  We have synthesised ceramic specimens of the tetragonal tungsten bronze
K3Li2Ta5O15 (KLT) and characterized its phase transition via X-ray, dielectric
permittivity, ultrasonic spectroscopy and heat capacity measurements. The space
group of KLT is reported as both P4/mbm or Cmmm with the orthorhombic
distortion occurring when there are higher partial pressures of volatile K and
Li used within the closed crucibles for the solid state synthesis. The data
show strong relaxor behaviour, with the temperature at which the two dielectric
relative permittivity peaks decreasing with 104 K $\geqslant$ Tm1 $\geqslant$
69 K and 69 K $\geqslant$ Tm2 $\geqslant$ 46 K as probe frequency f is reduced
from 1 MHz to 316 Hz. The data satisfy a Vogel-Fulcher model with an
extrapolated freezing temperature for {\epsilon}' and {\epsilon}" of Tf1 = +
15.8 and - 11.8 K and Tf2 = - 5.0 and - 15.0 K for f $\rightarrow$ 0 (tending
to dc). Therefore by tuning frequency, the transition could be shifted to
absolute zero suggesting KLT has a relaxor-type quantum critical point. In
addition, we have reanalysed the conflicting literature for Pb2Nb2O7 pyrochlore
which suggests that this is also a relaxor-type quantum critical point as the
freezing temperature from Vogel-Fulcher fitting is below absolute zero. Since
the transition temperature evidenced in the dielectric data at ca. 100 kHz
shifts below zero Kelvin for very low frequencies, heat capacity data collected
in the zero-frequency (dc) limit, should not indicate a transition. Both of
these materials show promise as possible new relaxor-type quantum critical
points within non-perovskite based structures as multiple compounds are
reported with low-temperature transitions.
