Abstract
Phase relations of rutile, freudenbergite, and hollandite structures were examined in the pseudobinary system NaCrO 2–TiO 2 (i.e., Na x Cr x Ti 8− x O 16) 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 = 1.7 and 1.75, and monoclinic for x = 1.8 and above. Single crystals of monoclinic hollandite Na 2Cr 2Ti 6O 16 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 β = 90.545 ( 9 ) ° 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.
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