AMoO4 (A = Mg, Ni) molybdates: Phase stabilities, electronic structures and chemical bonding properties from first principles

Stability of different phases of AMoO4 (A = Mg, Ni) molybdates versus A–O bonding and the corresponding electronic structures are examined from first principles. The energy-volume equations of state for three forms (β, α, ω), characterized by decreasing volumes in the sequence of Mg and Ni molybdates are established. While NiMoO4 is energy stabilized in the sequence β → α → ω, an opposite behavior is identified for the Mg molybdate. Charge analysis characterizing ionic Mg2+ versus covalent Ni+1.2 behaviors can explain the trend. Electronic band structure also shows large differences: MgMoO4 is insulating with a 2 eV band gap while in a magnetic state, NiMoO4 is a small gap (0.2 eV) semi-conductor. Chemical bonding properties show weak Mg and strong Ni bonding with oxygen, while identifying the Mo–O interaction as prevailing.