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 °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.

Graphical abstractDifference Fourier map at the y=0.5y=0.5 section for Na2Cr2Ti6O16 with a structure model containing no Na ions.Figure optionsDownload full-size imageDownload as PowerPoint slide