Temperature Dependent Luminescence Spectra of Synthetic and Natural Alexandrite (BeAl2O4:Cr3+)

doi:10.4236/msa.2011.24037

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Materials Sciences and Applicatio ns, 2011, 2, 284-287

doi:10.4236/msa.2011.24037 Published Online April 2011 (http://www.scirp.org/journal/msa)

Temperature Dependent Luminescence Spectra of

Synthetic and Natural Alexandrite (BeAl2O4:Cr3+)

1Neilo Marcos Trindade, 2Américo Tabata, 2Rosa Maria Fernandes Scalvi,

2Luis Vicente de Andrade Scalvi*

1Anhanguera Educacional, Sorocaba, Brazil

2UNESP- State University of São Paulo (D ept. Phy sics - FC), Bauru, Brazil

E-mail: [email protected]

Receives July 16th, 2010; revised October 11th, 2010; accepted March 15th, 2011.

ABSTRACT

Results of photoluminescen ce measurements for natural and synth etic alexandrite (BeAl2O4:Cr3+) are pre sented, wher e

the samples are excited by the 488 nm line of an Ar+ laser, at different temperatures. The main issu e is the analysis of

the Cr3+ transition in the chrysoberyl matrix (BeAl2O4), with major technological application as active media for laser

action. Results indicate anomalous behavior of Cr3+ transition depending on the measurement temperature. A simple

model to explain the phenomena is suggested.

Keywords: Alexandrite, Luminescence, Cr3+ Transitions, Optical Absorption

1. Introduction

The use of alexandrite as an active media for laser action,

with emission in the range 700 - 800 nm [1], has ap-

peared for the first time in 1978 [2]. Since then, this ma-

terial became technologically important. Recently, many

reports show wide application of the alexandrite laser, in

medicine as well as for atmospheric studies [3,4].

Alexandrite structure may be visualized as approxi-

mately hexagonal close packed (hcp), with unit cell of

four molecules, with eight Al3+ ions, occupying distorted

octahedral sites and four Be2+ ions, occupying distorted

tetrahedral sites, formed by oxygen located at plans per-

pendicular to c axis. The distortion from a perfect hcp

structure of oxygen ions give birth to the appearing of

two sites of distinct symmetries: Al1, located at inversion

sites and Al2, located at mirror sites [5,6]. Cr3+ ions are

incorporated in the octahedral of reflection and inversion

symmetries and may be denoted by and , re-

spectively.

Cr 3+

The octahedrally coordinated Al2 site is supposed to be

larger when compared to Al1, which can be explained by

the length of the bond Al-O (1.938 Å), which is longer

than for Al1 (1.890 Å), resulting on a larger polyhedral

volume [7]. The Al2 site, due to its lager size, is preferen-

tially occupied by Cr3+ ions, and then it is responsible for

the optical properties of alexandrite [8]. Then, sions

are characterized by a high probability of electric dipole

transition. The magnetic dipole transitions of the

do not contribute significantly to the optical absorption

and emission of this material. Besides, they are excluded

from the laser process, equilibrating or reducing the ex-

citation energy of sions [5]. Then, the ra tio of s

and i concentrations become a determinant parame-

ter for the alexandrite optical efficiency. EPR measure-

ments indicate that, in general, the Cr3+ ion enters in the

BeAl2O4 lattice in a ratio of about 75% of Al2 and 25%

of Al1, for the natural sample as well as for synthetic

material, showing variations depending on the relative

quantity of Cr2O3 in the sample [9].

In the luminescence spectra, the s lines are called

R1 and R2, and the lines of i are called S1 and S2. As

experimentally observed [6], the R lines show up pre-

cisely in the same wavelength, 680.4 and 678.5 nm, re-

spectively, at room temperature, in both spectra: optical

absorption and emission. In the emission spectra, the

lines S1 and S2 show up around 695.8 and 689.9 nm and

in the absorption spectra they are seen as narrow lines at

655.7, 649.3 and 645.2 nm [10,11].

2. Experimental

Alexandrite stones come from Minas Gerais state in Bra-

zil, and show a dark green color, due to the presence of

clusters. Samples have been cut such that parallel plans

between the faces were provided, leading to an average

Temperature Dependent Iuminescence Spectra of Synthetic and Natural Alexandrite (BeAlO:Cr3+) 285

2 4

thickness of 1.5 mm. The synthetic sample has been

grown by H. P. Jenssen and R. Morris (from Allied Sig-

nal Inc, U.S.A.), by the Czochralsky method as men-

tioned elsewhere [6]. The thickness is 2.3 mm and show

perfectly parallel faces.

For luminescence excitation it is used an Ar+ laser

tuned at 2.51 eVh



. In the excitation system, the

laser beam is irradiated through a chopper, being redi-

rected to the sample, which is placed inside a closed cy-

cle He cryostat. The emitted light from the sample (pho-

toluminescence) is sent to a spectrophotometer coupled

to a germanium detector, where the electrical signal is

amplified by a lock-in amplifier, linked to a computer

which controls the spectrophotometer and collects the

obtained data.

3. Results and Discussion

Figure 1 shows the Luminescence spectra for the syn-

thetic alexandrite sample measured at several tempera-

tures. Fig ure 1(a) evidences the lines R, and Figure 1(b),

lines S. Below room temperature, the luminescence

spectra is dominated by the R lines of individual Cr ions

and the S lines, originated from exchange-coupled pairs

of chromium ions [11].

The luminescence of R and S lines points towards a

dependency with the measurement temperature, as can be

observed in Figures 1(a) and 1(b), for the synthetic sam-

ple.

In Figure 1(a), the relative intensity of lines, R1

and R2, present always the same behavior, with the in-

tensity of line R1 being higher than for line R2, and the

difference between them decreases with temperature. As

the temperature is raised, the line R1 has an intensity de-

crease, whereas the line R2 show intensity increase until

135 K. Above this temperature, this line also shows a

gradual intensity decrease. For the lines, S1 and S2,

shown in Figure 1(b), the line S1 is more intense than

line S2. The variation in the intensities of these lines is

explained as an indicative that more then one type of

carrier relaxation mechanism is present [12]. Figure 1(b)

shows that line S1 decreases, whereas line S2 has a slight

intensity increase until about 135 K, and above that a

regular intensity decrease.

The variation in intensities of R lines as well as S lines

for natural alexandrite is shown in Figure 2. The varia-

tion of both lines occurs in a similar way for this sample,

as shown in the photoluminescence spectra of Figures

2(a) and 2(b). In the spectra, it is also possible to observe

that the lines R and S change position, starting from 135

K, shifting to higher wavelengths (lower energy), fol-

lowed by a wavelength broadening of these lines. More-

over, in a comparison of synthetic and natural samples, it

can be noticed that the lines position is practically in the

(a)

(b)

Figure 1. Luminescence spectra for the synthetic alexan-

drite sample as function of temperature. (a) Lines R, (b)

Lines S.

same wavelength.

Considering that either the natural as the synthetic

sample shows the same effect in the intensity of emission

lines measured at different temperatures, the explanation

concerning the behavior of lines R and S, can be done by

analogy. A simple model is proposed and is shown in

Figure 3, where the electron and hole ground states are

represented. Vertical lines stand for radiative transition

and oblique lines represent the electron transfer from an

electronic level to another. t represents the lifetime of the

carrier in the level and is associated with the transition

probability between two levels.

In this model, it is supposed that initially there is no

interaction between electronic levels R and S. Free carri-

ers of a site probably do not interact with the carriers

from another level, and with the associated levels of Cr3+

clusters. Then, the probability that carriers from level R

Temperature Dependent Iuminescence Spectra of Synthetic and Natural Alexandrite (BeAlO:Cr3+)

286 2 4

(a)

(b)

Figure 2. Luminescence spectra for natural sample as func-

tion of temperature (a) Lines R and (b) Lines S.

are transferred to level S and vice-versa is very low, as

well as from levels R or S to the levels associated with

Cr3+ clusters.

Then, at low temperature, electrons excited by optical

pumping lose energy by phonon emission (lattice vibra-

tion) and are trapped by levels R and S. Then, they re-

combine with holes from the ground state, generating the

lines R1, R2, S1 and S2. With the temperature increase, as

the intensity of lines R1 and S1 decrease due to interac-

tion between electrons and phonons, the emission associ-

ated to lines R2 and S2 show inverse behavior, and pre-

sent intensity increase. This behavior is observed until

135 K. Above that temperature, the intensities decrease

due to the electron-phonon coupling. This behavior is

explained as follows: as the temperature is increased,

some R1 carriers are excited back to R2. This is possible

if the lifetime t21 is long enough for the excitation or at

Figure 3. Diagram representing the transfer ratio between

emission levels and lifetime (t).

least of the same order of magnitude of t41, the lifetime

for excitation back of carriers from R1 to R2. On the other

hand, if the lifetime t31 is longer than t11, the carriers in

R2 tend to recombine radiatively with holes in the ground

state instead of finishing in the R1 level, making the

emission intensity associated with R2 to increase. With

higher temperature increase, more carriers in R1 will gain

energy to populate R2, and then, the intensity grows with

the temperature increase. However, above 135 K, the

electron-phonon coupling becomes very strong and the

relaxation via phonons (non-radiative) becomes domi-

nant. In this case, the intensity of lines R1 in the photo-

luminescence spectra decreases. The same process may

be applied to the tran sition involving S levels.

We have observed that it leads to some interesting

variation in the luminescence dependency with tempera-

ture. Lines R and S remains fixed at their wavelengths

until about 135 K, where a peak shift takes place to

higher wavelengths (lower energy), followed by a wave-

length broadening of theses lines, shown in the emission

spectra. This peak shift is as sociated with the increase of

the alexandrite unit cell with temperature.

The luminescence spectra also allow analyzing the dif-

ference between wavelengths of emission lines in these

samples, as can be seen in Table 1.

It can be verified in Table 1 that the wavelength sepa-

ration between S lines is about three times larger than the

Table 1. R1, R2, S1 and S2 lines position and separation (





)

between these lines at room temperature, for synthetic and

natural alexandrite samples.

Sample R2 (nm) R1 (nm) S2 (nm) S1 (nm)

Synthetic 678.5 680.3 689.9 695.7

1.8





 5.8





Natural 678.5 680.3 689.9 695.8

1.8



 5.9





Temperature Dependent Iuminescence Spectra of Synthetic and Natural Alexandrite (BeAl2O4:Cr3+)

287

separation between R lines. This behavior indicates a

stronger low symmetry component of the crystalline field

for ions located in a inversion center [13].

4. Conclusions

Photoluminescence measurements carried out at several

temperatures for alexandrite samples have shown up as

very relevant to the analysis of Cr3+ transition in this ma-

terial, because it is related to its potentiality to laser ac-

tivity. The temperature dependency of the emission spec-

tra exhibits an anomalous behavior for natural samples as

well as for the synthetic one, compared to most materials,

for instance, semiconductors. Then, a simple model is

proposed, based on electronic transitions of levels R and

S, characteristic of Cr3+ transitions.

5. Acknowledgements

This work was supported by CAPES, CNPq, FAPESP

and FUNDUNESP. The authors are grateful to Prof.

Tomaz Catunda for the synthetic sample.

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