Incommensurate cooperative tilting modes of MnF6 octahedra related to the structural phase transitions in BaMnF4

Abstract

The structural phase transitions of the layer compound BaMnF4 were studied by high-resolution X-ray diffraction using synchrotron radiation. The intensities and profiles of two kinds of superlattice reflections having incommensurate reduced wave vectors q(1) = (similar to +/- 1/5,0,0)p and q(2) = (similar to +/-2/5,1/2,1/2)(p), respectively, were measured as a function of temperature from 25K to 280K. These temperature dependencies show that incommensurate structural phase transitions of second order occur at 234K and 244 K. These structural phase transitions are interpreted as successive condensations of a folding-screen-like incommensurate plane-distortion mode and a commensurate anti-ferro-distortive tilting mode of the MnF6 octahedra around the primitive a(o)- and b(o)-axes, when cooled down. It is also found that there is another structural phase transition at about 45K related to a precursor structural distortion for the antiferromagnetic transition occurring at about 26 K.