Structural, electronic and magnetic properties of RNiO3 (R=Pr, Nd) thin films and layered structures

Abstract

Perovskites metal oxides make a fascinating class of materials due to the mutual interactions of charge, spin, and orbital degrees of freedom [1,2]. These oxides exhibit wealth physics of fundamental importance such as ferroelectricity,multiferocity, metal to insulator phase transitions, colossal magnetoresistance, high-temperature superconductivity, charge and spin orderings, etc[3–5]. Due to these intriguing properties, perovskite metal oxides have gained the attention of the scientific community for studying theoretical as well as technological aspects of these materials. These materials are identified to be capable of replacing conventional semiconducting materials such as silicon and emerging into a new field known as Mott –electronics [6,7]. Perovskites follow a chemical stoichiometric formula ABO3, where A is a large size cation and B is generally a transition metal cation. Ideally, a perovskite should have a cubic unit cell with B-O-Bbond angle 180° with one BO6 octahedral surrounded by eight A cations. The electronic and magnetic properties are influenced by the B-O-B bond length, angle and overlapping or gap between metal 3d band and oxygen 2p band [8,9]. Nevertheless, the structure may undergo a distortion at low temperatures and hence the structure acquires a lower symmetry [5-6]. This distortion can be quantified in terms of Goodenough tolerance factor (t) given as 𝑡= 𝑟𝐴−𝑂√2𝑟𝐵−𝑂……………..(1.1) Here, 𝑟𝐴−𝑂 and 𝑟𝐵−𝑂 are the ionic bond distances from oxygen to A and B cations, respectively. The value of t = 1 represents the ideal cubic crystal structure and a deviation from this value indicates a structural distortion causing a transition to a lower symmetry. This structural distortion changes the B-O-B angle, and profoundly modifies the electronic and magnetic properties of the system [10–12]. At 180° bond angle, the d orbital band generally overlaps with the oxygen p band and hence favors the conduction mechanism. Octahedral distortion in this structure reduces B-O-B bond angle and gradually forms a bandgap between the oxygen p band and transition metal d band. Hence, the structure-property relations generally govern the properties and functionalities in these oxides.