Low-temperature (c. 153 K) single-crystal X-ray
structure determinations, carried out on trivalent rare earth iodides
crystallized from aqueous solution at room temperature, have defined two
series of hydrates,
LnI3.nH2O.
For Ln = La–Ho, a nonahydrate phase (n
= 9) is defined, orthorhombic Pmmn,
a ~ 11.5, b ~ 8.0,
c ~ 8.8 Å, Z = 2, the
second phase (n = 10), monoclinic
P21/c,
Z = 4 being defined for Ln = Er–Lu,
a ~ 8.2, b ~ 12.8,
c ~ 17.1 Å, β ~ 103.7˚. Neither of these
phases is isomorphous with any of those pertinent to the previously studied
chloride or bromide (hydrated) arrays, nor, unlike those, does the halide
(iodide) in any case enter the coordination sphere of the lanthanoid. The
n = 9 phase takes the form
[Ln(OH2)9]I3,
the nine-coordinate lanthanoid environment stereochemistry being tricapped
trigonal-prismatic, while the n = 10 phase is
[Ln(OH2)8]I3.2H2O,
the eight-coordinate lanthanoid environment being square-antiprismatic.