Quantum chemical modelling of electron polarons and 'green' luminescence in PbTiO₃ perovskite crystals

In an extension of our previous study on the electron polarons and excitons in KNbO₃, KTaO₃ and BaTiO₃ (Kotomin E.A., Eglitis R.I. and Borstel G. 2000 J. Phys.: Condens. Matter 12 L557; Eglitis R.I., Kotomin E.A. and Borstel G. 2002 J. Phys.: Condens. Matter 14 3735) by the semiempirical Hartree-Fock method we present here results for free electron polarons in the PbTiO₃ perovskite crystal. We discuss the origin of the intrinsic visible band emission of PbTiO₃ perovskite oxides (so-called 'green luminescence') which has remained a topic of high interest during the last quarter of a century. We present a theoretical calculation modelling this emission in the framework of a concept of charge transfer vibronic excitons, i.e. as a result of radiative recombination of correlated (bound) self-trapped electron and hole polarons in the highly polarizable PbTiO₃-type matrix. The intermediate neglect of differential overlap method combined with the large unit cell periodic defect model was used for quantum chemical calculations and theoretical simulation of the green emission for a PbTiO₃ perovskite. The calculated 'green' luminescence energy for PbTiO₃ perovskite-type crystals agrees well with experimental measurements presented in this letter.