is also called symmetry non-restoration.26 ISB structural transformation has been indeed observed in some materials, e.g., Rochelle salt (KNaC4H4O64H2O),27 which goes, as the temperature increases, from a more symmetric orthorhombic crystalline structure to a less


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symmetric monoclinic structure at T = 255 K, liquid crystals,28 spin glass materials29 and (Pr, Ca, Sr)MnO3.30 To the best of our knowledge, however, it has never hitherto been reported in 1D spin system. Herein, we present an example of a nickel-bis-dithiolene salt, [Et3(n-Pr)N][Ni(dmit)2] (1), exhibiting 1D magnetic chain character in the crystal structure and a structural phase transition with lower lattice symmetry in HTP but not in LTP. Phenomenologically, the structural phase transition is similar to the ISB structural transformation. Moreover, such a structural phase transition in 1 leads to magnetic bistability with a surprisingly large thermal hysteresis of ~55 K above room temperature. Materials and methods Chemicals and Materials. All chemicals and reagents were purchased from commercial sources and used without further purification. 4, 5-di(thiobenzoyl)-1,3-dithiole-2-thione31 and [Et3(n-Pr)N]Br32,33 were prepared according to the published procedure. Preparation of [Et3(n-Pr)N][Ni(dmit)2] (1). 4, 5-di(thiobenzoyl)-1, 3-dithiole-2-thione (812 mg, 2 mmol) suspended in methanol (10 mL) was mixed with a methanol solution (10 mL) containing 184 mg (8 mmol) of metal sodium under nitrogen atmosphere at ambient temperature. The mixture was stirred for 30 minutes to give a wine red solution. NiCl2·6H2O (238 mg, 1 mmol) and (Et3PrN)Br (210 mg, 1 mmol) in methanol (20 mL) were added to the wine red solution with vigorously stirring, and the precipitation was immediately formed. The solution containing I2 (127 mg, 0.5 mmol) and NaI (150 mg, 1 mmol) in methanol (20 mL) was then added to the above mixture with strong stirring for 120 minutes. The microcrystalline product was collected by filtration, washed with methanol and dried in vacuum at 50 C. Yield: ~70%


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calculated according to the reactant 4, 5-di(thiobenzoyl)-1,3-dithiole-2-thione. Anal. Calc. for C15H22NNiS10 (1): C, 30.24; H, 3.72; N, 2.35%. Found: C, 30.21; H, 3.78; N, 2.28%. IR (KBr pellet, cm-1) and the assignments for the listed bands: 34 2966(m), 2872(s) attributed to the C-H of the alkyl chains; 1351(s) arose from the C=C of the dmit2- ligands; 1057(s) attributed to the C=S of the dmit2- ligands; 507(m), 503(m) for S-C-S of the dmit2- ligands. The dark green crystals of 1 were obtained by slow evaporation of the solution of 1 in acetone at ambient temperature over 7-10 days, and the crystals are suitable for X-ray single crystal diffraction analyses. The phase purity of crystals of 1 was inspected by means of powder X-ray diffraction technique. Physical measurements. Elemental analyses for C, H and N were performed with an Elementar Vario EL III analytic instrument. Powder X-ray diffraction (PXRD) data were collected on a Bruker D8 diffractometer operating at 40 kV and 40 mA using Cu Kα radiation with λ = 1.5418 Å at room temperature. The 2 angles span from 5 to 50° with 0.01° step−1.The variable temperature PXRD measurements were performed using a Shimadzu XRD-6100 diffractometer operating with a Cu-Kα radiation source (λ = 1.5418 Å) in the temperature range of 303 to 423 K (30-150 C). During measurements of the temperature-dependent PXRD data, the temperature changing rate is 10 K min−1. It is worth mentioning that PXRD measurement at a certain temperature starts after the sample is kept at the set temperature for 15 min to ensure that the sample and probe have the same temperature. Other parameter settings are the same as those in the PXRD measurement at room temperature. FT-IR spectroscopy were recorded on a Bruker FT-IR Vertex 80 (4000-400 cm-1) spectrophotometer with KBr pellets. Differential scanning calorimetry (DSC) was carried out on


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PerkinElmer DSC 8500 calorimeter for powdered samples in the range of 98-413 K (between 175 and 140 C) and the warming/cooling rate is 10 K min-1 during the thermal cycles. Magnetic susceptibility data were measured for polycrystalline samples on a Quantum Design MPMS-5 superconducting quantum interference device (SQUID) magnetometer, such a measurement was performed over the temperature range of 1.8-400 K in both cooling and warming modes, and the applied magnetic field is 10000 Oe. The diamagnetism was not subtracted from the experimental magnetic susceptibility. X-ray crystallography. Single-crystal X-ray diffraction data were collected for 1 at 293, 330, 340, 350, 350, 293, 100 K using the graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å) on a CCD area detector (Bruker SMART Apex II). Data reductions and absorption corrections were carried out with SAINT and SADABS software packages,35 respectively. Structures were solved by the direct method and refined by the full-matrix least-squares procedure on F2 using SHELXL-97 program.36 The non-Hydrogen atoms were anisotropically refined using the fullmatrix least-square method on F2. All hydrogen atoms were placed in calculated positions and refined as riding on the parent atoms. The crystallographic details about data collection and structural refinement at different temperatures are summarized in Table S1 and S2. Results and discussion Magnetic property. Temperature dependent m in both cooling and warming modes in the range of 1.8-400 K are displayed in Figure 1a for 1, where m is the molar magnetic susceptibility of one S = ½ [Ni(dmit)2] anion per formula unit of 1. Surprisingly, as shown in the plot of m vs. T, a striking magnetic phase transition appears above room temperature with an extremely large thermal hysteresis loop. Upon cooling, the m value drops abruptly at ~325 K,


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indicating that a magnetic transition undergoes. In the heating run, the magnetic susceptibility jumps sharply at ~375 K, and the thermal history dependent magnetic susceptibility is observed in the range of 300-390 K in 1, and this magnetic behavior demonstrates that 1 shows magnetic thermal memory effect, and the hysteresis loop in 1 is ~55 K calculated from the peak temperatures in the d(m)/dT vs. T plot (Figure 1b). Usually, it is common that the huge thermal hysteresis loop is observed in the transition metal SCO complexes,10,14 the cyanide-bridged heterometallic valence tautomeric complexes,16,17 and organic radical compounds.4-7 It is rare to observe the huge thermal hysteresis loop in the metal-bis-dithiolene complexes. To the best of our knowledge, only a few metal-bis-dithiolene complexes have hitherto been reported to display a more than 30 K thermos-hysteretic loop, including ~40 K in [Cp2Co][Ni(tfadt)2],20 ~49 K in [4’-CF3bzPy][Ni(mnt)2],21 ~37 and ~60 K in two [NO2bzql][Ni(dmit)2] polymorphs.37

(a) (b)

Figure 1. (a) Plot of m vs. T for 1, where the red and navy solid squares represent m upon cooling and heating (the inset shows the enlarged thermal hysteresis loop), respectively. (b) Plot of d(m)/dT vs. T in the warming (red line) and cooling mode (navy line) for 1. The critical temperature of the transition TC and the thermo-hysteresis loop (TC = TC↑−TC↓) are technologically important parameters, especially, TC near room temperature with TC  50 K is desirable for magnetically bistable material in practical applications.4 The thermo-hysteresis


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loop in a magnetically bistable system is dependent on the energy barrier of the transition state and two magnetic states, which are relevant to the magnetic transition, e.g., the high- and lowspin states in a transition metal SCO complex, the different valence tautomeric states in a cyanide-bridged heterometallic valence tautomeric complex as well as the gapless and dimerized states in a spin-Peierls-type compound. In a SCO/valence tautomeric complex, the coordination bond lengths (M-X) of metal center (M) to coordination atom (X) are distinctive in the high- and low-spin states/different valence tautomeric states. As a result, the SCO/valence tautomeric transformation process undergoes synergically with the change of bond lengths of M-X. In a spin-Peierls-type transition system, the magnetic transition is associated with the distortion or deformation of spin chain. Thus, the transition states are closely related to the local lattice deformation in the SCO/valence tautomeric transformation and the spin-Peierls-type transition process. The increasement of the local lattice deformation energy in the conversion process of magnetic bistable states should improve the thermal hysteresis loop. The crystal structure analysis reveals that the [Ni(dmit)2] anions form irregular stack in both HTP and LTP (see next section of crystal structure). Consequently, combining the crystal structure analysis and magnetic susceptibility character, an S = ½ AFM Heisenberg linear alternating chain model with J and J was chosen to analyze the magnetic behavior of 1 in both phases, where J and J represent the exchange constants within a spin chain between a spin and its left neighbor, and between a spin and its right neighbor (J < 0 and 0    1),38,

39

respectively. The experimental molar magnetic susceptibility m in both phases is represented with Eq. (1) when the paramagnetic impurity, the diamagnetism of atoms core and the possible van Vleck-


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type temperature-independent paramagnetism are further considered. The paramagnetic impurity originates from the uncoupling lattice defects, and the van Vleck-type temperature-independent paramagnetism arises from the coupling of the ground and excited states through a magnetic field.21 C

(1)

𝜒𝑚 = 𝜒𝑐ℎ𝑎𝑖𝑛 + T + 𝜒0 and 𝜒𝑐ℎ𝑎𝑖𝑛 =

𝑁𝑔2𝜇2𝐵 𝑘𝐵𝑇

𝐴 + 𝐵𝑥 + 𝐶𝑥2

∙ 1 + 𝐷𝑥 + 𝐸𝑥2 + 𝐹𝑥3

(2)

In Eq. (1), the chain represents the magnetic susceptibility contributed from spin chains, the C/T is the term of paramagnetic impurity and the term of 0 includes all temperature independent magnetic susceptibility (diamagnetism d and temperature independent paramagnetism TIP). In Eq. (2), x = J/kBT, J  0 and the parameters A-F are constants. Noticeably, two different sets of parameters A-F correspond to the cases of 0    0.4 and 0.4 <   1, respectively.38, 39 The fit was firstly performed for the m data in the range of 2-278 K in LTP using Eq. (1) and Eq. (2) (Figure S3), and two different sets of A-F parameters were tried in Eq. (1), respectively. The  value obtained via fitting the magnetic susceptibility data is still less than 0.4 even if the A-F parameters, corresponding to 0.4 <   1, are chosen (Figure S3b). The best fit was achieved using the A-F parameters, corresponding to 0    0.4, to give the parameters α = 0.074(5), J/kB = 266(2) K, g = 2.07(1), C = 2.92(3)10-3 emu K mol-1, and 0 = -3.8(3)10-4 emu mol-1. The 0 is comparable to the value of diamagnetic susceptibility calculated from the Pascal’ constants in this 1D spin chain compound, meaning that the temperature independent magnetic susceptibility in 1 arises mainly from the diamagnetism of atoms core. On the basis of the Curie constant, the molar fraction of uncoupled magnetic impurity in the crystal is estimated as ~0.78%. The small alternating parameter α indicates that 1 can be described as a singlet-triplet magnetic system.


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However, we failed to get the reasonable parameters when we fitted the magnetic susceptibility data in LTP using the singlet-triplet magnetic exchange model. In HTP, there are two types of irregular stacks of anions (see the crystal structure section). The different anion stack should show differently magnetic behaviors. Structurally, the Ni2 anion stack is analogous to the anion stack in LTP, and we suppose this type of anion stack displaying the magnetic behavior of an S = ½ AFM Heisenberg linear alternating chain, while the Ni1 anion stack showing singlet-triplet magnetic feature owing to it being strongly dimerized. Unfortunately, we tried to fit the magnetic susceptibility data in HTP using the S = ½ AFM Heisenberg linear alternating chain together with singlet-triplet model, but failed to obtained the reasonable parameters. For simplicity, two types of anion stacks are approximately considered to have the same parameters of J and α. Additionally, the magnetic impurity and the 0 term in HTP are thought as the same as those in LTP since they are not significantly affected by the phase transition. The best fit was made for temperature dependent m data in the range of 335395 K in HTP to give J/kB = 208(2) K and g = 2.11(1), respectively, and α = 0.032 (fixed). The magnetic data analysis revealed that the phase transition leads to both J and αJ increasing in LTP than those in HTP (Figure S3a). DSC measurements. As shown in Figure 2 and Figure S4b, DSC plot of 1 in the region of 100-413 K shows a pair of endothermic and exothermic peaks in a heating-cooling cycle. The corresponding endothermic and exothermic peaks coincide to each other well in two continuous thermal cycles, with peak temperature of TC↑ ≈ 376 K and TC↓ ≈ 314 K, respectively. Both TC↑ and TC↓ are close to the TC values obtained from the magnetic susceptibility measurement. The enthalpy difference (H) was estimated as 5.35 kJ mol-1 in the heating run. The entropy change


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(S) was calculated as 14.23 J K-1 mol-1 using the equation of S = H/TC, where TC is the critical temperature of the phase transition and herein is equal to 376 K. In statistical mechanics, the phase transition S value including the structure and the spin entropy differences between phases 1 and 2 can be expressed as Ω2

ΔS = RlnΩ1

(3)

where R (8.31 J K-1 mol-1) is the gas constant, and i (i = 1 and 2) represents the state numbers in phases 1 and 2, respectively. The ratio of state numbers in HTP and LTP, HT/IT, was estimated to be 4.35. Large HT/IT value may suggest the significantly structural deformations or molecular dynamics associated with the phase transition. On the basis of crystal structure analysis in LTP and HTP, the components of both cation and anion do not show dynamic disorder. However, the asymmetric unit changing from one pair of ion-pair in LTP to two pairs of ion-pair in HTP leads to the state number contributed from structure increasing by four times (both anion and cation increases by two times, respectively). The HT/IT value of 4.35 estimated from DSC measurement is close to 4, indicating that the entropy variation between LTP and HTP contributes primarily from the structure change.

Figure 2. DSC curves of 1 in the second heating and cooling cycles.


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Single crystal structures. To better understand the crystal structure change, the crystal structures in both LTP and HTP was compared with each other using the X-ray single crystal diffraction data at the same temperature (350 K). In LTP, the crystal of 1 belongs to monoclinic space group P21/c, as shown in Figure 3a. The asymmetric unit is comprised of one pair of anion and cation. From LTP to HTP, the crystal structure of 1 transforms from space group P21/c to P1. As displayed in Figure 3b, the asymmetric unit switches from one pair to two pairs of ionpairs. The a-axis shows the same orientation and comparable crystallographic axis length in both LTP and HTP, whereas the orientations of b- and c-axes in HTP correspond approximately to the orientation of c-axis and the opposite orientation of b-axis in LTP (Figure 3c, 3d), respectively. Moreover, the lengths of b- and c-axes in HTP are individually close to those of c-axis and the baxis in LTP. The bond lengths and angles (listed in Table S3 and S4) in both anions and cations in LTP and HTP at the selected temperatures are comparable to those in other [Ni(dmit)2]− compounds.22,23,37 (a)

(b)


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(c)

(d)

Figure 3. (a) Oak Ridge Thermal-Ellipsoid Plot (ORTEP) view with non-hydrogen atom labelling; the thermal ellipsoids set at 30% probability level in (a) LTP at 350 K (b) HTP at 350 K. A unit cell in (c) LTP (d) HTP where the cations with N2 and the anions with Ni2 represent in translucent manner, respectively, for 1. The crystallographic point group corresponding to space group P21/c is C2h, which contains four symmetry elements (E, C2, i and h), while the crystallographic point group corresponding to space group P-1 is Ci, which is the subgroup of C2h and has only two symmetry elements (E and i). From LTP to HTP, the loss of twofold screw axis and c-glide plane leads to the crystallographically equivalent anions/cations in LTP splitting into two groups of crystallographically inequivalent anions/cations in HTP, respectively. Although the cell parameters in HTP are close to that in LTP (Table S1, S2), and the alignment fashions of both anions and cations in HTP are also similar to that in LTP as well, that is, the anions form irregular stack and the cations, occupied the space between the neighboring anion stacks, form the equal distance linear arrangement (Figure 3c, 3d), the distinctions in crystal structure are observable between HTP and LTP. One type of irregular anion stack and one type of regular cation alignment in LTP switch into two types of irregular anion stacks and two styles of regular cation arrangements in HTP, respectively (Figure S5, S7). As depicted in Figure 3d, one irregular


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anion stack is built from Ni1 anions (in normal fashion) and another one is constructed from Ni2 anions (in translucent manner). The crystallographically inequivalent cations with N1 and N2 form separately regular alignments. By comparison of the anion stacks in LTP and HTP, it is found that the arranging manner of Ni2 anion stack in HTP is rather similar to that of anion stack in LTP (Figure 4a, 4b). Two types of Ni…Ni (4.244 and 4.319 Å) and plane–to–plane (3.665 and 3.601 Å) distances between neighboring anions within the Ni2 anion stack in HTP are comparable to those in LTP (Ni…Ni distances of 4.194, 4.388 Å and plane–to–plane distances of 3.679, 3.625 Å). The mean-molecule-plane of anion is defined by four coordinated S atoms. However, the pronounced deformation occurs in the Ni1 anion stack (see Figure 4c). In contrast to the anion stack in LTP or the Ni2 anion stack in HTP, as shown in Figure 4c, Figure S5 and S7, in the Ni1 anion stack in HTP, the significantly relative slippage underwent between two neighboring anion dimers along the molecular short axis of anion, and this leads to the neighboring Ni…Ni distance between two slipped dimers sharply increasing ca. 2 Å, however, their plane-to-plane distance shrinking ca. 0.6 Å. It is worth noting that two types of cations remain uniform alignments (Figure 4 and Figure S9). As the giant distortion in Ni1 anion stacks from LTP to HTP needs high lattice deformation energy, it becomes a high energy barrier for the transition state. The high lattice deformation energy responds to the huge thermal hysteresis loop in m vs. T plot. (a)

(b)

(c)


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Figure 4. Illustration for the characteristic distances (Å) within an anion stack in (a) LTP at 350 K and (b, c) HTP at 350 K, where the Ni…Ni distances (blue color) and plane-to-plane distance (red color) of mean-plane of anion, defined by four coordinated S atoms. Noticeably, the structural transformation from LTP to HTP leads to the lattice symmetry reduction of 1. Such type of lattice symmetry change with temperature is phenomenologically similar to the ISB structural phase transition. In a 1D S = ½ Heisenberg AFM chain system, the strong spin-lattice interaction generally gives rise to a spin-Peierls-type transition, coupled to the spin chain distortion and lattice symmetry reduction in LTP. Thus, the lattice symmetry is higher in HTP than that in LTP in a spin-Peierls-type transition system. An amount of spin-Peierls-type systems with SB structural phase transition have been found in 1D S = ½ organic radical4-7,40,41 and S = ½ metal-bis-dithiolene spin systems.20,23 To the best of our knowledge, 1 is the first 1D AFM spin system with a magnetic transition, which couples to a structural phase transition and shows lower lattice symmetry in HTP but not in LTP. Temperature dependent PXRD. The variable temperature PXRD diffractions were collected in the range of 303-423 K and in both heating and cooling runs for 1. As illustrated in Figure 5, the PXRD patterns show high similarity in the range of 2 = 5−50°. This observation consists with the single crystal structure analysis, with both LTP and HTP showing analogous parameters of unit cell and arrangement of ions. However, some distinctions in the PXRD patterns of two phases are obvious (as displayed in Figure 5 and Figure 6a), e.g., the diffractions of (0 2 0) and (0 2 1) in LTP transforms into the diffractions of (0 0 -2) and (0 1 -2) in HTP since the b- and caxes in LTP change individually into the c-axis and the opposite orientation of b-axis in HTP. As shown in Figure 6a, these diffractions shift from 2 = 6.68° and 9.77° in LTP to 2 = 6.94° and 9.55° in HTP, respectively. The diffractions of (0 1 1) and (0 1 -1) are equivalent with 2 = 7.86°


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in LTP, whereas inequivalent in HTP, which split into two diffraction peaks and appear at 2 = 8.71° and 7.77° (Figure 6a), respectively. Interestingly, the diffractions arising from the LTP and HTP are clearly observed in the PXRD profiles at 378 and 383 K simultaneously (Figure 6b), revealing that the LTP and HTP coexist in a temperature region near the TC (ca. 376 K from DSC measurement) but not at TC in the heating process, and this observation demonstrates that the HTP is pregenerated in LTP upon heating. It is an uncommon phenomenon that the HTP/LTP nucleates in the LTP/HTP and finally percolates through the whole sample in a polymorphous transition material when the temperature changes and the examples with two phases coexisting are limited. Most recently, Bisht et al. also observed two coexisting phases above the temperature near the metal-insulator transition (MIT) in NdNiO3 film.42 The bulk material of NdNiO3, which electronic properties have been hitherto widely studied, shows a MIT at ~130 K, and its HTP is metallic phase while its LTP is insulating phase. Additionally, it has been discovered that there exist competition between the strain and dimensionality effects on the MIT in NdNiO3 films, viz., the tensile strain increases the 3d band width of Ni and favors the metallic phase, while reducing dimensionality decreases the covalent band width and stabilizes the insulating phase in NdNiO3.43 Bisht et al. further found that the insulating phase is approached on cooling (T → TMIT) in NdNiO3 film even in the metallic phase of NdNiO3 (T > TMIT), making the sample electronically ‘inhomogeneous’ in nanoscale close to the transition.42 The mechanism of insulating and metallic phases coexisting is so complicated that it cannot be explained using a simple physical model in NdNiO3 film. Probably, the strong Coulomb repulsion between the conducting and localized polaronic electrons plays an important role in making the sample electronically ‘inhomogeneous’ in nanoscale near the transition. Additionally, the domains of substrate acting as nucleating centers also possibly leads synergistically to nanoscale phase


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separation.42 In the microcrystalline sample of [Et3(n-Pr)N][Ni(dmit)2], unlike the thin material, it can be excluded that the domains of substrate acting as nucleating centers gives rise to two phases coexistence near the ISB transition. In spite of this, the mechanism of two phases coexisting is complicated. At the present stage, it is difficult to provide a clear understanding for this observation. (a)

(b)

(c)

(d)

Figure 5. Variable-temperature PXRD patterns of 1 at the selected temperatures (a, b) in 363393 K in the heating run (c, d) in 403-303 K in the cooling process. The PXRD diffraction patterns of 1 at the selected temperatures in the range of 393-303 K upon cooling are shown in Figure 5c and 5d. The differences of PXRD patterns are also unambiguous between HTP and LTP. The transition occurs around 323 K in cooling process.


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The TC observed from the PXRD pattern change in both heating and cooling processes are also close to that detected from magnetic and DSC measurements. The relative intensity change with the temperature is analyzed for the three diffractions. The diffraction intensity is defined by the area of the diffraction peak. The diffractions of (0 1 -2) and (0 1 -1) in HTP correspond individually to the diffractions of (0 2 1) and (0 1 1) in LTP, and the relative intensity of the diffraction at the temperature T is defined as Int.(T)/Int.(303K). Regarding the diffraction pattern in LTP, the diffraction of (0 1 1) in HTP is a new peak, its relative intensity at the temperature T is defined as Int.(T)/Int.(423K). The relative intensity change against the temperature is plotted in Figure 6c, 6d and Figure S11b-11d. All show a ~55 K thermal hysteresis loop, revealing that the magnetic-bistability couples to the ISB structural transition. (a)

(b)

(c)

(d)


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Figure 6. (a) Variable-temperature PXRD patterns in the range of 2 = 6-18 at selected temperatures for 1 upon heating (b) variable-temperature PXRD patterns showing the coexistence of LTP and HTP (c, d) Temperature dependent relative intensity for the selected diffractions, showing ~55 K thermal hysteresis loop in heating and cooling cycle. Conclusion In summary, we observed a 1D S = ½ spin chain molecular crystal showing magnetic-bistable above room temperature with a wide thermal memory effect of ~55 K. We discovered that the magnetic-bistable transition couples to a structural phase transition with lower lattice symmetry in HTP but not in LTP, and this finding is unlike the observation that a spin-Peierls-type transition in 1D S = ½ spin chain is often associated with a SB structural phase transition. The wide magnetic thermal memory effect is relevant to the huge deformation of anion stack between LTP and HTP. We also found, firstly, that two phases coexist in a temperature region near TC but not at TC. This study may provide new insight for understanding the relationship between spinPeierls-type transition and structure phase transition, and will also offer a roadmap for searching magnetic-bistable materials with wide hysteresis above room temperature for practical application in future magnetic technologies. ASSOCIATED CONTENT Supporting Information The following files are available free of charge. Crystallographic data and refinement parameters, bond lengths and bond angles in both anions and cations at the selected temperatures upon heating and cooling; typical interatomic and plane-


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to-plane distances (Å) in an anion stack, between anion and cation at the selected temperatures. FT-IR spectrum, PXRD patterns at room temperature, fits of m vs. T in LTP and HTP, TG and DSC, crystal packing diagrams, H-bonds between cations and anions in LTP and HTP at 350 K, photograph of single crystal showing bendable at 400 K, variable-temperature PXRD patterns, variable temperature relative intensity for the selected diffractions showing ~55 K thermal hysteresis, Illustration for symmetry change between LTP and HTP for 1. (PDF) AUTHOR INFORMATION Corresponding Author * Email: [email protected] ORCID Xiao-Ming Ren: 0000-0003-0848-6503 Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (grant Nos.: 21801218, 21671100), the Natural Science Foundation of the Jiangsu Higher Education Institutions of China (Grant 17KJB150040). Ren thanks Prof. Chen-Jie Fang for reading this manuscript.


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A Wide Magnetic Thermal Memory Effect (~55 K) Above Room

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