The evolution of unit cell constants, and thereby the unit cell volume is an important consideration for these materials as they are considered for potential use in high-temperature SOC devices. Therefore, Rietveld refinements, to be discussed in detail in the next section, were performed using the GSAS/EXPGUI software package and it was found that L1P3N3 and L3P1N3 compositions studied by POWGEN crystallised in the low-symmetry orthorhombic space group Bmab at all temperatures except the L3P1N3 composition which adopts the high-symmetry tetragonal space group I4/mmm at 800˚C. L2P2N3 studied with the POLARIS diffractometer, however, crystallised with monoclinic symmetry. Table below summarises the room-temperature lattice constants obtained from Rietveld refinement of neutron diffraction data for the L3P1N3, L2P2N3 and L1P3N3 compositions and compares them with the two end members as reported by Zhang et al . The cell constant data obtained from the NPD measurements are in excellent agreement with our previous XRD studies .