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Chemical bonding in MeTe2, Me = Mn, Fe, Co, Ni
Volume 145, number 8,9
PHYSICS LETTERS A
30 April 1990
CHEMICAL BONDING IN MeTe2, Me=Mn, Fe, Co, Ni H. BINCZYCKA I, S.S. HAFNER, G. MOH ’ and J. STANEK ’ Institute of Mineralogy and Scientific Center of Materials Sciences, University of Marburg, 3550 Marburg, FRG
Received 9 November 1989; revised manuscript received 22 February 1990; accepted for publication 3 March 1990 Communicated by J.I. Budnick
A series of compounds with chemical formula MeTel (Me= 3d transition elements, Fe, Co, and Ni) were studied at 80 K using lz5Te Mijssbauer spectroscopy. The regular decrease of quadrupole splitting and increase of isomer shift with increasing atomic number of cations are interpreted in terms of charge transfer from metal to tellurium ions.
1. Introduction The element tellurium exhibits complicated and not well understood chemistry. Due to the electronic ground state configuration of 5s25p4, the nominal “valence state” of Te may vary from -2 to +6 . In this work, information on the effective electronic configuration of Te in 3d transition metal ditellurides was obtained from the 35.49 keV gamma ray resonant absorption of ‘25Te. MGssbauer spectroscopy of lzsTe is considered as a sensitive method for the investigation of chemical and dynamical properties of tellurium bearing compounds. However, the broad natural line width of 12’Teresonance (5.2 mm/s) and the requirement of holding the source at temperatures near 78 K or lower are reasons why in the past the measurements were not precise enough to find some tiny trends in hyperfine interaction parameters. As a consequence, the review of the literature on 125Te MSssbauer spectroscopy which counts about 700 positions, shows a frustrating artificial scattering of hyperfine parameters. In addition, the temperature dependences of hyperfine interactions and recoil free fractions,fl have been rarely studied in detail. However, some transition metal ditellurides had ’ Permanent address: Institute of Physics, Jagiellonian University, 30-059 Krakdw, Poland. * Permanent address: Mineralogisch-Petrographisches der Universitgt Heidelberg, 6900 Heidelberg, FRG.
been studied in the past [l-4] paying special attention to MnTe2 mainly because of its interesting magnetic properties at low temperatures. Recently, the application of a new source of 12’Te in Mg,TeOb [ 5 ] which, due to its unusually high Debye temperature can be operated at 300 K has opened new perspectives, as illustrated by this work.
2. Experimental For this study a source was prepared by activating lZSmTein 25 mg Mg,TeOb enriched in 124Teto 90% in a beam of 10’4/cm2s thermal neutrons for 20 days.The final (measured) source activity was about 150 mCi. The high Debye temperature, &,=350 K, of Mg,Te06 [ 61 yields a recoil-free fraction fs high enough for efficient resonant emission at 295 K: &x0.40 (at 4 KhzO.78). Using this source at 295 K, it was possible to record absorption MGssbauer spectra at 80 K in a simple gas flow cryostat. Telluride samples of stoichiometric composition were synthesized from high purity elements employing dry evacuated silica tube techniques. Ail charges were heated at elevated temperatures, quenched, reground and annealed at 300 ’ C for several weeks until complete equilibrium was obtained which was tested by X-ray diffraction.
Institut
467
PHYSICS LETTERS A
Volume 145, number 8,9
most regular dependencies found for any series of Te compounds.
3. Results
The spectra of FeTez, CoTeZ, and NiTez are shown in fig. 1. The solid lines present fits of Lorentzians assuming a symmetric quadrupole doublet in each case. The obtained isomer shifts, IS, quadrupole splittings, QS, and line widths, r, are collected in table 1. The IS values are given versus Mg,Te06 at 295 K. The gradual decrease of QS and increase of IS with increasing atomic number of cations are evident. It should be noted that this is one of the strongest and
0
1
0
8 1 P
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30 April 1990
0
4. Discussion The crystal structures of investigated compounds are different. NiTez has a hexagonal, CdI,-type structure, FeTe* and CoTeZ are orthorhombic with FeS2 (marcasite) structure, while MnTez, discussed here for comparison, is cubic [ 7 1. In all cases, however, the tellurium ions appear in the form of anion pairs. This is especially visible in the case of MnTe*. Therefore this compound may be described as being built up from Mn’+ cations and Te$- molecular ions. The surprisingly linear relationship between IS and QS (cf. fig. 2) calls for a simple interpretation. The electric field gradient at the Te nuclei within the TeZ molecular ion is produced by electron holes in the 5p shell. The decrease of QS with increase of atomic number from Mn to Ni results from a gradual charge transfer from the cations to Te, nearly completing the 5p shell in the case of NiTez. The increase in 5p shell population alone should lead to a decrease of s-electron density at the nucleus due to enhanced shielding. It was estimated [ 8 ] that one additional 5p electron causes a decrease of IS by 0.4 mm/s. For the compounds studied here, an increase of IS was observed with increasing population of 5p orbitals. This may be explained by direct population of the 5s shell. One 5s electron increases the
lB 0 2
4 -15
-lo
-5 VELOCITY
0
5
10
15
cntl/s)
Fig. 1. Mtissbauer spectra of 3d transition metals ditellurides measured at 80 K.
468
Fig. 2. QS-IS relationship for MeTe2 series, Me = Mn, Fe, Co, Ni.
PHYSICS LETTERS A
Volume 145, number 8,9
30 April 1990
Table 1 Hyperfine parameter, proposed Te electron configurations, effective charges of Te, cation formal effective charges and Te-Te distances within Tes molecule in investigated compounds. The errors of the measured IS and QS values are 0.03 mm/s. Compound
MnTel ‘) FeTer CoTe, NiTe*
IS (mm/s)
1.05 1.38 1.45 1.57
QS
l-
(mm/s)
(mm/s)
1.15 3.85 3.09 1.32
8.0 7.3 1.3 1.3
Tellurium electronic configuration
effective charge
5s’.635P5.‘5 5s1.*,5p>.67 5s’.s75P5.74 5s’.9’5Ps.90
0.98 1.50 1.61 1.85
Cation effective charge
Te-Te interatomic distance
+2 +3 +3 +4
2.14 2.93 2.93 3.45
a) Hyperline parameters taken from ref. [ 3 1.
IS by 2.4 mm/s [ 81. Thus, one can write the formal electron configuration of Te as 5~*-~5p~-~. In the case of MnTez the electron hole is in the pz orbital where z is the axis of the Te$- molecular ion [ 3 1. One missing 6p electron produces a positive V,, which corresponds to a quadrupole splitting of QS,= 12 mm/s [ 71. Thus, neglecting the depopulation of pXand py orbitals, for each compound studied, y=QS/QS,, where QS is the quadrupole splitting measured. For an estimate of the parameter x in the 5s2-“5p6-Y configuration of Te, the QS(IS) relation may be extrapolated down to QS= 0, i.e. to a hypothetical - 2 state 5s25p6. The resulting IS value is 1.66 mm/s. Then the measured IS can be expressed as IS,,= ISo- 2.4x+ 0.4~. The calculated electron configurations are collected in table 1 together with effective charges of Te and corresponding formal valence states of the cations. The postulated effective valence states are consistent with the crystal structure data. In the last column in table 1 the Te-Te interatomic distances within the Te2 molecular ion are presented. These data were taken from refs. [9], [lo] #I, [ 111, [ 121 for Mn, Fe, Co and Ni ditellurides, respectively. The increase in electron population of the tellurium 5s5p shell from MnTe2 to NiTe2 is well reproduced by the increase in the relevant Te-Te distances. It should be noted that our estimates confirm the - 1 state of Te in MnTe2, i.e. the Mn*+ state. However, the proposed +3 states for Fe and Co, as well as the + 4 state for Ni may be apparent on the basis of the present simplified interpretation. In FeTe2 iron a’ Calculated from atomic positions.
was described as being in low spin Fen state [ 13 ] or even in Fe’” state [ 131. However, the Fen state was concluded from a priori assumed - 1 valence state of Te comparable to MnTe2. In fact, the 57Fe Mossbauer spectrum of FeTe2 shows IS= 0.47 mm/s and QS=O.52 mm/s at 295 K, both values being typical for Fe3+ state. Previously, the high value of IS, i.e. the low electron density at Fe nuclei, was explained by strong delocalization of 3d electrons of the formal Fen configuration [ 15 1. In the case of NiTe2 the crystal structure may be used again as support for the proposed +4 state of nickel. NiTe2 belongs to a group of more than 50 compounds crystallizing in Cd12-type structure. These are mainly hydroxides, bromides, and iodides of divalent metallic ions as well as sulfides, selenides, and tellurides of tetravalent metallic ions [ 71. The simplified ionic conception used here may not be fully adequate in view of the expected substantial covalent participation. Moreover, only the Te-Te bonds were considered which are expected to be perturbed by covalent participation in the metal iontellurium bonds. In this connection, further study of ‘25Te resonance in 4d and 5d transition metal ditellurides may be useful.
References [ 1 ] A. Kjekshus and D.G. Nicholson, Acta Chem. Stand. 26 (1972) 3241. [2] Y. NishiharaandS. Ogawa, J. Phys. (Paris) 40 (1979) C2221. [ 31 M. Pastemak and A.L. Spikervet, Phys. Rev. I8 1 ( 1969) 574.
469
Volume 145, number 8,9
PHYSICS LETTERS A
[4] J.M. Hostings, L.M. Corliss, M. Blume and M. Pasternak, Phys. Rev. B 1 ( 1970) 3209. [5] H. Binczycka, S.S. Hafner, J. Stanek and M. Triimel, Phys. Lett. A 131 (1988) 135. (61 M. Pastemak, M. van Heyden and G. Langousche, Nucl. Instrum. Methods B 4 ( 1984) 152. [ 71 R.W.G. Wychoff, Crystal structures, Vol. 1 (Interscience, New York, 1963). [ 81 P. Dobud and C.M. Jones, J. Solid State Chem. 16 ( 1976) 201. [ 91Gmelins Handbuch der anorganischen Chemie (Springer, Berlin, 1976) p. 332.
470
30 April 1990
[ 101 S. Tanger, Z. Anorg. Allg. Chem. 239 (1938) 126. [ 111 H. Horaldsen, F. Gronvald and T. Hurlen, Anorg. Allg. Chem. 283 (1956) 143. [ 121 J. Borstad, F. Gronvald, E. Rost and E. Vestersjo, Acta Chem. &and. 20 (1966) 2865. [ 131 A.A. Temperly and H.W. Lefavre, J. Phys. Chem. Solids 27 (1966) 85. [ 141 N.N. Greenwood and T.C. Gibb, Mijssbauer spectroscopy (Chapman and Hall, London, 197 1) p. 284. [ 15) R. Ingals, F. van der Woude and G.A. Savatzky, in: MSssbauer isomer shifts, eds. G.K. Shenoy and F.E. Wagner (North-Holland, Amsterdam, 1978) p. 36 1.
PHYSICS LETTERS A
30 April 1990
CHEMICAL BONDING IN MeTe2, Me=Mn, Fe, Co, Ni H. BINCZYCKA I, S.S. HAFNER, G. MOH ’ and J. STANEK ’ Institute of Mineralogy and Scientific Center of Materials Sciences, University of Marburg, 3550 Marburg, FRG
Received 9 November 1989; revised manuscript received 22 February 1990; accepted for publication 3 March 1990 Communicated by J.I. Budnick
A series of compounds with chemical formula MeTel (Me= 3d transition elements, Fe, Co, and Ni) were studied at 80 K using lz5Te Mijssbauer spectroscopy. The regular decrease of quadrupole splitting and increase of isomer shift with increasing atomic number of cations are interpreted in terms of charge transfer from metal to tellurium ions.
1. Introduction The element tellurium exhibits complicated and not well understood chemistry. Due to the electronic ground state configuration of 5s25p4, the nominal “valence state” of Te may vary from -2 to +6 . In this work, information on the effective electronic configuration of Te in 3d transition metal ditellurides was obtained from the 35.49 keV gamma ray resonant absorption of ‘25Te. MGssbauer spectroscopy of lzsTe is considered as a sensitive method for the investigation of chemical and dynamical properties of tellurium bearing compounds. However, the broad natural line width of 12’Teresonance (5.2 mm/s) and the requirement of holding the source at temperatures near 78 K or lower are reasons why in the past the measurements were not precise enough to find some tiny trends in hyperfine interaction parameters. As a consequence, the review of the literature on 125Te MSssbauer spectroscopy which counts about 700 positions, shows a frustrating artificial scattering of hyperfine parameters. In addition, the temperature dependences of hyperfine interactions and recoil free fractions,fl have been rarely studied in detail. However, some transition metal ditellurides had ’ Permanent address: Institute of Physics, Jagiellonian University, 30-059 Krakdw, Poland. * Permanent address: Mineralogisch-Petrographisches der Universitgt Heidelberg, 6900 Heidelberg, FRG.
been studied in the past [l-4] paying special attention to MnTe2 mainly because of its interesting magnetic properties at low temperatures. Recently, the application of a new source of 12’Te in Mg,TeOb [ 5 ] which, due to its unusually high Debye temperature can be operated at 300 K has opened new perspectives, as illustrated by this work.
2. Experimental For this study a source was prepared by activating lZSmTein 25 mg Mg,TeOb enriched in 124Teto 90% in a beam of 10’4/cm2s thermal neutrons for 20 days.The final (measured) source activity was about 150 mCi. The high Debye temperature, &,=350 K, of Mg,Te06 [ 61 yields a recoil-free fraction fs high enough for efficient resonant emission at 295 K: &x0.40 (at 4 KhzO.78). Using this source at 295 K, it was possible to record absorption MGssbauer spectra at 80 K in a simple gas flow cryostat. Telluride samples of stoichiometric composition were synthesized from high purity elements employing dry evacuated silica tube techniques. Ail charges were heated at elevated temperatures, quenched, reground and annealed at 300 ’ C for several weeks until complete equilibrium was obtained which was tested by X-ray diffraction.
Institut
467
PHYSICS LETTERS A
Volume 145, number 8,9
most regular dependencies found for any series of Te compounds.
3. Results
The spectra of FeTez, CoTeZ, and NiTez are shown in fig. 1. The solid lines present fits of Lorentzians assuming a symmetric quadrupole doublet in each case. The obtained isomer shifts, IS, quadrupole splittings, QS, and line widths, r, are collected in table 1. The IS values are given versus Mg,Te06 at 295 K. The gradual decrease of QS and increase of IS with increasing atomic number of cations are evident. It should be noted that this is one of the strongest and
0
1
0
8 1 P
8
30 April 1990
0
4. Discussion The crystal structures of investigated compounds are different. NiTez has a hexagonal, CdI,-type structure, FeTe* and CoTeZ are orthorhombic with FeS2 (marcasite) structure, while MnTez, discussed here for comparison, is cubic [ 7 1. In all cases, however, the tellurium ions appear in the form of anion pairs. This is especially visible in the case of MnTe*. Therefore this compound may be described as being built up from Mn’+ cations and Te$- molecular ions. The surprisingly linear relationship between IS and QS (cf. fig. 2) calls for a simple interpretation. The electric field gradient at the Te nuclei within the TeZ molecular ion is produced by electron holes in the 5p shell. The decrease of QS with increase of atomic number from Mn to Ni results from a gradual charge transfer from the cations to Te, nearly completing the 5p shell in the case of NiTez. The increase in 5p shell population alone should lead to a decrease of s-electron density at the nucleus due to enhanced shielding. It was estimated [ 8 ] that one additional 5p electron causes a decrease of IS by 0.4 mm/s. For the compounds studied here, an increase of IS was observed with increasing population of 5p orbitals. This may be explained by direct population of the 5s shell. One 5s electron increases the
lB 0 2
4 -15
-lo
-5 VELOCITY
0
5
10
15
cntl/s)
Fig. 1. Mtissbauer spectra of 3d transition metals ditellurides measured at 80 K.
468
Fig. 2. QS-IS relationship for MeTe2 series, Me = Mn, Fe, Co, Ni.
PHYSICS LETTERS A
Volume 145, number 8,9
30 April 1990
Table 1 Hyperfine parameter, proposed Te electron configurations, effective charges of Te, cation formal effective charges and Te-Te distances within Tes molecule in investigated compounds. The errors of the measured IS and QS values are 0.03 mm/s. Compound
MnTel ‘) FeTer CoTe, NiTe*
IS (mm/s)
1.05 1.38 1.45 1.57
QS
l-
(mm/s)
(mm/s)
1.15 3.85 3.09 1.32
8.0 7.3 1.3 1.3
Tellurium electronic configuration
effective charge
5s’.635P5.‘5 5s1.*,5p>.67 5s’.s75P5.74 5s’.9’5Ps.90
0.98 1.50 1.61 1.85
Cation effective charge
Te-Te interatomic distance
+2 +3 +3 +4
2.14 2.93 2.93 3.45
a) Hyperline parameters taken from ref. [ 3 1.
IS by 2.4 mm/s [ 81. Thus, one can write the formal electron configuration of Te as 5~*-~5p~-~. In the case of MnTez the electron hole is in the pz orbital where z is the axis of the Te$- molecular ion [ 3 1. One missing 6p electron produces a positive V,, which corresponds to a quadrupole splitting of QS,= 12 mm/s [ 71. Thus, neglecting the depopulation of pXand py orbitals, for each compound studied, y=QS/QS,, where QS is the quadrupole splitting measured. For an estimate of the parameter x in the 5s2-“5p6-Y configuration of Te, the QS(IS) relation may be extrapolated down to QS= 0, i.e. to a hypothetical - 2 state 5s25p6. The resulting IS value is 1.66 mm/s. Then the measured IS can be expressed as IS,,= ISo- 2.4x+ 0.4~. The calculated electron configurations are collected in table 1 together with effective charges of Te and corresponding formal valence states of the cations. The postulated effective valence states are consistent with the crystal structure data. In the last column in table 1 the Te-Te interatomic distances within the Te2 molecular ion are presented. These data were taken from refs. [9], [lo] #I, [ 111, [ 121 for Mn, Fe, Co and Ni ditellurides, respectively. The increase in electron population of the tellurium 5s5p shell from MnTe2 to NiTe2 is well reproduced by the increase in the relevant Te-Te distances. It should be noted that our estimates confirm the - 1 state of Te in MnTe2, i.e. the Mn*+ state. However, the proposed +3 states for Fe and Co, as well as the + 4 state for Ni may be apparent on the basis of the present simplified interpretation. In FeTe2 iron a’ Calculated from atomic positions.
was described as being in low spin Fen state [ 13 ] or even in Fe’” state [ 131. However, the Fen state was concluded from a priori assumed - 1 valence state of Te comparable to MnTe2. In fact, the 57Fe Mossbauer spectrum of FeTe2 shows IS= 0.47 mm/s and QS=O.52 mm/s at 295 K, both values being typical for Fe3+ state. Previously, the high value of IS, i.e. the low electron density at Fe nuclei, was explained by strong delocalization of 3d electrons of the formal Fen configuration [ 15 1. In the case of NiTe2 the crystal structure may be used again as support for the proposed +4 state of nickel. NiTe2 belongs to a group of more than 50 compounds crystallizing in Cd12-type structure. These are mainly hydroxides, bromides, and iodides of divalent metallic ions as well as sulfides, selenides, and tellurides of tetravalent metallic ions [ 71. The simplified ionic conception used here may not be fully adequate in view of the expected substantial covalent participation. Moreover, only the Te-Te bonds were considered which are expected to be perturbed by covalent participation in the metal iontellurium bonds. In this connection, further study of ‘25Te resonance in 4d and 5d transition metal ditellurides may be useful.
References [ 1 ] A. Kjekshus and D.G. Nicholson, Acta Chem. Stand. 26 (1972) 3241. [2] Y. NishiharaandS. Ogawa, J. Phys. (Paris) 40 (1979) C2221. [ 31 M. Pastemak and A.L. Spikervet, Phys. Rev. I8 1 ( 1969) 574.
469
Volume 145, number 8,9
PHYSICS LETTERS A
[4] J.M. Hostings, L.M. Corliss, M. Blume and M. Pasternak, Phys. Rev. B 1 ( 1970) 3209. [5] H. Binczycka, S.S. Hafner, J. Stanek and M. Triimel, Phys. Lett. A 131 (1988) 135. (61 M. Pastemak, M. van Heyden and G. Langousche, Nucl. Instrum. Methods B 4 ( 1984) 152. [ 71 R.W.G. Wychoff, Crystal structures, Vol. 1 (Interscience, New York, 1963). [ 81 P. Dobud and C.M. Jones, J. Solid State Chem. 16 ( 1976) 201. [ 91Gmelins Handbuch der anorganischen Chemie (Springer, Berlin, 1976) p. 332.
470
30 April 1990
[ 101 S. Tanger, Z. Anorg. Allg. Chem. 239 (1938) 126. [ 111 H. Horaldsen, F. Gronvald and T. Hurlen, Anorg. Allg. Chem. 283 (1956) 143. [ 121 J. Borstad, F. Gronvald, E. Rost and E. Vestersjo, Acta Chem. &and. 20 (1966) 2865. [ 131 A.A. Temperly and H.W. Lefavre, J. Phys. Chem. Solids 27 (1966) 85. [ 141 N.N. Greenwood and T.C. Gibb, Mijssbauer spectroscopy (Chapman and Hall, London, 197 1) p. 284. [ 15) R. Ingals, F. van der Woude and G.A. Savatzky, in: MSssbauer isomer shifts, eds. G.K. Shenoy and F.E. Wagner (North-Holland, Amsterdam, 1978) p. 36 1.