Magnetic susceptibility and high field Mössbauer effect studies of some spin triplet iron(IV) compoundsW. T. Oosterhuis, Bruce Dockum, and William Michael Reiff Citation: The Journal of Chemical Physics 67, 3537 (1977); doi: 10.1063/1.435352 View online: http://dx.doi.org/10.1063/1.435352 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/67/8?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Mössbauer, magnetic susceptibility, and EPR studies of intermediate spin iron (III) dithiooxalato halides J. Chem. Phys. 69, 4411 (1978); 10.1063/1.436431 Mössbauer Study of Some 2–17 LanthanideIron Compounds J. Appl. Phys. 41, 910 (1970); 10.1063/1.1659014 MössbauerEffect Study of Erbium Spin Relaxation in Magnetically Ordered Compounds J. Appl. Phys. 39, 839 (1968); 10.1063/1.2163637 Magnetic Susceptibilities of Some Uranium (IV) Compounds J. Chem. Phys. 23, 1650 (1955); 10.1063/1.1742404 The Magnetic Susceptibilities of Some Uranium (IV) Compounds J. Chem. Phys. 16, 920 (1948); 10.1063/1.1747031

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Magnetic susceptibility and high field Mossbauer effect studies of some spin triplet iron (IV) compounds

W. T. Oosterhuis*t

Department of Physics. Carnegie-Mellon University, Pittsburgh, Pennsylvania 15213

Bruce Dockum and William Michael Reiff*

Department of Chemistry, Northeastern University, Boston, Massachusetts 02115 (Received 8 November 1976)

Miissbauer spectra of some spin triplet iron (IV) compounds with dithiocarbamate ligands have been observed in large magnetic fields at low temperatures. The magnetic and electric hyperfine interactions agree with the model put forth by Oosterhuis and Lang to explain the action of low symmetry crystal fields on a t i

g electronic configuration. A trigonal distortion together with the spin-orbit interaction in a

perturbation theory treatment is used as a model and gives the spin Hamiltonian parameters D. A. g. (efg). which simulate the data fairly well. The electric field gradient does vary with temperature while the paramagnetic interactions are essentially temperature independent in contrast to the behavior previously observed in tetragonal Fe(lV) arsine complexes. The temperature (1.6-300 K) and field dependence of the susceptibility of one of these salts Fe(diethyldithiocarbamate)iBF4) has also been studied using the Faraday method giving a value of D = 23.8 cm - I from least square fitting consistent with D values required for reasonable fits of the temperature dependence of high field Miissbauer spectra.

INTRODUCTION

Several Mossbauer effect experiments on 57 Fe in the unusual t~ electronic configuration have appeared in the literature recently. The magnetic hyperfine interac­tion (induced by an applied magnetic field) and the elec­tric quadrupole interaction can give information which characterizes the electronic ground state. The pre­vious examples of Fe(IV) such as [Fe(diars)2CI2] (BF 4)2,1 where diars is o-phenylene-bis-dimethylarsine, com­plex ES of cythochrome c peroxidase, 2 horseradish peroxidase,3 and Japanese radish peroxidase, 4 have tetragonal symmetry. The t~K configuration reflects the symmetry of the ligand field through the interactions with the magnetic field and the nuclear spin. To a first approximation the dithiocarbamate (DTC) iron (IV) com­plexes 5 Fe(DTC)3BF 4 with three bidentate ligands are ex­pected to have trigonal symmetry, and although this is an axial distortion as in the tetragonal case, the mag­netic hyperfine coupling tensor is quite different due to a change in sign of the dipole-dipole contribution6 as well as a change in sign of the electric field gradient. In this paper, several Fe(IV) complexes are studied in the presence of large applied magnetic fields to investi­gate the electronic energy level structure. Because of the very small paramagnetic hyperfine interactions seen in the Mossbauer spectra, magnetization measurements should be more sensitive to the temperature dependence of the magnetic moment. Although recent crystal struc­ture stUdies indicate local symmetry less than trigonal (D3 ) , the electronic structure of these compounds can be interpreted in terms of a trigonal model.

EXPERIMENTAL DETAILS

The Fe(IV) (DTC)3BF4 samples used as Mossbauer absorbers were a gift from Professor Darel Straub, who made an earlier investigation. 5 DTC is an ab­breviation for anyone of the N-N dis substituted dithio­carbamate ligands which are (1) PYR (pyrrolidyldithio­carbamate), (2) CHX (N, N-dicyclohexyldithiocarba-

mate), (3) IPR (N, N-di-isopropyldithiocarbamate), (4) ET (N, N-diethyldithiocarbamate), and (5) ME (N, N-di­methyldithiocarbamate). The samples were used as absorbers in the form of a finely divided powder which was mixed with wheat flour to disperse the sample even­ly over the area of the sample holder. The experiments were done in fields up to 60 kG with the field either parallel or perpendicular to the beam of y rays. A variable temperature device 7 with a heater and tem­perature sensor in a feedback system was used to ob­tain data in fields at temperatures up to 100 K. The temperature control was nominally ± 0.1 K. Some high field Mossbauer measurements (60 kG at 4.2 K) were al­so performed for Fe(diethyldithiocarbamate)3BF4 using the facilities of the Francis Bitter National Magnet Laboratory, Cambridge, MA. These experiments in­dicate that for this system, as well as for the other Fe(IV) compounds studied herein, V zz is negative and 71 is small.

A constant acceleration Mossbauer spectrometer was used with a source of 57CO diffused in a Pd foil main­tained at room temperature. An iron foil was used to calibrate the spectrometer and zero velocity is taken as the centroid of the calibration spectrum. The spectra are corrected for solid angle effects by fitting a para­bolic background to the data.

Variable temperature magnetic susceptibility mea­surements were made at Northeastern University on a Faraday balance composed of a Cahn RG electrobalance, a Varian model 4000 electromagnetic with a four inch constant force pole caps, and a Janis Super Vari-Temp. cryostat over the range 1. 5 to 300 K for ten fields be­tween 1. 6 and 5 kG. Temperature measurement and control was typically of the order ± O. 01 K or better and was achieved using a Leeds-Northrup K-4 potentiome­ter and a Lake Shore Cryotronics model DTC-500 set point controller, respectively, in conjunction with a calibrated silicon temperature sensor diode, a 10 J.J.A

The Journal of Chemical Physics, Vol. 67, No. B, 15 October 1977 Copyright © 1977 American Institute of Physics 3537

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3538 Oosterhuis, Dockum, and Reiff: Spin triplet iron (IV) compounds

TABLE I. Spin Hamiltonian parameters in trigonal Fe(IV) complexes from Mossbauer data.

Cvffiplex T(K) /ill(mm/s) A,,(mm/s) A,(mm/s) D(cm-') 6(mm/s)

Fe(CHX)3BF, 300 -2.00' SNP 0.48 195 - 2.1:3

77 -2.28 4.2 -2.40 +2.65±0.05 -0.05 20 "2

Fe(PYR)3BF, 300 -2.30' 0.48 195 -2.39

77 -2.52 4.2 -2.54+ + 2.80 ,0.13 25 ±2 0.50

Fe(ME)3BF, 300 -2.06' 0.45 195 -2.21

77 -2.45 4.2 -2.45' + 2. 50 +0.03 25 ±2 0.55

Fe(ET)3BF, 300 -2.23' 0.48 195 -2.40

77 -2.50 4.2 -2.51

FeUPR)3BF, 300 - 2. 27' 0.45 77 -2.50 4.2 -2.50

NotQ: gil ~ 1. 98, g~ ~ 2.21 in each case. The estimated devia­tions are indicated. (+ indicates data from Ref. 5.) The iso­mer shifts {, are with respect to sodium nitroprusside. Sub­tract 0.26 mm/ s from the values given in the table to reference relative to natural iron metal. The error in 6E values is es­timated to be ±0.03 mm/s.

constant current source, and an uncalibrated gallium arsenide control diode. Final temperature equilibration and stability were continuously monitored on a Leeds­Northrup Speedomax-XL 600 mV recorder that was used to read the error signal of the calibrated silicon diode after cancellation by the K-4 potentiometer. Tempera­tures below 4.2 K were measured via the vapor pressure of helium us ing Wallace- Tiernan models F A-160 and 61-050 absolute pressure gauges while pumping was precisely controlled with an L. J. Engineering model 329 vacuum regulator valve. Temperatures between 78 and 50 K were achieved by pumping on liquid nitrogen (Welch 1397) to well below the triple point on solid ni­trogen. Both the vapor pressure of nitrogen and a calibrated silicon diode were used to monitor the tem­perature. An F. W. Bell model 610 gaussmeter with a transverse Hall probe was used for measurement of magnetic fields.

The sample of Fe(diethyldithiocarbjl.Klate)3BF 4 used for susceptibility stUdies and M6ssbauer stUdies at Northeastern University was prepared by the method of Pasek and Straub5 and gave the required analysiS: found: C-30.80%, H-5.19%, N-7. 43% (Glabraith Laboratories, Inc .. , Knoxville, TN); calculated: C-30.68%, N-7.16%.

EXPERIIYIENTAL RESULTS

Mossbauer spectroscopy

Experiments were done for the several different di­thiocarbamate iron salts in zero field with the quadru­pole splitting listed as a function of T in Table I. Good agreement is obtained with the previous results of Pasek and Straub,5 and Golding et al. 8 (measure­ments to 78 K) for perchlorate salts of some Fe(IV) cations. It is important to note the strong temperature

dependence of DoE over the range 4.2-300 K for the Fe (IV)(DTC)3BF 4 complexes in contrast to the tempera­ture independence for [Fe(diars)2CI2](BF4)2.1 The sign of the principal component of the electric field gradient is found to be negative from Mossbauer spectra in ap­plied fields also in contrast to the results for tetragonal [Fe(diars)2CI2)(BF4)2' It should also be noted that the magnitude of the quadrupole splitting is significantly smaller than that found in [Fe(diars)2CI2](BF4)2' The isomer shifts 5 for the Fe (IV)(DTClsBF 4 compounds are near zero and seem to be nearly the same as [Fe(diars)2CI2)(BF 4)2, indicating a relatively large elec­tronic charge on the 57Fe nucleus in both cases. This is probably due to the smaller number of 3d electrons available to shield the 57Fe nucleus from the 3s and 4s electrons.

Experiments on these Fe (IV) complexes were also done in applied magnetic fields of 60 kG with the ex­perimental spectra in Fig. 1 taken with the applied field parallel to the gamma beam. These are data which in­dicate a negative field gradient in each case with axial symmetry. One of the complexes, Fe(CHXlsBF4, was also studied at elevated temperatures as indicated in

7.~·7·.~·~.·. . .' .. /:7:~ .. \ I~' ~ C\ t

CHX ·\t~</'\\'.J' .. J.ty\ I .: .. J \; .~

• I ., ·1 ., SOURCE IJElOCITY (mm/sec)

FIG. 1. Mossbauer spectra at T= 4.2 K in a field of 60 kG ap­plied parallel to the gamma beam for three different Fe(IV) complexes (powder samples). The solid curves were computed as described in the text using the parameters listed in Table 1.

J. Chern. Phys., Vol. 67, No.8, 15 October 1977

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Oosterhuis, Dockum, and Reiff: Spin triplet iron (IV) compounds 3539

Fig. 2, where a small temperature dependence was ob- "'Q 'I 'I

served. In this case, the applied field was perpendicular x

to the gamma beam direction. The spectra resemble the results expected for a diamagnet or rapidly relaxing paramagnet9 (Fig. 2), but the splitting of the low energy doublet is significantly reduced from the diamagnetic case, indicating a (negative) magnetic hyper fine inter­action with the paramagnetic electrons. This splitting of the low energy doublet is seen to decrease slightly on

lOOK ..

77 K

50K

20K ',. ," : .....

4.2 K ..... "", ........

... DIAMAGNETIC

·2.0 ·1.0 0 <1.0

SOURCE VELOCITY (mm/sec)

FIG. 2. Mossbauer spectra at variable temperature for the Fe(IV)(CHX)3BF4 complex with the field applied perpendicular to the gamma beam. Note the differences in the spectrum due to the orientation of the applied field. The solid curve at the bottom was calculated assuming no magnetic moment from the electrons. The continuous curves on the data are calculated using the electronic model discussed in the text. The spectra were taken in an applied field of 60 kG.

:J • 6

-E ~ <l> . <f> 0> 0 · -.S! 0 · i I _::<0 ~ -X 0 I CD r:- I ... f-::J iIi -f-D-W U 0 (f) 0 ::J Of-(f)

0 U 'T i= w z cc:> r-<! ::?'

I. I .. '\

'I " 10 TEMPERATURE (K)

100

-

-

-

I .1.1

FIG. 3. Log plot of the temperature and field dependence of the molar susceptibility (xli) over the range 1. 6 to 5.1 kG.

going from 4.2 to 20 K. Thus, there is a small but definite paramagnetic effect which appears to be nearly temperature independent. Again, this is in contrast to the [Fe(diarslaCl 2](BF 4)2 case where the magnetic inter­action is strongly dependent on temperature. 1

Magnetic susceptibility

The magnetic susceptibility of one of the iron (IV) complexes, Fe(diethyldithiocarbamate)3BF 4, has been determined over the range 1. 5 to 300 K. The data are shown in Fig. 3, where it is seen that the susceptibility is essentially field independent for T> - 3.0 K. In the log plot (Fig. 3), the data for ten fields (1. 6 to 5.1 kG) are superimposed on each other. A least squares fit to the temperature dependence of the molar suscepti­bility for T> 4. 2 is given in Fig. 4, solid curve, where the final bes t fit parameters are g %' g y' and g. = 2. 0, D=+23.8 cm-t, E=0.001 cm- 1

• The equation used in the least squares fitting procedure is

N{32 {g; XM= [ ( D+E) ( D-E)~ E

3 l+exp - kT +exp - ~~

X [exp(- Dk-;,E)_exp(_ Dk+TE)J+ D2:i

X[1-exp(- Dk+TE)} D2~k [l-exp(- ~;E]}. (1)

The foregoing equation was derived by Birker et al. 10

using the energy expressions given by Ballhausen. 11

The effect of such zero field splitting of S= 1 to an ex­cited M. = ± 1 and ground M. = 0 is more clearly seen in a plot of /-Leu vs T (Fig. 5), in which the moment drops off sharply below - 25 K. Some sample moment data are given in Table II, where it is seen that for tem­peratures greater than - 50 K the value of /-Leu is close to the spin only of {8 expected for a 3 A ground term. These values are similar to but cannot be directly com-

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3540 Oosterhuis, Dockum, and Reiff: Spin triplet iron (IV) compounds

to Q .: 8 ,.---"---,------,-----,---,----1,----,

E 0 .-'" 0 r/) co 0>

2

o

-

, , 1 I 40.00 80.00

TEMPERATURE (K)

FIG. 4. Fit to x.\t vs T (H = 5.1 kG) for zero field splitting D=+23.8 em-I.

pared with solution NMR (Evans method) data of Pasek and Straub, 5 who observe Il.tr = 3. 28 Il B for the ethyl system at 304 K. In any event, room temperature mo­ments close to the spin only are quite reasonable for a ground 3A based on dl/2 derived from sizable splitting of a 3T (d,.).

The Faraday method is basically a force method and owing to the large forces involved the low temperature measurements (Figs. 3-5) necessarily involved a small sample (6.85 mg) of the ethyl compound. A larger sample of the compound (25.7 mg) was used to check on the Curie-Weiss behavior over the range 50 to 300 K, where temperatures below 78 K were achieved by pump­ing on nitrogen. Before correction for temperature in­dependent paramagnetism (TIP), linear least squares

o o ,---,-----,----,,---,----r, r0

Jl' " LL

LL W

::::to o­

f- C\J Z W ~ o ~

u ~ W 0 Z 0.­(!) <l ~

o , I Ii

40.00 80.00 TEMPERATURE (K)

FIG. 5. Plot of !l.rr vs T (H=5.1 kG).

-

,

TABLE II. Sample magnetic moments.

T(K) Jleff(!lS)

302.91 2.94 228.20 2.94 188.41 2.92 104.17 2.85

45.50 2.81 24.92 2.70 19.93 2.60 16.08 2.48 12.89 2.33 10.33 2.16 8.25 1.98 6.55 1.80 4.25 1. 51 4.00 1.47 3.52 1.40 3.01 1.33 2.48 1. 23 2.00 1.13 1. 52 1.00

fitting of X~l vs T gave the following field weighted average of the parameters: Il eff = 3. 07 Il B' a Curie constant of C= 1. 07 emil/mole, and a paramagnetic Curie temperature 8 = - 21. 7 K. Nonlinear squares fitting of X:v= C/(T - 8) + TIP gave a TIP of 490x 10-6

cgs. Figure 6 shows a linear least squares fit to X';l vs T after correction for the preceding TIP. It appears that the data obey Curie-Weiss behavior with Il.ff = 2. 79 /lB' C=0.97 emil/mole, and8=-1.9K. Thus, theap­parent value of 8 and magnitude of nondilute magnetic interaction are too small to assess whether the be­havior for T< 3.5 K represents the onset of some type of magnetic order. Even so, this small value of 8 and the significant field dependence of x:V below T - 3.0 K, Fig. 3, may be related. To study this possibility, we have investigated zero field Mossbauer spectra of the

;,: f-:J ~o f-o [tic\! o ({) :::J ({)

I , I

-

-

I

300

FIG. 6. Fit of xlii vs T (H=5.1 kG) to Curie-Weiss behavior for T > 10 8 after correction for a TIP of 490 xIO-6 CGSU.

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Oosterhuis, Dockum, and Reiff: Spin triplet iron (IV) compounds 3541

z o i= Q.. 0::: o (f) OJ <!

W > ~ ~ W 0:::

~>~1i*~?\ :~ .1/?t~~;: .'\ f' . .-: . -~

'( . \ i. \ f f i I, I \ i i j

\ . \ ' .\ ) 11 i I \. i

1.46K I,· I \. 1· ., \i ~ .J

-4 -2 o 2 4

VELOCITY, mm/sec, RELATIVE TO IRON

FIG. 7. Zero-field Mossbauer spectrum of Fe(diethyldithio­carbamate)3BF 4 at 1. 46 K. The continuous curve is a least­squares fit to Lorentzian line shapes.

ethyl compound at T< 3. 0 K for evidence of hyperfine splitting and magnetic ordering. Other dithiocarbamate systems, e. g., Fe(III) (diethyldithiocarbamate)2CI, have recently l2.l3 been shown to order in this tempera­ture range. In the case of the preceding S= ~ chloride, ferromagnetic order occurs with the Curie temperature Tc= 2. 5 K.

Our lowest temperature Mossbauer spectrum (1. 46 K, Fig. 7) gave no direct evidence of slow relaxation broadening or magnetic hyperfine splitting from ordering phenomena. However, this does not mean that the latter process occurring at even lower temperatures cannot lead to the observed onset of field dependence in X~.

Since completion of this investigation another studyH of the temperature dependence of the magnetic moment of Fe(diethyldithiocarbamate)3BF 4 has come to our at­tention. The work also includes a magnetically per­turbed Mossbauer spectrum of the ethyl system of 4.2 K in an external field of 30 kG. The primary results of this work, Vzz negative, and evidence of zero-field splitting in the temperature dependence of the magnetic moment are in accord with our findings. However, there are some aspects of this work with which we dis­agree in basic interpretation. First of all, the fact that Vzz is negative is suggested as evidence for the con­figuration (d"., d~.)3d~. However, this configuration corresponds to a 3E ground term. On the other hand, the authors give a diagrammatic representation of the energy levels involved that shows a 3A2 term at lowest energy under a trigonal distortion. This gives rise to a further problem from the fact that the d"., d y • orbi­tals are not the appropriate ta, basis functions for a simple distortion (compression or elongation along C3 )

of trigonal symmetry.

Use of the set t~ = d.2, t'2,. = ..j2/3 d,,2.,2 -..j 1/3 d"., and ta, = V273 d>:y + v'l73""" dy • for quantization with respect to a threefold axis results in a negative quadrupole inter-

action with an orbital singlet ground term (3Al) based on d1l2. A ground singlet based on d>:y corresponds to a tetragonal distortion, Vzz positive and a 3E2 ground term. Furthermore, while the quadrupole interaction is negative for a 3 E based on the configuration (d"", d )3dl simple crystal field theory suggests that the

"16 'XY' magnitude is expected to be somewhat smaller, prob-ably -1. 5 to 2 mm/s, for such a term and counter to the observed limiting low temperature values, Table I.

Finally, the problem of going from a trigonally dis­torted six coordinate system (D3 symmetry) to trigonal prismatic coordination (D3h ) or somewhere in between (0° < cf:> < 60°) can be envisioned in terms of twisting one triangular face of the six coordinate polyhedron about a threefold axis while holding the opposite face fixed. Crystal field and angular overlap model calculations show that the one electron d orbitals in the trigonal prismatic extreme are in order of increasing energy: al (de2), e' (d>:y , d~.y2) e" (d"" , dye), that is, the v'173 coefficients of D3 symmetry have effectively vanished. It appears that d

ll2 is still the appropriate ground or­

bital for a complicated distortion consisting of a com­bined elongation and twist with respect to the C3 axis, as is probably the case for all of the Fe(IV) dithiocar­bamate systems of the present study.

ELECTRONIC MODEL

Fe(IV)(DTC!aBF4 is expected to have a tL- configura­tion with two unpaired electrons (S= 1) in near trigonal symmetry. The action of a trigonal distortion on the tL- configuration is considered in the paper by Oosterhuis and Lang6 with the electronic Hamiltonian

H= L: {~I+!;11·sl+{3H·(l1+2sl)}' (2) I

where!; is the one electron spin-orbit coupling parame­ter which is taken to be about + 400 cm·l , H is the ap­plied field, and ~, is the one electron crystal field in­teraction, which we take to be the same for each elec­tron. With a positive ~ the ground state of the system is left as an orbital singlet-spin triplet (t~t. tJ repre­sented by a spin Hamiltonian (S= 1) of the form

2 - S He = DS e+ {3H . g. . (3)

Once the electronic states are found for He' the hy­perfine interactions are given by the nuclear spin Hamil­tonian

The spin Hamiltonian parameters D, g, A, and Vzz are functions of the trigonal distortion ~ and the spin-orbit coupling constant!;, and have been calculated exactly for several different values of ~. 6 The perturbation theory expressions hold in the limit of ~»!; and give D=!; 2/4~ with

g~=2+!;/~+!;2/2~2 ,

gil = 2 - !; 2 / 4~ 2 ,

A~ =p[!;/~ - K(1+!; 2/4~2)

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3542 Oosterhuis, Dockum, and Reiff: Spin triplet iron (IV) compounds

-0.9

-Q8

-Q 7

-0.6

-0.5

-0.4

-Q 3

-0.2

-0.1

" "

- D ·20em- l

• __ • D : 25 em- l

.".--------" -------..... _----°0~~--~1~0-----2~0----~30~--~40~--~50~--~6~0----~

T(KI

FIG. 8. Calculation of (SX>T and (Seh using the orbital sin­glet-spin triplet model.

I 21 2 3?; 3?;2 ] . + 1 7 (1 -?; 2A) - 28A + 56A 2 ,

AII=P[+2/7-K- 2~~ _?;2/4A2] ,

where P= 2gn i3n i3"(r-3)",,, - 4.19 mmls in the nuclear ex­

cited state, K=0.35, and vzz =-4/7[1-?;2/8A2] (r- 3)KQ •

Since the Fe(DTC)3BF 4 samples were undiluted paramag­netic materials for T ~ 4. 2 K, we expect the spin-spin relaxation to be rapid with respect to the nuclear Lar­mor precession rate, so that th~ induced magnetic hy­perfine interaction is given by IA(S)T' where

(S)T= L: (nISln)e-En/~T / L: e-En/~T . n n

(5)

In an octahedral environment with a trigonal distor­tion, the A tensor is highly anisotropic with A~ '" 0 and All '" 2. 6 mmls (nuclear excited state) so that the induced hyperfine field is very small in the directions perpen­dicular to the trigonal axis.

For axial crystal field distortions of reasonable mag­nitude (say A - 5?;) we have D- 20 to 25 cm-1

• The ther­mal averages (SS)T and (S.)T are easily calculated and shown in Fig. 8 for two values of D. We see that (S.)T is quite small, whereas (Ss)T is fairly large at low temperatures.

However, since A~ is so small, the induced hyper-

fine field A~ (S~)T is very small with respect to the ap­plied field. The other component All is definitely not zero but the induced hyperfine field All (S,,)T is near zero because (S')T is so small.

Thus, for the field either parallel or perpendicular to the trigonal axis, our electronic model predicts very little paramagnetic contribution to the hyperfine structure. One would see a strong temperature depen­dence in a trigonal complex if D were small enough (-10 cm- 1

); but in that case the trigonal distortion would have to be very much stronger than is likely (> 8 ?;) and would lead to temperature independence of the field gradient through large separation of the orbital states.

COMPUTING MOSSBAUER SPECTRA FROM THE SPIN HAMILTONIAN MODEL

The solid curves in Figs. 1 and 2 are calculated from a model using the anisotropic spin Hamiltonian [Eqs. (3) and (4)]. First, the applied magnetic field is as­sumed to be in a given direction relative to the crystal axes. The electronic Hamiltonian [Eq. (1)] is diagonal­ized to obtain the electronic eigenstates and energies from which the hyperfine interactions are calculated. Then the nuclear Hamiltonian matrices are diagonalized to find the energies and amplitudes for the Mossbauer transitions. A linewidth of 0.3 mmls (full width at half maximum) is folded into a Lorentzian line shape for each transition and a spectrum is then calculated for that particular direction of the magnetic field relative to the crystal axes. In order to simulate random ori­entations of powder absorber, this calculation is re­peated for a hundred different orientations of the mag­netic field equally spaced over an octant of the unit sphere. This is done numerically on a computer and the computed Mossbauer spectra are compared to the ex­perimental data, in order to find the spin Hamiltonian parameters D, g, and A which give the best agreement. KQ is determined from the zero field quadrupole split­ting.

The spectrum computed in Fig. 2 for a diamagnet is done with tv". Q = - 1. 20 mmls, A~ = All = 0, and the magnetic field of 60 kG applied perpendicular to the gamma beam. Although the computer simulation for a diamagnet resembles the experimental data, there are significant differences in the low energy doublet which indicate the paramagnetism of the iron ion.

The magnitude of the quadrupole splitting in Fe(CHXhBF4 in zero field is found to be 2.40 mmls at 4.2 K. The sign of the EFG is negative and the asym­metry parameter is zero as found from the data in an applied field. The paramagnetic hyperfine interactions in the data are represented by the spin Hamiltonian parameters All = 2. 65 mmls and A~ = - O. 05 mmls (for the nuclear excited state), tVuQ= -1. 20 mmls, gil

= 1. 99, g~ = 2.12, and D"'" 20 cm- 1• The quality of the

fit can be judged by the comparison of the solid curves in Figs. 1 and 2 with the experimental data. It is seen that the required D value is similar to that found from fits to the temperature dependence of X~.

We find the induced hyperfine field A.(S)T to be near-

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Oosterhuis, Dockum, and Reiff: Spin triplet iron (I V) compounds 3543

ly temperature independent for the Fe (CHXhBF 4 com­pound from 4.2 to 100 K, which implies a large value jor D and a value for Il < 5 t. The temperature dependent behavior of Fe (CHXhBF 4 is not well reproduced by our spin Hamiltonian model, but the trends in the experi­mental spectra seem to follow the simulations. The largest paramagnetic contribution occurs in the 20 K data and provides the largest cancellation of the applied field.

A considerable amount of computational effort using other models was applied to the variable temperature data of the CHX sample with little satisfaction in obtain­ing a better fit. There are the possibilities of tempera­ture-dependent effects in the structure of the electronic states, and also that the assumption of fast relaxation of the electronic spin is not valid. Somewhat better representation of the data was obtained with D- 20 cm-l, All - 8. 15 mm/s, and A~ - - O. 080 mm/s. Unfortunately, these values are not consistent with any of the elec­tronic models considered. In any case, the 4.2 K data in Fig. 2 does reflect a magnetic interaction between the electronic and nuclear moments.

The data for the Fe(MEhBF4 complex at 4.2 Kin 60 kG (Fig. 1) are well represented by All = 2. 50 mm/s, A~=+0.03mm/s, tV..,Q=-1.29mm/s, gil = 1.98, g~ "'" 2. 2, and D"'" 2 5 cm- 1

• The results for Fe (PYR)3BF 4 at 4.2 K in 60 kG [Fig. l(c)] are A II =2.8 mm/s, A~=+0.13 mm/s, tV .... Q= -1. 29 mm/s, gil = 1. 98, g~ = 2. 2, and D-25 cm- 1

• In each of these complexes the computed spectra are not very sensitive to 5% variations in D but are fairly sensitive to the magnitudes of All and A~ • Reasonably good agreement between calculated spectra using the parameters in Table I and experiment is achieved. The values of A tensor deduced from the computer simulations of the experimental data agree well with the values calculated from the trigonal field model. 6 The results of fitting the experimental data are listed in Table 1.

In order to account for the temperature dependence of the quadrupole splitting, there must be some low­lying orbital states of the order of a hundred degrees above the ground state, which also implies a somewhat smaller positive Il and perhaps an additional rhombic term. A proper treatment should include these contri­butions from low-lying excited states. These observa­tions are consistent with a recent 1S single crystal x-ray structure determination of Fe(IV)(pyrrolidyldithiocar­bamate)3(C104). In this work, it is found that the Fe(IV) complex has a twist angle cp = 38 ° or therefore a coordi­nation intermediate to octahedral (cp = 60°) and trigonal prismatic (cp=OO). Moreover, the cation has exact C2 symmetry under which circumstances the excited 3E state eT ground in Oh-3A+3E trigonal) is probably al­so split to some extent by a rhombic component of the ligand field. Thus, exact agreement with calculations based on an axial (trigonal) model is not to be expected. If we assume ~ - 400 cm-l, then the perturbation theory expression for D= 1;2/41l gives Il "'" 1680 cm-1 for D = 23.8 cm-1•

Another possibility would be that the trigonal distor-

tion is negative leaving two low-lying orbital singlet states which have the same magnetic anisotropies as in the case for positive Il. But in this case the hyperfine field would enhance the applied field instead of opposing it and give splittings in large disagreement with experi­ment. The case with negative Il would fit in with a tem­perature dependent IlE and the temperature independent (S .. )T (if Il is small enough), but the sign of the EFG as well as that for the magnetic hyperfine contributions would both be contrary to experiment. Single crystal measurements of the principle susceptibilities over the temperature range 4-300 K would be very helpful in the determination of the orbital state separations and thus Il. Fits to the temperature variation of mean suscep­tibilities or moments as obtained from powder mea­surements are not that sensitive to the magnitude of 1l.16 Nevertheless, Figgis et al. 17 have studied the problem of the variation of mean powder moments with temperature for 3T1 ground terms perturbed by the com­bined action of axial ligand field components, spin-orbit coupling, and t2g electron delocalization (orbital angular momentum reduction factor k). Generally, the magni­tude and variation of magnetic moments of the present work, when viewed in terms of the foregoing study, are again only consistent with a large (-1000 cm- 1) positive value of Il and sizable delocalization, k - O. 6.

In conclUSion, the experimental results are consis­tent with a trigonal distortion Il between 1000 to 2000 cm- 1

, which leaves an orbital singlet-spin triplet 3A (d .. 2)

as the electronic ground state. This spin triplet is split through the spin-orbit interaction by an amount D'" + 20 cm- 1 and can account for the magnetic hyperfine effects observed. The sign of the EFG is negative as expected for this configuration and the EFG has near axial sym­metry although its magnitude is somewhat smaller than expected for a large Il. This may be related to de­localization-covalency effects involving the sulfur li­gands such that (r- 3

) is significantly reduced. In this context, it is worthwhile to point out that Golding and co-workers 18 have obtained reasonable fits for the tem­perature dependence of the quadrupole splitting (300-78 K) of some of the compounds studied herein using I; as small as - 280 cm-1

• If this is reasonable then a value of Il = 1;2/4D= 980 cm- 1 is obtained for D"'+ 20 cm- 1• A value of Il '" 1000 cm- 1 may be more consistent with the observed temperature dependence and magnitude of quadrupole splitting. This again points to significant covalence effects since 1;/1;0= 280/520'" O. 54. The an­isotropy (All» A~) of the magnetic hyperfine interaction is also expected from the electronic model and their magnitudes are in reasonable agreement with experi­ment.

It is interesting that the Fe (DTChBF 4 complexes ap­pear to experience a crystal field distortion of the same magnitude as the [Fe(diars)2CI2](BF 4h complex, but that the temperature dependence of the quadrupole and mag­netic hyperfine interactions are so different in the two cases. This difference is explained above as being due, in part, to the different values of All and A~ in different symmetries. These parameters will, of course, also be sensitive to the exact ligation involved, sulfur versus arsenic and chlorine.

J. Chem. Phys., Vol. 67, No. B, 15 October 1977

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3544 Oosterhuis, Dockum, and Reiff: Spin triplet iron (IV) compounds

Finally, we note that the small temperature depen­dence of the magnetic splitting of the low energy (Ie ,,± 3/2) doublet of the perturbed Mossbauer spectrum suggests that the internal field is always smaller than the applied field. Otherwise, in the limit as T gets large, we expect (S>r - 0 so that if the internal field were larger there would be a temperature at which the internal field matches the applied field and the lower doublet would be unsplit at that temperature.

ACKNOWLEDGMENTS

It is a pleasure to acknowledg the gift of some Fe(IV) Mossbauer samples from Professor D. Straub (Universi­ty of Pittsburgh), who also provided valuable insight in­to these complexes. Some of the data was taken at the A. E. R. E. in Harwell, United Kingdom. W. T. O. would also like to acknowledge helpful discussions and cal­culations by George Lang, K. Spartalian, D. Rhynard, and H. H. Wickman. W. T. O. acknowledges partial support of this research by the U. S. Public Health Ser­vice Grant AM-15-725-01 and the NSF while at CMU. W. M. R. acknowledges partial support of the Research Corporation, H. E. W. Grant No. RR 07143, and the NSF Grant No. DMR 75-13592AOl. He also thanks Dr. Herbert Wong for writing programs for least squares fitting of susceptibility data and the Francis Bitter Na­tional Magnet Laboratory for use of its high field Moss­bauer facilities.

*Authors to whom correspondence should be addressed. tpresent address: Division of Materials Research, National

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J. Chem. Phys., Vol. 67, No.8, 15 October 1977

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