Magnetoelectric Multiferroicity and Magnetic Anisotropy in Guanidinium Copper(II) Formate Crystal

Hybrid metal-organic compounds as relatively new and prosperous magnetoelectric multiferroics provide opportunities to improve the polarization, magnetization and magneto-electric coupling at the same time, which usually have some limitations in the common type-I and type-II multiferroics. In this work we investigate the crystal of guanidinium copper (II) formate [C(NH2)3]Cu(HCOO)3 and give novel insights concerning the structure, magnetic, electric and magneto-electric behaviour of this interesting material. Detailed analysis of crystal structure at 100 K is given. Magnetization points to the copper (II) formate spin-chain phase that becomes ordered below 4.6 K into the canted antiferromagnetic (AFM) state, as a result of super-exchange interaction over different formate bridges. The performed ab-initio colinear density functional theory (DFT) calculations confirm the AFM-like ground state as a first approximation and explain the coupling of spin-chains into the AFM ordered lattice. In versatile measurements of magnetization of a crystal, including transverse component besides the longitudinal one, very large anisotropy is found that might originate from canting of the coordination octahedra around copper (II) in cooperation with the canted AFM order. With cooling down in zero fields the generation of spontaneous polarization is observed step-wise below 270 K and 210 K and the effect of magnetic field on its value is observed also in the paramagnetic phase. Measured polarization is somewhat smaller than the DFT value in the c-direction, possibly due to twin domains present in the crystal. The considerable magneto-electric coupling below the magnetic transition temperature is measured with different orientations of the crystal in magnetic field, giving altogether the new light onto the magneto-electric effect in this material.

Incorporated Guanidinium Expands the CH3NH3PbI3 Lattice and Enhances Photovoltaic Performance

Guanidinium (GA) has been widely used as an additive in solar cells for enhanced performance. However, the size of the guanidinium cation is too large to be incorporated in the cage of the perovskite structure. Instead, GA forms a variety of structures with lead iodide, where its role in the perovskite crystal as well as solar cell devices is unclear. In this study, we demonstrate that GA can be incorporated into the structure of MAPbI₃ as (GA)_x(MA)_1-xPbI₃. From single-crystal X-ray crystallographic refinement, we observe lattice expansion and Pb-I bond elongation with GA incorporation similar to exerting "negative pressure", which weakens orbital overlap and widens the band gap from 1.49 to 1.53 eV. We find that the highest percentage of GA that can be incorporated into the 3D MAPbI₃ structure is 5.26%, as confirmed by nuclear magnetic resonance. The alloyed (GA)_x(MA)_1-xPbI₃ exhibits increased PL lifetimes from 154.4 to 266.3 ns after GA incorporation while the V_oc of (GA)_x(MA)_1-xPbI₃ devices enlarges from 1.05 to 1.11 V. High efficiencies in solar cell devices up to 20.38% with a J_sc of 23.55 mA cm^-2, V_oc of 1.11 V, and FF of 0.78 have been achieved, with stable photovoltaic performance for 900 h in air.

Insights on the Jahn-Teller distortion, hydrogen bonding and local-environment correlations in a promised multiferroic hybrid perovskite

Perovskite-type formates have recently emerged as potential candidates for multiferroics. Previous ab initio calculations have concluded that copper guanidinium formate (CuGF) is a promising multiferroic. Although ab initio design and characterisation of inorganic multiferroics is at a mature stage, this situation is not true for multiferroic metal-organic frameworks. Regardless of these limitations, such predictions have also been extended to other materials. Here, we present with deeper insights into the structural aspects based on single crystal neutron diffraction studies, questioning such predictions in CuGF. The comparative investigation of the polar CuGF and its centrosymmetric cousin, MnGF, uncovers the differences in the N-H⋯O bonds. Such differences are associated with the Jahn-Teller distortion, which balances its influence on the local-environment of the three sets of ND₂ in the guanidinium cation. Hydrogen bonds which were not dealt with in the previous ab initio studies have been carefully treated in this experimental study. Hence, providing a crucial understanding based on which an encouraging platform may be rendered for improving the ab initio calculations for hybrid perovskites catering various applications.

High-pressure phase transitions with group–subgroup disagreement in metal guanidinium formates

Neutron diffraction studies reveal high-pressure phase transitions in metal guanidinium formates. The change in symmetry disregards the group–subgroup association and yet exhibits reversibility.