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On the impurity trapped exciton luminescence in La2Zr2O7 : Bi3+
Journal of Luminescence 81 (1999) 293}300
On the impurity trapped exciton luminescence in LaZrO : Bi> A.M. Srivastava *, W.W. Beers GE Corporate Research and Development Center, KWB 316, 1 Research Circle, Niskayuna, NY 12309, USA GE Lighting, Nela Park, Cleveland, OH, USA Received 10 August 1998; received in revised form 21 December 1998; accepted 21 December 1998
Abstract The photoluminescence and the excitation spectra of Bi> in the pyrochlore La Zr O have been investigated in the temperature range of 15}300 K. At low temperatures, the photoluminescence spectrum of Bi> in LaZrO consists of two bands, one in the ultraviolet (385 nm) and the other in the visible (515 nm) part of the spectrum. Thermally activated intensity exchange between the two bands is observed. The ultraviolet emission is attributed to the P PS transition and the visible luminescence to an impurity trapped exciton-type emission. The low e$ciency of Bi> emission in LaZrO and in YO (#uorite structure) is tentatively ascribed to bonding within the tetrahedral BiO network that promotes photoionization of the luminescent center. 1999 Elsevier Science B.V. All rights reserved. Keywords: Impurity-trapped exciton luminescence; Bi>; Pyrochlore LaZrO
1. Introduction The spectroscopy of the Bi> ion which has the 6s electronic con"guration, has been investigated extensively in a variety of host lattices [1]. The ground state of the free ion is S whereas the 6s6p excited states give rise to the triplets levels [P, P, P] and the P singlet state. The SPP transition (A-band) becomes allowed as a result of spin}orbit coupling whereas the SPP transition (B-band) is forbidden but can be induced by coupling with unsymmetrical lattice vibrational modes. The SPP transition (C-band) is an * Corresponding author. Tel.: #1-518-387-7535; fax: #1518-387-5299; e-mail: [email protected].
allowed transition but the S PP is strongly forbidden. In addition to the above transitions another optical transition (a `D-banda) appears in the optical spectra of the Bi> ions. The D-bands are presumed to be a ligand to metal charge transfer transitions although an earlier view ascribed the band to a perturbed exciton [2]. In certain host lattices the ns ion exhibits an `anomalousa emission that di!ers considerably from the characteristic emission due to the P PS transitions. A photoionization process in which the luminescent center ejects an electron into the host lattice conduction band has been proposed to account for the `anomalousa emission. This interaction of the P levels of the ns ion with the host lattice conduction band results in the formation of an impurity bound exciton state in
0022-2313/99/$ } see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 3 1 3 ( 9 9 ) 0 0 0 0 4 - 6
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which the electron is delocalized in the host lattice and the hole trapped at the luminescent center. The `anomalousa emission in the optical spectra of ns ions is a result of the radiative decay of the exciton state. Results existing in the archival literature that are representatives of the ionization-induced `anomalousa emission in the optical spectrum of ns ions include Pb> activated CdCl [3], alkaline earth carbonates such as BaCO and SrCO [6], derivatives of the PbFCl structures [7] and Bi> activated InBO [4,5]. We wish to clarify two points before proceeding further. First, the equivalence in assigning the origin of the `anomalousa emission to an impurity bound exciton state or to the D-state (see above) is a consequence of our restricted understanding of the excited state composition responsible for such optical transitions. Second, the `anomalousa emission due to the ionization process is not con"ned to ns ions but has been observed in the luminescence of ions such as Eu> [8], Yb> [9], Tm> [9], Ce> [10}12] and Cu> [13]. The luminescence of Bi>activated La Zr O is examined in this paper. The host material crystallizes in the pyrochlore structure [14]. The cubic pyrochlore structure for the oxides A B O (space group Fd3m) can be considered to be derived from the #uorite structure (CaF ) by removing one eighth of the anions so that the composition can be written as A B O < where < is the anion va cancy. The large A-cations are eight coordinated whereas the small B-cations are in octahedral coordination. The pyrochlore structure can be viewed as interpenetrating network of BO octahedra and A O chains where the O are the axial oxygen ions that are exclusively coordinated to the rare earth ions.
2. Experimental Materials with general composition (La Bi ) Zr O were synthesized by blending stoi\V V chiometric amounts of the starting materials, La O (99.999%), ZrO (99.978%) and Bi O (99.9%) with 50% by weight Li SO and heating for 10 h in a covered crucible at 14003C. The #ux (Li SO ) promote the reaction between the refractory start-
ing materials and is removed from the sample by several washings in hot water. X-ray di!raction showed single phase formation. Luminescence measurements were done as previously described [15]. The spectra have been corrected for the wavelength-dependent variations in the Xe-lamp intensity and the photomultiplier response.
3. Results The excitation spectra of (La Bi ) Zr O at ¹"15 K is shown in Fig. 1. The emission spectra (j "290 nm) at several temperatures are shown in Fig. 2. The emission spectrum at 15 K exhibits two broad bands with maxima at 385 nm (UV) and 515 nm (VIS), respectively. The measured temperature dependence of the UV and VIS band intensities (Fig. 3) indicates an intensity exchange between the two bands in the 15}70 K temperature range. At ¹"70 K, the intensity of the UV band is at a minimum while that of the VIS band a maximum. Above 70 K the UV band recovers while the VIS band exhibits intensity loss. Thermal quenching of the UV emission occurs above 120 K. The room-temperature emission spectrum of (La Bi ) Zr O contains only the UV component with measurable but exceedingly weak intensity. The common feature in the excitation spectra of the UV and VIS emission bands is the broad band centered at 290 nm (4.27 eV). However, considerable di!erence is noted on the high-energy side of the excitation spectra (Fig. 1). Clearly, the VIS emission is more e$ciently excited by the high-energy band, the maximum of which is located below the detection limit of 250 nm of the instrument. The two bands in the excitation spectrum coincide with two absorption bands at 245 and 287 nm, respectively, in the room temperature diffuse re#ectance spectrum of (La Bi ) Zr O . The Bi>-induced absorption bands are absent in the di!use re#ectance spectrum of the pure material (in the 240}350 nm wavelength region). The UV emission exhibits a Stokes shift of about 1.06 eV. The estimated Stokes shift for the VIS emission is 2.66 eV (the excitation band for this emission is centered at about 245 nm).
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295
Fig. 1. Excitation spectra of (La Bi ) Zr O for (a) j "515 nm and (b) j "385 nm at ¹"15 K.
The emission spectrum for j "255 nm indi cates preferential excitation of the VIS emission by high-energy radiation (Fig. 4). This is in agreement with the excitation spectrum (see Fig. 1). A shift to higher energy of the UV emission band maximum with increasing temperature is observed. For example, the emission maximum shifts from 385 (3.22 eV) at 15}375 nm (3.31 eV) at 110 K. The maximum of the VIS emission is essentially temperature independent.
4. Discussions The intensity exchange between the UV and VIS bands and their identical excitation spectra (except for the variance on the high-energy side) suggests that the two emission bands originate from the same Bi> center in the La Zr O host lattice. We
would like to note that the pyrochlore crystal structure o!ers a single crystallographic site for the Bi> ion. At low temperatures, the UV band is assigned to the forbidden P PS transition on the Bi> ion. The VIS band is associated with the ionization process described previously in which the Bi> center injects an electron in the host lattice conduction band. Therefore, the VIS emitting state is an impurity centered exciton state that is populated by an electron transfer from the Bi> P excited states. This suggests the proximity of Bi> excited states (P ) with the bottom of the La Zr O conduction band. The two dominant radiative decay channels following Bi>S PP excitation are the emission from localized states (P ) and emission from the impurity trapped exciton level as a consequence of ionization. It is clear that even at low temperatures the emission is characterized by the competition
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Fig. 2. Emission spectra of (La Bi ) Zr O for j "290 nm at various temperatures.
between two emitting states. In the 15}70 K temperature range, the enhancement in the VIS emission intensity at the expense of the UV emission is a result of the thermally assisted ionization process becoming more e$cient than the radiative relaxation of the Bi>P excited states. The quench ing of the VIS emission for ¹'70 K is presumably related to a thermal ionization process of the exciton state [3]. The structure of the trapped exciton structure is of the hole localized on the luminescent ion and the electron delocalized over the surrounding cations. The trapped exciton in SrF :Yb>, for example, consists of a Yb> core with the electron delocalized over the 12 nearest neighbors Sr> ions [9]. As previously described, the pyrochlore structure can be perceived as an A O network (similar to the anticristobalite Cu O network) interpenetrat ing the BO framework. Consequently, the exciton
structure in La Zr O : Bi> can be considered to consist of the hole residing on the (La,Bi) O sublat tice and the electron in the ZrO framework [16]. We would like to mention that the VIS band can also be perceived as an emission arising from a charge transfer transition between the activator Bi> (6s) and the Zr> (4d) ions of the host lattice. Charge transfer transitions between ions with ns electronic con"guration and ions with nd electronic con"guration are fairly common and often observed [17]. The charge transfer state with an approximate charge distribution of ns}nd often exhibits very e$cient luminescence. The so-called D-state in the energy scheme of ns ions is thought to be of a charge transfer type. Therefore, the VIS emission in La Zr O : Bi> can be assigned to DPS radiative decay. A detailed report on the luminescence of LaOBr : Bi> is existing in the literature. There is
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297
Fig. 3. Temperature dependence of the relative intensities of the UV and VIS bands in (La Bi ) Zr O for j "290 nm.
a close correspondence between the luminescence of LaOBr : Bi> and La Zr O : Bi>. In the for mer system the outcome of Bi> S PP excita tion is UV emission at low temperatures with the VIS band emerging at the expense of UV band at higher temperatures [18]. The VIS emission is attributed to a charge transfer transition involving the Bi> and the Br\ ligands of the LaOBr lattice. Relative to the oxybromide system, one major difference is the absence of a signi"cant thermal barrier between the UV and VIS states in La Zr O : Bi>. Note the dominance of the VIS band even at low temperatures in the luminescence of La Zr O : Bi> (Fig. 2). An insigni"cant poten tial barrier between the Pb> [P ,P ] and the excitonic states is also observed in gallates with the magnetoplumbite structure (SrGa O ). It has been suggested that high covalency of the host lattice contributes to the low potential barrier [19].
So while not certain, it is feasible that the insigni"cant thermal barrier between the UV and VIS emitting states in La Zr O : Bi> is also a result of high host lattice covalency. The shift of the UV maximum to higher energy with increasing temperature can be explained as follows [20]. The Bi> luminescence results from the radiative decay of the P and P excited states to the S ground state. The P state is located below the P state. Hence, at low temperatures the P acts as an optical trap so that the emission is dominated by the strongly forbidden P PS transition. At higher temperatures, thermal population of the higher energy P state results in the occurrence of the P PS transition. This energy change arising from the two radiative transitions results in the temperature-induced shift of the UV maximum to higher energy. The band shift of about 90 meV is an approximate measure of the energy
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Fig. 4. Emission spectra of (La Bi ) Zr O for j "255 nm at ¹"15 K.
separation between the Bi>P and P states in La Zr O . The increase in the UV band intensity for ¹'70 K (Fig. 3) is in part due to the higher P PS transition probability. However, the intensity enhancement may also be due to a thermally induced energy back transfer from the excitonic state to the localized UV emitting state of the Bi> ion. The irradiation within the A band of ns ions in alkali halide crystals is known to produce two emission bands that are designated as A and A , in 6 2 order of increasing photon energy [2]. These bands, the intensities of which are also strongly temperature dependent, are ascribed to the coexistence of minima with two di!erent symmetries on the adiabatic potential energy surfaces of the relaxed excited state (APES). The origin of the two minima is the electron}phonon interaction of the orbital triplet with the e and q vibrational mode of the octahedral ns ion complex. The lumines-
cence properties of these systems are mainly governed by the ratio of the spin}orbit coupling (m) and the electron}lattice relaxation. If the ratio is very high or very low then only one minima is expected. The high atomic number of Bi> results in strong spin}orbit interaction so that only one minimum on the APES is anticipated [21]. Consequently, the doublet nature of Bi> luminescence in La Zr O may not be attributed to the Jahn}Teller e!ect. Also, the requirement of a symmetrical lattice site for the Jahn}Teller e!ect to be operative is not evident in the structure of the pyrochlore since the A-site coordination is that of a strongly distorted cube. We would like to note that the Stokes shift of impurity trapped excitonic state emission is usually much larger than that of the localized transitions [3,22]. Indeed, the Stokes shift of the exciton state in La Zr O : Bi> is nearly twice that of the local ized transition (P PS ).
A.M. Srivastava, W.W. Beers / Journal of Luminescence 81 (1999) 293}300
The nature of the high-energy band in the excitation spectrum of La Zr O : Bi> is now discussed. The assignment of the lowest energy band to the S PP (A-band) transition on the Bi> ion is straightforward. The higher energy absorption may be assigned to the allowed S PP (C-band) transition. This assignment would yield an energy di!erence between P and P states of about 6000 cm\. In solids, the P P energy separation usually amounts to about 10 000 cm\ [23,24]. Consequently, it is unlikely that the higher energy band corresponds to the S PP transition. The higher energy band is thus tentatively assigned to the absorption into the excitonic state. The rather low-energy position of this band in La Zr O may re#ect on the high host lattice covalency. The low e$ciency of the Bi> P PS (UV) emission at room temperature is inconsistent with the expectation of e$cient luminescence resulting from the small Stokes shift of emission (1.06 eV). We postulate that the intensity decrease of the UV band for ¹'120 K is due to thermal ionization of the localized levels and the decrease in the total emission intensity with increasing temperature is due to a thermal ionization process involving both the localized P and the excitonic states. We now proceed to compare the luminescence of Bi> activated Y O and La Zr O in order to gain insight into structure-photoionization issues. The low quantum e$ciency of Y O : Bi> has been attributed to enhanced radiationless transitions arising from the interaction between Bi> excited states (P ) and the host lattice con duction band [25]. We held the same mechanism responsible for the low e$ciency of La Zr O : Bi> system. A close examination of the crystal structures of Y O and La Zr O allows us to propose an explanation for the low e$ciency of Bi> luminescence in these materials. In the #uorite derived structure of Y O each oxy gen ion is surrounded by four Y> ion in a (distorted) tetrahedral arrangement and a network of such tetrahedral groups can be distinguished in the crystal structure. The pyrochlore structure may also be thought to be derived from the #uorite structure where the cations form face-centeredcubic array and the anions are located in the tetrahedral interstices of the cationic array. The three
299
tetrahedral interstices sites available for the anions in the pyrochlore structure are the 48(f ) positions with two A and B near neighbors, the 8(b) position having four B near neighbors (the site are unoccupied in the pyrochlore structure) and the 8(a) position with four A near neighbors forming a regular tetrahedron (A O). In this respect the pyrochlore structure resembles the Y O structure. Consequently, it is feasible that the bonding within the tetrahedral network promotes photoionization of the Bi> center in these materials. The ionization process is in turn held responsible for the low quantum e$ciency of Bi> luminescence. 5. Conclusion In this paper, we have observed emission from the impurity trapped exciton state in the La Zr O : Bi> system. This implies proximity of Bi> excited states (P ) with the bottom of host lattice conduction band. The impurity trapped exciton state exhibits di!erent excitation and emission spectra from the normally observed Bi> P S localized states. A signi"cant en ergy barrier between the P and the excitonic state is not observed in this host lattice. The low e$ciency of the Bi> luminescence in La Zr O at room temperature is attributed to thermal ionization of the P and the exciton states. A com monality in the crystal structures of Y O and La Zr O , namely the network of oxygen anions that are tetrahedrally coordinated by metal cations, is thought to promote photoionization of the Bi> center. Acknowledgements The authors are grateful to Dr. C. R. Ronda, Philips GmbH Forschungslaboratorium Aachen, Germany, for valuable discussions. References [1] G. Blasse, Prog. Solid State Chem. 18 (1988) 79. [2] P.W.M. Jacobs, J. Phys. Chem. Solids 52 (1991) 35 and references therein.
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[3] B. Moine, C. Pedrini, V. Ghiordanescu, J. Phys.: Condens. Matter 6 (1994) 4093. [4] V.P. Dotsenko, I.V. Berezovskaya, N.P. Efryushina, J. Phys. Chem. Solids 57 (1996) 437. [5] G. Blasse, C. de Mello Donega, I. Berezovskaya, V. Dotsenko, Solid State Commun. 91 (1994) 29. [6] H.F. Folkerts, G. Blasse, J. Phys. Chem. Solids 57 (1996) 303. [7] H.F. Folkerts, A. van Dijken, G. Blasse, J. Phys.: Condens. Matter 7 (1995) 10049. [8] B. Moine, C. Pedrini, B. Courtois, J. Lumin. 50 (1991) 31. [9] D.S. McClure, C. Pedrini, Phys. Rev. B 32 (1985) 8465. [10] W.M. Yen, M. Raukas, S.A. Basun, W. van Schaik, U. Happek, J. Lumin. 69 (1996) 287. [11] M. Raukas, S.A. Basun, W. van Schaik, W.M. Yen, U. Happek, Appl. Phys. Lett. 69 (1996) 3300. [12] C. Pedrini, F. Rogemond, D.S. McClure, J. Appl. Phys. 59 (1986) 1196. [13] S.A. Payne, D.S. McClure, J. Phys. Chem. 88 (1984) 1379. [14] A.W. Sleight, Inorg. Chem. 7 (1969) 1704.
[15] A.M. Srivastava, W.W. Beers, J. Electrochem. Soc. 143 (1996) L203. [16] H. Hamoumi, M. Wiegel, G. Blasse, J. Solid State Chem. 108 (1994) 410. [17] G. Blasse, Struct. Bonding 76 (1991) 154. [18] A. Wolfert, G. Blasse, J. Lumin. 33 (1985) 213. [19] H.F. Folkerts, F. Ghianni, G. Blasse, J. Phys. Chem. Solids 57 (1996) 1659. [20] G. Boulon, B. Moine, J.C. Bourcet, R. Reisfeld, Y. Kalisky, J. Lumin. 18/19 (1979) 924. [21] A. Wolfert, G. Blasse, J. Solid State Chem. 59 (1985) 133. [22] J. de Blank, G. Blasse, Eur.J. Solid State Inorg. Chem. 33 (1996) 295. [23] A.C. van der Steen, J.J.A. van Hesteren, A.P. Slok, J. Electrochem. Soc. 128 (1981) 1327. [24] J.L. Sommerdijk, J.M.P.J. Verstegen, A. Bril, Philips Res. Rep. 29 (1974) 517. [25] A.M. van de Craats, G. Blasse, Chem. Phys. Lett. 243 (1995) 559.
On the impurity trapped exciton luminescence in LaZrO : Bi> A.M. Srivastava *, W.W. Beers GE Corporate Research and Development Center, KWB 316, 1 Research Circle, Niskayuna, NY 12309, USA GE Lighting, Nela Park, Cleveland, OH, USA Received 10 August 1998; received in revised form 21 December 1998; accepted 21 December 1998
Abstract The photoluminescence and the excitation spectra of Bi> in the pyrochlore La Zr O have been investigated in the temperature range of 15}300 K. At low temperatures, the photoluminescence spectrum of Bi> in LaZrO consists of two bands, one in the ultraviolet (385 nm) and the other in the visible (515 nm) part of the spectrum. Thermally activated intensity exchange between the two bands is observed. The ultraviolet emission is attributed to the P PS transition and the visible luminescence to an impurity trapped exciton-type emission. The low e$ciency of Bi> emission in LaZrO and in YO (#uorite structure) is tentatively ascribed to bonding within the tetrahedral BiO network that promotes photoionization of the luminescent center. 1999 Elsevier Science B.V. All rights reserved. Keywords: Impurity-trapped exciton luminescence; Bi>; Pyrochlore LaZrO
1. Introduction The spectroscopy of the Bi> ion which has the 6s electronic con"guration, has been investigated extensively in a variety of host lattices [1]. The ground state of the free ion is S whereas the 6s6p excited states give rise to the triplets levels [P, P, P] and the P singlet state. The SPP transition (A-band) becomes allowed as a result of spin}orbit coupling whereas the SPP transition (B-band) is forbidden but can be induced by coupling with unsymmetrical lattice vibrational modes. The SPP transition (C-band) is an * Corresponding author. Tel.: #1-518-387-7535; fax: #1518-387-5299; e-mail: [email protected].
allowed transition but the S PP is strongly forbidden. In addition to the above transitions another optical transition (a `D-banda) appears in the optical spectra of the Bi> ions. The D-bands are presumed to be a ligand to metal charge transfer transitions although an earlier view ascribed the band to a perturbed exciton [2]. In certain host lattices the ns ion exhibits an `anomalousa emission that di!ers considerably from the characteristic emission due to the P PS transitions. A photoionization process in which the luminescent center ejects an electron into the host lattice conduction band has been proposed to account for the `anomalousa emission. This interaction of the P levels of the ns ion with the host lattice conduction band results in the formation of an impurity bound exciton state in
0022-2313/99/$ } see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 3 1 3 ( 9 9 ) 0 0 0 0 4 - 6
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which the electron is delocalized in the host lattice and the hole trapped at the luminescent center. The `anomalousa emission in the optical spectra of ns ions is a result of the radiative decay of the exciton state. Results existing in the archival literature that are representatives of the ionization-induced `anomalousa emission in the optical spectrum of ns ions include Pb> activated CdCl [3], alkaline earth carbonates such as BaCO and SrCO [6], derivatives of the PbFCl structures [7] and Bi> activated InBO [4,5]. We wish to clarify two points before proceeding further. First, the equivalence in assigning the origin of the `anomalousa emission to an impurity bound exciton state or to the D-state (see above) is a consequence of our restricted understanding of the excited state composition responsible for such optical transitions. Second, the `anomalousa emission due to the ionization process is not con"ned to ns ions but has been observed in the luminescence of ions such as Eu> [8], Yb> [9], Tm> [9], Ce> [10}12] and Cu> [13]. The luminescence of Bi>activated La Zr O is examined in this paper. The host material crystallizes in the pyrochlore structure [14]. The cubic pyrochlore structure for the oxides A B O (space group Fd3m) can be considered to be derived from the #uorite structure (CaF ) by removing one eighth of the anions so that the composition can be written as A B O < where < is the anion va cancy. The large A-cations are eight coordinated whereas the small B-cations are in octahedral coordination. The pyrochlore structure can be viewed as interpenetrating network of BO octahedra and A O chains where the O are the axial oxygen ions that are exclusively coordinated to the rare earth ions.
2. Experimental Materials with general composition (La Bi ) Zr O were synthesized by blending stoi\V V chiometric amounts of the starting materials, La O (99.999%), ZrO (99.978%) and Bi O (99.9%) with 50% by weight Li SO and heating for 10 h in a covered crucible at 14003C. The #ux (Li SO ) promote the reaction between the refractory start-
ing materials and is removed from the sample by several washings in hot water. X-ray di!raction showed single phase formation. Luminescence measurements were done as previously described [15]. The spectra have been corrected for the wavelength-dependent variations in the Xe-lamp intensity and the photomultiplier response.
3. Results The excitation spectra of (La Bi ) Zr O at ¹"15 K is shown in Fig. 1. The emission spectra (j "290 nm) at several temperatures are shown in Fig. 2. The emission spectrum at 15 K exhibits two broad bands with maxima at 385 nm (UV) and 515 nm (VIS), respectively. The measured temperature dependence of the UV and VIS band intensities (Fig. 3) indicates an intensity exchange between the two bands in the 15}70 K temperature range. At ¹"70 K, the intensity of the UV band is at a minimum while that of the VIS band a maximum. Above 70 K the UV band recovers while the VIS band exhibits intensity loss. Thermal quenching of the UV emission occurs above 120 K. The room-temperature emission spectrum of (La Bi ) Zr O contains only the UV component with measurable but exceedingly weak intensity. The common feature in the excitation spectra of the UV and VIS emission bands is the broad band centered at 290 nm (4.27 eV). However, considerable di!erence is noted on the high-energy side of the excitation spectra (Fig. 1). Clearly, the VIS emission is more e$ciently excited by the high-energy band, the maximum of which is located below the detection limit of 250 nm of the instrument. The two bands in the excitation spectrum coincide with two absorption bands at 245 and 287 nm, respectively, in the room temperature diffuse re#ectance spectrum of (La Bi ) Zr O . The Bi>-induced absorption bands are absent in the di!use re#ectance spectrum of the pure material (in the 240}350 nm wavelength region). The UV emission exhibits a Stokes shift of about 1.06 eV. The estimated Stokes shift for the VIS emission is 2.66 eV (the excitation band for this emission is centered at about 245 nm).
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295
Fig. 1. Excitation spectra of (La Bi ) Zr O for (a) j "515 nm and (b) j "385 nm at ¹"15 K.
The emission spectrum for j "255 nm indi cates preferential excitation of the VIS emission by high-energy radiation (Fig. 4). This is in agreement with the excitation spectrum (see Fig. 1). A shift to higher energy of the UV emission band maximum with increasing temperature is observed. For example, the emission maximum shifts from 385 (3.22 eV) at 15}375 nm (3.31 eV) at 110 K. The maximum of the VIS emission is essentially temperature independent.
4. Discussions The intensity exchange between the UV and VIS bands and their identical excitation spectra (except for the variance on the high-energy side) suggests that the two emission bands originate from the same Bi> center in the La Zr O host lattice. We
would like to note that the pyrochlore crystal structure o!ers a single crystallographic site for the Bi> ion. At low temperatures, the UV band is assigned to the forbidden P PS transition on the Bi> ion. The VIS band is associated with the ionization process described previously in which the Bi> center injects an electron in the host lattice conduction band. Therefore, the VIS emitting state is an impurity centered exciton state that is populated by an electron transfer from the Bi> P excited states. This suggests the proximity of Bi> excited states (P ) with the bottom of the La Zr O conduction band. The two dominant radiative decay channels following Bi>S PP excitation are the emission from localized states (P ) and emission from the impurity trapped exciton level as a consequence of ionization. It is clear that even at low temperatures the emission is characterized by the competition
296
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Fig. 2. Emission spectra of (La Bi ) Zr O for j "290 nm at various temperatures.
between two emitting states. In the 15}70 K temperature range, the enhancement in the VIS emission intensity at the expense of the UV emission is a result of the thermally assisted ionization process becoming more e$cient than the radiative relaxation of the Bi>P excited states. The quench ing of the VIS emission for ¹'70 K is presumably related to a thermal ionization process of the exciton state [3]. The structure of the trapped exciton structure is of the hole localized on the luminescent ion and the electron delocalized over the surrounding cations. The trapped exciton in SrF :Yb>, for example, consists of a Yb> core with the electron delocalized over the 12 nearest neighbors Sr> ions [9]. As previously described, the pyrochlore structure can be perceived as an A O network (similar to the anticristobalite Cu O network) interpenetrat ing the BO framework. Consequently, the exciton
structure in La Zr O : Bi> can be considered to consist of the hole residing on the (La,Bi) O sublat tice and the electron in the ZrO framework [16]. We would like to mention that the VIS band can also be perceived as an emission arising from a charge transfer transition between the activator Bi> (6s) and the Zr> (4d) ions of the host lattice. Charge transfer transitions between ions with ns electronic con"guration and ions with nd electronic con"guration are fairly common and often observed [17]. The charge transfer state with an approximate charge distribution of ns}nd often exhibits very e$cient luminescence. The so-called D-state in the energy scheme of ns ions is thought to be of a charge transfer type. Therefore, the VIS emission in La Zr O : Bi> can be assigned to DPS radiative decay. A detailed report on the luminescence of LaOBr : Bi> is existing in the literature. There is
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297
Fig. 3. Temperature dependence of the relative intensities of the UV and VIS bands in (La Bi ) Zr O for j "290 nm.
a close correspondence between the luminescence of LaOBr : Bi> and La Zr O : Bi>. In the for mer system the outcome of Bi> S PP excita tion is UV emission at low temperatures with the VIS band emerging at the expense of UV band at higher temperatures [18]. The VIS emission is attributed to a charge transfer transition involving the Bi> and the Br\ ligands of the LaOBr lattice. Relative to the oxybromide system, one major difference is the absence of a signi"cant thermal barrier between the UV and VIS states in La Zr O : Bi>. Note the dominance of the VIS band even at low temperatures in the luminescence of La Zr O : Bi> (Fig. 2). An insigni"cant poten tial barrier between the Pb> [P ,P ] and the excitonic states is also observed in gallates with the magnetoplumbite structure (SrGa O ). It has been suggested that high covalency of the host lattice contributes to the low potential barrier [19].
So while not certain, it is feasible that the insigni"cant thermal barrier between the UV and VIS emitting states in La Zr O : Bi> is also a result of high host lattice covalency. The shift of the UV maximum to higher energy with increasing temperature can be explained as follows [20]. The Bi> luminescence results from the radiative decay of the P and P excited states to the S ground state. The P state is located below the P state. Hence, at low temperatures the P acts as an optical trap so that the emission is dominated by the strongly forbidden P PS transition. At higher temperatures, thermal population of the higher energy P state results in the occurrence of the P PS transition. This energy change arising from the two radiative transitions results in the temperature-induced shift of the UV maximum to higher energy. The band shift of about 90 meV is an approximate measure of the energy
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Fig. 4. Emission spectra of (La Bi ) Zr O for j "255 nm at ¹"15 K.
separation between the Bi>P and P states in La Zr O . The increase in the UV band intensity for ¹'70 K (Fig. 3) is in part due to the higher P PS transition probability. However, the intensity enhancement may also be due to a thermally induced energy back transfer from the excitonic state to the localized UV emitting state of the Bi> ion. The irradiation within the A band of ns ions in alkali halide crystals is known to produce two emission bands that are designated as A and A , in 6 2 order of increasing photon energy [2]. These bands, the intensities of which are also strongly temperature dependent, are ascribed to the coexistence of minima with two di!erent symmetries on the adiabatic potential energy surfaces of the relaxed excited state (APES). The origin of the two minima is the electron}phonon interaction of the orbital triplet with the e and q vibrational mode of the octahedral ns ion complex. The lumines-
cence properties of these systems are mainly governed by the ratio of the spin}orbit coupling (m) and the electron}lattice relaxation. If the ratio is very high or very low then only one minima is expected. The high atomic number of Bi> results in strong spin}orbit interaction so that only one minimum on the APES is anticipated [21]. Consequently, the doublet nature of Bi> luminescence in La Zr O may not be attributed to the Jahn}Teller e!ect. Also, the requirement of a symmetrical lattice site for the Jahn}Teller e!ect to be operative is not evident in the structure of the pyrochlore since the A-site coordination is that of a strongly distorted cube. We would like to note that the Stokes shift of impurity trapped excitonic state emission is usually much larger than that of the localized transitions [3,22]. Indeed, the Stokes shift of the exciton state in La Zr O : Bi> is nearly twice that of the local ized transition (P PS ).
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The nature of the high-energy band in the excitation spectrum of La Zr O : Bi> is now discussed. The assignment of the lowest energy band to the S PP (A-band) transition on the Bi> ion is straightforward. The higher energy absorption may be assigned to the allowed S PP (C-band) transition. This assignment would yield an energy di!erence between P and P states of about 6000 cm\. In solids, the P P energy separation usually amounts to about 10 000 cm\ [23,24]. Consequently, it is unlikely that the higher energy band corresponds to the S PP transition. The higher energy band is thus tentatively assigned to the absorption into the excitonic state. The rather low-energy position of this band in La Zr O may re#ect on the high host lattice covalency. The low e$ciency of the Bi> P PS (UV) emission at room temperature is inconsistent with the expectation of e$cient luminescence resulting from the small Stokes shift of emission (1.06 eV). We postulate that the intensity decrease of the UV band for ¹'120 K is due to thermal ionization of the localized levels and the decrease in the total emission intensity with increasing temperature is due to a thermal ionization process involving both the localized P and the excitonic states. We now proceed to compare the luminescence of Bi> activated Y O and La Zr O in order to gain insight into structure-photoionization issues. The low quantum e$ciency of Y O : Bi> has been attributed to enhanced radiationless transitions arising from the interaction between Bi> excited states (P ) and the host lattice con duction band [25]. We held the same mechanism responsible for the low e$ciency of La Zr O : Bi> system. A close examination of the crystal structures of Y O and La Zr O allows us to propose an explanation for the low e$ciency of Bi> luminescence in these materials. In the #uorite derived structure of Y O each oxy gen ion is surrounded by four Y> ion in a (distorted) tetrahedral arrangement and a network of such tetrahedral groups can be distinguished in the crystal structure. The pyrochlore structure may also be thought to be derived from the #uorite structure where the cations form face-centeredcubic array and the anions are located in the tetrahedral interstices of the cationic array. The three
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tetrahedral interstices sites available for the anions in the pyrochlore structure are the 48(f ) positions with two A and B near neighbors, the 8(b) position having four B near neighbors (the site are unoccupied in the pyrochlore structure) and the 8(a) position with four A near neighbors forming a regular tetrahedron (A O). In this respect the pyrochlore structure resembles the Y O structure. Consequently, it is feasible that the bonding within the tetrahedral network promotes photoionization of the Bi> center in these materials. The ionization process is in turn held responsible for the low quantum e$ciency of Bi> luminescence. 5. Conclusion In this paper, we have observed emission from the impurity trapped exciton state in the La Zr O : Bi> system. This implies proximity of Bi> excited states (P ) with the bottom of host lattice conduction band. The impurity trapped exciton state exhibits di!erent excitation and emission spectra from the normally observed Bi> P S localized states. A signi"cant en ergy barrier between the P and the excitonic state is not observed in this host lattice. The low e$ciency of the Bi> luminescence in La Zr O at room temperature is attributed to thermal ionization of the P and the exciton states. A com monality in the crystal structures of Y O and La Zr O , namely the network of oxygen anions that are tetrahedrally coordinated by metal cations, is thought to promote photoionization of the Bi> center. Acknowledgements The authors are grateful to Dr. C. R. Ronda, Philips GmbH Forschungslaboratorium Aachen, Germany, for valuable discussions. References [1] G. Blasse, Prog. Solid State Chem. 18 (1988) 79. [2] P.W.M. Jacobs, J. Phys. Chem. Solids 52 (1991) 35 and references therein.
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