Low-temperature heat capacity measurements for MgCr
2O
4 have only been performed down to 52 K, and the commonly quoted third-law entropy at 298 K (106 J K
−1 mol
−1) was obtained by empirical extrapolation of these measurements to 0 K without considering the magnetic or electronic ordering
contributions to the entropy. Subsequent magnetic measurements at low temperature reveal that the Néel temperature, at which
magnetic ordering of the Cr
3+ ions in MgCr
2O
4 occurs, is at ∼15 K. Hence a substantial contribution to the entropy of MgCr
2O
4 has been missed. We have determined the position of the near-univariant reaction MgCr
2O
4+SiO
2=MgSiO
3+Cr
2O
3. The reaction, which has a small positive slope in
P-
T space, has been bracketed at 100 K intervals between 1273 and 1773 K by reversal experiments. The reaction is extremely sluggish,
and lengthy run times with a flux (H
2O, BaO-B
2O
3 or K
2O-B
2O
3) are needed to produce tight reversal brackets. The results, combined with assessed thermodynamic data for Cr
2O
3, MgSiO
3 and SiO
2, give the entropy and enthalpy of formation of MgCr
2O
4 spinel. As expected, our experimental results are not in good agreement with the presently available thermodynamic data.
We obtain Δ
f
H
∘
298=−1759.2±1.5 kJ mol
−1 and
S
∘
298=122.1±1.0 J K
−1 mol
−1 for MgCr
2O
4. This entropy is some 16 J K
−1 mol
−1 more than the calorimetrically determined value, and implies a value for the magnetic entropy of MgCr
2O
4 consistent with an effective spin quantum number (S') for Cr
3+ of 1/2 rather than the theoretical 3/2, indicating, as in other spinels, spin quenching.
Received: 9 May 1997 / Accepted: 28 July 1997
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