Heat Capacity and Thermodynamic Properties of α-Fe2O3 in the Region 300-1050 K. Antiferromagnetic Transition Entity Journal Article uri icon

DCO ID 11121/1422-1163-1344-8229-CC

year of publication

  • 1975

abstract

  • The heat capacity of synthetic α-Fe2O3, has been measured in the range 300-1050K by adiabatic shield calorimetry with intermittent energy inputs and temperature equilibration in between. A λ-type transition, related to the change from antiferro- to pammagnetism in the compound, is delineated and a maximum heat capacity of about 195 J K-1 mole-1 is observed over a 3 K interval around 955K. Values of thermodynamic functions have been derived and Cp (1000K), [H°(1000K) - H°(0)], and [S°(1000K) - S°(0)] are 149.0 J K-1 mole-1 , 115.72 J K-1 mole-1 , and 252.27 J K-1 mole-1 , respectively, after inclusion of earlier low-temperature results [X°(298.15K) - X°(0)]. The non-magneticheat capacity is estimated and the thermodynamic properties of the magnetic transition evaluated. The results are compared with spin-wave calculations in the random phase approximation below the Neel temperature and the Oguchi pair model above. An upper estimate of the total magnetic entropy gives 32.4  J K-1 mole-1 , which compares favorably with that calculated for randomization of five unpaired electron spins on each iron, ΔS = 2R ln6 = 29.79 J K-1 mole-1 for α-Fe2O3. The critical exponent α in the equation Cm = (A/α) [(|TN - T|/TN ) - 1] + B is - (0.50 ± 0.10)below the maximum and 0.50 ± 0.10 above, for TN = 955.0K. The high temperature tail is discussed in terms of short range order.

publication date

  • 1975

volume

  • 36