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ABO 3 perovskites oxides show fascinating functional properties, including ferroelectricity, magnetism, metal-to-insulator transition, half metallicity, polar metallic phase, etc. In particular, rare earth nickelates and alkaline earth nickelates within the orthorhombic crystal structures show an insulator–metal transition (IMT) and half-metallic (HM) nature respectively. Here, we report a series of ferromagnetic polar half metals by constructing LnNiO 3 /CaNiO 3 short period 1/1 superlattices (SLs) with Ln = La, Nd, Gd, Dy, Tm, and Lu within an orthorhombic phase. The polar distortion is induced by a hybrid-improper ferroelectric mechanism i.e., by constructing Ln/Ca layer-ordering. The system shows a ferromagnetic Ni-Ni interaction, whereas Ni-3 d 's are interacting antiferromagnetically with O-2 p. We explain that the origin of observed half metallicity and ferromagnetism is due to strong hybridization between Ni-3 d and O-2 p near the Fermi level and double exchange mechanism, respectively. • Ferromagnetic polar half-metals within LnNiO 3 /CaNiO 3 • Large spin polarization is expected. • Hole mediated double exchange mechanism drives the ferromagnetic ordering. • The systems exhibit polar nature through hybrid improper ferroelectric mechanism. • These systems are metallic due to strong hybridization between Ni-3 d and O-2 p. [ABSTRACT FROM AUTHOR]

Starting from a room temperature parent La 2 MnNiO 6 double perovskite as a prototype example, we ventured ferroelectricity and metal–insulator transition in the AA ′ BB ′ O 6 double perovskites. Using first-principles density functional calculations guided by symmetry, we have shown that by appropriate A-site substitution i.e., by forming AA ′ combinations, La 2 MnNiO 6 within Mn and Ni in rock-salt ordering can be transformed into both ferroelectric insulating or polar metallic phase. We found that the charge difference between the cations and the radius mismatch between A and A ′ are the key factors for deciding a particular ordering. The layered ordering becomes more stable when both charge difference and radius mismatch are large. The energy gain is more pronounced when the charge difference at the A-site is large. Further, we have identified and discussed the key structural distortions responsible for driving functional properties into the system. Our proposed mechanism is valid for other similar A 2 BB ′ O 6 systems within 3 d -3 d double perovskites architecture with B and B ′ sublattices in the rock-salt ordering. • A-site cation ordered double perovskites are not common in the literature. • A-site cation ordered double perovskites are designed. • Layered double perovskites are established multiferroics. • The additional carrier(s) can lead the systems to a metal–insulator transition. [ABSTRACT FROM AUTHOR]