  Water ice and spin ice are important model systems in which theory can
directly account for zero point entropy associated with quenched
configurational disorder. Spin ice differs from water ice in the important
respect that its fundamental constituents, the spins of the magnetic ions, can
be removed through replacement with non-magnetic ions while keeping the lattice
structure intact. In order to investigate the interplay of frustrated
interactions and quenched disorder, we have performed systematic heat capacity
measurements on spin ice materials which have been thus diluted up to 90%.
Investigations of both Ho and Dy spin ices reveal that the zero point entropy
depends non-monotonically on dilution and approaches the value of Rln2 in the
limit of high dilution. The data are in good agreement with a generalization of
Pauling's theory for the entropy of ice.
