- Email: [email protected]
Luminescence and photoconductivity of pure β-ThBr4 single crystal
Journal of Luminescence 40&41 (1988) 349 350 North-Holland, Amsterdam
349
LUMINESCENCE AND PHOTOCONDUCTIVITY OF PURE 5-ThBr~SINGLE CRYSTAL Solange HUBERT and Eric SIMONI Institut de Physique Nuclèaire, Laboratoire de Radiochimie, 91406 Orsay Cedex, BP n°1, France The luminescence properties of pure 5-ThBr
4 single crystals are reported. Blue emission extending from 340 to 520 nm occurs when the pure crystal is excited with U.V. radiation in the region 240 nm < A < 300 nni. Absorption, emission and decay time measurements as a function of temperature were investigated.
1. INTRODUCTION For several years 5-thorium tetrabromide has I
I
I
I
been extensively studied as host material for tetravalent actinides optical spectroscopy. In
300K
the pure material a bright blue luminescence has been observed at room temperature with
54K
ultra violet radiation by M. Genet et al.’’’, but the process leading to the emission was not so well known. In order to obtain more informations on the character of the optical transitions in 5-ThBr4, we have performed absorption, excitation, photoconductivity and emission measurements in the ultra violet and
I
I
44
visible region at different temperatures. The temperature dependence of the luminescence
I
46
I
I
48
Enerqy (eV)
intensity and radiative decay time have also been measured. The results of these investiga—
Absorption spectrum 5-ThBr~
~U~OlK and 1.54 K of
tions are presented in this paper. A description of the experimental techniques
energy when the temperature decreases and coin-
used will appear elsewhere.
cides with the excitation band, the maximum of which is at about 270 nm (4,6 eV).
2. RESULTS Absorption measurements were carried out at
We observed photoconduction in 5-ThBr~. The spectral dependence of the photocurrent at
different temperatures on very thin polished
295 K and 10 K shows similarities with the ex-
single crystal. The absorbance versus photon energy of a crystal of about 1/10 mm is given in figure 1. This spectrum shows that 5—ThBr5 has a strong absorption in the U.V. region with an
citation spectrum of the luminescence. Maximum in the photoconductivity was observed at 4.7 eV. When excited by 254 nm line of a mercury lamp, the sample produces a broad band emis— sion with maxima at 405 nm. Emission spectra
absorption edge which occurs at 4.48 eV at iOD K. This absorption edge shifts to higher
were recorded from 10 K to 400 K. At 10 K it consists of one broad band with a maxima at
0022 2313 88/$03.50 ~ Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
350
S. Hubert, E. Simoni
Luminescence and phozoconduciwity
418 nm (3 eV) with halfwidth of about 0.3 eV. When the temperature is raised, the maximum
red to be 0.45 eV. Absorption could be assigned to a band gap
shifts to higher energy (405 nm at 295 K) while the halfwidth increases (0.6 eV) as reported in figure 2. The emission is almost quenched at
transition from the highest occupied molecular orbital to empty molecular orbital of the cluster ThBr 8~. The emission process can be ex-
about 400 K.
plained either by the reverse process or by defect recombination centers. A theoretical stuof dy describing the cluster the ThBr8~ molecular is in orbitals progress.energies
C2K I
80K
c~
~I
I
160K
o
020K I
29016
5 4 3
L~ 350
400
:±~~‘ 200
450 50 A I in
FIGURE 2 Spectral energy distribution of the emission of 5-ThBr, at different temperatures. E — 4.88 eV. e xc
250
300
350
N
400
0 2
FIGURE 3 Emission intensity (. ) and decay curve C U) of the emission band of 13—ThBr6 as a function of temperature (excitation wavelength 285 nm). (o) represents calculated decay times. ACKNOWLEDGEMENTS
The intensity and the decay time of the fluorescence were measured as a function of temperature from 288 K to 390 K. All the decay curves are single exponentials. The decay time
The authors are grateful to Drs. C. Pedrini, B. Moine and d.C. Rivoal for valuable partici— pation in some experiments and to Pr. M. Genet for fruitful discussion.
varies from 5.7 ps to 0.35 ps. As reported in figure 3, both variations could be well fitted with the expression described below I
I~/1 + C exp(— AE/kT)
11 + — TR exp(— toE/kT). To and T CR The value of thermal activation energy toE appea-
REFERENCES 1. R. Carlier and NI. Genet : CR Acad. Sc. Paris 281C (1975)671. R. Carlier, Of lumines2. R. M. Guillaumont, Genet, M. Hussonnois, d.C.J. Krupa, cence 12/13 (1976)953.
349
LUMINESCENCE AND PHOTOCONDUCTIVITY OF PURE 5-ThBr~SINGLE CRYSTAL Solange HUBERT and Eric SIMONI Institut de Physique Nuclèaire, Laboratoire de Radiochimie, 91406 Orsay Cedex, BP n°1, France The luminescence properties of pure 5-ThBr
4 single crystals are reported. Blue emission extending from 340 to 520 nm occurs when the pure crystal is excited with U.V. radiation in the region 240 nm < A < 300 nni. Absorption, emission and decay time measurements as a function of temperature were investigated.
1. INTRODUCTION For several years 5-thorium tetrabromide has I
I
I
I
been extensively studied as host material for tetravalent actinides optical spectroscopy. In
300K
the pure material a bright blue luminescence has been observed at room temperature with
54K
ultra violet radiation by M. Genet et al.’’’, but the process leading to the emission was not so well known. In order to obtain more informations on the character of the optical transitions in 5-ThBr4, we have performed absorption, excitation, photoconductivity and emission measurements in the ultra violet and
I
I
44
visible region at different temperatures. The temperature dependence of the luminescence
I
46
I
I
48
Enerqy (eV)
intensity and radiative decay time have also been measured. The results of these investiga—
Absorption spectrum 5-ThBr~
~U~OlK and 1.54 K of
tions are presented in this paper. A description of the experimental techniques
energy when the temperature decreases and coin-
used will appear elsewhere.
cides with the excitation band, the maximum of which is at about 270 nm (4,6 eV).
2. RESULTS Absorption measurements were carried out at
We observed photoconduction in 5-ThBr~. The spectral dependence of the photocurrent at
different temperatures on very thin polished
295 K and 10 K shows similarities with the ex-
single crystal. The absorbance versus photon energy of a crystal of about 1/10 mm is given in figure 1. This spectrum shows that 5—ThBr5 has a strong absorption in the U.V. region with an
citation spectrum of the luminescence. Maximum in the photoconductivity was observed at 4.7 eV. When excited by 254 nm line of a mercury lamp, the sample produces a broad band emis— sion with maxima at 405 nm. Emission spectra
absorption edge which occurs at 4.48 eV at iOD K. This absorption edge shifts to higher
were recorded from 10 K to 400 K. At 10 K it consists of one broad band with a maxima at
0022 2313 88/$03.50 ~ Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
350
S. Hubert, E. Simoni
Luminescence and phozoconduciwity
418 nm (3 eV) with halfwidth of about 0.3 eV. When the temperature is raised, the maximum
red to be 0.45 eV. Absorption could be assigned to a band gap
shifts to higher energy (405 nm at 295 K) while the halfwidth increases (0.6 eV) as reported in figure 2. The emission is almost quenched at
transition from the highest occupied molecular orbital to empty molecular orbital of the cluster ThBr 8~. The emission process can be ex-
about 400 K.
plained either by the reverse process or by defect recombination centers. A theoretical stuof dy describing the cluster the ThBr8~ molecular is in orbitals progress.energies
C2K I
80K
c~
~I
I
160K
o
020K I
29016
5 4 3
L~ 350
400
:±~~‘ 200
450 50 A I in
FIGURE 2 Spectral energy distribution of the emission of 5-ThBr, at different temperatures. E — 4.88 eV. e xc
250
300
350
N
400
0 2
FIGURE 3 Emission intensity (. ) and decay curve C U) of the emission band of 13—ThBr6 as a function of temperature (excitation wavelength 285 nm). (o) represents calculated decay times. ACKNOWLEDGEMENTS
The intensity and the decay time of the fluorescence were measured as a function of temperature from 288 K to 390 K. All the decay curves are single exponentials. The decay time
The authors are grateful to Drs. C. Pedrini, B. Moine and d.C. Rivoal for valuable partici— pation in some experiments and to Pr. M. Genet for fruitful discussion.
varies from 5.7 ps to 0.35 ps. As reported in figure 3, both variations could be well fitted with the expression described below I
I~/1 + C exp(— AE/kT)
11 + — TR exp(— toE/kT). To and T CR The value of thermal activation energy toE appea-
REFERENCES 1. R. Carlier and NI. Genet : CR Acad. Sc. Paris 281C (1975)671. R. Carlier, Of lumines2. R. M. Guillaumont, Genet, M. Hussonnois, d.C.J. Krupa, cence 12/13 (1976)953.