Lots of investigators' interests

in the problem of an electron bound to a hydrogenic impurity and interacting with the longitudinal optical phonon field of an ionic crystal or a polar semiconductor

have been maintained over years.Since the 1980s

Takuda investigated the optical polaron bound in a Coulomb potential within the modified variational scheme of the LLP theory

some investigators started to study the bound optical polaron in variational physics models from theory or experiment facets

using different methods.Particularly

scholars are not only looking for the new means

but also paying more attention to the properties of the bound optical polaron in variational physics models recently.Chen et al.studied the ground state description of the bound polaron in parabolic quantum wires by the Feynman-Haken path integral.Bouhassoune et al.calculated the binding energy of a bound polaron in a quantum well wire.Es-sbai et al.investigated the impurity bound polaron in a cubic quantum dot with the use of the variational approach.Melnikov et al.applied the adiabatic variational method to calculate the polaron energy shift in a spherical quantum dot.Derieese et al.presented a multilateral theoretical study of bound polarons in oxide compounds

and Satori et al.attained the binding energy of a bound polaron to shallow donor impurity in spherical quantum dot using a vatiational approach within the effective mass approximation.One of this paper's authors by using linear combination operator and unitary transformation methods achieved the energy of the ground state and of the first excited state of the bound optical polaron for different values of the electro-phonon coupling.In the early 1970s

Huybrechts investigated the internal excited state of the bulk optical polaron by using linear combination operator method.The excitation energy of the stong coupling surface polaron was calculated by means of Huybrechts' method by one of the authors.However

few scholars studied the temperature dependences of the bound optical polaron with linear combination operator and unitary transformation methods so far.In this paper

we investigated the temperature dependences of the bound optical polaron in a Coulomb potential with this method.Using the parameters and the coupling constant of the RbCl crystal for the strong coupling

we calculated the vibration frequency

the ground state energy of the bound optical polaron and the mean number of optical phonon.The results show that the absolute values of the ground state energy

the vibration frequency and the mean number of optical phonon are all increase with the temperature's increasing.