A Molecular Crystal Shows Multiple Correlated Magnetic and Ferroelectric Switchings

Inorganic Anions Regulate the Phase Transition in Two Organic Cation Salts Containing [(4-Nitroanilinium)(18-crown-6)]+ Supramolecules

Open AccessCCS ChemistryRESEARCH ARTICLE1 Jul 2021A Nonlinear Optical Switchable Sulfate of Ultrawide Bandgap Yanqiang Li, Congling Yin, Xiaoyan Yang, Xiaojun Kuang, Jie Chen, Lunhua He, Qingran Ding, Sangen Zhao, Maochun Hong and Junhua Luo Yanqiang Li State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002 University of Chinese Academy of Sciences, Beijing 100049 , Congling Yin MOE Key Laboratory of New Processing Technology for Nonferrous Metals and Materials, Guangxi Key Laboratory of Optical and Electronic Materials and Devices, College of Materials Science and Engineering, Guilin University of Technology, Guilin, Guangxi 541004 , Xiaoyan Yang MOE Key Laboratory of New Processing Technology for Nonferrous Metals and Materials, Guangxi Key Laboratory of Optical and Electronic Materials and Devices, College of Materials Science and Engineering, Guilin University of Technology, Guilin, Guangxi 541004 , Xiaojun Kuang *Corresponding author: E-mail Address: [email protected] MOE Key Laboratory of New Processing Technology for Nonferrous Metals and Materials, Guangxi Key Laboratory of Optical and Electronic Materials and Devices, College of Materials Science and Engineering, Guilin University of Technology, Guilin, Guangxi 541004 , Jie Chen Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190 Songshan Lake Materials Laboratory, Dongguan, Guangdong 523808 , Lunhua He Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190 Songshan Lake Materials Laboratory, Dongguan, Guangdong 523808 , Qingran Ding State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002 University of Chinese Academy of Sciences, Beijing 100049 , Sangen Zhao *Corresponding author: E-mail Address: [email protected] State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002 University of Chinese Academy of Sciences, Beijing 100049 Fujian Science and Technology Innovation Laboratory for Optoelectronic Information of China, Fuzhou, Fujian 350108 , Maochun Hong State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002 and Junhua Luo *Corresponding author: E-mail Address: [email protected] State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002 University of Chinese Academy of Sciences, Beijing 100049 Fujian Science and Technology Innovation Laboratory for Optoelectronic Information of China, Fuzhou, Fujian 350108 https://doi.org/10.31635/ccschem.020.202000436 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesTrack Citations ShareFacebookTwitterLinked InEmail Nonlinear optical (NLO) switchable materials have attracted intense attention because of their promising applications in optoelectronic devices. However, previous studies are mainly limited to molecular-based compounds that usually exhibit narrow bandgaps. Here, we report all-inorganic Li9Na3Rb2(SO4)7 as an ultrawide-bandgap NLO switchable material. To the best of our knowledge, this sulfate shows the widest bandgap (6.70 eV) among NLO switchable materials. Furthermore, it undergoes a reversible phase transition with a high NLO switching contrast of about 180, which is almost the highest among known NLO switchable materials. Variable-temperature X-ray and neutron powder diffractions reveal that the exceptional NLO switching contrast can be attributed to the NLO-active [SO4]2− anions translating and rotating on the local scale. This work will provide insights into the development of ultrawide-bandgap NLO switchable materials, which may open up unprecedented opportunities for emerging applications, such as ultrawide-bandgap switches, sensors, and optoelectronics. Download figure Download PowerPoint Introduction Nonlinear optical (NLO) switchable materials, in which NLO responses are reversibly converted among diverse stable states under external stimulation, have attracted considerable interest owing to their prospective application in optoelectronic devices, such as switches, sensors, data storage, information processing of smart devices, and optical communication.1–5 Over the past decades, researchers have devoted great effort to studying NLO switchable materials and found a number of NLO switchable materials in the liquid phase.6,7 Meanwhile, a variety of efficient strategies have been put forward to explore solid-state crystalline NLO switchable materials, such as cation exchange, conformational reorganization, and "push–pull" molecules via photochromism.8–10 The resultant materials include metal–organic framework compounds such as [(H2NMe2)2Cd3(C2O4)4]·MeOH·2H2O and NH2-MIL-53 (Al), anil crystals such as N-(4-hydroxy)-salicylidene-amino-4-(methylbenzoate) and N-(3,5-di-tert-butylsalicylidene)-4-aminopyridine, as well as complexes.8,9,11,12 Unfortunately, these materials usually show low NLO switching contrasts with limited reversibility. A new scheme, relying on structural phase transitions, especially those from centrosymmetric (CS) structures to noncentrosymmetric (NCS) structures, has been recently employed to inspire sharp changes of NLO responses. In 2006, Mercier et al.13 first reported the transformation-induced NLO switchable phenomenon in a molecular-based compound, [{H3N(CH2)2SS(CH2)2NH3}PbI5]·H3O. Subsequently, a series of molecular-based NLO switchable materials were discovered, such as [α-(H3N(CH2)2S-S(CH2)2NH3)BiI5,14 (Hdabco+)(CF3COO−),15 bis(imidazolium) L-tartrate,16,17 2-(hydroxymethyl)-2-nitro-1,3-propanediol,18 (C4H10N)(CdCl3),19 dipropylammonium trichloroacetate,20 (R-CTA)2CuCl4,21 and (S-CTA)2CuCl4.21 However, despite excellent NLO switchable performance, their bandgaps are relatively narrow on account of π-conjugated electrons, d-d or f-f transition, such as [α-(H3N(CH2)2S-S(CH2)2NH3]BiI5 (∼1.84 eV),14 bis(imidazolium) L-tartrate (∼5.21 eV),17 and dipropylammonium trichloroacetate (∼3.82 eV).20 As a result, these molecular-based NLO switchable materials are merely used in visible and ultraviolet spectral regions. With respect to the deep ultraviolet spectral region (bandgap more than 6.20 eV), they play a unique and crucial role in a number of modern instruments, but it almost approaches the theoretical bandgap limit of NLO switchable materials.22,23 Hence, to date, there is still a huge opening for the application of ultrawide-bandgap NLO switchable materials. Recently, our group has reported a series of ultrawide-bandgap NLO sulfates, such as Li8NaRb3(SO4)6•2H2O24 and (NH4)2NaLi(SO4)2.25 Particularly, (NH4)2NaLi(SO4)2 exhibits considerable NLO responses aroused by nearly parallel nonbonding O 2p orbitals of NLO-active [SO4]2− groups, but (NH4)2NaLi(SO4)2 is not NLO switchable.25 As a result of our continuous studies on ultrawide-bandgap NLO sulfates, we successfully synthesized a new sulfate from the high-temperature melt, namely Li9Na3Rb2(SO4)7 ( I) with an ultrawide bandgap of 6.70 eV, which presents the widest working spectral range for NLO switchable materials ever known. Uniquely, I undergoes a reversible phase transition from the NCS state (P3) to CS state (P 3 ¯ ) with a high NLO switching contrast of about 180. Experimental Methods Synthesis of I Polycrystalline I was synthesized by the traditional high-temperature solid-state method using the Li2SO4•H2O (99.0%), Rb2SO4 (99.0%), and Na2SO4 (99.0%) in the stoichiometric molar ratio. The mixture was fully ground and transferred to a muffle furnace. After that, the mixture was preheated at 723 K for about 24 h. Subsequently, the products were gradually heated to 823 K and sintered at 823 K for about 96 h with several intermediate mixings and grindings. The purity of as-synthesized polycrystalline I samples was checked by powder X-ray diffraction (XRD) analyses, which were performed on a Rigaku MiniFlex 600 diffractometer with a Cu Kα radiation (λ = 1.5418 Å) in the 2θ range of 10°–70° with the scan rate of 1° min−1 at room temperature. Crystal growth of I Single crystals of I were grown by the top-seeded solution growth method. The polycrystalline I powders were put into a Φ 45 × 45 mm platinum crucible and melted at 973 K in a temperature-programmable furnace. This melt was held at 973 K for more than 24 h to form a transparent solution. Then, the solution was quickly cooled to 900 K, and a platinum wire was dipped into the solution. The solution was rapidly cooled to the saturation temperature, and then I crystals formed on the platinum wire via cooling the solution at a rate of 2 K h−1. After the crystal growth process was completed, I crystals were drawn out of the solution and served as seed crystals. In the following crystal growth procedure, we first determined the crystallized temperature of I samples by the tentative seed crystal strategy using as-grown I crystals. A seed of good optical quality was introduced into the solution at 10 K above the crystallized temperature. This temperature was held for about 60 min and then dropped to the crystallized temperature in 10 min. The centimeter-grade I single crystal was grown with the solution cooling down at a rate of 0.2–0.5 K per day. Finally, the I crystal was pulled out of the solution. Structure characterization The variable temperature (VT)-XRD analyses were carried out on a Panalytical X'Pert PRO diffractometer with an Anton Parr HTK 1200 N high-temperature attachment. The VT-XRD data over 10–80° 2θ range was collected from room temperature to 783 K using a temperature increment of 100 K except for the 733–783 K temperature range close to the phase transition region with an interval of 10 K. Time-of-flight neutron powder diffraction (NPD) data was collected from 10 to 783 K at several temperatures at the general-purpose powder diffractometer at the China Spallation Neutron Source. The VT-XRD data and time-of-flight NPD data were both refined by Rietveld methods using TOPAS-Academic software26 and Jana2006 software.27 The diffraction data of I at 100(2) K was collected by the single-crystal XRD on a Bruker APEX II CCD diffractometer with a monochromatic Mo Kα radiation. The program APEX3 was used to perform cell refinements and data reductions.28 The structure was established by the direct method with the program SHELXS and refined with the least-squares program SHELXL.29 Final refinements included anisotropic displacement parameters. The solved structure was verified by the program PLATON,30 and no higher symmetries were recommended. Property measurements Elemental analyses were performed by using a Jobin Yvon Ultima 2 inductively coupled plasma optical emission spectrometer. The thermal stability was investigated by the differential scanning calorimetric (DSC) and thermogravimetric (TG) analyses on a NETZSCH STA 449C simultaneous thermal analyzer instrument. About 22 mg I samples were placed in Al2O3 crucibles, heating and cooling at a rate of 10 K min−1 from room temperature to 973 K and from 660 to 820 K. The temperature dependence of the dielectric constant measurements performed on a TongHui TH2828 analyzer were in the temperature range of 680–790 K, with a heating and cooling rate of about 10 K min−1. The transmittance spectrum was acquired on the PerkinElmer LAMBDA 950 UV/Vis/NIR spectrophotometer in the wavelength range of 185–800 nm on a single-crystal plate. Powder second harmonic generation (SHG) measurements were carried out by the Kurtz–Perry method with a Q-switched Nd:YAG laser at the wavelength of λ = 1064 nm.31 Polycrystalline I samples were ground and sieved into a series of particle size ranges: 53–75, 75–125, 125–180, 180–250, and 250–300 μm. Infrared spectra were obtained on a LAMBDA 900 infrared instrument with the wavenumber in the range of 400–4000 cm−1. The bands ranging from 1006 to 1223 cm−1 can be attributed to the stretching vibrations of the [SO4]2− tetrahedra ( Supporting Information Figure S2). The absorption from 423 to 646 cm−1 was assigned to the bending modes of sulfate ( Supporting Information Figure S2). Results and Discussion Pure I samples were synthesized by the high-temperature solid-state method (Figure 1a). The inductively coupled plasma element analyses of I gave a molar ratio of Li:Na:Rb:S = 9.16∶2.71∶2.21∶6.78, which is well consistent with the compositions from the structural data. As illustrated in Figure 2a, DSC curves imply that I melts congruently around 874.8 K. As a result, bulk crystals of I were grown from its stoichiometric high-temperature melt by the top-seed technique. The as-grown I crystal was transparent with sizes of about 18 × 10 × 3 mm3 (Figure 1b). Interestingly, in the heating/cooling curves, a pair of relatively small peaks were observed at 771.7 (Tc) and 759.4 K (Figure 2a). At the same time, there was no obvious weight loss in the TG analysis (Figure 2a). Thus, we speculated that I undergoes a reversible phase transition at Tc, and then carried out additional DSC and TG analyses. As indicated by Figure 2b, DSC analyses can be reproducible, proving that a reversible phase transition occurs. For convenience, the phase above Tc was denoted as high-temperature phase (HTP), and the phase below Tc was marked as low-temperature phase (LTP). We also measured the temperature dependence of the dielectric constant of I under various frequencies. As shown in Figures 2c and 2d, the dielectric constant shows evident peak anomalies near Tc. Figure 1 | XRD patterns and crystal photograph of I. (a) Calculated and experimental XRD patterns of I. (b) Photograph of the as-grown I crystal. Scale bar, 10 mm. Download figure Download PowerPoint To further confirm the reversible phase transition, we collected single-crystal XRD data at 100 K and VT-XRD data from room temperature to 783 K. Considering neutrons are more sensitive to O atoms, and Li atoms have negative neutron scattering lengths in contrast with Na, Rb, and S atoms [neutron scattering lengths (fm) are: Li, −1.90; Na, 3.63; Rb, 7.09; O 5.803; S 2.847],32 the possible positions of O and Li atoms can be better determined with NPD data. We further collected time-of-flight NPD data of I at different temperatures. The VT-XRD data and time-of-flight NPD data were refined by the Rietveld method.33 As shown in Figure 3a, no obvious changes were seen in the VT-XRD patterns in the heating process. It indicated that there was no significant structural reorganization in the course of the phase transition. As illustrated in Figure 4a, the in situ time-of-flight NPD plots were similar from 10 to 763 K but changed dramatically at 783 K. Several intense reflections including (361), (171), (161), (251), and (440) nearly disappeared. Also, the background profiles displayed broad peaks below d = 1.5 Å below 763 K, and additional broad peaks formed within d = 2.0–2.7 Å at 783 K, which shows that structural disorder occurs initially at a local range of d = 1.0–1.5 Å and expands to a larger range of d = 2.0–2.7 Å with the increasing temperature. In view of the large neutron scattering length of O and the absence of broad peaks on the VT-XRD patterns, we can conclude that the structural disorder is most probably related to the thermally activated motion of [SO4]2− anions. In addition, the fitted lattice parameters and cell volumes from the VT-XRD (Figure 3b) and NPD (Figure 4b) data have abrupt changes around 743 K, consistent with the phase transition. Supporting Information Tables S1–S12 list the detailed crystallographic information. Figure 3 | VT-XRD data of I. (a) "Film plot" of VT-XRD data of I powders on heating. (b) VT cell parameters against the powder XRD data. The vertical dashed line marks the occurrence of phase transition. Download figure Download PowerPoint I crystallizes in the NCS space group of P3 (No. 143) in the LTP and the CS space group of P 3 ¯ (No. 147) in the HTP. As shown in Figure 5a, the crystal structure of LTP features a three-dimensional (3D) tetrahedral framework, in which tetrahedra have a basal plane nearly parallel to the ab plane and apical bonds parallel to the c-axis, composed of LiO4 and [SO4]2− tetrahedra. The Na+ and Rb+ are distributed in a well-ordered manner in the cavities to maintain the structural balance. There are nine crystallographically independent sites for S atoms in the LTP. The [S4O4]2− and [S6O4]2− tetrahedra are alternatively linked to LiO4 tetrahedra by sharing corners to construct hexagonal pillars (Figure 5b) around the three-fold axis. The [S5O4]2− tetrahedra occupy the central space of the pillars. The HTP can be considered as a locally melted phase of the LTP. In the structure of HTP, almost all atoms are partially occupied on two equivalent positions, disturbing the ordered alignment of [SO4]2− units. However, the local arrangements of [SO4]2− tetrahedra in the ab plane still retain the structural feature in the LTP, as shown in one possible [SO4]2− tetrahedra arrangement in Figure 5c. As compared to LTP (Figure 5d), the HTP structure features tilted [SO4]2− tetrahedra with the basal plane away from the ab plane. In addition, the [SO4]2− anions on the three-fold axis translate along the c-axis. Due to this shift, considerable lattice strains are formed in the ab plane, which leads to noticeable elongated [SO4]2− anions along the c-axis (Figures 5e and 5f). In order to reduce the strain, the hexagonal pillars formed by [SO4]2− anions on the ab plane are tilted locally to form a larger cavity, as reflected by its radius R increasing from 3.87 (LTP) to 4.03 Å (HTP). Owing to the reminiscent anionic arrangements in the LTP, the Li+, Na+, and Rb+ cations in the HTP are confined to occupy similar positions to the LTP, but they are more likely to move around and become more highly disordered on the local scale. Thus, the phase transition of I is similar to the melting behavior and can be called a quasi-melting phase transition. Figure 5 | Crystal structures of I. (a) Crystal structure of the LTP viewing along the c-axis. (b) The hexagonal pillars formed by [SO4]2− tetrahedra, highlighted by dashed yellow circle in (a). A possible local arrangement of [SO4]2− terahedra in the structure of HTP along (c) the c-axis and (e) the b-axis, in comparison with LTP along (d) the c-axis and (f) the b-axis. The green tetrahedra represent the [SO4]2− tetrahedra. R denotes the radius of the hexagonal pillar. For clarify, the Li+, Na+, and Rb+ cations are omitted in Figures 5c–5f. Download figure Download PowerPoint To obtain the accurate bandgap, a polished I crystal of about 5 × 5 × 1 mm3 (the inner picture in Figure 6a) was used for the test. As demonstrated in Figure 6a, I is transparent down to the spectral region below 185 nm, which corresponds to the bandgap of 6.70 eV. As far as we are aware, the bandgap of I is the widest among known NLO switchable materials, which far exceeds that of many molecular-based NLO switchable materials, such as [α-(H3 N(CH2)2 S-S(CH2)2NH3]BiI5 (∼1.84 eV),14 bis(imidazolium) L-tartrate (∼5.21 eV),17 2-(hydroxymethyl)-2-nitro-1,3-propanediol (∼6.20 eV),18 and dipropylammonium trichloroacetate (∼3.82 eV).20 Figure 6 | Optical properties of I. (a) Transmittance spectrum. The inner picture is a single-crystal I sample for measurements. (b) SHG intensity versus particle sizes at λ = 1064 nm. The red curve is drawn to guide the eyes and does not fit the data. (c) The VT-SHG intensity between SHG-on states and SHG-off states at λ = 1064 nm. The inner picture is the SHG signal at the SHG-off state. (d) Completely reversible and recoverable switching of SHG intensity. Download figure Download PowerPoint Given that I is CS in HTP and NCS in LTP, it is expected that I shows NLO switchable properties, that is, SHG-on states in LTP and SHG-off states in HTP. First, the SHG intensities of I were tested by the Kurtz–Perry method31 with a solid-state Q-switched Nd:YAG laser at 1064 nm at room temperature. As illustrated in Figure 6b, SHG intensities of I increase with increasing particle that I is in the same particle the SHG intensity of LTP is about that of the NLO ( Supporting Information Figure which is larger than that of reported ultrawide-bandgap NLO sulfates, such as × × and × After that, we performed VT-SHG measurements on a I single-crystal As shown in Figure the SHG intensities nearly constant over a temperature range Tc. the temperature to the SHG intensities rapidly and then except to previous the NLO switching contrast is as the ratio of SHG intensities at SHG-on states and SHG-off the NLO switching contrast of I is about 180. this NLO switching contrast is higher than that of most solid-state crystalline NLO switchable materials, including metal–organic framework compounds such as [(H2NMe2)2Cd3(C2O4)4]·MeOH·2H2O and NH2-MIL-53 such as and crystals such as 2-(hydroxymethyl)-2-nitro-1,3-propanediol and as well as compounds such as and The NLO switchable with higher NLO switching contrast than I in the NLO responses quickly 10 an excellent (Figure I is a as an ultrawide-bandgap NLO switchable because of its good thermal ultrawide bandgap, high NLO switching contrast as well as excellent reversibility. Figure | Time-of-flight NPD data of I. (a) Time-of-flight NPD patterns of I from 10 to 783 K. (b) cell parameters and volumes against the NPD data. The vertical dashed line marks the occurrence of phase transition. Download figure Download PowerPoint the HTP structure is highly it is to out To better for the NLO switching we for I by the method on anionic group [SO4]2− anions be the NLO-active the optical properties of which has also been by our previous the of [SO4]2− anions were are in the Supporting Information For LTP, over the [SO4]2− anions to an of in the cell along the that the NLO of the [SO4]2− anions are to the highly SHG-on state. the temperature to 783 K, a quasi-melting phase transition which [SO4]2− anions translate and on the local as compared with LTP. As a result, the structure of HTP and of [SO4]2− anions are out to be to the SHG-off state in HTP. it can be that the high NLO switching contrast is to the quasi-melting phase transition in the and of NLO-active [SO4]2− anions on the local scale. Figure 2 | DSC and dielectric measurements of I. DSC and TG curves for I measured (a) from to 973 K and (b) from 660 to 820 K. The dashed in the DSC peaks dependence of the dielectric constant for I (c) under various and (d) in the heating and cooling under 100 Download figure Download PowerPoint In we have synthesized a new all-inorganic NLO switchable of an ultrawide bandgap, namely This sulfate not has the widest bandgap of 6.70 in NLO switchable materials, but also exhibits a high NLO switching contrast of about that is almost the highest in known NLO switchable materials. Structural and theoretical analyses that the NLO switching contrast the quasi-melting phase transition with NLO-active [SO4]2− anions translating and rotating on the local scale. as a new of NLO switchable materials. We that this work also for ultrawide-bandgap NLO switchable materials with high NLO switching which may provide opportunities for applications in the such as ultrawide-bandgap switches, sensors, and optoelectronics. Supporting Information Supporting Information is of There is no of interest to Information This is by the National Science of China and the Research of the Chinese Academy of and the Science of Fujian the Key Research of of the Chinese Academy of the Science of Guangxi the National Key Research and of China Key Laboratory of and Technology, and Key Laboratory of New Processing Technology for Nonferrous Materials, of Key Laboratory of Optical and Electronic Materials and in a of Optical as of and and Yang Chen Nonlinear Optical with the Owing to its via and of Nonlinear Optical in Li Li Nonlinear Optical in an Open A Nonlinear Optical Li To Nonlinear Optical with Optical in the A on NLO from to on Mercier in of an Mercier Switchable NLO on and Luo Li Hong Nonlinear Optical with an Chen Luo Hong A Chen Zhao Hong Luo of Nonlinear Optical with in on Chen Hong Luo to Nonlinear Optical the in a He Chen Luo Chen of in a Nonlinear Optical Li Luo of Nonlinear Optical in a Li with He Chen of Nonlinear Optical the Crystal Li Zhao Li Ding Li Luo as a New Sulfate Nonlinear Optical Li Zhao Li Ding Hong Luo Nonlinear Optical for CCD Bruker of Structure with the Powder for the of Nonlinear Optical and Rietveld for and Li Hong Luo Nonlinear Optical of a New Crystal with a Li Zhao Luo of Nonlinear Optical in a Chen by Li Chen and Nonlinear Optical on the A Single Crystal and on K. of in a State Chen on an for the and SHG in of the Information Chinese bandgap

Structural changes in ferroelectric phase transitions of vinylidene fluoride-tetrafluoroethylene copolymers: 1. Vinylidene fluoride content dependence of the transition behaviour

Photo-induced charge-transfer phase transition of rubidium manganese hexacyanoferrate in ferromagnetic and paramagnetic states

Rubidium manganese hexacyanidoferrate, Rb0.97Mn[Fe(CN)6]0.99 ⋅ 0.5H2O, which has a three‐dimensional cyanido‐bridged Mn−Fe framework encapsulating Rb+ ions, shows a terahertz (THz) wave absorption property. This compound undergoes a charge‐transfer phase transition between FeIII‐CN‐MnII [high temperature (HT) phase] and FeII‐CN‐MnIII [low temperature (LT) phase] near room temperature. The HT phase exhibits a THz wave absorption at 1.06 THz due to the slow vibration of the heavy Rb+ ions encapsulated in the three‐dimensional framework. By contrast, the LT phase displays a higher resonance frequency at 1.13 THz. This shift to a higher frequency is attributed to the framework shrinkage, which decreases the space available for the trapped Rb+ ion. Indeed, the void volume accompanying the phase transition decreases by 16.4 % from 41.4 Å3 (HT phase) to 34.6 Å3 (LT phase). Tuning the THz wave absorption frequency is necessary for THz devices such as absorbers, switches, filters, and modulators, especially for THz technology with THz waves in the sub‐THz region or near 1 THz.

This study synthesized and characterized two zero-dimensional chiral organic-inorganic hybrid isomers (R-APH2)SnCl6 (1) and (S-APH2)SnCl6 (2) (where AP is 3-aminopyrrolidine). Their phase transition behaviors, chiral optical properties, and crystal structures were investigated via differential scanning calorimetry (DSC), vibrational circular dichroism (VCD), second harmonic generation (SHG) measurements, and high/low-temperature single-crystal X-ray diffraction analysis. The results showed that the two compounds undergo high-temperature reversible first-order phase transitions at 422/448 K and 418/448 K, respectively, with the high/low-temperature single-crystal symmetry exhibiting the rare characteristic of inverse temperature-induced symmetry breaking (ITSB). SHG tests revealed that during the phase transition, the compounds display a unique antisymmetric nonlinear optical switching effect: the low-temperature phase (chiral space group P212121) is in the "SHG-low" state, while the high-temperature phase (non-centrosymmetric space group P21) transitions to the "SHG-high" state. The symmetric signals of VCD spectra at specific wavenumbers confirm their enantiomeric properties. Further research reveals that the synergistic displacement of organic cations (R/S-APH22+) and the distortion synergy of inorganic metal frameworks ([SnCl6]2-) constitute the core phase transition mechanism that drives changes in crystal symmetry and optical properties. This study provides a reference for the development of low-dimensional chiral materials with high phase transition temperatures, facilitating their applications in optoelectronic devices, chiral sensing, and other fields.

Historically, the soft mode theory of ferroelectric phase transitions has been developed for the high-temperature (paraelectric) phase, where the phonon mode softens upon decreasing the temperature. In the low-temperature ferroelectric phase, a similar phonon softening occurs, also leading to a bosonic condensation of the frozen-in mode at the transition, but in this case the phonon softening occurs upon increasing the temperature. Here we present a soft mode theory of ferroelectric and displacive phase transitions by describing what happens in the low-temperature phase in terms of phonon softening and instability. A new derivation of the generalized Lyddane–Sachs–Teller (LST) relation for materials with strong anharmonic phonon damping is also presented which leads to the expression ɛ 0/ɛ ∞ = |ω LO|2/|ω TO|2. The theory provides a microscopic expression for T c as a function of physical parameters, including the mode specific Grüneisen parameter. The theory also shows that ωTO∼(Tc−T)1/2 , and again specifies the prefactors in terms of Grüneisen parameter and fundamental physical constants. Using the generalized LST relation, the softening of the TO mode leads to the divergence of ϵ 0 and to a polarization catastrophe at T c. A quantitative microscopic form of the Curie–Weiss law is derived with prefactors that depend on microscopic physical parameters.

Phase transition, structure and reorientational dynamics of H2O ligands and ReO4− anions in [Ba(H2O)3](ReO4)2⋅H2O

A MnIII spin crossover complex with atypical two-step hysteretic thermal switching at 74 K and 84 K shows rich structural-magnetic interplay and magnetic-field-induced spin state switching below 14 T with an onset below 5 T. The spin states, structures, and the nature of the phase transitions are elucidated via X-ray and magnetization measurements. An unusual intermediate phase containing four individual sites, where are in a pure low spin state, is observed. The splitting of equivalent sites in the high temperature phase into four inequivalent sites is due to a structural reorganization involving a primary and a secondary symmetry-breaking order parameter that induces a crystal system change from orthorhombic→monoclinic and a cell doubling. Further cooling leads to a reconstructive phase transition and a monoclinic low-temperature phase with two inequivalent low-spin sites. The coupling between the order parameters is identified in the framework of Landau theory.

We present a comprehensive study of the temperature dependence of the crystal structure using single-crystal X-ray diffraction and diffuse scattering, and electrical transport and magnetic properties as well as some optical properties at room temperature to elucidate the origin and the form of multiple ground states demonstrated in a previous study of the heat-capacity of the MMX chain compound, [Pt(II/III)(2)(n-PenCS(2))(4)I](∞). The present results confirm the presence of the two phase transitions, one reversible of first order at 207 K and the other nonreversible monotropic at 324 K, separating the low temperature (LT), room temperature (RT), and high temperature (HT) phases. The unit cell displays a 3-fold periodicity of -Pt-Pt-I- in the RT and HT phases because of the structural disorder which is exhibited by the dithiocarboxylato groups and the n-pentyl groups belonging to the central diplatinum unit. In addition, for the HT-phase all the dimers show this disorder. This compound undergoes a metal-semiconductor transition at T(M-S) = 235 K. The presence of diffuse streaks corresponding to 2-fold -Pt-Pt-I- periodicity in the HT and RT phases indicates dynamic valence ordering of the type -Pt(2+)-Pt(2+)-I(-)-Pt(3+)-Pt(3+)-I(-)-or-Pt(2+)-Pt(3+)-I(-)-Pt(3+)-Pt(2+)-I(-)-. For the LT-phase the diffuse scattering is condensed into clear Bragg diffraction peaks while keeping the 3-fold periodicity. This fact suggests further localization through dimerization of charges and spins confirming the diamagnetic state in the magnetic susceptibility and the low electrical conduction below 207 K. The present results are further discussed in relation to those of previous studies on the homologues, [Pt(II/III)(2)(RCS(2))(4)I](∞), R = methyl, ethyl, n-propyl, and n-butyl.

Bismuth titanate Bi4Ti3O12 (BTO) is a member of the layered Aurivillius phase perovskite ferroelectrics and has a structural phase transition at Tc of 675 C. Below Tc, BTO shows ferroelectricity with a large spontaneous polarization (30 C/cm) parallel to the layered structure. BTO attracted much attention as candidate material for ferroelectric memories because of the fatigue-free nature of the BTO-based mixed crystal Bi4 xLaxTi3O12 (BLT-x). Although the space group of BTO at room temperature is determined to be B2cb ð1⁄4 Aba2 1⁄4 C2cbÞ (Z 1⁄4 4) only from the X-ray and neutron diffraction patterns, its space group has finally been concluded to be Pc (Z 1⁄4 2) with monoclinic symmetry, taking into account the existence of the small spontaneous polarization (4 C/cm) perpendicular to the layered structure. Here, Z is the number of the chemical formula in the conventional unit cell. Various physical properties of BTO, such as a soft mode behavior and domain wall structures, have been intensively investigated. The Landau theory of phase transition in BTO was presented by assuming the point group 4=mmm in the hightemperature phase, where the phase transition of BTO from the tetragonal (4=mmm) phase to the monoclinic (m) phase was named the triggered phase transition. The splitting of the triggered phase transition, that is the existence of the intermediate orthorhombic phase (B2cb), was observed in the mixed crystal systems in Bi4 xRxTi3O12 (R = La, Nd, Sm, and Gd). It was reported that the prototype structure of BTO shows the space group I4=mmm (Z 1⁄4 4). It is conjectured, however, that neither the orthorhombic (B2cb) nor the monoclinic (Pc) phase can be directly induced from the tetragonal (I4=mmm) phase, on the basis of the group theoretical consideration. Even the primitive unit cell size in I4=mmm and B2cb phases is different. Note that the proper ferroelectric phase transition from I4=mmm to B2cb is impossible, because the latter is not any subgroup of the former at the -point. On the other hand, it was reported, on the basis of experimental results, that the Curie–Weiss behavior of the dielectric constant in the high-temperature phase is observed in BTO, indicating the proper ferroelectric phase transition. This seems to be in contradiction. In order to solve the above contradiction, Rae et al. and Perez-Mato et al. assumed the Fmmm symmetry to be the parent phase, where Fmmm is a subgroup of I4=mmm. If the high-temperature parent phase above 675 C is indeed orthorhombic as they claim, however, the Landau theory so far proposed of the triggered phase transition in BTO, which is based on the assumption that it is tetragonal, may fail. Thus, the symmetry of the high-temperature parent phase seems to be of crucial importance. Under these circumstances, to clarify the structural phase transition at 675 C and the symmetry of the parent phase of BTO, we investigate the domain wall structure in the high-temperature phase and discuss its symmetry on the basis of the group theory. Single crystals of BTO were grown by the flux method from a Bi2O3–TiO2 system. For domain wall observation, some c-plate crystals with an area of about 5mm and thicknesses of about 10–50 m were selected among asgrown crystals as samples. The temperature dependence of the domain wall structure in the c-plate of the BTO crystal was observed using a polarizing microscope with a hightemperature sample stage (Linkam TS-1500), where the possible temperature range of this stage is from room temperature to a maximum of 1500 C. Figures 1(a) and 1(b) show photographs of the c-plate crystal in BTO under crossed-Nicols below and above Tc, respectively, where the sensitive plate was used in Fig. 1(b). In Fig. 1(a), the 90 domain wall structure is observed, while no domain wall structure appears in Fig. 1(b), indicating that the c-plate sample of BTO is optically isotropic. Thus, it was confirmed that the symmetry of the high-temperature phase is tetragonal. In our experiment, the phase front was also observed near Tc, indicating the first-order phase transition. 200 μm A P

Electro-optical and polarization current measurements on 1,3-phenylene-bis[4-(3-fluoro-4-decyloxyphenyliminomethyl) benzoate] (3F-10-O-PIMB) revealed a second order phase transition between two antiferroelectric ‘smectic banana’ phases. The observations show that the switching between the ferroelectric states in the higher temperature (HT) phase requires higher thresholds than in the lower temperature (LT) phase. It is hypothesized that the HT phase has a lower (triclinic, C1) symmetry, than that of the LT phase (monoclinic, C2). It is also shown that electric fields can induce transitions between different ‘smectic banana’ phases.

Rubidium manganese hexacyanoferrate exhibits charge transfer phase transition from high temperature phase to low temperature phase at 230 K. This phase transition can also be triggered by light irradiation resonantly inducing charge transfer from Mn to Fe. In the present study, boundary sensitive Raman spectroscopy was performed for Rb0.94Mn[Fe(CN)6]0.98·0.2H2O in both cases of photoinduced and thermal phase transition. Since the frequencies of C≡N stretching vibration modes are very sensitive to the valence states of the adjacent metal ions, we can quantify the distribution of not only high and low temperature phase but also boundary configurations from the observed spectra. We obtained the time evolution of the fraction ratios of the valence states from the observed peak areas. In the case of photoinduced situation, the boundary increases up to 15% when high temperature phase diminishes to 55% of the initial fraction. This is quite different from the result in thermal phase transition where the boundary is created only 0.8% at the same high temperature phase fraction value. We conclude that many small domains are preferably created in photoinduced phase transition since the ratio of boundary is large, while large domains grow in thermal phase transition.

The reversible structural phase transition accompanied by the doubling of the unit cell parameter c and a change in the space group is found in the decamethylosmocene crystal C Os at a temperature of 243 ∠ 5 K. In the high-temperature (HT) phase (space group P 2 1 / m , Z = 2), the molecule adopts an eclipsed confor- mation and lies in the mirror symmetry plane. In the low-temperature (LT) phase (space group P 2 1 / c , Z = 4), the molecule occupies the general position with a small rotation of the ligands. The structural transformations from the high-temperature phase into the low-temperature phase at 120 K are described by the deviation of the molecular axis of fivefold symmetry by 3.2(2) i and the displacement of the center of mass of the molecule by 0.173(2) A with respect to their positions in the high-temperature phase, and also by the rotation of the Cp * ligands in the same direction by 3.0(2) i and 5.4(2) i , respectively. © 2000 MAIK "Nauka/Interperiodica". p 2 *

We report the discovery of a first-order phase transition at around 125 K in\nNbCrP, which is a nonsymmorphic crystal with Pnma space group. From the\nresistivity, magnetic susceptibility, and nuclear magnetic resonance\nmeasurements using the crystals made by the Sn-flux method, the\nhigh-temperature (HT) phase is characterized to be metallic with a\nnon-negligible magnetic anisotropy. The low-temperature (LT) phase is also\nfound to be a nonmagnetic metallic state with a crystal of lower symmetry. In\nthe LT phase, the spin susceptibility is reduced by ~30 % from that in the HT\nphase, suggesting that the phase transition is triggered by the electronic\ninstability. The possible origin of the phase transition in NbCrP is discussed\nbased on the electronic structure by comparing with those in other\nnonsymmorphic compounds RuP and RuAs.\n