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The nature of the luminescent centres in calcium uranate (Ca3UO6)
Solid State Communications,Vol. 19, pp. 779-781, 1976.
Pergamon Press.
Printed in Great Britain
THE NATURE OF THE LUMINESCENT CENTRES IN CALCIUM URANATE (Ca3UO6) G. Blasse
Physical Laboratory, State University, Sorbonnelaan 4, Utrecht, Netherlands
(Received 22 March 1976 by A.R. Miedema) The emission of Ca3UO 6 at 4 K is due to regular uranate octahedra as it is in Ca3WO6-U and Ca3TeO6-U. At higher temperatures, however, the excitation energy migrates through the lattice until it is trapped at either uranate groups near defects or at killer sites. 1. INTRODUCTION THE LUMINESCENCE of hexavalent uranium in solids is often ascribed to the linear uranyl group. Recently De Hair and Blasse have shown that in ordered perovskires the uranium luminescence is definitely due to the octahedral uranate group. 1 The emission contains a parity-forbidden transition. The spectrum consists of a zero-phonon line with a number of vibronics which are mainly due to coupling with the ungerade vibrational modes of the uranate octahedron. Concentration quenching occurred at room temperature in these systems at a few percent of uranium due to efficient U - U energy transport. In this paper we report on the luminescence of Ca3UO 6 which has also an ordered perovskite structure. 2 The U-concentration is now much higher than in the systems reported before. 1 Natansohn 3 has reported earlier on the luminescence of CaaUO6. He observed a shift of the emission band from 515 nm at 300 K to 570 nm at 78 K. In an earlier study of Ca3UO 6 we did not observe the latter emission. 4 It will be shown here that at 4 K the excitation energy remains localized in Ca3UO6, but that at higher temperatures this energy migrates through the lattice. Part of it is trapped at uranate octahedra near defects in the lattice. These emit the characteristic uranate emission which, however, is different from the emission of the regular uranate octahedra in Ca3UO 6. 2. EXPERIMENTAL Samples were prepared and characterized as described before. 1 This reference contains also a description of the apparatus used for the luminescence measurements. 3. RESULTS In Fig. 1. we have given the emission spectrum of Ca3UO 6 at 4 and 78 K. The vibrational fine structure 779
has been analyzed in Table 1 in comparison with the vibrational spectrum of Ca3UO6 .4'5 We measured also the emission spectrum of the uranium luminescence of the isomorphous CaaWO6-U (0.3%) and Ca3TeO6-U (0.3%). These spectra are identical. That of the tungstate at 4 K is also analyzed in Table 1. At 78 K this spectrum is similar apart from the occurrence of two hot lines (at 80 and 170 cm -1 higher energy than the zero-phonon line). The spectra of the diluted compounds do not depend on the preparation procedure. The 78 K emission spectra of Ca3UO 6 depend markedly on the preparation procedure in a way which we could not control. The 4 K emission of Ca3UO 6 does not depend on the preparation procedure. 4. DISCUSSION The luminescence of the diluted compounds is very similar to those reported earlier. 1 The emission must be ascribed to U 6÷ substituted for W6÷ or Te 6÷, i.e. to the UO66- octahedron. Note the dominant contribution to the total emission of the vibronics due to coupling with a single, ungerade phonon of the v3, u4 or u6 mode (see Table 1). The zero-phonon line and the vibronics due to coupling with more than one phonon are relatively weak. The emission spectrum of CaaUO 6 at 4 K resembles that of the dilute compounds very much. There is a shift of some 10 nm to longer wavelength from the diluted to the concentrated uranium system. This may be due to the somewhat larger lattice dimensions of CaaUO 6. The overall resemblance leads to the conclusion that at 4 K the emission is due to UO 6- groups at regular sites in the lattice of Ca3UO 6. This includes that at low temperatures the excitation energy is trapped at regular lattice sites and does not migrate through the lattice. This is in contradiction with the concentration quenching results mentioned in reference 1. We will return to this point below. Here we note the following differences between the emission pattern of Ca3UO6 and the diluted samples:
LUMINESCENT CENTRES IN CALCIUM URANATE
780
Vol. 19, No. 8
Table 1. Vibrational spectrum o f Ca3U06 and vibrational fine structure at 4 K o f the emission o f Ca3U06 and Ca3 WO6-U (0.3%) under long-wavelength u.v. excitation. Assignment in terms o f internal, octahedral UO~- modes and external mode Via+.All values in cm -1. The position o f the vibronics is relative to the zero-phonon ( 0 - 0 ) transition the position o f which is given in nm. The intensity o f the vibronics is indicated: s: strong, m: medium, w: weak
6
i c
; i-v-6
Vibrational spectrum Ca3UO6
\
i i
\2v~.v~ ^ "~--~V1
\ +Y 3
75 v~t" 150 170 200
Vibrational fine structure at 4 K CaaUO6
Ca3 WO6-U (0.3 %)
0-0
0-0
--130
505nm(w) (s)
496nm(m)
--80
(w)
-- 300 (s)
--280
(s)
~370 (510 P3 ~575 ~630 vl 725
- - 3 7 0 (s) - - 5 3 0 (s)
--380 --540
(s) (m)
Pl
--
v6 / 230 I
500
,
52.0
540 - -
---1,,,,,-
5 0
~260
nm
X
P4 1340 Fig. 1. Spectral energy distribution of the emission of Ca3UO 6 at 4 K (a) and at 78 K (b). Excitation wavelength 330 nm. The curve has not been corrected for photomultiplier response. (a) the zero-phonon line is less intense in Ca3UO 6 (b) the vibronics due to coupling with more than one phonon (ul or 2v I and one of the ungerade modes) have higher intenisty in Ca3UO 6. From these facts we conclude: (a) The spectral overlap of emission and absorption is very small in Ca3UO6 at 4K, because it is restricted to the weak zero-phonon line. This overlap determines the U - U energy transfer probability which, therefore, will be small at 4 K. (b) The vibrationalelectronic interaction in the case of Ca3UO6 is stronger than in the diluted samples. As a consequence the difference between the equilibrium distances of the emitting state and the ground state is larger in Ca3UO 6 than in the diluted compoundsfi This corroborates rules given by us before. 7 At temperatures above 4 K the emission of Ca3UO6 changes drastically, whereas that of the diluted samples remains the same apart from the occurrence of hot lines in the emission spectrum. This indicates that with increasing temperature the spectral overlap of emission and absorption increases so that in CaaUO 6 the excitation energy may become mobile. Our experimental results prove in fact that this occurs. The emission intensity of the regular UO66- octahedra decreases and longer-wavelength emission with a uranate-like appearance
occurs.
The migrating excitation energy is obviously trapped
7t- P l a t .
P l At- P 6 /)1 -[- I)4
vl+v3 2pl + Plat. 2Pl +/"6 2Pl + P3
-- 610 (w) -- 760 (m) 860 990 - - 1050 -- 1190 -- 1600 - - 1750 -- 1900 --
(m) (m) (w) (w) (w) (w) (w)
-- 960 (w) -- 1080 (w) -- 1220 (w)
at uranate octahedra situated near defect sites in the lattice. Because the overall intensity also decreases with increasing temperature, energy transport to killer sites seems to be competitive with that to the "defect" uranate octahedra. Since the nature of the defects will depend on the starting materials and perparation procedure it is not surprising that the emission of different CaaUO 6 samples is different at not too low temperatures. In view of work carried out previously in this laboratory s'9 and a recent paper on the influence of order on polarization energy in ordered perovskites, 1° one of the possibilities for such a "defect" uranate group is a slight deviation from complete crystallographic order on the octahedral sublattice of Ca3U06. Acknowledgement - The author is greatly indebted to G.P.M. van den Heuvel for skilful technical assistance.
Vol. 19, No. 8
LUMINESCENT CENTRES IN CALCIUM URANATE REFERENCES
1.
DE HAIR J.Th. W. & BLASSE G., J. Solid State Chem. (in press).
2.
RIETKERK H.M., Acta Cryst. 20, 508 (1966).
3.
NATANSOHN S., J. Electrochem. Soc. 120, 660 (1973).
4.
BLASSE G. & CORSMIT A.F., J. Solid State Chem. 6, 513 (1973).
5.
KEMMLER-SACKS.&SEEMANNI.,Z. Anorg. Allgem. Chem. 411,61 (1975).
6.
BALLHAUSEN C.J., Theor. Chim. Acta l, 285 (1963).
7.
BLASSE G., J. Chem. Phys. 51, 3529 (1969).
8.
BODE J.H.G. & VAN OOSTERHOUT A.B., J. Luminesc. 10, 237 (1975).
9.
MACKE A.J.H. (to be published).
10.
PATRAT G., BRUNEL M. & DE BERGEVIN F., J. Phys. Chem. Solids 37, 285 (1976).
781
Pergamon Press.
Printed in Great Britain
THE NATURE OF THE LUMINESCENT CENTRES IN CALCIUM URANATE (Ca3UO6) G. Blasse
Physical Laboratory, State University, Sorbonnelaan 4, Utrecht, Netherlands
(Received 22 March 1976 by A.R. Miedema) The emission of Ca3UO 6 at 4 K is due to regular uranate octahedra as it is in Ca3WO6-U and Ca3TeO6-U. At higher temperatures, however, the excitation energy migrates through the lattice until it is trapped at either uranate groups near defects or at killer sites. 1. INTRODUCTION THE LUMINESCENCE of hexavalent uranium in solids is often ascribed to the linear uranyl group. Recently De Hair and Blasse have shown that in ordered perovskires the uranium luminescence is definitely due to the octahedral uranate group. 1 The emission contains a parity-forbidden transition. The spectrum consists of a zero-phonon line with a number of vibronics which are mainly due to coupling with the ungerade vibrational modes of the uranate octahedron. Concentration quenching occurred at room temperature in these systems at a few percent of uranium due to efficient U - U energy transport. In this paper we report on the luminescence of Ca3UO 6 which has also an ordered perovskite structure. 2 The U-concentration is now much higher than in the systems reported before. 1 Natansohn 3 has reported earlier on the luminescence of CaaUO6. He observed a shift of the emission band from 515 nm at 300 K to 570 nm at 78 K. In an earlier study of Ca3UO 6 we did not observe the latter emission. 4 It will be shown here that at 4 K the excitation energy remains localized in Ca3UO6, but that at higher temperatures this energy migrates through the lattice. Part of it is trapped at uranate octahedra near defects in the lattice. These emit the characteristic uranate emission which, however, is different from the emission of the regular uranate octahedra in Ca3UO 6. 2. EXPERIMENTAL Samples were prepared and characterized as described before. 1 This reference contains also a description of the apparatus used for the luminescence measurements. 3. RESULTS In Fig. 1. we have given the emission spectrum of Ca3UO 6 at 4 and 78 K. The vibrational fine structure 779
has been analyzed in Table 1 in comparison with the vibrational spectrum of Ca3UO6 .4'5 We measured also the emission spectrum of the uranium luminescence of the isomorphous CaaWO6-U (0.3%) and Ca3TeO6-U (0.3%). These spectra are identical. That of the tungstate at 4 K is also analyzed in Table 1. At 78 K this spectrum is similar apart from the occurrence of two hot lines (at 80 and 170 cm -1 higher energy than the zero-phonon line). The spectra of the diluted compounds do not depend on the preparation procedure. The 78 K emission spectra of Ca3UO 6 depend markedly on the preparation procedure in a way which we could not control. The 4 K emission of Ca3UO 6 does not depend on the preparation procedure. 4. DISCUSSION The luminescence of the diluted compounds is very similar to those reported earlier. 1 The emission must be ascribed to U 6÷ substituted for W6÷ or Te 6÷, i.e. to the UO66- octahedron. Note the dominant contribution to the total emission of the vibronics due to coupling with a single, ungerade phonon of the v3, u4 or u6 mode (see Table 1). The zero-phonon line and the vibronics due to coupling with more than one phonon are relatively weak. The emission spectrum of CaaUO 6 at 4 K resembles that of the dilute compounds very much. There is a shift of some 10 nm to longer wavelength from the diluted to the concentrated uranium system. This may be due to the somewhat larger lattice dimensions of CaaUO 6. The overall resemblance leads to the conclusion that at 4 K the emission is due to UO 6- groups at regular sites in the lattice of Ca3UO 6. This includes that at low temperatures the excitation energy is trapped at regular lattice sites and does not migrate through the lattice. This is in contradiction with the concentration quenching results mentioned in reference 1. We will return to this point below. Here we note the following differences between the emission pattern of Ca3UO6 and the diluted samples:
LUMINESCENT CENTRES IN CALCIUM URANATE
780
Vol. 19, No. 8
Table 1. Vibrational spectrum o f Ca3U06 and vibrational fine structure at 4 K o f the emission o f Ca3U06 and Ca3 WO6-U (0.3%) under long-wavelength u.v. excitation. Assignment in terms o f internal, octahedral UO~- modes and external mode Via+.All values in cm -1. The position o f the vibronics is relative to the zero-phonon ( 0 - 0 ) transition the position o f which is given in nm. The intensity o f the vibronics is indicated: s: strong, m: medium, w: weak
6
i c
; i-v-6
Vibrational spectrum Ca3UO6
\
i i
\2v~.v~ ^ "~--~V1
\ +Y 3
75 v~t" 150 170 200
Vibrational fine structure at 4 K CaaUO6
Ca3 WO6-U (0.3 %)
0-0
0-0
--130
505nm(w) (s)
496nm(m)
--80
(w)
-- 300 (s)
--280
(s)
~370 (510 P3 ~575 ~630 vl 725
- - 3 7 0 (s) - - 5 3 0 (s)
--380 --540
(s) (m)
Pl
--
v6 / 230 I
500
,
52.0
540 - -
---1,,,,,-
5 0
~260
nm
X
P4 1340 Fig. 1. Spectral energy distribution of the emission of Ca3UO 6 at 4 K (a) and at 78 K (b). Excitation wavelength 330 nm. The curve has not been corrected for photomultiplier response. (a) the zero-phonon line is less intense in Ca3UO 6 (b) the vibronics due to coupling with more than one phonon (ul or 2v I and one of the ungerade modes) have higher intenisty in Ca3UO 6. From these facts we conclude: (a) The spectral overlap of emission and absorption is very small in Ca3UO6 at 4K, because it is restricted to the weak zero-phonon line. This overlap determines the U - U energy transfer probability which, therefore, will be small at 4 K. (b) The vibrationalelectronic interaction in the case of Ca3UO6 is stronger than in the diluted samples. As a consequence the difference between the equilibrium distances of the emitting state and the ground state is larger in Ca3UO 6 than in the diluted compoundsfi This corroborates rules given by us before. 7 At temperatures above 4 K the emission of Ca3UO6 changes drastically, whereas that of the diluted samples remains the same apart from the occurrence of hot lines in the emission spectrum. This indicates that with increasing temperature the spectral overlap of emission and absorption increases so that in CaaUO 6 the excitation energy may become mobile. Our experimental results prove in fact that this occurs. The emission intensity of the regular UO66- octahedra decreases and longer-wavelength emission with a uranate-like appearance
occurs.
The migrating excitation energy is obviously trapped
7t- P l a t .
P l At- P 6 /)1 -[- I)4
vl+v3 2pl + Plat. 2Pl +/"6 2Pl + P3
-- 610 (w) -- 760 (m) 860 990 - - 1050 -- 1190 -- 1600 - - 1750 -- 1900 --
(m) (m) (w) (w) (w) (w) (w)
-- 960 (w) -- 1080 (w) -- 1220 (w)
at uranate octahedra situated near defect sites in the lattice. Because the overall intensity also decreases with increasing temperature, energy transport to killer sites seems to be competitive with that to the "defect" uranate octahedra. Since the nature of the defects will depend on the starting materials and perparation procedure it is not surprising that the emission of different CaaUO 6 samples is different at not too low temperatures. In view of work carried out previously in this laboratory s'9 and a recent paper on the influence of order on polarization energy in ordered perovskites, 1° one of the possibilities for such a "defect" uranate group is a slight deviation from complete crystallographic order on the octahedral sublattice of Ca3U06. Acknowledgement - The author is greatly indebted to G.P.M. van den Heuvel for skilful technical assistance.
Vol. 19, No. 8
LUMINESCENT CENTRES IN CALCIUM URANATE REFERENCES
1.
DE HAIR J.Th. W. & BLASSE G., J. Solid State Chem. (in press).
2.
RIETKERK H.M., Acta Cryst. 20, 508 (1966).
3.
NATANSOHN S., J. Electrochem. Soc. 120, 660 (1973).
4.
BLASSE G. & CORSMIT A.F., J. Solid State Chem. 6, 513 (1973).
5.
KEMMLER-SACKS.&SEEMANNI.,Z. Anorg. Allgem. Chem. 411,61 (1975).
6.
BALLHAUSEN C.J., Theor. Chim. Acta l, 285 (1963).
7.
BLASSE G., J. Chem. Phys. 51, 3529 (1969).
8.
BODE J.H.G. & VAN OOSTERHOUT A.B., J. Luminesc. 10, 237 (1975).
9.
MACKE A.J.H. (to be published).
10.
PATRAT G., BRUNEL M. & DE BERGEVIN F., J. Phys. Chem. Solids 37, 285 (1976).
781