Group III Phosphates as Two-Dimensional van der Waals Materials

The ability of group III phosphates to adopt a two-dimensional van der Waals (2D VDW) structure observed for SiO2 was evaluated using density functional theory. The energies to form 2D hexagonal bilayers of corner-sharing tetrahedra did not follow a monotonic trend: the energies for AlPO4 and GaPO4 were similar to silica, while for BPO4 it was more than a factor of 2 larger and for InPO4 nearly another factor of 2 larger. The larger In atom favors octahedral coordination, accounting for the high energy of the 2D InPO4 structure. Meanwhile, boron's small size leads to a different favored bulk structure than AlPO4 or GaPO4 which competes much more successfully with the 2D phase. The implication is a sweet spot in the cation size for forming 2D tetrahedral oxides that spans Si to Ga. The 2D BPO4 and GaPO4 structures displayed alternating rotations of the B(Ga)O-4 and PO4 tetrahedra which allowed the B(Ga)-O-P bond angles to match those seen in the favored bulk compounds; no such rotations were required for 2D AlPO4 and SiO2 to match bond angles in bulk compounds. The interactions of AlPO4 and GaPO4 with Rh(111) as a prototypical growth substrate were also investigated with the results revealing adhesion dominated by VDW interactions. Alternate structures were considered with results mimicking those seen for SiO2 introducing larger rings of corner-sharing tetrahedra decreases the density, allowing the structure to be tuned by applying tensile strain. In comparison to SiO2, however, only even-membered rings are possible for the phosphates, restricting the range of structures and defects that can form. Finally, it was found "that Mg2+ could readily replace Al3+ in AlPO4 in the process, creating ion exchange sites. The results highlight the great promise for adding AlPO4 and GaPO4 to the family of 2D VDW materials.