Phase relations in NaxCrxTi8-xO16 at 1350°C and crystal structure of hollandite-like Na2Cr2Ti6O16

Phase relations of rutile, freudenbergite, and hollandite structures were examined in the pseudobinary system NaCrO2-TiO2 (i.e., NaxCrxTi8-xO16) at 1350 degrees 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 = 1.7 and 1.75, and monoclinic for x = 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), b = 2.9559(9), c = 9.9097(11)angstrom, and beta = 90.545(9)degrees with Z = 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. (c) 2006 Elsevier Inc. All rights reserved.