Abstract/Summary

The local nuclear and electronic structures and molecular dynamics of the ferroelectric lattice in selected geometric fluorides (BaMgF4, BaZnF4, BaMg1−xMnxF4 and BaMg1−xNixF4; x = 0.001 and 0.005) have been investigated. The 19F and 25Mg isotropic chemical shift δiso, 25Mg quadrupolar coupling constants (Cq) and asymmetry parameters (η) reflect the geometry of the coordination spheres. The zero-field splitting parameters |D| and |E| are consistent with distorted axial symmetry (low temperatures) and nearly rhombic symmetry (high temperatures) of octahedral Mn2+ coordination. The high resolution of the nuclear magnetic resonance, electron paramagnetic resonance and phonon spectra are consistent with the highly ordered crystallographic structure. Combined multi-technique data evidence the subtle discontinuous changes in the temperature dependences of |D| and |E|, isotropic chemical shifts δiso and signature parameters of Raman bands and suggest a discontinuous structural distortion of the fluoride octahedra. The temperature at which this change occurs depends on the ionic radius of the central ion of the octahedral site and is estimated to be ∼300 K for Zn2+ fluorides and ∼240 K for Mg2+ fluorides. This geometrical distortion modifies the lattice dynamics and originates from the rotation of the fluoride octahedra around a new direction approximately perpendicular to that related to the paraelectric–ferroelectric phase transition.