drogen iii the 0-OH bond, in addition to drawing electrons toward the oxygen attached to carbon, thus decreasing the polarization of the 0-0 bond enough t o permit the observed slow free-radical cleavage of peroxybenzoic acid. B R 0 K - S USI\7ERBITY
PROTIDEWE 12. RHODE ISLAXI) R E ( T I T E I > JI.LY ~
1087
COMMUXICATIOSS TO THE EDITOR
August, 1960
T= 2126°K
STEPHES R. C o m s Y JOHS0 . EDWARDS 1, 1!l6O
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Gij Ijeliar-iiient of ( ~ h e n i i s t r y ,S o r t l i e a s t e r n t - n i v e r s i t y , Boston 1.i SIassarliiisrt ts.
-+ Y
OS THIS W E O F T,1STAILUJI KSTJDSES (:E rm ISHIGH TEMPERATURE THEK3IODYS;IJIIC STUDIES OF OXIDES
Sir: It has been shown by several investigators' -5 that the rolat'ility of oxides is greatly enhanced in the prescnce of tantalum. Chupka, et u Z . ! ~ have shown t'hat this is due to the formation of TaO kind Ta02. 1nasmuc.h as heats of 011 of these species have heeii reportedq6 it might appear that t,antalum cells would still he useful for oxide studies provided the pre-beiice of gaseous tantalum oxides in the effwing vapors is taken intmoaccount. The purpose of this note to point out that such cxorrections 11-ill not, necessarily lead to menniugful equilibrium c~mstantsfor reactions hecw~seof the maiiner in which TaO(g) :11id T a 0 2 ( g i are formed and escape the Iinudsen cell. 1:or some time we have l ~ studj-iiig ~ n the vaporization of rare earth oxides by the Knudsen effusiou techiiiquv. These oxides vaporize much more readily from iautalum than from tungs'eii cruc*il~les.~ l'igure 1 shows the isot,hernial rate of weight loss of :I tantalum Knudsen rell' contaiiiing LazO, as a funrtion of the ratio of the tot,al weight of niater:al vaporized to the weight of the iiiitial charge of I,a& It should be noted that the steady r:rte of' vaporization persists beyond the poinf n-here rhr weight of material vaporized csveeds the weight of LasOa originally hi the crucible. This \viis assumed to result from the escape of TaO(5) and TaOz(g) through the orifice in equilihriurii n-ith the LaO(g) arising from La2Oa.j Subsequent niass spertrosmpic experiments, using a Beiidix time-of-flight iustrumriit, under idelltical coi1ditioi.s. ha\-e shon-11 that +his assumption \vas erroiieous. csperimciith, the on/!/ vapor ohser\-etl mi t'he Iiiiudsen orifice \vas TAO. Thc 1' La'+ varied with aniouiit vaporized iii :i n i a ~ i i i richitir:il ~ to that shou-ii in lig. 1. (1)
L.Brtjver and .i%aar(by. . .1.. I f f & . Chern. So(,., 7 3 , ::(OS
fSY5l). and R. .J. Thorn, zbid., 7 8 , 4 l l i g (18,Xjj. Anz. C ~ m 7 , : i r S o r .BwZ1.,38,2.3; (1Y.59). iiiika. .J. Rerkonvitz and AI. G . Ingliiaiii, J . Glum.
.
.itlsli. €1. \V. Goldstein and 1). \Tllite. J . Am. Ccrurn.
.
I n g h r s i i i . IT. .t. Cliulika a n d .1. Berkoiritz. J. C'hcm. f1937). ( 7 ) The oiiipty tantalriin crui,iblo was tliorouglily degiisscd a t 2400' i n uucuo j u t t lirior t o tliis set of runs.
I.
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b
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Weight vaporizc~d/initialwsight of L:dJ+ Fig. 1.
S o tant,alum oxides n-ere observed8uiitil beyond the flat, in the isotherm and then oiily when t.he teinperature was increased 100' or more. I+ is therefore concluded that most' of the oxygen from La20a is absorbed in the wall of the cell, through which It' diffuses with subsequent erolution of TaO(g) and Ta02(g)from the exterior surface of the c r ~ c i b l e . ~ 1T7e do not believe that any equilibrium involving ga,seous tant,aliim oxides is established 011 the cell. There is further evidence to support t'he conclusion reached here. Tant,alum Knudren cells containing TazOs lose weight at, a rate of 30 to 50 tinies that predicted from the reported thermodynamic properties of TaO(g'1 arid Ta02(gi6, become heavily etched on the orrt.side at the sample level. A "16" wucible wall has been eroded completely iu less t h m one hour at 2050°1roi\-art. G . De M a r i a , R . P. Burns a n d SI. c;. Inghraiii, J . C h r m . Phus., 34, 13Iili (ISliO). ( S I ) R. .J. .Ackerinann a n d R . J . T h o r n , .Arponne National Laliorat o r y Report :iKL-.i821, .Janiiarl-. 1858.
HA~T)IOISSIS OF FERROW 10s SOLI-TIOSS IS HE.11-Y IVLITEl
