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Submitted on 1 Jan 1979
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MÖSSBAUER STUDY OF 57Fe IN THE
PYRITE-TYPE DICHALCOGENIDES - ISOMER SHIFT OF DIVALENT IRON IN LOW SPIN STATE
Y. Nishihara, S. Ogawa
To cite this version:
Y. Nishihara, S. Ogawa. MÖSSBAUER STUDY OF 57Fe IN THE PYRITE-TYPE DICHALCO-
GENIDES - ISOMER SHIFT OF DIVALENT IRON IN LOW SPIN STATE. Journal de Physique
Colloques, 1979, 40 (C2), pp.C2-297-C2-298. �10.1051/jphyscol:19792103�. �jpa-00218472�
JOURNAL DE PHYSIQUE Colloque C2, supplkment au no 3, Tome 40,
mars
1979, page (22-297M ~ S S B A U E R
STUDYOF
5 7 ~ eIN THE PYRITE-TYPE
DICHALCOGENIDESISOMER SHIFT
OF
DIVALENT
IRONIN LOW SPIN STATE
Y. Nishihara and S. Ogawa
EZectrotechnicaZ Laboratory, Tanashi, Tokyo, Japan
Rbsum6.- Le dbplacement isombrique de 5 7 ~ e dans les dichalcogi5nures de m6taw de transition de la sbrie 3d
,
qui ont la structure de pyrite, est analysb. L'effet de distance interatomique sur le db- placement isombrique est comparable 1 celui de 1'6lectron8gativitd du ligand.Abstract.- The isomer shifts of 5 7 ~ e in the 3d-transition metal dichalcogenides with the pyrite struc- ture are analysed. The effect of atomic distance makes a comparable contribution to the isomer shift with that of the electronegativity of ligands.
1
.
Introduction.- Mijssbauer effect of 5 7 ~ e in ionic crystals has been investigated precisely in connec- tion with bonding properties / l / . As for covalent crystals with divalent iron in low spin state, Tem- perley and Lefevre /2/ have reported a linear rela- tion between the isomer shift and the reciprocal volume of unitcell for the marcasite-type crystals.On the other hand, Kjekshus and Rakke /3/ have shown that the isomer shifts of some binary and quasi-bina- ry compounds with the marcasite structure have a linear relation with their unitcell volume and that the compounds can be classified into two groups ac- cording to the principal quantum number of ligands.
Recently, McCann and Ward /4/ have investigated the relation between the isomer shift and the unitcell volume for the pyrite-structure compounds. However, the correlation of the isomer shift with the volume is not so good. More systematic work is needed to discuss the isomer shift of low spin iron.
The covalent interaction between metal and chalcogen atoms plays an important role in the 3d- transition metal dichalcogenides having the pyrite structure /5/. The Mijssbauer effect of 5 7 ~ e doped in these materials reveals that an iron remains in a low spin state throughout the system 16-]l/. Thus, t k system of 3d-transition metal dichalcogenides
(m2:
M=Fe,Co,Ni and X=S,Se,Te) with the pyrite structure is one of the most suitable systems to study the electronic state of divalent iron in low spin state.2. Experimental results.- NiSe2, CoSe2, CO N i S2 l-Y Y doped with 0.5 % 5 7 ~ e and Fe,-xCoxS2 were prepared by the sintering method /5,9/. The Mijssbauer spec- trum at room temperature is a single quadrupole dou- blet for all the samples. Examples are shown in fi- gure 1 .
-1 0 1 2
VELOCITY (mm/s)
Fig. 1 : Gssbauer spectra of 5 7 ~ e in Fel-,CoXS2 at room temperature.
The least squares fitting was made to all the spec- tra. The centre of the spectrum in sulphides changes by 20.127 mm/s from FeS2 to NiSz. The change of the second-order Doppler shift calculated with the Debye temperature obtained from the specific heat measure- ment /12/ is smaller than 0.02 mmls. Therefore, the
increase in centre shift from FeS2 to NiSz is attri- buted to the variation of the isomer shift.
The isomer shifts are plotted against the
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19792103
C2-298 JOURNAL DE PHYS~QUE lattice constant in figure 2. The values for FeSe,
and FeTez are those measured by Bither et al. /15/.
Fig. 2 : Isomer shift of " ~ e in the pyrite-type compounds plotted against lattice constant.
The isomer shift for sulphides has a good linear re- lation with the lattice constant. The result shows that the increase in atomic distance leads to the decrease in the electron-density argumentation for- med by the donation of electrons from the ligand to metal. This effect seems to be important for the isomer shift of low spin iron. The difference among sulphides, selenides and tellurides is probably due to the difference in the electronegativity of li- gands. The isomer shifts in figure 2 are expressed by the following equation including the above two effects.
AEIeS. = 0.467a + 0 . 5 4 4 ~
-
3.564 (mm/s)P (1)
where a is the lattice constant in
W
andX
is the P Pauling electronegativity 1131. Solid lines in the figure represent the equation (I) withX
correspon-P ding to S, Se and Te.
3. Discussion and conclusion.- In high spin compounds the decrease in electron donation from electronega- tive ligand to metal through a a-interaction of che- mical Fond effectively reduce the electron density at the metal nucleus /1,14/. Thus, the isomer shift increases as the electronegativity of ligand increa- ses. In addition to 0-interaction, T-interaction is important in low spin compounds /14/. The T-interac- tion has a role to get rid of excess charges accumu- lated on the metal through U-interaction. The increa- se in T-interaction decreases p and d electrons at the metal and leads to an increase in s electrons at the metal nucleus. Therefore, as the electronegati- vity of ligands decreases, the s electron density at the nucleus increases and the isomer shift decreases.
Figure 3 shows the isomer shift plotted against the electronegativity of ligand for various divalent iron compounds.
Fig. 3 : Isomer shift plotted against Pauling elec- tronegativity for divalent iron compounds. Open cir- cles are corrected ones for the effect of atomic distance according to equation (1) in the text.
Contrary to above expectation, the xomer shift of low spin compounds rapidly increases with the decrea- se in the electronegativity. However, as is shown by open circles, the isomer shift increases as the elec- tronegativity increases when the contribution of atomic distance is taken away using equation (I). It is concluded that both the atomic distance and the electronegativity of ligand make comparable contribu- tions to the isomer shift of low spin iron. In high spin iron, the contribution of atomic distance is smaller than that of electronegativity.
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