Crystal structure of lithium magnesium phosphate,LiMgPO4: Crystal chemistry of the olivine-type compounds

LiM_gPO₄ is orthorhombic, space groupPnma,a=10.147(2),b=5.909(2),c=4.692(2)×10^–10 m,D _m =3000(8),D _x =2980 kg m^–3,Z=4,R=4.8% for 1262 observed reflections. The structure contains tetrahedral PO₄ and octahedral LiO₆ and MgO₆ groups. It belongs to the ordered olivine-type structures, since the inversion site is occupied by Li⁺ ions and the mirror site is taken by Mg²⁺ ions. The distortion of polyhedra is caused mainly by the adjustment stresses between the different polyhedral dimensions and by the sharing of edges. Octahedral LiO₆ share six and MgO₆ share three common edges. The sharing of the O-O edges contributes to the stability of the bridging arrangements. The corresponding average distances of the O-O bridges are 2.432, 2.479, 2.883, and 3.028×10^–10 m, while the average O-O distances in the PO₄, MgO₆, and LiO₄ polyhedra are 2.512,2.921, and 3.015 × 10^–10 m, close to the ideal values 2.531, 2.970, and 3.026×10^–10 m respectively. The structure field of the olivine-type compounds plotted as a function of ionic radii in radius space is specified in relation to the-K₂SO₄, spinel, and K₄NiF₄ type structures. From the overlap of the structure fields, the high-pressure transformations of the olivine compounds are considered.