Optical properties of Cs2NaBiCl6

Journal of Luminescence 17 (1978) 61—72 © North-Holland Publishing Company

OPTICAL PROPERTIES OF Cs2NaBiCI6 F. PELLE * B. JACQUIER *

**

~‘,

J.P. DENIS

*

and B. BLANZAT *

Equipe de Chimie Metallurgique et Spectrosco pie des Terres Rarer, ER 209, CNRS, I p1., A. Briand 92190 Bellevue-Meudon, France Physico-Chimie des Matériaux Luminescents, ER 10, Université de Lyon 169621 Villeurbanne, France

Received 15 September 1977

Luminescence properties of Cs2NaBiC16 are reported. Absorption, excitation, steady and pulsed fluorescence spectra have been investigated over a wide temperature range. The temperature dependences of the integrated fluorescence intensities and lifetimes versus temperature were obtained under selective excitation in the main absorption bands. Absorption processes are described using a molecular orbital scheme. The behaviour of the absorption (intensity and splitting) with the temperature provide evidence of a strong interaction with the lattice as shown in alkali halides doped with mercury-like ions. Emission processes are discussed. The lifetime measurements do not agree3+ with the classicentres. Absorption andscheme emission results seem to support the hypothesis of a strong cal three-levels generally proposed for luminescent processes in Bi coupling with the vibrational modes. In addition, the behaviour of the decay times at low temperatures are consistent with migration and trapping sites effects.

1. Introduction In the present paper we report for the first time the luminescence properties of the phosphor Cs 2NaBiC16. This cubic Elpasolithe host Cs2NaMC16 is thecharge first system 3~occupies a six-coordination site without comin which theIn trivalent pensation. addition,ion thisMoctahedral site presents a perfect °H symmetry. This structure readily allows substitution in the octahedral site by rare earths or transition ions. For this reason, extensive studies using different techniques MCD [1,2], EPR [3,4], Raman [5] have been recently performed. Such high symmetry (OH) is unusual for bismuth compounds. Cs 2NaBiCl6 is an example of an intermediate bismuth phosphor between dilute activation compound and host material. 2. Experimental section The Elpasolithe hexachloride Cs2NaBiC16 was prepared by method X previously reported by Morss et al. [6], cooling slowly a hot aqueous HC1 solution of appropri61

62

F. Pelle eta!.

/ Optical properties of

Cs

2NaBiC16

ate cations. In this way transparent crystals of about 4 mm long up to 2 mm thick can be easily obtained. The X-ray diffraction patterns are similar to those previously reported for some isomorphous salts [5,7] and give no evidence of small amounts of CsC1 or NaC1. In addition, chemical analysis and electron microprobe measurements confirm the stoichiometry. 3~is surrounded crystallizes the cubic in which by Cs2NaBiC16 a regular octahedron of in chlorine ions.system It has Fm3m been shown thatBicertain members of the series Cs 2NaLnCl6 (Ln = lanthanide ions), isomorphous to Cs2NaBiCI6, present a crystalline phase transition between room temperature and liquid nitrogen temperature [8]. However, until now no proof of such phase transition has been verified for Cs2 NaBiC16. Spectroscopic studies have been investigated between 7 and 300 K. The samples were mounted in a stainless steel liquid helium dewar of standard design. A double beam Cary 17 spectrometer was employed for absorption measurements. Fluorescence spectra were recorded using a 0.5 m Jarrel Ash grating monochromator and a cooled S20 photomultiplier. For fluorescence work selective excitations were obtained at 2537 A with a low pressure mercury lamp, at 3371 A by a pulsed light from a nitrogen gas laser AVCO C 950, and at 3750 A with a Molectron tunable dye laser model DL 100. The PBD Molectron 70350 organic dye solution was employed. Excitation spectra were performed using the UV range of a xenon lamp (Osram XBO 450 W/4) dispersed by a Jobin Yvon grating monochromator. The relative spectral response of the detection system and the optical assembly were determined using a tungsten standard lamp. This lamp has been calibrated against standard lamps from the National Bureau of Standards. For lifetime measure-

PAR mod162 Boxcar

AVCO C950 N2 Laser

TE~1RONIx

SCOPE

lr,tePAR rotor PRE AMP

RTC

13

XV Recorder

561 VP

Motectron Dye Laser DL100

H.V.

TRW DJTEIUE4

H.V. L.FGene.

LIQUID He CRYOSTAT

JARREL ASH O.Sm Ebbert scanning SPECTROMETER

Fig. 1. Experimental apparatus for fluorescence and decay time measurements.

F. Pelle et al.

/ Optical properties of Cs2NaBiC16

63

ments, exciting flashes are generated by a TRW deuterium flash lamp or the pulsed N2 laser or the tunable dye laser. The signal detected with a RTC 56 TVP photomultiplier was fed to a PAR 162 boxcar integrator and the output displayed via an X— Y recorder (fig. 1).

3. Results 3.1. Absorption Absorption spectra were recorded at various temperatures using a thin film of powder. As shown in fig. 2, UV absorption consists of two ranges at higher and lower wavelength than 3000 A. At 7 K the higher range presents two intense bands 3230 A (3.83 eV) and 3500 A (3.54 eV) and a low intensity peak around 3750 A (3.30 eV); four bands 2000 A (6.19 eV), 2100 A (5.90 eV), 2200 A (5.63 eV) and 2600 A (4.76 eV) set the latter one. Increasing the temperature up to 35 K, the 3230 A band becomes more intense

lasJ.1 25, 7K

2O~

0

I “L

2000

Ii

-.—•—. —

fl

3000

35K 95K 280K b

24 4dOó~A

22

2

5000

6000

l~9 l~8, 17 7000

116 e~

~lAl

Fig. 2. UV absorption of Cs5NaBiCl6 at different temperatures. The arrows indicate the main absorption bands. No absorption was found in the visible range. Fig. 3. Emission spectra at 80 K (a), X exc

=

2537 A; (b), ?~exc

=

3371 A; (c), X exc

=

3750 A.

64

F. Pelle et al.

/ Optical properties of Cs2NaBiCI6

and is split into two components, 3150 A(3.93 eV) and 3270 A (3.79 eV). The total bandwidth of these two components is roughly equal to the 3230 A bandwidth at 7 K. The shoulder at 3750 A is more pronounced. The intensity and the splitting of these two components increase until 95 K and then become constant at higher temperature. Because of the high intensity and the shift to the long wavelengths of the 3270 A absorption band, the 3500 A band does not appear clearly at high temperature, while the intensity and the position of the 3750 A peak do not seem to change. In the lower range the absorption intensity of the 2600 A band increases with temperature and, on the other hand, the intensity and splitting of the band at higher energy is rather temperature independent.

3.2. Steady and pulsed fluorescence Emission spectra and fluorescence decays were recorded under selective excitation in the main absorption bands. As a general example fig. 3 indicates different emission spectra with 2537, 3371 and 3750 A excitation.

3.2.1. 2537 A excitation As shown in fig. 3, we observe a broad band (bandwidth = 0.49 eV at 80 K) peaking at 6200 A whatever the temperature. The temperature dependence of the intensity and decay times are represented in fig. 4. We note a sharp decrease of the intensity at higher temperature than 95 K. The temperature quenching is about 140 K. For the pulsed fluorescence we observe one short decay time which varies strongly with the temperature for T< 30 K and T> 100 K. 3.2.2. 3371

A excitation

Under 3371 A excitation the emission spectrum consists of a broad band which is shifted from 7200 to 6400 A with increasing temperature from 7 to 100 K, respectively (fig. 5). The behaviour of the bandwidth (fig. 5) and of the intensity (fig. 6) versus temperature, suggest a complex structure of this emission spectrum. The spectrum was fitted to two gaussian curves situated at 7125 A (1.740 eV) and 6240 A (1.986 eV), represented on fig. 7, using a non-linear least squares programme [9]. The temperature dependence of the intensity of these two components can be compared with the intensity of the global emission band. Indeed, until 60 K the 7125 A emission band is dominant. Both components become equivalent about 70 K, whereas above 80 K the 6240 A emission band becomes the more intense. Pulsed fluorescence of the emission band has been recorded: considering first the total emission, two decays have been observed at any temperature (fIg. 6b). Moreover, the lifetime measurements have been investigated using an interference filter which cuts off the spectral range above 7000 A. Using this method, the long lifetime (i- long) is mainly observed. In addition, only the long lifetime is seen at room tern-

F. Pelle et aL

/ Optical properties of Cs2NaBiCl6

65

maxi

15

Ia 7000

.

~(1) 25 6500

10

20

600C levI

15

10

50

100

150

OK

0.50 10

(2)

ico

C

i~o~

elK)3 5

0.3C 0.40 10

50

100

150 O(K

Fig. 4. Temperature dependences of the steady and pulsed fluorescences (6200 A) under 2537 A excitation. Fig. 5. Temperature dependences of the position (1) and the bandwidth (2) of the emission band under 3371 A excitation.

200

lou. (b)

(a)

mission pectrum (G2)

25

&

sb

6o

1

i~o

1~

I)

~lKl

50

100

150

Fig. 6. Temperature dependences of the steady (a) and pulsed (b) fluorescences under 3371 A excitation.

66

F. Pelle et al.

/ Optical properties of Cs2NaBiC16

8K

40

60K

40

2:

24ev 17

17

2

24ev

2

lo.~

1o.~ 71K

41

~

135K

~

2:~2o\

17

2

24ev

emission spectrum

17

2

21. cv

Gaussian curves 01— 02

Fig. 7. Gaussian analysis of emitting spectral energy distribution under 3371 A excitation. perature. As a consequence of the temperature dependence of the intensity and of

the decay time measurements, the long wavelength emission has a short decay tinie, while the short wavelength one has a long decay time. As shown on fig. 6b the long lifetime is roughly constant (210 jis) until 20 K and decreases sharply when increasing the temperature. The short decay time (13.5 ps) decreases sharply for T< 30 K, becomes constant until 60 K, and decreases again sharply between 60 and 90 K.

3.2.3. 3750

A excitation

The emission band obtained under 3750 A excitation is shown in fig. 3. The very weak intensity of this fluorescence (maximum at 7200 A) at lower temperature than 80 K, did not permit recording the emission spectrum. From 80 to 100 K the intensity increases, then becomes very weak above 100 K. Nevertheless, pulsed fluorescence has been studied between 7 and 150 K, as shown on fig. 8. As we saw for the

other excitations, we observe a very sharp decrease of the decay time at very low temperature (T< 100 K). 3.3. Excitation As we noted for the absorption spectra, we observe two main ranges when recording the excitation spectral distribution of the global emission. The positions of the

.E Pelle eta!. / Optical properties of C~

2NaBiQ6

67

40

0

~

1~0

~(K)

Fig. 8. Pulsed fluorescence under 3750 A excitation.

absorption and excitation bands are in good agreement (fig. 9). At 7 K the more intense band is around 3500 A, two other weaker bands peak at 3370 and 3120 A. The range around 2700 A seems weak. When increasing the temperature the 3750 A band appears and the 3370 and 2700 A bands become more intense, whereas the 3500 A band disappears.

lau

&

3000

3500

~

Fig. 9. Excitation spectra at different temperatures and absorption spectrum at 95 K.

68

F. Pelle eta!. /Optical properties of Cs

2NaBiC16

4. Discussion The isomorphous salt Cs2NaYC16 does not present any absorption between 1900 and 4000 A; this suggests that the absorption and fluorescence properties are due to the presence of bismuth in that host. From a molecular orbital description of the

mercury like luminescent centres [12], fig. 10 summarizes all the possible absorption mechanisms that we shall discuss in this section [13]. Let us consider a large cluster where bismuth is at the centre of symmetry (Oh)~

the first and second neighbours are respectively the chlorine anions, and the alkali earth Na and Cs. The Bi—Cl distance is 2.66 A while the shortest distance between two bismuth is 7.55 A. It is wellknown that the p states of Na and Cs are deep (around 30 and 20 eV, respectively), as well as the bismuth d states (35 eV); hence, they do not contribute efficiently to the valence band (VB). When decreasing the energy the first set of occupied molecular orbitals includes the bonding so and pa MOs (respectively aig and tiu) and the nonbonding chlorine for eg MO. The valence 3p atomic orbitals the nonbonding band is mainly composed of the chlorine

E (ev)

..............4....4..

)S

Na

ss

C~J

3P

Cl

a 19

~. conduction

band

~ 1

0

/E1IEIEIEI -~—-~...-‘

1213141

~ ,_[___J__L__J_

3

2eg

10

Bi

6s

7

3t~

_____________

tA \~-

\\

valence band

) 9

\\\~

2t 15 2a19 3s

20t

~

30

Cl

5p states of Cs

2pstates of Na

_______________

Sd states of Bu

3

ESCA on alkali halides. Fig. 10.measurements Molecular orbital energy levels estimated from calculations on (BiCI6)

cluster and

F. Pelle eta!.

/ Optical properties of Cs2NaBiC16

69

eg, t~g,t2g MOs, while the aig and tlu 3ajg MOs isare the top6sofand the VB an antibonding. admixture ofAtbismuth the highest occupied antibonding MO chlorine 3s and 3pa AOs. On the other hand, the empty s states of Cs and Na determine the conduction band.

The presumably large gap (E 4) of these salts

[141,and the strong antibonding

character of the 3 aig MO suggest that the main absorption process is E1 - Depending upon the position of the t1~MO we cannot rule out the E3 process. However, this mechanism would lead to a photoconduction property unknown at the present time.

Consequently, the E1 process gives the wellknown A, B, C absorption bands 1S

3Pi), E~and T

0) --~-~

3P

2),yT1~(’P1) 2 ions [151. The forbidden A for crystals highly doped with ns 1~state of the algtlu 3P MO excited configuration corresponds to the trap 0. The E2 mechanism gives the a~g~ ‘A19(

XTiu(

2~(

so-called D charge transfer band from ligand to metal. Comparing the absorption spectra of MC1 : Bi (M = Na, K, Rb) [16] and Cs2NaBiC16 one can see a similarity in the range of the A and C bands, the B band probably being close to the C band. The most surprising features are the high intensity and splitting of the A band, and their temperature dependence. Forgetting about the 3750 A peak (3.30 eV), the A band is split into two components at very low temperature: the intense one at 3230 A (3.83 eV) and a small one at 3500 A (3.54 eV). When the temperature is raised, both intensity and splitting (3150— 3270 A) of the 3230 component are increased. This anomalous behaviour of the thermal dependence of the absorption suggests that the transitions responsible for the A band, partially spin forbidden, are enhanced by the lattice vibrations. Moreover, as shown by Fukuda [171, the coupling of the orbital triplet xT1 ~ with the lattice (or localized) vibration modes produces a Jahn—Teller effect which can explain the splitting of the A band in alkali halides doped with mercury-like ions. In

addition, a Jahn—Teller effect has been observed [18] in isomorphous salts like Cs2NaPrC 16 in which the mode responsible for the Jahn—Teller has an energy appropriate to the rotary mode (T2) which was also responsible for the distortions of K2ReC16 [19] as the temperature is lowered from room temperature. The 2600 A band, which 3P has a similar temperature dependence, can be attributed to the E~and T2u levels ( 2), the coupling with the even vibration modes enhancing its intensity when T is raised. On the other hand, no drastic thermal dependence was

found 1P for the C band; this is compatible with the spin-allowed transition ‘Aig ~ yT1~( 1). According to the selection rules, the J = 0 J = 0 transition is strictly forbidden but it can be observed due to vibronic interaction. In addition, the energy oflevel. the So 1Aig(3Po) 3750 A low intensity is consistent with the position of the the 3750 peak may bepeak assigned to the ‘Aig(1So) ~ ‘A,g(3Po) transition. According to these assignments we can use the Sugano formula [201 to determine the dipole strength ratio of the C band with respect to the A band, the K electronic -÷

F. Pelle eta!. / Optical properties of Cs

70

2NaBiC16

exchange integral, and ~ the spin—onbit parameter:

12 l+x—[l+2x(1—x)]’/2’ R- fA fc _2—x+ [1 +2x(l —x)]’ where WB

—

WA

WC

—

WB

~=~(w~WA){2X —

(1)

—1

+

[1 + ~(1 —x)]’12},

and K=~-~—wB+~(wA+wB+wc)}.

(2)

As a consequence of the similar relative positions of the absorption bands we determine similar values of the K and ~ parameters involving the reduction factors of 0.4 and 0.5 with respect to the free ions values. The theoretical dipole strength R seems too large according to the absorption spectrum at 7 K (fig. 2). As an explanation of this disagreement we can argue that the spin—orbit coupling parameter is too reduced while the reduction of K is too small [12,21]. In addition, the s—o coupling can be quenched by a strong Jahn—Teller effect (see table 1).

Keeping in mind the cluster model where absorption is localized, radiative and non-radiative processes will occur only in the luminescent centre [22] As is wellknown for many phosphors doped by bismuth, emission is generally assigned to the lowest orbital triplet xTi~,the highest yT 1~(singlet spin state) relaxing thermally towards the XT1U level [23]. The A1~trap nearby the triplet xT1~complicates the -

radiative and non-radiative mechanisms. Pumping in xT1~or in a higher energy level,

there is a rapid relaxation between XT1U and the trap Aj~Boltzmann equilibrium can occur if the temperature is enough high with respect to the relative radiative probabilities of xTi~and A,~and the energy difference ~ between those two levels.

Because of the high site symmetry (Oh), we expect to determine a large gap ~ Table 1

3+ Comparison of the State energies and values of K, ~ and Rtheoret of Bi Wc(eV) wB(eV) wA(eV) K (eV) t (eV)

Rtheoret

Free ion Bi3~ KCI : Bi b)

14.211

11.956

9.415

1.435

2.105

8.8

5.98

5.08

3.81

0.56

1.00

7.2

563a)

4Tha)

354a)

0.57

1

7.3

Cs 2NaBiCI6

a) We took the lowest energy peak of each band at 7 K. b) Values from ref. 16.

F. Pelle eta!. / Optical properties of C5

2NaBiC16

71

3~[24], using decay time laws and (0.1 5—0.2 shift eV) as[25]. for CaO or MgO maximum However, in doped the casebyofBi a Jahn—Teller effect, the splitting of the xT,~adiabatic potential energy surface (APES) induced by the coupling to the trigonal or tetragonal distortions can reduce the effect of the s—o interaction on the 3T,~term; whence, two emissions will occur from each kind of minima ofthe T 1~ APES. The A1~trap nearby can be involved in the mechanisms in different ways [26]. Considering the emission band of the 3371 A excitation, the shift of the maximum and the variation of the bandwidth with temperature (fig. 5), and the gaussian analysis (fig. 7) in relation with the thermal dependence of the intensity (fig. 6) require that the emitting levels belong to the same centre. The presence of short and long fluorescence decays (fig. 5) suggests a three-levels mechanism [25], two excited states and a trap nearby. The emission of the trap will be seen below 80—100 K providing a trap depth roughly estimated at 0.15 eV [25], which is similar to the shift of the maximum of the emission band. A large Stokes shift and a strong difference in the curvature of the APES of the excited states with respect to the ground state can explain the thermal extinction of the trap (7200 A components of the emission band) before populating the nearest allowed level [fig. 5 (1)]. However, the selective fluorescence lifetime experiment (above 7000 A) seems to indicate that the highest energy band should have a long decay. As an alternative, in order to explain the structure of the 3371 A emission band, we can suppose a Jahn—Teller effect by the coupling with the trigonal and tetragonal vibrational modes, the xT1~APES exhibit two kinds of minima which can bring two near emission bands. However, the lifetime measurements exhibiting only one long decay for the highest energy band will not agree with a thermalization process between the two kind of minima [26]. In addition the sharp increase of all the decay times at very low temperature can be compared with similar results obtained on CaWO4 [27], Bi4Ge3O12 [11] and YVO4 : Bi [28]. Because of the large concentration of bismuth (25%), migration with trapping sites can occur providing the interpretation of the broadening of the emission bands (broader than those of isolated centres) and of the temperature dependence of the fluorescence decay times. Both aspects migration with trapping sites and Jahn—Teller components with trap level nearby have to be considered in the emission processes. Further experiments and the development of theoretical models are under way in order to ascertain the role of the two mechanisms in the fluorescence decay. —

—

References [1J R.W. Schwartz and P.N. Schatz, Phys. Rev. B8 (1973) 322. [2] R.W. Schwartz, lnorg. Chem. 15 (1976) 2817. [3] C.J. O’Connor, R.L. Carlin and R.W. Schwartz, J. Chem. Soc. Faraday Trans. 2 (3) (1977) 361.

72

[4] [5] [6] [7] [8] [9] [10] [11] 1121 [13] [14]

F Pel!e et al.

/ Optical properties of c’s2NaBia6

N.H. North and H.J. Stapleton, J. Chem. Phys. 66 (1977) 4133. H.D. Amberger, G.G. Rosenbauer and RD. Fisher, J. Phys. Chem. Solids 38 (1977) 379. L.R. Morss, M. Siegal, L. Stenger and N. Edelstein, Inorg. Chem. 9 (1970) 1771. L.R. Morss and J. Fuger, Inorg. Chem. 8 (1969) 1433. R.W. Schwartz, S.F. Watkins, Ci. O’Connor and R.L. Carlin, J. Clsem. Soc. Faraday Trans. 2, 72 (1976) 565. J. Sheldon and R.M. Stanley, Purdue University. CILB. Lushchik, N.E. Lushchik and IA. Muuga, Trudy. IFFA AN ESSSR 23 (1963) 22. R. Moncorge, B. Jacquier and G. Boulon, J. Lumin. 14 (1976) 337. B. Jacquier and J.W. Richardson, J. Chem. Phys. 63 (1975) 2442. Unpublished results. It can estimated roughly the same as CsC1.

[15] A. Fukuda, Sci. Light 13(1964) 3. [16] S. Radhakrishna and R.S. Srinivasa Setty, Phys. Rev. 14 (1976) 969. [17] A. Fukuda, Phys. Rev. B! (1970) 4161. [18] Cheng, Thesis (1976). [19] G. O’Leary and R.G. Wheeler, Phys. Rev. B! (1970) 4409. [20] S. Sugano, J. Chem. Phys. 36 (1962) 122. 121] D. Bramanti and M. Mancini, Phys. Rev. B3 (1971) 3670. [22] G. Boulon, R. Moncorge and F. Gaume, Coil. mt. Spectroscopie des iléments de transition Ct des éléments lourds dans les solides, Lyon, 1975 (Ed. CNRS, 1976) p. 163. [23] G. Bouion, J. Phys. 32 (1971) 333. [24] A.E. Hughes and G.P. Pclls, Phys. Status Sohdi (b) 71(1975) 707. [25] G. Boulon, C. Pedrini, M. Guidoni and Ch. Pannel, J. Phys. 36 (1975) 265.

[261 M. Bacci, A. Ranfagni, M.P. Fontana and G. Viliani, Phys. Rev. B! 1(1975) 3052. [27] M.J. Treadaway and R.C. Powell, J. Chem. Phys. 61(1974) 4003. [28] R. Moncorge, Thesis, Lyon (1976).