Diffuse reflectance spectra of protactinium(IV), uranium(IV) and neptunium(IV) tetrachlorides and tetraformates

F !n~tc ,ncU Chem, 1975, Vol 37, pp 249t-2495.

Pergamon Press.

Printed in Great Britain

DIFFUSE REFLECTANCE SPECTRA OF PROTACTINIUM(IV), URANIUM(IV) AN[) NEPTUNIUM(IV) TETRACHLORIDES AND TETRAFORMATES H. J. SCHENK, E. W. BOHRES and K. SCHWOCHAU l nstitut fi.irNuklearchemie der Kernforschungsanlage JiJlich GmbH, D-517 Jiilich, Germany (Received 6 December 1974)

Abstract--Diffuse reflectance spectra of 8-coordinate AnCL and An(HCOO), (An : Pa, U, Np) have been measured in the range of 18,000-2200 ,%at room temperature. The observed excited levels arise from 5f", 5f ~ ~6d ~ and 5f ~ 'rr electron configurations. The 5f" spectra of corresponding chlorides and formates are quite similar, shifted towards higher wave numbers in the latter case. Owing to the actinide contraction the crystal-field splitting of the 5f ~ levels decreases with increasing atomic number. The energy variation of the 5f ~ -~ 5f ~ '6d' transitions as a function of q is more pronounced in the predominantly ionic tetraformates than in the more covalent tetrachlorides. The cubic splitting parameter of the 6d shell is evaluated to be A~d = g k K and A,,, = 15kK in PaCL and Pa(HCOOL, respectively. The electron transfer spectra indicate the optical electronegativities X,,~, = 1.42 for protactinium(1VI and X , , , - 3.13 for the formate ion. INTRODUCTION .~BSORPTION spectroscopy has proved useful for enhanc•ng our knowledge of actinide systematics. Comparative ~pectroscopic studies allow correlations to be made between the atomic number and chemical properties such as the nature and strength of bonding[l, 2] and the relative oxidizing tendency of the actinide ions[3,4]. Such investigations, however, necessitate a series of homologous actinide compounds of known coordination number and molecular geometry in a chemically accessible valence state. Highly symmetric ligand frameworks are furthermore desirable in order to minimize the problems of interpretation. The tetraformates of protactinium(IV), uranium(IV) and nepttmium(IV) represent the electron configurations 5fL 5f 2 and 5 f 3, perturbed by a crystal field ~)f slightly distorted 8-fold cubic symmetry[5]. We have also measured the spectra of the analogous 8coordinate[6] tetrachlorides, because the spectra of the actinides, particularly in their higher oxidation states, are rather sensitive to the environment around the metal ion, this sensitivity being intermediate between d-group and lanthanide elements. Chlorine and formate are ligands with quite different spectrochemical and nephelauxetic properties[13] leading to spectral shifts and changes in intensity, which are expected to facilitate the assignment of absorption bands and the identification of excited qates. In the case of UCL the general t'eatures of our spectra are in satisfactory agreement with previously published data on polycrysmlline samples[7] and single crystals [8, 9, 20]. EXPERIMENTAL

"lhe preparation of anhydrous tetraformates of protactinium(IV!, uranium/IV) and neptunium(IV) is reported ,.:lsewhere[ 10j. We employed the reflectance method for measuring the absorption, since we were unable to obtain single crystals of sufficient size for optical studies in transmission since Jecrystallisation from solvents leads to decomposition or formation of addncts, and the thermal instability of the formates makes impossible cryqal growth from the melt or by sublimation lechniques. The spectra were recorded with a Cary-14 speclropboh~meter provided with a diffuse reflectance accessory. The samples (approx. 5 rag! were finely ground, mounted in ~,m~d[ smnple holders and covered by quartz plates in order to 2491

exclude moisture and oxygen and to avoid radioactive contamination. Such plates undoubtedly reduce the reflectance, but the position of the absorption maxima is known to be maintained [1 I]. All manipulations were carried out in an inert atmosphere glovebox. RESULTS

The internal ~:ransitions within the partly filled 5f shell of uranium(IV) and neptunium(IV) are shown in Figs. 1 and 2, where the logarithm of the Kubelka-Munk function, log F ( R ) (standard MgO), is plotted against the wave number ~. All individual maxima are listed in "Fables 1 and 2. In the case of 5f' protactinium(IV) we detected only two absorption bands centred at about 7000 and 10,000 cm '. The missing two bands are expected outside the spectral region available with our apparatus [12]. The intense 5 f f - ~ S f " - ' 6 d ~ and electron transfer spectra are shown in Fig. 3; the corresponding energies Table 1. 5f=-~5f '- absorption bands (cm ') of UCL (~1) and U(HCOOL (~..) (shoulders in parentheses) Peak No. l 2 3 4 5 6 7 8 9 I0 ii 12 13 d4 15 16 17 18 19 20 21 22 23 24 25

~i

32

5952 602# 6477 6662 68O3 6849 7545 7730 819o ~84oo ',, (877C) -)0,33 8928 ,', 9200) 9708 Ioooo 10204 10204 10417 ( iC600} 110~O ~.1300 (11560) (itS>o) (12270)(12580) 14597 14826 (14680) (14920) 14~48 (15210) 15291 i~5366 15557 15686 16393 (16) 3) (176oo) I~C'4& 17857 (18200) 19417 (20000) 20471 20512 20877 71500) 22727 22883

92

-

72 185 46 185 21c 263 272 292 o 183 250 270 ~iC 227 240 262 65 13A o 444 343 b83 42 623 155

Field-free ion 25+I_ ~j levels

91

3F2 3H 5

3F3, 3F~,

3H6

!G4

ID2 [

3p i_o 26

]P iII 3 6 P2

H. J. SCHENKet al.

2492

;,, b&}

-

10000 i

3F 2

l

3F31t.}

3H 5

5000 I

lot. ~O2 3P0

3H 6

116

&O00 I

3000 i

3P2

~PI

2,0

-°~

15

iai:o,,

///

I I,.'\,"~ l

1,0

,-.,o.

,..,~

":',,~

_o~

~

~

,,,

_,

10

$

..

ii ~ \ A ! [k ~, " # ",

/

L o.~

0,5

0.0

I

1 °'

10

15

20

25

30 ~(kK)

Fig. 1. Internal 5]'2- 5f: transitions of UCI4and U(HCOO)4.

10000 I 11

3

13

SO00 I

i 9

5

37

5

7

9

11

7

k (A)

4000 I ~q

5

3000 I

11 3 5

2,0 ill

:;

i

!

/ /

ij' i~1

T

i

17 n

1,5

1.0-

a

~', ~

/

I e I

,i

26

Sl

9

/

/

2s , ~7

/

0,5 T

0,5-

0,0-

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0,0

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'

I

i

,

,

l'

l

U

F

I

10

'

I

I H/,I

,

,

~

Y:' F

,'

.....

,

,

15

I

20

.

.

.

.

I

.

.

.

.

I

25

-0,5

NoIHCOOb

.......... '

,

30 ,"

9 (kK)

Fig. 2. Internal 5f~- 5 f transitionsof NpCh and Np(HCOOh. are collected in Table 3. In view of the high oscillator strength of these transitions it was necessary to dilute the samples with about 50% of MgO. DISCUSSION The complexity of actinide spectra is mainly due to the fact that the three major interactions: interelectronic repulsion, spin-orbit coupling and crystal-field influences are all of the same order of magnitude. This situation is intermediate between d-group elements, in which the spin-orbit interaction is small, and the lanthanides, in which the crystalline field is small. Therefore it is impossible to treat the crystal field as a perturbation on free-ion J levels, and any reliable assignment of individual bands presumes the simultaneous diagonalization of Coulomb interaction, spin-orbit coupling and crystalline field. This procedure, however, requires the identification of a sufficient number of excited states by experimental methods based on temperature shifts, polarization of electronic transitions and vibronic selection rules. Since

room-temperature reflectance spectra cannot yield such information we did not attempt a quantitative analysis of the spectra. Instead we discuss the general spectral features from a chemical point of view and try to reach conclusions which are valid even in the absence of detailed calculations. Internal 5fq~5f q transitions. The spectra of corresponding chlorides and formates are quite similar, shifted towards higher wave numbers in the latter case. Since the metal ions occupy sites of different symmetry in both compounds, D2d in AnC14[6] and $4(C~) in An(HCOO)4[5], the similarity of the spectra suggests that it is the eight nearest neighbour ligand atoms, which predominantly determine the crystalline field around the actinide ion and that the tetragonal distortion of the cubic ligand framework has only a small effect on the 5[ q energy levels. Different splitting patterns and transition intensities arising from different site-symmetries and selection rules are probably only detectable in lowtemperature spectra.

Spectra of actinide compounds

2493

Table 2. 5f~-~5f ~ absorption bands (cm ') of NpCh (~,) and Np(HCOOh (~)(shoulders in parentheses)

4QOO

3000

J

J-

~--

I Peak No. i

2 3

iI

~I

~2

5831 6079

5988 6231 6423 6711

157

7345

275

6452

(

)

Field-free ton J levels

j v~ 10 1 h i

vlbr. vlbr.

306

7605

/

/

15

q

......

/ Np(HCOOL

I / i/ # i 35 i [ i L

259

•

,

/

-

vlbr. 4

NpCL

2500

~ I

!

/ #

;

/ / ~

I

I

/

--__

~14(..J

,

vlbr. 5 6 7 8 9

8i63 90~o 911C 9456 (9430)[ 9775 ~43 I0225

lC iI 12

i0638 1 11025 11007 (i iSOC Zl7lO (1196c) 121~0 12i14 1246/

273 35o 346 345 582

3/2 13t2

387 495 25o

9/2

vibr.

:9860) i

~3 14 15 ~6 17

18

12903 1318133~2 ( 136oc ) i z3'~8 ! lg24C ) 14624 I

i#

i c-9 13888 14144

05 x

/

5/2

I

vlbr, 3~7 656 ?o4 802

7/2 3/2 7/2 vibr. v!br. vlbr.

15152

/

528

5/2

383 ",07

7/2

20

30

43

vlbr. 19 20

vibr. 21 22 23 24 25

39o 634 750 442 468

26

556

9/2

i I/2 ~ibr, vlbr. vlbr.

68,~ 19940

(20880 )

33

[~>38oo) ;4o96 ;:4~3C

3E

i840

31

38

6>59 8363

39

' 50100]

7/2 v~br. 9/2 5/2 i I/2 3/2 5/2 5/2

Fig. 3. Electron transfer and 5 f " ~ 5 f ~ '6d ~ spectra of protactinium(IV), uranium(IV) and neptunium(IV) tetrachlorides and tetraformates. we tentatively assigned it to the ~PI term which usually[18,20] is localized at about 19,000cm '. Otherwise the order of excited states of the free ions is taken from literature [ 15-18, 20]. The energy levels of uranium(IV) are labelled by their usual L S J - s y m b o h it must be kept in mind, however, that most of these states are not pure RusselI-Satmders states[14]. The Russell-Saunders approximation is even more inadequate in the case of neptunium(IV) where Table 3. Electron transfer and _~f" -> .~/q '6d ' transmons(ca "" ')In " protactinium(IV), uranium(IV) and neptuniumIIV) tetrachlorides and tetraformates (shoulders in parentheses) Energy

(cm -I)

Assignment

The higher wave numbers of the formate spectra are 26320 due to an increase in both interelectronic repulsion and crystal-field splitting, since oxygen containing ligands like (32250) formate are known to produce smaller nephelauxetic 41240 effects and stronger ligand fields than chlorine[13]. In 4545O going from C1 to HCOO the total width of each group of bands corresponding to just one field-free ion J level UCI 4 : increases by some 30%. 26500 Simultaneously the "baricentre" (simple, nqt weighted average of the observed levels) of the group is moved (31ooo) towards higher energies (see Figs. 1 and 2). The resulting (37030) blue shift of individual transitions is much more pronounced at the short wave length end of each group, NpCI : where both effects combine, than at the long wave length 295OO end, where the increase in Coulomb interaction is partly 34480 cancelled by the larger crystalline field (see Tables 1 and 2 39600 and Ref. [14]). Therefore we believe that band No. 23 of ~545O the uranium spectra cannot belong to the LI6 manifold and

(cm -I)

Pa(H300)4

PaCI 4 : 23120

Energy

Assignment

:

5f ----~6d-b 1

23810

5f "----@6d-a i

(30300)

5f ~

6d-a~

35090

5f ~

6d-e -

40000

5f ---~6d-b[

5f ~

6d

q~

~

5f

~Y ~

5f

5f "---¢ 6d-b,

U(HCOO) 4 : 5f -----~6d

34780

5f ---* 6d IY-----~ 5f?

Np(HCOO) 4 : '~--~

5f?

95400 (3968O)

5f ~

6d

H. J. SCIJENKet al.

2494

larger spin-orbit interactions require the employment of intermediate-coupling wave functions [15, 16]. Furthermore we cannot safely argue that a set of absorption bands which happen to be grouped together, do in fact arise from the same split J level. Both configurations 5f2[18-20] and 5f3[15, 21] contain excited J levels which lie so close to one another that the crystal-field splitting of each level may overlap with its neighbours. Levels of lower energy (<13-14kK) are generally better isolated from each other. In the case of such "isolated" levels of comparable degeneracy the Stark splitting is observed to decrease in going from uranium (Sf 2) to neptunium (5.:3) (see Figs. 1 and 2). This decrease in crystalline field strength gives spectroscopic evidence for the actinide contraction. The resulting smaller spatial extension of the 5f 3 as compared to the 5f 2 configuration leads to a weaker interaction between the neptunium ion and its environment and to a reduced crystal-field splitting of the 5f 3 terms. The vibrational structure of some 5f3-~5f 3 electron transitions involves the wave numbers 243 and 189 cm (Np(HCOO)4) and 332, 257, 217, 195 and 98 cm -~ (NpC14). These values compare favourably with known infrared and Raman data[10, 22]. An unambiguous discussion of the transitions within the 5f' configuration of protactinium(IV) is not possible at this stage, since our spectra do not include the region beyond 18,000 ~,. 5ff ~ 5fq-~6d~and electron transfer spectra. In PaC14 the separation between the lowest energy ¢r(Cl-)-)5f(Pa 4+) and 5f 1-~5f°6d j transitions can be extrapolated to be larger than 15 kK[23]. Hence both band systems (see Fig. 3) are well separated even at room temperature, the first electron transfer band occurring at 41.24 kK. According to J~rgensen's concept of optical electronegativities this energy is closely related to the oxidizing strength of the metal ion and to the reducing strength of the chlorine ligands [24]: ~corr 30. [Xop~(Cl-)- xop,(Pa4+)] =

with Xopt(C1-)=3.0124]. The actual wave number of ~7=41.24kK must be corrected for the effects of spin-pairing energy and other forms of interelectronic repulsion [3]:

~ o r r = 4 1 . 2 4 - [ - 8 D-9E3-~5/l. Assuming D~3.25kK[3], E3~0.3kK[3] and ~'5/~ 1"5 kK[25], the corrected and uncorrected optical electronegativities of protactinium(IV) are calculated to be Xopt(Pa4+)corr= 1.42 and Xoo~(Pa4+)...... = 1.63, respectively. In view of the actinide contraction both values compare favourably with the optical electronegativities already known for the heavier actinides in the tetravalent oxidation state [24]. Since oxygen containing ligands like formate have Xo~>3.1124], the first electron transfer band of Pa(HCOO)4 is expected at energies higher than 45 kK, that is, outside the spectral region available with our apparatus. Consequently the spectrum of Pa(HCOO)4 between 20 kK and 45 kK must arise exclusively from 5f ~~5f°6d ~ transitions. The tetragonal distortion of the cubic ligand framework, which is of minor influence on the 5if-levels, strongly affects the more extended 6d orbitals and produces further splitting of the subshells eg

and t2g. The orbital degeneracy of the e~-level is completely lifted, eg(Oh)~a + b(S4), whereas t2~ separates into an orbital singlet and an orbital doublet, t2g(Oh)-~ b + e(S4). Therefore the one-electron transition 5 f ~ 5f°6d ~gives rise to four absorption bands which are seen in Fig. 3. The corresponding level energies may approximately be expressed in terms of the two dodecahedral parameters A6a -(r4)/R 5, which is a measure of the cubic field strength, and 6 - (r2)/R 3, which is related to the tetragonal distortion[26]. The experimental energies as derived from the band maxima (see Table 3) indicate the values of A6e = 15kK and 6 = 2 k K . In the case of PaCh only three transitions are seen. The parameter A6d can hardly exceed 32.25- 23.12 ~ 9 kK < 15kK in agreement with the known spectrochemical order of C1 and HCOO-. The well resolved splitting of the transition to the 6d - eg subshell suggests 6 = 6 kK > 2 kK. Obviously the reflectance spectra indicate that the tetragonal distortion of the cubic geometry is less pronounced in Pa(HCOO)4 than in PaC14; this conclusion is supported by crystallographic[5] and magnetic[27] measurements. The energy separation of the 5f' and 6d ~ baricentres may be approximated by AE = ~ + [0'515A6d -- 0'076 ] -- 2~'5: where ~ is the wave number of the lowest energy 5 f ~ 5f°6d ~ transition and 2~r5~represents the spin-orbit stabilization of the ground state 2F5/2 of 5f 1[28]. The lowest level 2B~ of 6d ~is stabilized by [0-515A6~-0.076] in the dodecahedral crystal field. Inserting otherwise known values the Land6 parameter ~'5:= l.SkK of PAC162-[2] and neglecting the unknown crystal-field splitting of 2F5:2,the energy difference is calculated to be b E = 24 kK in PaCh and AE = 28 kK in Pa(HCOO)4. Since a higher energy separation of f and d orbitals is caused by a higher degree of ionization[29], this increase of AE reflects the lower degree of covalency of the metal-oxygen bond as compared to the metal-chlorine bond. However, the decrease in covalency is partly cancelled by the increase in crystal-field strength, explaining the near invariance of the lowest energy wave number: 23.12 kK in PaCh and 23.81 kK in Pa(HCOO)4. In the case of uranium and neptunium the larger nephelauxetic effect of chlorine is no longer compensated by the smaller crystal-field splitting. Owing to the actinide contraction the energy of the 5f orbitals decreases as a function of increasing atomic number leading to a blue shift of the 5ff ~ 5ff-~6d ~spectra and to a red shift of the electron transfer spectra. In PaCh, UCI4 and NpCI4 the 5ff~5fq-~6d ~ transitions shift regularly to higher wave numbers, the first band occurring at 23.12, 26.50 and 29.50kK, respectively. This blue shift is much more pronounced in the predominantly ionic formates: Pa(HCOO)4 has the first 5 f ~ 6 d ' band at 23.81kK, whereas the first 5f2~5f~6d ~ band of U(HCOO)4 is centred at 34.78 kK. From these values the lowest energy 5f~5f26d ~ transition of Np(HCOO)4 is extrapolated to occur at about 46 kK, that is, outside the measured region. Therefore the absorption band of Np(HCOO)4 at ~ = 35.40kK should correspond to an electron-transfer transition. From [3, 24] 35"40 = 30(Xopt- 1"75) - ~ D

Spectra of actinide compounds the optical electronegativity of the formate ion is calculated to be Xop, = 3.13 which is in the range predicted for oxygen containing ligands[24]. The electron transfer bands of NpCI4, U C h and U ( H C O O h , which are expected at 31, 36 and 40 kK, respectively, cannot unambiguously be distinguished from the more intense 5f ~ 6d transitions in these spectral regions.

4cknou,ledgement--The mental ',t,~sistance.

authors thank Mr. W. Wurtz for experi-

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2495

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