  A uniform distribution of La and Sr in lanthanum-strontium manganites would
lead to charged crystal planes, a charged surface, and arbitrarily large
surface energy for a bulk crystal. This divergent energy can be eliminated by
depleting the La concentration near the surface. Assuming an exponential form
for segregation suggested by experiment, the total electrostatic energy is
calculated, depending only upon the decay length and on an effective charge Z*
associated with the La ion. It is found to be lower in energy than
neutralization of the surface by changing Mn charge states, previously
expected, and lower than simply readjusting the La concentration in the surface
plane. The actual decay length obtained by minimizing this electrostatic energy
is shorter than that observed. The extension of this mechanism to segregation
near the surface in other systems is discussed.
