Mechanism of photostimulated luminescence in BaFX:Eu2+ (X=Cl,Br) phosphors

Jew sal el Luiiesesceece 31 & 32 1 984) 256-2t8 Net ti—Holland. Anisterdaei

266

MECHANISM OF PHOTOSTIMULATEO LUMINESCENCE IN BaFX:Eu PHOSPHORS

(X~Cl,Nr)

Kenji Takahaahi, Katauhiro Kohda, Junji Niyaltara Fuji Photo Film Co.,Ltd., Teohnology Oevelopment Center, Niyenodai Kaiaeimaohi, Aahigarekamigun, Kanegawa, 258 Japan Yoahihiko Kanemitau, Koji Amitani, Shigeo Shionoya The Inatitute for Solid State Phyaioa, The Univeraity of Tokyo Roppongi, Minatoku, Tokyo, 106 Japan 2~, the exoitation by X—ray or ultraviolet light and the In BaFX:Eu photostimulation of lumineaoenoe by viaible light are oauaed by the prooeaaea of Eu2~E±Euj+ and F+_oentera~±F.

1.

INTHOUCTTON RaFX(XoC1,Or) oryatala have the tetragonal atruoture of P6FC1 (PN/n~:O~ 6)

and two typea of F—oentera,

F(F) and

oan be oreated by X—ray expoaurel). It ia also ahown

that

theae

oentera turn

out to be the 2~. photoatimurable lumineaoenoe In thia papar, we would (PSL) to oentera like propoae

a band 2~ meohanism in BaFX:Eu .

2.

model

of

prooeaaea

I

F

‘

a000rp,ionspnctruc,

~ N *

apeotra

in

ermssion

---

PSL

EXPERIMENTAL RESULTS Varioua

EXCITATION PROCESS excitation of photoluMnesoonce

for

a BaFBr:Eu2

exoitation

aingle

Crystal

~

N

pl,otocondocooit 5

are

ahown in Fig.1. The exoitation 2 lumineaoenoe has the speotr~ for theThe Eu abaorption apeotrum han peak at 280nm.

8 t_~

a small peak at 27Onm oorreaponding to the exoitation

peak.

abaorption apeotrum

It

apeotrum, of

ia

aeen

the

that

°°\

the

~

exoitation

~

photooonduotivity,

and

the

~oo

-

tonoaooonpeotrionotPscoroecs

3)0 w000l000th/om

400

formation apeotruin of PSL oentera riae in nearly

the

wavelengths.

name

way

They seem

towards to have

shorter the

peaks

at about 200nm or less.

0022—2313/84/503 -00 © Elsevier Science Publishers By. (North-Holland Physics Publishing Division)

Fag.l

spectra for the excitation prorene in fliFnrPu single rryetele.

/ Mechanism

K. Takahashi et al.

Fig.2

shows

speotrs

of photostiniulated luounesceoce

for the PSL prooess

single crystal or BaFBr powder phosphor. conductivity spectrum,

the

267

in a heavily X—rayed BaFBr:Eu

2~

The absorption spectrum2), the photo— speotrum of the Eu 2’ luisineacenoe 2)

stimulation

and the fading speotrum of the ESE signal due to F—centers by light

exposure

/~I K )~

nearly oorreapond with one another.

3’~

Eu

PSI. PROCESS

A.

05

‘F

I, ESR~~

0.5

0$

I

700

500 600 wav&eigth/nm

7

360

I

300

~

~os

~

Hu~•~1

~

is 05

I

400

420

~ ~

440

500 600

wavelength/nm

Fig.2

~ 640

020

660

600

700

720

wavelength /nrn

Fig.3

apectra for the PSL proceaa

Changes of the Eu2+ and Eu3+ luminescence

in a heavily X—reyed ~Fft;Eu2+

intensity with CV and visible light

single crystal (absorption

irradistion

,

photoconductivity and stimul—

a. Thfore N

ation spectra) or ~FW

b. Aftsr N2lsser light irradistien.

2lsser light irradiation.

powder phosphor spectrum)

)

~R

fading

.

c. After the stimulation by He—Ne laser light.

BsFBr:Eu 2~ phosphors

prepared by firing in rather oxidizing stmosphere 3+ show weak red luminescence of s line spectrum due to Eu ions in addition to 2~ the Eu luminescence. Under strong irradiation by 337nm nitrogen laser light, it is found that the intensity of the Eu luminescenoe is decressed, and

3+

simultaneously the intensity of the Eu

luminescence is increased.

After the

K. Takahashi ci al. / Mochanisn,

268

0/

photosiu,iulatc~/luiiuncoeiuc

stimulation of the Eu luminescence is sufficiently made by visible light, 2~ the Eu luminescence intensity is found to increase to recovering the initial 3+ value and simultaneously the Eu luminescence intensity is returned to the initial value (Fig.3).

3

conductionband

~

DISCUSSION facts

provide the basis ing

the

4.....

—

Experimental

PSL

which

F~—~F

are

I

summarized as follows. 1)

Under

I

part of Eu changing

to

the

visible

they

return

8.3eV

strong irradiation

by CV light, the intensity of 2~ the Eu luminescence is decreased. That is to say, a 2+

I 3.2eV

4.6eV ?6eV

for discuss—

mechanism

~2eV

I

.

ions are 3+ ions.

Eu

light to

the

.

.

ionized

-

2+

EU valence band

After

0—---

initial

2) The formation spectrum of

PROCESS

PSI 11-0.4

The mechanism

states.

ExCITATION PROCESS

—~.

exposure,

of PSI

in

2+ PSFX~Fu

(The

onergy valuec apply to the case of

XPI)

PSL centers, the excitation spectrum of photoconductivity,

and the absorption spectrum nearly correspond with each

other. 3)

Under

photostimulable

conditions,

the stimulation spectrum,

the

photo—

conductivity spectrum, absorption spectrum, and the ESR fading spectrum nearly correspond with one another. Then, shown

the mechanism of PSL

is interpreted by an energy level

diagram as 2~ Eu ions are

in Fig.11.

ionized

By X—ray or CV light excitation, a part of 3+ changing to Eu ions and electrons are liberated to the conduction +

band.

These electrons

are trapped at F —centers.

By visible

light

irradiation

the trapped electrons are again liberated to the conduction band and return to 3+ 2~ Eu ions converting them to excited Eu ions. Thus the Eu luminescence is stimulated.

REFERENCES Chem. Solids 37

1)

M.Yuste, L.Taurel, M.Rahmani and D.Leymone, J. Phys. (1976) 961.

2)

K. Takahashi, Y. Shibahara and J. Miyahara, Electrochem. Soc. 163rd Meeting, 921RNP (1983), to be submitted to J. Electrochem. Soc.