Yoshito Onoda

ac ionic conductivity of hollandite type compounds from 100 Hz to 37.0 GHz

Abstract The frequency- and temperature-dependences of a.c. ionic conductivity of one-dimensional super-ionic conductors K-priderites with a hollandite type structures were investigated from 100 Hz to 37.0 GHz. Four kinds of K-priderite, K 1.6 Mg 0.8 Ti 7.2 O 16 , (K 1.3 , Li 0.1 ) Mg 0.7 Ti 7.3 O 16 , K 1.6 Al 1.6 Ti 6.4 O 16 and (K 1.3 , Li 0.2 ) Al 1.5 Ti 6.5 O 16 , were studied. An equivalent circuit to combine the data of the complex conductivity at low and high frequencies was proposed. The data of complex conductivity at low frequencies can be analyzed in terms of the moving box model proposed by Beyeler et al. The transport of K + ions at low frequencies is characterized by the cooperative motion of the K + ions with various mobilities and is accompanied with the polarization of the K + ions in the channels. The ion transport across intrinsic barriers at or above microwave frequencies is characterized by the frequency-independent ionic conductivity and is interpreted by the configurational model proposed by Beyeler et al. The height of intrinsic barriers is related to the lattice constants of a crystal.

Frequency-independent ionic conductivity of hollandite type compounds

Abstract The frequency- and temperature-dependences of a.c. ionic conductivity of one-dimensional superionic conductors K-priderites, K 1.6 Mg 0.8 Ti 7.2 O 16 (KMTO) and K 1.6 Al 1.6 Ti 6.4 O 16 (KATO), which were grown by using a flux with high grade purity, were investigated from 100 Hz to 10 MHz. The complex ionic conductivity along a c-axis was analyzed by using an equivalent circuit constructed by considering the frequency-dependent ionic conductivity σ mb and the frequency-independent ionic conductivity σ dc which was not observed for samples grown by using a flux with normal grade purity. The σ dc was thermally activated and the activation energies for KMTO and KATO were 0.26 and 0.30 eV respectively. The lateral conductivity was very small.

Growth and characterization of potassium- and rubidium-priderite single crystals with hollandite-type structure

Abstract Single crystals of K- and Rb-priderites, K( or Rb) x (B y Ti 8− y O 16 ), which is a one-dimensional ionic conductor were grown from a flux melt of the system K 2 O(or Rb 2 O)-MoO 3 . Chemical compositions, lattice parameters, IR spectra and the habit of the grown crystals are measured. The framework size of tunnel structure, which has remarkable influence on the ionic conductivity, is controlled by the ionic size of the B-elements such as Mg, Zn, Al, Ga and Nb. The variation of the framework size is observed by the change of a-axes and the shift of IR spectra. The influence of the Na ion in the tunnel is also observed to be an impurity barrier for conductive motion of the K ion.

Frequency dependence of ionic conductivity in one-dimensional ionic conductors K1.6Mg0.8Ti7.2O16 and K1.6Al1.6Ti6.4O16

Abstract Ion conduction in one-dimensional cationic conductors potassium priderite single crystals K 1.6 Mg 0.8 Ti 7.2 O 16 (KMTO) and K 1.6 Al 1.6 Ti 6.4 O 16 (KATO) was studied by impedance spectroscopy between 10 Hz to 99 GHz. The real part of the complex conductivity is approximately constant in the microwave frequency region. It is found that two partial thermally activated conduction processes contribute to the ion conduction at microwave frequencies. One corresponds to the ion conduction process across the perturbed intrinsic barriers with a period equal to the lattice constant along a c -axis.

Ion conduction in one-dimensional ionic conductors A1−xTi2+xB5−xO12(ATBO, ANa or K and BAl or Ga), x<1)

Ion conduction in one-dimensional alkali ionic conductors A1−xTi2+xB5−xO12(x<1) (ATBO, e.g. NTAO for ANa and BAl and KTGO for AK and BGa) has been studied in the frequency region between 100 Hz and 32.55 GHz. KTGO is a newly synthesized material. It is isostructural with NTAO which has been determined by another worker. Both the frequency-dependent and frequency-independent high ionic conductivities have been observed. The frequency dependence of the complex conductivity was almost the same as that of KMg-priderite with hollandite-type structure due to the existence of bottlenecks in tunnels; it was analyzed by Bernasconis random barrier model. Analytical results show that the attempt frequency may be dominated by the coupling of the vibration of mobile ions with the lattice vibration (Au mode) of the tunnel framework. The frequency independent ionic conductivity was thermally activated and the activation energy was 0.27 eV for NTAO and KTGO.

NMR study of one-dimensional ionic conductor with hollandite-type structure. II. Frequency dependence of spin-lattice relaxation time of 27Al

Abstract Frequency dependence of spin-lattice relaxation time T 1 of 27 Al in one-dimensional K + ion conductor, K-Al-priderite, was measured at 45 K in the frequency range from 10.1 MHz to 55 MHz. It is found that T 1 is proportional to ω 1.49±0.05 and agrres well with the ω 3 4 dependence derived by the continuum diffusion model. The intrinsic activation energy is determined to be 0.058 eV by doubling the slope E NMR =0.029 eV of the d(ln T 1 )/d T curve in the low temperature region. The frequency dependence of T 1 in the high temperature region measured in the frequency range from 11.5 MHz to 20.8 MHz shows a tendency that the frequency dependence becomes smaller than the ω 1 2 dependence as temperature is raised above 450 K.

Ionic conduction of new one-dimensional ionic conductors with large tunnels: Ax[Ga8Ga8+xTi16−xO56] (A=K, Rb, OR Cs, x≤2)

Ionic conduction in new one-dimensional (1-D) alkaline ionic conductors A x [Ga 8 Ga 8+ x Ti 16− x O 56 ] (AGGTO, A=K, Rb and Cs, x≤2 ) is studied by measuring the complex ionic conductivity between 100 Hz and 32.55 GHz. AGGTO has a tunnel structure with a smooth inside wall lacking bottlenecks . In the low-frequency region, the real and imaginary parts of the complex ionic conductivity indicate the dispersion due to the blocking effects by large impurity ions in tunnels on the motion of mobile ions. In the high-frequency region, the ionic conductivities at room temperature are large (e.g. ≈8 S/cm for RGGTO). Negative values of the dieletric constants for RGGTO and CGGTO are observed at microwave frequencies. The results show that ionic conduction in the microwave frequency region is dominated by collisions between mobile ions in the same tunnel. The ionic conduction in AGGTO can be explained in terms of the classical ion transport model.

Silver ionic conductor Ag9GaSe6 studied by Ag and Ga NMR

Abstract Temperature dependence of 109 Ag, 107 Ag, 71 Ga and 69 Ga NMR line shapes and relaxation times ( T 1 and T 2 ) have been measured for the ionic conductor Ag 9 GaSe 6 . In the high temperature cubic phase (Phase I), which is known to show X-ray diffuse scattering, line widths of 109 Ag signals have suggested that the motional narrowing is completed. T 1 of the all nuclei increases with increasing temperature, and T 1 , of 109 Ag and 107 Ag are independent of the magnetic field strength. The experimental T 1 ratio of 107 Ag to 109 Ag and the magnetic field independence of T 1 in the fast motion limit can be explained by the scalar coupling of first kind in terms of an exchange interaction. The temperature dependences of T 1 of mobile 109 Ag and 107 Ag are weaker than those of immobile 71 Ga and 69 Ga. 71 Ga and 69 Ga signals have shown that a new middle temperature phase (Phase II) exists in a narrow temperature range around 282 K. In this phase, Ag signals show the spectral width of 200 ppm. X-ray diffraction patterns have also shown diffuse scattering, and have been indexed on the basis of the trigonal lattice. In the low temperature cubic phase (Phase III), Ag signals have shown the chemical shift anisotropy width of which is about 1000 ppm.

Ionic conduction in (K,Rb)Alpriderites with hollandite structure

Abstract Dynamics of mobile ions in systems of priderites (K, Rb)Alpriderites with di-mobile species (DMS-priderites), (K 1−y ,Rb y ) x Al x Ti 8−x O 16 (x∼1.5 and y between 0 and 1) have been studied by analyzing data of the complex conductivity between 100 Hz and microwave frequency. The theoretical substructure of this study is a moving box model proposed by Beyeler to interpret data for KMgpriderite doped with a small amount of Cs as guest ions. Result shows that the complex conductivity function derived in terms of the improved moving box model is adequate even for the DMS-system with a large amount of Rb.

Ionic conduction in CuxAg1−xBrTe solid solutions

The ionic conduction of cation substituted CuBrTe, CuxAg1−xBrTe solid solutions, is investigated in the temperature range between 100 and 400 K. The change of electrical conductivities with temperature in the range of 0.94≤x<1 shows the characteristics of second-order rather than first-order phase transitions. The replacement of Cu with the Ag cation in CuxAg1−xBrTe leads to an increase in conductivity especially in the β and γ-phases.