PHOTOLYSIS AND RADIOLYSIS OF 2-METRYLBUTANE
`
`5139
`
`and it is, therefore, unlikely that after detachment of
`the first H atom, a second H atom would be eliminated.
`Little can be said concerning the modes of formation of
`the pentenes, except that they are related to those of
`hydrogen.
`
`ACKNOWLEDGMENT
`
`The authors would like to express their gratitude to
`the referee of this paper for his comprehensive com(cid:173)
`ments on the discussion.
`
`THE JOURNAL OF CHEMICAL PHYSICS
`
`VOLUME 47, NUMBER 12
`
`15 DECEMBER 1967
`
`Investigation of Some Cea+-Activated Phosphors
`
`G. BLASS£ AND A. BRIL
`Philips Researcl, Labrwalorus, N. V. Philips' Glceilampenfabrieken, Eind/rove,,, Netherland.s
`(Received 14 August 1967)
`
`The fluorescence of a number of new Cea+-activated phosphors is described and discussed. Only host
`lattices with a sublattice consisting of trivalent lanthanide ions are used to avoid charge compensation of the
`Cea+ ion. Usually the Ce'+ emission is in the near-ultraviolet region. YsAI.O,rCe and SrY,O,-Ce, however,
`show emission in the visible region with a maximum in the green. Conditions for visible Cet+ emission are
`indicated, viz., large crystal-field splittings (YaA40u--Ce) or a large Stokes shill (SrY,O,-Ce). In a number
`of cases we were able to observe all crystal-field components of the excited 5d level of Ce'+. The cubic
`crystal-field splitting of the 5d level varies strongly with host lattice from 7000 to 14000 cm-1• The position
`of the center of the 5d levels in oxides is about 30% lower than in the free ion. For some phosphors we
`observed more than one emission band at room temperatu.re. This is due to ftuorescence from higher excited
`levels. Efficient energy transfer from Ce'+ to Cr>+ was observed in YaA40u.
`
`I. INTRODUCTION
`
`II. EXPERIMENTAL
`
`The fluorescence of Ce3+ -activated compounds is well
`known.1•2 Usually the emission consists of a broad band
`with two peaks in the long-wavelength ultraviolet
`region. Since the Cea+ ion has a 4f configuration, the
`ground state consists of a doublet (2F6/2 and 2F112).
`The lower excited states are the crystal-field components
`of the Sd configuration. The transition 4f--+6s and
`charge-transfer transitions are at considerably higher
`energies3 and are not considered in this paper. The Cea+
`emission is due to a Sd-+4/ transition. This is an allowed
`electric dipole transition and therefore the decay time
`of the fluorescence is very short ( < 10-7 sec) .1 •4
`Recently we have also reported on a Ce3+-phosphor
`which emits in the visible, viz., Y81\4012-Ce.4 This
`peculiar behavior prompted us to study the Ce»
`fluorescence in other oxides of the trivalent lanthanides
`(Laa+, Gda+, Yl+, Sc:3+). Surprisingly enough, these
`materials have not widely been investigated, although
`these host lattices offer the possibility of introducing
`Cea+ without charge compensation.
`The present paper gives the results of this investiga(cid:173)
`tion. The complete crystal-field splitting of the Sd
`configuration of the Ce» ion was found in a number of
`cases. Requirements for Cea+ emission in the visible
`region are indicated.
`1 A. Bril and H. A. Klasens, Philips Res. Rept. 7, 421 (1952) .
`• J. W. Gilliland and M. S. Hall, Electrochem. Tech. 4, 378
`(1966).
`1 E. Loh, Phys. Rev. 154, 270 (1967).
`(cid:141) G. Blasse and A. Bril, Appl. Phys. Letters 11, 53 (1967).
`
`Samples were prepared by firing intimate mixtures of
`high-purity oxides (or compounds, which on decompo(cid:173)
`sition yield oxides) at appropriate temperatures in
`nitrogen. The Ce3+ ions were introduced by replacing
`part of the trivalent lanthanide ions by Ce3+ ions. The
`Ce3+ concentration was 1-2 at. %, Products were
`checked by x-ray analysis. The performance of the
`optical measurements has been described previously.~
`
`m. REFLECTION AND EXCITATION SPECTRA
`A. Results
`Tables I-III contain the absorption bands of the
`Ce3+ ion in a number of host lattices in the uv region.
`These host lattices do not absorb in the optical region
`studied except for the Ga-containing oxides. The data
`were obtained from excitation and diffuse reflection
`spectra. In some cases only a continuous absorption
`was found, so that the absorption bands remain un(cid:173)
`known (Table IT). Figures 1-3 show the excitation and
`diffuse reflection spectra of Y AlaB,012: Ce, YaAl5012: Ce,
`and ScB03: Ce. These figures illustrate two types of
`excitation spectra, viz., those in which the quantum
`efficiency of the Ce"' fluorescence is roughly independ(cid:173)
`ent of the exciting wavelength (YAl 3B,012:Ce, Fig. 1)
`and those in which the quantum efficiency decreases
`
`• A. Bril and W. L. Wanmaker, J. Electrochem. Soc. 111, 1363
`(1964).
`• G. Blasse and A. Brit, J. Chem. Phys. 46, 2579 (1967) .
`
`(cid:47)(cid:50)(cid:58)(cid:40)(cid:54) 1026, Page 1
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`

`5140
`
`G. BLASSE AND A . B RI L
`
`TABLE I. Efficiencies for ultraviolet and cathode ray excitation, positions of emission and excitation bands
`and Stokes shift of the emission of some Ce•+ phosphors.
`
`Composition•
`
`~t%)b q.,u(%)•
`
`Emission bands
`tlO' cm-•)d
`
`Excitation bands
`(103 cm-•)•
`
`Sc,Si,Or-Cc
`ScBO.-Ce
`YBO,-Ce
`LaBO.-Ce
`YPO.-Ce
`LaPO,- Ce
`Y Al,B,O1,-Ce
`YaAJ,O,,-Ce
`YOCl-Ce
`LaOCl-Ce
`LaOBr- Ce
`
`1.5
`2
`2
`0.2
`2.5
`2.2
`2
`3.5
`3.5
`0.4
`0.2
`
`65
`70
`so
`35
`30
`40
`40
`~70
`60
`30
`25
`
`25.4; 29.4
`26.8; ~30.S
`23.8; 25.4
`26.6; 28.4; 31.5
`28.3; 30.0
`29.8; 31.5
`27.2; 29. 1
`18.2; 27 .8; ~29.2
`26.3
`27. 8
`22.8
`
`29.0; 33.3; ~43.5
`28.0;31.2;36. 1;38.S
`27.4; (~29);40.8; (~43.5)
`30.8; 36.9; 41.5
`32.8; (34.2); 39. 6
`36.2; 38. 8; 41.6
`31.0; 36.6; 39.2
`22.0; 29.4; ~37; ~44
`31.6; 35.8
`(~35) ;35.8;39.7
`28.4; 34.7
`
`Stokes
`shift
`(103 cm- •)•
`
`3.6
`1.2
`2.0
`2.4
`2. 8
`4.7
`1.9
`3.8
`5.3
`5.2
`5.6
`
`• Cc•• concentration i, 1- 2 at.%,
`b Radiant efficiency for cathode ray excitation (20 kV).
`0 Quantum efficiency for excitation in the low-est excited level.
`d Position of the emission peaks.
`
`e Position of the excitation bands; shoulders between brackets.
`1 Stokes 6hift of the emis~ion, i.e., difference between the vositiona of the
`lowest excited level and the short. wavelength companent of the emiss:ion
`from this leveJ.
`
`drastically with decreasing exciting wavelength
`(Y3Ah012-Ce, Fig. 2 and ScBOa- Ce, Fig. 3) . All the
`excitation spectra we obtained belong to either the
`former or the latter type. In this section we discuss
`the number and position of the absorption bands and a
`possible explanation for the two types of excitation
`spectra. A comparison between the excitation spectra
`of Ces+- and TbH-activated phosphors is also included.
`
`B. Discussion
`The occurrence of more than one Ce3+ absorption
`band in the region between 25 000 and 50 000 cm- 1 is
`due to the crystal-field splitting of the Sd (2D) state.
`As argued before in the case of Tb3+ we can neglect the
`effect of spin-orbit splitting, since the spin-orbit param(cid:173)
`eter lf>d is about 101 cm-1, whereas the crystal-field
`splitting amounts to 104 cm-1 or more.7 For nearly all
`compounds mentioned in the tables the symmetry of
`the lanthanide site is known, so that it is possible to
`compare the experimentally observed number of Sd
`levels and the number expected from the site symmetry.
`Not in all cases a reasonable agreement was found.
`
`!l
`
`t
`
`l
`" I
`t 0
`
`JSO
`
`JOO
`
`l'.Xl
`l
`-~
`j
`t
`
`0
`200
`
`A(mp) .1:3..
`Flc. 1. Diffuse reflection spectrum (broken line) and relative
`e:<citation specll"Um (solid line) of the Ce>+ fluorescence of
`Y o.1tCeo.01A4B,O.,.
`
`7 G. Blasse and A. Bril, Philips Res. Repts. 22, 481 (1967),
`
`Consider, for example, YOCl-Ce and YPO.-Ce. The
`respective site symmetries are C,. and Du, so that 3
`and 4 levels are expected. The experimental numbers
`(Table I ) are 2 and 3, respectively. It cannot be
`excluded, however, that there are levels beyond the
`spectral limit of our apparatus ( ~220 mµ) or that lower
`symmetrical splittings remain hidden in the broad
`bands observed experimentally. We never observed
`more bands than expected from the site-symmetry.
`In Table IV we have tabulated those host lattices for
`which the number of levels observed experimentally
`agrees reasonably well with the number expected from
`site symmetry.
`From these data the cubic crystal-field splitting /l
`and the center of gravity of the Sd levels can be found,
`at least approximately. In YaA16012-Ce the Ce3+ ion
`occupies a distorted cube, so that the Sd level is split
`into a lower doublet and a higher triplet by the cubic
`component of the crystal field. Noncubic components
`cause a further splitting. Unfortunately we observed
`only two components of the triplet, which causes some
`uncertainty in the value of fl and the position of the
`center of gravity. In ScB03-Ce and Sc2Si20,Ce the
`}~
`!
`l
`j so
`t 0
`
`~
`
`l
`i
`sol
`t
`
`........
`' '
`
`... ___
`
`(,/)()
`
`Fie. 2. Dilluse reflection spectrum (broken line) and relative
`excitation spectrum (solid line) of the Ce>+ fluorescence of
`Y, ... Ce,.,.AJ,O... The dotted line represents the diffuse reflection
`spectrum of unactivated Y,Al,O12•
`
`(cid:47)(cid:50)(cid:58)(cid:40)(cid:54) 1026, Page 2
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`VIZIO Ex. 1026 Page 0002
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`

`

`I 1 VEST I GA TIO N OF SOME Ce H- - ACTIVATED PHOSPHORS
`
`5141
`
`TABLE II. Position of emission 11.nd eJ1:citation bands and Stokes shift of the emission of some c.,a+ phosphors
`(inefficient at room temperature ).•
`
`Composition
`
`Y,Al,0,- Ce
`SrY,0.-Cc
`Y.0.-Ce
`. aYO.-Ce
`NaGdO,-Ce
`LaAIO,-Ce
`
`Emissio,a bands
`( 103 cm- •)
`
`Eitcitation bands
`I 10' cm-•)
`
`Stokes shift
`(lo> cm- •)
`
`l~28) ; 31.2
`17 . 4b
`19.6; (~24)
`(~21.3) ; 22 .8
`21.3
`
`32.6; ~34; ~41.5
`25.2;?
`?
`
`~4.5
`~8
`
`24.2; 31.2; -40•
`
`• For explanation of columnu sec Table 1.
`
`b At 77°K.
`
`0 Data from n:flectlon :;IJCCt.rum.
`
`Ce3+ ion occupies distorted octahcdra, so that the triplet
`is lower. In YAl:iB4O1:-Ce the Ce:i+ ion is in a trigonal
`prism of 011- ions. The 2D level splits into three levels
`with symmetry species E', A1', and E" (31 000, 36 600,
`and 39 200 cm-1, respectively) . This corresponds to a
`cubic crystal-field splitting of 6500 cm- 1 (see Appendix).
`Finally Table IV contains analogous data for
`SrF2: Cc3+ recently given by Loh.3 Although our results
`are not accurate, Table IV gives some interesting results.
`The 4f- 5d distance is 51 000 cm-1 in the free Cea+ ion.
`This value decreases to 48 000 cm-1 in fluorides and to
`roughly 35 000 cm-1 in oxides, a reduction of 6% and
`30%, respectively. This fact must be ascribed to the
`reduction of the interelectronic repulsion parameters
`by covalency effects (nephelauxetic ellect8) . WiLhin
`the group of oxidic host lattices there is even a varialion
`of this value. For example, ScBO3-Ce has the center of
`the Sd level at ,.._,32 500 cm-1 . On the other hand, the
`value in the case of LaPO.-Ce must be considerably
`higher than the 35 000 cm-r mentioned above (see
`Table I). In our paper on TbH-activated phosphors7
`we have already drawn attention to this fact. The
`Tb3+-activated phosphors show excitation bands due to
`
`TABLE Ill. Efficiency for cathode-ray excitation and position
`of the two lower absorption bands and of the emission band in
`the visible region for some Ce'+ -acti vatcd garnets.
`
`Composition
`
`,, (%)
`
`Emission
`Position lower
`absorpt ion bands bands in the
`(difference hetween
`visible
`brackets)
`region
`(\(}'I cm-•)
`( JO'cm-1 ) •
`
`Y,.,Gd,.,Al,01.-Ce
`Y.Al,0,.-Ce
`Y,Al,GaO,.-Ce
`Y,Al,Ga.D,.-Ce
`Y,Al,Ga..01.-Ce
`Y,Ga,0 ,,-Ce
`
`. .. b
`3.5
`1.9
`1. 7
`I. 2
`
`21.S, 29.6 (8. 1)
`22.0, 29.4 (7 .4)
`22. s, 29 .1 (6. 6)
`23.0, 28.8 (5.8)
`23.3, 28.6(5.3)
`23 .8, 28.1 14 .3)
`
`17.4
`18. 2
`18 .. 5
`19.2
`19.6(18. 3)
`0 I•·
`
`"' Shoulders between parentheses.
`b Emissjon contains Eu1+ contributions d\le to the starting matcria,I
`Gd.O.. which contain. a slight arnount of europium.
`c No fluo~nce .
`
`• C. K. J¢rgensen, Absorption Spectra a1Jd Chem ical Bu11di11g
`(Pergamon Press, Inc., New York, 1962) .
`
`4f->5d transitions in the TbH ion. These bands are at
`higher wavenumbers than those of Ce3+ and partly
`beyond the shorter-wavelength limit of our apparatus,
`so that il was not possible to find the center of gravity
`in the case of TbH.
`Table IV also shows that the cubic crystal-field
`splitting t. varies strongly. For a cube the absolute
`value of t. is expected to be 8/ 9 times the value of t.
`for an octahedron with the same distances between
`central ion and ligand.9 With this in mind all values of
`A in Table IV can be compared with each other. The
`t. in Y3AlbO1rCe is relatively large, whereas A in
`YA}aB4O12-Ce is relatively small. That t. is strongly
`influenced by the nature of the next-nearest neighbors
`is a well-known phenomenon.10-11 We further note that
`t. in the Ouorides has the same order of magnitude as in
`the ox ides. This has also been found for the transilion(cid:173)
`melal ions.8
`The value of the noncubic <.:rysta!-field spliuing varies
`considerably. For some host lallices it is possible to
`compare the splitting of the lower cubic crystal-field
`level of the Sd state of CeH and the 4f1Sd stale of Tb'4 •
`The agreement is very good (Table VJ.
`The phenomenon that the efficiency of Cea+ -a<.:tivaled
`phosphors is more or less independent of the position
`of the excitation band in some cases (Fig. 1), but
`decreases strongly for excitation in bands with higher
`
`a_100
`l
`¾
`1 so
`i
`4'
`t
`
`0
`m
`
`100
`l
`.Ii
`i
`t
`
`,t
`
`0
`200
`
`F1c. 3. Diffuse reflection spectrum (broken line) and relative
`excitation spectrum (solid line) of the Ce"T fluorescence of
`ScuoCeo.01 BO,.
`
`9 J . S. Griffith, Tlte TheM"y of Transition-Metal Ions (Cambridge
`University Press, Cambridge, 1961 ).
`•• J. Ferguson, K. Knoit, and D. C. Wood, J . Chem. Phy~.
`35, 2236 (1961); 37, 193 (1962) .
`11 C. Blasse, J. Tnorg. Nucl. Chem. 29 1 1817 ( 1967) .
`
`(cid:47)(cid:50)(cid:58)(cid:40)(cid:54) 1026, Page 3
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`

`5142
`
`G . BLASSE AND A . BRIL
`
`TABLE IV. The 5d levels and cubic crystal-field splitting (.o.) for cei+ in several host lattices. All values in l()' cm-1•
`
`Composition
`
`Y ,AJ.Ou--Ce
`YAlaB,O,rCe
`ScBOrCe
`
`Sc.Si,OrCe
`
`SrFrCeb
`Free ionb
`
`Coordination
`of Ce>+
`
`5d levels
`(observed) •
`
`Deduced
`cubic levels
`
`Cubic crystal-
`field splitting (.0.)
`
`Spherical
`5d
`
`Distorted cube (D,)
`Trigonal prism (Da1,)
`Distorted octahedron
`(D1c1)
`Distorted octahedron
`(C.)
`Distorted cube
`
`22 .0 29.4 I 37 44
`31.0 36.6 39.2
`28 .0 31.2 I 36. 1 38. 5 ~29 . 5 (1. , ) , 37. 3 (e,)
`
`25 . 7 (e,), ~40 (k,)
`
`~14
`~6. 5•
`~8
`
`~34.5
`35.4
`~32.5
`
`29.0 33.3 j 43 . 5
`
`~31 (1,, ) , ~43 . 5 (e,)
`
`~12 .5
`
`~ JS
`
`33.6 48 .8 I 50.3 53 . 4
`
`4l.2 (e,), 52.4(1,,)
`
`11.2
`
`48
`51
`
`• Vertical bar indicate, separation bctw,en lower and higher cubic level._
`b After Ref. 3.
`
`• Value of the oct ahedral crystal-field 81)liU.in1 derived from the poslUon
`of tho level• for Ce•+ In prbmat!c coordination nalng a point-diar11< model
`(5"' Appendix) .
`
`wavenumbers in other cases (Figs. 2 and 3), is probably
`due to the position of the configuration coordinate
`curves relative to each other. Let us call the configura(cid:173)
`tion coordinate curves of the ground state and the two
`lowest excited states g, e1, and e2, respectively (e1 is
`below e2) . If the curve e1 crosses the curve e2 within the
`con.figuration coordinate curve of g, excitation into e1
`and e, may both result in efficient fluorescence from e1.
`If on extrapolation the curves e1 and e2 should cross
`outside the curve g, however, excitation into e1 gives
`efficient fluorescence from e1, but excitation into e,
`cannot give efficient fluorescence from e1, since either
`fluorescence from e2 or radiationless transition to g
`results. If the "crossings" of e, with e1 and g approach
`each other, weak fluorescence from e1 and e2 may be
`expected upon excitation into e2• From this simple
`model it follows that a Cea+-activated phosphor with an
`excitation spectrum in which the efficiency decreases
`with increasing excitation energy, must show fluores(cid:173)
`cence from et upon excitation into e2 ( or higher levels).
`This was in fact observed (see below) .
`
`IV. EMISSION
`
`A. Results
`
`The positions of the emission peaks of our samples
`are collected in Tables I- III. Figures 4-11 show the
`spectral energy distribution of the emission of a number
`
`too...----,,,,- - - - ---,
`
`I
`
`t
`
`of compounds. Usually the emission is in the near-uv
`region, but not always: Y3(Al, GahOu-Ce and SrY20,(cid:173)
`Ce (Figs. 5 and 10) show emission in the visible. The
`half-width value of the emission bands does not depend
`on their position and amounts to roughly 4000 cm- 1
`for all materials. lo some cases the• emission band is
`split, the difference between the two•peaks being some
`2000 cm- 1 (LaB03-Ce, Fig. 4; YB03- Ce, Fig. 9;
`YA13B401:- Ce and YPOr Ce, Table I) . In many cases
`more than one emission band is present (LaBOa-Ce,
`Fig. 4; YaAl.012- Ce, Fig. 5; ScBOa-Ce, Fig. 6; Sc2Si20,
`Ce, Fig. 7; and Y4Al20r Ce, Fig. 11) .
`
`B. Discussion
`
`The fluorescence emission of the Cea+ ion originates
`from a transition from one or more of the 5d levels
`(often only the lowest level) to the 2F ground state.
`Since the ground state of the Ce* ion is a doublet
`(2F 512 and 2F112) with a separation of about 2000 cm- 1,
`each emission band is expected to show two peaks as
`found by us in many cases (compare also Ref. 12) . This
`doublet character of the emission band depends on
`temperature and Cea+ concentration (self-absorption)
`and is not always found.13
`In the case of oxidic host lattices this emission usually
`peaks in the near uv. As a matter of fact there are two
`
`- - ---:..--:-?-<"-
`
`-
`
`-
`
`-
`
`-,
`
`IOO. - --
`I.
`
`t
`
`J50
`
`400
`-MmpJ
`FJG. 4. Spectral energy distributio11 of the fluorescence of
`Lao.ooCea.01B0. under 254-mµ excitation. In Figs. 5--12 theiradiant
`power per constant wavelength interval in arbitrary units (/ ) is
`plotted along the ordinate.
`
`450
`
`600
`-..i(~)
`
`Flo. 5. Spectral energy distribution of the fluorescence of
`Ys.NCeo,..Al.Oa (solid line) and Y,.14Ceo . ..Al,Ga.Oi, (broken
`line) under 254-mµ excitatio11.
`
`"F. A. Kroger and J. Bakker, Physica 8,628 (1941).
`11 Th. P. J. Botden, Philips Res. RepL 7, 197 {1952) .
`
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`VIZIO Ex. 1026 Page 0004
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`

`

`INVESTIGATION OF SOME Ce>+ - ACTIVATED PHOSPHORS
`
`5143
`
`TABu: V. Splitting of the lower cubic ci:y11tal-field component
`of the 5d level of Cel+ and the 4f Sd level of Tb,.. (all values in
`lC)lcm-1).
`
`Host lattice
`
`YaA40u
`YaGa.Ou
`YB01
`Sc,Si.O,
`
`cea+ •
`
`7.4
`4.3
`~l.6
`4.3
`
`Tb'+b
`
`7.5
`4.0
`1.4
`4. 6
`
`• From Tables f and Ill.
`b From Ref. 7.
`
`cases in which the emission wiU be at lower wave mun(cid:173)
`bers.
`
`(a) If the lowest 5d level lies exceptionally low.
`(b) If the Stokes shift of the emission is exceptionally
`large.
`
`We have found one example of each of these two cases.
`In Y aAlbO12--Ce the lowest 5d level is exceptionally
`low, viz., at 22 000 cm-1 (see Table I). The YgAI&Ou-Ce
`is the only Ce3+-activated phosphor that has not a
`white, but a yellow body color, whereas the host lattice
`itself is white. Therefore, the emission lies also at very
`low wavenumbers. Note in Table I that the Stokes
`shift of this emission does not have a value deviating
`from what is normally found. Table IV shows, why the
`lowest 5d level lies so very low. The center of the Sd
`levels of CeH in Y3Al5O12 has a normal value. In Sec. Ill
`we have found that the crystal-field splitting in YaA\;O12
`is very large. Moreover, the lower cubic level is the e0
`level which is ¾ll below the center. In an octahedron
`the l~ level is lower (only fll below the center) . This
`means that the lower cubic crystal-field component is
`relatively low in Y3AhO12 (25 700 cm- 1, compared with
`,-,.,30 000 cm-1 for the lower l2q level of ScBO.Ce and
`Sc2Si2O.,-Ce). There is yet another effect, viz., the large
`noncubic splitting of the e0 level of Ce* in YaAlsO12,
`which brings the lowest level at 22 000 cm- 1. Visible
`Ce3+ emission can therefore be expected, if the Sd
`crystal-field splitting and the lower symmetrical split(cid:173)
`ting are large. Cubic eight coordination is also favorable
`to obtain visible emission, because the lower cubic level
`will then be at relatively low energies.
`.
`It is obvious that long-wavelength emission will.also
`
`---c:c--- -- - - - - ,
`
`,00.---...--
`I
`t
`
`100.----=----;r...._--------,
`I
`t
`
`JSO
`
`Fm. 7. Spectra.I energy distribution of the fluorescence of
`Sc1•11Ce. . ..Si.O7 under 254-m1,1 excitation.
`
`result in the case of a very large Stokes shift. This is
`the case for SrY2O.Ce. The lowest 5d level is not
`exceptionally low (25 200 cm-1), but the Stokes shift
`is very large ( ~8000 cm-1, compare Tables I and II),
`so that visible emission results (Fig. 10), Such a large
`Stokes shift can only occur if the difference between
`the equilibrium distance of the excited state and that
`of the ground state is larger than usual. Elsewhere!• we
`have argued that the increase of the equilibrium dis(cid:173)
`tance upon excitation will be large if the ions of the
`host lattice are rather weakly bounded together, i.e.,
`if large and low-charged ions are involved ( this implies
`ions with relatively highly polarization or, as they are
`sometimes called, "soft" ionsu). Such a situation exists
`in Sr Y 20 4• We also noted weak fluorescence in the visible
`region in the case of Y 2OrCe, Na YO2-Ce, and NaGdO2-
`Ce (Table II). These are also lattices with large cations.
`The same holds for YOCl-Ce, LaOCl-Ce, and LaOBr(cid:173)
`Ce (Table I), but the oxychlorides emit in the uv. Note,
`however, that the Stokes shift of these materials is also
`large.
`Another consequence of a large increase of the
`equilibrium distance upon excitation is a low quenching
`temperature of the fluorescence. For SrY2Oc-Ce we
`found that the light output at 200°K is only 20% of
`the output at 100°K. Such a marked temperature
`dependence agrees with out model. In a number of
`other lattices with large ions (Sr2LaAIO.-Ce, SrLaAlO,(cid:173)
`Ce, LaAIO,-Ce) the Ce* emission was absent or only
`very weak, even at liquid-nitrogen temperature.
`
`I
`
`t
`
`50
`
`YOCl: Ce
`
`~).~)1)450
`
`Fee. 6. Spectra.I energy distribution of
`fluorescence of
`tl!e
`ScueCeo.o,BO, under 254-mµ exCJtation.
`
`Fie. 8. Spectra.I energy distribution of the fluorescence of
`Yo.nCe,.01OCI under 254-mµ excitation.
`
`14 G. Blasse and A. Bril, Z. Physik. Chem. (Frankfur:t). "T~e
`influence of crystal structure on the fluorescence of 0X1d1c mo(cid:173)
`bates and related compounds" (to be published).
`"Compare C. K. J¢rgensen, Coord. Chem. Rev. 1, 164 (1966) .
`
`(cid:47)(cid:50)(cid:58)(cid:40)(cid:54) 1026, Page 5
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`

`5144
`
`G. BL.'\SSE AND
`
`.'\. BRIL
`
`100r----"?"t""""-----~
`I
`t
`so
`
`S00
`
`F!G. 9. Spectral en(cid:173)
`ergy distribu lion of the
`fluorescence of
`Y •. .,Ceo.o,BO.
`under 254-mµ excita(cid:173)
`tion.
`
`Finally we note that also the nature of the ligands
`influences the position of the emission bands. For
`fluorides the center of the 5d level is at high energies
`resulting in emission at high energies (LaF3- Ce: 35 iOO
`e: 32 200 cm- 1 2) . In the Jess
`and 33 300 cm-1, Srf2-
`electronegative chlorides
`the emission
`lies
`lower
`(LaCh-Ce: 28 500 cm-1 2) . A similar phenomenon
`occurs in LaOCI- Ce and LaOBr- Ce (Table I) . If the
`Cl- ion of LaOCl is replaced by the less electronegative
`Br ion, the emission shifts to the visible region (due to
`the decreasing 4f-5d energy difference) .
`In many cases more than one Ce3"1" -emission band is
`present. The band at higher wavenumbers is undoubt(cid:173)
`edly due to fluorescence from higher 5d levels. This
`phenomenon is not unknown, but has only been found
`at low temperature.12 •16 In our investigations it was
`observed at room temperature in the case of LaBOa-Ce,
`ScBOa-Ce, Y1(Al,Ga)5012- Ce,
`Sc2Si201- Ce,
`and
`Y,Al20e-Ce (Tables I and II ). It is noteworthy that
`the two emission bands are better separated, if the
`energy difference between the two lowest SJ levels
`increases. The YaAl50 1rCe shows this very markedly
`(Table I) : the lowest levels are at 22 000 and 29 400
`cm- 1, the emission bands at 18 200 and 27 800 (and
`~29 200) cm- 1.
`The compounds with more than one emission band
`also exhibit the type of excitation spectrum in which
`the efficiency decreases on increasing excitation energy.
`The compounds with only one emission band have the
`other type of excitation spectrum. In the preceding
`section it was already shown that the former combina(cid:173)
`tion is not unexpected. At the moment our knowledge
`of the configuration coordinate curves of systems like
`those under consideration is inadequate to discuss this
`
`so
`
`~s,ino _ __ soo;};;;---""'5so~ --c600~ - -6c-!s-o __ _j
`-}.(mp)
`100
`
`Fm. 10. Spectral energy distribution of
`the fluorescence of
`SrY1.uCeo.020, at 77°K under 36S-mµ excitation.
`
`•• G. R. Fonda and F. J. Studer, J. Opt. Soc. Am. 38, 1007
`(1948).
`
`phenomenon further. As shown in Fig. 11, Y.ALiOrCe
`shows mainly fluorescence from ~ (31 200 cm-1) and
`only weak fluorescence from e1 (28 000 cm-1). In
`Sc2Si20-,-Ce both emission bands have equal intensity,
`in LaB03-Ce, Y3Al;;0,2- Ce, and ScB03- Ce
`the e1
`fluorescence is more intense. The intensity ratio of the
`two emission bands may be obscured by self-absorption
`or energy transfer of the type e2+g-g+e,. The latter
`mechanism results in e1 fluorescence upon excitation into
`
`V. THE SYSTEM Ys(Al,Ga)60 1r Ce
`The system Y 3(Al, Ga),01rCe was studied in more
`detail. Results are given in Table III and Fig. 6. The
`\'3Ga:,0 12-Ce does not fluoresce under long- or short(cid:173)
`wave uv or cathode-ray excitation. The energy of all
`these types of excitation is mainly absorbed by the
`host lattice. The absence of CeH fluorescence is probably
`due to inefficient energy transfer from lattice to activa(cid:173)
`tor. This fact may also explain the decrease of the
`efficiency for cathode-ray excitation with increasing Ga
`content (Table III). Figure 5 shows that with increas(cid:173)
`ing Ga content the ratio of uv to visible emission de-
`
`'10,--- - - , . . . - - - -- -~
`I
`
`t
`
`Fie. I I. Spectral energy
`distribution of the fluores(cid:173)
`cence of Yu eCe0 ... Ah01
`under 254-mµ excitation.
`
`creases. This again may be due to absorption of the uv
`emission of the Ce1+ activator by the Ga3+ ions of the
`host lattice.
`Table III shows that the splitting of the lower cubic
`crystal-field component decreases regularly from 7400
`cm-1 for CeH in YaAl,012 to 4300 cm- 1 for CeH in
`Y,Ga60,2. The visible emission band shifts therefore to
`the blue with increasing Ga content. That the deviation
`from cubic site symmetry is larger in YaAl~0,2 than in
`YaGas012 has also been found from studies on Pr'+(cid:173)
`activated gamets,17 Tba+-activated garnets,7 and from
`crystallographic investigations. 18 As argued above, the
`large deviation from cubic symmetry in the case of
`YaAl~012 shifts the fluorescence emission into the visible
`region. Replacing Y3+ by GdH results in an even larger
`splitting (see Table Ul / .
`Elsewhe1e we have reported that under cathode-ray
`excitation YaAl.012- Ce shows only a very weak uv
`emission in addition to the efficient visible fluorescence.'
`In the present model this can only be explained by
`assuming that the excitation energy of the lattice is
`transferred directly to the lowest excited state ( e1) .
`"F. N. Hooge, J. Chern. Phys, 45, 4505 (1966).
`11 F. Euler and J. A. Bruce, Acta Cryst. 19, 971 (1965) .
`
`(cid:47)(cid:50)(cid:58)(cid:40)(cid:54) 1026, Page 6
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`

`[NV EST I GA TIO N OF SOME Ce>+ - ACTIVATED PHOSPHORS
`
`5145
`
`VI. ENERGY TRANSFER FROM Cea+ TO
`OTHER IONS
`
`Energy transfer from Ce3+ to Ce1+ and to SmH, Eua+,
`Tua+, and Dr+ in some of the host lattices mentioned
`in this paper has been described and discussed else(cid:173)
`where.19 Transfer from Cea+ to Cr+ in Y A13B,012 was
`mentioned recently.20 Because of the anomalous emis(cid:173)
`sion of Cea+ in Y ;iAl60a energy transfer from Cel+ to
`Sml+, Eua+, Tbl+, Dy3+, and Cr+ was also studied in
`this host lattice. This was done by exciting YaA16012-
`Ce, X (X= Sm3+, Eua+, Tb3, Dyl+, or Cr3+) in the
`Cel+-excitation band at 29 400 cm-1 • The absorption
`of Ce3+ in this region is much more intense than that of
`X, so that nearly all excitation energy is absorbed by
`Ce3+. If Xis a lanthanide ion, the emission under these
`circumstances is still mainly Ce* emission, which indi(cid:173)
`cates an inefficient trans.fer from Cea+ to the other four
`lanthanide ions; if X=Cr+, however, the emission is
`mainly Cr+ emission,21 so that the Cel+-+Cr# transfer
`is efficient. These results agree with our previous con(cid:173)
`siderations19•20: if the Ce3+ emission overlaps the absorp(cid:173)
`tion lines of the 4f-4f transitions of other lanthanide
`ions (which is here the case), energy transfer occurs by
`exchange interaction and is not efficient. In the case of
`Cel+-Cr3+ in YaA150 12 the visible and uv emission of
`the Ce3+ ion overlaps the Cr+ absorption bands con(cid:173)
`siderably: the visible mission overlaps
`the broad
`absorption band of Cr+ between 15 000 and 20 000
`cm-1 ('Ar4 T 2), the uv emission that between 21000
`and 27 000 cm-1 (• Ar+4T1) •22 This gives rise to efficient
`energy transfer as in Y AhB1012: Ce, Cr .20 In the borate,
`however, only the •A2-+'T1 absorption is overlapped by
`the Ce* emission.
`
`Note added in proof: We have recently found that
`at 4.2°K the visible emission band of Y~50irCe con-
`" G. Blasse and A. Bril, J. Chem. Phys. 47, 1920 (1967).
`• G. Blasse and A. Bril, Phys. Letters, 25, A29 (1967).
`21 The emission of Y,A40,.-Cr has been reported by G. Burns,
`E. A. Geiss, B. A. Jenkins, and M. I. Nathan, Phys. Rev. 139,
`A1687 (1965).
`11 D. L. Wood, J. Ferguson, K. Kno;i:, and J. F. Dillon Jr., J.
`Chem. Phys. 39, 890 (1963).
`
`,,-EN
`,
`I
`---/:,. -At
`'
`' ' "'---E'
`
`Fzc. 12. Trigonal-prismatic coordination of Ce•+ in YAI,B,011
`(left-hand side) and splittiDg of the 'D level of Ces+ under Du.
`symmetry (right-band side). The black circle represents Ce>+,
`open circled represents QI-.
`
`sists also of two bands located at 19 100 and 17 400
`cm-1, respectively.
`
`ACKNOWLEDGMENTS
`
`The authors are greatly indebted to Miss M. P. Bo!,
`Miss D. J.M. Trumpie and Mr. C. J. Loyen, Mr. J. A.
`de Poorter, and Mr. J. de Vries for their assistance in
`the experimental part of the work.
`
`APPENDIX
`
`In a trigonal prism (D311 symmetry) the fivefold
`degenerate 2 D level of the Ce3+ ion splits into three
`levels (E' , A{, and E", see Fig. 12). The energy differ(cid:173)
`ence between E' and E" is
`( 4-ir)-1/2(0.37-ymF+ 1.78-yJ')
`and that between E' and Ai'
`( 4r)-li2(0.50y21fl-1.78-yef'),
`whereas in a regular octahedron the splitting is
`
`Here 'Y are parameters characteristic of the surroundings
`and fl'= (R.1 I r' I R.1 ), where R.1 denotes the radial
`one-electron wavefunctions. Using these formulas it is
`possible to deduce the cubic crystal-field splitting from
`the levels observed experimentally for the prismatic
`coordination.
`
`(cid:47)(cid:50)(cid:58)(cid:40)(cid:54) 1026, Page 7
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`
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