ANOMALOUS LATTICE SPECIFIC-HEAT OF LIINSE2 AT LOW-TEMPERATURES

The molar heat capacity at constant pressure of the ternary chalcopyri tes LiInSe2 and CuInSe2 have been determined at temperatures from 2K t o 300 K with an adiabatic microcalorimeter. From the low temperature d ata we derive the following Debye temperatures at 0 K: theta(0)(LiInSe 2) = 265 (10) K and theta(0)(CuInSe2) = 228(3) K. The standard entropi es and enthalpies at 298.15 K are calculated by integration of the spe cific 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, measure d 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, display s 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 sugge sting that the Li atom could reside in a double-well potential. Inspec tion of previously reported data for LiInTe2 hint at a similar effect in that compound.