It is instructive to examine the refined structures in detail; the structural changes on the transition from the orthorhombic to tetragonal structure primarily reflect changes in the NiO6 octahedra as demonstrated in Fig. . The tilt in the NiO6 octahedra for the sample measured at 250 ˚C is clearly visible when compared to the tetragonal structure obtained from the data set obtained at 800 ˚C. Similarly, the other two compositions, L2P2N3 (Fig. ) and L3P1N3 (Fig. ) were best refined in orthorhombic Bmab symmetry at both low and high temperature, and no transition to higher symmetry structures on heating was observed in either of the two compositions. However, as mentioned above L2P2N3 adopts monoclinic symmetry at RT, so it appears that there is a transition to higher symmetry orthorhombic structure at 250 ˚C, at least for the L2P2N3 composition. The occupancies of all the oxygen sites were refined and it was observed that vacancies at room temperature were limited with a total oxygen stoichiometry of 9.96. These vacancies were confined to the O(1) site for the sample measured at 25 ˚C agreeing well with the DFT calculations (Fig. ). In cases where occupancies refined to 1.0, these were fixed in subsequent refinement cycles. As was expected the total oxygen stoichiometry decreased with increasing temperature, due to the reduction of the B-site cation, pointing towards the expected decrease in the B-site cation average oxidation state. The oxygen stoichiometry of 9.54 at 600 ˚C agrees well with previously published reports in related higher-order Ruddlesden-Popper materials by our group and others in the community the vacancies predominantly prefer perovskite units of the structure . The atomic displacement parameters for the oxygen atoms were best fit with an anisotropic model and the thermal parameters show a considerable increase with the increase in temperature, which is expected (Table ). The large ab plane anisotropy of the apical O(1) sites, particularly at high temperatures, points towards faster oxygen diffusion in these materials. This apical oxygen position is indeed known to exhibit an anisotropic Debye-Waller factor and has been widely reported for lower-order Ruddlesden-Popper K2NiF4 materials .