  The nebular phase of lanthanide-rich ejecta of a neutron star merger (NSM) is
studied by using a one-zone model, in which the atomic properties are
represented by a single species, neodymium (Nd). Under the assumption that
beta-decay of r-process nuclei is the heat and ionization source, we solve the
ionization and thermal balance of the ejecta under non-local thermodynamic
equilibrium. The atomic data including energy levels, radiative transition
rates, collision strengths, and recombination rate coefficients, are obtained
by using atomic structure codes, GRASP2K and HULLAC. We find that both
permitted and forbidden lines roughly equally contribute to the cooling rate of
Nd II and Nd III at the nebular temperatures. We show that the kinetic
temperature and ionization degree increase with time in the early stage of the
nebular phase while these quantities become approximately independent of time
after the thermalization break of the heating rate because the processes
relevant to the ionization and thermalization balance are attributed to
two-body collision between electrons and ions at later times. As a result, in
spite of the rapid decline of the luminosity, the shape of the emergent
spectrum does not change significantly with time after the break. We show that
the emission-line nebular spectrum of the pure Nd ejecta consists of a broad
structure from $0.5\,\mu m$ to $20\,\mu m$ with two distinct peaks around
$1\,\mu m$ and $10\,\mu m$.
