Experimental studies of structurally incommensurate crystal phases

Magnetic resonance and relaxation in structurally incommensurate systems

NMR-NQR studies of structural phase transitions

Dielectric relaxation and electret polarisation effects in KTaO3:Li have been investigated as a function of Li concentration. The data reveal the presence of polarised clusters which are stabilised by Li impurities and permit a determination to be made of the coherence length for ferroelectric order in KTaO3. Based on the polarisation results, the KTaO3:Li system exhibits characteristics which indicate that it represents an electric dipole analogue of a spin glass and therefore should be termed a 'polar glass'.

Stabilisation of polarised clusters in KTaO3 by Li defects: formation of a polar glass

A comprehensive NMR−NQR study of carbon monoxide enriched 24.5% in 17O (I = 5/2) is reported, which provides interesting information on the motions of the CO dipoles in the liquid and in the a and b solid phases. In the liquid phase the 17O NMR spin‐lattice relaxation due to the tumbling rotational motions of the molecular axis, combined with the experimental evaluation of the rigid electric field gradient (EFG) at the 17O site has been utilized to achieve a direct and precise measurement of the reorientational correlation time of the dipole and its temperature dependence. In the b phase, the quadrupole perturbed NMR spectra and the spin‐lattice relaxation indicate the occurrence of a precessional motion of the dipole around the c axis. An effective EFG is consequently present, whose order of magnitude is about 103 smaller than the static one. The precessional correlation time is derived, which is surprisingly very close to the reorientational correlation time of the liquid phase, suggesting a quasifree rotation character for the precession. 17O NQR signals in the a phase have been detected and resonance frequencies and linewidths have been measured. From the temperature behavior of the NQR frequency, information on the local order parameter for the librational motion of the dipoles is obtained. The linewidth exhibits a peculiar temperature dependence, with a maximum around 37 K and a dramatic divergence for T→29 K. While an orientational static disorder could be expected in view of the residual entropy, it is shown how the linewidth, as well as some pure quadrupole relaxation measurements, seem to indicate the occurrence in the a phase of a dynamical disorder, with sudden head‐to‐tail reorientations of the dipole, at least for T≳29 K. In the framework of such a dynamical model, a theoretical picture for the motional contribution to the NQR allows one to obtain insights on the features of the orientational disordering processes. The hypothesis of a phase transition of antiferroelectric type below T?37 cannot be ruled out. Finally, some evidence of softening of the libronic frequencies on approaching the a–b phase transition is obtained.

17O NMR–NQR study of molecular motion in the liquid, and α and β solid phases of CO

$^{14}\mathrm{N}$ nuclear quadrupole resonance and relaxation measurements in NaN${\mathrm{O}}_{2}$ around the ferroelectric transitions have been performed by pulse and Fourier-transform technique, with high-temperature resolution and accuracy In the narrow temperature range in which the antiferroelectric phase exists and where previous authors failed to detect the nuclear-quadrupole-resonance (NQR) signals, the splitting of the ${\ensuremath{\nu}}_{+}$ and ${\ensuremath{\nu}}_{\ensuremath{-}}$ lines in two components were observed A theory is developed to connect the NQR parameters to the torsional vibrations of the N${\mathrm{O}}_{2}^{\ensuremath{-}}$ ions as well as to the order-disorder reorientational fluctuations driving the transitions The critical fluctuations are described through a kinematical Ising model It is proven that the reorientational order-disorder variable is reversed by an angle which is not exactly $\ensuremath{\pi}$ By analyzing the experimental results in the light of the theoretical treatment interesting information on the critical dynamics is obtained

N 14 NQR and relaxation at the ferroelectric transition in NaN O 2

EPR study of arene-solvated cobalt clusters as catalysts in the co-cyclization of α,ω-diynes and nitriles

The temperature dependence of the intensity, frequency, and line shapes of the $^{14}\mathrm{N}$ nuclear-quadrupole-resonance spectrum in potassium tetracyanoquinodimethane (K-TCNQ) showed that the Peierls transition consists of a complex set of transformations that on heating leads to a first-order transition at 130 \ifmmode^\circ\else\textdegree\fi{}C with hysteresis. Above 130 \ifmmode^\circ\else\textdegree\fi{}C, the high-temperature (HT) phase containing fluctuating charge-density waves (CDW) becomes incommensurate, transforms into a multisolitonic phase with narrow solitons above 141.5 \ifmmode^\circ\else\textdegree\fi{}C, and near the decomposition temperature changes towards the single-soliton limit. On cooling from above 150 \ifmmode^\circ\else\textdegree\fi{}C, K-TCNQ transforms directly from the HT multisolitonic to a HT commensurate phase at 141.4 \ifmmode^\circ\else\textdegree\fi{}C that coexists with the low-temperature phase from 128 \ifmmode^\circ\else\textdegree\fi{}C but only disappears below 90 \ifmmode^\circ\else\textdegree\fi{}C. The previously undetected transition observed at 141.5 \ifmmode^\circ\else\textdegree\fi{}C on heating and cooling is attributed to a change in the effective dimensionality in the ordering of the CDW's along the TCNQ stacks. The complex set of transitions found in the fully ionic K-TCNQ showed that the Peierls transition is likely to be a two-step process even in simple salts as predicted theoretically by Zhou and Gong [J. Phys. C 21, L561 (1988)].

N 14 nuclear-quadrupole-resonance study of solitons and charge-density waves during the Peierls transition in potassium tetracyanoquinodimethane

The relevance of the nuclearity of the metal in the cocyclization of α, ω-diynes and nitriles catalyzed by mesitylene solvated cobalt atoms; preliminary EPR studies

Arturo Colligiani

Papers

N 14 NQR and relaxation at the ferroelectric transition in NaN O 2

EPR study of arene-solvated cobalt clusters as catalysts in the co-cyclization of α,ω-diynes and nitriles

N 14 nuclear-quadrupole-resonance study of solitons and charge-density waves during the Peierls transition in potassium tetracyanoquinodimethane

The relevance of the nuclearity of the metal in the cocyclization of α, ω-diynes and nitriles catalyzed by mesitylene solvated cobalt atoms; preliminary EPR studies