Correlation effects and phonon modes softening with doping in Ba1-xKxBiO3

TL;DR

This study uses DFT+U calculations to analyze how correlation effects influence phonon mode softening and structural transitions in Ba1-xKxBiO3, aligning well with experimental observations and shedding light on superconductivity mechanisms.

Contribution

The paper demonstrates that including correlation effects via DFT+U is essential to accurately reproduce phonon softening and structural transitions in Ba1-xKxBiO3, advancing understanding of its superconductivity.

Findings

Phonon frequencies match experimental data when correlation effects are included.

Cubic structure becomes stable at x=0.30, close to experimental transition at x=0.37.

Correlation effects are linked to phonon softening and superconductivity in the material.

Abstract

Monoclinic crystal structure of the undoped BaBiO3 can be described as a cubic perovskite distorted by a frozen breathing and tilting phonon modes of BiO6 octahedra. The phonon mode softening is experimentally observed [M. Braden et al., Europhysics Letters (EPL) 34, 531 (1996)] in Ba1-xKxBiO3 through potassium doping followed by a transition into an ideal cubic perovskite structure at x = 0.37 close to the appearance of superconductivity. In our previous paper [D. Korotin et al., Journal of Physics: Condensed Matter 24, 415603 (2012)] we demonstrated that it is necessary to take into account correlation effects by DFT+U method in Wannier functions basis to obtain a good agreement between the calculated and experimental values of crystal structure distortion and energy gap in BaBiO3. In the present work with the same method we calculated the breathing mode phonon frequencies as a function of potassium doping level in Ba1-xKxBiO3. Obtained frequencies are in a good agreement with experimental values and the breathing mode softening with doping effect is reproduced while calculations without consideration of correlation effects failed to do so. We shown that the cubic crystal structure becomes stable at x = 0.30 in agreement with the experimental transition to cubic perovskite at x = 0.37. The possible connections between the correlation effects, phonon mode softening, and superconductivity in Ba1-xKxBiO3 are discussed.