Low-temperature thermal expansion in CuInSe2

The thermal expansion coefficients αa and αc of the lattice parameters a and c of CuInSe2 are measured in the temperature range from 30 to 300 K. Both αa and αc decrease with decreasing temperature below 300 K and become negative at about 60 and 80 K, respectively. The results are discussed in terms of the principal Grüneisen parameters of chalcopyrite structure crystals. It is concluded that there are low-energy lattice vibrational modes with negative Grüneisen parameters in CuInSe2.