To explore the origin of the suppression of the charge density wave (CDW) in ${\mathrm{Bi}}^{2-}$ square sheet of $R_2{\mathrm{O}}_2\mathrm{Bi}$ ($R$: rare-earth elements), we have investigated the band structures and phonon dispersions of ${\mathrm{La}}_2{\mathrm{O}}_2\mathrm{Bi}$ and ${\mathrm{Er}}_2{\mathrm{O}}_2\mathrm{Bi}$. We have found that the large spin-orbit coupling of Bi atoms together with the chemical pressure effect reduces the Fermi surface nesting, which results in the suppression of the CDW instability in $R_2{\mathrm{O}}_2\mathrm{Bi}$. The pressure effect has been checked for a similar compound, ${\mathrm{Er}}_2{\mathrm{O}}_2\mathrm{Sb}$, which shows a series of structural distortions in the Sb layer under volume contraction, from herringbone type, to 1D-ladder type, and to square sheet. Furthermore, we show that the observed local disorder in the Bi square sheet of ${\mathrm{La}}_2{\mathrm{O}}_2\mathrm{Bi}$ can be explained by the phonon softening anomaly, which is also expected to describe the anomalous resistivity upturn observed in ${\mathrm{La}}_2{\mathrm{O}}_2\mathrm{Bi}$ upon cooling.

• Received 13 November 2015
• Revised 9 February 2016

DOI:https://doi.org/10.1103/PhysRevB.93.125116