This report presents a brief review on the crystal and magnetic structures of high-Tc copper oxides. The structural instabilities or short range atomic fluctuations observed in a large number of the high-Tc superconductors are described. Several unresolved problems on the relations between the structural instability and the superconductivity are discussed.
The new members of mixed-valence sodium titanates; NaTi2O4, Na2.08Ti4O9, Na1.68Ti6O11, Na0, 57Ti2O4 and NaTi8O13 were synthesized and their crystal structures were determined by the single-crystal X-ray diffraction method. Most of them are classified into the “rutile-related type” tunnel structure group, different from the previously reported “Wadsley-Andersson type” one. Every sodium titanate of the rutile-related type belongs to either the mixed-valence compound or the sodiumdefect type one. The mixed-valence octahedral sites and sodium defects in the structure of these new rutile-related compounds appear to be related to the sodium vapor pressure in their synthetic condition.