Temperature- and pressure-dependent studies of a highly flexible and compressible perovskite-like cadmium dicyanamide framework templated with protonated tetrapropylamine

Abstract

We report temperature-dependent X-ray diffraction, thermal, dielectric, electron paramagnetic resonance (EPR), Raman and IR studies of [(C₃H₇)₄N][Cd(N(CN)₂)₃] ([TPrA][Cd(dca)₃]) exhibiting a rich sequence of temperature-induced phase transitions occurring at T₃ = 386 K, T₂ = 362 K, T₁ = 241 K and T₀ = 228 K. The X-ray diffraction study confirms the I4/mcm, P2₁c and P2₁/n symmetry of the high-temperature phase, the phase stable between T₂ and T₁ and the low-temperature phase stable below T₀, respectively. Our data show that the driving forces for the phase transitions at T₃ and T₂ are the partial ordering of the tetrapropylammonium cations (TPrA⁺) and dicyanamide (dca) linkers as well as the off-center shifts of TPrA⁺ cations and the tilting of the CdN₆ octahedra. X-ray diffraction data reveal that two low-temperature (LT) structural phase transitions at T₁ and T₀ are associated with strong monoclinic distortion of the [Cd(dca)₃]⁻ framework and further ordering of the TPrA⁺ cations and dca linkers. Raman and EPR data confirm the strong distortion of the framework, while dielectric data are consistent with the partial ordering of the TPrA⁺ cations at T₁. Dielectric and Raman data also prove that the phase transition at T₀ is associated with further freezing of the propyl groups’ motions of the TPrA⁺ cations, whereas EPR data indicate that this phase transition is also associated with some distortion of the cadmium-dicyanamide framework. In the family of [TPrA][M(dca)₃] compounds (M = Cd²⁺, Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺), phase transitions into monoclinic phases are unique only for the Cd analogue, and our data indicate that this behaviour can be attributed to the higher flexibility of the [TPrA][Cd(dca)₃] framework compared to the other analogues. We also report high-pressure X-ray powder diffraction and Raman studies of [TPrA][Cd(dca)₃]. Fitting of the pressure dependence of the unit cell volume to the third order Birch–Murnaghan equation of state gives a bulk modulus of 6.2(6) GPa and its pressure derivative B′ = 12(2). The small value of the bulk modulus demonstrates the highly compressible nature of [TPrA][Cd(dca)₃]. X-ray diffraction and Raman data show that this compound undergoes a pressure-induced phase transition between 0.3 and 0.4 GPa into a monoclinic structure, probably the same as that observed at ambient pressure below T₀.