Ab-initio Band Structure Calculations and 61 Ni-Mössbauer Studies of BaNiO2 , BaNiO3 and CaNiN

Abstract

The electron density distribution of the nickel compounds BaNiO2 , BaNiO3 and CaNiN has been investigated experimentally by 61Ni Mössbauer spectroscopy and theoretically by band structure calculations using the FP-LMTO (Full Potential Linear Muffin-Tin Orbital) method. For all compounds good agreement is found between the experimental and theoretical values of the electric field gradient qexp. and qtheor. at the nickel site. BaNiO2 contains nickel in a square-planar coordination forming puckered chains of edge-sharing NiO4 squares. |q| at nickel is large: qexp = - 15.7(1.5) • 1021 Vm-2 and qtheor. = - 15.59 • 1021 Vm-2 . The principal axis z is perpendicular to the NiO4 squares. The crystal structure of BaNiO3 contains face-sharing chains of NiO6 octahedra. In BaNiO3 q(Ni) is small: qexp. = ± 3.6(2.0) • 1021 Vm-2 and qtheor. = - 1.86 • 1021 Vm-2 . Because of the small broadening of the Mössbauer resonance line the sign of q could not be determined experimentally. The nitridoniccolate CaNiN contains infinite linear chains 1 ∞[NiN2/2 ] which run perpendicular to the c axis. Unexpectedly, |q(Ni)| in CaNiN is small: qexp. = 0.0 (2.0) • 1021 Vm-2 and qtheor. = - 3.05 • 1021 Vm-2. Again the sign of q(Ni) could not be determined experimentally. It is found theoretically that the small value of q(Ni) is caused by severe cancellation between σ and π contributions.