FULL PAPER Metallic Interfaces www.advtheorysimul.com Designing Nonpolar Metallic Interfaces using Insulating Transition Metal Olivine Phosphates Ajit Jena, Devaraj Murali, and Birabar Ranjit Kumar Nanda* Through density functional calculations, we have demonstrated that ferromagnetic and metallic (FM-M) phase can be tailored in superlattices consisting of two dissimilar antiferromagnetic and insulating olivine phosphates LiMPO 4 and LiMʹPO 4 where M and Mʹ are 3d transition metals. The proposed tailored superlattices are stable and differ from the regular superlattices through broken and missing PO 4 tetrahedra. As a result, the p–d covalent bondings become reasonable and transition metal ions are forced to stabilize in fractional charge state instead of the integer-charge state observed in bulk. These result in partially occupied parabolic dispersive bands to favor the metallic phase and therefore open up the possibilities to go beyond the conventional layered perovskite polar interfaces to create metallic heterostructures out of insulating oxides. Out of all M-Mʹ combinations, we find that Cr-Mn, Cr-Co, Cr-Ni, Mn-Co and Mn-Ni combinations yield the FM-M phase as ground state in the tailored superlattices. 1. Introduction In recent years, successful fabrication of atomically sharp inter- faces between two dissimilar transition metal oxides (TMOs) has gained significant attentions as the interfacial phases exhibit ex- otic electronic and magnetic phenomena, constructed by inter- play between spin, charge and orbital degrees of freedom, which are not found in the bulk constituents of the heterostructures. [1–7] Formation of high-mobility electron gas at the interface of two insulators LaAlO 3 and SrTiO 3 or LaMnO 3 and SrMnO 3, in order to quench the polar catastrophe introduced by charged layers of LaAlO 3 or LaMnO 3 , so far remains the highlight of the research in oxide interfaces. [2,3] In addition, formation of canted mag- netism through electron leakage from the paramagnetic metal CaRuO 3 to the antiferromagnetic insulator CaMnO 3 , [4] modula- tion of magnetic order at the interface between LaSrMnO 3 and BaTiO 3 by tuning the polarization direction of the latter, [5] and possible coexistence of superconductivity and magnetism at the interface of YBCO and LSMO [6,7] are a few of the other impor- tant results the research on oxide interfaces has offered. The common feature of these compounds, mostly single and dou- ble perovskites, constituting the interface is the layered (planar) Dr. A. Jena, Dr. D. Murali, Dr. B. R. K. Nanda Condensed Matter Theory and Computational Lab Department of Physics Indian Institute of Technology Madras Chennai 600 036, India E-mail: nandab@iitm.ac.in The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adts.201700007 DOI: 10.1002/adts.201700007 geometry (e.g., LaO and AlO 2 planes in LaAlO 3 ) they possess which facilitates the natural growth of such interfaces. However, restricting the synthesis of in- terfaces/superlattices only to the planar compounds limits the scope of research in this area and also acts as a hindrance to explore application of other complex ox- ides which do not possess the planar ge- ometry. One such family of oxides is the olivine phosphates of the form LiMPO 4 , M being a 3d transition metal. This fam- ily, though widely known for efficient Li ion diffusion, [8–10] has the disadvantage of having negligible electron mobility as these flat band materials have conductivity of the order 10 −4 S cm −1 or less. [11–13] There- fore, for practical purposes, exterior car- bon coating is done to inject delocalized electrons for conduction necessary for completing the battery circuit. [14] Recently, Liu et al. [15] have promisingly shown that layer by layer growth of LiFePO 4 can be achieved through atomic layer deposition (ALD) technique. By sequential deposition of Fe 2 O 3 , PO x , and Li 2 O layers through vapor-based surface reac- tion, instead of solid-state reaction mechanism, the thickness and film composition can be controlled with precision at the atomic level. Keeping this experimental development in mind, in this paper, we have designed heterostructures in the form of (LiMPO 4 ) 1 /(LiMʹPO 4 ) 1 superlattices, which now onward will be referred to as M-Mʹ superlattices, in search of conducting inter- faces which can enhance the electron mobility in these olivine compounds. Utilizing the results from the pseudopotential based density- functional calculations, as implemented in Quantum ESPRESSO [16] within the framework of GGA+U, here we propose the stable interfaces out of dissimilar olivine phosphates and found that some of them exhibit magnetized 2D electron gas (2DEG) and the mechanism of the formation of 2DEG is differ- ent from that of polar catastrophe led 2DEG in LaAlO 3 /SrTiO 3 or LaMnO 3 /SrMnO 3 interfaces. 2. Tailored Superlattices and Their Stability The four-formula unit LiMPO 4 crystal structure, as shown in Figure 1, can be best explained as the periodic stack of non- planar LiO, PO, and MO 2 units in the following order: . . . {PO- Li 2 O 2 -PO}-M 2 O 4 -{PO-Li 2 O 2 -PO}-M 2 O 4 . . . Therefore, the regu- lar M-Mʹ superlattice, shown in Figure 2 (upper panel), has the Adv. Theory Simul. 2018, 1700007 C  2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1700007 (1 of 6)