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Electron paramagnetic resonance of Fe3+ in natural topaz
PHYSICS
Volume 24A, number 8
LETTERS
CdO.8SrO.40. A similar behaviour was found for the reciprocal of the Hall constant ~/RH. The value of l/RH at room temperature was 4.8 Asec/cms corresponding to He = 3 X 1019cm-8 for x = 0.9 and 5.1 X 10sg Asec/cm8 for x = 0.6. Whereas the conductivity is only slowly decreasing with x for 1 2x 2 0.7, a strong dependence occurs at x-values lower than 0.7. Within the range from x = 1 to x = 0.75 we calculated the mobility from electrical conductivity and Hall effect assuming predominating n-type conductivity. The results are shown in fig. 1. A preliminary interpretation of these data leads to the assumption of predominant electron scattering by ionized impurities. For the lower electron densities at x = 0.775 and x =0.75 we find the typical Tt-dependence, according to the Conwell-Weisskopf formula for non-degenerate semiconductors, whereas the degenerate samples x = 1 and x = 0.9 show a behaviour similar to that expected by Koenig [6] for ionized impurity scattering in degenerate semiconductors. The considerable decrease of molibity with decreasing x is probably caused by changes in band structures as to be expected from the X-ray diffraction experiments. Assuming parabolic band structure and scattering by ionized impurities we determined the Fermi energy of the samples
10 April 1967
from thermoelectric power. The effective mass, estimated by comparison of the Fermi energy with the electron density, decreases from 0.10 m. at x = 0.9 to 0.05 m. at x = 0.75. Comparing investigations of the system Cd&al+0 lead to similar results. The present investigations will be extended to lower x, including optical measurements.
References and A. I. Ranyuk, Optics and 1. V. K. Miloslaviskii Spectr. 11 (1961) 536; T. K. Lakshmanan, J. Electrochem. Sot. 110 (1963) 548; H. Finkenrath, H. K’dhler and M. Lochmann, 2. angew. Phys. 21 (1966) 512. 2. Y. Colin and R. Tufeu, C. R. Acad. Sci. 256 (1963) 4195; Z.M. Jarzebski, Act.phys.pol. 29 (1966) 37; M. von Ortenberg, Diplomarbeit Darmstadt (1966). 3. E. Mollwo and R. Stumpp, 2. Phys. 184 (1965) 286. 4. R. Scholder, Angew. Chemie 66 (1965) 461. Gazz. chi&. Ital. 59 (1929) 5. G. Natta and L. Passerini, 129; G. Natta and L. Passerini, Gazz. chim. Ital. 60 (1930) 535. 6. S. H. Koenig, in: Report of the Intern. Conf. on the Physics of Semiconductors Exeter 1962, p. 5.
*****
ELECTRON
PARAMAGNETIC
RESONANCE
OF
Fe3+
IN NATURAL
TOPAZ
*
A. B. DENISON, T. C. ENSIGN ** and L. J. SIMS University of Wyoming, Laramie, Wyoming, USA. Received
14 March
1967
The EPR spectra of Fe3+, the dominant par amagnetic impurity in natural topaz, showed marked “superhyperfine” structure allowing the crystal site to be determined. Angular dependent spectra revealed strong zero-field splitting as predicted by an appropriate spin-Hamiltonian.
We have examined single crystals of natural topaz +** by using electron paramagnetic techniques and have learned the site of the paramagnetic impurity and something of the interactions responsible for the observed spectra. It was established early by X-ray fluorescence and chemical analysis that the crystal contains iron and the EPR spectra gave every indication that the impurity was Fe3+ (i. e. intensity and effective
g value). Topaz is an aluminum fluorosilicate [Al(F,OH) ]2SiO4 [l], the structure of which is an interweaving of silica tetrahedra and octahedral sites of the aluminum. The aluminum sites are paired as shown in * Research aided in part by Grant-in-Aid from Society of Sigma Xi. ** NASAFellow. *** Thomas Mountains, Juab Co., Utah, U.S.A.
405
PHYSICS
Volume 24A, number 8 b
v----Y ’ i
0
ALUMINUM
(IRON)
FLUORINE
0
OXYGEN
10 April 1967
LETTERS
The Al valence is 3+ as is Fe3+, and the fact that we have been able to obtain strong signals at room temperature and that the effective g value is near 4.2 leads us to believe that Fe3+ is the impurity and that it occupies the Al sites. The observed spectra (fig. 2) give strong evidence that our assumption is correct. These are two sets of triplets, one slightly more intense than the other. Each triplet corresponds to one of the sites mentioned and arises from the ligand hyperfine interaction of the Fe3+ with the neighboring fluorine nuclei. The relative intensities (1:2:1) of the triplet components indicate that each neighboring fluorine contributes an identical ligand hyperfine interaction. The strength of the isotropic hyperfine interaction is found to be A, = = 32 f 1 gauss, with a maximum anisotropic component of Aa = 10 f 1 gauss. In addition the center of the triplet has an angular dependence in agreement with the proposed site. Performing the appropriate angular transformation from the crystal axes (a, b, c) to the site axes (x, y, z) we obtain gx = 4.73
X.Y. 2
=
CRYSTALLINE
a, b, c
=
CRYSTAL
SITE
AXES
AXES
Fig. 1. Two adjacent octahedral sites in topaz. fig. 1. In general, there are eight magnetically inequivalent Al sites, however, these reduce to two when the crystal is rotated about the a-axis and the magnetic field is kept in the bc-plane. Each Al has two adjacent fluorines and four oxygens. For this site we have chosen the z-axis through the Al splitting the fluorines. It turns out that this axis is in fact the axis of highest symmetry (C2v), and a principal axis of the Hamiltonian.
IS’00
gz = 3.61 .
Fe3+ is a 6~~ ground state ion and must be under the influehce of strong zero-field splitting to give the large effective g values observed. This behavior has been observed in a number of substances [2.3] and is treated by F. Holuj [4] with the following Hamiltonian; H =D[S;-+S(S+l)]+E(S;-S;)
+@S*n.
For our case we mu_st also add the ligand hyperfine term CiS .A .Ii where the summation is over the neighboring fluorines. We have diagonalized this Hamiltonian using the axes of quantiza-
GAUSS
Fig. 2. Typical observed
406
gy = 4.64
spectrum
of Fe 3
in natural topaz for magnetic is 9.49 GHz.
field in bc plane.
The resonant
frequency
Volume 24A, number 8
PHYSICS
tion as the site axes acd transforming the external magnetic field into these axes. For the case E > H one obtains for the S=s three sets of Kramers The center Kramers doublet, the mainly cllblej;. 2s-z states, give the most isotropic and consistently strong spectra. It is from this pair of levels that the reported data are obtained. We have been able to fit gZ exactly and g, andgy quite well with E/D=O.45 and E =2 000 gauss. The full spectra, when the spectrometer is operated in a more sensitive mode, shows many lines. The further data are found to be in qualitative agreement with this general strong zero-field splitting picture and “+ = -6” and “-$= -z” lines are observed as well as the hyperfine “forbidden”
ELECTRON
LETTERS
10
April 1967
transitions [6]. Work is in progress at this time to complete the analysis. References
1. W.A.Deer et al., Rock Forming Minerals,
2. 3* 4 5:
Vol. 1 (Longmans, Green and Co., Ltd., London, 1962): The replacement of OH for F (for Utah Topaz) occurs only to a cery limited extent with 1.94 percent of the (F, OH) group being taken up with OH. T . Castner , Jr. , G.S.Newell, W.C.Ho1tonandC.P. Slichter, J.Chem.Phys.32 (1960) 668. H. Wickman, M.P.Klein and D.A.Shirley, J. Chem. Phys.42 (1965) 2113. F.Holuj, Ca.n.J. of Phys.44 (1966) 503. B. Bleaney and R.S.Rubins, Proc. Phys.Soc. (London) 77 (1961) 103.
AND PHONON EFFECTS IN SUPERCONDUCTING FCC LEAD-BASED ALLOYS * J. G. ADLER, J. E. JACKSON and T. A. WILL ** Department of Physics and Condensed State Center, Case Institute of Technology and Western Reserve University, Cleveland, Ohio 44106, U.S.A. Received
Electron tunneling measurements have been carried electron-phonon coupling strengths.
Using alloys of bismuth, thallium, and indium in lead, one can study fee lead-based alloys over a wide range of electron concentration. Since the atomic masses of bismuth and thallium are about the same as that of lead, changes in the electronphonon interaction in Pb-Bi and Pb-Tl arise primarily from changes in the electron concentration. On the other hand the atomic mass of indium is about half that of lead; thus in Pb-In alloys both the solute mass and electron concentration play an important role in determining the electronphonon interaction and phonon spectra. In this letter we report electron tunneling measurements in fee alloys of Pb with Bi, Tl, and In (over the range from 4.1 electrons/atom (e/a) to 3.3 e/a) and relate these measurements to * Work supported by the U.S. Air Force Office of Scientific Research through grant 565-66. ** National Aeronautics and Space Administration Predoctoral Trainee.
4 March
196’7
out in fee lead-based
alloys
over a wide range of
Table 1 Alloy Pb Ph Pb Pb Pb
e/a
A0 (meV)
2Ao/kT,
+ 20 at% Tl + 40 at% Tl + 60 at% Tl $
4.1 4.0 3.8 3.6 3.4
1.42 1.35 1.14 0.94 0.68
4.40 4.34 3.96 3.71 3.4
2at%In 3 at% In 5 at% In 12 at% In 15 at% In 16 at% In 26 at% In 27 at% In 50 a& In 70 at% In
3.98 3.97 3.95 3.88 3.85 3.84 3.74 3.73 3.50 3.30
1.36 1.36 1.35 1.33 1.32 1.32 1.29 1.28 1.20 1.18
4.34 4.38 4.37 4.35 4.33 4.34 4.36 4.35 4.20 4.20
+ 10 at% Bi
Pb+ Pb+ Pb + Pb + Pb + Pb + Pb f Pb + Pb + Pb +
OTh e gap for the Pb4Tl6 specimen was found by comparing the end point of the phonon spectrum observed by Ng and Brockhouse [8] with the corresponding singularity in da/d V.
407
Volume 24A, number 8
LETTERS
CdO.8SrO.40. A similar behaviour was found for the reciprocal of the Hall constant ~/RH. The value of l/RH at room temperature was 4.8 Asec/cms corresponding to He = 3 X 1019cm-8 for x = 0.9 and 5.1 X 10sg Asec/cm8 for x = 0.6. Whereas the conductivity is only slowly decreasing with x for 1 2x 2 0.7, a strong dependence occurs at x-values lower than 0.7. Within the range from x = 1 to x = 0.75 we calculated the mobility from electrical conductivity and Hall effect assuming predominating n-type conductivity. The results are shown in fig. 1. A preliminary interpretation of these data leads to the assumption of predominant electron scattering by ionized impurities. For the lower electron densities at x = 0.775 and x =0.75 we find the typical Tt-dependence, according to the Conwell-Weisskopf formula for non-degenerate semiconductors, whereas the degenerate samples x = 1 and x = 0.9 show a behaviour similar to that expected by Koenig [6] for ionized impurity scattering in degenerate semiconductors. The considerable decrease of molibity with decreasing x is probably caused by changes in band structures as to be expected from the X-ray diffraction experiments. Assuming parabolic band structure and scattering by ionized impurities we determined the Fermi energy of the samples
10 April 1967
from thermoelectric power. The effective mass, estimated by comparison of the Fermi energy with the electron density, decreases from 0.10 m. at x = 0.9 to 0.05 m. at x = 0.75. Comparing investigations of the system Cd&al+0 lead to similar results. The present investigations will be extended to lower x, including optical measurements.
References and A. I. Ranyuk, Optics and 1. V. K. Miloslaviskii Spectr. 11 (1961) 536; T. K. Lakshmanan, J. Electrochem. Sot. 110 (1963) 548; H. Finkenrath, H. K’dhler and M. Lochmann, 2. angew. Phys. 21 (1966) 512. 2. Y. Colin and R. Tufeu, C. R. Acad. Sci. 256 (1963) 4195; Z.M. Jarzebski, Act.phys.pol. 29 (1966) 37; M. von Ortenberg, Diplomarbeit Darmstadt (1966). 3. E. Mollwo and R. Stumpp, 2. Phys. 184 (1965) 286. 4. R. Scholder, Angew. Chemie 66 (1965) 461. Gazz. chi&. Ital. 59 (1929) 5. G. Natta and L. Passerini, 129; G. Natta and L. Passerini, Gazz. chim. Ital. 60 (1930) 535. 6. S. H. Koenig, in: Report of the Intern. Conf. on the Physics of Semiconductors Exeter 1962, p. 5.
*****
ELECTRON
PARAMAGNETIC
RESONANCE
OF
Fe3+
IN NATURAL
TOPAZ
*
A. B. DENISON, T. C. ENSIGN ** and L. J. SIMS University of Wyoming, Laramie, Wyoming, USA. Received
14 March
1967
The EPR spectra of Fe3+, the dominant par amagnetic impurity in natural topaz, showed marked “superhyperfine” structure allowing the crystal site to be determined. Angular dependent spectra revealed strong zero-field splitting as predicted by an appropriate spin-Hamiltonian.
We have examined single crystals of natural topaz +** by using electron paramagnetic techniques and have learned the site of the paramagnetic impurity and something of the interactions responsible for the observed spectra. It was established early by X-ray fluorescence and chemical analysis that the crystal contains iron and the EPR spectra gave every indication that the impurity was Fe3+ (i. e. intensity and effective
g value). Topaz is an aluminum fluorosilicate [Al(F,OH) ]2SiO4 [l], the structure of which is an interweaving of silica tetrahedra and octahedral sites of the aluminum. The aluminum sites are paired as shown in * Research aided in part by Grant-in-Aid from Society of Sigma Xi. ** NASAFellow. *** Thomas Mountains, Juab Co., Utah, U.S.A.
405
PHYSICS
Volume 24A, number 8 b
v----Y ’ i
0
ALUMINUM
(IRON)
FLUORINE
0
OXYGEN
10 April 1967
LETTERS
The Al valence is 3+ as is Fe3+, and the fact that we have been able to obtain strong signals at room temperature and that the effective g value is near 4.2 leads us to believe that Fe3+ is the impurity and that it occupies the Al sites. The observed spectra (fig. 2) give strong evidence that our assumption is correct. These are two sets of triplets, one slightly more intense than the other. Each triplet corresponds to one of the sites mentioned and arises from the ligand hyperfine interaction of the Fe3+ with the neighboring fluorine nuclei. The relative intensities (1:2:1) of the triplet components indicate that each neighboring fluorine contributes an identical ligand hyperfine interaction. The strength of the isotropic hyperfine interaction is found to be A, = = 32 f 1 gauss, with a maximum anisotropic component of Aa = 10 f 1 gauss. In addition the center of the triplet has an angular dependence in agreement with the proposed site. Performing the appropriate angular transformation from the crystal axes (a, b, c) to the site axes (x, y, z) we obtain gx = 4.73
X.Y. 2
=
CRYSTALLINE
a, b, c
=
CRYSTAL
SITE
AXES
AXES
Fig. 1. Two adjacent octahedral sites in topaz. fig. 1. In general, there are eight magnetically inequivalent Al sites, however, these reduce to two when the crystal is rotated about the a-axis and the magnetic field is kept in the bc-plane. Each Al has two adjacent fluorines and four oxygens. For this site we have chosen the z-axis through the Al splitting the fluorines. It turns out that this axis is in fact the axis of highest symmetry (C2v), and a principal axis of the Hamiltonian.
IS’00
gz = 3.61 .
Fe3+ is a 6~~ ground state ion and must be under the influehce of strong zero-field splitting to give the large effective g values observed. This behavior has been observed in a number of substances [2.3] and is treated by F. Holuj [4] with the following Hamiltonian; H =D[S;-+S(S+l)]+E(S;-S;)
+@S*n.
For our case we mu_st also add the ligand hyperfine term CiS .A .Ii where the summation is over the neighboring fluorines. We have diagonalized this Hamiltonian using the axes of quantiza-
GAUSS
Fig. 2. Typical observed
406
gy = 4.64
spectrum
of Fe 3
in natural topaz for magnetic is 9.49 GHz.
field in bc plane.
The resonant
frequency
Volume 24A, number 8
PHYSICS
tion as the site axes acd transforming the external magnetic field into these axes. For the case E > H one obtains for the S=s three sets of Kramers The center Kramers doublet, the mainly cllblej;. 2s-z states, give the most isotropic and consistently strong spectra. It is from this pair of levels that the reported data are obtained. We have been able to fit gZ exactly and g, andgy quite well with E/D=O.45 and E =2 000 gauss. The full spectra, when the spectrometer is operated in a more sensitive mode, shows many lines. The further data are found to be in qualitative agreement with this general strong zero-field splitting picture and “+ = -6” and “-$= -z” lines are observed as well as the hyperfine “forbidden”
ELECTRON
LETTERS
10
April 1967
transitions [6]. Work is in progress at this time to complete the analysis. References
1. W.A.Deer et al., Rock Forming Minerals,
2. 3* 4 5:
Vol. 1 (Longmans, Green and Co., Ltd., London, 1962): The replacement of OH for F (for Utah Topaz) occurs only to a cery limited extent with 1.94 percent of the (F, OH) group being taken up with OH. T . Castner , Jr. , G.S.Newell, W.C.Ho1tonandC.P. Slichter, J.Chem.Phys.32 (1960) 668. H. Wickman, M.P.Klein and D.A.Shirley, J. Chem. Phys.42 (1965) 2113. F.Holuj, Ca.n.J. of Phys.44 (1966) 503. B. Bleaney and R.S.Rubins, Proc. Phys.Soc. (London) 77 (1961) 103.
AND PHONON EFFECTS IN SUPERCONDUCTING FCC LEAD-BASED ALLOYS * J. G. ADLER, J. E. JACKSON and T. A. WILL ** Department of Physics and Condensed State Center, Case Institute of Technology and Western Reserve University, Cleveland, Ohio 44106, U.S.A. Received
Electron tunneling measurements have been carried electron-phonon coupling strengths.
Using alloys of bismuth, thallium, and indium in lead, one can study fee lead-based alloys over a wide range of electron concentration. Since the atomic masses of bismuth and thallium are about the same as that of lead, changes in the electronphonon interaction in Pb-Bi and Pb-Tl arise primarily from changes in the electron concentration. On the other hand the atomic mass of indium is about half that of lead; thus in Pb-In alloys both the solute mass and electron concentration play an important role in determining the electronphonon interaction and phonon spectra. In this letter we report electron tunneling measurements in fee alloys of Pb with Bi, Tl, and In (over the range from 4.1 electrons/atom (e/a) to 3.3 e/a) and relate these measurements to * Work supported by the U.S. Air Force Office of Scientific Research through grant 565-66. ** National Aeronautics and Space Administration Predoctoral Trainee.
4 March
196’7
out in fee lead-based
alloys
over a wide range of
Table 1 Alloy Pb Ph Pb Pb Pb
e/a
A0 (meV)
2Ao/kT,
+ 20 at% Tl + 40 at% Tl + 60 at% Tl $
4.1 4.0 3.8 3.6 3.4
1.42 1.35 1.14 0.94 0.68
4.40 4.34 3.96 3.71 3.4
2at%In 3 at% In 5 at% In 12 at% In 15 at% In 16 at% In 26 at% In 27 at% In 50 a& In 70 at% In
3.98 3.97 3.95 3.88 3.85 3.84 3.74 3.73 3.50 3.30
1.36 1.36 1.35 1.33 1.32 1.32 1.29 1.28 1.20 1.18
4.34 4.38 4.37 4.35 4.33 4.34 4.36 4.35 4.20 4.20
+ 10 at% Bi
Pb+ Pb+ Pb + Pb + Pb + Pb + Pb f Pb + Pb + Pb +
OTh e gap for the Pb4Tl6 specimen was found by comparing the end point of the phonon spectrum observed by Ng and Brockhouse [8] with the corresponding singularity in da/d V.
407