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Luminescence of iron(III)
Volume 129, number 1
CHEMICAL PHYSICS LETTERS
15 August 1986
LUMINESCENCE OF IRON@) IN ZIRCON-STRUCTURED PHOSPHATES MPO, (M = SC, Lu, Y) E.W.J.L.
OOMEN,
K. VAN DER VLIST,
W.M.A.
SMIT and G. BLASSE
Solid State Department, Physical Loboratory, State Unwerstty of Utrecht, P.O. Box 80.000, 3508 TA Utrecht, The Netherlands
Received 29 April 1986; in final form 4 June 1986
Luminescence and decay time measurements are presented for Fe3+doped ScP04, LuP04 and YPO+ The Fe3+ ion gives a red luminescence. A consistent interpretation of the luminescence data is possible by assuming that the Fe3+ ion in ScPO4 is surrounded by four oxygen ions within a relatively weak crystal field and in LuP04 and YP04 by eight oxygen ions within a relatively strong crystal field.
1. Introduction
2. Experimental
Ions with a d5 configuration can act as efficient activators for luminescence. The Mn2+ ion is a wellknown example and many Mn2+-activated compounds have been proposed as luminescent materials with high efficiencies [ 1] . However, only a few Fe3+activated compounds with high luminescence efficiency are known; for example, I_iAlO,-Fe3+, LiCaO,Fe3+ [2], a-Ga203-Fe3+ [3] , Li&08-Fe3+ [4:5] and LiGa,08_-Fe3+ [6] . In these compounds the Fe3+ ion occupies tetrahedral and/or octahedral sites and replaces a host cation with a radius which is smaller than the radius of the Fe3+ ion itself [7] . In this paper the luminescence of the Fe3+ ion in ScPO,, LuPO4 and YPO4 is described. As far as we are aware, this is the first time that Fe3+ luminescence is observed for a composition in which the Fe3+ ion replaces ions with a larger radius than itself [7] . Evidence from EPR measurements on MPO,-Fe3+ (M = SC, Lu, Y) reveals that the Fe3+ ion substitutes for a trivalent host cation [8] . At this site the Fe3+ ion is surrounded by eight oxygen ions forming two distorted tetrahedra, in such a way that four oxygen ions are at a shorter and four at a longer distance from the Fe3+ ion [9,lO] (site symmetry DZd).
Starting materials were Y2O3, Lu2O3 (both Highways 99.999%), Sc2O3 (Highways 99.99%), Fe203 (Baker p.a.) and (NHd),HPO, (Merck, pa.). Fe203, M2O3 (M = SC, Lu, Y) and (NH,),HPO, (excess 30 mol%) were grinded and, subsequently, fired in air at 900°C for 5 h followed by a 20 h firing period at 12OO’C. The weighted-in amounts of Fe3+ were 450 ppm for ScP04-Fe3+, 100 ppm for LuP04-Fe3+ and 150 ppm and 1000 ppm for YP04-Fe3+. The Fe3+ concentrations were kept low, because the F$+ luminescence is known to quench rapidly due to concentration quenching [2,3] . All samples were checked by X-ray powder diffraction using Cu Ka radiation. The instrumentation used for the luminescence and decay time measurements was the same as described in ref. [ 1 l] . The spectra were corrected according to the procedures given in ref. [ 1 l] .
0 OO9-2614/86/$ 03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
3. Results All three compositions show at room temperature (RT) a red luminescence under UV excitation. The emission and excitation spectra of MPO,-Fe3+ (M = SC, Lu, Y) at liquid-helium temperature are shown in fig. 1. The excitation spectra consist of a broad intense 9
Volume 129, number 1
CHEMICAL PHYSICS LETTERS
Fig. 1. Emission (broken lines) and excitation (full lines) spectra of the luminescence of Fe3+ in (a) ScPO4, (b) LuPO4 and (c) YPO4. All spectra are recorded at liquid-helium temperature. 0 denotes the radiant power per constant energy interval in arbitrary units. qr gives the relative quantum output in arbitrary units. X 10 denotes that these excitation bands are ten times enlarged for clarity. Note that the excitation bands around 14000 cm-’ are rather inaccurate (see text).
band around 30000 cm-l. Using wide slits and high sensitivity it is possible to observe two other excitation bands: one very weak band around 22000 cm-l, and another weak band around 14000 cm-l. The shape and position of these weak excitation bands cannot be 10
15 August 1986
determined accurately because of their low intensity. For all three compositions the emission consists of one band. The emission maxima are at about 12000 cm-l for YP04-Fe3+ and LuPO4-Fe3+ and at about 15000 cm-l for ScP04-Fe3+. The luminescence intensity of YP04-Fe3+ (1000 ppm) is comparable with the intensity of YPO,-Fe3+ (150 ppm). This might indicate that concentration quenching occurs at low Fe3+ concentrations, in accordance with other observations [2,3] . However, one must be careful in drawing conclusions about concentration quenching from the present measurements, because the true built-in amount of Fe3+ in these lattices is unknown. Temperature quenching is observed for all three compounds. The temperature at which quenching starts decreases in the order ScPO,-Fe3+ (220 K), LuP04-Fe3+ (150 K), YP04-Fe3+ (100 K). For all three compounds the maximum of the emission band at RT is somewhat shifted to higher energy (about 200-300 cm-l) as compared with the maximum at liquid-helium temperature (LHeT). The position of the strong excitation band is independent of temperature. The weak excitation bands around 22000 and 14000 cm-l cannot be observed at higher temperature due to temperature quenching. For all three compounds the decay times are measured as a function of temperature in the region 4.2300 K. All decay curves are exponential. At LHeT the decay times are 960 * 10 ps for’ScP04-Fe?+, 1600 + 100 ~.lsfor LuP04-Fe3+ and 2150 f 50 ps for YP04 -Fe3+. The decay times remain constant up to the temperature at which quenching starts, while at increasing temperature the decay times decrease in accordance with the decrease in emission intensity.
4. Discussion The observed emission and excitation maxima at LHeT of all three compounds are collected in table 1. The Fe3+ ion has a 3d5 configuration. All transitions within the d shell are both spin and parity forbidden [ 12,131. The intensities of the excitation bands around 22000 and 14000 cm-l indicate that these bands are due to forbidden transitions. In agreement with literature data on similar systems [6,14,15] these bands are assigned as follows: 22000 cm-1 : 6A,
15 August 1986
CHEMICAL PHYSICS LETTERS
Volume 129, number 1
Table 1 Emission and excitation maxima at liquid-helium temperature (M = SC, Lu, Y) Compound
Exqitation maxima (cm-‘)
ScP04-Fe3+ LuPO4 -Fe3+ YPO4-Fe”+
32800 * 100 29400 f 100 28600 * 100
for the luminescence of Fe’+ in zircon-structured
phosphates MP04
Emission maximum (cm-l) 22500 f 300 21600 f 300 21300 f 300
+ 4T2, 14000 cm-l : 6A1 + 4T,. The broad band around 30000 cm-l is certainly not a transition within the d shell because of its high intensity. Furthermore, this band is much too broad to represent a transition on the Fe3+ ion itself. Therefore, this band is ascribed to a Fe3+ f 02- charge-transfer transition. The position of this band will be discussed below. Excitation in each of the three absorption bands results in the same emission band, indicating that the emission has to be ascribed to the lowest crystal field transitions, i.e. the 4T, + 6A, transition. The forbidden character of this transition is reflected by the rather long decay times. The small shift of the emission maximum towards higher energy at increasing temperatures can be explained by the population of higher levels in the relaxed excited state of the Fe3+ ion at higher temperatures and to a weakening of the crystal field due to increasing Fe3+-02- distances. The emission and excitation maxima of Fe3+ in ScPO, are at higher energies than those of Fe3+ in LuPO, and YP04 ; the largest differences being observed for the charge-transfer band and the emission band. The position of the charge-transfer band is correlated to the coordination number of the Fe3+ ion; it shifts to higher energy for lower coordination number. For four-coordinated Fe3+ in Liti the chargetransfer band is found at about 35000 cm-l [14], while for 6coordinated Fe3+ in cu-Ga203[3] or GdAl03 [16] it is observed at about 30000 cm-l. Therefore, the position of the charge-transfer band in the present samples indicates that the effective coordination number of the Fe3+ ion in YPO, and LuPO, is higher than in ScPO,. According to the Tanabe-Sugano diagram for a d5 ion the energy of the emission band is expected to decrease for increasing crystal field [ 12 ,131 . For tetrahedrally coordinated Fe3+ at low crystal-field,
15400 f 500 14200 f 500 14200 * 500
e.g.
14900 f 100 12150 f 300 11950 f 300
LiGaSO8--Fe 3+ [6] and ordered L&08-Fe3+
[ 141 the emission maximum is at 14700 cm-l. For
Fe3+ at octahedral sites (i.e. stronger crystal field), as in disordered LiA1508-Fe3+ [14], GdA103-Fe3+ [ 161, LiGaO, -Fe3+ and LiA102-Fe3+ [2 151, the emission maxima have been observed between 13500 and 13800 cm-l, while for six-coordinated Fe3+ in QIGa203 the emission maximum is observed at 10500 cm-l [3] . Therefore we conclude from the emission maxima of the present samples that the Fe3+ ion in YP04 and LuPO, has a higher coordination number (i.e. a stronger crystal field) than in ScP04, in agreement with the conclusion drawn from the position of the charge-transfer band. We realize, however, that some care must be taken in drawing conclusions about the strength of the crystal field from coordination numbers of dopant ions, especially at low concentrations The Fe3+-02- distances might be slightly different from the M3+-02- distances given in table 2, since some lattice relaxation may be expected upon substitution of a smaller ion. However, the nice agreement of the structural data of table 2 with the above conclusions drawn from the positions of the charge-transfer band and the emission band gives some confidence in this interpretation. In
Table 2 Host cation-oxygen distances in some zircon-structured phosphates. rr represents the shorter cation-oxygen distance and rz the longer one. The distances are calculated from the structural data given in ref. [ 81 Lattice
rr (A)
r2
YPO4 LuPO4 ScPO4
2.23 2.21 2.09
2.25 2.21 2.31
uu
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CHEMICAL PHYSICS LETTERS
YPO, and LuP04 the two M3+-02distances are nearly equal, leading to an eightfold coordination of the M3+ ions. In ScPO,, however, the two Sc3+--02distances differ substantially and the Sc3+ ion may be considered to be on a site with (pseudo) fourfold coordination. As can also be seen from table 1 the position of the crystal-field excitation bands is related to the coordination number of the M3+ ions. The Stokes shift of the Fe3+ luminescence (given by the energy difference between the maximum of the 6A1 + 4TI excitation band and the maximum of the 4T1 + 6A1 emission band) increases in the order ScP04-Fe3+, LuP04-Fe3+, YP04-Fe3+. The same ordering can be seen in the temperature quenching of the emission, the highest quenching temperature being found for ScPO,-Fe3+ (see section 3), showing that Stokes shift and temperature quenching behave in accordance with a simple configuration coordinate picture.
5. Conclusions As far as we are aware this is the first time that luminescence of the Fe3+ ion is reported in lattices in which the Fe3+ ion is substituted for host-lattice cations which are larger than the Fe3+ ion itself. The luminescence phenomena of the Fe3+ ion in zirconstructured phosphates can be explained by assuming that the dopant ion experiences almost the same environment as the host-lattice cations. This leads to pseudo-fourfold coordination and a weak crystal field
12
15 August 1986
in the case of ScPO,-Fe3+, and to near-eightfold coordination and a stronger crystal field for LuPO,Fe3+ and YP04-Fe3+,
References [ 11 K.H. Butler, Fluorescent lamp phosphors (The Pennsylvania State Univ. Press, University Park, 1980) ch. 15. [2] J.G. Rabatin, J. Electrochem. Sot. 125 (1978) 920. [3] G.T. Pott and B.D. McNicol, J. Luminescence 6 (1973) 225. [4] N.T. Melamed, F. de S. Barros, P.J. Viccaro and J.O. Artman, Phys. Rev. B5 (1972) 3377. [5] T. Abritta, F. de S. Barros and N.T. Melamed, J. Luminescence 33 (1985) 141. [6] C. McShera, P.J. Coleran, T.J. Glynn, G.F. Imbusch and J.P. Remeika, J. Luminescence 28 (1983) 41. [7] R.D. Shannon, Acta Cryst. A32 (1976) 751. [8] M. Rappaz, J.O. Ramey, L.A. Boatner and M.M. Abraham, J. Chem. Phys. 76 (1982) 40. [9] R.W.G. Wyckoff, Crystal structures, Vol. 2 (Wiley, New York, 1963). [lo] A.T. Aldred, Acta Cryst. B40 (1984) 569. [ll] A. Wolfert and G. Blasse, J. Solid State Chem. 59 (1985) 133. 1121 A.B.P. Lever, Inorganic electronic spectroscopy (Elsevier, Amsterdam, 1968). [ 131 S. Sugano, Y. Tanabe and H. Kamlmura, Multiplets of transition metals in crystals (Academic Press, New York, 1970). [14] N.T. Melamed, P.J. Viccaro, J.O. Artman and F. de S. Barros, J. Luminescence l/2 (1970) 348. [ 151 D.T. Palumbo, J. Luminescence 4 (1971) 89. [16] A.J. de Vries, W.J.J. Smeets and G. Blasse, to be published.
CHEMICAL PHYSICS LETTERS
15 August 1986
LUMINESCENCE OF IRON@) IN ZIRCON-STRUCTURED PHOSPHATES MPO, (M = SC, Lu, Y) E.W.J.L.
OOMEN,
K. VAN DER VLIST,
W.M.A.
SMIT and G. BLASSE
Solid State Department, Physical Loboratory, State Unwerstty of Utrecht, P.O. Box 80.000, 3508 TA Utrecht, The Netherlands
Received 29 April 1986; in final form 4 June 1986
Luminescence and decay time measurements are presented for Fe3+doped ScP04, LuP04 and YPO+ The Fe3+ ion gives a red luminescence. A consistent interpretation of the luminescence data is possible by assuming that the Fe3+ ion in ScPO4 is surrounded by four oxygen ions within a relatively weak crystal field and in LuP04 and YP04 by eight oxygen ions within a relatively strong crystal field.
1. Introduction
2. Experimental
Ions with a d5 configuration can act as efficient activators for luminescence. The Mn2+ ion is a wellknown example and many Mn2+-activated compounds have been proposed as luminescent materials with high efficiencies [ 1] . However, only a few Fe3+activated compounds with high luminescence efficiency are known; for example, I_iAlO,-Fe3+, LiCaO,Fe3+ [2], a-Ga203-Fe3+ [3] , Li&08-Fe3+ [4:5] and LiGa,08_-Fe3+ [6] . In these compounds the Fe3+ ion occupies tetrahedral and/or octahedral sites and replaces a host cation with a radius which is smaller than the radius of the Fe3+ ion itself [7] . In this paper the luminescence of the Fe3+ ion in ScPO,, LuPO4 and YPO4 is described. As far as we are aware, this is the first time that Fe3+ luminescence is observed for a composition in which the Fe3+ ion replaces ions with a larger radius than itself [7] . Evidence from EPR measurements on MPO,-Fe3+ (M = SC, Lu, Y) reveals that the Fe3+ ion substitutes for a trivalent host cation [8] . At this site the Fe3+ ion is surrounded by eight oxygen ions forming two distorted tetrahedra, in such a way that four oxygen ions are at a shorter and four at a longer distance from the Fe3+ ion [9,lO] (site symmetry DZd).
Starting materials were Y2O3, Lu2O3 (both Highways 99.999%), Sc2O3 (Highways 99.99%), Fe203 (Baker p.a.) and (NHd),HPO, (Merck, pa.). Fe203, M2O3 (M = SC, Lu, Y) and (NH,),HPO, (excess 30 mol%) were grinded and, subsequently, fired in air at 900°C for 5 h followed by a 20 h firing period at 12OO’C. The weighted-in amounts of Fe3+ were 450 ppm for ScP04-Fe3+, 100 ppm for LuP04-Fe3+ and 150 ppm and 1000 ppm for YP04-Fe3+. The Fe3+ concentrations were kept low, because the F$+ luminescence is known to quench rapidly due to concentration quenching [2,3] . All samples were checked by X-ray powder diffraction using Cu Ka radiation. The instrumentation used for the luminescence and decay time measurements was the same as described in ref. [ 1 l] . The spectra were corrected according to the procedures given in ref. [ 1 l] .
0 OO9-2614/86/$ 03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
3. Results All three compositions show at room temperature (RT) a red luminescence under UV excitation. The emission and excitation spectra of MPO,-Fe3+ (M = SC, Lu, Y) at liquid-helium temperature are shown in fig. 1. The excitation spectra consist of a broad intense 9
Volume 129, number 1
CHEMICAL PHYSICS LETTERS
Fig. 1. Emission (broken lines) and excitation (full lines) spectra of the luminescence of Fe3+ in (a) ScPO4, (b) LuPO4 and (c) YPO4. All spectra are recorded at liquid-helium temperature. 0 denotes the radiant power per constant energy interval in arbitrary units. qr gives the relative quantum output in arbitrary units. X 10 denotes that these excitation bands are ten times enlarged for clarity. Note that the excitation bands around 14000 cm-’ are rather inaccurate (see text).
band around 30000 cm-l. Using wide slits and high sensitivity it is possible to observe two other excitation bands: one very weak band around 22000 cm-l, and another weak band around 14000 cm-l. The shape and position of these weak excitation bands cannot be 10
15 August 1986
determined accurately because of their low intensity. For all three compositions the emission consists of one band. The emission maxima are at about 12000 cm-l for YP04-Fe3+ and LuPO4-Fe3+ and at about 15000 cm-l for ScP04-Fe3+. The luminescence intensity of YP04-Fe3+ (1000 ppm) is comparable with the intensity of YPO,-Fe3+ (150 ppm). This might indicate that concentration quenching occurs at low Fe3+ concentrations, in accordance with other observations [2,3] . However, one must be careful in drawing conclusions about concentration quenching from the present measurements, because the true built-in amount of Fe3+ in these lattices is unknown. Temperature quenching is observed for all three compounds. The temperature at which quenching starts decreases in the order ScPO,-Fe3+ (220 K), LuP04-Fe3+ (150 K), YP04-Fe3+ (100 K). For all three compounds the maximum of the emission band at RT is somewhat shifted to higher energy (about 200-300 cm-l) as compared with the maximum at liquid-helium temperature (LHeT). The position of the strong excitation band is independent of temperature. The weak excitation bands around 22000 and 14000 cm-l cannot be observed at higher temperature due to temperature quenching. For all three compounds the decay times are measured as a function of temperature in the region 4.2300 K. All decay curves are exponential. At LHeT the decay times are 960 * 10 ps for’ScP04-Fe?+, 1600 + 100 ~.lsfor LuP04-Fe3+ and 2150 f 50 ps for YP04 -Fe3+. The decay times remain constant up to the temperature at which quenching starts, while at increasing temperature the decay times decrease in accordance with the decrease in emission intensity.
4. Discussion The observed emission and excitation maxima at LHeT of all three compounds are collected in table 1. The Fe3+ ion has a 3d5 configuration. All transitions within the d shell are both spin and parity forbidden [ 12,131. The intensities of the excitation bands around 22000 and 14000 cm-l indicate that these bands are due to forbidden transitions. In agreement with literature data on similar systems [6,14,15] these bands are assigned as follows: 22000 cm-1 : 6A,
15 August 1986
CHEMICAL PHYSICS LETTERS
Volume 129, number 1
Table 1 Emission and excitation maxima at liquid-helium temperature (M = SC, Lu, Y) Compound
Exqitation maxima (cm-‘)
ScP04-Fe3+ LuPO4 -Fe3+ YPO4-Fe”+
32800 * 100 29400 f 100 28600 * 100
for the luminescence of Fe’+ in zircon-structured
phosphates MP04
Emission maximum (cm-l) 22500 f 300 21600 f 300 21300 f 300
+ 4T2, 14000 cm-l : 6A1 + 4T,. The broad band around 30000 cm-l is certainly not a transition within the d shell because of its high intensity. Furthermore, this band is much too broad to represent a transition on the Fe3+ ion itself. Therefore, this band is ascribed to a Fe3+ f 02- charge-transfer transition. The position of this band will be discussed below. Excitation in each of the three absorption bands results in the same emission band, indicating that the emission has to be ascribed to the lowest crystal field transitions, i.e. the 4T, + 6A, transition. The forbidden character of this transition is reflected by the rather long decay times. The small shift of the emission maximum towards higher energy at increasing temperatures can be explained by the population of higher levels in the relaxed excited state of the Fe3+ ion at higher temperatures and to a weakening of the crystal field due to increasing Fe3+-02- distances. The emission and excitation maxima of Fe3+ in ScPO, are at higher energies than those of Fe3+ in LuPO, and YP04 ; the largest differences being observed for the charge-transfer band and the emission band. The position of the charge-transfer band is correlated to the coordination number of the Fe3+ ion; it shifts to higher energy for lower coordination number. For four-coordinated Fe3+ in Liti the chargetransfer band is found at about 35000 cm-l [14], while for 6coordinated Fe3+ in cu-Ga203[3] or GdAl03 [16] it is observed at about 30000 cm-l. Therefore, the position of the charge-transfer band in the present samples indicates that the effective coordination number of the Fe3+ ion in YPO, and LuPO, is higher than in ScPO,. According to the Tanabe-Sugano diagram for a d5 ion the energy of the emission band is expected to decrease for increasing crystal field [ 12 ,131 . For tetrahedrally coordinated Fe3+ at low crystal-field,
15400 f 500 14200 f 500 14200 * 500
e.g.
14900 f 100 12150 f 300 11950 f 300
LiGaSO8--Fe 3+ [6] and ordered L&08-Fe3+
[ 141 the emission maximum is at 14700 cm-l. For
Fe3+ at octahedral sites (i.e. stronger crystal field), as in disordered LiA1508-Fe3+ [14], GdA103-Fe3+ [ 161, LiGaO, -Fe3+ and LiA102-Fe3+ [2 151, the emission maxima have been observed between 13500 and 13800 cm-l, while for six-coordinated Fe3+ in QIGa203 the emission maximum is observed at 10500 cm-l [3] . Therefore we conclude from the emission maxima of the present samples that the Fe3+ ion in YP04 and LuPO, has a higher coordination number (i.e. a stronger crystal field) than in ScP04, in agreement with the conclusion drawn from the position of the charge-transfer band. We realize, however, that some care must be taken in drawing conclusions about the strength of the crystal field from coordination numbers of dopant ions, especially at low concentrations The Fe3+-02- distances might be slightly different from the M3+-02- distances given in table 2, since some lattice relaxation may be expected upon substitution of a smaller ion. However, the nice agreement of the structural data of table 2 with the above conclusions drawn from the positions of the charge-transfer band and the emission band gives some confidence in this interpretation. In
Table 2 Host cation-oxygen distances in some zircon-structured phosphates. rr represents the shorter cation-oxygen distance and rz the longer one. The distances are calculated from the structural data given in ref. [ 81 Lattice
rr (A)
r2
YPO4 LuPO4 ScPO4
2.23 2.21 2.09
2.25 2.21 2.31
uu
11
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YPO, and LuP04 the two M3+-02distances are nearly equal, leading to an eightfold coordination of the M3+ ions. In ScPO,, however, the two Sc3+--02distances differ substantially and the Sc3+ ion may be considered to be on a site with (pseudo) fourfold coordination. As can also be seen from table 1 the position of the crystal-field excitation bands is related to the coordination number of the M3+ ions. The Stokes shift of the Fe3+ luminescence (given by the energy difference between the maximum of the 6A1 + 4TI excitation band and the maximum of the 4T1 + 6A1 emission band) increases in the order ScP04-Fe3+, LuP04-Fe3+, YP04-Fe3+. The same ordering can be seen in the temperature quenching of the emission, the highest quenching temperature being found for ScPO,-Fe3+ (see section 3), showing that Stokes shift and temperature quenching behave in accordance with a simple configuration coordinate picture.
5. Conclusions As far as we are aware this is the first time that luminescence of the Fe3+ ion is reported in lattices in which the Fe3+ ion is substituted for host-lattice cations which are larger than the Fe3+ ion itself. The luminescence phenomena of the Fe3+ ion in zirconstructured phosphates can be explained by assuming that the dopant ion experiences almost the same environment as the host-lattice cations. This leads to pseudo-fourfold coordination and a weak crystal field
12
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in the case of ScPO,-Fe3+, and to near-eightfold coordination and a stronger crystal field for LuPO,Fe3+ and YP04-Fe3+,
References [ 11 K.H. Butler, Fluorescent lamp phosphors (The Pennsylvania State Univ. Press, University Park, 1980) ch. 15. [2] J.G. Rabatin, J. Electrochem. Sot. 125 (1978) 920. [3] G.T. Pott and B.D. McNicol, J. Luminescence 6 (1973) 225. [4] N.T. Melamed, F. de S. Barros, P.J. Viccaro and J.O. Artman, Phys. Rev. B5 (1972) 3377. [5] T. Abritta, F. de S. Barros and N.T. Melamed, J. Luminescence 33 (1985) 141. [6] C. McShera, P.J. Coleran, T.J. Glynn, G.F. Imbusch and J.P. Remeika, J. Luminescence 28 (1983) 41. [7] R.D. Shannon, Acta Cryst. A32 (1976) 751. [8] M. Rappaz, J.O. Ramey, L.A. Boatner and M.M. Abraham, J. Chem. Phys. 76 (1982) 40. [9] R.W.G. Wyckoff, Crystal structures, Vol. 2 (Wiley, New York, 1963). [lo] A.T. Aldred, Acta Cryst. B40 (1984) 569. [ll] A. Wolfert and G. Blasse, J. Solid State Chem. 59 (1985) 133. 1121 A.B.P. Lever, Inorganic electronic spectroscopy (Elsevier, Amsterdam, 1968). [ 131 S. Sugano, Y. Tanabe and H. Kamlmura, Multiplets of transition metals in crystals (Academic Press, New York, 1970). [14] N.T. Melamed, P.J. Viccaro, J.O. Artman and F. de S. Barros, J. Luminescence l/2 (1970) 348. [ 151 D.T. Palumbo, J. Luminescence 4 (1971) 89. [16] A.J. de Vries, W.J.J. Smeets and G. Blasse, to be published.