Structures of Na 0.74 WO 3

The structures of the four phases exhibited by Na 0.74 WO 3 , 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 3 m (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.