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Abstract

The electron density distribution of the nickel compounds BaNiO2, BaNiO3 and CaNiN has been investigated experimentally by Ni-61 Mossbauer 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 q(exp.) and q(theor.) 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: q(exp.) = -15.7(1.5) . 10(21) Vm(-2) and q(theor.) = -15.59 . 10(21) V-m-2. The principal axis z is perpendicular to the NiO, squares. The crystal structure of BaNiO, contains face-sharing chains of NiO6 octahedra. In BaNiO3 q(Ni) is small: q(exp) = +/- 3.6 (2.0) . 10(21) Vm(-2) and q(theor.) = - 1.86 . 10(21) Vm(-2). Because of the small broadening of the Mossbauer resonance line the sign of q could not be determined experimentally. The nitridoniccolate CaNiN contains infinite linear chains The nitridoniccolate CaNiN contains infinite linear chains 1 over infinity [NiN2/2] which run perpendicular to the c axis. Unexpectedly, \q(Ni)/ in CaNiN is small: q(exp.)=0.0(2.0). 10(21)Vm(-2) and q(theor.)=-3.05 . 10(21)Vm(-2). Again the sign q(Ni) could not be determined experimentally. It is found theoretically that the small value of q(Ni) is caused by severe cancellation between sigma and pi contributions. [NiN2/2] which run perpendicular to the c axis. Unexpectedly, \q(Ni)\ in CaNiN is small: q(exp) = 0.0 (2.0) . 10(21) Vm(-2) and q(theor.) = -3.05 . 10(21) 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 sigma and pi contributions.