  Rietveld profiles, Moessbauer spectra and x-ray absorption fine structure
(XAFS) were analyzed through the structural phase transition at Ts = 355 K in
HoBaCo4O7. Excess of the oxygen content over O7 was avoided via annealing the
samples in argon flow at 600 degree C. Space groups (S.G.) Pbn21c and P63mc
were used to refine the structure parameters in the low- and high-temperature
phases, respectively. Additionally, the Cmc21 symmetry was considered as a
concurrent model of structure of the low-temperature phase. In the
high-temperature phase, severe anisotropy of thermal motion of the major part
of the oxygen atoms was observed. This anisotropic motion turns to be quenched
as the sample is cooled below Ts. The variation of quadrupole splitting near Ts
is not similar to a steplike anomaly frequently seen at the charge-ordering
transition. We observe instead a dip-like anomaly of the average quadrupole
splitting near Ts. Narrow distribution of the electric field gradient (EFG)
over different cobalt sites is observed and explained on the basis of
point-charge model. XAFS spectra show no evidence of significant difference
between YBaCo4O7 (T > Ts) and HoBaCo4O7 (T < Ts). The origin of the transition
at Ts is ascribed to the condensation of the libration phonon mode associated
with the rigid-body rotational movements of the starlike tetrahedral units, the
building blocks of kagome network. It is shown that the condensation of the
libration mode is not compatible with translation symmetry for the hexagonal
S.G., but compatible for the orthorhombic S.G. The orthorhombic lattice
parameters and EFG components (Vxx, Vyy, Vzz) vary smoothly with temperature at
approaching Ts and closely follow each other.
