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

The pressure-induced phase transition sequence in the title compound, potas sium 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 t o the lower pressure P-c1 of 2.75 (5) GPa compared to 3.12 GPa in the paren t compound KMnF3. The transition is illustrated by the critical behaviour o f the unit-cell dimensions, the pressure-dependent evolution of the MnF6- o ctahedral rotation and related macroscopic spontaneous strain. As far as pr ecision 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 MnF6- octahedral tiltings, leading to the orthorhombic and monoclinic phases, has not been observed up to 8.33 GPa.