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

Abstract

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.

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Article information


Submitted
01 Jun 2020
Accepted
29 Jun 2020
First published
30 Jun 2020

Dalton Trans., 2020, Accepted Manuscript
Article type
Paper

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

D. G. Kellerman, A. P. Tyutyunnik, N. I. Medvedeva, A. Yu. Chufarov, A. D. Fortes, A. Gibbs, N. V. Tarakina, M. Kalinkin, N. Zhuravlev and O. Leonidova, Dalton Trans., 2020, Accepted Manuscript , DOI: 10.1039/D0DT01963A

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