The influence of azide and imidazole on the properties of Mn- and Cd-based networks: conductivity and nonlinear phenomena

Abstract

We report a study on a family of four new Mn- and Cd-azide-imidazolate-based compounds with various crystal architectures. Notably, three of these compounds display noncentrosymmetric crystal arrangements at room temperature, a rare phenomenon in hybrid organic–inorganic materials. Both nonlinear optical (NLO) and electrical phenomena in these compounds are observed. The NLO processes include second and third harmonic generation, while the electrical nonlinear phenomena include the presence of polarization and a coercive field in the capacitance measurements (1–2 μC cm⁻²), and a piezoelectricity with ca. d₃₃ = 0.2 × 10⁻¹² m V⁻¹ coefficient. Additionally, the presence of the second and third harmonics is detected in DC conductivity measurements. The phase transition (PT) present in Mn-based compounds at ca. 370 K is confirmed by DSC, X-ray diffraction, Raman spectroscopy, dielectric, DC conductivity and capacitance measurements. The PT is triggered by the motions of imidazole molecules around a nitrogen-metal bond with minor adjustment of azide bridges in response to this motion as derived from the X-ray diffraction and spectroscopic data. Mn-analogues also feature antiferromagnetic order. Both Cd- and Mn-imidazolate-azides exhibit a conductivity, with a mixed electron/proton hopping transport mechanism, in the case of Cd- similar to those of classic semiconductors.