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Abstract:
Two 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.
