We undertake a detailed high-resolution diffraction study of a plain band insulator, La2MgO4, which
may be viewed as a structural surrogate system of the undoped end-member of the high-Tc superconductor
family La2−x−yA2+
x R3+
y CuO4 (A = Ba, Sr; R = rare earth). We find that La2MgO4 exhibits the infamous lowtemperature orthorhombic (LTO) to low-temperature tetragonal (LTT) phase transition that has been linked to the
suppression of superconductivity in a variety of underdoped cuprates, including the well-known La2−xBaxCuO4
(x = 0.125). Furthermore, we find that the LTO-to-LTT phase transition in La2MgO4 occurs for an octahedral
tilt angle in the 4◦–5◦ range, similar to that which has previously been identified as a critical tipping point
for superconductivity in these systems. We show that this phase transition, occurring in a system lacking
spin correlations and competing electronic states such as charge density waves and superconductivity, can be
understood by simply navigating the density functional theory ground-state energy landscape as a function of
the order parameter amplitude. This result calls for a careful reinvestigation of the origins of the phase transitions
in high-Tc superconductors based on the hole-doped, n = 1 Ruddlesden-Popper lanthanum cuprates.