Yoshitaka Michihiro

On the diffusion of Li+ defects in LiCoO2 and LiNiO2

Abstract NMR spectra of 7 Li nucleus have been measured in LiCoO 2 and LiNiO 2 . Line widths of the spectra decreased with increasing temperature above 400 K. The decrease obviously indicates the motional narrowing originating from diffusing Li + defects. A simple ion hopping model revealed that the diffusion constant of LiCoO 2 with an activation energy of 0.3 eV is larger than that of LiNiO 2 with the activation energy of 0.6 eV. This suggests that lithium ions are easier to diffuse in LiCoO 2 than in LiNiO 2 at high temperatures.

Temperature Dependence of the Nuclear Quadrupole Relaxation Rate in Silver and Sodium Halide Crystals.

The results of the temperature dependence of nuclear relaxation rate, 1/ T 1 measured for 23 Na, 35 Cl and 79 Br in AgX and NaX with X being Cl and Br are interpreted in terms of the quadrupole relaxation due to lattice vibrations and defect motions. The data of 1/ T 1 in the low temperature region are compared with a model calculation based on an ionic model, in which the degree of covalency is estimated. The result is 24.5% in AgBr and 20.5% in AgCl, which are found to be an order of magnitude larger than the values in sodium halides. On the other hand, from an analysis of 1/ T 1 data in the high temperature region, the migration and formation energies of defects are evaluated and compared with the results of previous conductivity measurements. The obtained activation energies for migration are 0.27 eV in AgBr, 0.23 eV in AgCl and 0.60 eV in NaCl.

Nuclear magnetic relaxation and ionic conductivity in LiNaSO4

The temperature dependence of the spin-lattice relaxation time for 7 Li has been measured in a single crystal of LiNaSO 4 over the temperature range of 450 K–770 K. The result can be explained by a mechanism of vacancy diffusion. The activation energies for the hopping of Li + and Na + vacancies are estimated to be E 1 =0.70 eV and E 2 =1.20 eV, respectively. In addition, the ionic conductivity has been measured as a function of frequency from 500 Hz to 20 MHz and the value of 1.59 eV is deduced as the activation energy, E m for the ionic diffusion in the extrinsic region. These values of E 1 , E 2 , E m are well compared with the corresponding values of 0.70 eV, 1.24 eV and the intermediate value between 1.74 eV for Li + and 1.90 eV for Na + ions evaluated from the previous NMR measurements.

Ultrasonic Measurements in the Ionic Conductor β-Ag3SI

The velocities of 10 MHz-longitudinal and 15 MHz-transverse waves have been measured by a pulse-echo-overlap method from the transition temperature (157 K) to 500 K in a polycrystal of β-phase Ag 3 SI. The attenuation measurements of longitudinal waves have also been done at 10, 30 and 50 MHz over the temperature range 157 K–300 K. The results are compared with a theory taking into account the coupling between the pseudospin fluctuations and the acoustic phonons. The activation energy of 0.15 eV and the attempt frequency of 1.8×10 12 Hz are obtained for the hopping of mobile silver ions.

Changes in NMR spectra and knight shifts due to the CDW transition in CuV2S4

Abstract The NMR Spectrum and Knight shift of 51 V, 63 Cu and 65 Cu were measured in a copper sulfide-spinel CuV 2 S 4 . Abrupt changes in the line widths and in the Knight shifts were observed at about 90 K. The Cu NMR signals disappear suddenly below 83 K. These facts are considered to be due to the formation of a charge density wave (CDW) at 90 K. They are explained by the model that an energy gap is formed and the 3 d electrons with opposite spins are trapped in each CDW wavelength. The spatial distribution of the electric field gradient is increased drastically by the CDW at 90 K.

NMR investigation on the motion of Li+ defects in LiCoO2 and LiNiO2

Abstract The spin–lattice relaxation time, T 1 , for 7 Li has been measured in LiCoO 2 and Li x NiO 2 ( x =1, 0.5) from 77 K to 680 K. The data in the high temperature region are interpreted by the relaxation mechanism of the diffusive motion of Li + ion defects. The activation energy for the diffusion of the Li + in LiCoO 2 and LiNiO 2 is estimated to be almost same value, 0.08 eV. From the analysis based on the fluctuating field model (BPP model) with the diffusion of Li + defects, the dominant mechanism of the spin–lattice relaxation in LiCoO 2 would be the quadrupole interaction at the high temperature region. On the other hand, the relaxation in LiNiO 2 would be governed by the magnetic dipole interaction between Li nucleus and paramagnetic Ni ions in Li layer.

Ultrasonic Attenuation of β-Ag3SI

The ultrasonic attenuation of ionic conductors is studied in a model taking into account the mutual coupling between mobile ions in addition to the coupling between mobile ions and lattice ions. The result is compared with the experiments in a polycrystal β-Ag 3 SI. The measurements are taken at 10, 30, 50 and 70 MHz for longitudinal waves from room temperature to the β-γ transition temperature employing a standard pulse echo technique. The activation energy and the attempt frequency are obtained for the hopping of mobile Ag + ions. The authors also estimate the magnitude of the interaction parameter of mutual coupling between mobile ions relative to that between lattice ions and mobile ions.

11B Nuclear Spin-Lattice Relaxation in Itinerant Weak Antiferromagnetic SmRh3B2

11 B nuclear spin-lattice relaxation time T 1 has been measured in order to study the anomalous properties of SmRh 3 B 2 from the microscopic viewpoint. No anomaly has appeared at the temperature where the electric resistivity changes drastically. The temperature dependence of 1/ T 1 T in the paramagnetic state ( T >100 K), where the uniform magnetic susceptibility hardly changes with temperature, has been found to show the typical weak antiferromagnetic case of itinerant electrons.

NMR study on ionic transport in silver halides

Abstract The temperature dependence of 1/ T 1 , for 23 Na, 35 Cl, 79 Br and 81 Br in AgX and NaX ( X = Cl , Br ) has been measured from 77 K to 700 K, and the results of AgX are compared with those of NaX. With the Bloembergen-Purcell-Pound model, we obtain the activation energies from which the effective radius of the mobile Ag + ion is evaluated to be of the order of 1 A.

NMR study of mobile ions in CuxSe and CuxS

Abstract The nuclear spin-lattice relaxation time T 1 has been measured for 63 Cu nucleus in Cu x Se and Cu x S (1.8≦×≦2. 0) with a standard pulse NMR technique. The measurements have been done at 8.0 MHz from 300 K to 700 K. The results are compared with the previous data of NMR line width and the ionic conductivity, and interpreted in terms of the activation energy E for diffusive copper ions and the formation energy E 0 for the defects. Almost the same values are obtained for both E and E 0 on measured compounds, which suggests that the diffusion mechanism is essentially the same for Cu x Se and Cu x S in the high temperature phase.