A method of forming a nitridophosphate is disclosed, the method including forming a precursor mixture by combining a metal source material, a phosphorus source material, and a nitrogen source material, and heating the precursor mixture at a maximum temperature between 800 C. and 1300 C. in an atmosphere including nitrogen gas at a pressure between 2 MPa and 500 MPa.

An aircraft includes fuselage walls, a passenger cabin between the fuselage walls, rows of seating arranged with an aisle through the rows, an emergency exit in the fuselage walls, an access path between the rows of seating to the emergency exit from the aisle, and a marker forming tracks on a floor of the aisle and access path, to direct passengers to the emergency exit. The marker includes a photoluminescent material exhibiting persistent luminescence and comprising a pigment that emits blue visible light in response to excitation by light of wavelength between 250-500 nm. The photoluminescent material has an emission spectrum providing light which appears blue to a viewer, with a maximum peak intensity between 400-510 nm. The marker includes a cover over a top surface of the photoluminescent material that partially blocks light of some UV range wavelengths whilst being transparent to other wavelengths in the UV range.