Ionic Phase Transition Research Articles

Quantum Neutral–Ionic Phase Transition as Investigated by Raman Scattering and X-ray Diffraction

The neutral–ionic (N–I) phase transitions of the molecular valence state have been investigated by Raman spectroscopy and x-ray diffraction for a series of isostructural charge-transfer complexes, ...

Transformation between Monovalent and Divalent Ionic Solids: An Ionic(I)–Ionic(II) Phase Transition in a Biferrocene–F1TCNQ Charge-Transfer Complex

An “ionic(I)–ionic(II)” charge-transfer transition has been observed in a new molecular solid composed of dineopentylbiferrocene (D) and fluorotetracyanoquinodimethane (A). The material is a monova...

Ionic conductivities and phase transitions of lanthanide rare-earth substituted La 2Mo 2O 9

The ion conductivities and phase transitions of lanthanum molybdate (La 2Mo 2O 9) substituted with lanthanide rare-earths are investigated using impedance spectroscopy, dilatometry, and X-ray powder diffraction. Among the substituted La 2Mo 2O 9 of 10 mol% Ce, Nd, Sm, Gd, Dy, Er, Yb, the specimens containing Er, and Dy exhibit depressed α–β phase transformation and high conductivities. Their 700 °C conductivities are approximately five to seven times that of La 2Mo 2O 9, around 0.26 S cm −1, comparable with those of (LaSr)(GaMg)O 3 and Gd-substituted CeO 2. Among the three compositions of 10 mol% Gd, Dy, Er showing depressed phase transition, Er- and Dy-substituted La 2Mo 2O 9 possess relatively low thermal expansion coefficient 11×10 −6 K −1, compared with that of the Gd-substituted La 2Mo 2O 9, 18×10 −6 K −1, which is near that of La 2Mo 2O 9. Hence, Dy and Er are valuable dopants in improving the La 2Mo 2O 9 properties. Across the lanthanide series, 10 mol%-substituted La 2Mo 2O 9 demonstrates systematic variations in the conductivity–temperature relation. Hysteresis phenomena in both of conductivity and thermal expansion are also observed in those compositions which display phase transition.

Phase transition through intermediate formation?

The ionic conductivity and phase transitions of HZr2(PO4)3.H2O and H0.98Zr1.98Nb0.02(PO4)3.H2O were studied using impedance spectroscopy and high temperature X-ray powder diffraction.

High lithium ionic conductivity in the lithium halide hydrates Li3-n(OHn)Cl (0.83 < or = n < or = 2) and Li3-n(OHn)Br (1 < or = n < or = 2) at ambient temperatures.

Lithium ionic conductivity and phase transitions in a series of lithium halides hydrates and hydroxides with general formula Li3-n(OHn)X (0.83 < or = n < or = 2; X = Cl,Br) were studied using impedance measurements and 1H and 7Li NMR spectroscopy. All compounds studied in this work crystallize in the antiperovskite structure or are closely related to this structure type. With the exception of LiCl. H2O, all compounds with integer lithium content exhibit good lithium ionic conductivity in their high temperature cubic phases above T = 33 degrees C. Lithium doping of samples LiX.H2O and Li2(OH)X leads to a suppression of the phase transition into the noncubic phases and the good ionic conductivity is extended down to lower temperatures (T < 0 degree C). Thus, lithium doping of the lithium halide hydrates provides a promising tool for tailoring the ionic conductivity at ambient temperatures to its optimum value.

Novel magneto-lattice transition in the mixed-stack charge-transfer complexes with inter-columnar networks

We report on a novel spin–lattice phase transition in an ionic mixed-stack charge-transfer (CT) compound of (BEDO-TTF)(Cl 2TCNQ). We found that the rotational modes for rigid-body displacements of molecules afforded an appreciable quantitative variation in the exchange interactions along the mixed stacks, which resulted in the symmetry- invariant first-order magneto-lattice phase transition at around 110 K. It was found in the electron paramagnetic resonance (EPR) spectra that the low-temperature strongly interacting spin phase was followed by a magnetic order at 16 K. We discuss the origin of the transition at 110 K in terms of the competition of free energy between the respective spin states. We also present neutral–ionic (NI) phase transition in the isomorphous compound of (BEDT-TTF)(ClMeTCNQ) under pressure.

Cooperative and non-linear phenomena at the neutral–ionic phase transition

The complex interplay among on-site energy, Hubbard U, and coupling to Holstein and Peierls phonons at the neutral–ionic phase transition is discussed using diagrammatic valence bond calculations. The charge transfer and dimerization amplitudes and the infrared intensity of molecular vibrations are studied in the transition region.

Design of quantum neutral–ionic phase transition in organic charge-transfer complexes

Neutral–ionic (NI) phase transition has been investigated by various chemical modifications in the prototype complexes of TTF (or DMTTF)- p-chloranil. Successive lattice expansion due to the enlarged molecular size serves to lower the NI transition temperature. From the dielectric properties and the molecular ionicity, we demonstrate the novel examples of the NI transition, which is affected by the quantum fluctuation effect.

Optical spectroscopy of the neutral–ionic phase transition in a charge-transfer complex: TTF and tetrahalo- p-benzoquinones

Far-infrared reflectivity spectra have been investigated for a charge-transfer complex of tetrathiafulvalene and 2-bromo-3,5,6-trichloro- p-benzoquinone, which undergoes a neutral–ionic (N–I) transition at T c=68 K. With decrease in temperature toward T c, the far-infrared reflectivity spectrum is gradually transformed into a Drude-like high reflectance band edging around 100 cm −1, and the optical conductivity shows an anomalous peak around 10 cm −1. The far-infrared anomaly is related to the gigantic dielectric response just above T c, which are both discussed in terms of dynamics of N–I domain walls with fractional charge.

Electric field effect on charge and lattice dynamics in TTF-QCl 4 and TTF-QBrCl 3

Electroreflectance (ER) spectra are measured on the mixed-stack charge transfer (CT) complexes, tetrathiafulvalane (TTF)-2-bromo-3,5,6-trichloro- p-benzoquinone (QBrCl 3) and TTF- p-chloranil (QCl 4), showing the neutral–ionic (N–I) phase transition. In the N phase of TTF-QBrCl 3, we observe a large ER signal, which originates from the electric field-induced I domains in the N stacks. In contrast, no ER signal is obtained in the N phase of TTF-QCl 4. The difference of the ER signal intensity between the two compounds is discussed by taking account of the difference in the degree of the valence instability.

Temperature and pressure studies of the continuous neutral–ionic borderline in ClMePD–DMeDCNQI

Temperature- and pressure-induced neutral–ionic phase transitions in the mixed stack charge transfer crystal ClMePD–DMeDCNQI are studied and compared. Using vibrational spectroscopy, we show that the crossing of the neutral–ionic interface is different from previously studied cases. The ionicity change is continuous, and the onset of the stack dimerization precedes, rather than accompany, the neutral–ionic crossing. Moreover temperature- and pressure-induced phase transitions appear to be slightly different, suggesting that different mechanisms are involved.

Neutral to Ionic Pressure-Induced Phase Transition in the Mixed Stack Charge-Transfer Crystal ClMePD-DMeDCNQI

In the present work we report a spectroscopic investigation of the pressure-induced neutral to ionic (NI) phase transition in the mixed stack charge-transfer crystal 2-chloro-5-methyl-p-phenylenediamine 2,5-dimethyl-dicyanoquinonediimine, ClMePD-DMeDCNQI. The pressure induced phase change is studied by calculating, from spectroscopic data, the degree of ionicity, 𝜌 , which undergoes a continuous change as a function of pressure, reaching a value of about 0.65 at 3 GPa. The NI crossing reveals unique properties with respect to previously studied cases on similar compounds.

Ionic Mobility, Structure, Phase Transition, and Conductivity in (NH4)3Sb4F15

Fluoride and ammonium ion dynamics in (NH4)3Sb4F15 was studied by 1H and 19F NMR spectroscopy in the range 180-440 K. Types of ionic motion were determined, and their activation energies were estimated. The crystal structure of a single crystal of (NH4)3Sb4F15 (space group \(P\bar 1\)) was solved. In the range ∼\(430 \pm 2\) K, a reversible phase transition has been found. Based on the experimental values of conductivity \(\sigma \) of (NH4)3Sb4F15 (\(\sigma \) ≈ \(9 \cdot 10^{ - 3} \) S/cm at T = 440 K), this antimony(III) fluoride is classified as a superionic conductor.

Ionic Mobility, Phase Transitions, and Electric Conductivity in Ammonium Tetrafluoroantimonate and Heptafluorodiantimonate(III)

The dynamics of the fluoride and proton sublattices and the electrophysical properties of NH4SbF4 (I) and NH4Sb2F7 (II) in the temperature range 210-435 K were studied by 19F and 1H NMR and impedance spectroscopy. Types of ionic motion were determined and their activation energies were estimated. The structural phase transitions in I and II form the high-temperature modifications β-NH4SbF4 and β-NH4Sb2F7, having high ionic (superionic) conductivity in the range 425-435 K (σ∼1.9-1.5×10-3 S/cm).

Ferroelectric valence transition and phase diagram of a series of charge-transfer complexes of 4,4'-dimethyltetrathiafulvalene and tetrahalo-p-benzoquinones.

Variation of the ferroelectric phase transition has been investigated for a series of isomorphous donor (D)-acceptor (A) charge-transfer complexes composed of 4,4'-dimethyltetrathiafulvalene (DMTTF) and tetrahalo-p-benzoquinones by measurements of dielectric susceptibility, X-ray diffraction, and infrared molecular vibrational spectra. The neutral-ionic phase transition of DMTTF-p-chloranil at 65 K accompanies a dielectric peak anomaly associated with the DA stack dimerization. Successive halogen replacement by bromine in the component tetrahalo-p-benzoquinone molecule expands the lattice along the DA stack of the DMTTF complex, and reduces the critical temperature steeply toward zero temperature in an analogous way to the critical behavior of quantum ferroelectrics. The 2,6-dibromo-substituted compound showing the behavior of the quantum paraelectricity is located in the immediate vicinity of the quantum critical point of this phase diagram as signified also by enhancement of the ionicity, the dielectric susceptibility, and the dynamical dimeric distortion at the lowest temperature.

Effect of nanocrystalline alumina on ionic conductivity and phase transition in CsCl

Conductivity, structure and thermodynamic parameters of CsCl in (1− x)CsCl– xAl 2O 3 composites are investigated. As the heterogeneous dopant, highly-dispersed alumina with specific surface area 210 m 2/g is used. Maximum conductivity 9×10 −4 S/cm at 400°C occurs at x=0.6. As x rises, conductivities of bcc and fcc phases tend to level off and at x>0.6 no conductivity drop due to the phase transition occurs. The phase transition enthalpy decreases with x and at x>0.6 no bcc–fcc phase transition is detected by DSC technique. New peaks attributed to fcc-phase with the lattice parameter a=0.694 nm are observed on X-ray powder diffractograms. At high x conductivity enhancement is likely to be due to the presence of the high-temperature fcc phase of CsCl stabilized on CsCl–Al 2O 3 interfaces.

Photoinduced Cooperative Charge Transfer in Low-Dimensional Organic Crystals

We demonstrate that the cooperative charge transfer among constructive molecules in organic crystals of tetrathiafulvalene-chloranil, i.e., the bidirectional transition between the neutral and ionic phases, can certainly be induced by irradiation of a laser pulse with an 80-fs width. We also report the dynamics of the photoinduced neutral−ionic phase transition by means of the time-resolved spectroscopic technique. Studies using various excitation intensities have shown that the photoconverted fraction (Φ) depends nonlinearly on the excitation intensity. In addition, it has been found that the growth dynamics of the macroscopic domain of the metastable phase strongly depends on the excitation intensity as well. The role of the cooperative charge-transfer interaction in the driving process of this unusual photoeffect is discussed from both the experimental and the theoretical viewpoints.

Ionic conduction and structural phase transitions of Na 2SO 4 doped with various impurities

The metastable room-temperature phase III of Na 2SO 4 crystal transforms from III→I on heating and I→II→III on cooling. Phase II exists in a narrow temperature interval (less than 7°C). To obtain better microscopic understandings about the existence and nature of the intermediate phase II, electrical conductivity and Raman spectrum were measured for mixed crystals of Na 2(SO 4) 1− x (SeO 4) x and Na 2(1− x) Ag 2 x SO 4 with x≤0.1. On cooling, the intermediate phase II is stabilized in a wider temperature interval with the incorporation of guest ions. In the case of Na 1.8Ag 0.2SO 4, phase II persists as wide as ∼59°C. On heating, phase II, which is otherwise forbidden on heating in pure Na 2SO 4 crystal, reappears when x is large.

CONDENSATION OF SELF-TRAPPED CHARGE-TRANSFER EXCITATIONS AND INTERPLAY BETWEEN QUANTUM AND THERMAL EFFECTS AT THE NEUTRAL-TO-IONIC TRANSITION

The cooperativity between self–trapped electronic excitations is carried to extreme in the case of the neutral–to–ionic phase transition which is observed in some organic mixed–stack charge–transfer crystals. This electronic–structural phase transition manifests itself by a change of the degree of charge–transfer and a dimerization distorsion along the stacking axis in the ionic phase. Thermal charge–transfer excitations associated with the formation of structurally relaxed ionic strings along neutral chains are at the heart of mechanism of this uncommon phase transition. Symmetry and thermodynamics analysis of the neutral–to–ionic transition in the prototype compound, tetrathiafulvalene–p–chloranil, in particular the recent determination of the pressure–temperature phase diagram, make possible to present a consistent picture of this phase transition. Supported by a phenomenological approach, taking into account the quasi–one–dimensional nature of the system and the interplay between quantum and thermal effects, the experimental results show that the neutral–to–ionic transition results from the condensation and the ordering (crystallization) of charge–transfer excitations, following a phase diagram analogous to the solid–liquid–gas one.

Ionic motions and phase transitions in solid trimethylethylammonium tetrafluoroborate studied by 1H, 19F, and 14N NMR, powder X‐ray diffraction, and differential scanning calorimetry

Abstract1H, 19F, and 14N NMR, powder X‐ray diffraction, and differential scanning calorimetry (DSC) were studied for trimethylethylammonium tetrafluoroborate, (CH3)3NC2H5BF4. Five solid phases, named I, II, III, IV, and V in the order of decreasing temperature, were obtained in the temperature range 77‐630 K. The transition temperatures and the corresponding enthalpy changes determined by DSC were 222 K (0.16 kJ mol−1), 239 K (1.8 kJ mol−1), 293 K (2.1 kJ mol−1), and 362 K (4.5 kJ mol−1). X‐ray powder patterns showed that Phase I and II form an NaCl‐type cubic lattice (a = 10.14 Å, Z = 4) and a tetragonal lattice (a = 8.59, c = 6.24 Å, Z = 2), respectively. The NMR study revealed that the cationic orientation in Phase I and II is dynamically disordered. Moreover, since the self‐diffusion of anions and the isotropic rotation of cations were observed in Phase I, this phase can be considered as a mesophase very close to the plastic phase. In Phases II, III, IV, and V we have detected the isotropic reorientation of anions, the anisotropic tumbling of cations, the C'3 reorientation of the whole (CH3)3N group about C‐N bond axis, and the C3 reorientation of the CH3 groups. The motional parameters were evaluated for these cationic and anionic motions.