Heat-capacity C(T) data on the title materials, as well as on CuO for comparison, were obtained with use of a differential scanning calorimeter to a precision of ≃0.1–1%. The measurements were carried out (i) to characterize the thermal anomaly at the tetragonal-orthorhombic transition temperature ${\mathit{T}}_0$ of single-crystal ${\mathrm{La}}_2$${\mathrm{CuO}}_4$ and polycrystalline ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_4$ (0≤x≤0.20) and (ii) to search for thermal anomalies at the Néel temperature ${\mathit{T}}_{\mathit{N}}$ of ${\mathrm{La}}_2$${\mathrm{CuO}}_4$ (≃300 K). We find a cusp-shaped anomaly with a peak at 523 K (≊${\mathit{T}}_0$) for a single crystal of ${\mathrm{La}}_2$${\mathrm{CuO}}_4$ with ${\mathit{T}}_{\mathit{N}}$=304 K and find the transition to be second order, consistent with previous neutron- and x-ray-diffraction measurements. ${\mathit{T}}_{\mathit{N}}$ was determined from measurements of the anisotropic magnetic susceptibility on the same crystal, which are also presented. The C(T) anomaly at ${\mathit{T}}_0$ is smeared out somewhat in polycrystalline ${\mathrm{La}}_2$${\mathrm{CuO}}_4$.

The size of the anomaly decreases with x in polycrystalline ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_4$ until the anomaly is no longer observable for x≥0.10. For 0≤x≤0.08, ${\mathit{T}}_0$ decreases linearly with x at a rate ${\mathit{dT}}_0$/dx=-(2430±50) K. This variation of ${\mathit{T}}_0$ with x is consistent with that found previously using x-ray and neutron diffraction. No features in C(T) were observed at the Néel temperature of single-crystal or polycrystalline ${\mathrm{La}}_2$${\mathrm{CuO}}_4$. This is consistent with the expectation that the magnetic entropy is very small at ${\mathit{T}}_{\mathit{N}}$, since dynamic short-range intraplanar antiferromagnetic ordering, which begins at much higher temperatures, is well developed at ${\mathit{T}}_{\mathit{N}}$. A calculation shows that the expected size of the anomaly at ${\mathit{T}}_{\mathit{N}}$ is well below the resolution of the measurements.

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