Synthesis, crystal structure, and ionic conductivity of a new layered metal phosphate, Li₂Sr₂Al(PO₄)₃

A new layered metal phosphate, Li₂Sr₂Al(PO₄)₃, was synthesized in the form of either a single-crystal or polycrystalline powder using the molten hydroxide flux method or a solid-state reaction, respectively. Li₂Sr₂Al(PO₄)₃ crystallizes to the P2₁/n (Z=4) monoclinic space group with lattice parameters a≈4.95 Å, b≈22.06 Å, c≈8.63 Å, and β≈91.5°. The structure is composed of stacked [LiSrAl(PO₄)₂] layers alternating regularly with [LiSrPO₄] layers. In the [LiSrAl(PO₄)₂] sublattice, the AlO₆ octahedra and PO₄ tetrahedra are tilted cooperatively to form an anionic, corrugated, two-dimensional [Al(PO₄)₂]³⁻ framework that can be regarded as a "distorted-glaserite" structure. The [LiSrPO₄] sublattice is that of a layered block containing a six-membered ring formed from alternating linkages of LiO₄ and PO₄ tetrahedra. The six-membered rings show a boat-type arrangement with the up(U) or down(D) pointing sequence, UUDUUD. The interspace between the two sublattices generates a two-dimensional pathway for Li⁺ ion conduction. The impedance measurement indicated that Li₂Sr₂Al(PO₄)₃ had a moderate ion conductivity (σ≈1.30×10⁻⁴ S cm⁻¹ at 667 K), with an activation energy E_a≈1.02 eV. - Graphical abstract: Polyhedral view of Li₂Sr₂Al(PO₄)₃. Li⁺ ions are represented by green spheres, Sr atoms by white spheres, AlO₆ groups by octahedra, and PO₄ groups by tetrahedra. - Highlights: • New compound Li₂Sr₂Al(PO₄)₃ is reported. • The crystal structure is investigated by single-crystal XRD analysis. • The structure is formed by the alternate stacking of two different sublattices. • Correlation between the crystal structure and ionic conductivity is discussed.