Mn(V) polyhedron size in Ba10((P,Mn)O4)6F2: vibrational spectroscopy and EXAFS study

Abstract

This paper reports the study of manganese substituted barium fluoroapatite Ba₁₀(P_1−xMn_xO₄)₆F₂ with 0 ≤ × ≤ 0.20. X-rays diffraction investigation indicates that the phosphate-manganate solid solution is complete, at least for this composition range, and follows Vegard's law, while diffuse reflectance and fluorescence spectra confirm that Mn(V) substitutes for P(V) in the barium apatite lattice. EXAFS experiment, at the manganese K edge, performed on the x = 0.2 compound, indicates that the Mn(V)-O distance is 1.73 Å, a much larger value than the P-O distance equal to 1.54 Å.

The (MnO₄)³⁻ vibrational mode frequencies, deduced from the IR and Raman spectra are almost independent of the x value. This indicates that, even at very low manganese content (x = 0.001), the Mn-O distance is also very close to 1.73 Å.

The Mn(V) ion distorts the barium apatite lattice to a large extent when it substitutes for P(V) and imposes its own size to the host sites.