  We report the first principle investigations on the structural, electronic,
magnetic and ferroelectric properties of a Pb free double perovskite
multiferroic Bi2NiTiO6 using density functional theory within the general
gradient approximation (GGA) and GGA+U method. Our results show that Bi2NiTiO6
will be an insulator with G-type magnetic ordering in its ground state with
Ni2+ in a high spin state and a spin moment of 1.74\mu_B. The paraelectric
phase stabilizes in nonmagnetic state with Ni2+ in low spin configuration
showing that spin state transition plays an important role in strong
magnetoelectric coupling in Bi2NiTiO6. The bonding characteristics of the
constituents are analyzed with the help of partial density of states and Born
effective charges. The presence of Ti ions at Ni sites suppresses the
disproportionation observed in case of BiNiO3 and results in a
noncentrosymmetric crystal structure. The coexistence of Bi 6s lone pair and
Ti4+ d0 ions which brings covalency produces a polarization of 32 \muCcm-2.
