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(3) over bar 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(+) (293K)under right arrow a(+)a(+)a(0) (343K)under right arrow a(0)a(0)a(+) (430K)under right arrow a(0)a(0)a(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.
|Pages (from-to)||584-587 Part 5|
|Journal||Acta Crystallographica Section B Structural Science|
|Early online date||1 Sept 2003|
|Publication status||Published - 25 Sept 2003|