Modulated structures of some alkali molybdates and tungstates

Abstract

Some of the high-temperature phases of K2MoO4, K2WO4 and Rb2WO4 are shown to have modulated structures. The modulation wave vector k is rationally independent of the basic vectors of the unit cell. The diffraction patterns have symmetry 6/mmm. The k vector points in the direction of the a* axis; k= tea*. For K2MoO4 the value of x changes continuously from 0.297 at 360°C to 0.290 at 430°C. The values for K2WO4 and Rb2WO4 are 0-250 at 395 °C and 0.363 at 435°C respectively. A model for the modulation is proposed. In this model the modulation is assumed to be a periodic change in the orientations of the anion tetrahedra.

Phase relations, dopant effects, structure, and high electrical conductivity in the Na2WO4Na2MoO4 system

“…Calibration was on Zn, as given by the "American Institute of Physics Handbook". For the technique used in the structure determination by Van den Berg et al we will chiefly confine ourselves to referring to their latest work on a related compound (9). X-ray powder diffraction exposures as a function of temperature were made with a Guinier-Len& camera.…”

“…This makes an interpretation of the powder data very difficult. Essentially, however, this could imply the possibility of performing single crystal measurements, for instance with the technique used previously by Van den Berg (9).…”

Modulated phase ofK2MoxW1−xO4mixed compounds using orthorhombic symmetry: A powder di

“…Although preliminary inspections might detect the presence of peaks at the positions of reflections ͑2 3 0͒, ͑4 1 0͒, and ͑4 1 2͒, a further detailed examination reveal that these peaks correspond to the most intense satellite reflections identified by van den Berg et al 11 using a hexagonal lattice. These peaks would correspond to the satellites ͑4 2 2 1͒, ͑4 1 0 0͒, and ͑2 0 3 1͒, and ͑2 2 2 1͒, respectively, indexed in the orthorhombic setting.…”

“…It has to be noted that the values obtained for q in each compound are approximately twice as big as those given in the literature, [11][12][13] as expected by the use of the orthorhombic domains model.…”

“…In most of the works so far, hexagonal symmetry was used to describe the modulated phase of both potassium molybdate and potassium tungstate. 5,[11][12][13] Based on precession photos, van den Berg et al 11 interpreted the modulated phase using hexagonal symmetry and the satellite reflections were indexed by the wave vectors q = q͑ma ‫ء‬ + nb ‫ء‬ ͒, with ͑m , n͒ = Ϯ ͑1,0͒, Ϯ͑0,1͒, or Ϯ͑1,1͒, where a ‫ء‬ and b ‫ء‬ are the unit vectors of the reciprocal hexagonal lattice. Based on x-ray powder diffraction results, Warczewski 13 proposed a commensurate modulated phase for K 2 WO 4 , and an incommensurate modulated and a lock-in phase for K 2 MoO 4 using an equivalent indexation.…”