Structures of Na0.74WO3

Abstract

The structures of the four phases exhibited by Na0.74WO3, sodium tungstate, with a change in temperature have been refined from powder diffraction patterns obtained on the high-resolution powder diffractometer (HRPD), ISIS, Rutherford Appleton Laboratory, England. At temperatures above 430 K, the stable phase has the ideal perovskite structure, with space group Pm\bar 3m (No. 221). Below that temperature, the octahedra are tilted about pseudocubic 〈100〉 directions; the tilts are produced by the condensation of the normal modes of vibration of the octahedra in the high-temperature cubic phase. Using Glazer's notation, the tilt system undergoes the following sequence:a^+a^+a^+ \buildrel {\rm 293\, K} \over \longrightarrow a^+a^+a^0 \buildrel {\rm 343\, K} \over \longrightarrow a^0a^0a^+ \buildrel {\rm 430\, K} \over \longrightarrow a^0a^0a^0 .The structural refinements presented here confirm the sequence of the transitions first proposed by Clarke [(1977). Phys. Rev. Lett. 39, 1550–1553]. This is the first time that a structure determination of a perovskite with the tilt system a + a + a 0 has been reported in the literature. In addition, we evaluate the weights, or importance, of the condensed modes in each low-temperature phase.