Ni complexes in diamond

Spin-polarised local density functional cluster calculations are carried out on isolated interstitial (Ni-i(+)) and substitutional (Ni-s(-), Ni-s(+)) impurities in diamond and on Ni-i(+)-B-s(-), Ni-s(+)-B-s(-) and Ni-s(-)-N-s(+) complexes. Ni-i(+) (S=1/2) lies at the T-d site. Ni-s(-) (S=3/2) is shown to be stable and remains on-site in agreement with experimental results. Donor-acceptor complexes of Ni, namely Ni-i(+)-Br-s(-), Ni-s(-)-N-s(+) and Ni-s(+)-B-s(-) are stable trigonal defects which have a number of gap levels. Optical transitions are expected between some of these levels. We argue that there is no convincing evidence for Ni-i defects.