The physical properties of the sintered sample of Ga2O3(ZnO)9, a member of the homologous series Ga2O3(ZnO)m and recently found to have new structures, were investigated. The material was found to be a new transparent conducting oxide, and is composed of relatively abundant and inexpensive elements compared to indium. The electrical conductivity of a sintered low 57% density sample, 13 S cm−1 as fired at 1723 K, could be varied by postheating in air or a reducing gas (H2 3%:Ar 97%) flow. The changes in conductivity were associated with the variations of absorbance in the visible light range, while the optical band gap, or the absorption edge, was almost unchanged. The thermoelectric properties showed n-type behavior. It was relatively easy to vary the thermoelectric properties through redox treatments, and reversibility was also observed. The maximum figure of merit Z reaches a value of close to 10−4 K−1 at 660 K for a sample with density of only 73%. The potential of Ga2O3(ZnO)9 as a thermoelectric material appears to be similar to or even greater than the related In2O3(ZnO)m system, and since Ga2O3(ZnO)9 has an advantage in the abundance of constituent elements, it is revealed to be a promising system for further investigations.

Possible local structures of K ions in the tunnel of a hollandite superstructure KxMg(8 + x)/3Sb(16 − x)/3O16 (x ≈ 1.76) were presented, and the validity of the models was confirmed by the structure refinement using single-crystal X-ray diffraction data. Additional constraint conditions were introduced in refinements so that the average structure obtained from the refinement is consistent with assumed microscopic pictures. Joint-probability density functions were calculated for specific K ions concerning the hopping process between neighboring cavities, and converted to the one-particle potentials. Three types of the energy barrier for the hopping process were seen, which were uniformly reduced applying the anharmonic atomic displacement parameters.Research highlights► Possible local structures of K ions in a hollandite superstructure were presented. ► Validity of the models was confirmed by the single-crystal X-ray diffraction study. ► Transport properties of K ions were investigated from structural parameters obtained. ► Three types of the energy barrier for the hopping process were seen.

The structure of a one-dimensional (1d) K ion conductor KxMgx/2Ti8 − x/2O16 has been refined at 250 K and 150 K by a single-crystal X-ray diffraction technique. Probability density functions (PDFs) for the K ion were obtained from the structure model considering possible local structures in a hollandite 1d tunnel. The joint-PDFs were calculated for specific K sites concerning the hopping process in the tunnel. Temperature dependence of the joint-PDF was explained by a simple thermal activation, indicating no phase transition.

Single crystals of KxMg(8+x)/3Sb(16−x)/3O16 (x≈1.76) with a hollandite superstructure were grown. Ordering schemes for guest ions (K) and the host structure were confirmed by the structure refinement using X-ray diffraction intensities. The space group is I4/m and cell parameters are a=10.3256(6), c=9.2526(17)Å with Z=3. Superlattice formation is primarily attributed to the Mg/Sb occupational modulation in the host structure. Mg/Sb ratios at two nonequivalent metal sites are 0.8977/0.1023 and 0.1612/0.8388. Two types of the cavity are seen in the tunnel, where parts of K ions deviate from the cavity center along the tunnel direction. Probability densities for K ions in the two cavities are different from each other, which seems to have arisen from the Mg/Sb modulation.Mg/Sb occupational modulation in a superstructure of hollandite KxMg(8+x)/3Sb(16−x)/3O16 (x≈1.76) was clarified by the X-ray diffraction technique. The Mg/Sb ratio is 0.8977/0.1023 at the M1 site and 0.1612/0.8388 at the M2.

Phase relations of rutile, freudenbergite, and hollandite structures were examined in the pseudobinary system NaCrO2–TiO2 (i.e., NaxCrxTi8−xO16) at 1350 °C. The hollandite structure was obtained in the composition range 1.7⩽x  ⩽2.0. The symmetry of the samples at room temperature was tetragonal for x=1.7x=1.7 and 1.75, and monoclinic for x=1.8x=1.8 and above. Single crystals of monoclinic hollandite Na2Cr2Ti6O16 were grown and the structure refinement has been carried out using an X-ray diffraction technique. The space group was I2/m   and cell parameters were a=10.2385(11)a=10.2385(11), b=2.9559(9)b=2.9559(9), c=9.9097(11)Åc=9.9097(11)Å, and β=90.545(9)°β=90.545(9)° with Z=1Z=1. The Na ion distribution in the tunnel was markedly deformed from that in the tetragonal form. It was suggested that Cr/Ti ratios were different between the two framework metal sites.Difference Fourier map at the y=0.5y=0.5 section for Na2Cr2Ti6O16 with a structure model containing no Na ions.

Molecular dynamics simulations have been performed to investigate the temperature dependence of static and dynamical properties of Na+ ions in a one-dimensional tunnel of the hollandite structure. Structural data including the Na+ ion distribution obtained from diffraction experiments were well reproduced at room temperature and 773 K. Microscopic pictures and Fourier transforms of the velocity autocorrelation function clarified the diversity in dynamical behavior of Na+ ions in a range between 300 and 1273 K. The difference between Ti–Na and Cr–Na pair correlation functions, suggesting the tendency of Na+ ions to be closer to Cr3+ rather than Ti4+ in the framework structure, was significant up to at least 1273 K. Correlation of ionic motions is so strong at high temperature as to cause the collective translation of Na+ ions in a few tunnels at 773 K and above.