Luminescence of Exchange Coupled Pairs of Transition Metal Ions

Transcription

1 Journal of The Electrochemical Society, E313-E /2001/148 7 /E313/8/$7.00 The Electrochemical Society, Inc. Luminescence of Exchange Coupled Pairs of Transition Metal Ions A. P. Vink, a,b, z M. A. de Bruin, a S. Roke, a P. S. Peijzel, a and A. Meijerink a a Department of Condensed Matter, Debye Institute, 3508 TA Utrecht, The Netherlands b Radiation Technology Group, Interfaculty Reactor Institute, 2629 JB Delft, The Netherlands E313 The influence of exchange interaction between paramagnetic ions on the luminescence properties is investigated for Mn 2 -Mn 2, Cr 3 -Cr 3,Cr 3 -Gd 3, and Mn 2 -Gd 3 pairs. Two effects are reported: shortening of the lifetime of the spin-forbidden emission and a shift of emission lines or bands to a longer wavelength. The shortening of the lifetime of the spin-forbidden emission is important in Mn 2 doped phosphors. For application of Mn 2 -doped phosphors in displays the long lifetime of the Mn 2 emission is a problem. Shortening of the lifetime is possible by exchange coupling with Mn 2 neighbors, or possibly other paramagnetic neighbors. The results reported here show that the lifetime shortening depends on the exchange-coupling parameter J. The strongest coupling is observed in Cr 3 -Cr 3 pairs and the lifetime shortening is strong e.g., inlaalo 3 :Cr 3 J 63 cm 1 and the lifetime is about 30 times shorter for the pair emission. The coupling in Mn 2 -Mn 2 and Cr 3 -Gd 3 pairs is weak. Typically, values for J are around 1 cm 1 and the lifetime is reduced by a factor of 2-10 e.g., J is 1.0 cm 1 for exchange coupling between Cr 3 and Gd 3 ; the lifetime of the spin-forbidden Cr 3 emission is 14 ms in GdAlO 3 compared to 56 ms in LaAlO 3. The lifetime shortening for the Mn 2 emission is not related to the observed red shift for the pair emission. For the Mn 2 -Gd 3 pair the exchange interaction is very weak and no significant lifetime shortening could be measured for Gd 3 -Mn 2 pairs 24 ms for the Mn 2 emission in YF 3 vs. 22 ms in GdF The Electrochemical Society. DOI: / All rights reserved. Manuscript submitted May 15, 2000; revised manuscript received March 5, Available electronically June 6, z avink@iri.tudelft.nl The luminescence properties of pairs of ions can be different from the luminescence properties of single ions, for example, due to energy transfer processes within the pair 1,2 or pair interactions, which shift the positions of energy levels. 3,4 Research on the luminescence properties for ion pairs is not only interesting from a fundamental point of view. Some applications rely on the interactions within pairs of optically active ions. A recent example is the possibility of quantum cutting through down conversion which is based on energy transfer processes within a pair of Gd 3 -Eu 3 ions. 5 Another issue is the long lifetime of the spin-forbidden emission from Mn 2. For some applications, the long lifetime of the Mn 2 emission is prohibitive. 6 However, due to magnetic interactions in pairs of Mn 2 ions, the spin selection rule can be relaxed resulting in lifetime shortening. 7-9 In this paper, various luminescent properties of exchange coupled paramagnetic ions are studied. Luminescent materials based on the luminescence of Mn 2 have a long history. In the first generation of commercial fluorescent lamps, Zn,Be 2 SiO 4 :Mn 2 was used. In 1948, calcium halophosphate activated with Sb 3 and Mn 2 was discovered. 10 For decades, this material has been applied as a phosphor in fluorescent lamps until lanthanide-doped phosphors have partly replaced calcium halophosphate because of the higher efficiency and stability. In recent years, the interest in the well-known green-emitting Zn 2 SiO 4 :Mn 2 phosphor willemite has revived because of the potential application in plasma display panels PDPs. Zn 2 SiO 4 :Mn 2 is an efficient luminescent material under vacuum ultraviolet VUV excitation which is generated in the xenon plasma of a PDP. The luminescence of Mn 2 is due to a spin-forbidden transition within the 3d 5 configuration of the Mn 2 ion. Because of the forbidden character of the transition, the lifetime of the Mn 2 emission is long, typically 10 ms. In lighting applications, this long lifetime is not a problem but in displays the long lifetime results in an observable afterglow. To prevent this afterglow, some recent studies have investigated the shortening of the life time of the Mn 2 emission by increasing the concentration of Mn 2 ions. 8,9 A qualitative study on the influence of the Mn 2 concentration on the lifetime of the Mn 2 emission was reported by Barthou et al. 8 Zn 2 SiO 4 :Mn 2 samples were studied in which the Mn 2 concentration relative to Zn 2 was varied between 0.05 and 12.5 mol %. Luminescence measurements showed a shift of the emission wavelength from 519 to 526 nm upon increasing the concentration. Lifetime measurements revealed a long lived emission at low concentrations ( 15 ms). With increasing concentrations, the lifetime decreases and the lifetime measurements show a nonexponential decay behavior. At the highest concentrations, a single exponential decay time of 1.8 ms is reached. Based on the results it was concluded that the shortening of the lifetime was not the result of concentration quenching, but rather the effect of magnetic interactions between Mn 2 neighbors. A more quantitative explanation was presented by Ronda and Amrein who showed that exchange interaction between Mn 2 ion can be responsible for a red shift of the Mn 2 emission and a shortening of the luminescence lifetime. 9 It was suggested that the shortening of the lifetime is only observed at room temperature. 9 This is remarkable since also at low temperatures exchange coupling is expected to relax the spin selection rule resulting in lifetime shortening at low temperatures. In this paper the concentration dependence of the luminescence of Mn 2 in other host lattices than Zn 2 SiO 4 is reported. Luminescence and lifetime measurements for Mn 2 in MgAl 2 O 4 and ZnGa 2 O 4 are performed as a function of Mn 2 concentration. In the different host lattices, the distance between Mn 2 neighbors in pairs of ions is different. From the results, insight can be obtained in the influence of magnetic coupling between Mn 2 ions on the luminescence lifetime and shift in emission wavelength. In the case of Mn 2, the broad emission bands of single ions and pairs of ions have a large overlap and cannot be spectrally resolved. The 2 E 4 A 2 emission of Cr 3 is a sharp line and emission from single ions and Cr 3 pairs can be spectrally resolved. The 2 E 4 A 2 emission within the 3d 3 configuration of Cr 3 is spinforbidden, just as is the 4 T 1 6 A 1 emission for Mn 2. The line emission for Cr 3 pairs is shifted to longer wavelengths. For Cr 3 the influence of magnetic coupling in the pairs can be probed by selectively measuring the lifetime for the pair emission line. Results are reported for exchange coupled Cr 3 pairs in LaAlO 3. Due to the stronger exchange coupling in Cr 3 pairs, the influence on the luminescence lifetime is stronger for the Cr 3 pair emission than for the Mn 2 pair emission. A potential problem in the case of lifetime shortening by increasing the Mn 2 or Cr 3 concentration is concentration quenching.

2 E314 Journal of The Electrochemical Society, E313-E This problem can be avoided if coupling with a different type of paramagnetic ion, which does not quench the Mn 2 or Cr 3 emission, and can be used to lift the spin-selection rule. A possible candidate is the lanthanide ion Gd 3 4f 7 which has a large magnetic spin moment (S 7/2). For both the 2 E 4 A 2 emission of Cr 3 and the 4 T 1 6 A 1 emission of Mn 2 the influence of coupling with nearest Gd 3 neighbors on the luminescence lifetime is investigated. Theory Two paramagnetic transition metal ions which are close typical distance of a few angstroms show magnetic interaction. The Heisenberg Hamiltonian, H AB, describing the spin interaction between two transition metal ions A and B, is shown in Eq. 1 9 H AB J S A S B j S A S B 2 1 In theory, this exchange interaction can occur for both the 3d n and 4f n transition metal ion pairs. Because the electrons of the 4f n lanthanides are shielded from their surrounding by the filled 5s and 5p orbitals, the interaction is much weaker for pairs of lanthanide ions. The interaction between 4f n and 3d n transition metal ion pairs is expected to be in between and is investigated in this paper. The second term of Eq. 1 usually is very small and when this term is neglected, the energy values associated with H AB can be determined, which is shown in Eq E S J S S 1 S A S A 1 S B S B 1 2 From Eq. 2, it can be observed that the energy is dependent on the total spin, S, and on the exchange coupling strength parameter J. The S can take all integer values between the sum and difference of the values of S A and S B. States with different values for S are at different energies. As an example, the pair splitting of Mn 2 is discussed here. The ground state of Mn 2 3d 5 is 6 A 1 (S 5/2) and first excited state 4 T 1 (S 3/2). When both ions are in the ground state, 6 A 1, both spins have the same value: S A S B 5/2. Therefore, the values of S for the exchange coupled pair can be: (S A S B ),..., (S A S B ), so S 5, 4, 3, 2, 1, 0. When one of the Mn 2 ions is in the first excited state, 4 T 1, and the other ion is in the ground state, 6 A 1, the total S value can be S 4, 3, 2, and 1. The sign of J in Eq. 2 is determined by the type of magnetic interaction, J 0 for a ferromagnetic and J 0 for an antiferromagnetic interaction. 10 As an example for the splitting pattern for a Mn 2 pair with both ions in the ground state and one ion in the excited state, the interaction has been taken as an antiferromagnetic interaction for both the 6 A 1 6 A 1 ground state and the 6 A 1 4 T 1 excited state Fig. 1. The value of J is dependent on the distance between ions in the pairs. Emissions originating from the pairs can be ascribed to transitions from the lowest excited spin state to different spin components of the ground state. Transitions where S 0 are spin allowed. 10 From Fig. 1, it can be observed that there are always transitions present, where S 0. The influence of the crystal field on the position of the energy levels for the 3d 5 transition metal ion Mn 2 is given in the Tanabe- Sugano diagram for 3d 5. A Tanabe-Sugano diagram for Mn 2 in the high spin state low crystal fields, 6 A 1 ground state is shown in Fig. 2. Experimental Synthesis. The XAlO 3 :Cr 3 (X:La 3, Gd 3 and La 3 /Gd 3 lattices were synthesized using usual solid-state techniques. The starting materials which were used are NH 4 Al SO H 2 O, X 2 O 3, and Cr 2 O 3. They were milled for 30 min in a planetary ball mill and fired in a tube furnace in a N 2 atmosphere for 3 h at 1000 C and 5 h at 1500 C. The Mn 2 -doped MgAl 2 O 4 was synthesized by mixing Figure 1. Schematic representation of the splitting for energy levels in 6 A 1 6 A 1 ground state and 6 A 1 4 T 1 state resulting from exchange coupling for antiferromagnetic exchange-coupled Mn 2 pairs. The vertical dotted line shows the spin allowed 1 1 transition ( S 0). Al 2 O 3, MgO, and MnCO 3, and firing the mixture for 4hinaN 2 /H 2 atmosphere 75/25 at 1350 C. ZnGa 2 O 4 :Mn 2 was synthesized taking Ga 2 O 3, ZnO, and MnCO 3 as starting materials and firing the mixed starting materials in a reducing atmosphere (N 2 /H 2, ratio 75/25%, first for 3hat1000 C and later for 5hat1200 C. The Mn 2 -doped YF 3 and GdF 3 were synthesized using a high frequency furnace. In the furnace, the starting materials YF 3,GdF 3, and MnF 2 were in a glassy carbon crucible, which was placed in a quartz tube. The tube was first flushed with nitrogen. After heating to about 540 C, the system was flushed with SF 6 and then, again in a nitrogen atmosphere, heated until the melting point at about 1400 C. The samples were slowly in about 4 h cooled down to room temperature. A polycrystalline powder was obtained after grinding. All samples were found to be single phase, when checked with X-ray powder diffraction on a Philips PW 1729 X-ray generator, using Cu K radiation. Luminescence and time resolved measurements. The luminescence measurements for LaAlO 3 :Cr 3 were performed using a Spex Fluorolog spectrofluorometer equipped with two double grating 0.22 m SPEX 1680 monochromators and a 450 W Xenon lamp as excitation source. The excitation wavelength was at 408 nm exciting in the 4 A 2 4 T 1 spin-allowed absorption The emission was detected with a cooled Hamamatsu R928 photomultiplier. The time resolved measurements for the Cr 3 -doped aluminates were performed on a frequency doubled Quanta Ray Nd:yttrium-aluminum-garnet YAG laser. The excitation wavelength was at 532 nm exciting in the 4 A 2 4 T 2 spin-allowed absorption. The emission spectra were measured by a Spex 1269 monochromator 1.26 m and light was detected with a cooled Hamamatsu R928 photomultiplier. The decay curves were taken using a Tectronix 2430 digital oscilloscope. The low temperature measurements were done using an Oxford Instruments liquid helium flow cryostat.

3 Journal of The Electrochemical Society, E313-E E315 Figure 3. Emission spectra ( exc 308 nm, T 298 K of Mn 2 -doped MgAl 2 O 4 for different Mn 2 concentrations 0.2, 2, and 5 mol %. Figure 2. Tanabe-Sugano diagram of a 3d 5 transition metal ion. The emission spectra for the Mn 2 -doped spinel samples were measured on the Nd:YAG-laser described above using the Quanta Ray PDL-2 DyeLaser with a sulforhodamine dye range: nm. The light of 612 nm was frequency doubled with a Quanta Ray WEX-1 wavelength extender to an excitation wavelength of 306 nm. The emission was detected by an ARC Spectro Pro-300i monochromator length: 0.3 m and a Princeton Instruments charge coupled device CCD camera. Lifetime measurements for the Mn 2 -doped spinel samples were performed using a Lambda Physik LPX-100 XeCl-excimer laser for excitation at a wavelength of 308 nm. The samples were cooled with an Oxford Instruments liquid helium flow cryostat. The emission was spectrally resolved with a Jobin-Yvon 1 m monochromator and detected with a cooled Hamamatsu R928 photomultiplier. The decay curves were measured using a Tectronix 2440 digital oscilloscope. The luminescence measurements for the Mn 2 -doped YF 3 and GdF 3 samples were performed on the Spex Fluorolog setup described above, whereas the time resolved measurement were done using the Nd:YAG laser setup described above using an excitation wavelength of 355 nm third harmonic of the 1064 nm wavelength. The emission was collected using the monochromator with the CCD camera described above. For lifetime measurements an RCA C31034 photomultiplier tube, on the other port of the monochromator, was used and the decay curves were recorded with a Tectronix 2440 digital oscilloscope. Results and Discussion Luminescence of Mn 2 pairs. Luminescence spectra for Mn 2 in MgAl 2 O 4 for different Mn 2 concentrations 0.2, 2, and 5 mol % of Mn 2 are shown in Fig. 3. For the lowest Mn 2 concentration the emission has a maximum at nm. The position of the emission band is in agreement with values reported in literature 11 and the emission can be assigned to the 4 T 1 6 A 1 transition on Mn 2. The excitation spectrum not shown here shows different weak absorptions ascribed to transitions within the 3d 5 configuration of Mn 2 all spin-forbidden and a strong charge transfer band around 308 nm. Upon increasing the Mn 2 concentration, the position of the Mn 2 emission shifts to longer wavelengths. For the sample with highest Mn 2 concentration 10 mol % the emission band has shifted to nm. In Fig. 4, the position of the Mn 2 emission left axis is plotted as a function of the Mn 2 concentration. To investigate if pair formation at higher Mn 2 concentrations gives rise to a shortening of the lifetime of the Mn 2 emission, the lifetime of the Mn 2 emission was measured as a function of the Mn 2 concentration. In Fig. 5, decay curves of the Mn 2 emission are plotted. For the lowest Mn 2 concentration, the emission shows a single exponential decay behavior with a lifetime of 7.2 ms. At higher concentrations, the decay becomes nonexponential and faster. Lifetimes were determined by fitting the experimentally observed luminescence decay between 2 and 20 ms to a single exponential function. The decrease of lifetime at increasing Mn 2 concentration is shown in Fig. 4. The lifetime of the Mn 2 emission decreases as a function of the Mn 2 concentration from 7.2 ms for the sample with 0.2 mol % of Mn 2 to 4.5 ms for the sample doped with 10 mol %. The shortening of the lifetime can be explained by magnetic interactions exchange coupling. To exclude a contribution from concentration quenching to the shortening of the luminescence life- Figure 4. Peak position nm and decay time ms of the Mn 2 emission as a function of the Mn 2 concentration in MgAl 2 O 4 :Mn 2.

4 E316 Journal of The Electrochemical Society, E313-E Figure 5. Decay curves (T 298 K of Mn 2 -doped MgAl 2 O 4 0.2, 2, and 5 mol % showing a decrease of radiative lifetime at increasing Mn 2 concentration. Figure 6. Excitation spectra left side, em 501 nm for 1 mol % and 503 nm for 5 mol % and emission spectra right side, exc 287 nm of ZnGa 2 O 4 -doped with 1 and 5 mol % Mn 2 measured at T 4 K. time, the light output was measured as a function of the Mn 2 concentration. Up to 5 mol % Mn 2 the light output increases linearly with the Mn 2 concentration, indicating that concentration quenching does not contribute to the observed lifetime shortening in this concentration range. At low Mn 2 concentrations, the decay curves show a single exponential decay since there is only one type of luminescent center single Mn 2 ions. At higher concentrations, the decay becomes slightly nonexponential due to the fact that now also emission from Mn 2 pairs occurs which has a shorter lifetime. The lifetimes reported here are obtained by fitting a nonexponential decay curve to a single exponential function in the time regime between 2 and 20 ms. This procedure gives an average lifetime. Due to the presence of different types of Mn 2 pairs e.g., nearest and next nearest neighbors and some single step energy transfer from isolated Mn 2 ions to Mn 2 pairs, there is a range of lifetimes and it is not possible to fit the decay curves to a simple biexponential assuming only single ions and ion pairs are present. In Ref. 8, the nonexponential decay curves of the Mn 2 in Zn 2 SiO 4 :Mn 2 were fitted to the Yokota-Tanimoto model for luminescence decay in the presence of diffusion limited energy migration to traps. Good fits were obtained and information was derived from the fit parameters. Although it seems to be more correct to fit the nonexponential decay curves to a model which can describe an observed nonexponential decay behavior, we think that in the case of the emission from Mn 2 in MgAl 2 O 4 and also Zn 2 SiO 4, the Yokota-Tanimoto model is not valid. The model describes the luminescence decay of donor ions in the case of isotropic diffusion limited energy migration to deep traps. 12 The Mn 2 pairs can indeed act as traps, but due to the strong overlap between the emission from pairs and single ions, the luminescence decay curves represent the decay for both the single ions and pairs of ions i.e., donors and traps. Therefore, the Yokota-Tanimoto model cannot be applied to this situation. Also, the energy migration will be anisotropic since the donors do not occupy all lattice sites the Mn 2 concentration is not 100% and the donors are randomly distributed over the lattice sites. Finally, the traps are not only populated by energy transfer to traps, but also by direct excitation the excitation spectra of the single ions and pairs are very similar. For this situation, the Yokota-Tanimoto model, or any other model which describes the donor or acceptor decay in the presence of direct energy transfer like Inokuti-Hirayama and/or energy transfer via energy migration like the models developed by Burshtein or Huber, cannot be applied. 12,13 The values for the lifetime derived from the fit to a single exponential decay is, although unsatisfactory, the best indication for the shortening of the lifetime due to exchange interaction. The observed decrease from 7.2 to 4.5 ms is less than observed for Mn 2 in Zn 2 SiO 4 where the lifetime decreases from 15 to 1.8 ms. This can be explained by a smaller exchange coupling between the Mn 2 ions in MgAl 2 O 4. This is probably due to the larger Mn-Mn distance in nearest neighbor NN pairs of MgAl 2 O 4. The Mn-Mn distances in Mn 2 -doped Zn 2 SiO 4 are 3.30 Å, 14 whereas Mn-Mn distances in Mn 2 -doped spinels are much larger (MgAl 2 O 4 :5.71 Å and ZnGa 2 O 4 :5.90 Å. 15 The red shift of the emission from nm at low concentrations to nm for 10% Mn 2 is 350 cm 1, which is slightly larger than the red shift reported for Mn 2 in Zn 2 SiO cm 1. The red shift of the Mn 2 emission as a function of concentration can be explained by a difference in exchange coupling for both ions in the ground state and for the situation where one ion is in the excited state. 9 Apparently, this difference in exchange coupling is somewhat larger for Mn 2 pairs in MgAl 2 O 4. The concentration dependence of the Mn 2 emission in ZnGa 2 O 4 was investigated by measuring luminescence spectra and luminescence decay curves for samples with 0.1, 1, and 5 mol % Mn 2. The low temperature excitation and emission spectra for 1 and 5 mol % Mn 2 are shown in Fig. 6. No evidence for concentration quenching is found in the concentration regime up to 5 mol % of Mn 2. The emission spectra show a zero phonon line at 501 nm and vibronic lines at longer wavelengths, in agreement with previous publications. 16,17 Both the zero phonon line and the vibronic lines have shifted to slightly longer wavelengths for the sample with 5 mol%mn 2. The red shift is about 75 cm 1. The luminescence decay curves, shown in Fig. 7, are nonexponential, even for the lowest Mn 2 concentrations. Probably, this is due to energy transfer to defects. Also in previous publications, a faster initial decay has been observed in the luminescence decay curves of the Mn 2 emission in ZnGa 2 O 4 at low Mn 2 concentrations. 16,17 The luminescence lifetime derived from the tail of the decay curves (t 5 ms) is 7 ms for the sample with 0.1 mol % Mn 2 and 4 ms for the sample with 5 mol % Mn 2. The results presented above show that the shortening of the lifetime as a result of exchange coupling between Mn 2 occurs in both MgAl 2 O 4 and ZnGa 2 O 4. The reduction in lifetime is smaller than what has been reported for Mn 2 in Zn 2 SiO 4. This is probably due

5 Journal of The Electrochemical Society, E313-E E317 Figure 7. Decay curves of the Mn 2 emission for ZnGa 2 O 4 :Mn 2 for two concentrations: 0.1 mol % Mn 2, and 5.0 mol % Mn 2. exc 308 nm and em is 500 nm 0.1 mol % or 503 nm 5 mol %. The temperature is 4 K. Figure 8. Emission spectra of LaAlO 3 :Cr 3 ( exc 408 nm, T 4 K for different Cr 3 concentrations 0.1, 0.5, 1, and 2 mol %. The emission spectra are scaled to the intensity of the R-line. 14 to smaller exchange interaction between Mn 2 ions in MgAl 2 O 4 and ZnGa 2 O 4 due to a larger distance between nearest Mn 2 neighbors. The shorter lifetime for the Mn 2 pairs is also observed at low temperatures. The red shift of the Mn 2 emission is not related to the change in lifetime. The largest change in lifetime is found in Zn 2 SiO 4 :Mn 2 from 15 to 1.8 ms with a red shift of about 250 cm 1. 8,9 For the Mn 2 emission in MgAl 2 O 4 and ZnGa 2 O 4, the lifetime is reduced from about 7 ms to 4 ms in both host lattices while the red shift of the Mn 2 emission is larger 350 cm 1 in MgAl 2 O 4 or smaller 75 cm 1 in ZnGa 2 O 4 than the red shift reported for Mn 2 in Zn 2 SiO cm 1. This result is consistent with the model presented by Ronda and Amrein: the shortening of the lifetime is due to exchange interaction, while the red shift of the emission results from a difference in the exchange interaction in a pair of ions both in the ground state and one in the ground state and one in the excited state. 9 Luminescence of Cr 3 pairs. The magnetic coupling between Cr 3 ions is stronger than between Mn 2 ions. Typical values for J for exchange coupled Mn 2 pairs are below 5 cm 1, 18 while for Cr 3, values around 50 cm 1 are commonly observed. 19 Based on the stronger exchange coupling, a stronger reduction of the luminescence lifetime can be expected for pairs of Cr 3 ions. In Fig. 8, emission spectra of LaAlO 3 doped with 0.1, 0.5, 1.0, and 2.0 mol % of Cr 3 are shown. For the lowest concentration, a sharp 2 E 4 A 2 emission line is observed around 735 nm. This emission from single Cr 3 ions is known as the R-line emission. In a higher resolution spectrum not shown here it can be observed that the R-line emission is split in a R 1 andar 2 line separated by 5.4 cm 1. On the longer wavelength side, weak vibronic lines are observed. The present results are in agreement with earlier reports on the luminescence of Cr 3 in LaAlO When the Cr 3 concentration is raised, new emission lines appear in the emission spectrum. These lines can be attributed to emission from Cr 3 pairs. As can be expected for pair lines, the relative intensity of the pair emission lines strongly increases with increasing Cr 3 concentration. In Table I, the positions of the various emission lines are tabulated. In addition to the emission lines from the single ions zero phonon line and vibronics pair lines are observed peaks 5, 8, and 11, which can be assigned to an antiferromagnetically coupled nearest neighbor NN pair of Cr 3 ions. The strong line at nm corresponds to the 2 2 transition and, at higher and lower energy, the 2 1 and 2 3 lines are observed. From the positions of the emission lines and with Eq. 2, the value for J can be estimated to about 63 cm 1. These observations are in good agreement with previous results on LaAlO 3 :Cr 3, where a value of 66.6 cm 1 was calculated for the antiferromagnetically coupled nearest Cr 3 neighbors in LaAlO The emission lines at 752.1, 745.4, and nm have not previously been identified. The positions fit the spectral positions expected for a ferromagnetically coupled Cr 3 pairs with J 64 cm 1. Since ferromagnetic interaction is predicted for next nearest neighbor NNN Cr 3 pairs in LaAlO 3, 20 the emission lines can probably be attributed to emission from NNN Cr 3 pairs. In Fig. 9, the luminescence decay curves are shown for the single Cr 3 ions R-line emission and the pair emission lines. The lifetime of the emission of the single ions is long ( 56 ms at room temperature, 81 ms at T 4 K 1. The long lifetime of the Cr 3 emission is due to a combination of the spin-selection rule and the parity selection rule in inversion symmetry the transition within the 3d 3 configuration is parity forbidden. For the Cr 3 pairs, the emission lifetime is much shorter, around 2 ms. The shortening of the decay time can be explained by a partial lifting of the spin-selection rule. For the exchange coupled Cr 3 pair, the 2 2 and 1 1 transitions are formally spin allowed which results in a shortening of the lifetime of the emission from Cr 3 pairs similar to the observation for Mn 2 pairs described above. The stronger exchange cou- Table I. Assignment of the emission peaks observed for LaAlO 3 :Cr 3 2mol%,T 4 K, w: weak, m: moderate, s: strong intensity. Peak em nm v experimental cm 1 Assignment v literature cm w Vibronic - 2 w Pair line vibronic w R-line vibronic m R-line vibronic m NN w R-line vibronic s NNN - 8 s NN m NNN - 10 m R-line vibronic - 11 w NN w NN m NNN - 14 s R 1 R 2 -lines 13625

6 E318 Journal of The Electrochemical Society, E313-E Figure 9. Decay curves for single ion emission (R 1 -line, em nm, nearest neighbor pair (2 2, em nm and next nearest neighbor pair (1 1, em nm emission for Cr 3 in LaAlO 3 :Cr 3 2 mol %, exc 532 nm and T 7 K. The time scale for the single ion emission (R 1 ) should be multiplied by a factor of 10 i.e., the decay shown is the decay between 0 and 60 ms. pling between Cr 3 ions can explain the observation that the influence of the exchange coupling on the decay time is larger for the Cr 3 pairs than for the Mn 2 pairs. In addition, a distortion of the inversion symmetry by replacing a NN or NNN Al 3 ion by Cr 3 can also lead to a shortening of the decay time for Cr 3 pairs. A deviation from inversion symmetry will allow for the admixture of opposite parity states in the 3d 3 states as a result of the presence of odd parity crystal field components. Due to the admixture, the 2 E 4 A 2 transition will become partially electric dipole allowed. It is not possible to separate or estimate the contribution of the lifting of the spin and parity selection rule on the lifetime shortening in Cr 3 pairs. Luminescence of Cr 3 -Gd 3 and Mn 2 -Gd 3 pairs. In the previous two sections, the shortening of the luminescence lifetime of spin-forbidden transitions by magnetic coupling between paramagnetic ions of the same type was described. It is interesting to investigate if exchange coupling between Cr 3 or Mn 2 with a different type of paramagnetic ion can also result in a shortening of the lifetime. Lifetime shortening due to concentration quenching is avoided if exchange coupling with a different type of paramagnetic ion occurs. Especially for Mn 2, it is interesting to investigate if the problem of the long luminescence lifetime can be solved by exchange coupling with a different type of paramagnetic ion which does not quench the Mn 2 emission. An obvious candidate for magnetic interaction is Gd 3 4f 7. The lanthanide ion Gd 3 has a large magnetic spin moment (S 7/2) and the energy gap between the ground state ( 8 S 7/2 ) and the first excited state ( 6 P 7/2 ) is large, 32,000 cm 1. The high energy position of the first excited state prevents energy transfer from Mn 2 or Cr 3 to Gd 3 and no shortening of the lifetime by energy transfer is possible. To investigate the influence of magnetic coupling in pairs of Cr 3 or Mn 2 and Gd 3 isostructural compounds of Gd 3 paramagnetic and Y 3 or La 3 diamagnetic doped with Cr 3 or Mn 2 were synthesized and the lifetime of the spin-forbidden emission from the 3d n transition metal ion was measured. In the past, some work has been done on the magnetic coupling between Cr 3 and Gd 3 in GdAlO 3. 20,21 The line shape of the 2 E 4 A 2 transition could be explained by magnetic coupling between Gd 3 and Cr 3. The coupling constant, J, was calculated to be 1 cm 1 for Cr 3 in the 4 A 2 ground state and 0.55 cm 1 for Cr 3 in the 2 E excited state. 20 These values are of the same order of magnitude as values Figure 10. Decay curves of the 2 E 4 A 2 emission for Cr 3 in LaAlO 3 ( em 735 nm), La 0.94 Gd 0.06 AlO 3 ( em 735 nm) and GdAlO 3 ( em 727 nm) measured for exc 532 nm at room temperature. reported for the exchange interaction in Mn 2 ion pairs. To study the influence of magnetic coupling on the spin selection rule, LaAlO 3 and GdAlO 3 doped with 0.05% of Cr 3 were investigated. Both GdAlO 3 and LaAlO 3 have a slightly different perovskite-like structure in which the Cr 3 ion, which substitutes for an Al 3 ion, is in a center of inversion symmetry. The emission spectra of Cr 3 -doped LaAlO 3 and GdAlO 3 are quite similar. The 2 E 4 A 2 emission is at slightly shorter wavelength for Cr 3 in GdAlO nm vs. 735 nm in LaAlO 3 indicating that the Racah parameter, B, is somewhat larger for Cr 3 in GdAlO 3. In Fig. 10, the luminescence decay curves are plotted for the Cr 3 2 E 4 A 2 single ion emission in LaAlO 3, La 0.94 Gd 0.06 AlO 3, and GdAlO 3. It is clearly observed that the decay of the emission is faster for Cr 3 in GdAlO 3. Fits of the observed decay behavior to a single exponential function gives a lifetime of 56 ms for Cr 3 in LaAlO 3 and 14 ms for Cr 3 in GdAlO 3. The shortening of the lifetime can be explained by lifting of the spin-selection rule by magnetic coupling between Cr 3 and Gd 3. The order of magnitude of the shortening a factor of four is similar to what is observed for exchange coupled pairs of Mn 2 ions reported in the Results and Discussion section on Luminescence of Mn 2 pairs. For Cr 3 pairs, a stronger reduction of the lifetime is observed the Results and Discussion section on Luminescence of Cr 3 pairs. This can be understood qualitatively. Due to the stronger magnetic coupling between Cr 3 neighbors J around 50 cm 1, the spin selection rule is lifted to a larger extent than for Gd 3 -Cr 3 or Mn 2 -Mn 2 pairs J below 5 cm 1 and the lifetime shortening is more pronounced. The weaker coupling in the Gd 3 -Cr 3 pair compared to the Cr 3 -Cr 3 pair is due to the shielding of the 4f 7 electrons in Gd 3 by the filled 5s 2 and 5p 6 shells. Due to the shielding, the interaction between the 4f n configuration of lanthanides with the environment is weak. Due to the weak coupling of the 4f n configuration of lanthanides, the magnetic coupling in pairs of lanthanides is very weak. For example, in GdAlO 3 the exchange interaction parameter J is only 0.05 cm 1 for the antiferromagnetic interaction between nearest Gd 3 neighbors. 21 The significant shortening of the lifetime of the Cr 3 emission by magnetic interaction with Gd 3 neighbors indicates that it may be possible to shorten the Mn 2 lifetime by coupling with Gd 3 neighbors. On the other hand, the weaker interaction between Mn 2 neighbors in comparison to Cr 3 neighbors indicates that the magnetic coupling with Gd 3 neighbors may be smaller for Mn 2 than for Cr 3, resulting in a small or negligible effect on the luminescence lifetime of the spin-forbidden Mn 2 emission. We are not

7 Journal of The Electrochemical Society, E313-E E319 Figure 12. Luminescence decay time curves for Mn 2 -doped GdF 3 and YF 3. Both curves were measured at T 298 K using exc 355 nm and em 523 nm. agreement with the value of 21 ms reported by Poort. 22 The small difference in decay time between Mn 2 -doped YF 3 and GdF 3 may be due to magnetic interactions but may also be due to other factors. Clearly, the influence of magnetic interactions between Mn 2 and Gd 3 on the luminescence lifetime of the Mn 2 emission is small or absent. This shows that the exchange coupling between Gd 3 and Mn 2 is small; J is probably smaller than 0.1 cm 1. As a result of the weak coupling, lifetime shortening of the long lived Mn 2 emission by magnetic coupling with Gd 3 ions does not seem a promising route to solve the problem of the long lifetime of the green Mn 2 emission in display applications. Figure 11. Excitation spectra solid lines and emission spectra dotted lines of A YF 3 :Mn 2, em 522 nm and exc 335 nm, and B GdF 3 :Mn 2, em 522 and exc 273 nm measured at room temperature. The intensity for the excitation spectrum of GdF 3 :Mn 2 has been multiplied between 320 and 450 nm by a factor of 100. aware of estimates in the literature on the magnitude of the Mn 2 -Gd 3 exchange interaction parameter J. To investigate the influence of the presence of nearest Gd 3 neighbors on the lifetime of the Mn 2 emission, the luminescence of Mn 2 was studied in YF 3 and in GdF 3. In Fig. 11, the excitation and emission spectra for Mn 2 in GdF 3 and YF 3 are shown. To avoid an influence of the formation of Mn 2 pairs, the Mn 2 -concentration was low 0.05 mol %. The excitation spectra of YF 3 :Mn 2 shows the spin-forbidden Mn 2 absorptions, whereas the GdF 3 :Mn 2 excitation spectrum is dominated by the 8 S 7/2 6 P J ( exc 310 nm) and 8 S 7/2 6 I J ( exc 275 nm) excitation lines. The emission spectra show the 4 T 1 6 A 1 emission around 523 nm for Mn 2 in both host lattices. In both emission spectra, defect emission is present at an emission wavelength between 400 and 450 nm. Defect emission is well known for fluoride crystals and may be related to F vacancies. Possible, the concentration of F vacancies is relatively high since F vacancies can be used for charge compensation required by replacing a trivalent (Y 3 or Gd 3 ion by a divalent Mn 2 ion. In Fig. 12, the luminescence decay curves of the Mn 2 emission are depicted. Comparison of the two decay curves shows that the difference between the luminescence decay for the Mn 2 emission in YF 3 :Mn 2 and GdF 3 :Mn 2 is very small. Fits of the experimentally observed decay to a single exponential decay function gives a lifetime of 24 ms for the Mn 2 emission in YF 3 :Mn 2 and 22 ms for Mn 2 in GdF 3 :Mn 2. The value of 22 ms for GdF 3 :Mn 2 is in good Conclusions Luminescence spectroscopy of exchange coupled pairs of transition metal ions has been reported for different types of pairs. For Cr 3 -Cr 3 pairs the exchange coupling is strong. As a result, the emission spectra are different from the single ion spectra and the lifetime of the spin-forbidden 2 E emission is strongly reduced. For Mn 2 -Mn 2 and Cr 3 -Gd 3 pairs the exchange interaction is weaker than for Cr 3 -Cr 3 pairs. Still, a shift in the emission wavelength and a shortening of the lifetime of the spin forbidden 4 T 1 emission for Mn 2 or 2 E emission for Cr 3 is observed. For Mn 2 -Gd 3 pairs, the exchange coupling is too weak to measure a significant change in the spin-forbidden Mn 2 emission. Acknowledgments We would like to thank M. Heeroma and J. Meijer for the lifetime measurements performed on Mn 2 -doped MgAl 2 O 4. References 1. H. Siebold and J. Heber, J. Lumin., 23, J. Hegarty, D. L. Huber, and W. M. Yen, Phys. Rev. B, 25, T. Tsuboi, Phys. Rev. B, 29, J. Derkosch and W. Mikenda, J. Solid State Chem., 22, R. T. Wegh, H. Donker, K. D. Oskam, and A. Meijerink, Science, 283, C. R. Ronda, J. Alloys Compd., 225, A. L. N. Stevels and A. T. Vink, J. Lumin., 8, C. Barthou, J. Benoit, P. Bennalloul, and A. Morell, J. Electrochem. Soc., 141, C. R. Ronda and T. Amrein, J. Lumin., 69, B. Henderson and G. F. Imbusch, Optical Spectroscopy of Inorganic Solids, p. 475, Oxford University Press, Oxford, U.K R. Clausen and K. Petermann, J. Lumin., 40,41, M. Yokota and O. Tanimoto, J. Phys. Soc. Jpn., 22, B. Henderson and G. F. Imbusch, Optical Spectroscopy of Inorganic Solids, p. 445, Oxford University Press, Oxford K. H. Klaska, J. C. Eck, and D. Pohl, Acta Crystallogr., 34,

8 E320 Journal of The Electrochemical Society, E313-E R. W. G. Wyckoff, Crystal Structures, p. 75, 2nd ed., John Wiley & Sons, New York S. H. M. Poort, D. Cetin, A. Meijerink, and G. Blasse, J. Electrochem. Soc., 144, C. F. Yu and P. Lin, Jpn. J. Appl. Phys., Part 1, 35, D. S. McClure, J. Chem. Phys., 39, A. L. Shawlow, D. L. Wood, and A. M. Clogston, Phys. Rev. Lett., 3, J. P. van der Ziel, Phys. Rev. B, 4, T. Kita and Y. Tanabe, J. Phys. Soc. Jpn., 54, S. H. M. Poort, A. Meijerink, and G. Blasse, Solid State Commun., 103,