Recently we have reported an experimental study of the heat capacity of NdAlO 3 in the 0 to 900 K temperature range by combined low-temperature adiabatic calorimetry and high-temperature drop calorimetry  . The two sets of measurements were in good agreement and the enthalpy and entropy for this compound were derived by integration of the heat capacity curve. However, it was not taken into account that the lanthanide compounds often exhibit an excess term to heat capacity and entropy due to the electronic contribution, which will affect the entropy recommendation. Nd 3+ has a 4 I 9/2 ground state that is split due to the crystal field in most ionic crystals. The ground state of NdAlO 3 was found to split into five Kramer’s doublets at 0, 122, 148, 595 and 595 cm −1  . Also data for the higher states ( 4 I 11/2 , 4 I 13/2 and 4 I 15/2 ) of Nd 3+ are available  . The heat capacity and entropy calculated from these data are shown in Fig. 1 . The heat capacity curve shows a maximum around 50 K caused by the ground state splitting, followed by a gradual increase after about 500 K due to the contribution of the higher electronic states. The excess entropy S xs varies from 5.763 J K −1 mol −1 at low temperatures to about 20.5 J K −1 mol −1 at 900 K. The former value is equal to R ln ( g 0 ) where g 0 =2, the degeneracy of the ground state. Calorimetric measurements for NdAlO 3 in the 0.2–6 K range by Bartolomé et al.  have confirmed this value. These authors found a transition at (0.93±0.01) K, below which an antiferromagnetic ordering of the Nd 3+ ions occurs, with a total entropy change equal to Δ S= Rln (2) . The ‘practical’ entropy of NdAlO 3 is then obtained by adding R ln (2) to the entropy derived from the calorimetric measurements (99.3±0.3 J K −1 mol −1 ) to give S 0 ( NdAlO 3 , cr , 298.15 K )=(105.0±0.3) J K −1 mol −1 </P>
Thermochimica Acta – Elsevier
Published: Jul 20, 2001
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