New Li -Mg phosphates with 3D framework: experiment and ab initio calculations
TTwo new lithium-magnesium phosphates LiMg6(PO4)3(P2O7) and Li(Mg5.62Sc0.19Li0.19)(PO4)3(P2O7) were synthesized by the solid-phase method. Using high-resolution time-of-flight neutron powder diffraction (TOF NPD) and X-ray powder diffraction (XRPD), we established that these phosphates have Pnma orthorhombic structure with the cell parameters a=9.14664(5) Å, b=18.83773(8) Å, c=8.27450(4) Å, V=1425.71(1) Å3 and a=9.14516(5) Å, b=18.84222(9) Å, c= 8.28204(4) Å, V=1427.12(1) Å3, respectively. The crystal structures can be described by stacking of the [Mg6O18] or [Mg5.62Sc0.19Li0.19O18] wavy layers, which are parallel to the (100) direction and interconnected through PO4 tetrahedra and P2O7 groups to a 3D-framework. Li atoms are located in large tunnels formed in a 3D lattice, which contributes to lithium diffusion. AC impedance spectroscopy shows that LiMg6(PO4)3(P2O7) and Li(Mg5.62Sc0.19Li0.19)(PO4)3(P2O7) have Li ion conductivity of 3.6 10-4 S/cm and 1.7 10-4 S/cm at 950 °C, with activation energy of 1.28 eV and 1.55 eV, respectively. NMR MAS studies confirmed a coexistence of pyro- and orthogroups in the structure of both phases and two lithium positions in Li(Mg5.62Sc0.19Li0.19)(PO4)3(P2O7). The first-principles method was used to study the electronic structure and stability of the two phases. The calculated formation enthalpies demonstrated that Sc is a stabilizing impurity in LiMg6(PO4)3(P2O7), while there exists a strong destabilization of olivine LiMgPO4 upon doping with Sc. This explains the failure to synthesize the Sc-doped olivine. The new phosphate LiMg6(PO4)3(P2O7) is a dielectric with band gap of 5.35 eV, which decreases to 4.85 eV due to a localized Sc 3d peak upon doping with Sc. These findings are consistent with the results obtained from UV-Vis spectroscopy. The new phase may be a good optical matrix similar to LiMgPO4.