Neutron Diffraction Studies of the Negative Thermal Expansion in a Layered Indium Selenide Crystal

The neutron diffraction patterns have been analyzed for a layered single crystal and a powder of the gamma-polytype of indium selenide in the temperature range 10-300 K. In the temperature range 10-50 K, the excitation of bending vibrations due to the charge density waves changes the phonon spectrum and gives rise to a negative thermal expansion in the plane of layers, i.e., alpha(parallel to c) = -2.2 x 10(-6) K-1, which is characteristic of two-dimensional structures. The average (over the range T = 50-300 K) coefficients of thermal expansion along the principal crystallographic directions have been calculated: (alpha) over bar (perpendicular to c) = 10.48 x 10(-6) K-1 and (alpha) over bar (parallel to c) = 12.97 x 10(-6) K-1, which agree with the X-ray diffraction data previously obtained by the authors at T = 290 K.