Cation segregation to yttrium aluminum garnet (111) surfaces

Single crystals of yttrium aluminum garnet doped with various cations were annealed in air at different temperatures for varying amounts of time. Dopants were chosen to probe the effects of size, charge and site occupancy on surface segregation. Of a number of dopants (Ca, Nd, Cr, Si, Sr) Ca was the only dopant observed to consistently segregate upon annealing in air. Ca enrichment at the (111) surface was measured with Auger electron spectroscopy and the segregation enthalpy was determined as Delta H-seg = -32.33 kJ mol(-1). Since variation in size did not cause segregation while variations in valence did, it is suggested that surface segregation is electrostatically driven. Theoretical calculations explain the noted lack of Si segregation in co-doped samples in terms of the vacancy formation energy dependence of the isoelectric point. It is predicted that an excess of negative charge, on the order of 600 ppm, is required to observe a compositional isoelectric point. (C) 1998 Published by Elsevier Science B.V. All rights reserved.