New Li -Mg phosphates with 3D framework : experiment and ab initio calculations

Kellerman, Dina Georgievna; Tyutyunnik, Alexander P.; Medvedeva, Nadezhda Ivanovna; Chufarov, Aleksandr Yu.; Fortes, Andrew Dominic; Gibbs, Alexandra; Tarakina, Nadezda V.; Kalinkin, Mikhael; Zhuravlev, Nickolai; Leonidova, Olga

doi:10.1039/D0DT01963A

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New Li -Mg phosphates with 3D framework : experiment and ab initio calculations

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Tarakina, Nadezda V.
Nadezda V. Tarakina, Kolloidchemie, Max Planck Institute of Colloids and Interfaces, Max Planck Society;

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引用

Kellerman, D. G., Tyutyunnik, A. P., Medvedeva, N. I., Chufarov, A. Y., Fortes, A. D., Gibbs, A., Tarakina, N. V., Kalinkin, M., Zhuravlev, N., & Leonidova, O. (2020). New Li -Mg phosphates with 3D framework: experiment and ab initio calculations. Dalton Transactions, -. doi:10.1039/D0DT01963A.

引用: https://hdl.handle.net/21.11116/0000-0006-AA9C-B

要旨

Two new lithium-magnesium phosphates LiMg₆(PO₄)₃(P₂O₇) and
Li(Mg_5.62Sc_0.19Li_0.19)(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 [Mg_5.62Sc_0.19Li_0.19O₁₈]_∞ 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(Mg_5.62Sc_0.19Li_0.19)(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(Mg_5.62Sc_0.19Li_0.19)(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.