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Verwey transition in Eu4As3
338
of Magnetism and Magnetic Materials 54-57 (1986) 33X-340
Journal
VERWEY
TRANSITION
G. WORTMANN, lk~rl~lulftir
Atom
IN Eu,As,
E.V. SAMPATHKUMARAN und Festkijrperph.vtk.
* and G. KAINDL
Freie UmoersrtBt Berlin, Arnintdlee
14. D 1000 Berlin 33, German?
The mixed-valent compound Eu,As, is studied by “’ Eu Miissbauer spectroscopy in the temperature range from 4.2 to 575 K. The Mossbauer spectra are characterized by distinct Eu’+ and Eu’+ resonances below = 340 K and a single intermedtate resonance at higher temperatures. The data indicate the existence of a Verwey transition at about 340 K.
Mixed-valent rare-earth (RE) compounds exhibiting a Verwey transition are of considerable interest with respect to their electronic structure [l]. Above a certain temperature, rv, the RE ions, which occupy crystallographically equivalent sites in such compounds, fluctuate between two valence states due to electron-hopping processes; below T,, however, they are characterized by charge ordering, which may cause crystallographic distortions. Such charge-ordering transitions are well established for Eu,S, and Sm,S, [2-71, and it is believed that they may even occur in EuPd,B [8] as well as in non-stoichiometric europium bromide [9]. In this contribution. we report on the results of “‘EuMossbauer measurements over a wide temperature interval on Eu,As,. a compound crystallziing in the anti-Th,P, structure [lo]. The results strongly suggest the existence of charge ordering in this system at temperatures below T, - 340 K. The present work on Eu,As, was motivated by Hulliger’s observation of a crystallographic distortion from the cubic Th,P, structure [lo] followed by the speculation of Nagarajan et al. on the possible existence of charge ordering below a relatively high temperature [ll]. In consistency with the ionic chemical formula Eu:+EuitAs3, the ratio of intensities of the Eu*’ and Eu3+ Mossbauer lines was found to be close to 3 at 300 K [ll]. Subsequently, Lorenz gave an estimate of TV = 520 K on the basis of Madelung energy calculations [ 121. which is considerably higher than the experimental result of TV = 340 K obtained in the present work. The studied sample was prepared by melting stoichiometric amounts of the constituent elements in a sealed MO crucible at about 1700°C. DebyeeScherrer analysis revealed the presence of an unidentified second phase. The Mossbauer measurements with the 21.6 keV resonance of 15’Eu were carried out in the temperature range from 4.2 to 575 K using a lS’SmF, single-line source at room temperature. Due to its sensitivity to air. the sample was handled in purified argon atmosphere. For with
the
high
a vacuum
temperature of 10-e
studies Torr
a high-vacuum
Fig. 1 shows some representative Mossbauer absorption spectra of Eu,As, at various temperatures. The results obtained at 300 and 77 K are in good agreement with those previously reported [ll]. Below 340 K. the spectra exhibit essentially two resonance features, one around zero Doppler velocity (from Eu3+) and a second relatively broad peak around - 11 mm/s (from Eu*’ ).
J VELOCITY
at various temperaFtg. 1. “‘Eu Mossbauer spectra of Eu,As, tures. The results of the least-squares fit analysis are included. Long-dashed: divalent component; dash-dotted: trivalent component; for T > T,, dotted: mixed-valent relaxation subspectrum: short-dashed: divalent impurity phase.
was employed.
* On leave of absence from Tata Institute of Fundamental Research. Homi Bhabha Road. Bombay-400005. India.
0304-8853/86/$03.50
0 Elsevier
Science
lmmlsl
oven
Publishers
B.V.
A least-squares fit of the data with a superposition of Lorentzian lines clearly shows that this broad resonance consists of two absorption lines, which - at 77 K - are at ~ 10.3 mm/s (main resonance) and at - 12.3 mm/s (weak satellite), respectively. Above 340 K. the two absorption lines at - 10 mm/s and at 0 mm/s. assigned to Eu,As, because of their intensity ratio 3: 1. have collapsed into a single line at an intermediate position at - 8.5 mm/s, while the position of the line at - 12 mm/s remains essentially unchanged. This extra line is assigned to an additional phase of a stable divalent Eu compound in the studied sample (see below). The very faint absorption visible in the spectra above 340 K around +l mm/s is attributed to minute oxidation of the sample. The linewidths of the absorption peaks assigned to Eu,As, are rather constant below 300 K. then sudden increase. before a smooth decrease is observed in the collapsed line above 340 K. Fig. 2 summarizes the results of the fit analysis for both isomer shift S and linewidth W. The experimental observations are interpreted in terms of a thermally driven valence transition in Eu,As, at 340 K, quite similar to the one previously reported for Eu,S, [2-61. The compound is in a charge-ordered state in the temperature range below 340 K giving rise to two separate Mossbauer absorption lines from Eu” and Eu3+ ions, respectively. On approaching the charge ordering temperature at TV = 340 K, slow electron hopping processes start to occur. At temperatures above 340 K, the Eu ions fluctuate randomly in their valence by fast electron hopping. This is evidenced by the linewidth of the common absorption line. which decreases with temperature in a way typical for a fast and temperature-activated relaxation between two ionic states of Eu [2]. It should be stressed that the observed collapse of 22i ..
2- ,,__---o-----__y_____ O1
3_k_ 1 T,=‘3LOK
’
-
9
I ,m-a---_o----_~ I
2’
rn”
~_______.,____--_r--r’ ,2nd phase -;:----__~~______~__~~_~----*-___.3_ J-
_ _+
38
‘-------‘--____*-*$
F3L
2’
%.
.‘.$
:
-.E 30 3 26
o____---*-___3_+_~~~ I
1
100
1
mv . . . .
0 I
I
200 300 100 TEMPERATURE i K)
I
I
500
Fig. 2. Isomer shift S and the linewidth W (fwhm) as a function of temperature for the subspectra of the “‘ELIMiissbauer resonance of Eu,As,.
the Eu,As, resonance lines around 340 K cannot be described as the transition between slow and fast relaxation, where the characteristic Miissbauer measuring time 7 M=A/AS=10~ys,withAS=S(3+)-.S(2+).isequal to 7,. the inverse electron-hopping frequency [2.3]. Such a transition is observed in Eu,S, well above TV. where all features of the Mossbauer spectra can be described by a constant electron-hopping activation energy E., [2]. In the present case of Eu,As,, analysis of the data of fig. 2 yields rC > lOmy s below 340 K and r, < lo- “’ s above 340 K. This points to a sudden increase of E, at this transition temperature, in accordance with the observations in Eu,S,. where a drop of E, at TV is observed [4]. There seems to be one major difference, however. in the nature of the charge ordering transitions of Eu,As~ and the previously studied Eu,S,. In Eu,S,, the isomer shift of the intermediate absorption line above TV (= 186 K for Eu,S,) is consistent with the weighted mean value expected from the chemical formula Euf ’ Eu:+ S, [2], while this is not so for Eu,As,. In this latter case, the weighted mean value of S known from the present low-temperature data in the charge-ordered regime as well as from the chemical formula is -7.7 mm/s. whereas a value of S = -8.5 mm/s is observed for temperatures above 340 K. The deviation from the expectation of a simple model makes Eu,As, a highly interesting compound. Also, the experimentally observed value of TV is significantly different from that derived by Lorenz [12]. These discrepancies could be due to significant changes in the electronic structure of this compound at the charge-ordering transition. Further experimental work, particularly on a single-phase sample, would be highly desirable. Both Eu sites in Eu,As, order magnetically below Tc = 20 K (see fig. 1). The magnetic hyperfine fields at 4.2 K are found to be BClf(EuZ+)= 240 kOe and B,,,(Eu”) = 230 kOe. In comparison to the Eu3S, case. B,,, at the trivalent site is surprisingly large; in Eu,S,, &(Eu’+) was found to be only = 50 kOe [5.6]. In addition, a considerably stronger magnetic exchange in Eu,As, as compared to Eu,S, is revealed from a comparison of the magnetic ordering temperatures: 7; = 3.1 K was previously observed for Eu,S, [6]. We should finally comment on the additional Mossbauer absorption line at = - 12 mm/s. which does not move with temperature apart from the expected thermal redshift. It is presumably due to Eu,As> phase in the sample, as suggested both by the observed isomer shift and the magnetic ordering temperature (q. = 18 K) [lo]. This work was supported by the Sonderforschungsbereichof the Deutsche Forschungsgemeinschaft. [I] J.B. Goodenough. Brown
therein.
(Reidel.
in: Mixed-Valence Compounds. ed. D.B. Dordrecht. 1980) p. 413. and references
12) 0. Herkoo~. M. Malamud and S. Shtrikman. Solid State C‘ommun. 6 (196X) 1x5. [3] J. Riihlrr and G. Kaindl. Solid State Common. 36 (IWO) 1055. [4] R. Pott. G. Guntherodt. W. Wichclhau\. M. Ohl and H. Bach, in: Valence Instabilities. eds. P. Wachtcr and 11. Hoppart (North-Holland, Amsterdam. lYX2) p. 565. [5] E. Giirlvzh. H.U. Hrynkiewcr. R. Kmicc. K. Lathn and K. Tomala. Phys. Stat. Sol. (h) 64 (1974) K147. [h] 0. Mawact. J.M.D. Coey and F. Holtzherg, J. dc Phy\. (‘4 (1976) 297.
171 I. Morkc. G I’ravagllm and I’. Wxhter. 111rcl’. 14). p. 571. [X] S.K. MaIrk. R. NagaraJan. SK. Malik. R. ViJaynragha\an. M.M. Ahd-1:lmcguid and H. Micklitr. Phvh. KC\. 2YH (19X4) 5953. IO6 [Y] S.J. I.yle and WA. Westall. J. I. e\\ C’cmmon Metal\. (lYX5) 109. [IO/ F. Hulllger. Mat. Reh. Hull. 14 (1979) 37. [I I] R. NagaraJan. EV. S~lmpathkutllaran. R. VIJ+yaragh:l\an ,rnd Hhaktdur\han. Phw. Stat. Sol. (a) 75 (19X3) K14Y. [I?] Il. L.oruv. Phys. Stat. Sol. (h) 125 (19X4) 375.
of Magnetism and Magnetic Materials 54-57 (1986) 33X-340
Journal
VERWEY
TRANSITION
G. WORTMANN, lk~rl~lulftir
Atom
IN Eu,As,
E.V. SAMPATHKUMARAN und Festkijrperph.vtk.
* and G. KAINDL
Freie UmoersrtBt Berlin, Arnintdlee
14. D 1000 Berlin 33, German?
The mixed-valent compound Eu,As, is studied by “’ Eu Miissbauer spectroscopy in the temperature range from 4.2 to 575 K. The Mossbauer spectra are characterized by distinct Eu’+ and Eu’+ resonances below = 340 K and a single intermedtate resonance at higher temperatures. The data indicate the existence of a Verwey transition at about 340 K.
Mixed-valent rare-earth (RE) compounds exhibiting a Verwey transition are of considerable interest with respect to their electronic structure [l]. Above a certain temperature, rv, the RE ions, which occupy crystallographically equivalent sites in such compounds, fluctuate between two valence states due to electron-hopping processes; below T,, however, they are characterized by charge ordering, which may cause crystallographic distortions. Such charge-ordering transitions are well established for Eu,S, and Sm,S, [2-71, and it is believed that they may even occur in EuPd,B [8] as well as in non-stoichiometric europium bromide [9]. In this contribution. we report on the results of “‘EuMossbauer measurements over a wide temperature interval on Eu,As,. a compound crystallziing in the anti-Th,P, structure [lo]. The results strongly suggest the existence of charge ordering in this system at temperatures below T, - 340 K. The present work on Eu,As, was motivated by Hulliger’s observation of a crystallographic distortion from the cubic Th,P, structure [lo] followed by the speculation of Nagarajan et al. on the possible existence of charge ordering below a relatively high temperature [ll]. In consistency with the ionic chemical formula Eu:+EuitAs3, the ratio of intensities of the Eu*’ and Eu3+ Mossbauer lines was found to be close to 3 at 300 K [ll]. Subsequently, Lorenz gave an estimate of TV = 520 K on the basis of Madelung energy calculations [ 121. which is considerably higher than the experimental result of TV = 340 K obtained in the present work. The studied sample was prepared by melting stoichiometric amounts of the constituent elements in a sealed MO crucible at about 1700°C. DebyeeScherrer analysis revealed the presence of an unidentified second phase. The Mossbauer measurements with the 21.6 keV resonance of 15’Eu were carried out in the temperature range from 4.2 to 575 K using a lS’SmF, single-line source at room temperature. Due to its sensitivity to air. the sample was handled in purified argon atmosphere. For with
the
high
a vacuum
temperature of 10-e
studies Torr
a high-vacuum
Fig. 1 shows some representative Mossbauer absorption spectra of Eu,As, at various temperatures. The results obtained at 300 and 77 K are in good agreement with those previously reported [ll]. Below 340 K. the spectra exhibit essentially two resonance features, one around zero Doppler velocity (from Eu3+) and a second relatively broad peak around - 11 mm/s (from Eu*’ ).
J VELOCITY
at various temperaFtg. 1. “‘Eu Mossbauer spectra of Eu,As, tures. The results of the least-squares fit analysis are included. Long-dashed: divalent component; dash-dotted: trivalent component; for T > T,, dotted: mixed-valent relaxation subspectrum: short-dashed: divalent impurity phase.
was employed.
* On leave of absence from Tata Institute of Fundamental Research. Homi Bhabha Road. Bombay-400005. India.
0304-8853/86/$03.50
0 Elsevier
Science
lmmlsl
oven
Publishers
B.V.
A least-squares fit of the data with a superposition of Lorentzian lines clearly shows that this broad resonance consists of two absorption lines, which - at 77 K - are at ~ 10.3 mm/s (main resonance) and at - 12.3 mm/s (weak satellite), respectively. Above 340 K. the two absorption lines at - 10 mm/s and at 0 mm/s. assigned to Eu,As, because of their intensity ratio 3: 1. have collapsed into a single line at an intermediate position at - 8.5 mm/s, while the position of the line at - 12 mm/s remains essentially unchanged. This extra line is assigned to an additional phase of a stable divalent Eu compound in the studied sample (see below). The very faint absorption visible in the spectra above 340 K around +l mm/s is attributed to minute oxidation of the sample. The linewidths of the absorption peaks assigned to Eu,As, are rather constant below 300 K. then sudden increase. before a smooth decrease is observed in the collapsed line above 340 K. Fig. 2 summarizes the results of the fit analysis for both isomer shift S and linewidth W. The experimental observations are interpreted in terms of a thermally driven valence transition in Eu,As, at 340 K, quite similar to the one previously reported for Eu,S, [2-61. The compound is in a charge-ordered state in the temperature range below 340 K giving rise to two separate Mossbauer absorption lines from Eu” and Eu3+ ions, respectively. On approaching the charge ordering temperature at TV = 340 K, slow electron hopping processes start to occur. At temperatures above 340 K, the Eu ions fluctuate randomly in their valence by fast electron hopping. This is evidenced by the linewidth of the common absorption line. which decreases with temperature in a way typical for a fast and temperature-activated relaxation between two ionic states of Eu [2]. It should be stressed that the observed collapse of 22i ..
2- ,,__---o-----__y_____ O1
3_k_ 1 T,=‘3LOK
’
-
9
I ,m-a---_o----_~ I
2’
rn”
~_______.,____--_r--r’ ,2nd phase -;:----__~~______~__~~_~----*-___.3_ J-
_ _+
38
‘-------‘--____*-*$
F3L
2’
%.
.‘.$
:
-.E 30 3 26
o____---*-___3_+_~~~ I
1
100
1
mv . . . .
0 I
I
200 300 100 TEMPERATURE i K)
I
I
500
Fig. 2. Isomer shift S and the linewidth W (fwhm) as a function of temperature for the subspectra of the “‘ELIMiissbauer resonance of Eu,As,.
the Eu,As, resonance lines around 340 K cannot be described as the transition between slow and fast relaxation, where the characteristic Miissbauer measuring time 7 M=A/AS=10~ys,withAS=S(3+)-.S(2+).isequal to 7,. the inverse electron-hopping frequency [2.3]. Such a transition is observed in Eu,S, well above TV. where all features of the Mossbauer spectra can be described by a constant electron-hopping activation energy E., [2]. In the present case of Eu,As,, analysis of the data of fig. 2 yields rC > lOmy s below 340 K and r, < lo- “’ s above 340 K. This points to a sudden increase of E, at this transition temperature, in accordance with the observations in Eu,S,. where a drop of E, at TV is observed [4]. There seems to be one major difference, however. in the nature of the charge ordering transitions of Eu,As~ and the previously studied Eu,S,. In Eu,S,, the isomer shift of the intermediate absorption line above TV (= 186 K for Eu,S,) is consistent with the weighted mean value expected from the chemical formula Euf ’ Eu:+ S, [2], while this is not so for Eu,As,. In this latter case, the weighted mean value of S known from the present low-temperature data in the charge-ordered regime as well as from the chemical formula is -7.7 mm/s. whereas a value of S = -8.5 mm/s is observed for temperatures above 340 K. The deviation from the expectation of a simple model makes Eu,As, a highly interesting compound. Also, the experimentally observed value of TV is significantly different from that derived by Lorenz [12]. These discrepancies could be due to significant changes in the electronic structure of this compound at the charge-ordering transition. Further experimental work, particularly on a single-phase sample, would be highly desirable. Both Eu sites in Eu,As, order magnetically below Tc = 20 K (see fig. 1). The magnetic hyperfine fields at 4.2 K are found to be BClf(EuZ+)= 240 kOe and B,,,(Eu”) = 230 kOe. In comparison to the Eu3S, case. B,,, at the trivalent site is surprisingly large; in Eu,S,, &(Eu’+) was found to be only = 50 kOe [5.6]. In addition, a considerably stronger magnetic exchange in Eu,As, as compared to Eu,S, is revealed from a comparison of the magnetic ordering temperatures: 7; = 3.1 K was previously observed for Eu,S, [6]. We should finally comment on the additional Mossbauer absorption line at = - 12 mm/s. which does not move with temperature apart from the expected thermal redshift. It is presumably due to Eu,As> phase in the sample, as suggested both by the observed isomer shift and the magnetic ordering temperature (q. = 18 K) [lo]. This work was supported by the Sonderforschungsbereichof the Deutsche Forschungsgemeinschaft. [I] J.B. Goodenough. Brown
therein.
(Reidel.
in: Mixed-Valence Compounds. ed. D.B. Dordrecht. 1980) p. 413. and references
12) 0. Herkoo~. M. Malamud and S. Shtrikman. Solid State C‘ommun. 6 (196X) 1x5. [3] J. Riihlrr and G. Kaindl. Solid State Common. 36 (IWO) 1055. [4] R. Pott. G. Guntherodt. W. Wichclhau\. M. Ohl and H. Bach, in: Valence Instabilities. eds. P. Wachtcr and 11. Hoppart (North-Holland, Amsterdam. lYX2) p. 565. [5] E. Giirlvzh. H.U. Hrynkiewcr. R. Kmicc. K. Lathn and K. Tomala. Phys. Stat. Sol. (h) 64 (1974) K147. [h] 0. Mawact. J.M.D. Coey and F. Holtzherg, J. dc Phy\. (‘4 (1976) 297.
171 I. Morkc. G I’ravagllm and I’. Wxhter. 111rcl’. 14). p. 571. [X] S.K. MaIrk. R. NagaraJan. SK. Malik. R. ViJaynragha\an. M.M. Ahd-1:lmcguid and H. Micklitr. Phvh. KC\. 2YH (19X4) 5953. IO6 [Y] S.J. I.yle and WA. Westall. J. I. e\\ C’cmmon Metal\. (lYX5) 109. [IO/ F. Hulllger. Mat. Reh. Hull. 14 (1979) 37. [I I] R. NagaraJan. EV. S~lmpathkutllaran. R. VIJ+yaragh:l\an ,rnd Hhaktdur\han. Phw. Stat. Sol. (a) 75 (19X3) K14Y. [I?] Il. L.oruv. Phys. Stat. Sol. (h) 125 (19X4) 375.