The crystal structure, lattice dynamics and specific heat of M(C2H8N2)Cl2 (M = Zn, Cu) metal-organic compounds

Comparative analysis of structural and physical properties of Cu(en)Cl2 and Zn(en)Cl2 (en = ethylenediamine) metal-organic compounds was performed. In Cu(en)Cl2 the ethylenediamine coordinates as a typical chelating ligand with carbon disorder, while single crystal X-ray study of Zn(en)Cl2 revealed untypical bridging en ligand with the trans conformation. Infrared and Raman spectra indicate sensitivity of Zn(en)Cl2 to the preparation method. Specific heat of both compounds at temperatures from 2 to 300 K is governed preferentially by low-frequency vibrations of metal chromophores. The appearance of a boson-like peak in Zn(en)Cl2 and observation of Debye-like behavior in Cu(en)Cl2 specific heat suggest qualitative differences in acoustic/optical phonon branches within the Brillouin zone. The appearance of the boson peak in perfect crystals as a potential hallmark of anomalous phonon scattering and its impact on the thermal transport and magnetic properties is discussed.