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.