Investigation of NMR shielding, EFG and structural relaxation of all-inorganic Pb-based Ruddlesden Popper halide perovskites

In this article the structure variations and relaxations, electric field gradient (EFG) and nuclear magnetic resonance (NMR) shielding of Cs2Pb(X1–uYu)4 (X/Y = Cl, Br, I and u = 0, 0.25, 0.5, 0.75, 1) are studied using density functional based calculations. The structure symmetry (space group) of the studied compounds are varying with the increment of halides at different sites. The space group of Cs2PbX4 is I4/mmm, while Cs2PbX2Y2 exist in the space group I4/mmm and P4/mmm, depending upon the halogen atomic coordinates. Similarly, Cs2PbX3Y and Cs2PbXY3 exist in three different crystal symmetries having space groups I4/mmm, P4/mmm and Pmm2. The ground state energies of Cs2Pb(X1–uYu)4 are lower in I4/mmm space group compared to P4/mmm and Pmm2, which show that these compounds are more stable in I4/mmm space group. NMR shielding increases with the increase in band gap energy, because the interaction of valence electrons and external magnetic field decreases. Density of states plot reveals that the Pb-5d orbital exists in the core region (σ-sphere about − 17.5 eV) and play a key role in NMR shielding, while the peaks of Cs-5p and X/Y-s orbitals exist at − 8 eV and − 12 eV respectively having contribution in NMR shielding. The negative contribution of the I-p state reduces the magnetic shielding effect (anti-bonding character increases), hence the applied external field strengthens. The electronic charge densities of Cs and Pb are elongated along z direction and make oblate distribution due to their negative electric field gradient ((V_zz^tot)), while the (V_zz^tot) values of halogens are positive, predict that the density of electronic charge compressed along the z direction and make prolate distribution.