VoL 8, No. 2, February 1969
THELOWERHALIDESO F HAFNIUM 227
As an oxidant, perbromate exhibits properties intermediate between those of perchlorate and those of periodate. However, because of their slowness, the reactions shed little or no light on the thermodynamic stability and oxidizing power of perbromate. The instability of concentrated perbroniic acid warrants detailed study to elucidate the mechanism of decomposition. It is interesting to note that some perbromic acid can be distilled in spite of this decomposition, confirming the early observation of vapor-phase HBr04 in the mass spectrometer.ll The properties found for the perbromates do not explain why these compounds have been so hard to prepare. , Although the thermal decomposition studies (11) M. H. Studier, J . Am. Chem. Soc., 90, 1901 (1966).
CONTRIBUTION No. 2359
FROM THE
show that efforts to make perbromates by disproportionation of bromates are most likely doomed to failure, it is not a t all clear why electrolytic synthesis is so unsatisfactory.* It may be that the stumbling block is a prohibitively high activation energy caused by an extremely unstable Br (VI) intermediate through which the synthesis must proceed. Acknowledgment-The author is indebted to Mr. Frank Gallagher and Mr. William Johnson for the thermogravimetric measurements, to Mr. Kenneth Jensen for the potassium analyses, to Mrs. Donald Engelkemeir for measurement of oxygen isotope ratios, and to Mr. Jerome Lech for the analyses by emission spectrography.
IKSTITUTE FOR ATOMICRESEARCH A N D DEPARTMENT OF CHEMISTRY, I O W A STATE UNIVERSITY, AMES, IOWA 50010
The Lower Halides of Hafnium. A Nonstoichiometric Hafnium Triiodide Phase1 BY A R T H U R W. STRUSS
AND
JOHN D. CORBETT
Received J u l y 5, 1968
A hafnium iodide phase of the composition HfI3.2-8.3 which is free of unreacted metal is obtained by the reaction of liquid HfI4 with metal foil (600 ppm of Zr) in a sealed tantalum container at 500-550'. T h e lower composition limit established from equilibration with separate metal powder is Hf13.0,a t 425-575'; the phase in equilibrium with HfId(1) varies from Hf13.4e at 475" t o Hf13.zDa t 550'. Neither equilibration of HfI4 or HfI3 with metal at u p to 1000' nor disproportionation of HfIa under high vacuum gives any indication of the existence of still lower iodide, specifically HfI2. X-Ray powder data indicate t h a t the a and c dimensions of the previously suggested hexagonal unit cell2 should probably be doubled and quadrupled, respectively. T h e composition Hf13.s5 shows a small temperature-independent paramagnetism corresponding t o a peff value of 0.68 a t 300°K; the susceptibilities are considerably smaller than reported earlier3 for impure samples.
Introduction Stoichiometric amounts of tetrahalide and metal powder sealed in glass ampoules were heated up to 700" using a The lower halides of hafnium have been only partially bomb to counteract the internal pressures. This procharacterized, principally because reductions of the cedure gave up to 80-85yo conversion to the tribromides tetrahalides under diverse conditions are characterisand triiodides of both hafnium and zirconium but only tically slow and incomplete. To some extent the interabout 40% for the chlorides. The unreacted tetrapretation of what is known has been based on the ashalide sublimbed off during the quenching process, and sumption that there should be a close parallel with the solutions of the residues in water gave appropriate analbetter known lower halides of zirconium as there is in yses for the trihalides after a precipitate of unreacted many ways in the chemistries of the tetravalent states. metal was filtered off. Trihalides of greater than 99% The latter, of course, arises from the well-known effect of purity were obtained when the initial products were rethe lanthanide contraction, a t least as it affects properground and reheated with large excesses of the approties largely determined by radii of the tetrapositive priate tetrahalide, although analytical data were not state, but it is less clear how the interjection of the 4f reported. I n later work such an HfIB product was elements should influence the stabilities of lower oxidafound to contain a residue of 6.4% metal, with larger tion states. amounts in the chloride and bromide.2 The hafnium The direct reduction of hafnium(1V) to hafnium(II1) (and zirconium) trihalides all appeared to disproporhalides (except fluoride) by hafnium metal itself has been brought to completion best by Larsen and L e d d ~ . ~ tionate incompletely into gaseous tetrahalide and a presumed dihalide when heated above 450' in sealed, evac(1) Work was performed in t h e Ames Laboratory of t h e U. S. Atomic uated tubes, and the reaction was followed either by Energy Commission. Presented a t t h e 156th National Meeting of the American Chemical Society, Atlantic City, N. J., Sept. 6-13, 1968. loss or in a high-temperature X-ray camera. tetrahalide (2) L. F. Dahl, T. Chiang, P. W. Seabaugh, and E. M. Larsen, I m v g . The same synthesis reactions a t 37-60 atm and ca. 500" Chem., 3, 1236 (1964). (3) W. A. Baker, Jr., and A. R. Janus, J . Inovg. N z d . Chem.. 26, 2067 gave 2 99% conversion to the zirconium(II1) chloride (1964). and bromide5r6 but unreproducible compositions with (4) E. M. Larsen a n d J. J. Leddy, J. Am. Chem. SOC.,'78, 5983 (1956).
228
ARTHURU'. STRUSSAND JOHN D. CORBETT
the same hafnium halides.' I n contrast to the present results, the reaction of excess ZrI4 with a metal sheet a t 510" resulted in a nonadhering ZrI3 product and this under effusion conditions first disproportionated to Zr12.86 and then to Zr12.8 Aluminum has also been used as the reductant following its application in the zirconium systems. Aluminum with excess HfBr4 in a bomb a t 60 atm and 470" for 100 hr gave pure hafnium(II1) bromide (Br/ Hf = 3.00-3.03) in 1-1.5-g amounts after removal of the AlBr3 and excess HfBrr by ~ u b l i m a t i o n . ~On the other hand, the product of the reduction of HfI4 with aluminum powder a t only 3C10-385~for 2-14 days followed by a like sublimation still contained unreacted aluminum, and the material gave p e f f values in excess of the spin-only value for a d' salt.3 Dahl, et n1.,2 interpreted X-ray powder data for zirconium(II1) chloride, bromide, and iodide and hafnium(II1) iodide in terms of a common structure, the P-TiC13 type. I n this the formally octahedral holes between hcp halogen atoms are occupied such t h a t infinite chains of MXs octahedra sharing opposite faces are oriented parallel to the c axis. The metal atoms are evidently equally spaced in the chains according t o the extinction of h01 reflections with 1 $ 2n. Wattss has confirmed this condition for p-ZrC13. A basically similar structure has also been reported for Ti13 by von SchneringlO although very weak superlattice diffractions found with single-crystal (but not powder) methods require a doubling of the a axis. In the present study hafnium metal foil has been used as the reductant to allow ready physical separation of an uncontaminated triiodide phase from unreacted metal. The results show that (1) hafnium(II1) iodide is the only reduced iodide, ( 2 ) the phase has a substantial homogeneity range on the hafnium(1V) side, and (3) the triiodide gives X-ray and magnetic susceptibility results somewhat different from what have been reported previously.
Experimental Section Materials.-Crystal bar hafnium (Nuclear Materials and Equipment Corp.) was used in much of the preliminary work. This had listed impurities of 3y0 Zr and, in ppm: C, -50; H , -12; 0, -50; Mo, 30; Si, 40, with other metallic elements below the limit of spectrographic detection. The metal was cold rolled into 10-15-mil foil for the preparation of HfI3. T h e larger surface area of 325-mesh powder (Wan Chang) was employed for low-limit equilibrations; this contained, in ppm: 0, 790; X, 121; C,
