With the goal of finding new lithium solid electrolytes
by a combined
computational–experimental method, the exploration of the Li–Al–O–S
phase field resulted in the discovery of a new sulfide Li3AlS3. The structure of the new phase was determined through
an approach combining synchrotron X-ray and neutron diffraction with 6Li and 27Al magic-angle spinning nuclear magnetic
resonance spectroscopy and revealed to be a highly ordered cationic
polyhedral network within a sulfide anion hcp-type
sublattice. The originality of the structure relies on the presence
of Al2S6 repeating dimer units consisting of
two edge-shared Al tetrahedra. We find that, in this structure type
consisting of alternating tetrahedral layers with Li-only polyhedra
layers, the formation of these dimers is constrained by the Al/S ratio
of 1/3. Moreover, by comparing this structure to similar phases such
as Li5AlS4 and Li4.4Al0.2Ge0.3S4 ((Al + Ge)/S = 1/4), we discovered
that the AlS4 dimers not only influence atomic displacements
and Li polyhedral distortions but also determine the overall Li polyhedral
arrangement within the hcp lattice, leading to the
presence of highly ordered vacancies in both the tetrahedral and Li-only
layer. AC impedance measurements revealed a low lithium mobility,
which is strongly impacted by the presence of ordered vacancies. Finally,
a composition–structure–property relationship understanding
was developed to explain the extent of lithium mobility in this structure
type.
The
synthesis of BiAgOCh (Ch = S or Se) compounds has been successfully
achieved via the ion exchange of copper with silver in aqueous solutions,
starting from the copper parent phase. Optical and electrical measurements
of BiAgOCh powders confirm an increase in both the bandgap and the
electrical resistivity, as compared to those of the copper compounds.
The structure of the BiAgOS phase has been clearly examined. X-ray
diffraction synchrotron measurements coupled with advanced high-resolution
transmission electron microscopy analysis evidenced a Ag-deficient
structure, as well as Bi-rich defects, both types of defects being
oppositively charged. Silver atoms are also found in interstial sites,
which explains the two-dimensional ionic conductivity. This structural
study combined with theoretical calculations explains the intrinsic
conductivity behavior of these semiconductors linked to the mutual
compensation of both defect types in the structure and to the increase
in the hole effective mass. This study shows the feasibility of modifying
the optoelectronic properties of the BiMOCh compounds, with the goal
of integrating them in heterojunction solar cells. Moreover, it provides
very precise insight into the complexity of the relationship between
structural defects and optoelectronic properties.
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