Far infrared spectra of vaporized neodymium trihalides

Chemical Physics 24 (1977) 391-397 0 North-Holland

Publishing

Company

FAR INFRARED SPECTRA OF VAPORIZED NEODYMIUM TRiHALIDES + John C. WELLS Jr. * Department

of Physics, Tennessee Tech~zological University,

Cookeville,

Tennessee 38501,

USA

John B. GRUBER Department of Physics, College of Science and Mathematics, North Dakota State University, Fargo, North Dakota 58102.

USA

and

Milton LEWIS ** Donald IV. Douglas Laboratories, McDotmell Douglas Corporation,

McDonnell Douglas Astronautics Company, Richland. Washbtgton 99352, USA

Received 7 March 1977

Par infrared absorption spectra of vaporized NdC13, NdBr3, and NdI3 have been examined between 50 and 400 wavenumbers at 1OOO’C using a Beckman FS-720 far infrared Fourier spectrophotometer. High temperature far infrared spectra create experimental difficulties that can be overcome by experimental design. Earlier electron diffraction studies of vaporized neodymium halides have been interpreted in favor of planar D3h molecules rather thau the pyramidal Cav molecules. We have examined the infrared spectra on the possibility that either symmetry may be appropiiate. However, we conclude that Djh symmetry is a reasonable approximation even though all fundamental modes of vibration appear to be infrared active.

1. Introduction Until rather recent times there has been relatively little experimental data and interpretation of the vapor spectra of the rare earth trihalides [l-4]. The high temperatures (nearly 1000°C) needed to volatilize sufficient quantities for detection and the long sample path lengths and accompanying experimental configuration required for sufficient resolution of the spectra, have discouraged many experimentalists from designing and carrying out measurements *. Vapor spectra of the tribromides and triiodides of Pr, Nd, Er and Tm, measured between 4000 and 30 000 * Supported

in part by USAEC (later ERDA) contracts, AT(45-l)-2225 and AT(45-l)-2221’T6 (1973, 1974). * NORCUS Re&ch Participant, Summers 197 3.1974, DonaId IV. Douglas Laboratories. RichIand, WA. ** Present Address: Columbia Engineers Services, Inc., Richland, WA., USA.

cm -’ have been reported by Gruen and his coworkers in 1966 and 1967 [ 1,2]. While “LJ levels were identified in part and oscillator strengths calculated for some observed bands, there still remained uncertainty regarding the detailed crystal-field structure of each observed [SL] J manifold. To study such fine detail, Clifton et al. [6] examined the fluorescence spectra of matrix-isolated PrIf in Ar and Xe matrices. More recently there hai been growing interest in other more easily vaporized rare earth molecules. Current work is underway to explore the suitability of rare earth molecular vapors as a cIass of laser media for advanced fusion applications [7-9]_ Higher volatility is possible when the rare earth trihalides are complexed with such volatile compounds as AlCl, [lo]. However, in all such studies reported so far, we have seen no presentation of the infrared spectra * See ret [Sl and references therein describing difficulties to be overcome in obtaining quality vapor spectra.

J.C. WeIls Jr. et aI./Far infrared speck

392

-: between-50 and 400 cm-t for the vaporized molecules of the rare earth trihalides bther than references to our earlier work reported at an APS meeting [4]. Because of obvious interest in potential application, we have endeavored to complete our earlier analysis and assignments of vibrational spectra and to present these results here.

.2. Experimental

details

Each of the neodymium trihalides was prepared from neodymium oxide by the following procedure reported by Reed et al. [ 1 I]. An excess amount of the appropriate ammonium halide was reacted with the neodymium oxide at 190°C. To remove the excess ammonium halide without the formation of oxyhalide compounds, the mixture was heated to 320°C under vacuum. Chemical analysis of material as well as tests for solubility in water indicated that the neodymium trihalides had been prepared properly. To provide sufficient vapor pressure, the spectroscopic measurements were made in the neighborhood of IOOO”C. Vapor pressure data for rare earth trihalides by Brewer [ 121 and by Polyachenok and Novikov [!3] suggest that at this temperature vapor pressures exceed 10B3 atm for NdCl, and NdBr,, nnd low2 atm for NdI,. The far infrared spectra of NdCl, , NdBr3, and NdI3 in the vapor state were investigated, using a Beckman FS-720 far infrared Fourier interference spectrophotometer. Measurements were made between 50 and 400 wavenumbers. Distinct spectral bands were observed for each molecular vapor and are reported in table 1. Measurements of the vapor concentration of MX, predicted by the vapor pressure equation versus (1/Z’)104 and absorbance against temperature for a specific band supported the fact that the spectra are of gaseous monomeric rare earth trihalide [S]. Unfortunately complete spectra taken at different temperatures were not possible since vapor concentrations were low over the usable experimental temperature range 900- 11 50°C. At 1000 -C 50°C observed bands were reproducible over different scanning times and different runs. Since the spectrophotometer is of modular design, it was taken apart and a tube furnace with sufficient insulation was inserted between the interferometer

of vaporized n&~dymiuh

trjialides

:

’

and.Golay detector modules. Additional polyethylene lenses Were used to collimate the infrared radiation so, as to reduce the amount of radiation loss from the : source. Before taking data the spectrophotometer, sample tube and tube furnace were Flushed with dry argon to eliminate any traces of water vapor. The instrument was calibrated between 40 and 500 cm-’ using HCl vapor at roam temperature. Our spectrum of HCl agreed to within a 0.1 cm-’ with acepted values. The infrared spectrum of water vapor was also taken at room temperature covering the same wavelength range_ Another reason for taking the vapor spectra of HCl and H20 in the infrared was to assure us in identifying the halide spectra from these common impurities. A reference interferogram with argon in the system was taken with the furnace set at 1000°C. Following this procedure the system was then co&d down and approximately a gram of the neodymium trihalide was placed in a sample tube in the center of the furnace. The system was again flushed with dry argon and the sample heated to 1000°C where an interFerogram was obtained. Spectra were determined From the interferograms by a direct Fourier transform computer code. Spectral intensities were determined at intervals of 1.25 cm-‘. To reduce the effects of noise in the spectra, adjacent spectral intensities were combined in a way that smoothed out the spectra. The smoothing was performed in the following way: In a given spectrum, each spectral intensity I(v,) was replaced by the sum

with a = 0.17. This smoothing was applied to each sample and reference spectrum before takiig the ratios to obtain the transmission spectrum of the trihalide.

3. Observed spectra The infrared spectrum for each of the three neodymium trihahdes is shown in fig. 1 and reported in table l_ Several spectra were recorded for each vaporized rare earth trihalide at 1000 t 50°C. The spectra shown here are representative of the general features

v3

or vt

or v3

3 5 6

8 3

1

331 ?~2 349 f 2 364r3

3 3 6

1 9 6

28lk2 301 f 1 325 f 2

7 4 8 6

10 8

Band. width c, (cm-* )

2 8 4 6 5

2 10

Rel. I b,

169 *2 177 f I 190*2 201*3 221 f 2

103*3 120*2

Energy (cm+)

.

D3

D2

h

c3

c2

Cl

B5

B4

B3

B2

Bl

A2

Al

L.

or ~3

u3 or v1

v1

v2

*4

41a)

NdBr3 (1000°C)

-_.-

-

.__.

-

e) Assigned fundamental mode of vibratjon; see ret I15 1, b, Relative intensity from 1 to 10 approptiatc to each sepi\rntc spectrum. c, Bandwidth at half the height of the band from baseline.

D3

D2

fh

C3

c2

Cl

ki

VI

v2

Bl BZ 03

04

v4

+I 3)

A2

AI

L.

NdC13(1000°C)

Tablc 1 Infrared absorption of veporizcd neodymium trihrlides

.

_. _

(sltouldcr) 220 f. 3 242*2

17113 188 A5 194 +3

93 f 1 104+2 11122 120r2 132*2

65i2 8Oi2

Energy (cm-’ )

8 2

5 ;’

1 3 3 10 4

2 8

-

-

lo 4

18

5

3 6 5 4

6 5

Rcl, BandI b, width c, (cm-’ )

__

D3

D2

h

co

C1 CZ

B5

B4

Bz

B2

Bt

A2

At

L.

~3

..^

or VI

vt or v3

v2

“4

VII=)

Nd13 (I 000°C)

._

182i2 195i2 203 f 2

133 t 3 1411:2 15S*3

98+2 (shoulder) 118 t2 (shoulder)

60*2 12k2

lhrgy (cm-‘)

3 10 3

8 6 3

4 1 -

3 4

Rel. I b)

8 -

5 4 8

_ 4 3 -

4 4

Bondwidth C) (cm-‘)

g w

J.C. We& Jr. er &/Far infrared spectra of vaporized neodymium

,394

40

NOI,

20 clc 40

SO

120

160 200 240 230 WAVENUMBER km-‘1

320

360

400

Fig_ I. The far infrared absorption spectra of vaporized NdCls. NdBrs and Ndls taken at 1000°C.

observed. Signal-to-noise ratio was a problem during data taking and base line variations in transmission could run as much as 10% in certain regions. These variations do not, however, greatly effect the location of the center of the absorption bands which were reproducible to within a few percent. Base line uncertainties, however, do effect the total area under the absorption curve. On the average this effects our report of relative intensities by approximately 10%. We view the observed infrared spectra for all thfes compounds in terms of four general groupings with each grouping having definite structure. The exact structiire or profile for each grouping should be viewed with caution due in part to the signal-tonoise ratio problem mentioned earlier. In f&t at 1000°C we would Ilot expect a well-resolved

enve-

lope. The structure of each grouping corresponds to a combination of rotational-vibrational transitions. Even over the narrow temperature

range investigated

there appears evidence for existence of “hot” bands. At 1000°C we expect From Boltzmann considerations that transitions will originate not only from molecules in the ground state, but from excited vibrational states as well. If there are any anharmonicities in vibration, similar to those observed for vaporized UF, and UC14, these “hot” bands will not precisely coincide with the ground state bands. Due to experi$ental conditions such as the beam

wihaiides

splitter thickness, the entire spectral range was not covered for each of the trihalides. The spectral range was selected for each moIecuIar species in such a way as to include the expected values for all of the fundamental vibrational frequencies for that molecule. The infrared spectrum of vaporized NdC13 (see fig. 1 and table 1) was taken between 90 and 375 wavenumbers at approximately 1000°C. The spectrum consists of four groupings of bands with the first strong band appearing at 120 cm-t, the second grouping with peaks at 177,190,201 and 221 cm-‘; the third grouping with peaks at 301 and 325 cm-’ ; and the fourth grouping containing peaks at 349 and 364 cm-’ . To the low-energy side of each of the four groupings we find considerably weaker peaks at 103,169,287 and 331 cm-‘. The possibility of water vapor and HCl contributing to the observed spectrum was explored carefully. At 1000°C HCl would have a series of rotational bands covering the wavelength investigated, spaced 20.5 cm-t apart, with the strongest absorption expected at 145 cm-’ . We observe no absorption between 123 and 166 cm-‘, nor do we observe absorption between 225 and 275 cm-’ _Since water vapor is also expected to have strong absorption bands between 225 and 275 cm-t , we conclude that to within the sensitivity of the instrument neither HCl nor water vapor were found in our sample at 1000°C. An earlier measurement using the vapor pressure equation and variation of absorbance over a narrow temperature interval suggested a single monomeric species of NdCl,. The infrared spectrum of vaporized NdBr3, taken between 70 and 250 wavenumbers at approximately 1000aC (see fig. 1 and table 1) also shows a similar grouping of bands shifted to lower energy and somewhat more compacted. There are a few differences in relative intensities within the band groupings. The band lowest in energy appears at 80 cm-’ ; with a second band grouping having peaks at 104,120 and 132 cm-‘; a third grouping at 171 and 188 cm-’ and a fourth band at 220 cm-l. Again to the lowenergy side of each band or band grouping either an extremely weak peak or shoulder would appear separated in energy by less than observed in the vaporized NdC13 spectrum. The infrared spectrum of vaporized NdI3, taken between 40 and 200 cm-’ at approximately 1000°C

J. C. Welts A. et &/Far

infrared spectra of vaporized neodymium

395

triahlides

Table 2

Fundamental frequencies and force constants for vaporized SbXs, NdXs. BiXs (C3v symmetry) MOlCCUk

Ref.

Force

Fundarnentat frequencies (cm-’ )

ccinstants

(X 105 dyne/cm)

Bond

angle SlJClJ NdCls BiClSbB;s NdBr3 Bilks Sbi3 Nd13 Bils

[ISI

1181 I171 t171 (171 r171

vl(Ar)

uz(A~)

r%(E)

r%(E)

fr

frr

fa

f 0xX

360 349 288 244 220 196 177 182 145

165 177 130 110 107 104 89 98 90

320 306 242 226 1.88 lb9 147 141 115

134 120 100 91 80 90 71 72 71

1.78 1.72 1.19 1.47 1.17 1.06 0.81 0.86 0.65

0.16 0.21 0.17 0.10 0.18 0.16 0.15 0.20 0.14

0.18 0.17 0.13 0.16 0.14 0.16 0.16 0.18 0.18

0.02

99.5”

0.05 0.025

(99.6”)

0.01 0.02 0.015 0.015 0.03 0.02

=

a)

loo.o” 97.0° (97.8”) a) 100.0” 99.0” (99.3”) 3 loo.oO

a) Estimated value.

also

can be described in terms of four observed band groupings with the peaks lowest in energy appearing around 60 and 72 cm-‘. Asecond band is found at 98 cm-’ and two stronger band groupings are found at 133 and 155 cm-‘, and at 182 and 195 cm-l respectively. Here again we observe a shift to lower frequencies (energies) as the anion becomes heavier in going from C1 to Br to I. 4. Discussion Any interpretation of the observed infrared spectra must be viewed with caution without additional temperature dependent studies on each molecule and higher resolution of the bands, which we conclude is going to be difficult to obtain even if a tunable infrared laser source is available [8,14]. We have not as yet had the opportunity to take the Raman spectra which hopefully could provide additional independent evidence

for determiniig

the molecular

symmetry

WI.

Electron diffraction studies of vaporized neodymium halides have been interpreted in favor of planar Dlh molecules rather than the pyramidal C,, molecules 1161. While in both cases four fundamental modes of vibration are expected, we find different infrared optical properties are expected [ 151. For C,, symmetry all four modes are active in both the infrared and Raman spectrum. However, for DSh symmetry the only totally symmetric vibration is inactive in the infrared while the an&symmetric vibration is

in the Raman effect. There is a number of infrared spectra of vaporized molecules MX, having C,, symmetry and molecular weights both lighter and heavier than NdX3 with which we can compare our data [ 15, 17,18]. However, we have not been as successful in finding infrared spectra of MX, molecuIes of Djh symmetry having comparable molecular weight [ 151. Reports are usually made for much Iighter MX3 species. Moreover, earlier high resolution studies on infrared bands of vaporized MX, (M = rare earth) and UCI, show that rotati0na.I and Coriolis interactions involving different cation and anion isotopes Iead to anharmonicities in vibrational motion resulting in a time-averaged position of the cation just out of the pIane of anions in MX, [8,14]. This effect, difficult to ascertain from the electron diffraction data, does allow all four modes of vibration to become infrared and Raman active as expected for C,, symmetry. To better ascertain the appropriate symmetry, we have tried to interpret the infrared data first on the basis of a pyramidrd model, then on a planar model and followed by our conclusions and comments. For the pyramidal model (C,, symmetry) we assumed a four-constant potential function first derived by Howard and Wilson [ 191. We Further assumed that if the vaporized neodymium trihalides were pyramidal, their fundamental frequencies would probably lie between those found for the vaporized trihalides of antimony and bismuth previously reported [15,17,18]. . We then selected the most pronounced peak or band center from each of the four groupings in the observed

1-C. WeIls Jr. et aI./Far tifiaredspectra

396 Table 3 Fundamental Molecule

frequencies

and forceconstantsforvaporized

Fundamental vn (cm-‘)

frequencies

a)

of vaporized neodymium trikalides

NdXJ (Dph symmetry) Force constants (lo5 dyne/cm)hf.

[ 15))

h3 + x,

U-HS) (ref. [ 15) ) a)

(II, 21I)CRHS) (ref. [Is])a)

u1

u2

u3

“4

kl

b/l2

kg/l2

NdCIg

301

177

349

120

1.90

0.08

0.38

0.080

0.082

NdBr3

171

107

220

80

I.38

0.06

0.20

0.032

0.036

Nd13

133

98

195

72

1.32

0.08

0.20

0.025

0.028

a) if, according to symmetry considerations only ~2, “3 and “4 are observed, we predict values of v1 as follows: NdC13 (vI = 297 cm-‘). kl= 1.85,ksfl’= O.O~,X-A/~~ =0.38;NdBr-, (u, = 159cm-'). kl = 1.19, ksf12 = 0.07,kA/12 = 0.20;and forNd13 (~1 = llicm-I). k;= 0.96, k6/12: 0.09, kaf12 = 012Xs

and identified it as vr , v2, v3, or v4 by its relation to one of the known fundamental frequencies of SbX3 and BiX3. Further details regarding this method of making assignments and the method of calculating force constants by this method are given elsewhere [4,15]. In table 2 we summarize the results and include for comparison the fundamental frequencies and force constants for SbX3 and BiX3 which represent lighter and heavier vaporized molecules respectively both having reported C3v symmetry [ 15]_ The force constants for SbX, and BiX, appearing in table 2 were determined using the same method we used for NdX,. We see that force constants for vaporized neodymium halides based on C,, symmetry’lie close to the corresponding force constants for the other molecules. Only in one case, faa for NdCl, , is the force constant significantly larger. The force constant is sensitive to the frequencies of the two deformafQ(2 tion modes, v2 and u4. In order to have f,, = 0.02 X lo5 dyne/cm for NdCl3 with the other force constants unchanged, it would be necessary to have v2=154cm-Iandv4= 134 cm-l. As no absorption was observed in these regions, we conchrdk that the larger value off,, may be real. Our concern with this calculation however rests with the predicted bond angle (Ywhich is determined from the isotopic masses as well as the chosen frequencies vu. Our present conception based on the earlier work [8,14] eluded to suggest the molecules should be somewhat flatter or closer to a planar configuration (D3r, symmetry). Our second calculation was made to determine if spectra

the assumption of D3,., planar molecules would result in force constants that were also reasonable and consistent with observed vaporized NdX3 spectra. The method of analysis we used is described by Herzberg [ 15, pp. 177-1791. Valence forces are assumed which involve only three force constants. See table 3. We assign v2, v3 and v4 in table 3, assuming that with increasing energy the observed four groupings in the infrared spectra bear the same order in fundamental frequency assignments as those published for other vaporized MX, molecules having D3, symmetry. We then calculated force constants based on these assignments and determined the energy level for v1 which strictly speaking is infrared inactive. However, if vibrational anharmonicities resulting from rotational and Coriolis interactions are present we still expect to see band structure around the vl level. Our predictions for VI turn out to be rather close to spectra expected around a v1 level. In table 3 we list calculated values for VI with estimates from the data in parentheses. On the other hand, if we proceed as does Herzberg [ZS] , in the knowledge of the energy of VI, we can determine the left-hand side (LHS) and the right-hand side (RI-IS) of eqs. (II, 210)-(II, 212) of ref. [15, p_ 1781 -The last two columns in table 3, namely X3 + A, (LHS) and RHS, show better agreement than do many of the D3, molecules in table 44 of Herzberg [lS]. We come to the conclusion that vaporized MX3 molecules can be described in terms of D3h symmetry even though they are not strictly planar. We feel

J.C. Wells Jr. et al/Far infiared spectra of vaporized neodymium

justified in the length of discussion concerning the two calculations reported since assignments of vn to high temperature infrared vapor data are difficult to make. Nevertheless, these data appear consistent with the general interpretation of the symmetry suggested for the vaporized rare earth hihalides.

Acknowledgement The authors are grateful to Mr. Robert Unden for designing the high temperature cell, and to Mr. Robert Schumacher for assistance in setting it up. Also, thanks are due Mr. Kenneth Stine of Beckman Instruments for helpful discussions on the operation of the spectrophotometer.

[I]

[2] [ 31 [4] [S] ]6]

D.M. Gruen and C-W. DeKock, J. Chem. Phys. 45 (1966) 455. D.M. Gruen, C.W. DeKock and R.L McBcth, Am. Chcm. Sot., Advan. Chem. Ser. Ml (1967) 102. D.M. Gruen, Progr- Inorg. Chem. 14 (197 1) 119. J.C. Wells Jr., M. Lewis and J.B. Gruber, Bull. Am. Phys. Sot. II 19 (1974) 1108. J.R. Morrey. D.G. Carter and J.B. Gruber, J. Chem. Phys. 46 (1967) 804. J.R. Clifton, D.hl. Gruen and A. Ron, J. Mol. Spectry. 39 (1971) 202.

trihalides

397

[7] W.F. Krupke, Prospects for Trivalent Rare Earth Molecular Vapor Lasers for Fusion, Proceedings of the 12th Rare Earth Research Conference, Vol. II, S-4 (1976) 1034. [S] J.B. Gruber, Spectroscopic Studies of Actinide Ions in Crystalline Solids, USAEC Ann. Prog. Rep., RLO-2221T6-13 (1972); RLQ2221-T6-20 (1974). [9] W.F. Krupke, Dynamics of Trivalent Rare Earth nmb chr Vapor Lasers, KID-16993, LRL (1976). f 101 _ - H.A. Dye and D.M. Gruen, J. Am. Chem. Sot. 91 (1969) 2229. [ll] J.B. Reed, B.S. Hopkins and L-F. Audrieth, J. Am. Chem. Sot. 57 (1935) 1159. [ 121 L. Brewer, The chemistry and metallurgy of miscellaneous materials, ed. L.L. Quill (McGraw-Hill, New York, 1950) p. 198. [ 131 0-G. Polyachenok and G.I. Novikov. Russ. J. [norg. Chem. 8 (1963) 793. [ 141 J.B. Gruber, Laser Induced Chemical Isotopic Separation Process, US Department of Commerce Patent Office. ~530,567 (December 5, 1974). Submitted by ERDA on behalf of the author through ERDA contract AT(45-l)-2221-T6. 1151 G. Henberg. Molecular spectra and molecular structure, Vol. 2, Infrared and Raman spectra of polyatomic molecules, 12th Ed. (Van Nostrand Company, New York, 1966). (161 P.A. Akishin, V.A. Naumov and V.hl. Tatevskii. Nauch. Dokl. Vyssh. Shk. (1959) 229; Chem. Abstr. 53 (1959) 19493e. Acta 21 iI71 T-R. Manley and D.A. Williams, Spectrochim. (1965) 1773. and S. Sundaram, Proc. Phys. Sot. [I81 K. Venkateswarlu A69 (1956) 180. 1191 J.B. Howard and E.B. Wilson Jr., J. Chem. Phys. 2 (1934) 630.