Néel temperature of stoichiometricLa2CuO4

Following doping with ${}_{}{}^{111}{}_{}{}^{}\mathrm{In}$ in ${\mathrm{O}}_2$ gas at 1273 K, samples of ${\mathrm{La}}_2$ ${\mathrm{CuO}}_{4+\mathrm{y}}$ were subjected to vacuum anneals of varying lengths to adjust the oxygen content. Susceptibility studies show that vacuum anneals of ∼13 h, starting at 1083 K and terminating at 873 K, consistently produce magnetic ordering at Néel temperature ($T_N$)=317(3) K, the highest value yet published. Perturbed-angular-correlation (PAC) studies of the same samples, using the 171–245-keV γγ cascade of ${}_{}{}^{111}{}_{}{}^{}\mathrm{Cd}$ populated via the decay of ${}_{}{}^{111}{}_{}{}^{}\mathrm{In}$, exhibit a combined magnetic-dipole–electric-quadrupole interaction. Analysis of this yields a magnetic hyperfine field and an electric-field-gradient asymmetry that follow the expected temperature dependence of the local magnetization and orthorhombic distortion, respectively. The samples have uniquely static and homogeneous hyperfine interactions with undetectable PAC line broadening. We argue that this implies a ‘‘defect-free’’ probe environment which we identify as stoichiometric with y≃0. Assuming that any residual defects are randomly distributed on oxygen lattice sites, we find ‖y‖≤0.016.