Investigation of nanocrystalline FeF2 by Mössbauer spectroscopy

0038-1098/90 $3.00 + .00 Pergamon Press plc

Solid State Communications, Vol. 74, No. 8, pp. 851-855, 1990. Printed in Great Britain.

I N V E S T I G A T I O N OF N A N O C R Y S T A L L I N E FeF2 BY M O S S B A U E R SPECTROSCOPY S. Ramasamy,* J. Jiang, H. Gleiter, R. Birringer and U. Gonser Werkstoffwissenschaften, Universit~it des Saarlandes, D-6600 Saarbriicken, Federal Republic of Germany

(Received 5 January 1990 by B. Miihlschlegel) The structure of nanocrystalline FeF2 specimens (average crystal size 10 nm, produced by inert gas condensation) was studied by M6ssbauer spectroscopy. The spectrum shows a quadrupole distribution. With the assumption of an average value for Q.S. distribution, two doublets were identified having nearly equal isomer shift. One doublet corresponds to bulk FeF2. The second doublet exhibits a broad distribution of quadrupole splittings with an average value of 1.97 mm s -1 and it is suggested to originate from the atoms in the grain boundaries. A similar distribution was noticed in the hyperfine field of nanocrystalline ~-Fe suggesting the non-lattice arrangement of atoms in the grain boundaries of metallic as well as ionic nanocrystalline materials. 1. I N T R O D U C T I O N N A N O C R Y S T A L L I N E (n-)metallic systems like Fe, Pd, Cu have been studied by various techniques like EXAFS, X-ray scattering, specific heat measurement, M6ssbauer spectroscopy, positron annihilation spectroscopy etc. The results of all these investigations have led to the conclusion that in nanocrystalline metals, a large concentration of grain boundaries with a different local arrangement of atoms exists, giving these materials special-microscopic and macroscopicproperties [1-5]. But there seems to be a contradicting information concerning this type of non-lattice arrangement in ionic systems. For example in nanocrystalline TiO2, it has been concluded from laser Raman spectroscopy investigations that such an arrangement does not exist [6] whereas the recent M6ssbauer spectroscopy result on n-a- and n-7-Fe203 showed a hyperfine field distribution indicating the possibility of different atomic arrangements in the grain boundaries and in the bulk [7]. This paper aims to address to the question of this controversy. Again, M6ssbauer spectroscopy was used for the investigation. And FeF2 has been chosen as a model system for the reason that the effect of the electric field gradient (EFG) on the splitting of the nuclear energy levels in ferrous ion has been dealt with extensively both theoretically and experimentally [8-11]. Further n-FeF2 can be prepared as stoichiometric compound. In our study M6ssbauer spectroscopy for a FeF2 pellet

*Permanent address: Dept. of Nuclear Physics, University of Madras, A.C. College Campus, Madras600025, India.

made of commercial powder (comm. FeF 2 pellet), a n-FeF2 pellet coated with paraffin, n-FeF2 powder suspended in paraffin and n-FeF2 powder in a vacuum container were measured at room temperature. 2. E X P E R I M E N T A L Nanocrystalline FeF2 was prepared from 99.9°0% pure commercial FeF2 polycrystals by an inert gas condensation process [12] by directly evaporating the substance from a tungsten boat in a helium atmosphere. The powder was pressed as a pellet (of size 8 mm diameter and approx. 0.1 mm thick) in vacuum with an uniaxial pressure of 1 GPa. The pellet was collected in a quartz tube containing molten paraffin to avoid the contamination of the sample by moisture which gives Fe 3+ compounds [13]. Then the paraffin was frozen and a paraffin coated n-FeF 2 pellet was taken out for the M6ssbauer spectroscopy studies. It was found essential to heat the paraffin at least for 30 min. in vacuum to get rid-off the water content in it. The average grain size of the as-prepared sample was determined by X-ray line broadening and TEM (Fig. l(a)) and found to be 10nm. The TEM micrograph shows that the grains attach to one another before consolidation into a pellet (Fig. l(b)). A 25 mCi57Co in a Rh matrix single line source was used. All isomer shift values are given relative to the centre of ~-Fe at room temperature. 3. R E S U L T S A N D DISCUSSION The M6ssbauer spectra recorded for the commercial FeF 2 pellet and for the nanocrystalline FeF 2 pellet

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I N V E S T I G A T I O N OF N A N O C R Y S T A L L 1 N E FeF2

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Fig. 1. TEM micrography of n-FeF2. (a) Dark field image showing individual grains. (b) Bright field image showing agglomeration of grains. are shown in Fig. 2. It is clearly seen that the spectrum for the comm, pellet sample consists of only one quadrupole doublet whereas the spectrum for the n-FeF2 shows a quadrupole splitting distribution. The spectrum for the commercial sample was fitted with one 1. O0

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Fig. 2. (a) and (b): Q.S. doublet fittings for the M6ssbauer spectra of commercial and n-FeF2 pellets. The inner doublet of n-FeF2 spectrum was assumed to have an average value of Q.S.

quadrupole splitting (Q.S.) doublet. Although the spectrum for the n-FeF2 shows a Q.S. distribution, it was fitted with two Q.S. doublets with the assumption that the Q.S. distribution has an average value. The fitting parameters are given in Table 1. In n-FeF2 the isomer shifts (I.S.) for both of the doublets are nearly equal. The outer doublet in n-FeF 2 has a Q.S. value very close to that for the commercial sample. However the slightly lower value of the I.S. of the outer doublet in the n-FeF 2 compared to that of the commercial sample suggests that there is a stretching of Fe-F bonds near the grain boundaries in n-FeF2. Similar stretching has been observed in a twin grain boundary in NiO [14]. The inner quadrupole doublet has an average Q.S. value of 1.97 mm s ~. It is instructive to note that the line-width of this inner doublet is nearly two times of the line-width of the outer doublet. This inner doublet is suggested to originate from the atoms in the grain boundaries. In order to get more information, fitting with a Q.S. distribution function [15] was performed. The distribution for commercial and n-FeF 2 samples are shown in Fig. 3. The distribution of the Q.S. for n-FeF2 has a tail extending to lower Q.S. values similar to the hyperfine field distribution for a n-~-Fe sample observed by Jing [7]. The broader distribution of the Q.S. compared to the polycrystalline sample seems to indicate the non-lattice arrangement of atoms in the grain boundaries of n-FeF2. Whether or not the two maxima at the lower Q.S. value side of the Q.S. distri-

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I N V E S T I G A T I O N O F N A N O C R Y S T A L L I N E FeF 2 'l

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Fig. 3. Mfssbauer spectra and the Q.S. distributions of (a) commercial and (b) n-FeF2 pellets. bution shown in Fig. 3(b) have physical significance remains a question. Aging of the n-FeF2 samples at ambient temperature leads to the formation of hydrated FeF 2 [13]. The

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spectra for the n-FeF2 aged for 2.5 weeks and 6 weeks are shown in Fig. 4. Obviously, the intensity of the Fe 3+ subspectrum increases with aging time. After prolonged aging, the volume fraction of the interfacial component is reduced. The Fe 3+ component increased whereas there is little change of the bulk component suggesting absence of grain growth. In fact, the Fe 3+ component seems to grow in the grain boundaries during aging. This observation supports our suggestion that the inner doublet with broad distribution of Q.S. splittings in n-FeF 2 originates from the interfaces, Figure 5 shows the Q.S. distribution fitting of the as-prepared and the aged n-FeF2 samples. In the latter case the Fe 3÷ component has been subtracted. Two observations can be made from the Q.S. distribution: firstly, the tail at lower Q.S. values reduces and secondly, the main peak narrows with aging. The reasons could be either the growth of Fe 3÷ compounds or some kind o f structural relaxations in the grain boundaries. Further n-FeF 2 powder coated with paraffin was studied. In Fig. 6 the spectrum of the paraffin coated powder and the spectrum of the n-FeF2 pellet are shown. In the former case the Fe 3+ component has been subtracted. The Q.S, distributions are also included in this figure for comparison. The Q.S. distribution as well as the hyperfine parameters (Table 1)

Table 1. MOssbauer hyperfine parameters for commercial and n-FeF2 samples Sample

I.S. (mms -I)

Q.S. (rams t)

L.W. (mms -~)

Area (%)

Bulk

1.328

2.783

0.309

100

Bulk Int.

1.289 1.267

2.733 1.968

0.352 0.550

75 25

n-FeF: pellet 2.5 weeks Bulk Int. Fe 3÷

1.336 1.327 0.417

2.766 2.118 0.562

0.322 0.551 0.246

73 22 5

n-FeF 2 pellet 6 weeks Bulk Int. Fe 3+

1.331 1.376 0.450

2.768 2.057 0.619

0.331 0.536 0.369

74 14 12

1.344 1.316

2.746 1.972

0.322 0.484

70 22

0.420

0.746

0.332

8

comm. FeF 2 pellet n-FeF 2 pellet

n-FeF 2 powder Bulk Surf ( + Int.) Fe 3+

Note: I.S. ~ Isomershift; Q.S. ~ Quadrupole splitting; L.W. ---, Line width; Int. ---, Interfacial component. Surf. ~ Surface component.

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I N V E S T I G A T I O N OF N A N O C R Y S T A L L I N E FeF 2 I

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Fig. 5. Q.S. distribution fitting for M6ssbauer spectra of as-prepared and aged n-FeF 2 pellets: the Fe 3+ component was subtracted in the latter case. (a) asprepared, (b) aged for 2.5 weeks, (c) aged for 6 weeks. I

indicate not much of a difference suggesting that the investigated n-FeF2 powder and pellet are structurally similar. The reason for this similarity may be the high degree of agglomeration and/or aggregation of the as-produced n-FeF 2 powder as can be seen in Fig. l(b). The M6ssbauer spectrum for loose n-FeF2 powder sealed in a vacuum container gives very little resonance absorption. The experimental conditions were similar to that o f v o n Eynatten and B6mmel [16] for freely dispersed Fe microcrystats between two Be windows. Since the investigated sample contains grains in the nanometer scale, the arguments namely the additional low frequency phonons from the oscillations of these nanocrystals or from the softening of surfaces [16] for the little resonance absorption should hold good for the present results also. 4. C O N C L U S I O N In the M6ssbauer spectrum of nanocrystalline FeF2, unlike in commercial polycrystals where only one doublet is observed, a quadrupole splitting distribution is observed. With the assumption that the Q.S.

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INVESTIGATION OF NANOCRYSTALLINE FeF 2

distribution has an average value, a two Q.S. doublet fitting was made and they are found to have same isomer shift. The outer doublet corresponds to the bulk component. The inner doublet having an average Q.S. value of 1.97mms -I is suggested to originate from the interfaces. The fitting with the distribution of Q.S. has a tail at the lower Q.S. value region as was previously observed for the hyperfine field distribution of n-e-Fe. The broadening of the Q.S. distribution implies the possibility of non-lattice arrangement of atoms in the grain boundaries in n-FeF2. The Q.S. distribution in n-FeF2 and the hyperfine field distribution in n-e-Fe are similar. This similarity suggests that there is little difference in the local arrangement of atoms in the grain boundaries of nanocrystalline ionic and metallic systems.

Acknowledgement - T h e authors are grateful to Mr U. Herr, Mr J. Weissmiiller and Dr J. Jing for fruitful discussions. They are also thankful to Mr M. Schuler for his assistance in taking microscopy pictures and to Mr H.J. Schwarz and Mr S. Trapp for their technical assistance. One of the authors, Dr S. Ramasamy, wishes to place on record his thanks to Alexandervon-Humboldt-Foundation, Federal Republic of Germany, for awarding a research fellowship. The financial support of the Deutsche Forschungsgemeinschaft is gratefully acknowledged. REFERENCES .

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