Quantum Beam Center, National Institute for Materials Science
Na-hollandite is a candidate for the one-dimensional Na ion conductor, as the frequency-independent conduction was observed by impedance spectroscopy measurements . In the present study, hollandite structures were obtained in the pseudobinary system NaCrO2-TiO2 (i.e. NaxCrxTi8-xO16) of the composition range 1.7=<x<=2.0 at 1350 C. The symmetry of the samples at room temperature was tetragonal for x=1.7 and 1.75, and monoclinic for x=1.8 and above. Furthermore, 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), b=2.9559(9), c=9.9097(11)Å, and beta=90.545(9). There are three characteristics in Na2Cr2Ti6O16 related to tetragonal-monoclinic transformation of hollandite structures; (i) distortion of the framework structure, (ii) deformation of the Na ion distribution in the tunnel, and (iii) different Cr/Ti ratios between the two metal sites in the framework structure. The first one is common in hollandite structures showing the tetragonal-monoclinic transformation. The second one was observed in the low-temperature phase of NaxCrxTi8-xO16 (x=1.7)  and more prominent in Na2Cr2Ti6O16. The third one was first pointed out in this study.
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