Structural Origin of the Anisotropic and Isotropic Thermal Expansion of K₂NiF₄‑Type LaSrAlO₄ and Sr₂TiO₄

K₂NiF₄<i>-</i>type LaSrAlO₄ and Sr₂TiO₄ exhibit anisotropic and isotropic thermal expansion, respectively; however, their structural origin is unknown. To address this unresolved issue, the crystal structure and thermal expansion of LaSrAlO₄ and Sr₂TiO₄ have been investigated through high-temperature neutron and synchrotron X-ray powder diffraction experiments and ab initio electronic calculations. The thermal expansion coefficient (TEC) along the <i>c</i>-axis (α_<i>c</i>) being higher than that along the <i>a</i>-axis (α_<i>a</i>) of LaSrAlO₄ [α_<i>c</i> = 1.882(4)­α_<i>a</i>] is mainly ascribed to the TEC of the interatomic distance between Al and apical oxygen O2 α­(Al–O2) being higher than that between Al and equatorial oxygen O1 α­(Al–O1) [α­(Al–O2) = 2.41(18)­α­(Al–O1)]. The higher α­(Al–O2) is attributed to the Al–O2 bond being longer and weaker than the Al–O1 bond. Thus, the minimum electron density and bond valence of the Al–O2 bond are lower than those of the Al–O1 bond. For Sr₂TiO₄, the Ti–O2 interatomic distance, <i>d</i>(Ti–O2), is equal to that of Ti–O1, <i>d</i>(Ti–O1) [<i>d</i>(Ti–O2) = 1.0194(15)<i>d</i>(Ti–O1)], relative to LaSrAlO₄ [<i>d</i>(Al–O2) = 1.0932(9)<i>d</i>(Al–O1)]. Therefore, the bond valence and minimum electron density of the Ti–O2 bond are nearly equal to those of the Ti–O1 bond, leading to isotropic thermal expansion of Sr₂TiO₄ than LaSrAlO₄. These results indicate that the anisotropic thermal expansion of K₂NiF₄-type oxides, <i>A</i>₂<i>B</i>O₄, is strongly influenced by the anisotropy of <i>B</i>–O chemical bonds. The present study suggests that due to the higher ratio of interatomic distance <i>d</i>(<i>B</i>–O2)/<i>d</i>(<i>B</i>–O1) of <i>A</i>₂^2.5+<i>B</i>³⁺O₄ compared with <i>A</i>₂²⁺<i>B</i>⁴⁺O₄, <i>A</i>₂^2.5+<i>B</i>³⁺O₄ compounds have higher α­(<i>B</i>–O2), and <i>A</i>₂²⁺<i>B</i>⁴⁺O₄ materials exhibit smaller α­(<i>B</i>–O2), leading to the anisotropic thermal expansion of <i>A</i>₂^2.5+<i>B</i>³⁺O₄ and isotropic thermal expansion of <i>A</i>₂²⁺<i>B</i>⁴⁺O₄. The “true” thermal expansion without the chemical expansion of <i>A</i>₂<i>B</i>O₄ is higher than that of <i>AB</i>O₃ with a similar composition