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Mo5+ in synthetic zircon crystals
Volume 160, number 1
Mo5+ IN SYNTHETIC
CHEMICAL PHYSICS LETTERS
28 July 1989
ZIRCON CRYSTALS
K. EFTAXIAS University ofAthens, Department of Physics, Solid State Section, 104 Solonos Street, 10680 Athens, Greece
P.E. FIELDING Department of Physical and Xnorganic Chemistry University of New England, Armidale. NSW 2351, Australia
and G. LEHMANN Institutfir Physikalische Chemie, UniversitiitMiinster. Schlosspiatr 4, D-4400 M&aster, Federal Republic of Germany Received 21 April 1989; in final form 16 May 1989
In synthetic zircon crystals grown from Li20-Moo3 and simultaneously doped with U,O. and either Euz03 or Tbb203.Mo5+ was detected by EPR after UV illumination at 15 K. The site symmetry is monoclinic, but the deviation from orthorhombic symmetry is very small. MO’+ is most likely formed as a radiation defect of low thermal stability from Moe+ initially incorporated on Si sites. The g factors and satellite hypertine splitting constants for the mixture of “MO and 9’M~ nuclei were determined.
1. Introduction Although a number of radiation defects including Ti3+, Zr3+, Hf’+, Nb4+, SiOg- and SiOr as electron centers and Tb4+ or O- adjacent to different cations as hole centers are reported for natural zircons [ 1 ] the cause of the color of the natural red variety hyacinth has not yet been determined beyond all doubt. The strong natural radioactivity due to the high content of uranium in this mineral may cause extensive structural damage transforming the crystals into a so-called metamict state. It is conceivable that the uranium impurities are also directly involved in the formation of radiation defects. In order to test this possibility crystals doped with both uranium and different rare earth ions were synthesized. Color centers were readily produced following illumination with UV light of 365 nm wavelength. Different types could be characterized by their optical absorption spectra depending on the illumination [2] temperature. Although tentative models were proposed, structural elucidation requires more powerful methods such as EPR. Here, we report our 36
initial results for Mo5+ which is formed as an electron center of low thermal stability following W illumination at low temperatures.
2. Experitnentd Details of the crystal growth have been reported elsewhere [2]. For the present study samples simultaneously doped with U,Os and either Euz03 or Tbz03 were used, and most measurements were taken on a sample doped in the molar ratio U:Tb=7.8. EPR spectra were recorded for rotations of the crystals around the II and c axes on a Bruker X-band spectrometer model B-ER 414s at a temperature of about 15 K using a closed-cycle refrigeration system model 202 G of Air Products and Chemicals Ltd., Allentown, PA, USA. Measurements were made after the sample had been illuminated with a 150 W xenon lamp at 15 K for 6 h. The crystal was again illuminated after warming to room temperature prior to re-orienting in the second position. The magnetic fields were measured with an NMR gaussmeter while
0 009-26 14/89/s 03.50 Q Elsevier Science Publishers ( North-Holland Physics Publishing Division )
B.V.
Volume 160, number 1
the microwave frequency ceine as standard [ 3 1.
CHEMICAL PHYSICS LETTERS
was determined
using pi-
3. Results and discussion In addition to quartets of Tb4+ already visible at room temperature before UV illumination [ 4 ] several groups of EPR signals appeared after UV illumination. A valence state of MO as one source is easily recognized due to the characteristic sextet hyperfine satellites from the g5M~ and “MO nuclei with I= 5/2 and a calculated intensity ratio of 5.6% relative to the main signal from the even isotopes. Due to their similar magnetic moments and linewidths in the range 0.4 mT, the hypefine splittings of these two isotopes overlap. Fig. 1 shows a spectrum with the magnetic field along the c axis. At other orientations splittings into magnetically nonequivalent sets occurred. Splittings of up to four such sets at arbitrary orientations show that the true site symmetry is monoclinic. For rotation around the c axis a splitting into only two sets occurred with extreme values of both the g factor and the hyperfine splitting
along the rz axes showing that the deviation from orthorhombic site symmetry is very small. Fig. 2 shows the angular variation of the g factors; the principal values of the g matrix and the hyperfine splitting tensor are listed in table 1. The lower than axial site symmetry is almost certainly caused by local charge compensation. MOO:- is most likely initially incorporated on SiO!- sites since no charge compensation would be required for incorporation of Mo4+ on Zr sites. The low temperatures maintained during and after UV illumination prevent the migration of ions following photochemical change of oxidation state. Formation of Mo5+ is most likely and is consistent with the observation of only one fine structure transition with gfactors smaller than that of the free electron. According to the crystal structure data [ 5 ] the SiO, groups are significantly elongated along the c axis. Thus for MO’+ the d,l_,,l orbital should be lowest in energy, and to a first approximation the
Q
1.88
L
Y WO
36a
h
n
Y
Y
B,,/mT
28 July 1989
c
0
30
a
90
60 cngb
Jo
from cando
0
caisl’
Fig. 2. Angular variation of the g factors for MO’+ in zircon for rotations around thea (left, rotation from 0’ to 90” ) and e axes (right, rotation from 90” to 0”). Coincidence of the principal values with both c and (I shows that the site symmetry is practically orthorhombic. Table 1 Spin-Hamiltonian
Fig. I. A portion of the EPR spectrum at 9.28 GHz and about 15 K for B& after 6 h of UV illumination at the same temperature showing the main signal and hypefine satellites ofMo5+. A complicated pattern preceding and a single line following the first hyperfine satellite, both of unidentified origin, have been omitted.
a
I 60
parameters for MO’+ in zircon
i
&?A
&
x
1.9256(3) 1.9372( 3) 1.8863(3)
43.9(2) 42.7(2) 78.7(2)
Y I
(IO-4cm-L)
Orientation iI& llQ2 Ilc
31
Volume 160, number 1
CHEMICAL PHYSICS LETTERS
energies of the split components be calculated from the principal trix according to g,, = 2.0023 - lU/AE,,
,
g,,,,=2.0023-W/AE, , g, = 2.0023 - 2n/AE,,
of the tz orbital can values of the g ma-
(la) (lb)
.
served after low-temperature UV illumination. Since these ligand-field bands can only be of moderate intensity, the low concentration of this radiation defect of at most 100 ppm per lattice site allows for small intensities only. The main contribution must thus originate from allowed transitions of one or more as yet unidentified species.
(ICI
The spin-orbit coupling constant ,I for free MO’+ is 1013 cm-’ [ 61, and assuming a reduction to 900 cm- * due to covalency effects in the Si sites in zircon, this results in values of 62100,275OO and 23500 cm- ’ for g,,, g,,Yand g,, respectively. This indicates a large distortion from cubic symmetry, but for the elongated georhetry the dxy level is expected to lie below the other two. Very similar g factors were observed for the isoelectronic Nb4+ ion (in tetragonal sites) in zircon [ 71, although this larger ion almost certainly occupies Zr sites. The axial elongation [ 51 causes the same ordering of the energy levels. However, smaller values for the energy differences result from the smaller spin-orbit coupling constant of 742 cm- ’ for the free Nb4+ ion [ 61. For Mo5+ in the rutile [8,9] and brookite [IO] structures of TiO, as well as for other complexes of MO’+ [ 11,121, rather similar g factors and hyperfine splitting constants are observed. Also the value of A,, is roughly twice as large as the other two principal values, a characteristic of the d’ ions MoS+, Nb4+ as well as V4’ for which the largest body of experimental data exists. Two of the predicted energy levels for Mo5+ in zircon fall into the visible range, and their absorption bands may thus contribute to the blue color ob-
38
28 July 1989
Acknowledgement This work was supported by a grant from the Stiftung Volkswagenwerk to two of us (ICE and GL).
References [ 11AS. Marfunin, Spectroscopy, luminescence and radiation centers in minerals (Springer, Berlin, 1979) p. 283. [2] P.E. Fielding, Australian J. Chem. 23 (1970) 1513. [ 3 ] B. Schmitz, M. Jakubith and G. Lehmann, Z. Naturforsch. 34a (1979) 906. [4] D. Hutton and B. Milne, J. Phys. C 2 (1969) 2297. [ 5 ] IL Robinson, G.V. Gibbs and P.H. Ribbe, Am. Mineral. 56 (1971) 782. [6] S. Fraga, J. Karwowaki and K.M.S. Saxena, Handbook of atomic data (Elsevier, Amsterdam, 1976) p. 280. [7] V.M. Vinokurov, M.M. Zaripov, V.G. Stepanov, G.K. Chirkin and L.Y. Shekun, Soviet Phys. Solid State 5 ( 1963) 1487. [8] R.T. Kyi, Phys. Rev. 128 (1962) 151. [9] T.T. Chang, Phys. Rev. 136A (1964) 1413. [ IO ] VS. Grunin and M.V. Razumeenko, Soviet Phys. Solid State 22 (1980) 1449. [ 111N.M. Atherton, G. Danti, M. Ghedini and C. Oliva, J. Magn. Reson. 43 (1981) 167. [ 12 ] G.R. Hanson, G.L. Wilson, T.D. Bailey, J.R. Pilbrow and G.R. Wedd, J. Am. Chem. Sot. 109 (1987) 2609.
Mo5+ IN SYNTHETIC
CHEMICAL PHYSICS LETTERS
28 July 1989
ZIRCON CRYSTALS
K. EFTAXIAS University ofAthens, Department of Physics, Solid State Section, 104 Solonos Street, 10680 Athens, Greece
P.E. FIELDING Department of Physical and Xnorganic Chemistry University of New England, Armidale. NSW 2351, Australia
and G. LEHMANN Institutfir Physikalische Chemie, UniversitiitMiinster. Schlosspiatr 4, D-4400 M&aster, Federal Republic of Germany Received 21 April 1989; in final form 16 May 1989
In synthetic zircon crystals grown from Li20-Moo3 and simultaneously doped with U,O. and either Euz03 or Tbb203.Mo5+ was detected by EPR after UV illumination at 15 K. The site symmetry is monoclinic, but the deviation from orthorhombic symmetry is very small. MO’+ is most likely formed as a radiation defect of low thermal stability from Moe+ initially incorporated on Si sites. The g factors and satellite hypertine splitting constants for the mixture of “MO and 9’M~ nuclei were determined.
1. Introduction Although a number of radiation defects including Ti3+, Zr3+, Hf’+, Nb4+, SiOg- and SiOr as electron centers and Tb4+ or O- adjacent to different cations as hole centers are reported for natural zircons [ 1 ] the cause of the color of the natural red variety hyacinth has not yet been determined beyond all doubt. The strong natural radioactivity due to the high content of uranium in this mineral may cause extensive structural damage transforming the crystals into a so-called metamict state. It is conceivable that the uranium impurities are also directly involved in the formation of radiation defects. In order to test this possibility crystals doped with both uranium and different rare earth ions were synthesized. Color centers were readily produced following illumination with UV light of 365 nm wavelength. Different types could be characterized by their optical absorption spectra depending on the illumination [2] temperature. Although tentative models were proposed, structural elucidation requires more powerful methods such as EPR. Here, we report our 36
initial results for Mo5+ which is formed as an electron center of low thermal stability following W illumination at low temperatures.
2. Experitnentd Details of the crystal growth have been reported elsewhere [2]. For the present study samples simultaneously doped with U,Os and either Euz03 or Tbz03 were used, and most measurements were taken on a sample doped in the molar ratio U:Tb=7.8. EPR spectra were recorded for rotations of the crystals around the II and c axes on a Bruker X-band spectrometer model B-ER 414s at a temperature of about 15 K using a closed-cycle refrigeration system model 202 G of Air Products and Chemicals Ltd., Allentown, PA, USA. Measurements were made after the sample had been illuminated with a 150 W xenon lamp at 15 K for 6 h. The crystal was again illuminated after warming to room temperature prior to re-orienting in the second position. The magnetic fields were measured with an NMR gaussmeter while
0 009-26 14/89/s 03.50 Q Elsevier Science Publishers ( North-Holland Physics Publishing Division )
B.V.
Volume 160, number 1
the microwave frequency ceine as standard [ 3 1.
CHEMICAL PHYSICS LETTERS
was determined
using pi-
3. Results and discussion In addition to quartets of Tb4+ already visible at room temperature before UV illumination [ 4 ] several groups of EPR signals appeared after UV illumination. A valence state of MO as one source is easily recognized due to the characteristic sextet hyperfine satellites from the g5M~ and “MO nuclei with I= 5/2 and a calculated intensity ratio of 5.6% relative to the main signal from the even isotopes. Due to their similar magnetic moments and linewidths in the range 0.4 mT, the hypefine splittings of these two isotopes overlap. Fig. 1 shows a spectrum with the magnetic field along the c axis. At other orientations splittings into magnetically nonequivalent sets occurred. Splittings of up to four such sets at arbitrary orientations show that the true site symmetry is monoclinic. For rotation around the c axis a splitting into only two sets occurred with extreme values of both the g factor and the hyperfine splitting
along the rz axes showing that the deviation from orthorhombic site symmetry is very small. Fig. 2 shows the angular variation of the g factors; the principal values of the g matrix and the hyperfine splitting tensor are listed in table 1. The lower than axial site symmetry is almost certainly caused by local charge compensation. MOO:- is most likely initially incorporated on SiO!- sites since no charge compensation would be required for incorporation of Mo4+ on Zr sites. The low temperatures maintained during and after UV illumination prevent the migration of ions following photochemical change of oxidation state. Formation of Mo5+ is most likely and is consistent with the observation of only one fine structure transition with gfactors smaller than that of the free electron. According to the crystal structure data [ 5 ] the SiO, groups are significantly elongated along the c axis. Thus for MO’+ the d,l_,,l orbital should be lowest in energy, and to a first approximation the
Q
1.88
L
Y WO
36a
h
n
Y
Y
B,,/mT
28 July 1989
c
0
30
a
90
60 cngb
Jo
from cando
0
caisl’
Fig. 2. Angular variation of the g factors for MO’+ in zircon for rotations around thea (left, rotation from 0’ to 90” ) and e axes (right, rotation from 90” to 0”). Coincidence of the principal values with both c and (I shows that the site symmetry is practically orthorhombic. Table 1 Spin-Hamiltonian
Fig. I. A portion of the EPR spectrum at 9.28 GHz and about 15 K for B& after 6 h of UV illumination at the same temperature showing the main signal and hypefine satellites ofMo5+. A complicated pattern preceding and a single line following the first hyperfine satellite, both of unidentified origin, have been omitted.
a
I 60
parameters for MO’+ in zircon
i
&?A
&
x
1.9256(3) 1.9372( 3) 1.8863(3)
43.9(2) 42.7(2) 78.7(2)
Y I
(IO-4cm-L)
Orientation iI& llQ2 Ilc
31
Volume 160, number 1
CHEMICAL PHYSICS LETTERS
energies of the split components be calculated from the principal trix according to g,, = 2.0023 - lU/AE,,
,
g,,,,=2.0023-W/AE, , g, = 2.0023 - 2n/AE,,
of the tz orbital can values of the g ma-
(la) (lb)
.
served after low-temperature UV illumination. Since these ligand-field bands can only be of moderate intensity, the low concentration of this radiation defect of at most 100 ppm per lattice site allows for small intensities only. The main contribution must thus originate from allowed transitions of one or more as yet unidentified species.
(ICI
The spin-orbit coupling constant ,I for free MO’+ is 1013 cm-’ [ 61, and assuming a reduction to 900 cm- * due to covalency effects in the Si sites in zircon, this results in values of 62100,275OO and 23500 cm- ’ for g,,, g,,Yand g,, respectively. This indicates a large distortion from cubic symmetry, but for the elongated georhetry the dxy level is expected to lie below the other two. Very similar g factors were observed for the isoelectronic Nb4+ ion (in tetragonal sites) in zircon [ 71, although this larger ion almost certainly occupies Zr sites. The axial elongation [ 51 causes the same ordering of the energy levels. However, smaller values for the energy differences result from the smaller spin-orbit coupling constant of 742 cm- ’ for the free Nb4+ ion [ 61. For Mo5+ in the rutile [8,9] and brookite [IO] structures of TiO, as well as for other complexes of MO’+ [ 11,121, rather similar g factors and hyperfine splitting constants are observed. Also the value of A,, is roughly twice as large as the other two principal values, a characteristic of the d’ ions MoS+, Nb4+ as well as V4’ for which the largest body of experimental data exists. Two of the predicted energy levels for Mo5+ in zircon fall into the visible range, and their absorption bands may thus contribute to the blue color ob-
38
28 July 1989
Acknowledgement This work was supported by a grant from the Stiftung Volkswagenwerk to two of us (ICE and GL).
References [ 11AS. Marfunin, Spectroscopy, luminescence and radiation centers in minerals (Springer, Berlin, 1979) p. 283. [2] P.E. Fielding, Australian J. Chem. 23 (1970) 1513. [ 3 ] B. Schmitz, M. Jakubith and G. Lehmann, Z. Naturforsch. 34a (1979) 906. [4] D. Hutton and B. Milne, J. Phys. C 2 (1969) 2297. [ 5 ] IL Robinson, G.V. Gibbs and P.H. Ribbe, Am. Mineral. 56 (1971) 782. [6] S. Fraga, J. Karwowaki and K.M.S. Saxena, Handbook of atomic data (Elsevier, Amsterdam, 1976) p. 280. [7] V.M. Vinokurov, M.M. Zaripov, V.G. Stepanov, G.K. Chirkin and L.Y. Shekun, Soviet Phys. Solid State 5 ( 1963) 1487. [8] R.T. Kyi, Phys. Rev. 128 (1962) 151. [9] T.T. Chang, Phys. Rev. 136A (1964) 1413. [ IO ] VS. Grunin and M.V. Razumeenko, Soviet Phys. Solid State 22 (1980) 1449. [ 111N.M. Atherton, G. Danti, M. Ghedini and C. Oliva, J. Magn. Reson. 43 (1981) 167. [ 12 ] G.R. Hanson, G.L. Wilson, T.D. Bailey, J.R. Pilbrow and G.R. Wedd, J. Am. Chem. Sot. 109 (1987) 2609.