X-Ray Study of Phase Transition in RbNO₃

Abstract

The phase transition in RbNO 3 from the NaCl type cubic phase I to the rhombohedral phase II is investigated with X-ray diffraction method. In the phase I, from the observations of X-ray temperature diffuse scattering, it is found that c 44 / c 11 has abnormally small value 0.03 and a diffuse intensity concerning only c 44 increases at the rate of 8% per 10°C as temperature decreases to the I→II transition temperature. A lattice relationship is found at the transition: In the phase II, the lattice is distorted by only an increase in angle between axes with decreasing temperature. A model of orientational disorder among eight orientations of NO 3 - is proposed in both phases. These results suggest that the coupling between the orientation of NO 3 - and the shear strain plays an important role in this transition.