Polymorph of LiAlP₂O₇: Combined Computational, Synthetic, Crystallographic, and Ionic Conductivity Study

We report a new polymorph of lithium aluminum pyrophosphate, LiAlP₂O₇, discovered through a computationally guided synthetic exploration of the Li-Mg-Al-P-O phase field. The new polymorph formed at 973 K, and the crystal structure, solved by single-crystal X-ray diffraction, adopts the orthorhombic space group Cmcm with a = 5.1140(9) Å, b = 8.2042(13) Å, c = 11.565(3) Å, and V = 485.22(17) ų. It has a three-dimensional framework structure that is different from that found in other LiM^IIIP₂O₇ materials. It transforms to the known monoclinic form (space group P2₁) above ∼1023 K. Density functional theory (DFT) calculations show that the new polymorph is the most stable low-temperature structure for this composition among the seven known structure types in the A^IM^IIIP₂O₇ (A = alkali metal) families. Although the bulk Li-ion conductivity is low, as determined from alternating-current impedance spectroscopy and variable-temperature static ⁷Li NMR spectra, a detailed analysis of the topologies of all seven structure types through bond-valence-sum mapping suggests a potential avenue for enhancing the conductivity. The new polymorph exhibits long (>4 Å) Li-Li distances, no Li vacancies, and an absence of Li pathways in the c direction, features that could contribute to the observed low Li-ion conductivity. In contrast, we found favorable Li-site topologies that could support long-range Li migration for two structure types with modest DFT total energies relative to the new polymorph. These promising structure types could possibly be accessed from innovative doping of the new polymorph.