- Email: [email protected]
High pressure Mössbauer study of hyperfine interactions in europium metal
Solid State Communications, Vol. 18, Pp. 291—293, 1976.
Pergamon Press.
Printed in Great Britain
HIGH PRESSURE MOSSBAUER STUDY OF HYPERFINE INTERACTIONS IN EUROPIUM MIaTAL* U.F. Klein, G. Wortmann and G.M. Kalvius Physik Department der Technischen Universitãt Mtinchen, D-8046 Garching, Germany (Received 8 August 1975 by P.H. Dederichs) has been studied at 4.2 K in The 21.6 keV Mbssbauer resonance ‘51EuThe europium metal at pressures up to 16ofkbar. measured pressure coefficient of the isomer shift dS/dp = 4.8(5) x 10—2 mm/sec kbar can be understood as a congruent compression of 6s conduction electrons. The magnetic hyperfine splitting decreases under pressure. The slope of the (negative) hyperfine field dB/dp = + 0.133(8) T/kbar must arise predominantly from an increase in the conduction electron polarization. MOSSBAUER SPECTROSCOPY at high pressure has long been recognized as an important technique to study the volume dependence of electronic structure in solid materials.1 To date, measurements have been carried out mainly using the resonance in 57Fe and ‘91Au. Detailed data on the pressure variation of isomer shifts in metallic and non-conducting compounds of iron and gold are available.2 In contrast, only few measurements of the volume dependence of magnetic hyperime fields were performed in d-transition metals, the most detailed studies were done on metallic iron.1”3 The rare earth elements contain a large number of MOssbauer isotopes but no systematic high pressure studies have yet appeared, In an earlier communication4 we reported briefly on measurements on the nonconducting Eu-chalcogenides at pressures up to 80 kbar using the 103 keV resonance of 153 Eu. In this letter we report on a high pressure study of magnetically ordered Eu metal at helium temperature by means of the 21.6 keV Mossbauer resonance of 151 Eu. This resonance gives a much better resolution for magnetic studies and is more suitable for materials with a low Debye temperature like Eu metal. The present investigation was undertaken in order to gain further insight into the contribution of conduction electrons to contact charge and spin densities at the nucleus. Measurements were performed using a newly designed high pressure cell which allows Mossbauer absorber experiments to be performed in normal transmission geometry at gamma ray energies down to 14 keV and at liquid helium temperature. This cell is based on design principles outlined earlier5 but uses specially shaped B 4C anvils which permit sufficient of of 6 A belt transmission system consisting gamma rays of low energy. two concentric supporting rings provides a nearly *
quasihydrostatic pressure at the absorber. The pressure gradient is less than 10%. The diameter of the pressurized absorber was 8 mm. The mean pressure as well as the the pressure profile at the absorber were determined in situ by a superconducting lead manometer.7 Europium metal (b.c.c.) exhibits a large compressibiity due to the divalent state of the Eu-ions.8 The pressure needed for a 10% volume change is only about 16 kbar. Figure 1 shows Mossbauer spectra of Eu metal at 4.2 K at several applied pressures. Contaminations of our sample by tn- and divalent oxides were negligibly small. The absorber foil had a thickness of 0.2 mm at atmospheric pressure. The observed variation in isomer shift was —
+ 4.8(5) x 102 mm/sec kbar
The magnetic h.f. splitting reduces with increasing pressure. Since the magnetic h.f. field at atmospheric pressure is negative,9 this leads to a positive pressure coefficient
Work supported by the Deutsche Forschungsgemeinschaft.
—
=
+ 0.133(8)T/kbar.
Using the pressure volume relation given in reference 8 one obtains the following volume coefficients of the h.f. parameters (since the Neel temperature of Eu metal at 90K is pressure independent up to 40kbar,8 the hyperfine fields at 4.2 K can be considered to represent the saturation value) / a o’” ~ ‘ = 21 (3)a~3 (1) \a ln V,/T
)
and
I/ a~ I
=
—
21.0(F.2) T.
(2)
\a in V/T
To calculate the volume coefficient of the contact charge density p(0) [equation (1)] we have used an 291
292
HYPERFINE INTERACTIONS IN EUROPIUM METAL
Vol. l~ No. 3
s d transfer of conduction electrons undei pressurc
Particularly for the case of Fe it has been difficult to
Eu-meta: ,.
f~
t I
j
3 4 I
estimate the latter contribution reliably because of the large uncertainty in the isomer shift calibration constant 3 of the hand structure of h c APW calculations’ Eu metal produce energy bands winch resemble ch~e!s those of the metals of the first d-transition series (V to
4
1
‘
Fe) consisting of an s-band with a small p adm1\tur’~
V
~‘
j
~
and a d-band. From X-ray photoemission studies14 t concluded that the density of d states near the Fermi
‘
38
BE
4
surface is fairly low in Eu. The 4/states are considered strongly localized in the rare earth metals. The data o
“4
‘3 4
‘84
reference 13, however, imply that in Eu metal the occupied 4fstates are only about I eV below the Feinit surface and a small admixture of 4/electrons to the
Ir’
op
.••4
f
~
tainty. conduction Theand band-states difference beshift excluded between with ionic r ~u 2f salts Eu metalincannot isisomer generally considered tocci arise
!~
8.,
solely from the contribution of conduction electrons to the contact charge density.’°Using published values of these shifts leads to p~~(°)I 7(2)a 0 ~. a value which is well confirmed by a band structure calculation of conduction electron density at the 5 nucleus fin tito. o.c.~ Fu Inserting metal under atmospheric pressure.’ numbei for pce(O) into equations (1) and (3) gis es
.1
S
1
___________________
Fig. 1. Mossbauer spectra of Eu metal at 4.2 K at vanous pressures. The source is Eu 2 03, also at 4.2 K. The solid line through the data is a least squares fit using a sum of Lorentzians. The hyperuIne parameters returned from the fits are given on the right side. The relative errors for the variation of B are less than 0.1 T.
y 1 2(2). One concludes that the major contribution to the volume dependence of isomer shift arises front a congruent compression of conduction electrons and that charge transfer effects in the investigated pressure lange must be small, if present at all. The hf. fIeld in Eu metal is the sum of three contributions: ~
isomer shift calibration constant of
~
0.353a~mm/sec 2) 18.5 x 10 ~ fm’ for the which corresponds 21.6 keV transitiontoof ~(r ~‘ Eu. Thus value is confirmed to witlun 25~by various calculations of free ion electron densities, bywith optical measurements and by comparison theisotope muonic shift gamma ray shift.1°The isomer shift of europium can be considered to be the most reliably calibrated one of all Mossbauer isotopes. In d-transition metals the pressure induced variation of isomer shifts is interpreted as arising mainly from a compression the conduction electrons with S cliarac2’11’12 In of tIns case we may write ter.
/ öp(0) \ ~ in
~
)
(i) Core-election polarization B1 34.0 (2.0) T. (ii) Conduction electron polari7ation by own 4/ electronsB~. + l9.0(2.0)T. (iii) Conduction electron polarization by neighbour ing atoms B, 11.5(2.0) T. 2~is1 an S-state an orbital hyperfine field does Since Eu not exist. The observed decrease of hf. splitting under press. ure indicates that the overriding contribution to equation (2) must arise from B~ 1. The core field B~is con sidered to be only weakly dependent on chemical oi volume Littlethat can both be said about tileremain behaviour of Br,. Ifchanges. one assumes B~ and B~ conthe applied volume change then equation (2) suggests an almost uniform compression of contact spin density in analogy to our conclusions concerning the contact charge density A rough correlation between BCE and the isomer shift has been established from measurements on vanous intenmetallic compounds.’6 Under the assuniptioii~ given above, a combination of equations (I) and (2)1 stant under
i.,,
~
(3)
wheie pce(O) is the contact density of conduction electrons at atmospheric pressure. A value of y I means that the contact charge s-conduction electrons are congruently compressed. An additional term to the volume coefficient of contact charge density can arise from an
Vol. 18, No. 3
HYPERFINE INTERACTIONS IN EUROPIUM METAL
keeping with these findings. A more conclusive interpretation of equation (2) awaits the measurement of the volume dependence of h.f. fields in dilute interin
293
metallic compounds and alloy systems of Eu. The work will be continued in this direction.
REFERENCES 1.
DRICKAMER H.G., INGALLS R. & COSTON C.J., Physics ofSolids at High Pressures, p. 313. Academic
2.
Press, NY (1965). WILLIAMSON D.L., Mossbauer Isomer Shifts (Edited by SHENOY G.K. & WAGNER F.E.). North Holland, Amsterdam (to be published).
3.
RAIMOND D.L. & JUR.A G., J. Appi. Phys. 38,2133 (1967) and references cited therein.
4.
KLEIN U.F., WORTMANN G. & KALVIUS G.M., Proc.
Nauka, Moscow (1974).
mt. Conf Magnetism, Moscow
1973, Vol. IV, p. 149.
5.
SCHILLING J.S., KLEIN U.F. & HOLZAPFELW.B.,Rev. Sci. Instrum. 45, 1353 (1974).
6. 7.
KLEIN U.F. (to be published). EICHLER A. & WITTIG J., Z. Angew. Phys. 25, 319 (1968).
8.
MCWHAN I).B., SAUERS P.C. & JURA G., Phys. Rev. 143, A385 (1966).
9. 10.
HUFNER S. & WERNICK J.H., Phys. Rev. 173, 448 (1968). BAUMINGER R.E., NOWIK I. & KALVIUS G.M., MOssbauer Isomer Shifts (Edited by SHENOY G.K. &
11.
WAGNER F.E.). North Holland, Amsterdam (to be published). INGALLS R., Phys. Rev. 155, 157 (1967).
12.
ROBERTS L.D., PATTERSON DO., THOMSON J.O. & LEVY R.P., Phys. Rev. 179, 656 (1969).
13.
FREEMAN A.J., Magnetic properties ofRare Earth Metals, p. 270. Plenum Press, NY (1972).
14. 15.
MARIOT J.M., KARNATAK R.C. & BONELLE C., J. Phys. Chem. Solids 35, 657 (1974). KOBAYASI S., FUKUCHI M. & NAGAI S., Solid State Commun. 13, 727 (1973).
16.
NOWIK I., DUNLAP B.D. & WERNICK J.H., Phys. Rev. B8, 238 (1973).
Pergamon Press.
Printed in Great Britain
HIGH PRESSURE MOSSBAUER STUDY OF HYPERFINE INTERACTIONS IN EUROPIUM MIaTAL* U.F. Klein, G. Wortmann and G.M. Kalvius Physik Department der Technischen Universitãt Mtinchen, D-8046 Garching, Germany (Received 8 August 1975 by P.H. Dederichs) has been studied at 4.2 K in The 21.6 keV Mbssbauer resonance ‘51EuThe europium metal at pressures up to 16ofkbar. measured pressure coefficient of the isomer shift dS/dp = 4.8(5) x 10—2 mm/sec kbar can be understood as a congruent compression of 6s conduction electrons. The magnetic hyperfine splitting decreases under pressure. The slope of the (negative) hyperfine field dB/dp = + 0.133(8) T/kbar must arise predominantly from an increase in the conduction electron polarization. MOSSBAUER SPECTROSCOPY at high pressure has long been recognized as an important technique to study the volume dependence of electronic structure in solid materials.1 To date, measurements have been carried out mainly using the resonance in 57Fe and ‘91Au. Detailed data on the pressure variation of isomer shifts in metallic and non-conducting compounds of iron and gold are available.2 In contrast, only few measurements of the volume dependence of magnetic hyperime fields were performed in d-transition metals, the most detailed studies were done on metallic iron.1”3 The rare earth elements contain a large number of MOssbauer isotopes but no systematic high pressure studies have yet appeared, In an earlier communication4 we reported briefly on measurements on the nonconducting Eu-chalcogenides at pressures up to 80 kbar using the 103 keV resonance of 153 Eu. In this letter we report on a high pressure study of magnetically ordered Eu metal at helium temperature by means of the 21.6 keV Mossbauer resonance of 151 Eu. This resonance gives a much better resolution for magnetic studies and is more suitable for materials with a low Debye temperature like Eu metal. The present investigation was undertaken in order to gain further insight into the contribution of conduction electrons to contact charge and spin densities at the nucleus. Measurements were performed using a newly designed high pressure cell which allows Mossbauer absorber experiments to be performed in normal transmission geometry at gamma ray energies down to 14 keV and at liquid helium temperature. This cell is based on design principles outlined earlier5 but uses specially shaped B 4C anvils which permit sufficient of of 6 A belt transmission system consisting gamma rays of low energy. two concentric supporting rings provides a nearly *
quasihydrostatic pressure at the absorber. The pressure gradient is less than 10%. The diameter of the pressurized absorber was 8 mm. The mean pressure as well as the the pressure profile at the absorber were determined in situ by a superconducting lead manometer.7 Europium metal (b.c.c.) exhibits a large compressibiity due to the divalent state of the Eu-ions.8 The pressure needed for a 10% volume change is only about 16 kbar. Figure 1 shows Mossbauer spectra of Eu metal at 4.2 K at several applied pressures. Contaminations of our sample by tn- and divalent oxides were negligibly small. The absorber foil had a thickness of 0.2 mm at atmospheric pressure. The observed variation in isomer shift was —
+ 4.8(5) x 102 mm/sec kbar
The magnetic h.f. splitting reduces with increasing pressure. Since the magnetic h.f. field at atmospheric pressure is negative,9 this leads to a positive pressure coefficient
Work supported by the Deutsche Forschungsgemeinschaft.
—
=
+ 0.133(8)T/kbar.
Using the pressure volume relation given in reference 8 one obtains the following volume coefficients of the h.f. parameters (since the Neel temperature of Eu metal at 90K is pressure independent up to 40kbar,8 the hyperfine fields at 4.2 K can be considered to represent the saturation value) / a o’” ~ ‘ = 21 (3)a~3 (1) \a ln V,/T
)
and
I/ a~ I
=
—
21.0(F.2) T.
(2)
\a in V/T
To calculate the volume coefficient of the contact charge density p(0) [equation (1)] we have used an 291
292
HYPERFINE INTERACTIONS IN EUROPIUM METAL
Vol. l~ No. 3
s d transfer of conduction electrons undei pressurc
Particularly for the case of Fe it has been difficult to
Eu-meta: ,.
f~
t I
j
3 4 I
estimate the latter contribution reliably because of the large uncertainty in the isomer shift calibration constant 3 of the hand structure of h c APW calculations’ Eu metal produce energy bands winch resemble ch~e!s those of the metals of the first d-transition series (V to
4
1
‘
Fe) consisting of an s-band with a small p adm1\tur’~
V
~‘
j
~
and a d-band. From X-ray photoemission studies14 t concluded that the density of d states near the Fermi
‘
38
BE
4
surface is fairly low in Eu. The 4/states are considered strongly localized in the rare earth metals. The data o
“4
‘3 4
‘84
reference 13, however, imply that in Eu metal the occupied 4fstates are only about I eV below the Feinit surface and a small admixture of 4/electrons to the
Ir’
op
.••4
f
~
tainty. conduction Theand band-states difference beshift excluded between with ionic r ~u 2f salts Eu metalincannot isisomer generally considered tocci arise
!~
8.,
solely from the contribution of conduction electrons to the contact charge density.’°Using published values of these shifts leads to p~~(°)I 7(2)a 0 ~. a value which is well confirmed by a band structure calculation of conduction electron density at the 5 nucleus fin tito. o.c.~ Fu Inserting metal under atmospheric pressure.’ numbei for pce(O) into equations (1) and (3) gis es
.1
S
1
___________________
Fig. 1. Mossbauer spectra of Eu metal at 4.2 K at vanous pressures. The source is Eu 2 03, also at 4.2 K. The solid line through the data is a least squares fit using a sum of Lorentzians. The hyperuIne parameters returned from the fits are given on the right side. The relative errors for the variation of B are less than 0.1 T.
y 1 2(2). One concludes that the major contribution to the volume dependence of isomer shift arises front a congruent compression of conduction electrons and that charge transfer effects in the investigated pressure lange must be small, if present at all. The hf. fIeld in Eu metal is the sum of three contributions: ~
isomer shift calibration constant of
~
0.353a~mm/sec 2) 18.5 x 10 ~ fm’ for the which corresponds 21.6 keV transitiontoof ~(r ~‘ Eu. Thus value is confirmed to witlun 25~by various calculations of free ion electron densities, bywith optical measurements and by comparison theisotope muonic shift gamma ray shift.1°The isomer shift of europium can be considered to be the most reliably calibrated one of all Mossbauer isotopes. In d-transition metals the pressure induced variation of isomer shifts is interpreted as arising mainly from a compression the conduction electrons with S cliarac2’11’12 In of tIns case we may write ter.
/ öp(0) \ ~ in
~
)
(i) Core-election polarization B1 34.0 (2.0) T. (ii) Conduction electron polari7ation by own 4/ electronsB~. + l9.0(2.0)T. (iii) Conduction electron polarization by neighbour ing atoms B, 11.5(2.0) T. 2~is1 an S-state an orbital hyperfine field does Since Eu not exist. The observed decrease of hf. splitting under press. ure indicates that the overriding contribution to equation (2) must arise from B~ 1. The core field B~is con sidered to be only weakly dependent on chemical oi volume Littlethat can both be said about tileremain behaviour of Br,. Ifchanges. one assumes B~ and B~ conthe applied volume change then equation (2) suggests an almost uniform compression of contact spin density in analogy to our conclusions concerning the contact charge density A rough correlation between BCE and the isomer shift has been established from measurements on vanous intenmetallic compounds.’6 Under the assuniptioii~ given above, a combination of equations (I) and (2)1 stant under
i.,,
~
(3)
wheie pce(O) is the contact density of conduction electrons at atmospheric pressure. A value of y I means that the contact charge s-conduction electrons are congruently compressed. An additional term to the volume coefficient of contact charge density can arise from an
Vol. 18, No. 3
HYPERFINE INTERACTIONS IN EUROPIUM METAL
keeping with these findings. A more conclusive interpretation of equation (2) awaits the measurement of the volume dependence of h.f. fields in dilute interin
293
metallic compounds and alloy systems of Eu. The work will be continued in this direction.
REFERENCES 1.
DRICKAMER H.G., INGALLS R. & COSTON C.J., Physics ofSolids at High Pressures, p. 313. Academic
2.
Press, NY (1965). WILLIAMSON D.L., Mossbauer Isomer Shifts (Edited by SHENOY G.K. & WAGNER F.E.). North Holland, Amsterdam (to be published).
3.
RAIMOND D.L. & JUR.A G., J. Appi. Phys. 38,2133 (1967) and references cited therein.
4.
KLEIN U.F., WORTMANN G. & KALVIUS G.M., Proc.
Nauka, Moscow (1974).
mt. Conf Magnetism, Moscow
1973, Vol. IV, p. 149.
5.
SCHILLING J.S., KLEIN U.F. & HOLZAPFELW.B.,Rev. Sci. Instrum. 45, 1353 (1974).
6. 7.
KLEIN U.F. (to be published). EICHLER A. & WITTIG J., Z. Angew. Phys. 25, 319 (1968).
8.
MCWHAN I).B., SAUERS P.C. & JURA G., Phys. Rev. 143, A385 (1966).
9. 10.
HUFNER S. & WERNICK J.H., Phys. Rev. 173, 448 (1968). BAUMINGER R.E., NOWIK I. & KALVIUS G.M., MOssbauer Isomer Shifts (Edited by SHENOY G.K. &
11.
WAGNER F.E.). North Holland, Amsterdam (to be published). INGALLS R., Phys. Rev. 155, 157 (1967).
12.
ROBERTS L.D., PATTERSON DO., THOMSON J.O. & LEVY R.P., Phys. Rev. 179, 656 (1969).
13.
FREEMAN A.J., Magnetic properties ofRare Earth Metals, p. 270. Plenum Press, NY (1972).
14. 15.
MARIOT J.M., KARNATAK R.C. & BONELLE C., J. Phys. Chem. Solids 35, 657 (1974). KOBAYASI S., FUKUCHI M. & NAGAI S., Solid State Commun. 13, 727 (1973).
16.
NOWIK I., DUNLAP B.D. & WERNICK J.H., Phys. Rev. B8, 238 (1973).