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

• 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.

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 Ea≈1.02 eV.

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.Figure optionsDownload as PowerPoint slide