Heat capacities of alpha-iron(II) molybdate and iron(II) molybdate-II from 5 to 350 K

Abstract

Heat capacities of iron(II) molybdate-II (a metastable high-pressure phase) and of two samples of alpha-iron(II) molybdate were measured over the temperature range 5 to 350 K by means of adiabatic calorimetry. Rounding of lambda anomalies found at (43.0 +/- 0.1) K for FeMoO4-II and at (31.5 +/- 0.1) K and (31.75 +/- 0.1) K for two different samples of [alpha]-FeMoO4 is attributed to impurity introduced by hitherto unreported slow air oxidation of these substances. Excess entropies associated with magnetic ordering were estimated as 1.11 calth K-1 mol-1 for FeMoO4-II and 1.77 and 1.63 calth K-1 mol-1 for the two samples of [alpha]-FeMoO4. At 298.15 K, the values of Cp0, S0, "H0(T) - H0(0)', and -"G0(T) - H0(0)'/T, respectively, are 29.05 calth K-1 mol-1, 30.93 calth K-1 mol-1, 4852.3 calth mol-1, and 14.656 calth K-1 mol-1 for FeMoO4-II, and 28.31 calth K-1 mol-1, 32.7 calth K-1 mol-1, 4963 calth mol-1, and 16.1 calth K-1 mol-1 (selected composite values) for [alpha]-FeMoO4.