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

摘要

We report temperature-dependent X-ray diffraction, thermal, dielectric, electron paramagnetic resonance (EPR), Raman and IR studies of [(C3H7)(4)N][Cd(N(CN)(2))(3)] ([TPrA][Cd(dca)(3)]) exhibiting a rich sequence of temperature-induced phase transitions occurring at T-3 = 386 K, T-2 = 362 K, T-1 = 241 K and T-0 = 228 K. The X-ray diffraction study confirms the I4/mcm, P42(1)c and P2(1)/n symmetry of the high-temperature phase, the phase stable between T-2 and T-1 and the low-temperature phase stable below T-0, respectively. Our data show that the driving forces for the phase transitions at T-3 and T-2 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 T-1 and T-0 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 T-1. Dielectric and Raman data also prove that the phase transition at T-0 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 T-0.