X-ray Diffraction and Polarized X-ray Absorption Study of Single Crystal LiFePO$_{4}$

Large size LiFePO$_{4}$ single crystals have been grown by standard flux growth technique with the LiCl as the flux. Single crystal x-ray diffraction (XRD) and synchrotron polarized x-ray absorption spectroscopy (XAS) measurements have been performed on the crystals. The XRD measurements were performed at T = 293 K using Mo $K_{\alpha }$ radiation ($\lambda $ = 0.7107{\AA}) to a resolution corresponding to sin$\theta $/$\lambda $ = 0.6486 {\AA}$^{-1}$, with 2$\theta _{max}$ = 54.9\r{ }. A total number of 1330 reflections were processed with 350 unique data. The obtained crystal structure data were the following: lithium iron (II) phosphate, LiFePO$_{4}$, orthorhombic, space group \textit{Pnma}, lattice constants: $a$ = 10.3172 (11) {\AA}, $b$ = 6.0096(8) {\AA}, $c$ = 4.6775 (4) {\AA}, Z = 4, formula weight: 157.76, density: 3.613, $\mu $ = 55.562 cm$^{-1}$. The bond lengths between Fe and O and between P and O were obtained. The polarized XAS was performed at the Fe $K$-edge with the x-ray \textbf{E}-vector along the a-, b-, and c-axis. The XAS results show that the Fe ions in the LiFePO$_{4}$ single crystals are divalent. We also observed a big shift in both the energies of the pre-edge 1$s \quad \to $ 3$d$ transition feature and the main edge when the polarization direction of the \textbf{\textit{E}}-vector changes from along $a$-axis to along $c$-axis.