  We investigated the rare-earth transition metal oxide series, Ln2CuTiO6
(Ln=Y, Dy, Ho, Er and Yb), crystallizing in the hexagonal structure with
non-centrosymmetric P63cm space group for possible occurrences of multiferroic
properties. Our results show that while these compounds, except Ln=Y, exhibit a
low temperature antiferromagnetic transition due to the ordering of the
rare-earth moments, the expected ferroelectric transition is frustrated by the
large size difference between Cu and Ti at the B-site. Interestingly, this
leads these compounds to attain a rare and unique combination of desirable
paraelectric properties with high dielectric constants, low losses and weak
temperature and frequency dependencies. First-principles calculations establish
these exceptional properties result from a combination of two effects. A
significant difference in the MO5 polyhedral sizes for M = Cu and M = Ti
suppress the expected co-operative tilt pattern of these polyhedra, required
for the ferroelectric transition, leading to relatively large values of the
dielectric constant for every compound investigated in this series.
Additionally, it is shown that the majority contribution to the dielectric
constant arises from intermediate-frequency polar vibrational modes, making it
relatively stable against any temperature variation. Changes in the temperature
stability of the dielectric constant amongst different members of this series
are shown to arise from changes in relative contributions from soft polar
modes.
