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Dissociation energy of samarium monoxide and its relation to that of europium monoxide
Volume 48, number 2
CIIEWCAI.
PHYSICS LET’WRS
1 June 1977
DISSOCIATION ENERGY OF SAMARiUM MONOXIDE AND ITS RELATlON
TO THAT OF EUROPIUM
MONOXIDE*
D.L. IIILDENBKAND Stanford Research Inrtmrte. Menlo Park. Cahfornia 9402.5, USA Rccclved 22 Fcbrunry
1977
Bcc,n~sc of conflictme result\ tar the dlssociatlon cncrglcs (0:) of SmO nnd EuO determined by %%cral dlffcrcnt tcchniqucs. @(SmO) w&15rcdetcrmmed by rcfcrcncc to AlO,110, and CuO by means of high tcmpcrnturc ITKIFS spectrometry. Derived results for Dg(SmO) tram the exchange reactions with AK), TIO, and Ku0 were 135.1, 136.3, and 136.9 kcal/mol, rc%pcctivciy, lc.~rlm:; to the qclcctcd v.~luc 136.0 2 2 kc.ll/mol. Lvtcnsivc equilibrium measurements on the gascour reaction Al + SmO = A10 + SIN with both pulse counting and dc clcLtromctcr techmqucs gave close agreement between second and third law cnthalplcs, slgmtyin:: the cbtunnted thermodynamic funrtlon\ of SmO to bc rchablc. The new results dlffcr substanti,llly from prcvlously rcportcd d&i for the rcactron Eu + SmO = tu0 + Sm, and thereby rcsolvc puL7ling discrcpancles bctwccn the dissoci.ltion cncrgcs of SmO and tuO.
I. Introduction
The rare earths, as a famdy, are considered to form very stdblc gaseous monox~dcs, as cvidcnccd by the thermochemical studies of Ames et al. [ 11 which showed that, with the exception of YbO, the dissoclatlon energies were on the order of 6 to 8 eV. A double pcrlodicity m the MO dissociation energies and the metal heats of sublimation was observed [ 1] across the series, with mmima at Eu and Yb, althou& the minimum rn 0: was much less pronounced at ELI than at Yb. @(FuO) was derived from Knudsen effuslon measurements of the vaporization of Euz03(s) in a tungsten cell and from a study of the gaseous isomolecular rcactron Eu + SmO = EuO + Sm by mass spectromctry [ 11. In the latter case @(EuO) IS referenced to Di(SmO) through the derrvcd enthalpy change for the reaction. Dg(SmO) was determined by reference to @(YbO) and from the vaporization of Sm203. The results were interpreted to yield @SmO) = 142.0 ? 4.6 kcal/mol and @(EuO) = 133.8 + 4.6 kca.l/mol. Subsequently, Dickson and Zare [2] *This rcscarch was supported by the Air I’orce Office of Scientific Research, Air Force Systems Command, under Contract l-44620-73-C-0037.
340
reported new determinations beam-gas chcmiluminescent the lower bounds D@niO) and 1.fi(l-u0)
?
of these quantities by a reaction method, giving 2 135.5 2 0.7 kcal/mol
13 1.4 f 0.7 kcal/lnol,
consistent
with
thermochemical data. Contrary to the foregoing, Murad and Ihldenbrand [3,4] determined @(gEuO) with respect to AIO, BaO, and TIO, obtaining D,(EuO) = 111.9 + 2.4 kcal/mol, in sharp drsagreemcnt with the earlier determinatrons. New evidence was presented [4] to show that the results of Ames et al. [I] derived from the vaporization of Eu203(s) were invalid and that other thermochemical infor mation was consistent with Dt(EuO) = I 12 5 3 kcal/mol. This led us to question the derived thermochemical data [ 1] for the EuO-SmO exchange reaction which yielded AfG = @(SmO) - @(EuO) = 13.0 + 1.9 kcal/mol, and were therefore inconsistent wrth the new thermochemical data for EuO and a seemingly established value of @(SmO) near 140 kcal/ mol. In order to resolve these conflicts, new and independent determinations of Dg(SmO) were undertaken, along with a reinvestigation of the EuO-SmO isomolecular exchange reaction. the earlier
Volume 48, number 2
CHEMICAL PHYSICS LET-TI:KS
range 2000 to 2300 K, yielded gaseous Al, NO, Sut
2. Experimental TIICmass spectrometric method and the cxperimentai arrangement utdized in this research have been described previously [S]. Briefly, the mass spectrometer 1s a 30.5 cm radius, GO” sector mngnctic deflection instrument equipped with a Knudsen cell molecular source and a Nier-type ciectron bombardment ion source. GilSCoUS species in the molecular CffUslOn beam arc iomzcd by electron nilpact, and the resuitmg ions are mass analyzed and coiiectecl. Reaction equilibrium constants derived from the measured ion abundance ratios are then used to evaluate thcrmochemical data for the gaseous species being studlcd. In the past ion detection has been done cxciusiveiy with a convcntionai electron multiplier and dc eicctrornetcr arr‘mgcment. For some of the prcscnt studies, however, a pulse counting technique was employed in which ion pulses detected by the multiplier were sent to a discrmiinator and then to a counter. The counting equipment, which consisted of a preamplifier, ampiifier/dlscriminator, ratemetcr, and counter, was obtamed from OKTEC, Inc., Oak Ridge, Tcnnessce. In convertmg to pulse counting, the only illOd1fiCiltion rcqulred was Lo mcrcclsc tlic multiplicl ~1”’ to about 5x 106, .In order of nl.lgniturlc higher tlI.ln thnt normally used with the electrometer output. Signals dctccted by pulse counting are free of any potential discrimination effects rcsuiting from variations in multiplier sensitivity with ion ma5s or composition, and have significantly improved signal-to-noise chalacterIstics. Studies of the Sm-AI-O system were made by vapori/ing mixtures of Sm,O, and Al,O, from a molybdenum effusion ceil. Two complete sets of equilibrium measurements were made on the Sm-Al -0 system, one each by puisc counting and dc clcctromctcr methods of ion detection. For this work, two samples with somewhat different Sm203/A1203 ratio were used. For the Eu-Sm-0 and Sm-Ti-0 measurements, a molybdenum ceil containing a mixture of Eu,O,, Sm203, Ti, and TIOZ was empioycd. AI1 materials were of reagent grade quality or bcttcr.
3. Results The Sm203-Ai203
1 IUrIc IY77
mixture,
when heated
into the
and SmO. ldcntification of these species was made from the masses and the appearance p(>tenti;rIs OCAl’* (6.0 eV), AlOf( eV), Sm*(SS eV) and SmO” (5.5 CV). llicse threshold appearance potentials ‘tre in agreement with the known ioni;ration potcntiaIs of At, Al0 and Sm, and w$tli the expected value For SmO. A recent measurement by Ackermann et al. [6] gave IP(Sm0) = 5.55 eV, in accord with our threshotd appearance potential for SmO’. Two mdependcnt scrics of equilibnum measurements were made, usrng ionizing electron energies 5 eV &ove tlic respective thrcdLoids; one series was made by the conventional dc clectromctcr method and a second by ion pulse counting so that a direct comparison of the results could bc made. Iiqudibrium constants were evaluated from the raw ion abundance ratios, and the data were treated by second and third law methods, with the results shown in table 1. Thermodynamic functions used in the analysis were taken from sources noted in the appendix. As can bc seen from table 1, the data obtained for the gaseous reaction Al + SIllO = NO + sm
(i)
by the two different ion current accun&tion rncthads arc in close agreement, in regard to both the ruagnitude and the temperature dependence of the cquihbrium constant. A plot of the equilibrium data IF shown in fig. 1. In absolute magmtudc, the smoothed equilibrium constants obtained by least squ”rcs fitting for the two runs differ by Ices than six percent, and the slopes agree within the stamlrrrd dcviarions. Such accord indicates that there arc no SigilifiCiuLt systematic errors ip the 1011abundance mcasuremerrt~, and specifically that the detector introduces no drscermbie mass discrimination effects when uecd as an 10n current multiplier under OUi cxpcrimcntal cottdi-
tions. The comparison of second and third law heats of reaction (1) in table 1 indicates that the calculated thermodynamic functions of gaseous SmO, wliich contain estimated rotational and electronic contributions, are reasonably accurate. One would expect the heat content correction term a(Hzzoo - Hzg8) for reaction (1) to be small, within a few kcaI. so LEtat the _ direct comparison of the heats in table 1 is mcaningfui. Because of the large electronic heat capacity of 341
CHEMICAL PHYSICS LETTERS
Volunx 48. number 2 TiIblc 1 Equilibnum
data for tbc gaseous reaction Al + SnKl = Al0 + Sin __._ - -__._ _--
K
T(K)
-----
&Ii298 (kcal/mol) . ---.-
scrics 1 (elcctromctcr) 2087 2120 2140 2140 2lSI 2176 2189 2210 2224 2229 2234 2243 2243 2246 2260 2265 2273 2277 2298
K
T(K)
AH2s~(kcd~rnol) .-
scr:c\ II (pulse count@
0.219 0.230 0.234 0.233 0.238 0.222 0.250 0.241 0.256 0.267 3.256 0.270 0.264 0.258 0.274 0.285 0.266 0.294 0.295
12.9 13.0 13.1 13.1 13.1 13.6 13.2 13.6 13.4 13.3 13.5 13.3 13.4 13.6 13.4 13.3 13.6 13.2 13.4
2110 2124 2125 2132 2135 2144 2150 2161 2169 2176 2180 2181 2189 2189 2196 2211 2220 2231 2252
third Inw TV. 13.3 second law Aff22, I = 13.1 L 1.3
109 K =
1 June 1977
0.236 0.254 0.234 0.231 0.246 0.246 0.263 0.268 0.261 0.259 0.260 0.271 0.26 1 0.269 0.262 0.270 0.278 0.284 0.293
12.8 12.6
12.a 13.1 12.s 12.9 12.7 12.7 12.9 13.0 13.0 12.8 13.0 12.9 13.1 13.1 13.0 13.0 13.0
2254
0.284
13.2
2271 2271 2272 2288 2295
0.293 0.294 0.285 0.297
13.2 13.2 13.3 13.3
0.301
13.2
-(0_19R + 0.130) --(2865 + X8)/r tlnrd law av. 13.0 second law
-.
-.-.
---.__
it is difficult - If,,,).
The
to make
on SmO(g), ;~n accurate cstmlate
342
11.9 + 0.8
-
Sm + TiO = SmO + Ti
--
(2)
of
avcragc thlrci hW heat, AH&l) = 13.1 kcaljmol, IS assigned an uncertainty of 2.5 kcallmol, and can be combined with @(AlO)= I22.0 I 1 kcal/mol [7,81 to give f$(SmO) = 135.1 1 2.7 kcal/mol. Vaporiration of the mixture of Eu, Sm, and Ti oxides produced beams contaming the gaseous metal atoms and monoxides. The observed threshold appearance potentials of Eu+(SS ev), EuO+ (6.2 cV), SrrP (5.5 eV), SmO*(SS eV), TP(6.7 cV), and X0+(6.7 eV) provide positive identification of the parent neutrals. Equrlibrium constants for the gaseous reactions A(~~,,,,
=
log K = -(0.390 2 0.083) - (2597 + 181)/T --.----_ _--_ --------
---
Sm(g) and the lack of such informalion however,
Af1~,9,
and
Eu + SmO = EuO + Sm
(3)
were evaluated from ion abundances measured at 5 cV above thrcshoId and the results along with derived third law heats are given in table 2. The average values Mzg8(2) = 22.2 f 2.5 kcai/mol and A&j&) = 24.8 f 2.5 kcal/mol can bc coupled with Dg(TiO) = 158.4 + 1.5 kcal/mol [9,10] and @(EuO)= 111.9 + 2.4 kcal/ mol 141 to give L$(SmO) = 136.3 f 2.9 kcal/mol and 136.9 * 3.5 kcal/mol respectively.
CHEMICAL WIYSICS LITTERS
Voiumc 48, nurnbcr 2
T---
-.-----1
-1
_-_--r-_-7
-
-
--
1
1.a
2.0
0 Pulse i
Countinq Reed
n Vlbroting
l___
_-&--
Electrometer
___._I~
__
_4t6_-
_ _
-Aa
constant
for
_ ---fI;
4.3 104 -r
dcpcndcncc of equilibrium gnscous reaction Al + SmO = A10 + Sm. Fig.
1. ‘Tcmpcraturc
2120 2166 2191 7?16 2227 2212
_._----
A/1*9,(2) (kcnl/rnol) K2 x 10’ -_._ ---_.--_-----.--___--._-----1.Ot-l 1.12 1.45 1.67 1.58 1.62
_____
22.9 22.9 22.0 21.6 21.9 21.9
av. 22.2
__ ____ _.___
K1
A&9*(3) (kcal/mol)
X lo*
I.03
24.9 24.7 24.8 34.6 23.8 24.9
1.z5 1.31 1.48 1.4G 1.50
-_---___--_-__--
hid, differ substantially from tl~c results rcpmtd by Rlncs ct al. [ I], wit11 our crpilibrium constants king
more than a factor of ten lower. The diffcrcnce cannot be ascribed to their working at 25 eV ionizing en&es compared to our 5 eVahove thrcsho[d, since a check at 25 eV showed K to increase hy Icss than a factor of two. It must be assumed tlzat the caclicr measurements [ 11 contain a large systematic error, of prcscntly unknown origin. In any event, :hc.prescnt data lead to A@(3) = @(SmO) - D@uO) = 24.8 kcd/mol rather than 13.0 kcal/mol [I\, and are cntircly compatible with the newly detcrmincd vaIue @(EuO) = 1 11.9 2 2.4 kcal/mol [41_ This clears up any remaining discrepancies in the therrnocIremical data for gaseous
t,uO and S:nO,
and verifies
the suhstan-
tial drop in binding
‘I-;llllC2 Equilibrium data and third law hats for ga~cou~ Sm-X-0 and Sm-h-0 reactions __ __.. ___ _____.___ --_--__ _ ._--__ _.-. ----T(R) ----
I June 1977
a~.
24.8
-__
4. Discussion The Sn10 data obtdncd from the three independent equilibria are in good mutual agreement, leading to the selected value $(SmO) = 136.0 + 2 kcallmol. This result is in accord with D$(SmO) = 138.1 kcal/rrwl from ;I third law treatment of Sm203 effusion weight loss data [ l,l], but is significantly lower than the value D(d(Sm0) = 142.8 kcallmol obtained from mass spectrometric studies of the exchange reaction hctween YbO and SmO [ 11. Our result is also compatible with the lower bound @(SmO)> 135.5 +- 0.7 kcal/mol derived from observations of the chemilumincsccnt spectrum resulting from the reaction of atomic Sm and NO,. Our equilibrium data for reaction (3). on the other
energy in going from SmO to EuO; the latter must he associated with the attainment OF the half-filled 4f shell in Eu. A similar sharp dccrcase in 1): occurs in passing from TmO to YhO, here resulting from the comptction of the 4f sM1 in Yh. As noted by Ames et al. [ 11, the trends in E$ values of the lanthanide monoxides, along with the heats of subiimation of the metals, can he accounted for nearly quantitatively by variations in the WC -4ff1-- 1Sd transition energies of the divalcnt rare earth ions. This model assumes that the binding cnergics of the monoxides are nearly equal when the meta atom is in the 4f” -- 1 Sd configlracion, so that the Do Vallles for those n~ctals rccpirinl: the elf4 5d promotion are reduced below the baseline energy by the amount of this promotion energy_ For this purpose, the baseline is defineci by Lao, GdO, anti LuO in which the metal atom ground states have the 4f7*5d configuration. The model then predicts ~$(SmO) Z= 140 kcal/mol and DiS(EuO) = I I2 kcailmo[. in remarkably good agreement with our expcrinrental vatues. In retrospect, it is worth noting that AmCs et al. [ 1 ] were well aware that the D$(EuO) vaIur: predictem by this model was about 23 kcal/mol lower than their experimental value, in contrast to the gOOd agreement obtained with the other oxides; apparently the cvidencc at that time was not considered strong enough to prompt a cc-examination of the EuO data.
Acknowledgement The author
is indebted
to Dr. E. Xfurad for the ca 343
culatcd thermodynamic functions of SmO(g) and fcr helpfu! discussions of the work. In addition, the assistance of I&. K.IL Lau with some of the mcasurcmcnts is grutsfuily
Appendix functions
of gaseous
Al, AIO,
Ti, and TiO wcrc taken from the JANAF Tables [9], while those for Sm and Eu were taken from the tabulation of Stuli and Smke [ 121. Values for EuO(g) were taken from carlicr work [4]. For SmO&), thermodynamic functions were calculated from the following molecular constants: a VIbratlonal frequency [ 131 of 815 cm-l ; an estimated rotational constant B, = 0.40 cm- * ; and a ground state statistical weight of 10, estimated by analogy with neighboring rare earth monoxides. Free energy functions calculated from these constants arc withm 0.5 cal/mol deg of those given by Ames et al. [J 1.
344
References [I]
L.L. AI-nc~, P.N.
Wz3lsh
arlrl D. b%JfC,
J. i’hys.
Chcrn.
71
(19b7) 2707121 C.K. Dickson and R.N. Zare, Chem. Phys. 7 (1975) 361. [ 31 D.L thldcnbrand and L:‘.Murad, 2. Naturforsch. 30a
acknowledgc;ecl.
The themiodynan~ic
1 June 1977
CHEMICAL PIlYSICS LETTERS
Volume 48, number 2
(1975) 1087. [4] E. Murad and D.L. Ilildenbrand, J. Cbcm. Phys. 65 (1976) 3250. 1.51 . _ D.L thldcnbrand, J. Chcm. Phys. 48 (1968) 3657; 52 (1970) 5751. [6] R.J. Ackcrmann, F.G. Rduh and R.J. Thorn, J. Chcm. Phys. 65 (1976) 1027. [ 7j J. &wart, raraday Symp. Chcm. Sac. 8 (1973) 165. [S] P.J. Dngdigian, l1.W. Crucc and R.N. Zarc, J. Chcm. Phys. 62 (1975) 1824. 191 JANAF Thcrmochcmicnl Tnblcs, 2nd Ed., NSKDS-NBS 37 (US Govt. Printing Office, Washington, 1971); 1975 Suppl,, J. Phys. Chcm. Ref. Data 4 (1975) 1. [IO] D.L. Hrldcnbrmd, Chem. Phy~. Lcttcrs 44 (I 976) 281. [ 1 l] National Hurcau of Stand‘lrds Report NBSIR 74-600, 1 October 1974. [ 121 D.R Siull and G.C. Sinkc, Tbermodynnmic Propcrtlcs of tbc Elcmcnts, Advan. Chcm. Ser. 18 (Am. Qmn. Sot., Waslmgton. 1956). [ 131 IIL. DeKock and W. Wcltncr Jr., J. Pbys. Chcm. 75 (1971) 514.
CIIEWCAI.
PHYSICS LET’WRS
1 June 1977
DISSOCIATION ENERGY OF SAMARiUM MONOXIDE AND ITS RELATlON
TO THAT OF EUROPIUM
MONOXIDE*
D.L. IIILDENBKAND Stanford Research Inrtmrte. Menlo Park. Cahfornia 9402.5, USA Rccclved 22 Fcbrunry
1977
Bcc,n~sc of conflictme result\ tar the dlssociatlon cncrglcs (0:) of SmO nnd EuO determined by %%cral dlffcrcnt tcchniqucs. @(SmO) w&15rcdetcrmmed by rcfcrcncc to AlO,110, and CuO by means of high tcmpcrnturc ITKIFS spectrometry. Derived results for Dg(SmO) tram the exchange reactions with AK), TIO, and Ku0 were 135.1, 136.3, and 136.9 kcal/mol, rc%pcctivciy, lc.~rlm:; to the qclcctcd v.~luc 136.0 2 2 kc.ll/mol. Lvtcnsivc equilibrium measurements on the gascour reaction Al + SmO = A10 + SIN with both pulse counting and dc clcLtromctcr techmqucs gave close agreement between second and third law cnthalplcs, slgmtyin:: the cbtunnted thermodynamic funrtlon\ of SmO to bc rchablc. The new results dlffcr substanti,llly from prcvlously rcportcd d&i for the rcactron Eu + SmO = tu0 + Sm, and thereby rcsolvc puL7ling discrcpancles bctwccn the dissoci.ltion cncrgcs of SmO and tuO.
I. Introduction
The rare earths, as a famdy, are considered to form very stdblc gaseous monox~dcs, as cvidcnccd by the thermochemical studies of Ames et al. [ 11 which showed that, with the exception of YbO, the dissoclatlon energies were on the order of 6 to 8 eV. A double pcrlodicity m the MO dissociation energies and the metal heats of sublimation was observed [ 1] across the series, with mmima at Eu and Yb, althou& the minimum rn 0: was much less pronounced at ELI than at Yb. @(FuO) was derived from Knudsen effuslon measurements of the vaporization of Euz03(s) in a tungsten cell and from a study of the gaseous isomolecular rcactron Eu + SmO = EuO + Sm by mass spectromctry [ 11. In the latter case @(EuO) IS referenced to Di(SmO) through the derrvcd enthalpy change for the reaction. Dg(SmO) was determined by reference to @(YbO) and from the vaporization of Sm203. The results were interpreted to yield @SmO) = 142.0 ? 4.6 kcal/mol and @(EuO) = 133.8 + 4.6 kca.l/mol. Subsequently, Dickson and Zare [2] *This rcscarch was supported by the Air I’orce Office of Scientific Research, Air Force Systems Command, under Contract l-44620-73-C-0037.
340
reported new determinations beam-gas chcmiluminescent the lower bounds D@niO) and 1.fi(l-u0)
?
of these quantities by a reaction method, giving 2 135.5 2 0.7 kcal/mol
13 1.4 f 0.7 kcal/lnol,
consistent
with
thermochemical data. Contrary to the foregoing, Murad and Ihldenbrand [3,4] determined @(gEuO) with respect to AIO, BaO, and TIO, obtaining D,(EuO) = 111.9 + 2.4 kcal/mol, in sharp drsagreemcnt with the earlier determinatrons. New evidence was presented [4] to show that the results of Ames et al. [I] derived from the vaporization of Eu203(s) were invalid and that other thermochemical infor mation was consistent with Dt(EuO) = I 12 5 3 kcal/mol. This led us to question the derived thermochemical data [ 1] for the EuO-SmO exchange reaction which yielded AfG = @(SmO) - @(EuO) = 13.0 + 1.9 kcal/mol, and were therefore inconsistent wrth the new thermochemical data for EuO and a seemingly established value of @(SmO) near 140 kcal/ mol. In order to resolve these conflicts, new and independent determinations of Dg(SmO) were undertaken, along with a reinvestigation of the EuO-SmO isomolecular exchange reaction. the earlier
Volume 48, number 2
CHEMICAL PHYSICS LET-TI:KS
range 2000 to 2300 K, yielded gaseous Al, NO, Sut
2. Experimental TIICmass spectrometric method and the cxperimentai arrangement utdized in this research have been described previously [S]. Briefly, the mass spectrometer 1s a 30.5 cm radius, GO” sector mngnctic deflection instrument equipped with a Knudsen cell molecular source and a Nier-type ciectron bombardment ion source. GilSCoUS species in the molecular CffUslOn beam arc iomzcd by electron nilpact, and the resuitmg ions are mass analyzed and coiiectecl. Reaction equilibrium constants derived from the measured ion abundance ratios are then used to evaluate thcrmochemical data for the gaseous species being studlcd. In the past ion detection has been done cxciusiveiy with a convcntionai electron multiplier and dc eicctrornetcr arr‘mgcment. For some of the prcscnt studies, however, a pulse counting technique was employed in which ion pulses detected by the multiplier were sent to a discrmiinator and then to a counter. The counting equipment, which consisted of a preamplifier, ampiifier/dlscriminator, ratemetcr, and counter, was obtamed from OKTEC, Inc., Oak Ridge, Tcnnessce. In convertmg to pulse counting, the only illOd1fiCiltion rcqulred was Lo mcrcclsc tlic multiplicl ~1”’ to about 5x 106, .In order of nl.lgniturlc higher tlI.ln thnt normally used with the electrometer output. Signals dctccted by pulse counting are free of any potential discrimination effects rcsuiting from variations in multiplier sensitivity with ion ma5s or composition, and have significantly improved signal-to-noise chalacterIstics. Studies of the Sm-AI-O system were made by vapori/ing mixtures of Sm,O, and Al,O, from a molybdenum effusion ceil. Two complete sets of equilibrium measurements were made on the Sm-Al -0 system, one each by puisc counting and dc clcctromctcr methods of ion detection. For this work, two samples with somewhat different Sm203/A1203 ratio were used. For the Eu-Sm-0 and Sm-Ti-0 measurements, a molybdenum ceil containing a mixture of Eu,O,, Sm203, Ti, and TIOZ was empioycd. AI1 materials were of reagent grade quality or bcttcr.
3. Results The Sm203-Ai203
1 IUrIc IY77
mixture,
when heated
into the
and SmO. ldcntification of these species was made from the masses and the appearance p(>tenti;rIs OCAl’* (6.0 eV), AlOf( eV), Sm*(SS eV) and SmO” (5.5 CV). llicse threshold appearance potentials ‘tre in agreement with the known ioni;ration potcntiaIs of At, Al0 and Sm, and w$tli the expected value For SmO. A recent measurement by Ackermann et al. [6] gave IP(Sm0) = 5.55 eV, in accord with our threshotd appearance potential for SmO’. Two mdependcnt scrics of equilibnum measurements were made, usrng ionizing electron energies 5 eV &ove tlic respective thrcdLoids; one series was made by the conventional dc clectromctcr method and a second by ion pulse counting so that a direct comparison of the results could bc made. Iiqudibrium constants were evaluated from the raw ion abundance ratios, and the data were treated by second and third law methods, with the results shown in table 1. Thermodynamic functions used in the analysis were taken from sources noted in the appendix. As can bc seen from table 1, the data obtained for the gaseous reaction Al + SIllO = NO + sm
(i)
by the two different ion current accun&tion rncthads arc in close agreement, in regard to both the ruagnitude and the temperature dependence of the cquihbrium constant. A plot of the equilibrium data IF shown in fig. 1. In absolute magmtudc, the smoothed equilibrium constants obtained by least squ”rcs fitting for the two runs differ by Ices than six percent, and the slopes agree within the stamlrrrd dcviarions. Such accord indicates that there arc no SigilifiCiuLt systematic errors ip the 1011abundance mcasuremerrt~, and specifically that the detector introduces no drscermbie mass discrimination effects when uecd as an 10n current multiplier under OUi cxpcrimcntal cottdi-
tions. The comparison of second and third law heats of reaction (1) in table 1 indicates that the calculated thermodynamic functions of gaseous SmO, wliich contain estimated rotational and electronic contributions, are reasonably accurate. One would expect the heat content correction term a(Hzzoo - Hzg8) for reaction (1) to be small, within a few kcaI. so LEtat the _ direct comparison of the heats in table 1 is mcaningfui. Because of the large electronic heat capacity of 341
CHEMICAL PHYSICS LETTERS
Volunx 48. number 2 TiIblc 1 Equilibnum
data for tbc gaseous reaction Al + SnKl = Al0 + Sin __._ - -__._ _--
K
T(K)
-----
&Ii298 (kcal/mol) . ---.-
scrics 1 (elcctromctcr) 2087 2120 2140 2140 2lSI 2176 2189 2210 2224 2229 2234 2243 2243 2246 2260 2265 2273 2277 2298
K
T(K)
AH2s~(kcd~rnol) .-
scr:c\ II (pulse count@
0.219 0.230 0.234 0.233 0.238 0.222 0.250 0.241 0.256 0.267 3.256 0.270 0.264 0.258 0.274 0.285 0.266 0.294 0.295
12.9 13.0 13.1 13.1 13.1 13.6 13.2 13.6 13.4 13.3 13.5 13.3 13.4 13.6 13.4 13.3 13.6 13.2 13.4
2110 2124 2125 2132 2135 2144 2150 2161 2169 2176 2180 2181 2189 2189 2196 2211 2220 2231 2252
third Inw TV. 13.3 second law Aff22, I = 13.1 L 1.3
109 K =
1 June 1977
0.236 0.254 0.234 0.231 0.246 0.246 0.263 0.268 0.261 0.259 0.260 0.271 0.26 1 0.269 0.262 0.270 0.278 0.284 0.293
12.8 12.6
12.a 13.1 12.s 12.9 12.7 12.7 12.9 13.0 13.0 12.8 13.0 12.9 13.1 13.1 13.0 13.0 13.0
2254
0.284
13.2
2271 2271 2272 2288 2295
0.293 0.294 0.285 0.297
13.2 13.2 13.3 13.3
0.301
13.2
-(0_19R + 0.130) --(2865 + X8)/r tlnrd law av. 13.0 second law
-.
-.-.
---.__
it is difficult - If,,,).
The
to make
on SmO(g), ;~n accurate cstmlate
342
11.9 + 0.8
-
Sm + TiO = SmO + Ti
--
(2)
of
avcragc thlrci hW heat, AH&l) = 13.1 kcaljmol, IS assigned an uncertainty of 2.5 kcallmol, and can be combined with @(AlO)= I22.0 I 1 kcal/mol [7,81 to give f$(SmO) = 135.1 1 2.7 kcal/mol. Vaporiration of the mixture of Eu, Sm, and Ti oxides produced beams contaming the gaseous metal atoms and monoxides. The observed threshold appearance potentials of Eu+(SS ev), EuO+ (6.2 cV), SrrP (5.5 eV), SmO*(SS eV), TP(6.7 cV), and X0+(6.7 eV) provide positive identification of the parent neutrals. Equrlibrium constants for the gaseous reactions A(~~,,,,
=
log K = -(0.390 2 0.083) - (2597 + 181)/T --.----_ _--_ --------
---
Sm(g) and the lack of such informalion however,
Af1~,9,
and
Eu + SmO = EuO + Sm
(3)
were evaluated from ion abundances measured at 5 cV above thrcshoId and the results along with derived third law heats are given in table 2. The average values Mzg8(2) = 22.2 f 2.5 kcai/mol and A&j&) = 24.8 f 2.5 kcal/mol can bc coupled with Dg(TiO) = 158.4 + 1.5 kcal/mol [9,10] and @(EuO)= 111.9 + 2.4 kcal/ mol 141 to give L$(SmO) = 136.3 f 2.9 kcal/mol and 136.9 * 3.5 kcal/mol respectively.
CHEMICAL WIYSICS LITTERS
Voiumc 48, nurnbcr 2
T---
-.-----1
-1
_-_--r-_-7
-
-
--
1
1.a
2.0
0 Pulse i
Countinq Reed
n Vlbroting
l___
_-&--
Electrometer
___._I~
__
_4t6_-
_ _
-Aa
constant
for
_ ---fI;
4.3 104 -r
dcpcndcncc of equilibrium gnscous reaction Al + SmO = A10 + Sm. Fig.
1. ‘Tcmpcraturc
2120 2166 2191 7?16 2227 2212
_._----
A/1*9,(2) (kcnl/rnol) K2 x 10’ -_._ ---_.--_-----.--___--._-----1.Ot-l 1.12 1.45 1.67 1.58 1.62
_____
22.9 22.9 22.0 21.6 21.9 21.9
av. 22.2
__ ____ _.___
K1
A&9*(3) (kcal/mol)
X lo*
I.03
24.9 24.7 24.8 34.6 23.8 24.9
1.z5 1.31 1.48 1.4G 1.50
-_---___--_-__--
hid, differ substantially from tl~c results rcpmtd by Rlncs ct al. [ I], wit11 our crpilibrium constants king
more than a factor of ten lower. The diffcrcnce cannot be ascribed to their working at 25 eV ionizing en&es compared to our 5 eVahove thrcsho[d, since a check at 25 eV showed K to increase hy Icss than a factor of two. It must be assumed tlzat the caclicr measurements [ 11 contain a large systematic error, of prcscntly unknown origin. In any event, :hc.prescnt data lead to A@(3) = @(SmO) - D@uO) = 24.8 kcd/mol rather than 13.0 kcal/mol [I\, and are cntircly compatible with the newly detcrmincd vaIue @(EuO) = 1 11.9 2 2.4 kcal/mol [41_ This clears up any remaining discrepancies in the therrnocIremical data for gaseous
t,uO and S:nO,
and verifies
the suhstan-
tial drop in binding
‘I-;llllC2 Equilibrium data and third law hats for ga~cou~ Sm-X-0 and Sm-h-0 reactions __ __.. ___ _____.___ --_--__ _ ._--__ _.-. ----T(R) ----
I June 1977
a~.
24.8
-__
4. Discussion The Sn10 data obtdncd from the three independent equilibria are in good mutual agreement, leading to the selected value $(SmO) = 136.0 + 2 kcallmol. This result is in accord with D$(SmO) = 138.1 kcal/rrwl from ;I third law treatment of Sm203 effusion weight loss data [ l,l], but is significantly lower than the value D(d(Sm0) = 142.8 kcallmol obtained from mass spectrometric studies of the exchange reaction hctween YbO and SmO [ 11. Our result is also compatible with the lower bound @(SmO)> 135.5 +- 0.7 kcal/mol derived from observations of the chemilumincsccnt spectrum resulting from the reaction of atomic Sm and NO,. Our equilibrium data for reaction (3). on the other
energy in going from SmO to EuO; the latter must he associated with the attainment OF the half-filled 4f shell in Eu. A similar sharp dccrcase in 1): occurs in passing from TmO to YhO, here resulting from the comptction of the 4f sM1 in Yh. As noted by Ames et al. [ 11, the trends in E$ values of the lanthanide monoxides, along with the heats of subiimation of the metals, can he accounted for nearly quantitatively by variations in the WC -4ff1-- 1Sd transition energies of the divalcnt rare earth ions. This model assumes that the binding cnergics of the monoxides are nearly equal when the meta atom is in the 4f” -- 1 Sd configlracion, so that the Do Vallles for those n~ctals rccpirinl: the elf4 5d promotion are reduced below the baseline energy by the amount of this promotion energy_ For this purpose, the baseline is defineci by Lao, GdO, anti LuO in which the metal atom ground states have the 4f7*5d configuration. The model then predicts ~$(SmO) Z= 140 kcal/mol and DiS(EuO) = I I2 kcailmo[. in remarkably good agreement with our expcrinrental vatues. In retrospect, it is worth noting that AmCs et al. [ 1 ] were well aware that the D$(EuO) vaIur: predictem by this model was about 23 kcal/mol lower than their experimental value, in contrast to the gOOd agreement obtained with the other oxides; apparently the cvidencc at that time was not considered strong enough to prompt a cc-examination of the EuO data.
Acknowledgement The author
is indebted
to Dr. E. Xfurad for the ca 343
culatcd thermodynamic functions of SmO(g) and fcr helpfu! discussions of the work. In addition, the assistance of I&. K.IL Lau with some of the mcasurcmcnts is grutsfuily
Appendix functions
of gaseous
Al, AIO,
Ti, and TiO wcrc taken from the JANAF Tables [9], while those for Sm and Eu were taken from the tabulation of Stuli and Smke [ 121. Values for EuO(g) were taken from carlicr work [4]. For SmO&), thermodynamic functions were calculated from the following molecular constants: a VIbratlonal frequency [ 131 of 815 cm-l ; an estimated rotational constant B, = 0.40 cm- * ; and a ground state statistical weight of 10, estimated by analogy with neighboring rare earth monoxides. Free energy functions calculated from these constants arc withm 0.5 cal/mol deg of those given by Ames et al. [J 1.
344
References [I]
L.L. AI-nc~, P.N.
Wz3lsh
arlrl D. b%JfC,
J. i’hys.
Chcrn.
71
(19b7) 2707121 C.K. Dickson and R.N. Zare, Chem. Phys. 7 (1975) 361. [ 31 D.L thldcnbrand and L:‘.Murad, 2. Naturforsch. 30a
acknowledgc;ecl.
The themiodynan~ic
1 June 1977
CHEMICAL PIlYSICS LETTERS
Volume 48, number 2
(1975) 1087. [4] E. Murad and D.L. Ilildenbrand, J. Cbcm. Phys. 65 (1976) 3250. 1.51 . _ D.L thldcnbrand, J. Chcm. Phys. 48 (1968) 3657; 52 (1970) 5751. [6] R.J. Ackcrmann, F.G. Rduh and R.J. Thorn, J. Chcm. Phys. 65 (1976) 1027. [ 7j J. &wart, raraday Symp. Chcm. Sac. 8 (1973) 165. [S] P.J. Dngdigian, l1.W. Crucc and R.N. Zarc, J. Chcm. Phys. 62 (1975) 1824. 191 JANAF Thcrmochcmicnl Tnblcs, 2nd Ed., NSKDS-NBS 37 (US Govt. Printing Office, Washington, 1971); 1975 Suppl,, J. Phys. Chcm. Ref. Data 4 (1975) 1. [IO] D.L. Hrldcnbrmd, Chem. Phy~. Lcttcrs 44 (I 976) 281. [ 1 l] National Hurcau of Stand‘lrds Report NBSIR 74-600, 1 October 1974. [ 121 D.R Siull and G.C. Sinkc, Tbermodynnmic Propcrtlcs of tbc Elcmcnts, Advan. Chcm. Ser. 18 (Am. Qmn. Sot., Waslmgton. 1956). [ 131 IIL. DeKock and W. Wcltncr Jr., J. Pbys. Chcm. 75 (1971) 514.