Temperature-dependent studies of a new two-dimensional cadmium dicyanamide framework exhibiting an unusual temperature-induced irreversible phase transition into a three-dimensional perovskite-like framework

Abstract

We report the synthesis, crystal structure, and thermal, dielectric, optical and phonon properties of a new two-dimensional (2D) cadmium(II) complex [(C₃H₇)₄N][Cd(N(CN)₂)₃]. Our results show that this compound crystallizes in a two-dimensional monoclinic structure, with the space group P2/n, with ordered tetrapropylammonium cations and disorder of some dicyanamide linkers. It undergoes a structural phase transition at 245 K into another low-temperature (LT) monoclinic structure, with the space group P2₁/n. X-ray diffraction, dielectric, IR and Raman studies show that freezing of the dca motions stands at the origin of the phase transition. Optical studies indicate that this material has an energy band gap of 4.83 eV and exhibits intense bluish-white emission under 266 nm excitation. Upon heating, this compound undergoes an irreversible phase transition near 390 K associated with significant bond rearrangement. The high-temperature (HT) phase has a three-dimensinal (3D) perovskite-like structure. [(C₃H₇)₄N][Cd(N(CN)₂)₃] is, therefore, the first example of a hybrid organic–inorganic dicyanamide exhibiting a temperature-induced reconstructive transition from a 2D (layered) structure to a 3D (perovskite-like) structure.