  Tm1-xYbxB12 dodecaborides represent model objects for the studies of quantum
critical behavior, metal-insulator transition and complex
charge-spin-orbital-phonon coupling phenomena. In spite of intensive
investigations, the mechanism of the ground state formation in this strongly
correlated electron system remains a subject of active debates. We have
performed first systematic measurements of temperature-dependent spectra of
infrared conductivity of Tm0.19Yb0.81B12 at frequencies 40-35000 cm-1 and in
the temperature interval from 300 K down to 10 K. Analysis of the temperature
evolution of the observed absorption resonances is performed. Their origin is
associated with the Jahn-Teller instability in the cubic lattice which results
in the rattling modes of the rare earth ions and leads to emergence of both the
intra-gap mixed-type collective excitations and the dynamic charge stripes.
Temperature dependent effective mass of charge carriers is determined and a
picture is presented of the multiple relaxation channels and the transformation
of the manybody states at different temperatures. We argue in favor of
electronic phase separation scenario which is valid both for the
metal-insulator transition and for the formation of the nanoscale filamentary
structure in Tm1-xYbxB12 compounds.
