Temperature dependence of structural parameters in the perovskite Na0.74WO3

PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS(2005)

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Abstract
NaxWO3 undergoes three phase transitions involving tilting of the oxygen octahedra about < 100 > pseudocubic directions. We have measured the temperature dependence of the lattice parameters between 200 and 750 K - a temperature range that spans the three transitions - and determined the evolution of the structure within each phase as temperature changes. The experiments were performed on the highresolution powder diffractometer HRPD, Rutherford Appleton Laboratory, UK, using time-of-flight neutron diffraction. The critical exponent that describes the temperature dependence of the total tilt angle has been evaluated. An unexpected change in the thermal expansion coefficient in the cubic phase is not understood: an argument based on Landau's theory of phase transitions is used to demonstrate that the change is not the result of co-existence of two phases.
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Key words
time of flight,phase transition,thermal expansion coefficient,neutron diffraction,lattice parameter,critical exponent,high resolution
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