Nuclear quadrupole resonance of barium inBaBiO3andBaPbO3

The nuclear quadrupole resonance (NQR) on Ba nuclei was studied in four samples of ${\mathrm{BaBiO}}_{3}$ prepared in different ways and, in addition, in ${\mathrm{BaPbO}}_{3}.$ The spectrum of ${}^{137}\mathrm{Ba}$ at $T=4.2 \mathrm{K}$ consists of a relatively broad line centered near 18 MHz for all ${\mathrm{BaBiO}}_{3}$ samples and near 13 MHz for ${\mathrm{BaPbO}}_{3}.$ The integrated intensity of the ${}^{137}\mathrm{Ba}$ resonance in the ceramic sample of ${\mathrm{BaBiO}}_{3}$ synthesized at $800\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ is approximately twice as large as that in ceramic samples and a single crystal prepared at $930--1080\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}.$ The decrease of the NQR signal can be attributed to the partial disordering of charge-disproportionated Bi ions on the two inequivalent sites. The broadening of the resonance indicates that local distortions of the crystal structure exist in both compounds. The point-charge model was used to analyze the electric-field gradient on the Ba sites.