Structural properties of a mixed valence compound Fe₂ BO₄

We have performed X-ray powder diffraction measurements in a mixed valence compound of Fe₂ BO₄ using synchrotron radiation. The diffraction patterns obtained at several temperatures between 85 and 386 K were analyzed by the Rietveld method. The monoclinic-orthorhombic transition, ascribed to charge-ordering of Fe^{2 +} and Fe^{3 +} ions, was confirmed by the temperature dependence of the monoclinic angle β. No superlattice reflection due to the charge-ordering was detected. The temperature dependences of the lattice parameters show anomalies around 270 K as well as the monoclinic-orthorhombic transition temperature of 327-347 K. The structural change around 270 K is attributable to the onset of the partial charge-delocalization.