Physical Properties of Metal-Organic Zeolitic Imidazolate Frameworks

Zeolitic imidazolate frameworks represent new materials falling into the large group of metal-organic frameworks with wide application potential. The present work is focused on the study of fundamental properties of imidazolate compound [{Zn(mIm)(2)center dot 2H(2)O}(infinity)],which is referred in literature as ZIF-8. Measurements of infrared spectra in the mid-infrared and far-infrared regions at room temperature as well as heat capacity in the range from 2 to 300 K were performed. The character of the infrared spectra suggests why the specific heat data values are so small, and even at 300 K the values are much lower than the classical value of 3sR resulting from the equipartition principle. Analysis of the specific heat of the ZIF-8 powder sample within the Debye model including only the contribution of three acoustic phonon branches revealed deviations of the model from experimental data already above 20 K. Low-frequency optical modes as indicated by infrared spectra are responsible for the observed deviations.