  We report thermal conductivity and Seebeck effect measurements on
Mn(Bi1-xSbx)2Te4 (MBST) with x = 0.26 under applied magnetic fields below 50 K.
Our data shows clear indications of the electronic structure transition induced
by the antiferromagnetic (AFM) to ferromagnetic (FM) transition driven by
applied magnetic field as well as significant positive magnetothermal
conductivity in the Weyl semimetal state of MBST. Further, by examining the
dependence of magnetothermal conductivity on field orientation for MBST and
comparison with the magnetothermal conductivity of MnBi2Te4 we see evidence of
a contribution to thermal conductivity due to Weyl fermions in the FM phase of
MBST. From the temperature dependence of Seebeck coefficient under magnetic
fields for MBST, we also observed features consistent with the Fermi surface
evolution from a hole pocket in the paramagnetic state to a Fermi surface with
coexistence of electron and hole pockets in the FM state. These findings
provide further evidence for the field-driven topological phase transition from
an AFM topological insulator to a FM Weyl semimetal.
