Structural Variation in the Lithiophilite–triphylite Series and Other Olivine-group Structures

The crystal structures of five natural samples of the lithiophilite–triphylite series [Li(Mn,Fe)PO4; Li = M1, (Mn,Fe) = M2] were refined to determine structural variation along the Mn (r = 0.83 Å) ⇔ Fe (r = 0.78 Å) solid-solution series, and to elucidate variations in the atomic arrangement of Pbnm olivine. The refinements converged to R ≤ 0.017. Bonds at the O3 site are fundamental in understanding the response of the atomic arrangement as Fe concentration increases. The M2–O3a bond shortens by more than 0.06 Å, and the M2–O3b bond shortens by ~0.02 Å over the series. This shift of the O3 oxygen toward the two coordinating M2 sites is commensurate with an increase in the M1–O3 bond length by approximately 0.03 Å, and an increase in the distortion of the M1 site. Much previous work has focused on polyhedron distortions in the olivine structure. The angle variance for the M1, M2, and T polyhedra were calculated for each phosphate sample in this study and published silicate and germanate olivine structures. In each case, the angle variance of the phosphate olivines was found to be smaller in the M1 octahedron, which is in contrast to the other olivine-structure phases examined in this study. However, if the size difference in the radius of the M1 and M2 site cations is ≥0.17 Å, the distortion is greater in the octahedral site that is occupied by the larger cation. The structural differences along the lithiophilite–triphylite solid-solution series may have significant effects on its solid electrolyte properties, including rates of lithium diffusion and activation energies, and thus are important in the development and design of Li-olivine storage cathodes.