August, 1962
THEVANADIUM-VANADIUM CARBIDE; SYSTEX
give some indication of the electronic structure of the complexes. In general, the complexes with the lowest dissociation const,ants are those with the highest bond order for the dative bond from iodine to oxygen or nitrogen, but the number of available dissociation const,ants is too small for any detailed analysis of these relationships. The fractional electron transfer is larger for the IC1 complex than for the corresponding IBr com-
1401
plex, as would be expected from the greater electronegativity of chlorine. Although the IF5 complex shows a smaller fractional electron transfer than the corresponding IC1 complex, the geometry is considerably djff erent. Presumably the nitrogen occupies roughly the sixth corner of an irregular octahedron about iodine and both steric and electronic factors may lengthen the I-N bond and weaken the complex relative to the IC1 complex.
THE VANADIUM-VANADIUM CARBIDE SYSTEM' BY E. I(.STORMS AND R. J. MCXEAI? Los Alarnos XcientiJic Laboratory, Los Alamos, New Mexico Received January 86, 196.9
The solid portion of the V-VC phase diagram has been determined above 1000'. The following characteristic temperatures were measured: the melting point of vanadium metal at 1888 =k lo', a eutectic melting between VCo.09 and VCo.33 at 1630", peritectic melting of the VzC-phase between VC0.56 and VCo.80 a t 2165", and peritectic melting of the VC-phase beginning a t VCo.s5and extending beyond VCl.0 a t 2650". The VC-phase does not extend to VCl.00, as has been assumed in the past, but terminates a t VC0.88 a t 1000" and moves to lower carbon contents as the temperature is increased. The variation OF lattice parameter of the VC-phase over its homogeneity range was determined. Evidence for an additional phase in the region between V2C and VC is shown. Vanadium evaporates preferentially from all compositions investigated.
Introduction Vanadium shares with niobium and tantalum the formation of two carbides, a hexagonal configuration having the nominal formula VZC and a face-centered-cubic (XaCl-type) structure which will be refemed to as VC. Both of these compounds exist over a range of composition. Gurevich and Ormont3 have compiled a rather complete bibliography on the subject and may be consulted for information regarding earlier work. As yet, however, a complete phase diagram for this system has not been reported. It is the purpose of this paper to show the relationship between the solid phases which exist in the region between vanadium metal and VCl.oo,and in the temperature interval between 1000° and l,he melting point of the material. Experimental Vanadium metal4 and AUC graphite (99.4% carbon) were used as starting materials. The original 20 mesh metal was made brittle by several methods in order that a 325 mesh product could be obtained. When pure metal was desired, the stock vanadium was hydhded by slow cooling from 800" in hydrogen and the hydride was reduced to 325 mesh in a steel mortar. Iron was removed by magnetic separation or by dilute HCI, after which the powder was dehydrided in u a c w at 600 The impurities remaining in the metal after this treatment are listed in Table I. To prepare starting material for the carbide samples, the stock metal was mixed with outgassled graphite, reacted in a graphite crucibk in vacuo, ground to 325 mesh, and again heated in a graphite crucible until essentially no additional gas was evolved (to 5 X 10 -e torr.). As with the hydride, iron was removed by reflux extraction with constant boiling HCl. Further heating produced a dried and uniform product. This powder then way mixed with either powdered vanadium or graphite, in appropriate amounts, to give the desired composition. Each mixture waq cold pressed into a plug s',8 in. in diameter and a / d in. long. A hole was drilled into each end,
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(1) This work done under the auspices of the Atomic Energy Commission. (2) Guest scientist during the L.A.S.L. Summer Program. (3) M. A. Gurevich and E. F. Ormont, Russ. J . Inorg. Chem., 2, 1586
(1957). (4) High purity metal was furnished b y Union Carbide Metals Comoany, Niagara Falls, New York.
one for black-body temperature measurement (0.040 X a / ~ in.) and the other to take a 0.060 in. diameter T a or W support rod. The absence of a crucible eliminated possible contamination and allowed rapid removal of CQ from the sample environment.
TABLE I IMPURITIES CONTAINED IN THE VANADIUM METAL Element
Sample 1
Sample 2
Oxygen 0.33 wt. % 0.51 wt. % Nitrogen 0.078 0.12 Hydrogen 25 p.p.m. 75 p.p.m. Carbon 90 p.p.m. .... Boron
