Accurate heat capacity measurements on powdered samples using a Quantum Design physical property measurement system

We have evaluated the accuracy of the heat capacity option of a Quantum Design physical property measurement system (PPMS) by measuring the heat capacity of various types of conducting and insulating samples over the temperature range from (2 to 300) K. In particular, the accuracy of measurements on a copper pellet was determined to be +/-2% for 2 K < T < 20K and +/-0.6% for 20K < T < 300 K, however similar measurements on a powdered sample of benzoic acid had errors as high as 20%. A new method for heat capacity measurements of powdered samples using a PPMS system has been developed that allows us to obtain heat capacity measurements for both insulating and conducting powdered samples with an accuracy of +/-1% from (20 to 300) K and +/-2% to +/-5% for T < 20K. Since the heat capacity of substances (and corresponding entropy contribution) is small at low temperatures for lattice-only contributions, the accuracy of +/-2% to +/-5% below 20 K is considered acceptable. As a test of the new method, the heat capacity of powdered bulk hematite has been measured in the temperature range from (2 to 300) K with the PPMS, and its standard entropy at T = 298.15 K was calculated to be (87.33 and 87.27) J . K-1 mol(-1), which deviates -0.08% and -0.15% from the accepted reference value, respectively. We recommend that this new method become the standard for accurate heat capacity measurements on insulating powdered samples using a PPMS system and the corresponding thermodynamic calculations. (C) 2010 Elsevier Ltd. All rights reserved.