W. Medycki

Structure, phase transitions and molecular motions in 4-aminopyridinium perchlorate

The crystal structure of the 4-aminopyridinium perchlorate (4-apyH)ClO4 has been determined at 100 K by means of x-ray diffraction as monoclinic, with space group P 21, with Z = 8. The crystal undergoes two structural phase transitions: one of first-order type, reversible, at 241/243 K (on cooling/heating respectively) and one of weakly first-order type, irreversible, at 277 K (on heating). The crystal dynamics is discussed on the basis of the temperature dependence of the 1 H nuclear magnetic resonance second moment (M2) and spin–lattice relaxation time T1. Both phase transitions are interpreted in terms of the changes in the motional state of (4-apyH)+ cations and ClO4− anions. The dielectric dispersion studies disclose a relaxation process over the high-temperature phase (above 241 K) in the audio-frequency region. The dielectric results are described by a Cole–Cole equation. The title crystal reveals pyroelectric properties below 241 K. The ferroelastic domain structure of (4-apyH)ClO4 is observed over the whole temperature range studied.

Structure, phase transition and molecular motions in (C5H5NH)BiCl4

The crystal structure at 293 K of the new pyridinium compound, (C5H5NH)BiCl4, has been determined by X-ray diffraction as monoclinic, space group Cc, Z = 4. The crystal is built up of one-dimensional (BiCl4−)n chains and pyridinium C5H5NH+ cations. A structural phase transition of first-order type is detected by differential scanning calorimetry (DSC) and dilatometric measurements at 114/110 K (on heating–cooling, respectively). Proton NMR second moment and spin–lattice relaxation time studies confirmed the order–disorder mechanism of the phase transition at 110 K. It was connected with the reorientational motion of the pyridinium cation. Dielectric investigations reveal absorption and dispersion in the audio-frequency region in both the high and low temperature phases. The experimental results were analysed in the high temperature phase in terms of the Havriliak–Negami formula. In the low temperature phase the Cole–Cole relation for a single relaxator was applied. Above the phase transition point the macroscopic relaxation time exhibits non-Arrhenius behaviour, whereas below Tc it is perfectly described by a pure Arrhenius relation.

Synthesis, crystal structure and phase transitions of a series of imidazolium iodides

The reaction of imidazole with hydroiodic acid leads to three products crystallizing as ionic salts; [C3N2H5+][I−], [C3N2H5+]2[I42−] and [C3N2H3I2+][I−]. All the analogs were characterized by single-crystal X-ray diffraction, while the first two were additionally studied by calorimetric, dilatometric, dielectric and proton magnetic resonance methods. At room temperature (RT), [C3N2H5+][I−] adopts the centrosymmetric, trigonal space group (R). The crystal structure consists of disordered imidazolium cations and discrete I− ions. [C3N2H5+][I−] undergoes two discontinuous phase transitions (PTs) at 180/185 K and 113/123 K (cooling–heating), both of them governed by the imidazolium cation dynamics. [C3N2H5+]2[I42−] consists of disordered imidazolium cations and quite rare and exotic [I4]2− tetraiodide counterion. It undergoes continuous PT at 204 K of the ferroelastic type with a symmetry change from orthorhombic Fddd to monoclinic C2/c. The mechanism of PT is complex and consists of ‘order–disorder’ and ‘displacive’ contributions that are assigned to the dynamics of cations and to the distortion of the [I42−] rods, respectively. [C3N2H3I2+][I−] is built up of discrete 4,5-diiodoimidazolium cations and isolated I− ions. A characteristic feature of this compound is the presence of a layered structure in which moieties are held together by strong I⋯I halogen interactions and N–H⋯I hydrogen bonds.

Field cycling methods as a tool for dynamics investigations in solid state systems: recent theoretical progress.

In this paper physical mechanisms and theoretical treatments of polarization transfer and field-dependent relaxation in solid state systems, containing mutually coupled spins of spin quantum numbers I=12 (spins 12) and S1 (quadrupolar spins), are presented. First, theoretical descriptions of these effects are given in detail for an illustrative, simple system. Next, it is shown how to generalize the theories to much more complex spin systems. The polarization transfer and relaxation effects are illustrated by several examples. Typical misunderstandings regarding their physical origins are clarified. This paper reviews recent theoretical descriptions of the polarization transfer and relaxation phenomena. Its goal is to popularize the proper theoretical treatments with the intention to establish them as standard tools for analyzing field cycling data.

Structure–property relationships in hybrid (C3H5N2)3[Sb2I9] and (C3H5N2)3[Bi2I9] isomorphs

Two hybrid crystals imidazolium iodoantimonate(III) and iodobismuthate(III), (C3H5N2)3[Sb2I9] (ImIA) and (C3H5N2)3[Bi2I9] (ImIB), have been synthesized and characterized in a wide temperature range (100–350 K) by means of X-ray diffraction, dielectric spectroscopy, proton magnetic resonance (1H NMR), FT-IR spectroscopy and optical observations. They undergo two temperature induced solid–solid structural phase transitions. The first one, quasi-continuous (with temperature hysteresis below 1 K), occurs at 324 K in ImIA and 327 K in ImIB, and the second one, clearly of the first order, at 273/278 (cooling/heating) and 291/295 K, in ImIA and ImIB, respectively. Ferroelastic properties are maintained in low-temperature phases. Both materials are isomorphic in the corresponding phases. High temperature phase I has a hexagonal P63/mmc symmetry, and phase II has orthorhombic Cmcm. The crystal architecture is composed of discrete, face-sharing bioctahedra [M2I9]3− (M: Sb, Bi) and imidazolium cations which are highly disordered over phases I and II. The dynamics of the imidazolium cations has a prominent impact on the stability of the particular phases.

Dynamical disorder in 2-methyl-4-nitroaniline and its deuterated analogue crystals studied by Fourier transform infrared and nuclear magnetic resonance.

The Fourier transform infrared spectra of the thin layers of 2-methyl-4-nitroaniline (MNA) and its deuterated analog were recorded in the 500-4000 cm(-1) region in the 10-300 K temperature range. Activation energies of the -CH(3), -NH(2), and -NO(2) groups reorientations were estimated. The (1)H-NMR spin-lattice relaxation time, T(1), and the second moment of (1)H-NMR resonance line, M(2), measured in the 80-298 K temperature range, were used to determine the parameters of the -CH(3) group motion. The experimental potential barriers for the amine, nitro, and methyl group reorientations are considered in the context of strengths of the N-H([ellipsis (horizontal)])O, C-H([ellipsis (horizontal)])O intermolecular hydrogen bonds, and other short contacts, recognized recently [U. Okwieka et al., J. Raman Spectrosc. 39, 849 (2008)], and they agree with the barriers calculated by quantum chemical methods. The dynamical disorder found in the MNA crystal in the large temperature range seems to be important from the point of view of its nonlinear optical and other properties.

The structure, phase transition and molecular dynamics of [C(NH2)3]3[Sb2Br9]

The crystal structures of [C(NH2)3]3[Sb2Br9] (Gu3Sb2Br9) at 300 K and of [C(NH2)3]3[Sb2Cl9] (Gu3Sb2Cl9) at 90 and 300 K are determined. The compounds crystallize in the monoclinic space group: C 2/c. The structure is composed of Sb2X93− (X = Cl, Br) ions, which form two-dimensional layers through the crystal, and guanidinium cations. In Gu3Sb2Br9 the structural phase transformation of the first-order type is detected at 435/450 K (on cooling/heating) by the DSC and dilatometric techniques. The dielectric relaxation process in the frequency range between 75 kHz and 5 MHz over the low temperature phase indicates reorientations of weakly distorted guanidinium cations. The proton 1H NMR second-moment and spin–lattice relaxation time, T1, temperature runs for the polycrystalline Gu3Sb2Br9 sample indicate a complex cation motion. A significant dynamical non-equivalence of two guanidinium cations was found. The possible mechanism of the phase transition in Gu3Sb2Br9 is discussed on the basis of the results presented.

Structure and properties of [2-NH2C5H4NH][SbCl4] and [2-NH2C5H4NH][SbBr4]

The crystal structures of [2-NH 2 C 5 H 4 NH][SbCl 4 ] (2-APyHSbCl 4 ) and [2-NH 2 C 5 H 4 NH][SbBr 4 ] (2-APyHSbBr 4 ) are determined at 100 K. Both compounds crystallize in the monoclinic space group: P2 1 /c. The structure is composed of SbX - 4 (X = Cl, Br) ions which form infinite chains through the crystal via halogen linkages. The structural phase transformations are detected by the differential scanning calorimetry and dilatometric techniques: at 402 K, close to continuous, and at 412 K, clearly discontinuous, in 2-APyHSbCl 4 and 2-APyHSbBr 4 , respectively. Dielectric relaxation studies in the frequency range between 1 kHz and 25 MHz indicate reorientations of the 2-aminopyridinium (2-APyH) cations in both compounds in the low temperature phases. The proton NMR second moment, M 2 , and spin-lattice relaxation time, T 1 , for 2-APyHSbCl 4 and 2-APyHSbBr 4 measured between 78 and 430 K reveal the in-plane reorientation of the 2-APyH cations. The possible mechanism of the phase transitions in the title crystals is discussed on the basis of the results presented.

Polar and antiferroelectric behaviour of a hybrid crystal – piperazinium perchlorate

Monoprotonated piperazinium perchlorate, [NH2(CH2)4NH][ClO4], appeared to be a novel room temperature polar material (P1). Its acentric symmetry was confirmed by single-crystal X-ray diffraction, second harmonic generation (SHG) and pyroelectric measurements. Differential scanning calorimetry (DSC) measurements revealed a complex sequence of phase transitions above room temperature: I ↔ II at 433/422 K (heating–cooling), II ↔ III at 417/411 K, III ↔ IV at 403/395 K and IV ↔ V at 397 K (the lowest temperature phase transition recorded only upon heating). The characteristic feature of the structure of [NH2(CH2)4NH][ClO4] is the presence of two parallel cationic chains which are connected with each other by strong N–H⋯N hydrogen bonds. In phase V, these strongly polar non-equivalent chains contribute to spontaneous polarization. 1H NMR measurements disclosed the reorientational motions of the piperazinium ([NH2(CH2)NH]+) cations as well as the proton motion in the N–H⋯N hydrogen bonds along the piperazine chain. Over phase I, the overall motions of the ClO4− anions and reorientational motion of cations are postulated. The dielectric response, e′(T), accompanying the PT I ↔ II indicates possible antiferroelectricity in phase II.

Complex molecular dynamics of (CH3NH3)5Bi2Br11 (MAPBB) protons from NMR relaxation and second moment of NMR spectrum.

Molecular dynamics of a polycrystalline sample of (CH(3)NH(3))(5)Bi(2)Br(11) (MAPBB) is studied on the basis of the proton T(1) (55.2 MHz) relaxation time and the proton second moment of NMR line. The T(1) (55.2 MHz) was measured for temperatures from 20K to 330 K, while the second moment M(2) for those from 40K to 330 K. The proton spin pairs of the methyl and ammonium groups perform a complex stochastic motion being a resultant of four components characterised by the correlation times τ(3)(T), τ(3)(H), τ(2), and τ(iso), referring to the tunnelling and over the barrier jumps in a triple potential, jumps between two equilibrium sites and isotropic rotation. The theoretical expressions for the spectral densities in the cases of the complex motion considered were derived. For τ(3)(H), τ(2), and τ(iso) the Arrhenius temperature dependence was assumed, while for τ(3)(T)-the Schrödinger one. The correlation times τ(3)(H) for CH(3) and NH(3) groups differ, which indicates the uncorrelated motion of these groups. The stochastic tunnelling jumps are not present above the temperature T(tun) at which the thermal energy is higher than the activation energy of jumps over the barrier attributed to the hindered rotation of the CH(3) and NH(3) groups. The T(tun) temperature is 54.6 K for NH(3) group and 46.5 K for CH(3) group in MAPBB crystal. The tunnelling jumps of the methyl and ammonium protons are responsible for the flattening of T(1) temperature dependence at low temperatures. The isotropic tumbling is detectable only from the M(2) temperature dependence. The isotropic tumbling reduces the second moment to 4 G(2) which is the value of the intermolecular part of the second moment. The motion characterised by the correlation time τ(2) is well detectable from both T(1) and M(2) temperature dependences. This motion causes the appearance of T(1) minimum at 130 K and reduction of the second moment to the 7.7 G(2) value. The small tunnelling splitting ω(T) of the same value for the methyl and ammonium groups was estimated as 226 MHz from the Haupt equation or 80 MHz from the corrected by us Haupt equation. These frequencies correspond to 0.93 μeV and 0.34 μeV tunnel splitting energy.