In general, anharmonicity of potential-energy function, which can be detected by vibrational frequency shift in the infrared and Raman spectra by applied stress, is responsible for the thermal expansion. The thermal expansion, i.e.,separation between the molecules with increasing temperature, occurs owing to the asymmetry
of the potential function. When a material has strong anharmonicity with low modulus, it shows marked thermal expansion [28–34]. Since the modulus becomes lower
by the addition of TCP, pronounced thermal expansion is expected.