Temperature- and pressure-dependent studies of niccolite-type formate frameworks of [NH3(CH2)4NH3][M2(HCOO)6] (M = Zn, Co, Fe)

Abstract

We report temperature-dependent electric, IR and Raman studies of niccolite-type formate frameworks templated by protonated 1,4-diaminobutane. Our results show that the zinc-analogue exhibits a first-order phase transition close to 240 K. Single-crystal dielectric data show a much stronger anomaly at the phase transition for ε′ along the a-direction compared to the c-direction. They also reveal that dipole relaxation exists in bnZn. Pronounced temperature-dependence observed for bending and torsion modes of the NH₃⁺ groups proves that ordering of protonated amine plays a major role in the phase transition mechanism. The ordering is associated with distortion of the zinc formate framework but the number of observed vibrational modes is much smaller than expected assuming 36-fold multiplication of the unit cell below T_C. It is also much smaller than reported for the Mn-analogue, which exhibits only a two-fold increase of the unit cell below T_C. We discuss the origin of this behavior. Our results also show that the Co-analogue exhibits a similar phase transition to its Zn-counterpart. However, the observed narrowing and splitting of the corresponding bands is significantly smaller, suggesting weaker distortion of the framework and the presence of some disorder for this compound even at 5 K. The Raman and IR spectra of the Fe-analogue show weak narrowing of bands upon cooling, indicative of statistical freezing of the protonated amine at low temperatures. We also report high-pressure Raman scattering studies of the zinc-analogue. This study revealed a pressure-induced reversible phase transition between 3.4 and 4.1 GPa. Large shifts and splitting of modes corresponding to the vibrations of HCOO⁻ ions associated with weak changes of the protonated amine prove that the major contribution to the phase transition mechanism comes from distortion of the zinc formate framework.