Computational studies of low dimensional functional materials

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Gupta, Kanika (2020). Computational studies of low dimensional functional materials. University of Birmingham. Ph.D.

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Abstract

This thesis describes the application of advanced atomistic simulation techniques to study the defect properties, dopant substitution and ion insertion reactions in two novel materials FeSb $_2$ O $_4$ and Ca $_5$ Ir $_3$ O $_{12}$ . New pair potentials were empirically derived to fit both the parent phases and a series of related phases. The crystal structures predicted using these potentials were in good agreement with the observed values. These potentials were then used to investigate the defect behaviour (both intrinsic and extrinsic) in FeSb $_2$ O $_4$ and Ca $_5$ Ir $_3$ O $_{12}$ . For FeSb $_2$ O $_4$ , the calculations predict that the preferred mode of intrinsic disorder in the system would be through the creation of oxygen Frenkel defects. The dopant substitution calculations predict that among the species that were examined, the most favourable dopant species at Fe site is Mg $^{2+}$ and at Sb site is, Pb $^{2+}$ . The preferred charge compensation mechanism for this substitution was predicted to be through the oxidation of Fe $^{2+}$ to Fe $^{3+}$ in the chains of linked octahedra. For oxygen and fluorine insertion, the calculations predict that the oxygen insertion reactions are energetically more favourable. In the case of oxygen insertion reaction, the simulations predict that the interstitial oxygen ions occupy sites in the one-dimensional channels bonding to Sb ions in the channel walls. The extra oxygen intake is balanced by the oxidation of Fe $^{2+}$ to Fe $^{3+}$ in the chains of octahedra and Sb $^{3+}$ to Sb $^{5+}$ in the channel walls. The calculations predict the formation of a defect cluster comprising of three 4-coordinate Sb $^{3+}$ ions, two O $^{2-}$ interstitials and one 6-coordinate Sb $^{5+}$ ion. Similarly, for fluorine insertion, the calculations predict that the interstitial ions occupy the central position in the channels and are bonded to two 4-coordinate Sb $^{3+}$ ions in the channel walls. For Ca $_5$ Ir $_3$ O $_{12}$ , the defect calculations suggest that the preferred mode of the intrinsic disorder will be through the creation of calcium Frenkel defects. The energetics of Na insertion in Ca $_5$ Ir $_3$ O $_{12}$ that the Na goes to Ca site 1 to form Ca $_4$ NaIr $_3$ O $_{12}$ and goes to Ca2 site in Ca $_3$ Na $_2$ Ir $_3$ O $_{12}$ .