From 57Fe Mössbauer spectrum of FeTe2 taken in high external magnetic field (B = 4.6 T), the sign of electric field gradient was determined as negative, with an asymmetry parameter of 0.2. A comparison of these data with results of calculation of the electric field gradient within point charge model suggests the lattice character of electric field gradient with some contribution from covalency effects. The effective magnetic field acting on 57Fe is less than the applied, which points out the diamagnetic Fe+2 state of iron. This "electronic state" is in a contradiction to Fe+3 "charge state" concluded from 125Te experiments on 3d transition metal ditellurides as well as from 57Fe quadrupole splitting and isomer shift

Fornal, Piotr

Stanek, Jan

Nukleonika

English

Stanek, J.

Fornal, P.

Biblioteka Nauki - repozytorium artykuÅÃ³w

Local electronic and charge state of iron in FeTe2

Jagiellonian Univeristy Repository

NUKLEONIKA 2004;49(Supplement 3):S63−S65PROCEEDINGSIntroductionA theoretical and experimental study of the relationsbetween crystal structures and local electronic states oftheir elemental constituents is a key for understandingmany basic problems of solid state physics. Althoughpresent numerical methods are able to reproduce, in somecases, the electronic properties of broad classes of com-pounds with known structure, the experiments which aresensitive to local electronic states are necessary for testifyingassumptions applied in the calculation.The present work is a continuation of the study of aninteresting group of materials, 3d transition metal ditellu-rides, which exhibit gradual changes in crystal structurefrom pyrite type, through marcasite to CdI2-type withincreasing number of 3d electrons of cations. PreviousMössbauer experiments on 125Te [1, 5] supplied the dataon charge states of Te which determine as well the chargestates of metals. This charge state of metal may be directlychecked only in case of FeTe2. However, by substitutionaldoping 57Fe into other ditellurides it appears feasible toobserve how the crystal structure forces the charge andelectronic states of this probe [4].In this paper we concentrate on valence and chargestates of iron in FeTe2. The problem is not trivial. Themagnetic measurements clearly show the diamagnetic stateof iron, with formal low spin 3d6 configuration (S = 0).This was, at first look, in a contradiction with the 125TeMössbauer investigations as described in Discussion.Experimental and resultsThe crystal structure of FeTe2 is orthorhombic marcasitetype (Pnnm). Each tellurium atom has one Te atom as thenearest neighbour at a distance of 0.293 nm, iron is octa-Local electronic and charge stateof iron in FeTe2Jan Stanek,Piotr FornalJ. StanekM. Smoluchowski Institute of Physics,Jagiellonian University,4 Reymonta Str., 30-059 Kraków, Poland,Tel.: +48 12/ 633 63 77 ext. 5537, Fax: +48 12/ 633 70 86,e-mail: jstanek@if.uj.edu.plP. FornalInstitute of Physics,Cracow University of Technology,1 Podchorążych Str., 30-083 Kraków, PolandReceived: 28 June 2004,  Accepted: 20 September 2004Abstract From 57Fe Mössbauer spectrum of FeTe2 taken in high external magnetic field (B = 4.6 T), the sign of electric fieldgradient was determined as negative, with an asymmetry parameter of 0.2. A comparison of these data with results of calculationof the electric field gradient within point charge model suggests the lattice character of electric field gradient with somecontribution from covalency effects. The effective magnetic field acting on 57Fe is less than the applied, which points out thediamagnetic Fe+2 state of iron. This “electronic state” is in a contradiction to Fe+3 “charge state” concluded from 125Teexperiments on 3d transition metal ditellurides as well as from 57Fe quadrupole splitting and isomer shift.Key words Mössbauer •  chemical bonding •  charge state •  telluridesJ. Stanek, P. FornalS64hedrally coordinated by Te atoms. The distorted Te6octahedron is compressed, which leads to quasi-linear coor-dination of Fe. The details of  the local surrounding of Feare shown in Fig. 1. In general, the structure of FeTe2 maybe described as formed of  Fe cations and weakly boundedTe2 molecular anions.Sample of FeTe2 was synthesised from high purityelements employing dry evacuated silica tube technique.The specimen was heated, quenched, reground and annealedat 400°C several times until complete, single phaseequilibrium, tested by X-ray diffraction, was obtained. Thedetails of the preparation are given in [1]. The 57FeMössbauer spectra were recorded at a temperaturebetween 293 K and 4.2 K. In addition, at 4.2 K the spectrumwas recorded in an external magnetic field of 4.6 T, parallelto gamma radiation.The spectra taken without magnetic field are symmetricquadrupole doublets with quadrupole splitting, ∆ = 0.52mm/s, practically invariant between room temperature and4.2 K and isomer shift, δ = 0.47 mm/s at room temperature,see Fig. 2. The spectrum taken in external field, shown aswell in Fig. 2, was numerically evaluated using the Gabriel-Ruby procedure. The fit gives negative sign of the electricfield gradient tensor, Vzz < 0, which is obvious from visualinspection of the spectrum, and asymmetry parameter η= 0.2(1). Moreover, the fitted effective magnetic field wasless by 2% than the applied one.DiscussionFrom the previous 125Te experiments on MeTe2 series, (Me= Mn, Cr, Fe, Co, Ni) a linear correlation between ∆ and δwas found. The isomer shift may be expressed by deficit aand b in 5s and 5p orbitals, respectively, as: δ = δ0 − 2.4a +0.4b [2] where δ0 = 1.66 mm/s is the isomer shift in thehypothetical Te−2 anion with 5s25p6 configuration. In addi-tions, the quadrupole splitting is: ∆ = b∆0 , where ∆0 =12 mm/s is the quadrupole splitting produced by oneelectron hole in the 5pz orbital [3]. These relations allowedthe determination of electron configuration of Te in formof 5s2−a 5p6−b in this group of materials and lead to theeffective charges of Te: −0.98, −1.12, −1.50, −1.61 and −1.96for MnTe2, CrTe2, FeTe2, CoTe2 and NiTe2, respectively.Consequently, the charge states of cations were determinedas well. In particular, the Fe charge state of +3 waspredicted for FeTe2.Indeed, the ∆ and δ values of 57Fe in FeTe2 are typicalfor high spin Fe+3 state. Moreover, for such iron state theelectric field gradient (efg) has only lattice origin, beingweakly temperature dependent, which is confirmed byexperiment. To study this problem more accurately, thelattice efg was calculated in the point charge model.Certainly, such approach is a significant approximation andcannot give reliable values of quadrupole splitting but, atleast, the sign and asymmetry parameter of efg are reliable.The calculations were carried out considering the spheresurrounding Fe atom with the radius up to 2 nm; it turnedout that the values of efg are determined by charges placedup to 1.5 nm from Fe atom, see Fig. 3.The results of calculations are interesting. The sign ofVzz is negative, as expected from the quasi linear coordi-nation of Fe. However, the difference in distances betweenlinear and planar tellurium neighbours is small (0.005 nm)and the efg coming from Te6 non axially distorted octa-hedron is positive. The next neighbours of the Te atoms inthis structure change the sign of efg again to the negativeone. The final asymmetry parameter is η = 0.5, significantlyhigher than the experimental one, η = 0.2(1). This differencepoints out some covalency effects: the covalent bonding ofFig. 1. The octahedral coordination of Fe in FeTe2 unit cell. Ironhas two linear nearest and four planar next-nearest telluriumneighbours. The distances are in Å. The octahedron is distorted:the marked angles are: α = 88.1°, β = 96.0°.Fig. 2. 57Fe Mössbauer spectra of FeTe2 taken at 297 K (above)and at 4.2 K in external magnetic field B = 4.6 T (below). Solidline is the fit.Local electronic and charge state of iron in FeTe2 S65Fe with two nearest Te ligands increases axial electrondensity and reduces the asymmetry of the lattice efg.Summarising, the 125Te and 57Fe measurements givea consistent picture of the charge transfer from Fe to Te inFeTe2. The crystal is built from (Te2)−3 molecules and Fe+3ions with some residual Te-Fe-Te covalent bonds. However,from the text books of inorganic chemistry it is well knownthat Fe in FeTe2 is diamagnetic, which leads to Fe+2 (lowspin, S = 0) state. Indeed, the spectrum taken in externalmagnetic field shows that the effective field at 57Fe nucleiis reduced in comparison with the applied field thatconfirms the S = 0 state of iron.In fact, this is only an apparent discrepancy of linguisticcharacter. Three lowest electronic levels of Fe are occupiedby paired electrons which give the spin S = 0. However,these levels are only partly filled, there is a significantcharge transfer from iron to tellurium. In consequence, thelocalised charge is such as in typical Fe+3 state.SummaryThis simple minded work points out that the terms“electronic state”, “charge state” and “valence state” shouldnot be used equivalently. In the case studied, iron in FeTe2has low spin “electronic state” with nominal 3d6 configur-ation. Such configuration means only that electronic levelsare populated by paired electrons. However, if the chargelocalised on Fe is considered, due to the significant electronflow from Fe to Te, directly measured by 125Te resonancethe “charge state” of Fe should be described as Fe+3 whichis in agreement with the observed isomer shift of 57Fe. Inother words, in FeTe2 the low spin Fe+2 orbitals are usedto form +3 charge state of iron; its electronic configurationmay be written as 3(d6)x, x = 5/6.References  1. Binczycka H, Hafner SS, Moh G, Stanek J (1990) Chemicalbonding in MeTe2, Me=Mn,Fe,Co,Ni. Phys Lett 145:467−469  2. Cheyne BM, Johnstone JJ, Jones CH (1972) An inter-pretation of 125Te Mössbauer shifts and quadrupole splittingsin some tellurium compounds. Chem Phys Lett 14:545−548  3. Qaim SM (1967) Mössbauer effect of 57Fe in various hosts:isomer shifts of the 14.4 keV gamma line of 57Fe in differentmetallic lattices. Proc Phys Soc 90:1065−1076  4. Stanek J, Hafner SS, Fornal P (2001) Metal-tellurium bondsin 3d-transition metal ditellurides. Hyperfine Interact C5:355−358  5. Stanek J, Khasanov AM, Hafner SS (1992) Electronic statesof tellurium in 3d-transition-metal ditellurides. Phys Rev B45:56−60Fig. 3. Components of electric field gradient tensor (Vxx, Vyy, Vzz)in principal axes system calculated in point charge approximationvs. radius of the sphere surrounding Fe included for calculations.

Local electronic and charge state of iron in FeTe_{2}

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Local electronic and charge state of iron in FeTe_{2}

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Local electronic and charge state of iron in FeTe_{2}

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