Luminescence of Eu(III) activated niobates

Specmchimia Rintcd

AC&. Vol. 41.4, No. 9, pp. 1041-1046,

in Great

1985

Britain.

6

Luminescence

0584-8539/85 53.00 + 0.00 1985 Pergnmon Rcss Ltd.

of Eu(II1) activated niobates

HARRY G. BRITTAIN* and WILLIAM A. MCALLISTER? *Department of Chemistry, Seton Hall University, South Orange, NJ 07079, U.S.A. and tNorth American Philips Lighting Corporation, Bloomfield, NJ 07003, U.S.A. (Received 4 January 1985; injinalform

26 February 1985)

Abstract-The luminescence properties of a series of Eu(III) doped niobate phosphors were obtained at cryogenic temperatures under high resolution conditions. Substitution of Eu(II1) into a CdNb,O, host takes place at the Cd sites, and results in the production of a single emitting Eu(III) species. Luminescence was found to take place out of the sD, and ‘0, excited states, and all spectral transitions could be interpreted with the assumption of a Czu point group for the Eu(III) ion. In the (Cd, Ca)Nb,O, host system, emission was found to occur at defect sites as well as at the intrinsic C,, sites. Within the CaNb,O, host system, the luminescence associated with the defect sites was found to dominate the spectra. The largest number of luminescent Eu(III) species was observed in the pure Ca(I1) niobate host system.

only materials consisting of a single phase were used in the luminescence measurements. All spectra were obtained on a high-resolution emission spectrometer constructed at Seton Hall University. Samples were excited by the 350 nm output (50 mW) of an argon-ion laser, and the emission was analyzed at 2 cm- ’ resolution by a l-m grating monochromator (Spex model 1704). The emission was then detected by a cooled photomuhiplier tube (S-20 response), and processed through the Spex digital photometer. In all work, the sample temperature was held between 8.5-9.0K, with the relatively low excitation power being desirable in that sample heating could be minimixed. The phosphor materials were mounted on a Cu block bolted onto the cold stage of a closedcycle cryogenic refrigator system (Lake Shore Cryotronics model LTS-21).

1. INTRODUCTION of lanthanide ions in niobate host systems has been of interest, since LiNbOJ has found use in a variety of electro-optic devices. While the emissive properties of lanthanide ions doped into LiNbOJ seem to be well understood [ 1,2], much less work has featured the luminescence properties of lanthanide ions doped into other niobate host systems. MCALLISTER has reported the room temperature luminescence properties of several lanthanide ions doped into calcium and cadmium niobates, which have the general formula (Ca, Cd)NbzOb [3]. While the Cd host system was found to efficiently activate lanthanide ion emission (especially that of Et?+), the Ca host system did not exhibit such sensitization. In the present study, we have focused on the (Cd, Ca)Nb206 : Eu3 + phosphor system, as the particularly simple spectroscopy of this lanthanide ion allows for detailed data interpretation. The spectra were obtained under high-resolution conditions at cryogenic temperatures. In an attempt to learn about the possible existence of defect sites, phosphors containing systematic variations in both the Cd/Ca ratio and in the dopant concentration level were prepared and studied. The luminescence

3. RESULTS AND DISCUSSION

2. EXPERIMENTAL The phosphor host material was prepared in the manner described previously [3]. SL grade calcium carbonate (GTE) and cadmium oxide (99.99%, American Metals and Chemicals) were reacted with optical grade niobium pentoxide (Kawecki-Berylco). The Eu 3+ dopant was introduced as the 99.99 % oxide (Molycorp). Na’ was introduced as a charge compensating species, since substitution took place at the divalent Ca*+ site. Firing was performed in air for 2 h at llOO-14OO”C, the higher temperature being required for phosphors containing high percentages of calcium (greater than 50 mole per cent). In the high calcium formulations, a slight excess of Nb205 was included so as to avoid the phases present when stoichiometric amounts are fired [4]. Product purity was determined through X-ray diffraction studies of the powders (using a Philips model APD instrument), and

The crystal structure of CaNblOs has been determined to be orthorhombic in nature [5], and has been found to be quite similar to that of columbite (FeNbrO,, point group Pbcn). Each niobium octahedron shares two edges with adjoining niobium octahedra, and thus chains of octahedra exist through the solid. The chains of niobium octahedra are connected to each other through the calcium atoms. This process forms a highly distorted polyhedron of eight oxygen atoms about the calcium atom, with this distortion being so severe that the polyhedron has been described as an irregular cuboid [6]. The structure of CdNbzOd is essentially identical with that of CaNb206, with the unit cell parameters differing by less than 2 % [5]. A full normal coordinate analysis has been carried out for CaNb20s [7], and this analysis included the cohesive effects due to the presence of the calcium ions.

3.i. Luminescence of CdNb206 : Et?+ Within the CdNbzOe host system, the luminescence associated with the Eu(III) centers was found to be extremely intense. At cryogenic temperatures, emission was found to originate from both the ‘Do and ‘0, excited states. The luminescent transitions all ter-

1041

1042

HARRYG. BRI-ITAINand

WILLIAM A. MCALLISTER

minate in the various J states of the ground ‘F, level, with the present work concentrating on the simpler transitions to the J = t&4 levels. Assignment of the origin of the various band systems is simple, as the lack of covalency in the bonding ensures that all transitions will occur at essentially the same energies as those of the free ion. The luminescence spectra were found to be concentration independent up to Eu(II1) doping levels of 12 %, indicating that substitution of Eu(II1) at the Cd(I1) sites was a well-behaved process. Examples of the spectra observed within the various ‘D, -+ ‘FJ band systems may be found in Figs 1 and 2, and the

wavelengths and energies associated with each peak are located in Table 1. The identification of all emitting levels associated with the Eu(II1) ion was made on the basis of the luminescence data. A single, sharp ‘D, -+ ‘F, peak was observed at 5809.5 A, thus placing the energy of the ‘D,, state at 17.213 kK. A pair of ‘D, + ‘F. peaks were noted at 5266 and 5274.5 A, and these identify two levels of the ‘D, excited state at 18.990 and 18.959 kK. Several band systems were found to contain broad bands which were located at lower energies than were the sharp peaks, and these have been assigned as being

I

o-o

I-O l-2

!

Ii

530

J

-1

I/I’

525

L l-3

535

540

550

555

560

Woveiength

565

575

580

/

585

(nm)

Fig. 1. 9.0 K luminescence spectra corresponding to the ‘D, --* ‘FJ Eu(III) transitions within the CdNb,O, host system.-The various spectral transitions are labeled by the initial and final J quantum numbers. The samples were doped with Eu(II1) at the 5 “,‘, level.

o-4

610

615 Wavelength

650

/

660

(nrr.1

Fig. 2. 9.0 K luminescence spectra corresponding to the ‘D, + ‘FJ Eu(II1) transitions within the CdNb,O, host system. The various spectral transitions are labeled by the initial and final J quantum numbers. The samples were doped with Eu(II1) at the 5 TjAlevel.

590

1043

Luminescence of activated niobates Table 1. Wavelengths, energies and peak assignments for the luminescence associatied with CdNbtO, : Eu3+ Assignment

Wavelength (A)

Energy (kK)

5266 5275 5358 5368 5408

18.990 18.959 18.664 18.629 18.491

Dl (b)-FO (a) Dl (abF0 (a) Dl (a)-Fl (a) Dl (akF1 (b) Vibronic

5520 5529 5538 5546 5553 5624 5642

18.117 18.087 18.059 18.031 18.008 17.781 17.723

Dl (bbF2 Dl (a)-F2 Dl (a)-F2 Dl (abF2 Dl (a)-F2 Vibronic Vibronic

5810

17.213

DO(aHO (a)

5837 5848 5862 5874 5887 5892

17.132 17.101 17.059 17.024 16.987 16.973

Dl Dl Dl Dl Dl Dl

'Do + 'F I

5911 5924 5956 5963 5973

16.918 16.882 16.790 16.770 16.742

DO(abF1 (a) DO(a)_Fl (b) Vibronic Vibronic Vibronic

5D, -* 'F,

6119 6129 6142 6148 6237

16.342 16.317 16.282 16.265 16.034

DO(a)_F2 (a) DO(a)-F2 (b) DO(a)-F2 (c) DO(a)-F2 (d) Vibronic

'0,

'F,

6511 6530 6546 6561 6566

15.358 15.315 15.278 15.241 15.230

Do (a)-F3 (a) DO(a)_F3 (b) DO(a)_F3 (c) DO(a)_F3 (d) Do (a)_F3 (e)

'Do -+ 'F,

7036 7043 7051 7060 7064 7078 7087

14.213 14.199 14.182 14.165 14.156 14.128 14.111

DO(a)-F4 (a) DO(a)_F4 (b) DO(a)_F4 (c) DO(a)_F4 (d) DO(a)_F4 (e) DO(a)--F4 (f) DO(a)_F4 (g)

Band system

'0, + 'F2

-+

(a) (a)

(b) (c) (d)

(b)-F3 (a) (bbF3 (b), Dl (a)-F3 (a) (bbF3 (c), Dl (a)-F3 (b) (b)-F3 (d), Dl (a)-F3 (c) (a)-F3 (d) (a)-F3 (e)

The peak assignments were made on the basis of the energy level sequence of Table 2.

in nature. Confirmation of vibronic assignments was possible through systematic variation in the sample temperature, since the vibronic peaks exhibited a much greater temperature dependence than did the pure electronic lines. Since the 5Do + ‘F. transition cannot be split by any crystal field, the existence of a single sharp peak at 580 nm may be taken as evidence that only a single Eu(II1) species is responsible for the series of luminescent transitions. The number of peaks within each of the sD, --) ‘F, band systems must therefore equal the number of true ‘F, crystal field components plus the number of vibronic bands. Fortunately, one may readily separate the true crystal field components of each ‘F, level from the vibronic bands. Recognition vibronic

that each genuine crystal field component of the ‘F, levels would represent the termination point of emission out of both the 5D, and ‘De states permits the deduction of an energy level scheme for the Eu(III) ion as would exist in the CdNbz06 host. These energy levels are summarized in Table 2, and the spectral assignments of Table 1 are based on the levels as shown in Table 2. Correlation of the number of observed luminescence peaks with the number predicted on the basis of symmetry considerations allows one to determine the molecular point group of the emitting Eu(lI1) ion. FORSBERG [S] has worked out the selection rules governing the ‘D,, + ‘F, emissive transitions, and a comparison of these results with those of the present

HARRY G. BRI?TAIN and WILLIAMA. MCALLISTER

1044

Table 3. Selection rules governing the ‘D, --* ‘FJ luminescence transitions of Eu(II1) in C,,, symmetry

Table 2. Energy levels and state assignments for the crystal-field components of Eu(II1) in CdNb,O,

‘FJ level

Energy (cm-‘)

‘FO

0

State FO

‘F,

5F,

7F,

295 331

Y

Y

J=l

A B B

Y

Y

Y

Y N

Y N Y

2A A B B

N

A 2A 28 2B

N

3A 2A 28 2B

N

~‘2 (4 4~2(b) f.2 (4 172(4

1855 1898 1935 1972 1983

F3 (a) F3 (b) F3 (c) F3 (d) ~‘3 (e)

J=4

3000 3014 303 1 3048 3057 3085 3102

F4 F4 F4 F4 F4 F4 F4

)D,

17213

DO (a)

*D,

18959 18990

Dl (a) Dl (b)

work reveals that in CdNbzO, the Eu(III) ion occupies a site of C,, symmetry. The selection rules governing emission out of the ‘D, level have been worked out, and are presented in Table 3. One finds in every case that far fewer than the predicted number of transitions are actually observed. However, almost all of the 5D, bands originate with the lowest energy component of this level (at 18.959 kK). Emission bands associated with the other ‘D, component are invariably much weaker in intensity, and simply have not been observed in many cases. It is highly significant to note that should the 18.959 kK level be of either B, or B2 symmetry, then the number of peaks found to originate from this level agrees exactly with the predictions made on the basis of CZV symmetry. Thus, it may be concluded that in the CdNb,Ob host system a single emitting Eu(II1) species exists, and this ion occupies an effective site characterized by GIL’ symmetry. This result is somewhat surprising since consideration of the crystal structures would indicate that the Eu(II1) site symmetry ought to be C3L.[ 1, 21. However, C,, site symmetry should lead to the observation of one CM peak, one O-1 peak, three &2 peaks, three O-3 peaks and five O-5 peaks [S], and one simply does not observe this pattern in the luminescence spectra. It may be concluded that the effective crystal field experienced by the Eu(II1) is distorted from that predicted on the basis of crystallography. An opposite

B, excited state

N

FO (4 Fl@) F1 lb)

Y

Y

Y Y N

N

Y

Y Y

Y Y

Y N

N

Y Y Y N

Y

Y Y

Y Y Y

Y Y Y

Y Y

Y

N implies a forbidden allowed transition.

(a) (b) (c) (d) (e) (f) (g)

B, excited state

A

J=3

871 896 931 948

A, excited state

J=O

J=2

‘Fl

Symmetry

transition,

Y Y N

Y

while Y signifies

an

crystal field trend was noted recently for Eu(II1) in K2EuCI,, where the effective site symmetry was found to actually be higher than that predicted from the crystal structure [9]. 3.ii. Luminescence

of (Cd, Ca)Nb,O,

: Eu3’

While all existing structural information indicates that the replacement of Cd(I1) by Ca(II) should be isomorphous, the luminescence data obtained on mixed Cd, Ca niobate systems suggests otherwise. In the host system defined as (Cd,,,, Ca0..,)Nb206, the luminescence spectra were found to exhibit a dependence on Eu(II1) concentration. This behavior is most evident in the 5D, + 7Fo/5D, --t ‘F3 spectral region (57(t590 nm), and data obtained in this sequence of studies may be found in Fig. 3.

1

I

I 575

I

I

5eo Woveiength

585 (nm

590

1

Fig. 3. 9.0 K luminescence spectrum obtained within the Eu(II1) *Do-+ ‘F, and 5D, + ‘F, transitions for (Cd, Ca)Nb,O, : Eu3+. The Cd(I1) concentration was fixed at 60”,;, and the Eu(II1) dopant levels were 5 (trace A), 10 (trace B), and 12”” (trace C).

Luminescence of activated niobates One may note from Fig. 3 that at the lowest Eu(II1) doping level, the observed spectrum is essentially the same as had been obtained in the pure CdNbrO, system. Increasing the doping level from 5 to 10% results in a slight broadening of all spectral features, and to the development of a new spectral feature at 5753A (17.382kK). Further increase in the Eu(II1) doping level to 12 % resulted in a strong intensification of this feature. The other Eu(II1) emission band systems (not shown) were found to become considerably more complicated than has been shown in Figs 1 and 2. The C,, Eu(II1) site described in the previous section still existed, as proven by the existence of a sharp 5D, -) ‘F, luminescence peak at 5810A. These results indicate that a second type of emitting Eu(II1) species is present at the highest dopant concentration. Since the X-ray diffraction analysis of the material did not indicate the presence of any additional phases, it is concluded here that the new species represents a Eu(II1) ion which occupies a defect site. Further increases in Eu(II1) dopant concentrations were not possible without causing the development of hetereogeneous materials. However, it was possible to produce a niobate having equal amounts of Cd(I1) and Ca(II), and to dope this material with 12 % Eu(II1). The luminescence obtained within the 5D, + ‘FJ5D, + ‘F, spectral region is shown in Fig. 4 for this phosphor. One may note in this spectrum that the defect emission has become stronger than the luminescence associated with the CzV site, although both types of luminescence are still observed.

3.iii. Luminescence of (Ca)Nb206 : Eu3 + These studies were carried to their logical conclusion through the preparation of a phosphor consisting of pure CaNbzOs doped at the 12% level with Eu(II1). The luminescence spectrum obtained for this material within the 5D,, + ‘Fo15D, + ‘IFa spectral region is shown in Fig. 5. The emission has become even more complicated than shown in the previous figure, and it is

I 575

I

I

580 Wavelength

585

I 590

(nm)

Fig. 4. 9.0 K luminescence spectrum obtained within the Eu(III) ‘D, +‘Fo and ‘D, +‘F3 transitions for (Cd,Ca)Nb,O, : EuJ+. The Cd(I1) concentration was fixed at 36:/, and the Eu(II1) dopant level was 12:/,.

I

I 575

1045

I 580 Wavelength

I 585

L

5s

(nm)

Fig. 5. 9.0 K luminescence spectrum obtained within the Eu(II1) ‘0, + ‘F, and ‘D, + ‘F transitions for CaNbsO, : Eu3+, at a Eu(II1) dopait level of 12 y/..

clear that several different defect sites are possible in this phosphor. Analysis of the data is not possible, since it is not possible to differentiate between peaks due to 5D, and 5D, origins. 4. CONCLUSIONS

The results presented here provide a considerable amount of insight into the luminescence properties of Eu(III)-substituted NbzO, phosphor systems. Substitution of Eu(II1) into a CdNb206 host [which takes place at the Cd(I1) sites] results in the production of a single emitting Eu(II1) species, whose luminescence spectrum is essentially concentration independent. Luminescence was found to take place out of the 5D,, and ‘DI excited states, and all spectral transitions could be interpreted with the assumption of a CzV point group for the Eu(II1) ion. While one would expect that replacement of Cd(I1) by Ca(I1) in the (Cd, Ca)NbzO, host system would not affect the nature of the Eu(II1) sites [S], quite different results were observed. Even though the Eu(II1) substitution still took place at the Cd,Ca sites, increasing either the Ca(I1) content or the Eu(II1) dopant level resulted in the generation of defect sites. Luminescence was still observed originating from Eu(II1) ions occupying the intrinsic CzV sites, but this emission became less pronounced as the Ca(I1) content of the phosphor increased. The lack of either spectral or temporal resolution prevented a more definitive study of the nature of these defect sites. Acknowledgement-This work was supported by the Camille and Henry Dreyfus Foundation, through a Teacher-Scholar award to H.G.B. 5. REFERENCES [l] D. M. KROL, G. Bussnand 73, 163 (1980).

R. C. POWELL,J. them. Phys.

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HARRY G. BRITAIN and WILLIAMA. MCALLISTER

[2] J. K. TYMINSKI,C. M. LAWSONand R. C. POWELL,J. them. Phys. 77,431s (1982). [3] W. A. MCALLISTER, J. Elecrrochem. SOC. 131, 1207 (1984). [4] E. M. LEVIN,C. R. ROBBINSandH. F. MCMURDIE,Phase fkgrams for Ceramisls, 1975 Supplement, p, 106. American Ceramic Society. [S] A. WACHTEL,JI. Electrochem. Sot. 111, 534 (1964).

[6] J. P. CuMMINGsand S. H. SIMONSEN, Am. Mineral. 55,90 (1970). [7] E. HUSSON,Y. REPELIN, N. Q. DAO and H. BRUSSET,J. them. Phys. 66, 5173 (1977). [8] J. FORSBERG,Coord. Chem. Rev. 10, 195 (1973). [9] H. G. BRITAIN and G. MEYER,J. Solid Stare Chem. 54, 156 (1984).