Growth of a novel K0.4Rb0.6Pb2Cl5 crystal and theoretical and experimental studies of its electronic and optical properties

Khyzhun, O. Y.; Vu, Tuan V.; Lavrentyev, A. A.; Gabrelian, B. V.; Denysyuk, N. M.; et al. // Optical Materials//

We report on successful growth by Bridgman method of an optical quality K0.4Rb0.6Pb2Cl5 crystal and determination of its crystal structure and electronic and optical properties. In particular, the present results indicate that the K0.4Rb0.6Pb2Cl5 crystal crystallizes in monoclinic space group P21/c, with the unit-cell parameters as follows: a = 8.9484(2) Å, b = 7.9802(2) Å, c = 12.5359 Å, and β = 90.1220(10)°. For the K0.4Rb0.6Pb2Cl5 crystal, we use X-ray photoelectron spectroscopy (XPS) to measure binding energies of the core-level electrons for constituting atoms and to reveal the energy distribution of the valence electronic states. Our XPS measurements indicate very low hygroscopicity of the K0.4Rb0.6Pb2Cl5 crystal surface and partial alteration from Pb2+ ions to Pb0 when using bombardment with 3 kV Ar + ions. To find peculiarities of filling the valence band and the conduction band of the K0.4Rb0.6Pb2Cl5 crystal by partial densities of electronic states associated with the composing atoms, we apply different approaches for exchange-correlation potential using model K0.5Rb0.5Pb2Cl5 solid solution. We have found that the finest agreement of the experimental and theoretical data is derived when in the calculating procedure we use Tran-Blaha modified Becke-Johnson (TB-mBJ) potential involving spin-orbit coupling and the Hubbard parameter U (TB-mBJ + U + SOC approach). The present theoretical TB-mBJ + U + SOC results predict that K0.5Rb0.5Pb2Cl5 is a non-direct material with energy band gap of 4.167 eV. The optical properties of K0.5Rb0.5Pb2Cl5 are elucidated theoretically in detail based on first-principles calculations within the TB-mBJ + U + SOC model.