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Luminescence of mercuric molybdate and tungstate
Journal of Luminescence 9 (1974) 74-78. © North-Holland Publishing Company
LUMINESCENCE MOLYBDATE
OF MERCURIC
AND TUNGSTATE
G. BLASSE and G.P.M. van den HEUVEL Physical Laboratory, State University, Sorbonnelaan 4, Utrecht, The Netherlands
Received 2 April 1974
The compounds HgMoO4 and HgWO4 show characteristic molybdate and tungstate luminescence respectively. The emission colour of HgMoO4 is orange, that of HgWO4 blue-green. The thermal quenching temperature of their luminescence is relatively high.
1. Introduction The crystal structure of the isomorphous compounds HgMoO 4 and HgWO 4 has been reported recently [1 ]. Swindells [2] has reported a blue-white luminescence for HgWO 4 with a broad emission band peaking at about 490 nm. No further details were given. In the course of our studies on the luminescence of mixed metal oxides containing highly-charged transition metal ions (like Nb 5+, W 6+) it seemed interesting to study these compounds in more detail for two reasons. In the first place the octahedral coordination of Mo(W) in HgMo(W)O 4 is very irregular with two long Mo(W) O distances. In the second place the mercuric ions with a linear two-coordination by oxygen might influence the properties of the molybdate or tungstate group in an unexpected way. In this paper the luminescent properties of HgMoO 4 and HgWO 4 are reported. Their vibrational spectra will be discussed elsewhere [3].
2. Experimental method Powder samples with a near-white body colour were prepared as described before [3]. The way in which the luminescent properties were measured has been given in ref. [4].
3. Results Under ultraviolet excitation at room temperature HgMoO 4 shows an orange emis74
G. Blasse, (;.P.M. van den Hem, el, Molybdate and tungstate luminescence
l III
75
Ill
295 Kt/
I'/"\ IIII
I
,/
~6o
~,~o
58o
6~o
6~o
76o~
Fig. 1. Spectra] energy distribution of the emission of HgMoO4 at 4 and 295°K. Excitation wavelength 340 nm. @Xgives the radiant power per constant wavelength interval in arbitrary units. i
i
• -,\
F
~x
i
i
\ 295 K
T
420
46o
5do
~o
5ao
6)o ~
Fig. 2. Spectral energy distribution of the emission of HgWO 4 at 4 and 295°K. Excitation wavelength 300 nm.
G. Blasse, G.P.M. van den Heuvel, Molybdate and tungstate luminescence
76
Table 1 Position of the maxima of excitation and emission bands of HgWO 4 and HgMoO 4. All values in kK (= 1000 c m - 1 ) . Temperature (°K)
Maximum of excitation band
Maximum of emission band
Stokes shift
HgWO4
5 295
34.5 33.0
19.6 19.2
15 14
HgM°O4
5 295
29.3 29.3
16.3 16.8
13 12.5
sion of medium intensity. At liquid nitrogen temperature the emission intensity is higher. The compound HgWO4 shows under these conditions a strong blue-green emission. In figs. 1 and 2 we have drawn the emission spectra of these compounds at liquid helium and room temperature. Note that the temperature dependence of the position of the emission bands is different. The shift to longer wavelengths upon cooling the sample of HgMoO 4 is contradictory to what is usually observed for tungstates and molybdates. Both compounds show one broad excitation band in the ultraviolet spectral region. The position of its maximum is given in table 1, together with the emission maxima and the Stokes shift derived therefrom. In fig. 3 the temperature.dependence of the luminescence output is given. Roughly speaking these curves coincide with those reported for the analogous calcium compounds reported many years ago by
~
Hg MO 04
I
I -
1 0 0
I
I
-
5o
I
8
I
I
5o
I
t
100
~
t
150
--- T (°C)
Fig. 3. Temperature dependence of the relative luminescence output of HgMoO 4 and HgWO4. Excitation wavelength 340 and 300 nm respectively.
G. Blasse, G.P.M. van den Heuvel, Molybdate and tungstate luminescence
77
Kr6ger [5]. The quenching temperatures as defined by Kr6ger are 80°C for HgMoO4 and 130°C for HgWO4.
4. Discussion
Obviously the broad emission band of HgMo(W)0 4 may be ascribed to the molybdate (tungstate) group. In the first place the resemblance of this emission band to those of other molybdates (tungstates) [5, 6] is striking; in the second place characteristic emission from the Hg 2+ ion with d 10 configuration is unlikely [7]. This assignment has already been suggested by Swindells [2]. It is remarkable that the position of the emission and excitation bands of CdMoO 4 [5] and HgMoO4 on the one hand and CdWO 4 [5] and HgWO4 on the other hand are practically identical. The Cd 2+ ion (4d I 0) has a similar electron configuration to the Hg 2+ ion (5d10). The crystal structures, however, are different: CdWO 4 has the wolframite structure [8] and CdMoO 4 the scheelite structure [9], the former with WO 6 octahedra, the latter with MoO 4 tetrahedra. The vibrational spectra of CdWO4, HgMoO4 and HgWO4 show, however, that the molecular octahedral group in these compounds can be considered as a tetrahedral one [3]. This is also substantiated by the fact that two of the six M o ( W ) - O distances is relatively long [1, 8]. From these arguments we conclude that the molybdate (tungstate) emission of compounds with the wolframite or HgMoO 4 structure can be ascribed to electronic transitions within a tetrahedral Mo(W)04 group. The large Stokes shift of the mercuric compounds is also in line with this description, since the Stokes shift for the tetrahedral group ( > 12 000 cm -1) is much larger than for the octahedral group (< 1 0 0 0 0 c m -1) [5, 10]. This description implies that efficient molybdate (tungstate) luminescence has only been observed for tetrahedrally coordinated molybdenum (tungsten). The tungstates with ordered perovskite structure are in fact the only oxides with octahedrally coordinated W 6+ ions that luminesce with reasonable efficiency (but below room temperature) [5, 10]. It is also interesting to note that the quenching temperature of the luminescence of HgMo(W)O 4 is relatively high compared with those for other molybdates (tungstates). In fact HgMoO4 shows a quenching temperature of 80°C to be compared with 75°C for CaMoO 4 [5], the highest quenching temperature known up till now for molybdate luminescence. As a matter of fact this difference is not significant. At first sight this high quenching temperature seems to disagree with a rule proposed earlier [11 ], viz. the quenching temperature should increase if the surrounding cations are smaller, higher-charged or less polarisable. This describes the decrease of the quenching temperature if Ca in CaMoO 4 (Tq = 75°C) is replaced by the more polarisable Cd (Tq = 12°C) [5]. A further replacement of Cd by Hg with larger ionic radius than Cd should give a still lower Tq. The unexpected high value obtained for
78
G. Blasse, G.P.M. van den Heuvel, Molybdate and tungstate luminescence
HgMoO 4 can probably be related to the peculiar coordination of the Hg 2÷ ion in HgMoO 4 [I ]: Hg forms its characteristic two short collinear H g - O bonds (2.026 A), whereas the other H g - O distance are m u c h longer (2.672 and 2.766 A). These short, strong b o n d s are directed towards the m o l y b d a t e groups so that their surroundings are stiff compared to the situation in CdMoO 4. As a consequence the difference between the equilibrium distances of the ground and excited state of the luminescent centre is small and Tq relatively high [ 11 ]. Further studies on the influence of mercuric ions on m o l y b d a t e and tungstate luminescence are in progress.
References [ 1] [2] [3] [4] [5] [6] [7] [8] [9] [ 10]
W. Jeitschko and A.W. Sleight, Acta Cryst. B29 (1973) 869. F.E. Swindells, J. Opt. Soc. Amer. 41 (1951) 731. G. Blasse, J. lnorg. Nucl. Chem., in press. G. Blasse and G.P.M. van den Heuvel, Physica Star. Sol. (a) 19 (1973) 111. F.A. Kr6ger, Some Aspects of the Luminescence of Solids (Elsevier, Amsterdam, 1948). G. Blasse and A. Bril, J. Solid State Chem. 2 (1970) 291. G. Blasse, J. Chem. Phys. 48 (1968) 3108. A.P. Chichagov, V.V. llyukhin and N.V. Below, Dokl. Akad. Nauk SSSR 166 (1966) 87. A.W. Sleight, Acta Cryst. B28 (1972) 2899. G. Blasse, A.I:. Corsmit and M. van der Pas, Luminescence of crystals, Molecules and Solutions, ed. F. Williams (Plenum Press, New York, 1973) p. 612. [11] G. Blasse, J. Chem. Phys. 51 (1969) 3529.
LUMINESCENCE MOLYBDATE
OF MERCURIC
AND TUNGSTATE
G. BLASSE and G.P.M. van den HEUVEL Physical Laboratory, State University, Sorbonnelaan 4, Utrecht, The Netherlands
Received 2 April 1974
The compounds HgMoO4 and HgWO4 show characteristic molybdate and tungstate luminescence respectively. The emission colour of HgMoO4 is orange, that of HgWO4 blue-green. The thermal quenching temperature of their luminescence is relatively high.
1. Introduction The crystal structure of the isomorphous compounds HgMoO 4 and HgWO 4 has been reported recently [1 ]. Swindells [2] has reported a blue-white luminescence for HgWO 4 with a broad emission band peaking at about 490 nm. No further details were given. In the course of our studies on the luminescence of mixed metal oxides containing highly-charged transition metal ions (like Nb 5+, W 6+) it seemed interesting to study these compounds in more detail for two reasons. In the first place the octahedral coordination of Mo(W) in HgMo(W)O 4 is very irregular with two long Mo(W) O distances. In the second place the mercuric ions with a linear two-coordination by oxygen might influence the properties of the molybdate or tungstate group in an unexpected way. In this paper the luminescent properties of HgMoO 4 and HgWO 4 are reported. Their vibrational spectra will be discussed elsewhere [3].
2. Experimental method Powder samples with a near-white body colour were prepared as described before [3]. The way in which the luminescent properties were measured has been given in ref. [4].
3. Results Under ultraviolet excitation at room temperature HgMoO 4 shows an orange emis74
G. Blasse, (;.P.M. van den Hem, el, Molybdate and tungstate luminescence
l III
75
Ill
295 Kt/
I'/"\ IIII
I
,/
~6o
~,~o
58o
6~o
6~o
76o~
Fig. 1. Spectra] energy distribution of the emission of HgMoO4 at 4 and 295°K. Excitation wavelength 340 nm. @Xgives the radiant power per constant wavelength interval in arbitrary units. i
i
• -,\
F
~x
i
i
\ 295 K
T
420
46o
5do
~o
5ao
6)o ~
Fig. 2. Spectral energy distribution of the emission of HgWO 4 at 4 and 295°K. Excitation wavelength 300 nm.
G. Blasse, G.P.M. van den Heuvel, Molybdate and tungstate luminescence
76
Table 1 Position of the maxima of excitation and emission bands of HgWO 4 and HgMoO 4. All values in kK (= 1000 c m - 1 ) . Temperature (°K)
Maximum of excitation band
Maximum of emission band
Stokes shift
HgWO4
5 295
34.5 33.0
19.6 19.2
15 14
HgM°O4
5 295
29.3 29.3
16.3 16.8
13 12.5
sion of medium intensity. At liquid nitrogen temperature the emission intensity is higher. The compound HgWO4 shows under these conditions a strong blue-green emission. In figs. 1 and 2 we have drawn the emission spectra of these compounds at liquid helium and room temperature. Note that the temperature dependence of the position of the emission bands is different. The shift to longer wavelengths upon cooling the sample of HgMoO 4 is contradictory to what is usually observed for tungstates and molybdates. Both compounds show one broad excitation band in the ultraviolet spectral region. The position of its maximum is given in table 1, together with the emission maxima and the Stokes shift derived therefrom. In fig. 3 the temperature.dependence of the luminescence output is given. Roughly speaking these curves coincide with those reported for the analogous calcium compounds reported many years ago by
~
Hg MO 04
I
I -
1 0 0
I
I
-
5o
I
8
I
I
5o
I
t
100
~
t
150
--- T (°C)
Fig. 3. Temperature dependence of the relative luminescence output of HgMoO 4 and HgWO4. Excitation wavelength 340 and 300 nm respectively.
G. Blasse, G.P.M. van den Heuvel, Molybdate and tungstate luminescence
77
Kr6ger [5]. The quenching temperatures as defined by Kr6ger are 80°C for HgMoO4 and 130°C for HgWO4.
4. Discussion
Obviously the broad emission band of HgMo(W)0 4 may be ascribed to the molybdate (tungstate) group. In the first place the resemblance of this emission band to those of other molybdates (tungstates) [5, 6] is striking; in the second place characteristic emission from the Hg 2+ ion with d 10 configuration is unlikely [7]. This assignment has already been suggested by Swindells [2]. It is remarkable that the position of the emission and excitation bands of CdMoO 4 [5] and HgMoO4 on the one hand and CdWO 4 [5] and HgWO4 on the other hand are practically identical. The Cd 2+ ion (4d I 0) has a similar electron configuration to the Hg 2+ ion (5d10). The crystal structures, however, are different: CdWO 4 has the wolframite structure [8] and CdMoO 4 the scheelite structure [9], the former with WO 6 octahedra, the latter with MoO 4 tetrahedra. The vibrational spectra of CdWO4, HgMoO4 and HgWO4 show, however, that the molecular octahedral group in these compounds can be considered as a tetrahedral one [3]. This is also substantiated by the fact that two of the six M o ( W ) - O distances is relatively long [1, 8]. From these arguments we conclude that the molybdate (tungstate) emission of compounds with the wolframite or HgMoO 4 structure can be ascribed to electronic transitions within a tetrahedral Mo(W)04 group. The large Stokes shift of the mercuric compounds is also in line with this description, since the Stokes shift for the tetrahedral group ( > 12 000 cm -1) is much larger than for the octahedral group (< 1 0 0 0 0 c m -1) [5, 10]. This description implies that efficient molybdate (tungstate) luminescence has only been observed for tetrahedrally coordinated molybdenum (tungsten). The tungstates with ordered perovskite structure are in fact the only oxides with octahedrally coordinated W 6+ ions that luminesce with reasonable efficiency (but below room temperature) [5, 10]. It is also interesting to note that the quenching temperature of the luminescence of HgMo(W)O 4 is relatively high compared with those for other molybdates (tungstates). In fact HgMoO4 shows a quenching temperature of 80°C to be compared with 75°C for CaMoO 4 [5], the highest quenching temperature known up till now for molybdate luminescence. As a matter of fact this difference is not significant. At first sight this high quenching temperature seems to disagree with a rule proposed earlier [11 ], viz. the quenching temperature should increase if the surrounding cations are smaller, higher-charged or less polarisable. This describes the decrease of the quenching temperature if Ca in CaMoO 4 (Tq = 75°C) is replaced by the more polarisable Cd (Tq = 12°C) [5]. A further replacement of Cd by Hg with larger ionic radius than Cd should give a still lower Tq. The unexpected high value obtained for
78
G. Blasse, G.P.M. van den Heuvel, Molybdate and tungstate luminescence
HgMoO 4 can probably be related to the peculiar coordination of the Hg 2÷ ion in HgMoO 4 [I ]: Hg forms its characteristic two short collinear H g - O bonds (2.026 A), whereas the other H g - O distance are m u c h longer (2.672 and 2.766 A). These short, strong b o n d s are directed towards the m o l y b d a t e groups so that their surroundings are stiff compared to the situation in CdMoO 4. As a consequence the difference between the equilibrium distances of the ground and excited state of the luminescent centre is small and Tq relatively high [ 11 ]. Further studies on the influence of mercuric ions on m o l y b d a t e and tungstate luminescence are in progress.
References [ 1] [2] [3] [4] [5] [6] [7] [8] [9] [ 10]
W. Jeitschko and A.W. Sleight, Acta Cryst. B29 (1973) 869. F.E. Swindells, J. Opt. Soc. Amer. 41 (1951) 731. G. Blasse, J. lnorg. Nucl. Chem., in press. G. Blasse and G.P.M. van den Heuvel, Physica Star. Sol. (a) 19 (1973) 111. F.A. Kr6ger, Some Aspects of the Luminescence of Solids (Elsevier, Amsterdam, 1948). G. Blasse and A. Bril, J. Solid State Chem. 2 (1970) 291. G. Blasse, J. Chem. Phys. 48 (1968) 3108. A.P. Chichagov, V.V. llyukhin and N.V. Below, Dokl. Akad. Nauk SSSR 166 (1966) 87. A.W. Sleight, Acta Cryst. B28 (1972) 2899. G. Blasse, A.I:. Corsmit and M. van der Pas, Luminescence of crystals, Molecules and Solutions, ed. F. Williams (Plenum Press, New York, 1973) p. 612. [11] G. Blasse, J. Chem. Phys. 51 (1969) 3529.