  We present low-temperature anelastic and dielectric spectroscopy measurements
on the perovskite ionic conductor BaCe(1-x)Y(x)O(3-x/2) in the protonated,
deuterated and outgassed states. Three main relaxation processes are ascribed
to proton migration, reorientation about an Y dopant and tunneling around a
same O atom. An additional relaxation maximum appears only in the dielectric
spectrum around 60 K, and does not involve H motion, but may be of electronic
origin, e.g. small polaron hopping. The peak at the lowest temperature,
assigned to H tunneling, has been fitted with a relaxation rate presenting
crossovers from one-phonon transitions, nearly independent of temperature, to
two-phonon processes, varying as T^7, to Arrhenius-like. Substituting H with D
lowers the overall rate by 8 times. The corresponding peak in the dielectric
loss has an intensity nearly 40 times smaller than expected from the classical
reorientation of the electric dipole associated with the OH complex. This fact
is discussed in terms of coherent tunneling states of H in a cubic and
orthorhombically distorted lattice, possibly indicating that only H in the
symmetric regions of twin boundaries exhibit tunneling, and in terms of
reduction of the effective dipole due to lattice polarization.
