Dependence of the accuracy of a continuous phase transition temperature on angular resolution in powder diffractometry

We have investigated in situ a continuous transition between the orthorhombic and tetragonal phases in perovskite-structured La0.63(Ti0.92,Nb0.08)O3 by three X-ray powder diffractometers with different Δd/d resolutions of 0.03%, 0.06% and 0.10%. The d and Δd denote the λ/(2·sinθ) and peak width where λ and θ are wavelength of X-ray and Bragg angle. Only the highest-resolution diffractometer of Δd/d=0.03% was able to detect the peak splitting between 400 and 040 reflections in the temperature range of 327–339°C. It was found that the accuracy of the transition temperature is considerably improved with decreasing of Δd/d value. The maximum temperature where the peak splitting between 400 and 040 reflections is detectable increases, while the transition temperature determined by a power law decreases with decreasing of Δd/d value.