“X Factor” in the Structure and Anion Exchange of Layered Yttrium Hydroxides

The
anion-exchangeable family of layered rare-earth hydroxides
RE2(OH)5X·nH2O (LREH-X, RE = rare-earth elements such as Y, X– = monovalent anions, n ≈ 1.5) exhibits considerable
potential in various applications. However, their structures remain
controversial or even unresolved. Herein, a comprehensive study involving
1D and 2D 89Y solid-state NMR spectroscopy, powder X-ray/neutron
diffraction, and periodic DFT calculations was conducted to solve
the structure of LYH-X (X– = Cl–, Br–, NO3–), one
of the most extensively used and studied subjects of the LREH-X family.
The anion-exchange behaviors were then investigated, and the results
clearly demonstrate strong influences of X– on the
structure and anion exchange: first, the structure of LYH-Cl is orthorhombic P21212, whereas LYH-Br and LYH-NO3 are monoclinic P21 structures;
second, the conversion from the high-symmetry P21212 phase to the low-symmetry P21 phase can be conducted quite well via anion exchange,
but not vice versa; finally, the foreign anions site-specifically
interact with host layers. Such strong influences are the unique characteristics
of LYH-X and thus should be considered in future studies and applications
of these materials.