Effect of pressure on phase transitions in K1−xNaxMnF3(x = 0.04)

Abstract

The pressure-induced phase transition sequence in the title compound, potassium sodium fluoromanganate, has been investigated by single-crystal X-ray diffraction using synchrotron radiation and a diamond anvil pressure cell. Na^+ ions at 4% of the K^+ sites shift the ferrodistortive phase transition to the lower pressure P_{c1} of 2.75 (5) GPa compared to 3.12 GPa in the parent compound KMnF3. The transition is illustrated by the critical behaviour of the unit-cell dimensions, the pressure-dependent evolution of the MnF_6 ^- octahedral rotation and related macroscopic spontaneous strain. As far as precision of the present experiment allows, the observations show that the 4% of Na^+ admixture at the K^+ sites does not substantially change the nature of the transition at P_{c1}. The main effect of pressure is to stabilize the tetragonal phase II. The expected further evolution of the MnF_6 ^- octahedral tiltings, leading to the orthorhombic and monoclinic phases, has not been observed up to 8.33 GPa.