The synthesis, structural characterization, luminescence
properties, and proton conduction performance of a
new family of isostructural cationic 2D layered compounds are
reported. These have the general formula [Ln(H4NMP)-
(H2O)2]Cl·2H2O [Ln = La3+, Pr3+, Sm3+, Eu3+, Gd3+, Tb3+,
Dy3+, Ho3+, H6NMP = nitrilotris(methylphosphonic acid)],
and contain Cl− as the counterion. In the case of Ce3+, a 1D
derivative, [Ce2(H3NMP)2(H2O)4]·4.5H2O, isostructural with
the known lanthanum compound has been isolated by simply
crystallization at room temperature. The octa-coordinated
environment of Ln3+ in 2D compounds is composed by six
oxygen atoms from three different ligands and two oxygens
from each bound water. Two of the three phosphonate groups
act as both chelating and bridging linkers, while the third phosphonate group acts solely as a bridging moiety. The materials are
stable at low relative humidity at less at 170 °C. However, at high relative humidity transform to other chloride-free phases,
including the 1D structure. The proton conductivity of the 1D materials varies in a wide range, the highest values corresponding
to the La derivative (σ ≈ 2 × 10−3 S·cm−1 at RH 95% and 80 °C). A lower proton conductivity, 3 × 10−4 S·cm−1, was measured
for [Gd(H4NMP)(H2O)2]Cl·2H2O at 80 °C, which remains stable under the work conditions used. Absorption and
luminescence spectra were recorded for selected [Ln(H4NMP)(H2O)2]Cl·2H2O compounds. In all of them, the observed
transitions are attributed solely to f−f transitions of the lanthanide ions present, as the H4NMP2− organic group has no
measurable absorption or luminescence properties.
