Anomalous lattice specific heat of LiInSe2 at low temperatures

The molar heat capacity at constant pressure of the ternary chalcopyrites LiInSe2 and CuInSe2 have been determined at temperatures from 2K to 300 K with an adiabatic microcalorimeter. From the low temperature data we derive the following Debye temperatures at 0 K: theta(0)(LiInSe2) = 265 (10) K and theta(0)(CuInSe2) = 228(3) K. The standard entropies and enthalpies at 298.15 K are calculated by integration of the specific heat curves: S-0 = 146.6(1.5) J (mol K)(-1) and H-H-0 = 20.17(0.3) J K-1 for LiInSe2, and S-0 = 158.2(1.6) J (molK)(-1) and H-H-0 = 21.32(0.25) J mol(-1) for CuInSe2. Comparisons are made with previously reported values for the molar heat capaciy of these compounds, measured above 200 K, and with other chalcogenides of the type A(I)B(III)C(2)(VI), with A = Cu, Ag, B = Ga, In and C = S, Se, Te. It is shown that the averaged atomic heat scales to one general curve within 3% for ail known chalcogenide compounds except the Li compounds. LiInSe, displays an excess heat below about 80K that is represented by a (two-level) Schottky anomaly with an energy splitting of Delta/k = 71.4(4) K suggesting that the Li atom could reside in a double-well potential. Inspection of previously reported data for LiInTe2 hint at a similar effect in that compound.