Cooperative Displacements of Ba2+ Ions in the Incommensurate Structural Phase of Piezoelectric Layer Compound BaMnF4

Abstract

Structural properties of BaMnF 4 has been studied in the incommensurate phase (ICP) below about 250 K by X-ray diffraction. The ICP structure has a unit cell of N a p ×2 b p ×2 c p ; N ∼5, where a p , b p and c p mean lattice constants at room-temperature commensurate phase (CP). The structure is determined at about 200 K in addition to that at room temperature, by using a four-circle X-ray diffractometer. The large distortion of each MnF 6 octahedron, which jointly owns two F - ions along the a -axis and a F - ion along the c -axis with a neighbor octahedron, is roughly similar in the ICP phase with those in the CP phase. In the ICP phase, the MnF 6 octahedra show different tilting-scheme between nearest-neighboring two-dimensional MnF 6 octahedra networks (folding screens), which are stacked along the b -axis and alternatively displaced each other with a glide vector of (0,1/4,1/4) in the unit cell of the ICP phase. There are two types of folding screens which are stacked along the b p -axis alternately...