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) angstrom, b = 8.2042(13) angstrom, c = 11.565(3) angstrom, and V = 485.22(17) angstrom(3). It has a three-dimensional framework structure that is different from that found in other (LiMP2O7)-P-III materials. It transforms to the known monoclinic form (space group P2(1)) above similar to 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(I)M(III)P(2)O(7) (A = alkali metal) families. Although the bulk Li-ion conductivity is low, as determined from alternating-current impedance spectroscopy and variable-temperature static Li-7 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 A) 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.

Automated summary

A new polymorph of lithium aluminum pyrophosphate, LiAlP2O7, with a unique three-dimensional framework structure, has been discovered at 973 K. Density functional theory (DFT) calculations show that this new polymorph is the most stable low-temperature structure for this composition among seven known structure types. Although the bulk Li-ion conductivity is low, there are potential avenues for enhancing conductivity through analysis of structure topologies. Promising structure types with favorable Li-site topologies have been identified for possible long-range Li migration.