Heat capacity of single-crystal La2CuO4 and polycrystalline La2−xSrxCuO4 (0≤x≤0.20) from 110 to 600 K

Abstract

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 congruent-to 0.1-1%. The measurements were carried out (i) to characterize the thermal anomaly at the tetragonal-orthorhombic transition temperature T(o) of single-crystal La2CuO4 and polycrystalline La2-xSr(x)CuO4 (O less-than-or-equal-to x less-than-or-equal-to 0.20) and (ii) to search for thermal anomalies at the Neel temperature T(N) of La2CuO4 (congruent-to 300 K). We find a cusp-shaped anomaly with a peak at 523 K ( almost-equal-to T(o)) for a single crystal of La2CuO4 with T(N) = 304 K and find the transition to be second order, consistent with previous neutron- and x-ray diffraction measurements. T(N) was determined from measurements of the anisotropic magnetic susceptibility on the same crystal, which are also presented. The C(T) anomaly at T(o) is smeared out somewhat in polycrystalline La2CuO4. The size of the anomaly decreases with x in polycrystalline La2-xSr(x)CuO4 until the anomaly is no longer observable for x greater-than-or-equal-to 0.10. For 0 less-than-or-equal-to x less-than-or-equal-to 0.08, T(o) decreases linearly with x at a rate dTo/dx = -(2430 +/- 50) K. This variation of T(o) with x is consistent with that found previously using x-ray and neutron diffraction. No features in C(T) were observed at the Neel temperature of single-crystal or polycrystalline La2CuO4. This is consistent with the expectation that the magnetic entropy is very small at T(N), since dynamic short-range intraplanar antiferromagnetic ordering, which begins at much higher temperatures, is well developed at T(N). A calculation shows that the expected size of the anomaly at T(N) is well below the resolution of the measurements.