  The problem of losses in resonators is a key issue in the design and use of
modern plasmonic and photonic meta-devices. We propose a spectroscopic method
for determining material and radiation losses in structures with bound states
in the continuum using as example the phase-change chalcogenide Ge2Sb2Te5. The
complex refractive index for the amorphous and crystalline Ge2Sb2Te5 phases was
measured experimentally, and the resonance optical properties were calculated
for a telecommunication wavelength of 1500 nm. To demonstrate the validity of
the method, two types of resonators were considered - a cylinder and a ring
which support bound states in the continuum according to the Friedrich -
Wintgen mechanism. We have determined the value of the material quality-factor
for the amorphous Ge2Sb2Te5 as Qmat = 46 and the values of the radiation
quality-factors Qrad = 608 for the cylinder and Qrad = 332 for the ring of a
pair of TE0,1,2 and TE0,2,0 modes in the regime of bound states in the
continuum. We also demonstrated two mechanisms for efficient switching of the
scattering spectra between the amorphous and crystalline phases of Ge2Sb2Te5.
