Abstract
The reduction of hafnium tetraiodide, HfI4, with aluminum at 600 °C or 850 °C in the presence of a NaI flux resulted in black single crystals of Hf0.86(1)I3. This composition corresponds well to the upper end of the non-stoichiometry range 0.89 ≤ x ≤ 1.00 previously reported for HfxI3. The crystal structure (a = 1250.3(2), c = 1999.6(3) pm, R-3m, Z = 18) is made up of hexagonal closest packed layers of iodide ions. One third of the octahedral holes would be filled as in TiI3 or ZrI3 if it were Hf1.00I3. In Hf0.86(1)I3, one out of six octahedral holes along [001] are, however, only occupied by 16%. In contrast to TiI3-I and ZrI3, one striking structural feature is in the formation of linear hafnium trimers with identical Hf―Hf distances of 318.3(2) pm rather than the formation of dimers. These may be associated with Hf―Hf bonding although only 2.64 electrons are available for one Hf5.16I18 column.
Highlights
The group four elements, titanium, zirconium and hafnium, are, in principle, capable of forming iodides in the +2, +3 and +4 oxidation states
Powder X-ray diffraction established that HfI3, as the trichlorides, bromides and iodides of titanium and zirconium, belongs structurally to an ever growing family of trihalides with hexagonally closest packed halide layers, between which one-third of the octahedral voids are occupied by metal atoms [13]
A number of important conclusions were drawn from a closer inspection of the phase equilibria in the Hf–HfI4 system [14]: First, no lower iodide of hafnium than hafnium triiodide, HfI3, exists in equilibrium
Summary
The group four elements, titanium, zirconium and hafnium, are, in principle, capable of forming iodides in the +2, +3 and +4 oxidation states. The di-, tri- and tetraiodides of titanium [1,2,3,4,5] and zirconium [6,7,8,9,10] appear to be well characterized At least, their crystal structures and the importance. Powder X-ray diffraction established that HfI3, as the trichlorides, bromides and iodides of titanium and zirconium, belongs structurally to an ever growing family of trihalides with hexagonally closest packed halide layers, between which one-third of the octahedral voids are occupied by metal atoms [13]. A number of important conclusions were drawn from a closer inspection of the phase equilibria in the Hf–HfI4 system [14]: First, no lower iodide of hafnium than hafnium triiodide, HfI3, exists in equilibrium. The crystals have a clearly resolved superstructure with the statistical under-occupation of one out of six hafnium sites
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