  The investigation of crystallographic, electronic, and magnetic
characteristics, especially the mixed valences of Eu$^{2+}$ and Eu$^{3+}$ under
pressure of a novel europium-based bismuth selenide compound,
Eu$_4$Bi$_6$Se$_{13}$, presented. This new compound adopts a monoclinic crystal
structure classified under the P$2_1$/m space group (#11). It exhibits
distinctive structural features, including substantial Eu-Se coordination
numbers, Bi-Se ladders, and linear chains of Eu atoms that propagate along the
b-axis. Electronic resistivity assessments indicate that
Eu$_{4}$Bi$_{6}$Se$_{13}$ exhibits weak metallic behaviors. Magnetic
characterization reveals uniaxial magnetic anisotropy, with a notable spin
transition at approximately 1.2 T when the magnetic field is oriented along the
b-axis. This behavior, coupled with the specific Eu-Eu interatomic distances
and the magnetic saturation observed at low fields, supports the identification
of metamagnetic properties attributable to the flipping of europium spins. The
Curie-Weiss analysis of the magnetic susceptibility measured both perpendicular
and parallel to the b-axis and high-pressure partial fluorescence yield (PFY)
results detected by X-ray absorption spectroscopy (XAS) reveal the tendency of
the material to enter a mixed valent state where the trivalent state becomes
more prominent with the pressure increase or temperature decrease.
