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Magnetic and ESR studies of Er3+ in lanthanum dihydride LaH2
Solid State Communications, Vol. 36, pp. 973—975. Pergamon Press Ltd. 1980. Printed in Great Britain. MAGNETIC AND ESR STUDIES OF Er3 IN LANTHANUM DIHYDRIDE LaH2 i-I. Drulis, K.P. Hoffmann and B. Staliñski Institute for Lo~Temperature and Structure Research, Polish Academy of Sciences 50-950 Wrocfaw, Poland (Received 26 June 1980 by P. Wachter) 3~electron spin resonance ESR and magnetic susceptibility have been Er studied in metallic lanthanum dthydride host. The ESR spectrum contains a single asymmetrical line with g-factor g = 6.68 ±0.05 close to that expected for r 7 as ground state. The experimental magnetic susceptibility was interpreted on the base of LLW cubic crystal field Hamiltonian. The best fit of the experimental data has been obtained 3K;B for the following5K B4 andB6 support which crystal the fieldanionic-like parameters:character B4 = —5.2 hydridic x 10 model 6= of 3.8 hydrogen x 10 atoms in this hydride. THE LIGHT LANTHANIDE ELEMENTS form
At 4.2 K Er3~ions in the dihydride LaH
non-stoichiometric hydride phases within a wide range of composition from LnH190 up to LnH3 [I]. Two interesting aspects of the hydrides are their dramatic decrease of the electrical conductivity and the loss of magnetic ordering while going from the rare-earth metal through dihydride up to trihydride [21. For further study of these compounds it would be plausible to usc the ESR technique which is a sensitive method to refine the electronic and crystalline-structure of substances because the electronic ground states of the rare-earth ions in hydrides are determined by the local symmetry of the paramagnetic ion and strength of the CEF generated by hydrogen neighbours. This paper concerns the crystal field effect 3~ inion lanthanum dihydride examined by means as a magnetic probe. In the perfect Lal-1 of Er 2-structure of the CaF2 type eight hydrogen atom placed in the tetrahedral sites form the first coordination sphere of the magnetic ion thus generating a cubic crystal field. The more distant octahedral sites are occupied by hydrogen atoms only when 3-site H/Lndepends ratio exceeds strongly 2. onThe the point way in symmetry at Er~ which the octahedral sites are occupied. The neutron diffraction studies of LaDs and CeD~revealed the existence of the tetragonal superstructure when x >2.3. For more detailed studies of this superstructure the cubic crystal field parameters for La!-!2 dihydride should be found. The powder samples were obtained by reaction of the La—Er solid solution containing 1% of Er with purified hydrogen in a vacuum glass system. The hydride composition was volumetrically determined as La!-!2 02 ESR measurements were carried out in the temperature range 2.5—30 K using RE 1301 X band spectrometer.
20 reveal the microwave frequency absorption in the magnetic field which corresponds to the spectroscopic splitting factor g = 6.68 ±0.05. The observed single asym. metrical resonance line without noticeable hyperfine structure has a shape typical for the ESR spectra registered in metallic substances (A/B ratio equal Ca. 2.2). The interaction between the local magnetic moment and a conduction electron leads to a linear Korringa dependence of the resonance lincwidth versus temperature Dl-! = a + bTwith a = 43 Oe and b = 3.30 Oc/K as is shown in Fig. 1. Low temperature susceptibility measurements were carried out by application of the Faraday method in the temperature range 4.2—150K. temperature dependence of the reciprocal magneticThe susceptibility of L.aH2: 1% at. Er corrected for that of the La!-!2 -host is shown in Fig. 2 One may notice that the susceptibility obeys the Curie-Weiss law x = C/T—O, between 50—150K. Below 50 K no Curie—Weiss behaviour is observed. This fact is attributed to the crystal field splitting of the electronic ground term. In LLW parametrization [41the crystal field Hamiltonian for the cubic symmetry is given by: ~‘l HCEF = W (O~+ 5O~)4..\ X / (O~ 210) F 4 F6 (1) The parameters Wand x defined by LLW are: —
-~-
B4F4 and h~l—lxi) with I
W•x
=
—
=
B6F6
—
where B4 and B6 are crystal field constants depending on the system in question. The cubic crystal field Hamiltonian equation 1 splits the 16-fold degenerate 973
-
STUDIES OF Er3~IN LANTHANUM DIHYDRIDE LaH2
974
DH (Ce) -
40
~
‘00 -
-j
—
20
—
0
I
I 0
values ofx. Determination of the electronic ground state allows us to draw the conclusion on the charge sign at hydrogen atoms. Comparing the value ofg = 6.68 found experimentally with those expected theoretically [5] for the doublets [“6 (g = 6.0) and F’7 (g = 6.77) one finds that in LaH F’7 is the most probable ground state of the3’~ doublet Er 20 host and consequently the x parameter should be larger than 0.46. There are several ways to evaluate x and W parameters: (i) from anisotropy and value ofg-factor (ii) from the magnetic susceptibility data. We find that we cannot obtain a precise value of x and W using the ESR method only since the Zeeman splitting of the F7 doublet is isotropic. Thus, we have established crystal fieldand splitting and the relative magnitudes the of the fourth sixth-order crystal field —
,,,77X
60
Vol. 36, No. 11
20
30
I
Fig. 1. The ESR linewidth of Er 1% at. in LaH2~as a function of temperature.
parameters from the experimental ~ versus T curve. Computer diagonalization of the Hamiltonian equation 1 for various values ofx and W gives the relevant eigenvalues and eigenvectors of crystal field levels [4]. These energy values are necessary to calculate the magnetic
0 15-
0 0
Oe
o0
300 200N~,/ 100
05 Coic. ShOWfl for scheme fl InSect
10
H~=5OKI r7
=0K
-100
-
-200
-
;?~,‘,,/~xpdata 50
00
ISO
1(K)
3~in Fig. 2. Reciprocal molar susceptibility of Er LaH 2~vs temperature within the range 4.2—150 K. to equation 2 andpoints; splitting diagram presented.according o, Experimental solid line, theoretical
ground term of the Er3~ion into three quartets F’~2~ I’~and two doublets F’ 6 and F’,. The resulting energies of these levels vary as a function the 3~ion of in xtheincubic manner shown Fig. 3. For Er crystal field theinparameter x isthenegative (coordination number 8). The sign of the W parameter depends on the effective electrostatic charge on hydrogen atoms generating this field. As shown in Fig. 3, for the case of W>0, i.e. for the hydridic model (H), B 4 <0, B6 > 0 and depending on thex value either I’6 or F’, doublet is the ground state. For W < 0 (protonic model H~), B4 <0 and the quartet r’~ is the ground state for all
~
~
0
3’~in cubic environ-
ment [41. diagram for Er Fig. 3.from Crystal-field
1~”(4I
~
‘ia,
/ / / / ——
5
-
2605
—,
F~ ‘~‘~
‘N \“..
~J~I6I
50K
r. 121
3~in LaH Fig. 4. Crystal-field level scheme for Er
2.02.
Vol. 36, No. 11
STUDIES OF Er~IN LANTHANUM DIHYDRIDE LaH2
975
susceptibility according to the fundamental Van Vieck equation.
field parameters (see Fig. 3). This change may be caused by the contraction of the ErH2 lattice constant in corn-
N ~p, e_E~~T X = ~ eEi~ (2) The calculated magnetic susceptibility (solid line in Fig.
parison to that for LaH2. This assumption has been ion where the lattice constant is about tested by us by3~ study of lutetium dihydride LuH2 doped with Er 11% smaller than that in LaH 2 and about 2% smaller than in ErH2. In this case (LuH2) we3~’ observe that the ion similarly as in F’, doublet is the ground state of Er dilute solid solutions of Er in other dthydrides. There-
2.) is compared with experimental results. The best fit experimental curve has been obtained for xof=the — 0.37 and W = 0.84 K. These values yield the following B 4 and B6 parameters: 3K, B 5K B4 = —5.2xl0 6 = 3.8x10 The resulting total splitting of the in 16-fold 3~is shown Fig. 4.degenerate This result ground Er of anonic-like character of stronglymultiplet favors theofidea hydrogen in accordance with the recently given interpretation of the Schottky anomaly in the low ternperature heat capacity of the light rare-earth dihydrides [6]. It is worthy to be noticed that the value of thex parameter is very close to the cross point of F’, and F’ 6 levels —0.46. MOssbauer and ESR measurements mdirate that in dihydrides like LaH2 (this work), 3~ions theYH2 ground [7, 8]ofand Er On the other hand state the HoH2 erbium[9]is doped the I”, with doublet. the Mössbauer data [101 showed that the F’ 6 doublet is the ground state in Er!-!2. In terms of the point charge model we might expect that the crystal field effect would be identical for all REH2 and La(RE)112 compounds neglecting some slight changes of the lattice constants. The change from F’6 to F, ground state on passing from La(Er)H2, Y(Er)H2 to Er!-!2 indicates a change in the ratio of fourth to sixth-order crystal
fore, one may suppose that the changes in the lattice stateoftheEr3~ionsinErH constant are not responsible for changes of the ground 2.
1.
2. 3. 4. 5. 6. 7. 8. 9. 10.
REFERENCES c.E. Holley, R.N. Mulford, Jr. F.H. Ellinger, W.C. Kohier & W.H. Zachariasen,J. Thys. Ozem. 59, 1226 (1955). G.G. Libowitz, Ber. Bunsenges. Phys. G~em.76, 837 C.G.(1972). Titcomb, A.K. Cheetham & B.E.F. Fender, I. Phys. C7, 2409 (1974). K.R. Lea, MJ.M. Leask & W.P. WoIf,J~Phys. Giem. So!. 23, (1962). A. Abragam & B.1381 Bleaney, Electron Pararna.gnetic Resonance of Transition Ions p. 330, Clarendon Press, Oxford (1970). Z. Biega?iski, Ber. Bunsenges. Phys. Qiem. 76, 1183 (1972). U. Drulis & J. Pyter, B. Staliñski (in press). J. Stöhr & J.D. Cashion, Phys. Rev. B12, 4805 (1975). J.M. Fricdt, B. Suits, G.K. Shenoy, B.D. Dunlap & R.G. Westlake,J. App!. Phys. 50, 2049 (1979). G.K. Shenoy, B.D. Dunlap, D.G. Westlake & A.E. Dwight, Phys. Rev. B14, 41(1976).
At 4.2 K Er3~ions in the dihydride LaH
non-stoichiometric hydride phases within a wide range of composition from LnH190 up to LnH3 [I]. Two interesting aspects of the hydrides are their dramatic decrease of the electrical conductivity and the loss of magnetic ordering while going from the rare-earth metal through dihydride up to trihydride [21. For further study of these compounds it would be plausible to usc the ESR technique which is a sensitive method to refine the electronic and crystalline-structure of substances because the electronic ground states of the rare-earth ions in hydrides are determined by the local symmetry of the paramagnetic ion and strength of the CEF generated by hydrogen neighbours. This paper concerns the crystal field effect 3~ inion lanthanum dihydride examined by means as a magnetic probe. In the perfect Lal-1 of Er 2-structure of the CaF2 type eight hydrogen atom placed in the tetrahedral sites form the first coordination sphere of the magnetic ion thus generating a cubic crystal field. The more distant octahedral sites are occupied by hydrogen atoms only when 3-site H/Lndepends ratio exceeds strongly 2. onThe the point way in symmetry at Er~ which the octahedral sites are occupied. The neutron diffraction studies of LaDs and CeD~revealed the existence of the tetragonal superstructure when x >2.3. For more detailed studies of this superstructure the cubic crystal field parameters for La!-!2 dihydride should be found. The powder samples were obtained by reaction of the La—Er solid solution containing 1% of Er with purified hydrogen in a vacuum glass system. The hydride composition was volumetrically determined as La!-!2 02 ESR measurements were carried out in the temperature range 2.5—30 K using RE 1301 X band spectrometer.
20 reveal the microwave frequency absorption in the magnetic field which corresponds to the spectroscopic splitting factor g = 6.68 ±0.05. The observed single asym. metrical resonance line without noticeable hyperfine structure has a shape typical for the ESR spectra registered in metallic substances (A/B ratio equal Ca. 2.2). The interaction between the local magnetic moment and a conduction electron leads to a linear Korringa dependence of the resonance lincwidth versus temperature Dl-! = a + bTwith a = 43 Oe and b = 3.30 Oc/K as is shown in Fig. 1. Low temperature susceptibility measurements were carried out by application of the Faraday method in the temperature range 4.2—150K. temperature dependence of the reciprocal magneticThe susceptibility of L.aH2: 1% at. Er corrected for that of the La!-!2 -host is shown in Fig. 2 One may notice that the susceptibility obeys the Curie-Weiss law x = C/T—O, between 50—150K. Below 50 K no Curie—Weiss behaviour is observed. This fact is attributed to the crystal field splitting of the electronic ground term. In LLW parametrization [41the crystal field Hamiltonian for the cubic symmetry is given by: ~‘l HCEF = W (O~+ 5O~)4..\ X / (O~ 210) F 4 F6 (1) The parameters Wand x defined by LLW are: —
-~-
B4F4 and h~l—lxi) with I
W•x
=
—
=
B6F6
—
where B4 and B6 are crystal field constants depending on the system in question. The cubic crystal field Hamiltonian equation 1 splits the 16-fold degenerate 973
-
STUDIES OF Er3~IN LANTHANUM DIHYDRIDE LaH2
974
DH (Ce) -
40
~
‘00 -
-j
—
20
—
0
I
I 0
values ofx. Determination of the electronic ground state allows us to draw the conclusion on the charge sign at hydrogen atoms. Comparing the value ofg = 6.68 found experimentally with those expected theoretically [5] for the doublets [“6 (g = 6.0) and F’7 (g = 6.77) one finds that in LaH F’7 is the most probable ground state of the3’~ doublet Er 20 host and consequently the x parameter should be larger than 0.46. There are several ways to evaluate x and W parameters: (i) from anisotropy and value ofg-factor (ii) from the magnetic susceptibility data. We find that we cannot obtain a precise value of x and W using the ESR method only since the Zeeman splitting of the F7 doublet is isotropic. Thus, we have established crystal fieldand splitting and the relative magnitudes the of the fourth sixth-order crystal field —
,,,77X
60
Vol. 36, No. 11
20
30
I
Fig. 1. The ESR linewidth of Er 1% at. in LaH2~as a function of temperature.
parameters from the experimental ~ versus T curve. Computer diagonalization of the Hamiltonian equation 1 for various values ofx and W gives the relevant eigenvalues and eigenvectors of crystal field levels [4]. These energy values are necessary to calculate the magnetic
0 15-
0 0
Oe
o0
300 200N~,/ 100
05 Coic. ShOWfl for scheme fl InSect
10
H~=5OKI r7
=0K
-100
-
-200
-
;?~,‘,,/~xpdata 50
00
ISO
1(K)
3~in Fig. 2. Reciprocal molar susceptibility of Er LaH 2~vs temperature within the range 4.2—150 K. to equation 2 andpoints; splitting diagram presented.according o, Experimental solid line, theoretical
ground term of the Er3~ion into three quartets F’~2~ I’~and two doublets F’ 6 and F’,. The resulting energies of these levels vary as a function the 3~ion of in xtheincubic manner shown Fig. 3. For Er crystal field theinparameter x isthenegative (coordination number 8). The sign of the W parameter depends on the effective electrostatic charge on hydrogen atoms generating this field. As shown in Fig. 3, for the case of W>0, i.e. for the hydridic model (H), B 4 <0, B6 > 0 and depending on thex value either I’6 or F’, doublet is the ground state. For W < 0 (protonic model H~), B4 <0 and the quartet r’~ is the ground state for all
~
~
0
3’~in cubic environ-
ment [41. diagram for Er Fig. 3.from Crystal-field
1~”(4I
~
‘ia,
/ / / / ——
5
-
2605
—,
F~ ‘~‘~
‘N \“..
~J~I6I
50K
r. 121
3~in LaH Fig. 4. Crystal-field level scheme for Er
2.02.
Vol. 36, No. 11
STUDIES OF Er~IN LANTHANUM DIHYDRIDE LaH2
975
susceptibility according to the fundamental Van Vieck equation.
field parameters (see Fig. 3). This change may be caused by the contraction of the ErH2 lattice constant in corn-
N ~p, e_E~~T X = ~ eEi~ (2) The calculated magnetic susceptibility (solid line in Fig.
parison to that for LaH2. This assumption has been ion where the lattice constant is about tested by us by3~ study of lutetium dihydride LuH2 doped with Er 11% smaller than that in LaH 2 and about 2% smaller than in ErH2. In this case (LuH2) we3~’ observe that the ion similarly as in F’, doublet is the ground state of Er dilute solid solutions of Er in other dthydrides. There-
2.) is compared with experimental results. The best fit experimental curve has been obtained for xof=the — 0.37 and W = 0.84 K. These values yield the following B 4 and B6 parameters: 3K, B 5K B4 = —5.2xl0 6 = 3.8x10 The resulting total splitting of the in 16-fold 3~is shown Fig. 4.degenerate This result ground Er of anonic-like character of stronglymultiplet favors theofidea hydrogen in accordance with the recently given interpretation of the Schottky anomaly in the low ternperature heat capacity of the light rare-earth dihydrides [6]. It is worthy to be noticed that the value of thex parameter is very close to the cross point of F’, and F’ 6 levels —0.46. MOssbauer and ESR measurements mdirate that in dihydrides like LaH2 (this work), 3~ions theYH2 ground [7, 8]ofand Er On the other hand state the HoH2 erbium[9]is doped the I”, with doublet. the Mössbauer data [101 showed that the F’ 6 doublet is the ground state in Er!-!2. In terms of the point charge model we might expect that the crystal field effect would be identical for all REH2 and La(RE)112 compounds neglecting some slight changes of the lattice constants. The change from F’6 to F, ground state on passing from La(Er)H2, Y(Er)H2 to Er!-!2 indicates a change in the ratio of fourth to sixth-order crystal
fore, one may suppose that the changes in the lattice stateoftheEr3~ionsinErH constant are not responsible for changes of the ground 2.
1.
2. 3. 4. 5. 6. 7. 8. 9. 10.
REFERENCES c.E. Holley, R.N. Mulford, Jr. F.H. Ellinger, W.C. Kohier & W.H. Zachariasen,J. Thys. Ozem. 59, 1226 (1955). G.G. Libowitz, Ber. Bunsenges. Phys. G~em.76, 837 C.G.(1972). Titcomb, A.K. Cheetham & B.E.F. Fender, I. Phys. C7, 2409 (1974). K.R. Lea, MJ.M. Leask & W.P. WoIf,J~Phys. Giem. So!. 23, (1962). A. Abragam & B.1381 Bleaney, Electron Pararna.gnetic Resonance of Transition Ions p. 330, Clarendon Press, Oxford (1970). Z. Biega?iski, Ber. Bunsenges. Phys. Qiem. 76, 1183 (1972). U. Drulis & J. Pyter, B. Staliñski (in press). J. Stöhr & J.D. Cashion, Phys. Rev. B12, 4805 (1975). J.M. Fricdt, B. Suits, G.K. Shenoy, B.D. Dunlap & R.G. Westlake,J. App!. Phys. 50, 2049 (1979). G.K. Shenoy, B.D. Dunlap, D.G. Westlake & A.E. Dwight, Phys. Rev. B14, 41(1976).