  A search for decays of pair-produced neutral long-lived particles (LLPs) is
presented using 139 fb$^{-1}$ of proton-proton collision data collected by the
ATLAS detector at the LHC in 2015-2018 at a centre-of-mass energy of 13 TeV.
Dedicated techniques were developed for the reconstruction of displaced jets
produced by LLPs decaying hadronically in the ATLAS hadronic calorimeter. Two
search regions are defined for different LLP kinematic regimes. The observed
numbers of events are consistent with the expected background, and limits for
several benchmark signals are determined. For a SM Higgs boson with a mass of
125 GeV, branching ratios above 10% are excluded at 95% confidence level for
values of $c$ times LLP mean proper lifetime in the range between 20 mm and 10
m depending on the model. Upper limits are also set on the cross-section times
branching ratio for scalars with a mass of 60 GeV and for masses between 200
GeV and 1 TeV.
