Confirmation of complex non-collinear Fe(II) spin ordering in the iron oxychalcogenide La${}_2$O${}_2$Fe${}_2$OSe${}_2$ by broad range ${}^{139}$La-NQR

The transition metal oxychalcogenide La${}_2$O${}_2$Fe${}_2$OSe${}_2$ exhibits a non-collinear spin-ice like antiferromagnetic ordering below $T_N\approx$ 90 K due to complex magnetic exchange interactions and axial single-ion anisotropy of Fe${}^{2+}$ spins. The magnetic structure was deduced/determined from combined ${}^{57}$Fe Mössbauer spectroscopy and ${}^{139}$La-NMR on polycrystalline specimen. In this work, we present zero-field ${}^{139}$La-NQR investigations on polycrystalline La${}_2$O${}_2$Fe${}_2$OSe${}_2$ in a broad frequency and temperature range between 2.8 - 20 MHz and 4.2 - 100 K. The measured resonances at ${\nu }_Q$, $2{\nu }_Q$, and $3{\nu }_Q$ are fully consistent with the previous NMR studies and also with the magnetic structure. A substantial modification of the electric field gradient at the La-position below $T_N$ is inconsistent with the current investigations.