REASSESSMENT OF THE MÖSSBAUER SPECTRA OF IRON(I I) PYRIDINE-N-OXIDE PERCHLORATE : EVIDENCE FOR SITE DISTORTIONS ENHANCED BY GRINDING

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REASSESSMENT OF THE MÖSSBAUER SPECTRA OF IRON(I I) PYRIDINE-N-OXIDE PERCHLORATE :

EVIDENCE FOR SITE DISTORTIONS ENHANCED BY GRINDING

B. Howes, D. Price, D. Mackey

To cite this version:

B. Howes, D. Price, D. Mackey. REASSESSMENT OF THE MÖSSBAUER SPECTRA OF IRON(I I) PYRIDINE-N-OXIDE PERCHLORATE : EVIDENCE FOR SITE DISTORTIONS EN- HANCED BY GRINDING. Journal de Physique Colloques, 1979, 40 (C2), pp.C2-286-C2-289.

�10.1051/jphyscol:19792101�. �jpa-00218470�

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JOURNAL DE PHYSIQUE Colloque C2, supplkment au no 3, Tome 40, mars 1979, page C2-286

REASSESSMENT OF THE MOSSBAUER SPECTRA OF IRON( I I

1

PYRIDINE-N-0x1 DE PERCHLORATE :

EVIDENCE FOR S I T E DISTORTIONS ENHANCED B Y GRINDING B.D. Howes, D.C. Price and D.J. ~ackey'

D e p t . o f Solid State Physics, Research School of Physical Sciences, Australian National University, Canberra, A. C. T., Aus t r a l i a

" ~ c h o o ~ of Natural Resources, University of. the South Pacific, Suva, F i j i

RCsum6.- Les spectres ~gssbauer de poudre de Fe(C5H5N0)6 (C101,)~ mettent en Bvidence l'existence de relaxations Blectroniques lentes 2 basse temp6rature et en champ nul, mais aucune structure magnBti- que hyperfine n'a pu Stre observ6e. Ces spectres sont bien compris B partir d'un modsle oii l'effet dominant est dO aux consBquences du traitement mscanique de mise en poudre sur les sites de Fe2+ qui sont des octasdres en distorsion trigonale. Le traitement mBcanique introduit une distorsion non- axiale suppldmentaire qui est reprBsent6e par un terme B;O$ dans 1'Hamiltonien du champ cristallin.

Une distribution des valeurs du paramltre B$ permet une meilleure simulation, avec un pic B B:

-

0.3 cm-' alors que pour le matdriau massif le pic de distribution se trouve 2 B$ = 0.

Abstract.- " ~ e dssbauer spectra of crushed powders of F~(C~H~NO)~(C~OI,)Z show evidence of slow electronic relaxation at low temperature in zero applied magnetic field, but resolved magnetic hyperfine structure is not observed. However, the spectra of small uncrushed crystals contain well-resolved magnetic hyperfine structure. These spectra have been well reproduced using a model in which the dominant effect of the mechanical crushing on the ~e'+ site, which is a trigonally distorted octahe- dron, is to introduce a non-axial distortion that is re resented in the crystal field Hamiltonian by a term ~$0:. Distributions of values of the parameter B2 enabled better simulation of the spectra in both cases, but while the distribution required for the spectrum of the uncrushed crystals had a

strong peak at B$ = 0, that for the powder was peaked at B:

-

^{0.3 cm-'.}

1 . Introduction.- In the light of recent work on

the relaxation of Fe2+ ions in ZnC03 / I / and MgC03 fa) ⁵ ^~ ^g

121, in which the cations are coordinated to six

i

oxygen ions in an octahedral configuration with a

l

trigonal elongation (point symmetry C,;), it is surprising that the "Fe ~gssbauer spectra of Fe(PyN0) 6 (ClOr) 2 (where PyNO CsHsNO) in zero applied field at low temperatures 131, / 4 / , /5/,do not show well-resolved paramagnetic hyperfine structure, because the cation sites have the same geometry in this compound as in the rhombohedral carbonates /3/,

/ 6 / . High-spin Fe2+ ions in sites such as this have

I bl

an orbital doublet 'Eg(=~zg) lowest which, when spin-orbit coupling is considered, results in a doublet ground state. Figure 1 shows the energy le- vel structure of Fe2+ in FeC03 (see _{/ l / )} which should be similar to that in Fe(PyNO)6(C10~)2: Sams and Tsin / 5 / estimated the excitation energy of the lowest singlet state in the latter case to be

- 1 1 0 cm-'. The states of the ground doublet are

highly magnetically anisotropic (g,

2

^{10, g1}= 0)

,

Fig. 1 : Schematic diagram of the electronic energy and for this reason spin-spin relaxation between levels of the high-spin Fe2+ ion in a crystal field them should be very slow and the dominant relaxation of trigonal symmetry as in FeC03 /l/ and in Fe

(PyN0)6(ClOr)2. (a) The effect of the cubic and tri- mechanisms are expected to be two-phonon processes gonal crystal fields on the sD free ion state. (b) involving excited states within the 'E multiplet The effect of the trigonal crystal field component

g and the spin-orbit coupling on the 5 ~ 2 g orbital tri-

/ l / . Such processes result in a relaxation rate plet. The values of the parameters used were B4 =

that is strongly temperature dependent and is very '55-56 cm-', B8 P -12-5 cm-' and h cm-'.

slow at low temperatures.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19792101

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However, this is not the sort of behaviour observed in Fe(PyN0)6(C101+)2 by Sams and Tsin /h/.

As the sample temperature decreased below "30K, they observed line broadening consistent with a slowing down of the relaxation rate, but the rate appeared to remain nearly constant below -15K and no resolved magnetic hyperfine structure was observed. They did find, however, that resolved hyperfine structure appeared when quite small (-0.1T) magnetic fields were applied to powder samples at 4.2K 151, but they were unable to satisfactorily explain the detailed behaviour.

The relaxation behaviour of Fe2+ ions in CaC03 and CdC03 has also been studied recently(/7/ and unpublished work) and, although these compounds are isomorphous with ZnCOs and MgC03, no magnetic hyperfine structure was observed in zero applied magnetic field, even at temperatures as low as 1.6K. It was suggested /7/ that this was due to a small distortion of the Fe2+ site from Csi symmetry. It will be shown in this paper that a similar explanation is consistent with previously reported results for Fe(~y~O)~(Cl0~)2 (131, /4/, /5/) and with the new results that will be presented below.

2. Experimental

.-

Crystals of Fe(PyN0) 6 (C104) 2 were grown from a methanol solution that was prepared by adding a solution of ferrous perchlorate in methanol (approx. 0.06M) to one of pyridine-N-oxide in methanol (approx. 0.4M). Small red rhombohedral crystals (with - 1 m sides) separated when the solution was left to stand for a few hours. 5 7 ~ e Mijssbauer spectra of some of the smaller of these crystals and of powders produced by grinding them, were recorded with the samples at 4.2K and 77K using a constant acceleration spectrometer 181 with a 5 7 ~ o g source. The spectrometer was calibrated using the spectrum of an iron foil absorber at room temperature and the data of Violet and Pipkorn 191.

Zero velocity was taken to be at the centroid of the iron foil spectrum.

3. Results and discussion.- Figure 2 shows typical Gssbauer spectra for a sample consisting of a large number of small Fe(PyNO)~(C104)2 crystals taken directly fromthe methanolsolution, and of a powder produced by mechanical grinding of crystals grown from the same solution, both recorded at 4.2K. Also shown for comparative purposes are 4.2K spectra of powder samples of 20% ~e'+ : ZnCO3 and 3.6% Fe2+ : CaC03 from references /l / and /7/ respectively. All spectra in figure 2 were recorded with the samples in zero applied magnetic field.

- S D + S

V E L O C I T Y < M M / S >

Fig. 2 : ~ijssbauer spectra of (a) a large number of small, uncrushed Fe(PyN0)6(ClOs)2 crystals and (b) a powder produced by mechanical grinding of similar crystals. (c) and (d) are the spectra of powder samples of -20% Fe2+ : ZnC03 and 3.6% i?e2' : CaC03 taken from references / l / and /7/ respectively. All spectra were recorded with the samples at 4.2K in zero applied magnetic field.

The two most significant features of these spectra are, firstly, the marked differences between the spectra of the unground Fe(PyN0)6(C10+)2 crystals and of the ground powder, the latter spectrum being effectively identical to the equivalent one reported by Sams and Tsin 131, /4/, 151, and, secondly, the appearance of peaks in the unground crystal spectrum reminiscent of the well-resolved hyperfine structure observed in Fe2+ : ZnC03. The loss of this hyperfine structure when the crystals are ground

indicates that lattice distortions play an important role. A simple model for the effect of such a lattice distortion is outlined below and it will be shown that the spectra of figure 2 can be described quite well in terms of it.

3.1. Model for Distortion of the Cation Site in Fe(PyN0) 6(ClO4)2.-

The same distortion model was used in this work as was used(/7/, 181) to explain the lack of magnetic hyperfine structure in ~Gssbauer spectra of Fe2+ : CaCOs. To the single-ion Hamiltonian of ~e'+ in a site of Cgi symmetry /I/ is added a rhombic distortion term of the form B ~ ( L ;

-

L2), and the parame-

Y

ter 322 is expected to be small compared with the trigonal splitting and the spin-orbit coupling. Such a distortion will mix and split the states of the

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C2-288 J O U R N A L DE PHYSIQUE

ground doublet of figure 1, resulting in two non- ~e'+ : CaCo, /7/ although in that case the B$ value

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

-

5 0 5

V E L O C I T Y I M M / S )

magnetic singlets $1 and $ 2 . is sufficiently large that no line-broadening is Because their splitting may be comparable with the observed due to the nuclear hyperfine interaction magnitude of the magnetic hyperfine interaction, as alone (Fig. 2).

indicated by the line-broadening observed in the

Fig. 3 : MEssbauer spectra calculated as described in the text for four different values of the distortion parameter B:. Note that the centre shift has not been included in these spectra.

spectrum of the ground powder (Fig. 2), these two terms (viz. the rhombic distortion and the magnetic hyperfine interaction) were applied together as a perturbation to coupled electron-nuclear states derived from the states of the ground doublet of fi-

gure 1 . This approach is justified in view of the z

0

excitation energy (-100 cm-') of the lowest excited ^V) 2 state. Spectra calculated in this way, using magne- I

V) z tic hyperfine constants derived from the work of 4

Sams and Tsin / 5 / are shown in figure 3. Figure 4

E

shows spectra calculated using distributions of values of the distortion parameter B;.

B;:O

3.2. Discussion.- While none of the spectra calculated with a single value of

BH

(Fig. 3) give a particularly good representation of the experimental spectra for either the ground or unground Fe(PyN0)s (C104)2 crystals, it can be seen that line broadening somewhat similar to that observed in the spectra of the ground sample results from small (but non-zero) B$ values. Put simply, the magnetic field from the nuclear moment tends to remagnetize the non-magnetic states when their splitting is small enough (of the same order of magnitude as the hyperfine interaction). This is similar in principle to the effect of a small applied magnetic field on

Fig. 4 : MEssbauer spectra calculated in the same way as those shown in figure 3 but using distribu-

tions of the distortion parameter B:. The distributions used are explained in section 3.2 of the text.

The spectrum calculated from an exponential distribution of values of B$ centred at zero and with a half-width of -0.15 cm-', and shown in figure 4 , does give a reasonably good description of the spectrum obtained from the unground crystals. It is not suggested that such a distribution accurately represents the physical situation, but a strong peak at B; = 0 was required by the data. The distribution that gives the closest approximation to thespectrum of the ground powder, however, is a Gaussian distribution centred at B: -0.3 cm-lwith a width of a = 0.45 cm-'.

V E L O C I T Y IMM/SI c:o 1 5 c m - l

For various reasons the actual distributions of B$ values used in calculating the spectra of figure 4 should not be taken too seriously, but it is believed that the qualitative conclusions are correct, i.e. that there is a distribution of cation- site distortions present in both the unground and ground samples, that there is a significant propor- tion of sites in the former sample that have effectively no distortion and bhat the average site distortion is increased markedly by mechanical grinding.

These conclusions would appear to indicate that the distortions are due to random strains in the crystals.

The magnetic field results of Sams and Tsin

0:=0 3 3 c m - l U = O L S C , T - (

-

Î ^f ¹ Î Î Î Î Î Î ^, ⁽ ^, ^,

- 5 0 -5

I

C

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/5/ appear to be at least qualitatively consistent with these conclusions. For large applied magnetic

fields (>I.OT) the Zeeman interaction is greater than both the non-axial crystal field and the magnetic hyperfine interaction, so a reasonable description of the spectra can be obtained assuming all sites are identical, albeit with gl # 0 as expected for distorted sites. That they did not detect any departure from axial symmetry in the electric qua- drupole interaction is not surprising in view of the very small values of the distortion parameter B$

thatare introduced above. Their spectra in small applied fields, however, indicate a non-uniqueness of the ~ e sites, but rather than this being due to ~ + a difference in spin-lattice relaxation rates as they suggest, their spectra could equally well result from a distribution of distortions as proposed here.

Mechanical grinding of samples to powder form is done routinely in Massbauer spectroscopy. These results show that spectra obtained from samples treated in this way should be interpreted warily.

References

/I/ Price, D.C., Johnson, C.E. and Maartense, I., J. Phys. C

10

(1977) 4843.

121 Srivastava, K.K.P., PhD Thesis, Australian Natio- nal University (1976).

/3/ Sams, J.R. and Tsin, T.B., Inorg. Chem.

3

⁽¹⁹⁷⁵⁾

1573.

/ 4 / Sams, J.R. and Tsin, T.B., J. Chem. Phys.

62

(1975) 734.

/5/ Sams, J.R. and Tsin, T.B., Chem. Phys.

15

(1976) 209.

/6/ Van Ingen Schenau, A.D., Verschoor, G.C. and Romers, C., Acta Cryst. B E (1974) 1686.

/7/ Price, D.C. and Srivastava, K.K.P., J. Physique Colloq.

2

(1976) C6-123.

/8/ Window, B., Dickson, B.L., Routcliffe, P. and Srivastava, K.K.P., J. Phys. E

1

(1974) 916.

/g/ Violet, C.E. and Pipkorn, D.N., J. Appl. Phys.

42 (1971) 4339.