Disorder and Nonstoichiometry in Synthetic Garnets A3B5O12 (A = Y, LuLa, B = Al, Fe, Ga). A Simulation Study

Atomistic simulation calculations have been used to study disorder and deviation from stoichiometry in synthetic garnets A3B5O12 as a function of A3+ (Y3+, Lu3+ to La3+) and B3+ (Al3+, Ga3+, Fe3+) ionic radius. Trends corresponding to the energy of several defect reactions have been obtained. Results compare favorably with the available, albeit limited, experimental data. Disorder in garnets is dominated by formation of antisite defects and the corresponding energy decreases with decreasing A cation size. Incorporation of excess Y2O3 or rare-earth oxide is energetically preferred with respect to incorporation of excess B2O3 (B = Al, Fe, Ga). Solution of excess binary oxide occurs by the formation of antisites rather than vacancies. Neither an excess of A2O3 or B2O3 seems to be the main source of oxygen nonstoichiometry in Al and Ga garnets. On the contrary, accommodation of oxygen nonstoichiometry in iron garnets is energetically more favored if accompanied by the reduction of Fe(III) to Fe(II). The energy of formation of oxygen vacancies is lower for Lu, Yb, and Er iron garnets. Interactions between divalent iron and oxygen vacancies are discussed. Within the limitations of the present approach, the calculated trends contribute to the understanding of defect chemistry of garnets and provide suggestions for future experimental studies.