Revisiting a perovskite-like copper-formate framework NH4[Cu(HCOO)3]: order-disorder transition influenced by Jahn-Teller distortion and above room-temperature switching of the nonlinear optical response between two SHG-active states

Abstract

Metal-formate frameworks comprising ammonium cations as guests are proven to be a fertile ground to study various phenomena associated with the temperature-induced changes in structural, dielectric, optical, and magnetic properties. In this contribution, we revisit NH4[Cu(HCOO)3], a member of metal formates that distinguishes itself in terms of its phase transition behavior and associated properties. New data on structural dynamics of all phases of NH4[Cu(HCOO)3] have been obtained with the use of variable-temperature Raman measurements. Smooth changes of band positions observed near 220 K attest to the postulated continuous nature of low-temperature phase transition, whereas apparent discontinuities at 355 K confirm the first-order type of transition between orthorhombic (II) and hexagonal (I) phases. Low-temperature Raman data were confronted with diffraction results, pointing to a significant effect of Jahn-Teller distortion on the vibrational properties the CuO6 subnetwork. In the high-temperature range, a significant broadening of bands is observed, confirming that phase I is highly disordered, with the strongest changes of full width at half maximum (FWHM) parameters being observed for bands corresponding to NH4+ cations. Dielectric investigations revealed the symmetric shape of the observed process indicating the Debye-like relaxation. Thus, the dielectric relaxation was characterized in terms of the dipolar relaxation model using the Cole–Cole relaxation function, leading to an Ea value of approximately 0.76 eV. Finally, temperature-resolved second harmonic generation (SHG) measurements unequivocally corroborate the noncentrosymmetric setting of phases II and I, as well as allowed us to realize temperature-induced switching of second-order nonlinear optical (NLO) responses. We demonstrate that NH4[Cu(HCOO)3] serves as a host to uncommon kind of quadratic NLO switching, which takes advantage of two SHG-active states: SHG-high state below Tc, and SHG-low state above Tc. The demonstrated SHG-high – SHG-low temperature-driven bistability stands out from the vast majority of molecular and coordination polymer NLO switches that employ binary SHG-on and SHG-off switching schemes.