  Pb_1.83Mg_0.29Nb_1.71O_6.39 (PMN) crystallizing in a cubic pyrochlore
structure exhibits, as the first dielectrics with pyrochlore structure, typical
feature of quantum paraelectrics - its permittivity continuously increases on
cooling and levels off below ~ 30K, without any signature of a structural phase
transition. Broad-band dielectric spectra do not show any dielectric dispersion
in the real part of permittivity up to 8.8 GHz. THz and infrared spectra reveal
a soft polar optic mode which is responsible for the temperature dependence of
the permittivity. The leveling-off of the permittivity at low temperatures
obeys the Barrett formula and the fitted vibrational zero-point energy
1/(2*k_B*T_1) corresponds to the measured soft mode frequency. The number of
observed infrared phonons exceeds that predicted from the factor-group analysis
which indicates that the structure is at least locally non-cubic.
