The electronic spectrum of cesium hexafluoromanganese(IV)

JOURNAL

OF MOLECULAR

SPECTROSCOPY

39, 8-20 (1971)

The Electronic Spectrum of Cesium Hexafluoromanganese(IV) MARTIN J. REISFELD, NICHOLAS A. University

of California,

MATWIYOFF, AND LARNED B.

Los Alamos Scienti$c

Laboratory,

Los Alamos,

N.

ASPREY M. 87644

A number of high purity samples of CszMnFs have been prepared and their electronic spectra studied, both in absorption and emission. The observed peaks were assigned to the crystalfield energy levels for an octahedral da configuration. A large number of vibronic components have been observed and correlated with data on the ground state vibrational frequencies. Evidence of spin-orbit splitting of several of the bands was adduced. Recent work on the MnF:- complex was examined and some of the discrepancies discussed. The crystal-field parameters have been derived, and a value for the optical electronegativity of Mn(IV) calculated. The results are: Dp = 2180 cm-l, B = 650 cm-l, C = 3972 cm-l, and xopt = 2.90. The nephelauxetic ratios are ~56= ~35= 0.61. I. INTRODUCTION In the course of a series of investigations on the preparation and properties of transition state complexes in which the central metal appears in an abnormally high or low oxidation state, we turned to a study of the fluoride complexes of tetravalent manganese. Some of the chemical properties of salts of the type AZMnFs have long been known (I), but only recently have structural parameters (2, S) and spectroscopic data become available. Since the first work on t’he electronic spectra of these salts (4), two papers have appeared (6, 6) in which several discrepancies are noted, and it is the purpose of this work to reexamine the absorption spectra as well as to present additional work which will hopefully clarify the situation. II. EXPERIMENTAL

Details of the preparations of the bright yellow salt C&MnFa have been previously given (7). Samples of the powder were intimately ground with Fluorolube S-30 (8) oil in an inert atmosphere, and the mull placed between CaFz discs utilizing tantalum spacers to obtain the desired thickness of preparation. These discs were then assembled to the cold junction of a Cary liquid helium dewar, these operations also being carried out in inert atmosphere. Electronic absorption spectra were taken on an Applied Physics Cary 14-MR spectrophotometer in the region from 2000 A (50 000 cm-‘) to 25 000 A (4000 cm-‘). 8

ELECTRONIC

SPECTRUM

9

OF CszMnFc

Emission data were also obtained on the above instrument utilizing an Osram XBO-450 Xe arc as a light :ource and a Farrand grating monochromator to supply narrow band (~100 A) excitation over a range of exciting frequencies. In all cases, spectra were recorded several times and refluorination of the product was carried out at least twice to insure complete conversion to the desired product. III.

RESULTS

The complete absorption spectrum obtained for CszMnFs at liquid nitrogen temperature with a cell having an 0.008’ Ta spacer is shown in Fig. 1. The following features should be noted (or are worthy of emphasis) : (a) A system of very low intensit.y peaks lying at approximately 6000 Li (16.6 kK); (b) Two peaks of high intensity, centered at 4600 A (21.8 kK) and 3550 8 (28.1 kK), respectively. The former peak shows a great deal of fine structure and the second band shows evidence of broader splitting; (c) An extremely high intensity peak is observed at about 2550 A (39.0 kK) with no trace of fine structure; (d) A weak band located at 3900 w (25.7 kK) present as a shoulder on the st#rong 3550 i peak. WAVENUMBER

40.0

35.0

30.0

25.0

I IO’ cm-‘)

20.0

15.0 I.6 1.5

3.5

14

3.4

13

33

1.2

3.2

1.1

; 3.1 9 3.0

1.0;

x

0.9:

2.9 ;a28 2.7

0.88

$ 2.6

0.6:

0.72 0.5:

2.5

0.4

2.4

0.3

2.3

0.2

2.2

0.1 WAVELENGTH

FIG. 1. Absorption

ness = 0.008”)

spectrum

Ii,

-

of CszMnFg mull at liquid nitrogen

temperature.

(Thick-

REISFELD,

10

MATWIYOFF,

AND

ASPREY

A “blow-up” of the 16.6 kK region at 77°K is shown in Fig. 2 (sample thickness ~0.100~ ). A tabulation of all the observed peaks in the absorption spectrum of solid CszMnFa is given in Table I. The absorption peaks listed in Table I are accurate to within 3 8 over the region 15 000-25 000 cm-‘. This corresponds to an error of less than &20 cm-‘. For the region above 25 000 cm-‘, t*he error is approximately f-100 cm-‘, due to the difficulty in ascertaining the centers of the broader bands. The emission spectrum of CszMnFs under excitation at 4500 A is shown in Fig. 3 and the observed peaks are also listed Table I. Cesium hexafluoromanganate (IV) adopts the KzPtC16 structure having the space group Oh5(Fm3m) with four molecules per unit cell and a crystal lattice parameter of 8.92 b (3). In this crystal structure are present regular octahedra of MnF%-. The X-ray powder patterns of our material are in agreement wit#hthese findings. Thus the electronic spectrum of CseMnFB is to be understood within the framework of 3 cE3ion in an octahedral environment of fluoride ions. The crystal field energy diagram for vuch an ion has been constructed by several workers (9-11) and the complete energy matrices have been tabulated by Eisenstein (10). For a d3 ion in a cubic field, the expected level distribution is: 4A2(I’S) (ground state),

%(P~), ?‘I@‘~, I?,>, 4T’t(r~,~,8),2Tz(~~,~), 4T~(~~,~,8,~) with the next

highest spin-allowed transition A 17.4 , I

17.2 ,

I

being to the 4Tl(~ 6,,,8,8)level arising from the

WAVENUMBER 1

170 /

I

16.8 ,

I

16.6 /

I

( IO3 16.4 ,

cm-‘) 16.2 I ,

1

16.0 l

1

15.8 I

’

I56 I

t-

E

:

cn z

E

z! ii 0

E

E

i u

~~“r”“~‘i”~~‘~l”lll’l”” 57

5.8

5.9

6.0 WAVELENGTH

6.1

6.2

6.3

6.4

(IO3 i J

FIG. 2. Absorption spectrum of Cs*MnF, mull at liquid nitrogen temperature. ness = 0.100”) The *AZ, + 2Eg, 2T1, t.ransitions. The lines marked E denote location observed in emission.

(Thickof peaks

ELECTRONIC

SPECTRUM TABLE

OBSERVEDABSORPTIONPEAKS AND Frequency (cm-‘) 15 15 15 15 15 15 16 16

4138 683 716a 782 795” 848 012 021”

16 071 16 234 16 260” 16 16 16 16 16 16 17 17

Assignment

2E,

353 501 625 734 828 949 036 145

17 300s 17 417 a Observed

Frequency (cm-l) 20 20 20 20 20 21 21 21 21 21 22 22 22 22 22 23 23 23 23 24

370 471 768 820 978 317 468 801 843 920 297 346 758 831 894 245 348 770 872 284

OF CszMnFe

11

I

ASSIGNMENTS FOR Cs,MnFe AT 75°K Assignment

Frequency (cm-l) 25 707

Assignment 2Tz,

27 716 28 100 28 530 28 986 Vz,

39 016

‘Tl,

T--+ tlg

2T1, in emission.

free ion 4P state. In addition, we expect to observe, at fairly high energies, the allowed “charge-transfer” bands of the ?r + IZs t’ype. We may thus expect a spectrum showing two very weak spin-forbidden bands at low-energies (4A2+ %, 4Az + 2T1); two medium intensity bands at higher energies (corresponding to the Laporte forbidden; spin-allowed 4A2 -+ 4T2 , 4A2-+ 4T~) ; a very weak spin-forbidden band (4A2 -+ ‘T2) in the region of the high intensit,y peaks; another mediumint,ensit(y band (“A2 -+ 4T~) at higher energies; and finally a broad charge t,ransfer band of very high intensity, perhaps masking the upper transitions. As the spectrum in Fig. 1 shows, we do have the expected distribution of peaks and will discuss then separately below. The very high intensity peak at 39.0 kK will be assigned t,o the charge-transfer band; the bwo peaks at 21.8 kK and 28.1 kK are assigned to the 4A2 + 4T2 and 4A2 4 4T1 transitions respect,ively and the numerous small peaks centered at 16.6 kK are assigned to the 4A~ + % and 4A2 -+ ‘TI levels. A. ELECTRONIC LEVELS

We turn our attention first to the location of the lowest-lying spin-forbidden bands arising from transitions to the % and ‘TI states. As may be noted from Fig. 2, this region of the spectrum is characterized by a large number of rela-

12

REISFELD,

MATWIYOFF,

AND ASPREY

tively weak peaks separated by spacings of approximately 100 cm-‘. It is impossible to locate the M) electronic transitions in the absorption spect’rum due to the Laporte forbiddeness of the transit’ion. Although the same is true for the mull emission spectrum shown in Fig. 3, the smaller number of peaks observed aids in the interpretation. This spectrum, which corresponds to excitation at 4500 fi is dominated by two main features. The first is a relatively broad weak band centered at 5780 B (17 300 cm-‘) while the second shows a sharp strong series of peaks with the highest frequency component lying at 6150 A (16 260 cm-‘). It was observed that as long as the exciting frequency lay above this region all sets of emission spectra gave the same results. We take the locat,ion of the higher lying 2T1 level as corresponding to the 17 300 cm-’ band, while the % ---f 4A2 emissions are considered to give rise t.o the sharp series of lines. Recent, work by Pfeil (12) on the emission of Mn4+ in hexagonal I<&InFG and Cs2MnFe places

WAVELENGTH

56 58 I/ I'

I

180 -

I

I

(IO3

H,

-

60 6.2 6.4 66 68 ! 'I 1 I' I 1 I'

I

17.0 WAVENUMBER

I

I

160

I

I

15.0

( IO3 cm-‘)

FIQ. 3. Emission spectrum of CszMnFg mull at liquid nitrogen temperature. Excitation at 4500 PI.

ELECTRONIC

SPECTRUM OF Cs2MnF6

13

the origin of this band at 16 024 cm-’ in the cubic Cs salt. Our works shows a peak at 16 021 cm-’ which we then take as the location of this transition, with the higher-lying peak at 16 290 being an Anti-Stokes band. In this context we may remark that for Oh symmetry the @O band is rigorously forbidden for an electric dipole transition. A magnetic dipole moment mechanism could enable this emission, but such transitions have only very rarely been observed. At t.his juncture, the mechanism for t.he appearance of this band is not clear. These assignments are in conflict with those of Kemeny and Haake who obtained the emission spectra of Mn4+ in a magnesium fluorogermanate matrix (IS), where the peaks were fitted to transitions from t’he *T2, -+ 4Az, levels; the upper state being split by a Jahn-Teller mechanism. Kemeny and Haake concluded that the energies observed at the emission maxima could only be fitted into the crystal-field diagram if transitions into doublet states were excluded. We find t,hat such is not the case, and that excellent agreement between calculated levels and observed data is obtained if the emission peaks are correlated with transitions of the 2E,, ‘T2, --+ 4A2, type. Once these states have been specified, we ma3 evaluate the crystal-field parameters, in terms of which the energy matrices are specified. To first, order (g-11 ) :

EfE, EfT1,

- 4A2,] = 9B + 3C - 90B2/10 Dq = 16 020 cm-‘, -

“E,] = 66B2/10 Dq = 17 300 -

16 020 = 1280 cm-‘,

(1) (2)

where B and C are the interelectronic repulsion Racah parameters and 10 Dq is the cryst.al-field parameter corresponding to the energy separation between the strong field electronic ground state (bg3) and the first excited state configuration (tzg2eg). From the two assignments we may ascertain B and C in terms of Dq. The first spin-allowed transition corresponds to excitation from t,he 4A2, ground state to the 4T2, level, with energy EfT2,

-

4A2g] = 10

Dq.

(3)

As noted previously the center of the first band which is composed of numerous sharp peaks lies at 21801 cm-‘. The progression spacing is about 500 cm-’ probably corresponding to the totally symmetric Al, vibration (which appears in this ground state at 608 cm-‘). With this shift in the equilibrium nuclear distance between the metal ion and the ligand relative to that in the ground state, we take the center of the band as that energy for which Dq is to be calculated and arrive at Dq = 2180 cm-l. With this parameter evaluated, we obt’ain B = 650 cm-’ and C = 3972 cm-‘(C/B = 6.11). The nephelauxetic ratio 0 = B/B0 is then 0.61. Utilizing the parameters, we may calculate the positions of all the remaing peaks. For Dhe second spin-allowed transition (4i42g+ 4T1,) we compute

E[4Tl, - 4A2,] = 15 Dq + 7.5 B - 0.5 [lo0 Dq2 - 180 Dq B + 225 B2]l” = 28 700 cm-‘.

(4)

14

REISFELD,

MATWIYOFF,

AND

ASPREY

From Fig. 1 and Table I we find the appearance of a second whose center lies at 28 090 cm-’ for the location of the 4T1, level. For the next highest allowed peak there are two possibilities:

strong

band

1) Transition to the other 4T1, level arising from the free ion “P stat,e, resulting in a predicted energy of 44 453 cm-l, and 2) A charge-transfer corresponding roughly to electron-transfer from ligand P type antibonding orbitals to a tzs nonbonding d orbital. The distinguishing features of this type of transition are a much higher transition probability than for d-d t’ransitions (and a concomitantly greater intensity) and a generally greater band half-width. J&gensen has related the position of the electrontransfer bands to the optical electronegativity for a given metal-ligand combination (14-16). This concept rests on the assumption that t’he electron transfer bands in MX,’ complexes have wavenumbers (corrected for spin-pairing energy and configuration interaction) proportional to the difference between t’he values of xopt ofM and X. A linear relation is found to exist between the opt,ical electronegativities of a metal ion in a given oxidation state (belonging to the same transition series) and the number of electrons in the partially filled shell. If a plot is made of xvs. the number of electrons, t’hen it is found that the lines for 3d, 4d, and 5d complexes have the same slope. If we take the value of xopt = 3.50 for the 3d6 ion Ni4+ (17), then we predict a value for Mn4+ of xopt = 2.90. Using this value for x (metal) with the tabulated value of XF = 3.90 (15) we obtain i&Ohs (cm-l)

E 30 000

(Xligand

-

X&al)

+

lo&

-

+

[
7(5$B

+

1))

+ C)/6

-

X(S

+

111

= 39 413 cm-‘.

As may be noted from the spectrum, the highest energy peak lies at 39 020 cm-’ in agreement with the predicted energy, assuming a charge-transfer transition. In addition, our data indicates an intensity ratio bet’ween this peak and bhe lower lying spin-allowed transitions of ~100, which lends furt,her support to the assignment of t,his peak as arising from electron transfer. In addit#ion to t’he bands assigned above, there is also present a shouldez on the 4A2, -+ 4T1, transition. This very weak peak is located at 3890 f 25 A or absorption does not appear in approximately 25 700 cm-‘. (This low intensity Fig. 1.) From the energy level diagram (Fig. 4) it may be seen that the ‘T,, level should occur in approximately this region. Utilizing crystal-field parameters as previously determined, we have calculated the expected position for the 4A2, -+ ‘T2, transition at 25 661 cm-l, in excellent agreement, with the observed locat’ion. The final assignments of the electronic transitions, along with bhe derived parameters are given in Table II. The observed peaks are shown schematically in Fig. 1 along with the energy levels as calculated from the parameters. The

ELECTRONIC

SPECTRUM

15

OF CszMnFe

Obs.

Calc.

Dq = 2180 B = 650

cm-’

C

cm-’

= 3972

cm-’

4’

-4T,p

-

-2T2p

-

-4T~o

-

-.-2T,o

*Eg -

FIG. 4. Comparison of observed . The overall absorption

in CstMnFs

levels have been calculated and Sugano (9).

and calculated energy levels for octahedral spectrum is shown at the right of the figure.

using the full energy-level

matrices

MnFi-

as given by Tanabe

B. FINE STRUCTURE i. The 4Ao, ---f 4T2, Transitim The most prominent feature of this band (centered at 21 801cm-‘) is the large number of strong, sharp peaks, each of which is further split having high and low frequency component,s in addition to the sharp prominence. We find that these features may be explained if we assume that there are present three distinct progressions of the totally symmetric vibration of the iMnF:- complex. In Table III we have tabulated the observed frequencies along with our assignments for the three progressions. The average value of ~1 found for all three progressions is maintained in the excited is 500 f 15 cm-l, If the Oh point group symmetry state, then the actual band origin must lie at some lower lying frequency than

16

REISFELD,

MATWIYOFF, TABLE

AND

ASPREY

II

ENERGY LEVEL ASSIGNMENT AND CRYSTAL FIELD PARAMETERS FOR CszMnF Level

a-@,

16 021S (VW) 17 300" (VW) 21 801 (s)

ZT1, 4T2, BT2, ‘T‘, p +

0

0

4A2,

25 707 28 090 39 016

tep

16 895 17 575 21 800

(sh) (9) (vs)

25 661 28 698 39 413b

Dq = 2180 cm-l B = 650 cm-l C = 3972 cm+ B From emission data. VW = very weak; s = strong; b Assuming x = 2.90

TABLE

OBSERVED PEAKS

AND ASSIGNMENTS

Frequency (cm-l) 20 370 20 471

III

FOR THE “Az~ + 4Tz, TRANSITION Assignment

E E” + VI +ul E’ + 2v,

21 317 21 468 21 843

E

E’ E

$2~1

E” + 2~1 E’ + 3v, E

+3r,

E” + 3v, E’ + 4~1 E

f4Vl

22 346 22 396 22 758

E” + 4v, E’ + 5~1

22 831 22 894 23 245

En + 5v, E’ + 6~1

23 348 23 419 23 770

-

vs = very strong

20 768 20 820 20 978

21 801 21 920 22 297

(E (E” -

sh = shoulder;

23 872 24 284 E’) = 66 f

23 cm-1 31 cm-1 (~1) = 500 f 15 cm-l

E) = 93 f

h’

+5~1

E

+6r1

E” + 6~1 E’ + 7~1 E

f7Vl

E” + 8v,

ELECTRONIC

SPECTRUM

OF CszMnFe

17

any observed unless a magnetic dipole mechanism is enabling the transition. If we assume that such is not the case, then the 0’ +- 0 transition lies at somewhat lower frequency than the first observed absorption at 20 370 cm-‘. This frequency (20 370 cm-‘) would then probably correspond to v3(T1,) + 0’ t 0 or v6(Tzu) + 0’ 6 0 and the gross band structure corresponds to transitions of the type vi + nvl + 0’ +-- 0, where vi is either an odd mode T1, or T,, vibration in the excited &ate and VI’ is the A, totally symmet.ric vibration in the excit.ed state. The 4T2, level is twelve-fold degenerate, composed of: one rs state (two-fold degenerate), one r~ state (two-fold degenerate) and two rs states (four-fold degenerate). Under spin-orbit coupling the spin degeneracy is lifted and the three I’; levels result (rs, rs and I’,,*) separated by 5X/4 and 2X/4. Thus the entire level width is 2X. The three progressions found in the 4A~, -+ *TzOtransition are separated by 159 cm-‘, giving a value of X = 80 cm-’ for the complex MnFi-. This corresponds to a spin-orbit coupling constant of f = 240 cm-’ and a resulting value of {/lo = 0.5S, where To is the free-ion value. It is to be expected t,hat { will be reduced from its free-ion value, since t’he spin-orbit constant is a function of the square of the effective nuclear charge and is therefore quite sensitive to the ligand field. This value of r/S_0 of 0.58 is in reasonable agreement, with value of B/B0 = 0.61 found previously. The origins of the reduction of crystal-field parameters and spin-orbit constants from their free-ion values have been ascribed Do delocalization effects arising from orbital formation with the ligands and to effective charge variation in the partly filled shell (16). If we then assume that it is t.he spin-orbit interaction which is responsible for the fine splittings, then the t,hree levels would be predicted to be split by 60 cm-’ and 80 cm-‘, respectively. The average observed spacings are 66 cm-’ and 93 cm-’ with a deviation of approximately 25 cm-‘. We may then understand the structure of the 4T2, band as arising from a spin-orbit splitbing of the level combined with a uniquantal promode. The levels E, E” and E” in gression of an excited st’ate A lg vibrational Table III t.hus correspond to the r?, I’S and I’,,* spin st.ates of the *T2, level. 2. The ‘A,, -+ ‘T1, Transition which is centered at 28 090 cm-’ shows clear eviThe 4Az, + 4T,, transition dence of four distinct peaks located at 28 986, 25 530, 28 100 and 27 716 cm-‘. The accuracy with which these peaks are determined is estimated to be &50 cm-‘. The spacings between the peaks in question are 456,440 and 374 cm-‘. If we consider only spin-orbit coupling to be responsible for the splittings, then to first order, quartet T1 st,ates are split into onIy three levels, corresponding to (4T~)1j2, (4Tl)312and (4T1)6,2with spacings of 5X/4 and 3X/4. For Mn4+ the spin-orbit coupling constant { = 415 cm-’ (18), leading to a value of X = l/3 = 135 cm-’ for t,he 4T level of the free ion. Therefore, we would expect’ only bhree peaks in this absorption region, separated at most by 173 cm-’ and 194 cm-‘. Clearly, spin-

18

REISFELD,

MATWIYOFF,

AND

ASPREY

orbit coupling alone cannot be responsible for the observed results. There are, however, two other mechanisms which we might consider. The first is a coupling of excited stat,e vibrational wavefunctions of the MnF26- complex wit’h the electronic wave function for the 4T, (&e,) state. In general, this vibronic interaction leads typically (for Oh symmetry) to the appearance of several peaks corresponding to uniquantal progressions of a totally symmetric vibration (Al,). In R’lnFi-, the ground state Raman-active Al, vibration occurs at 608 cm-’ and the observed average spacing of 423 cm-’ in the absorption spectrum may correspond to this mode in an excited state where the bond energies are somewhat weaker (the transition corresponds to excitation of the electron from a weakly antibonding A* level to a strongly antibonding Q* level). For the lower-lying 4T2g st,ate as mentioned in B1, this AI, frequency has a value of 500 cm-‘. Secondly, the JahnTeller (19) effect requires that if a nonlinear molecule possesses an orbitally degenerate state then a distortion will take place to remove the degeneracy, although under cert’ain circumstances the spin-orbit coupling may stabilize the symmetric structure. For an octahedral complex with a T1 or T2 electronic state, either tetragonal or trigonal distortions may occur, giving rise to several minima in the potential energy curve. For example, Van Vleck (90) estimated a JahnTeller fourfold splitting for the 4T state of Cr3+ (also a d3 case) of approximately 330 cm-’ for a trigonal dist’ortion. Such static distortions, coupled with a spinorbit mechanism might lead to the splitting pattern observed here, but, since no unique mechanism for the observation can be formulated we will not pursue these points further. S. The 4A2g+ 2E, and 4Az, -+ 2T,, Transition The analysis of the fine structure of the spin-forbidden bands is even more complex than that for the allowed transitions. An expanded picture of the region in question is shown in Fig. 2. This region of the spectrum contains 16 clearly discernible absorption peaks over a spectral regime of only 300 cm-‘. With the evidence of the emission spectra placing the %g and 2T, levels at -16.0 and 17.3 kIi, it is clear that the remaining large number of peaks can arise only from vibrational interactions. Under a spin-orbit interaction the ‘Eg (rs) level remains unsplit; while for a { of 240 cm-’ as found above, the 2T1, splits into a Is and I’S level separated by approximately 4 cm-’ as determined by a full spin-orbit crystalfield calculation. As proposed for the 4A2, -+ 4T1, transition above, bot#h JahnTeller and vibronic interactions can also lead to further splitting. However, we have been unable to find a consistent splitting pattern showing logical progressions of any vibrational frequency in our examination of this region of the spectrum. It is apparent that oriented single-crystal studies involving both Stark and Zeeman experiments will be desirable to resolve these difficulties. Even such a thoroughly studied d” system as Cr3’ shows discrepancies between observed and calculated levels, particularly with regard to the 4T,, (“I’) state, and the nature of

ELECTRONIC

SPECTRUM OF CsnMnFG

19

the splittings of the %, state in KCr (SOb)Z. 12H,O have not yet, been completely explained (21). IV. DISCUSSION

The results obtained here show that the main feature of t’he electronic spectrum of the hexafluoromanganese (IV) ion as found in the salt’ CszMnF, may be adequately described on t’he basis of elementary crystal-field theory. The fine structure of the spectra are rich in vibronic components, and the emission spectra have proved helpful in ascertaining the locat’ion of the electronic levels. As previously mentioned, the early work on the absorption spectrum of the MnFi- ion by Jorgensen (4) indicated t’he presence of the 4T1,, 4Tt,, and the spin-forbidden levels but the latter were not resolved, nor was the fine struct,ure observed on the main peaks, The electron-transfer band was not observed, and the spectra was fitted with crystal-field parameters Dq = 2180 cm-‘, B = 600 ratios ,856= 0.54 and p36 = 0.56 (4, 2%‘).A value of cm-l, and the nephelauxetic C/B = 1.0 was assumed. We have found that our data leads to results in excellent agreement for the primary parameters, namely Dq = 2180 cm-’ and B = 650 cm-’ but we find a value of C/B = 6.1 and that &6 = PSS= 0.61. We have found as well that if we assume that this value of /3 = 0.61 is applied to the spin-orbit coupling constant, then we arrive at { (MnFi-) = 254 cm-’ in comparison to our deduced value of 239 cm-‘. Therefore, the reduction in all the appropriate parameters map be adequat,ely described by a single value of the nephelauxetic rat,io. Following this earlier work, a paper appeared by Allen, El-Sharkarwy, and Warren (5) who obtained the diffuse reflectance spectrum of KzMnFG . They found the prominent, spin-allowed t,ransitions at 22.2 kK and 28.6 kK as before, but also found two weak bands at 14.0 kK and 19.3 kK. These were assigned as respectively. These results are the 4A?g-+ 'E, , ‘T1, and 4Az, -+ 2T2, transitions, not, in complete agreement, with our data which indicate 4A2, + ‘Eg = 16.02 kK, 4A2, --$ 2T1, = 17.30 kK, and 4A2, -+ 2T2, = 25.71 kK. The material used by these workers was obt,ained by reaction of aqueous HF with potassium manganite (K2MnOa). The product, after recryst,allization with 40 7c HF, was washed with acetic acid and acetone. This was the same preparative method as used by Jergensen. Recently, further work on this system was carried out by Novot,nJ and Sturgeon (S ) who prepared several hexafluoromanganates by direct fluorination of MnC12.4H?0 and RF and by fluorinat#ion of trivalent pentafluoride of the type A&InFb.H,O. The latter workers found spect,ra corresponding to that of both .J$rgensen and Allen, et al. depending on the method of preparation. They concluded that the cubic phase was responsible for t,he band at 19.3 kK while the weak and narrow lines at 16.6 kK arose from transitions enhanced by a hexagonal symmet#ry in K2MnFB. They also concluded that the 38-39 kK peak arose from the 4AY,-+ 4T1, (“P) transit’ion. As may be seen from Fig. li, however, we find that’ this latter peak is apparently a charge t’ransfer band and that the 4T1, (“I’) level

20

REISFELD,

MATWIYOFF,

AND

ASPREY

lies some 6000 cm-’ higher. The spectra of Novotny and Sturgeon show the 38 kK peak as having about the same intensity as does the 4T1, (“F) band, but it is not clear from their data whether the sharply increasing background in this region has been properly subtracted. Further, in the present work we find no evidence of the aforementioned 19.3 kK band despite the fact that Cs&iInFs is characterized by only one phase, a cubic structure. Therefore, we conclude that the band found by Allen, et al., and Novotny and Sturgeon, at 19.3 kli in &MnFs , and assigned to the ‘Tz, level arises from incomplete fluorination of the NIn2+ or Mn3+ ion (or reduction by solvents) and is not due to structural changes. In conclusion, therefore, we find that both the emission and absorption spectra of CszMnFG are explained on the basis of the crystal-field theory utilizing only one nephelauxetic ratio and that all the predicted transitions for a d3ion in octahedral symmetry are observed. The fme structure of the spectra may be understood by consideration of vibronic coupling, spin-orbit interaction and Jahn-Teller effects. RECEIVED:

April 4, 1970 REFERENCES

1. 2. 8. 4. 6.

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