Nonstoichiometry, Defect Structure and Energetics in T (La 2 MO 4 ; M=Cu, Ni) and Ts' (Nd 2 CuO 4 ) Structures
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NONSTOICHIOMETRY, DEFECT STRUCTURE AND ENERGETICS IN T (La 2MO 4; M=Cu, Ni) and T' (Nd 2CuO 4 ) STRUCTURES ANURAG DWIVEDI AND A. N. CORMACK, NYS College of Ceramics, Alfred University, Alfred, NY-14802
Abstract Atomistic computer simulation is performed to investigate the extent and nature of nonstoichiometry in La 2 CuO 4 , and La 2 NiO 4 . Results suggest anion Frenkel to be the dominant intrinsic defect in both compounds. Spontaneous oxidation is predicted for La 2 NiO 4 but not for La 2 CuO 4 ; however, both compounds are more easily oxidised than reduced, as opposed to Nd 2 CuO 4 , a superconducting compound with same generic formula. Oxidation takes place via accommodation of oxygen interstitials; however, the possibility of metal vacancies can not be completely ignored in La2CuO4. There is only one favorable oxygen interstitial site which is same in both the compounds La 2 MO 4 (M=Cu, Ni). We found a favorable oxygen interstitial site in T' structure too. Relaxed structures around an oxygen interstitial in these compounds are reported.
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INTRODUCTION
Undoped La 2 Cu0 4 and La2NiO 4 are insulators, but they exhibit superconductivity after La is substituted by a large cation such as Ba or Sr (x=0.15) [1]. Recent investigations have shown that same level of carriers may be produced when excess oxygen is incorporated into
the structure [2] [3]. Another important feature of these compounds lies in their intrinsic tendency to oxidise as opposed to the T' structured superconductor Nd 2 CuO 4 which is readily reduced. Oxygen nonstoichiometry, thus, seems to play an important role in the superconduc-
tivity phenomenon observed in La-Cu-O and La-Ni-O systems. Oxygen related defects are, thus, of immediate interest. Hong and Smyth [4] put forward a structural basis for the different directions of oxygen stoichiometry observed in the T and T' compounds. They argued that, the
primary distinction between these two compounds is that there is a favorable oxygen interstitial site in the La 2CuO 4 lattice but not in the Nd 2 CuO 4 lattice. In addition to 0i incorporation, some possible mechanisms for accommodating excess oxygen are metal vacancies, and the formation of Ruddlesden-Popper (R-P) type phases; they are discussed in detail for La 2 MO 4 (M=Cu, Ni), elsewhere[5]. Excess oxygen in the lattice of La 2CuO 4+6 might take various forms, such as oxide (02-) [6], peroxide (022-) [7], or superoxide 3 (02-) [7], ions all of which require formation of Cu + for electroneutrality. There are also reports of superconducting La 2CuO 4 to be La deficient with La/Cu ratios varying from 1.99 [8] [9], to 1.8 [10]. Although intergrowths of R-P type homologous series members, Lan+ICuO 3n+l have been reported up to n=7 by Davies and Tilley [11], other workers have failed to produce any phase pure compound of this series[12). It is noteworthy that first few members of R-P series have been synthesised in the La-Ni-O system [13] [14]. Computer simulation studies of Dwivedi et al [15] [16] provide a thermodynamic basis for the observe
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