Structure around the Tm3+ ion in a glass based on AlF3

Journal of Non-Crystalline Solids 331 (2003) 58–69

www.elsevier.com/locate/jnoncrysol

Structure around the Tm3þ ion in a glass based on AlF3

Hiroyuki Inoue a,*, Kohei Soga b, Akio Makishima c

a Institute of Industrial Science, The University of Tokyo, 4-6-1, Komaba, Meguro-ku, Tokyo 153-8505, Japanb Department of Advanced Materials Science, Graduate School of Frontier Sciences, The University of Tokyo,

7-3-1, Hongo, Bunkyo-ku, Tokyo 113-8656, Japanc Center for New Materials, Japan Advanced Institute of Science and Technology, 1-1, Asahidai, Tatsunokuchi,

Nomi, Ishikawa 923-1292, Japan

Received 10 July 2002; received in revised form 6 June 2003

Abstract

Absorption and emission spectra of a Tm3þ-doped glass based on AlF3 were estimated from crystal-field pa-

rameters based on crystal-field theory and Judd–Ofelt theory. On the basis of point charge approximation, two

kinds of crystal-field parameters were obtained from two classes of structural models prepared by using molecular

dynamics (MD) simulation. In the first class the structural models were prepared from initial coordinates given at

random. The calculated spectra were in agreement with the observed ones. In the second class the structural models

were prepared from fixed coordination polyhedra of Tm3þ ions. The splitting of the energy levels and emission

spectra of the Tm3þ ion were calculated from the second type of structural models. It was found that the splitting

of the 3F4 and 1G4 levels represented the coordination number of the Tm3þ ion, and the splitting of the 3F4 level

could be evaluated from the emission spectrum of the 1G4–3F4 transition. The observed emission spectrum of the

1G4–3F4 transition at 20 K resembled the emission spectrum calculated from the models of 8-fold coordinated Tm3þ

ions.

� 2003 Elsevier B.V. All rights reserved.

1. Introduction

Recently, there have been a number of research

efforts examining the development of rare earth

doped fiber amplifiers and lasers [1]. The opticalproperties of rare earth ions are strongly depen-

dent on glassy hosts. The splitting of the energy

* Corresponding author. Tel.: +81-3 5452 6315; fax: +81-3

5452 6316.

E-mail address: [email protected] (H. Inoue).

0022-3093/$ - see front matter � 2003 Elsevier B.V. All rights reserv

doi:10.1016/j.jnoncrysol.2003.09.026

levels of 4f electrons and the radiative transition

rate between them, both of which determine the

optical spectra of the 4f–4f transition of rare earth

ions, can be described by the crystal-field potential.

The crystal-field potential can be described by theelectrostatic field from the atomic arrangement

around rare earth ions. Therefore, it is important

for the design of the optical properties to clarify

the relation between the optical properties and the

atomic arrangement around rare earth ions in

glass as well as the chemical bond between the rare

earth ion and its ligand. The technique of laser-

induced fluorescence line-narrowing (FLN) has

ed.

mail to: [email protected]

H. Inoue et al. / Journal of Non-Crystalline Solids 331 (2003) 58–69 59

significant potential for the analysis of the atomicarrangement around rare earth ions. Brecher and

Riseberg [2,3] have reported that the local coor-

dination structure around the Eu3þ ion in glasses

can be analyzed from the emission spectra of 5D0–7FJ (J ¼ 1 and 2) transitions of the Eu3þ ion. In

spite of significant potential of the FLN technique,

their studies have recognized only a few successful

examples. Tikhomirov and Tikhomirova [4] foundthat the branching ratio of the transition (3P0–3H6):(

3P0–3F2) of the Pr3þ ion increased linearly

with the host polarizability in fluoride glass fami-

lies. The slope of the branching ratio-polarizability

varied between glass families, such as fluoroindate

fluoroaluminate, and fluorozirconate glasses. They

concluded that the symmetry of the Pr3þ ion be-

tween the glass families differed depending on theglass-network former structural units. This is a

typical example of optical properties showing

different atomic configuration around rare earth

ions even in fluoride glasses. However, the specific

atomic arrangement of rare earth ions in the

glasses has not been known.

The calculation of the energy levels of rare earth

ions in glasses has been proposed [5] and devel-oped [6,7]. Recently, we estimated the crystal-field

potential from structural models prepared by

using molecular dynamics (MD) simulation [8–10].

In fluoride glasses the optical spectra estimated

from the crystal field potential could reproduce the

observed spectra [11,12]. In various structural

factors, the coordination polyhedron is a charac-

teristic and dominant factor for the optical prop-erties of the rare earth ion. In this paper, the

method which extracts information on the local

structure around the rare earth ion in glasses from

the optical spectrum is examined by using struc-

tural models. We propose two approaches. The

former is to examine the correlation of individual

crystal-field parameters and the splitting of the

energy levels. The latter is to examine the relationbetween a fixed specific structural element in the

structural models and the estimated optical spec-

trum from the structural models. Special attention

is given to the local structure around the Tm3þ ion

in glass based on AlF3. A similar discussion will be

applied to other rare earth ions in other fluoride

glasses.

2. Experimental conditions

2.1. Preparation of the structural model containing

the Tm3þ ion

Two kinds of structural models were prepared

by using MD simulation. The former was the

structural model whose initial coordinates weredetermined at random. We referred to this struc-

tural model as Model-R. Three hundred and forty

eight ions (Mg2þ : 9, Ca2þ : 26, Sr2þ : 8, Ba2þ : 9,Tm3þ : 1, Y3þ : 12, Al3þ : 35, F� : 248) were placed

randomly in a cubic cell with periodic boundary

conditions. The cell parameter of 1.654 nm was

determined from the experimental density of the

glass. Simulations were carried out at a constantvolume. The potential of the Born–Mayer type

was used with formal ionic charges, and the values

of the potential parameters determined from the

radial distribution curves of AlF3–BaF2–CaF2

glass were used [13,14]. The values of the param-

eters for the remaining ionic pairs were estimated

from the ionic radii reported by Shannon and

Prewitt [15]. These values are listed in Table 1. TheCoulomb force was evaluated by the Ewald sum-

mation. To obtain the variation of the Tm3þ sites

in the glass structure, MD simulation was per-

formed for 300 different sets of random initial

coordinates. The temperature of the simulation

was lowered from 3000 to 300 K with a time step

of 1 fs for 10 000 time steps (10 ps). After 5000

time steps (5 ps) at 300 K, the coordinates of thelast step were used for further calculation.

The second type was a model in which the first

coordination polyhedron of the Tm3þ ion was

fixed during the simulation. We referred to this

structural model as Model-F. The typical first co-

ordination polyhedra (7-fold coordination poly-

hedra :mono-capped octahedron and pentagonal

bipyramids, 8-fold coordination polyhedra : cubic,anti-prism, and triangulated dodecahedron, and 9-

fold coordination polyhedron : trigonal prism with

the rectangular faces capped) are shown in Fig. 1.

The distance of the Tm–F pair was set at 0.22 nm

in the first coordination polyhedron. These first

coordination polyhedra, except the cubic, are

found in fluoride crystal structures [7]. The initial

coordinates of the remaining ions in a unit cell

Table 1

The potential and the parameters used in MD simulation

Mg Ca Sr Ba Tm Y Al F

Zi +2 +2 +2 +2 +3 +3 +3 )1

Aij (10�6 J) Mg Ca Sr Ba Tm Y Al F

Mg 1.44 2.75 3.45 5.65 3.78 2.82 1.64 1.22

Ca 5.65 7.22 12.22 7.72 5.57 2.99 2.48

Sr 9.28 15.84 9.85 7.06 3.72 3.16

Ba 27.46 16.60 11.74 5.97 5.32

Tm 10.55 7.65 4.14 3.39

Y 5.61 3.14 2.46

Al 1.93 1.30

F 0.843

Born–Mayer potential

Uij ¼e2

4pe0

ZiZj

rijþ Aij exp

�� rij

q

�; q ¼ 0:03 nm:

Table 2

The parameters hrki, ak , Nðk; kÞ and bðk; kÞ for the 4f electrons

[12]

k hrki ak

2 1.846· 10�21 m2 0.933

4 8.637· 10�42 m4 3.487

6 8.318· 10�62 m6 7.057

k, k Nðk; kÞ bðk; kÞ1, 2 )2.759· 10�4 m2 J�1 2.891

3, 2 1.253· 10�24 m4 J�1 7.410

3, 4 1.458· 10�24 m4 J�1 7.059

5, 4 )7.122· 10�45 m6 J�1 13.867

5, 6 )2.015· 10�44 m6 J�1 6.611

7, 6 7.444· 10�65 m8 J�1 12.411

Fig. 1. The first coordination polyhedrons of Tm3þ ions in

Model-F. The 7-fold coordination polyhedrons [mono-capped

octahedron (a) and pentagonal bipyramids (b)], 8-fold coordi-

nation polyhedrons [cubic (c), anti-prism (d) and triangulated

dodecahedron (e)] and 9-fold coordination polyhedron [trigonal

prism with the rectangular faces capped (f)].

60 H. Inoue et al. / Journal of Non-Crystalline Solids 331 (2003) 58–69

were determined at random. MD simulation wasperformed for 50 different sets of random initial

coordinates for each first coordination polyhe-dron. The simulation was carried out in the same

way as that of Model-R, except for the use of the

fixed first coordination polyhedron.

2.2. Calculation of the spectra of the Tm3þ ion in the

structural models

A detailed method of the calculation for Model-R has been described in previous reports [11,12].

The splitting of the 4f energy levels and the tran-

sition rates between them were estimated using the

parameters reported previously [12]. The values of

the parameters are listed in Table 2. The cross-

sections of absorption and emission were esti-

Fig. 2. The pair distribution curve and the accumulated coor-

dination curve for the Tm–F pair in Model-R.


mated from 300 structural models and were aver-aged.

The splitting of the 4f energy levels and

the transition rates were also estimated from the

crystal-field parameters obtained only from the

first coordination polyhedron in Model-F. Then

the splitting of the 4f energy levels and the tran-

sition rates were estimated from the crystal-field

parameters obtained from all coordinates inModel-F. The dependency of the first coordination

polyhedron on the splitting and transition rates for

each energy level was evaluated.

2.3. Preparation of glass and measurement of

spectra

Glass with a composition of 35AlF3 Æ 12YF3 Æ26CaF3 Æ 8.7MgF2 Æ 8.7SrF2 Æ 8.6BaF3 Æ 1TmF3 was

prepared. The powders were mixed and melted in a

platinum crucible at 1080 �C for 30 min under an

Ar atmosphere. The melt was cast into a preheated

brass mold. The glass obtained was cut into a

10 · 10 · 5 mm shape and polished.

Absorption spectrum was measured with a self-

recording spectrophotometer (U3410, Hitach) inthe wavelength range of 200–2600 nm at room

temperature.

Line-narrowed fluorescence is generated by the

excitation of a subset of the rare earth ions in

glass. The excited ions must have almost identical

emission spectrum. Therefore, it is highly desirable

that the excitation is accomplished through an

non-degenerated level which does not split by thecrystal-field. The 7F0–

5D0 transition of the Eu3þ

ion is ideally suited for such excitation. The 3H6–3P0 transition of the Tm3þ ion was well suited for

such excitation and analysis of the first coordina-

tion polyhedron. The characteristics of the energy

levels and the transitions will be discussed later.

The tunable laser around 280 nm is necessary for

the excitation to the 3P0 level. It is impossible forour laser equipment to generate the 280 nm laser

emission. Thus, we used the excitation to the 1G4

level to obtain the emission spectra at a low tem-

perature as a second choice. The emission spec-

trum was measured at 20 K by using an OPO laser

with excitation 465 nm (Mirage 500, Hoya Con-

tinuum Inc.) with a line width <0.1 nm. The

emission from the sample was focused on the en-trance slit of a spectrometer (1000 M, Spex In-

dustries Inc.) and detected with a photomultiplier

tube (R1477, Hamamatsu Corp.).

3. Results

3.1. Molecular dynamics simulation

The pair distribution function for the Tm–F

pair in Model-R structure model of the glass at 300

K is shown in Fig. 2 together with the accumulated

coordination number. The peak of the Tm–F pair

was at 0.229 nm with 0.020 nm of FWHM. The

valley of the peak was found around 0.31 nm.

Thus, we determined the F� ions within this dis-tance as the first coordination polyhedron. The

coordination number of the Tm3þ ions was dis-

tributed from 7 to 10. The 7-, 8-, 9- and 10-fold

coordinated Tm3þ ions in the 300 models of

Model-R were 10%, 46%, 36% and 8%, respec-

tively. The average coordination number was 8.33.

The accumulated coordination curves for the Tm–

F pair in Model-F are shown in Fig. 3. As can beseen from Fig. 3, the beginning of the second

neighboring F� ions was dependent on the first

coordination number and shape. The average

coordination number of the Tm3þ ions in the

Fig. 3. The accumulated coordination curves for the Tm–F pair

in Model-F. Mono-capped octahedron (� � �) and pentagonal

bipyramids (––) of 7-fold coordination polyhedrons (a). Cubic

(� � �), anti-prism (– Æ –) and triangulated dodecahedron (––) of 8-

fold coordination polyhedrons (b) and 9-fold coordination

polyhedron (c).

Fig. 4. The observed (a) and calculated (b) absorption cross-

section of Tm3þ ions.


mono-capped octahedron increased more than 7.5

by approaching another F� ion to the polyhedron.

The structural model was unsuitable to examine

the effect of the fixed coordination polyhedron. On

the contrary, the 7-fold coordination was retained

up to 0.31 nm in the pentagonal bipyramids.

3.2. The observed and calculated spectra of the

Tm3þ ion

The observed absorption cross-section is shown

in Fig. 4(a). The absorption bands can be ascribed

to the transitions from the ground state, 3H6, to

the upper levels of the Tm3þ ion. The parameters

hrki, ak, Nðk; kÞ and bðk; kÞ determined in the pre-

vious report [12] are listed in Table 2. The ab-sorption cross-section calculated from Model-R is

shown in Fig. 4(b). The position, width and height

of each absorption band were reproduced sub-

stantially. The values of the observed and calcu-

lated oscillator strength of the Tm3þ ion are listed

in Table 3. Since the parameter obtained from the

Tm3þ-doped ZBLAN glass was used, the repro-ducibility of the absorption spectrum of the Tm3þ

ion in the glass based on AlF3 was inferior to that

in ZBLAN glass. The composition dependence of

individual parameters is a problem for future

consideration.

The emission spectra under 465 nm (21 505

cm�1) and 475 nm (21 053 cm�1) excitation at 20 K

are shown in Fig. 5(a) and (b). The emissionspectra were assigned to the radiative transition

from the 1G4 to 3F4 level. The main peak was lo-

cated around 15 400 cm�1 with the widespread

band at the lower energy side to the main peak.

Since the 1G4 level, which was the terminal level of

the excitation, splits into 9 Stark components and

the absorption bands of the individual compo-

nents overlap, a complete site selective excitationwas not achieved. The typical emission spectrum is

shown in Fig. 5(a), which was excited at a 465 nm

Table 3

The observed and calculated oscillator strengths (10�6) of the

Tm3þ ion

Level Observed Calculated

MD ED Total

3F4 1.15 1.27 0.02 1.293H5 1.07 0.93 0.40 1.333H4 1.57 1.62 0.01 1.633F3

�2.46

1.81 0.00�

2.143F2 0.33 0.001G4 0.55 0.43 0.00 0.431D2 1.72 1.17 0.00 1.17

Fig. 5. The observed and calculated emission spectra of Tm3þ

ions at 20 K. The observed spectra excited at 465 nm (a) and

475 nm (b), and calculated spectrum from Model-R (c).


wavelength. However, a systematic change of theemission spectra was observed under the excitation

of the long wavelength wings of the absorption.

The main peak and side band were shifted to the

lower energy side. The width of the main peak was

sharpened. The calculated emission spectrum of

the 1G4–3F4 transition from Model-R at 20 K is

shown in Fig. 5(c). Though the width of the main

peak was slightly narrower, and the height of theside band was slightly higher than the observed

ones, the shape of the spectrum in Fig. 5(a) was

reproduced substantially.

4. Discussion

4.1. The relation between the splitting of the energy

levels and the crystal-field parameters

On the basis of the crystal-field theory and

point charge approximation, the value of the

crystal-field parameter Akq can be written as fol-lows:

Akq ¼ � e2

4pe0

Xj

qjRkþ1j

CkqðRjÞ; ð1Þ

where qj is the charge on the jth ion in the struc-

tural model, and Rj is the distance between the jthion and the Tm3þ ion. The value of the Akq term

with a higher k converges at the shorter distance

from the central rare earth ion. Therefore, the

value of the Akq term with a higher k is dominated

by the closer structure around the rare earth ions,

such as the first coordination number and shape.

In the previous section we showed that the calcu-

lation could reproduce the observed absorptionand emission spectra. The characteristics of the

splitting of each level for the Akq are examined

from the calculated energy splitting of 12 levels

between the 3H6 and 3P2 levels. The crystal-field

parameters were classified into three groups by

their order: A2q, A4q and A6q terms. The energy

splitting of each level calculated only from the

parameter of each group was compared with theoriginal splitting, which was calculated from full

sets of the crystal-field parameters. If the contri-

bution of a certain group of the crystal-field pa-

rameter is large for the splitting of an energy level,

the energy calculated only from the group must be

correlated with the original value of the energy

splitting [11]. The correlation coefficients of the

calculated value from each parameter group to theoriginal calculated value are listed in Table 4. It

was found that the relation between the splitting of

the energy level and the crystal-field is not identical

at each level. There were no levels in which the

value of CA6q was the largest among the values of

CA2q, CA4q and CA6q. The values of CA4q for3F4 and

1G4 levels were 0.908 and 0.862, respectively, and

these values were the largest values for the 3F4

and 1G4 levels. The values of CA2q for the3F4 and

Table 4

The correlation coefficients CA2q, CA4q and CA6q between the

energy positions of Stark components of each level calculated

from individual A2q, A4q and A6q terms and the energy positions

calculated form the full set of the crystal field parameters from

Model-R

Level CA2q CA4q CA6q

3H6 0.797 0.489 0.2133F4 0.141 0.908 0.2833H5 0.858 0.433 0.1043H4 0.726 0.483 0.3773F3 0.947 0.214 0.3213F2 0.879 0.408 0.0031G4 0.361 0.862 0.2541D2 0.994 0.117 0.0051I6 0.885 0.408 0.0473P0 – – –3P1 1.000 0.004 0.0003P2 0.828 0.471 0.005


1G4 levels were 0.141 and 0.361, respectively. The

values of CA2q for the remaining of the levels were

the largest. The larger value of CA2q indicates the

larger effect from the longer-range structure on the

splitting of the energy level. Therefore, it is ex-

pected that information on the short-range struc-

ture similar to the first-coordination sphere will

appear in the splitting of the 3F4 and 1G4 levels.

4.2. The calculated splitting of energy levels and

transition rate from Model-F

In order to verify the characteristics of the

splitting of the 3F4 and1G4 levels and to select the

transition for the measurement of the splitting of

the 3F4 level, the splitting of the energy levels andtransition rates between energy levels were calcu-

lated from Model-F. At first, the splitting of each

level, which is calculated from Model-F, is com-

pared with that calculated only from the first co-

ordination polyhedron. The distribution curves of

the energy levels calculated from Model-F whose

first coordination polyhedron was the anti-prism

are shown in Fig. 6 together with the position ofthe energy levels calculated only from the polyhe-

dron. The distribution curves were obtained from

50 models using the Gaussian shape function with

20 cm�1 of FWHM. If the splitting of a certain

level is dominated by the effect of the first coor-

dination polyhedron, the narrow distribution ofthe Stark component is expected at the position

obtained only from the first coordination polyhe-

dron with the height corresponding to the degree

of degeneracy. As can be seen from the figure, the

distribution curves showed the original splitting

for the first coordination polyhedron to some ex-

tent. The width of each Stark component of the3F4 level was narrower than those of other levels.It was found that the splitting of the 3F4 level was

dominated by the effect from the first coordination

polyhedron. Though the width of each Stark

component of the 1G4 level was wide, the shape of

the distribution curve of the 1G4 level corre-

sponded more closely to the original splitting than

those of the other levels except the 3F4 level. It is

confirmed that the splitting of both the 3F4 and1G4 levels, whose C4Aq coefficient values were lar-

ger, were dominated by the first coordination

polyhedron. There was a similar tendency for

other first coordination polyhedra. Therefore, to

analyze the first coordination polyhedron of the

Tm3þ ion, the splitting of the 3F4 and 1G4 levels

should be examined.

Next, the emission spectra from the lowestStark component of the individual levels to the 3F4

level were calculated from the distribution of the

Stark components and the transition rates between

the components for Model-F. The calculated

emission spectra from Model-F whose first coor-

dination polyhedron was the anti-prism are shown

in Fig. 7 together with the spectra calculated only

from the polyhedron. As can be seen from thefigure, the emission spectra of the 1G4–

3F4 and3P0–

3F4 transitions retained the spectra calculated

only from the polyhedron to some extent. The

emission spectra of the 1G4–3F4 and 3P0–

3F4

transitions for the other polyhedra are shown in

Fig. 8. It was found that the splitting width of the3F4 level decreases with an increase of the coor-

dination number. The position of the spectrumresembled each other on the spectra for the anti-

prism and dodecahedron configurations. The shape

on the 3P0–3F4 spectrum of 9-fold coordination

differed from the spectra for other coordination

polyhedra. The shortest wavelength of laser oper-

ation was 440 nm in the equipment used. There-

fore, we could not use the levels above the 1G4

Fig. 6. The distribution curves of the energy levels of Model-F, whose first coordination polyhedron was anti-prism. The straight lines

at the upper part show the positions of the Stark components of the energy levels calculated only from the anti-prism polyhedron. The

thin lines show the distribution of individual Stark components and the thick lines show the total of them. The S, D and Q indicate the

degeneracy of the components, (a) 3H6, (b)3F4, (c)

3H5, (d)3H4, (e)

3F3, (f)3F2, (g)

1G4 and (h) 1D2 levels.


Fig. 7. The emission spectra calculated from Model-F, whose first coordination polyhedron was anti-prism. The straight lines at the

upper part show the positions of the Stark components of the 3F4 level calculated only from the anti-prism polyhedron. The thin lines

show the spectra calculated only from the anti-prism polyhedron and the thick lines show the spectra calculated from Model-F.

(a) 3H5–3F4, (b)

3H4–3F4 (c) 3F3–

3F4, (d)3F2–

3F4, (e)1G4–

3F4, (f)1D2–

3F4, (g)1I6–

3F4 and (h) 3P0–3F4 transitions.


level. The measurement of the emission spectrumfor the 3P0–

3F4 transition and to the 1G4 level is a

problem for future consideration. Only the spec-

trum for the 1G4–3F4 transition is discussed.

4.3. Comparison of the observed emission spectra

and the calculated spectra from Model-R

The relation between the coordination numberof the Tm3þ ion and emission spectra for the 1G4–3F4 transition is discussed. It was shown in Fig. 8

that the splitting width of the 3F4 level calculated

from Model-F decreased with an increase in the

coordination number. The emission spectra for the1G4–

3F4 transition at 20 K calculated from Model-

R are shown in Fig. 9. The structural models of

Model-R were classified according to the coordi-nation number of the Tm3þ ion. It was found that

the width of the spectra calculated from Model-R

decreases with an increase in the coordination

number in the same manner as that of Model-F.

The observed emission spectra are shown in Fig.5(a) and (b). The spectrum in Fig. 5(a) is attributed

to the various sites of the Tm3þ ions because the

excitation wavelength is in the middle of the ab-

sorption band of the 1G4 level. The spectrum in

Fig. 5(b) was observed under the excitation of the

long wavelength wings of the absorption band.

Therefore, the shape of the spectrum in Fig. 5(b)

must be included in the spectrum in Fig. 5(a). Wetried to subtract the contribution of the spectrum

in Fig. 5(b) from the spectrum in Fig. 5(a), and the

residual spectrum is shown in Fig. 10. Though the

proportion of the contribution was not known, we

assumed the contribution of 30% from the shape

of the spectra. The shape of the spectrum in Fig.

10 was good agreement with the spectrum in Fig.

9(b), which was calculated from Model-R, inwhich the Tm3þ ions were 8-fold coordinated. On

the basis of the structural model and the spectral

calculation, it is confirmed that the 8-fold coordi-

nated Tm3þ ions exist in the glass based on AlF3.

Fig. 8. The emission spectra calculated fromModel-F. The thin

lines show the spectra calculated only from the coordination

polyhedron and the thick lines show the spectra calculated from

Model-F. (a) 1G4–3F4 and (b) 3P0–

3F4 transitions for the pen-

tagonal bipyramids, (c) 1G4–3F4 and (d) 3P0–

3F4 transitions for

the triangulated dodecahedron, (e) 1G4–3F4 and (f) 3P0–

3F4

transitions for the trigonal prism with the rectangular faces

capped.

Fig. 9. The emission spectra for the 1G4–3F4 transition calcu-

lated from Model-R, in which coordination number of Tm3þ

ions were (a) 7, (b) 8 and (c) 9.

Fig. 10. The residual emission spectrum (––) for the 1G4–3F4

transition obtained from the spectra in Fig. 5(a) and (b) and the

calculated spectrum (� � �) from the Tm3þ ions, of which the

coordination number was 8 in Model-R.


The remaining problem is the assignment of the

spectrum in Fig. 5(b), whose energy differencebetween the main peak and side band is slightly

wider and whose peak position is located at a 40cm�1 lower energy side than the spectrum in Fig.

5(a). There are alternative explanations for this.

One is attributed to the 7-fold coordinated Tm3þ

ion. The other is that the spectrum is due to the

distorted 8-fold coordinated Tm3þ ion. The 7- and

8-fold coordinated Tm3þ ions are excited at a

475 nm wavelength from the distribution of the

excitation energies for both ions calculatedfrom Model-R. The dependence of the excitation

energy on the spectra calculated from the 8-fold

Fig. 11. The calculated emission spectra for the 1G4–3F4 tran-

sition from the Tm3þ ions, of which the coordination number

was 8 in Model-R. The excitation wavelengths were (a) 466.6–

468.6 nm, (b) 468.8–471.0 nm, (c) 471.0–473.2 nm and

(d) 473.2–475.5 nm, respectively.


coordinated Tm3þ ions is shown in Fig. 11. The

width of the main peak decreases, and the side

band spreads to the lower energy side with a de-

crease in the excitation energy. The shapes of bothspectra of the 7-fold coordinated Tm3þ ions and

the 8-fold coordinated Tm3þ ions excited at the

lower energy side are almost the same and similar

to the spectrum in Fig. 5(b). Therefore, neither

explanation can be denied. However, the calcu-

lated position of the main peak is not shitted. The

near-neighbor structures of several rare earth ions

in fluorozirconate glasses have been studied byextended X-ray absorption fine structure (EXAFS)

[16]. The near-neighbor F� ions are classified into

three subshells according to the distances between

rare earth ions. There are 7–8 F� ions at 0.24 nm

in the first two subshells. The number of the F� ion

of these two subshells appears in the emission

spectrum of the 1G4–3F4 transition, when a similar

coordination structure is assumed in the glassbased on AlF3. However, the proportion of the 9-

fold coordinated Tm3þ ion in Model-R was 36%.

The spectrum corresponding to the 9-fold coordi-

nated Tm3þ ion was not observed. It is not clear

whether there is the 9-fold coordinated Tm3þ ion

in the glass, or whether the spectrum due to a 9-fold coordinated Tm3þ ion is different from the

calculated one. These problems can be clarified by

examining the emission spectrum for the 3P0–3F4

transition and the emission spectrum to the 1G4

level.

5. Conclusion

On the basis of point charge approximation,

crystal-field parameters were obtained from

structural models prepared by using moleculardynamics simulation. Absorption and emission

spectra of Tm3þ-doped glass based on AlF3, which

were estimated from crystal-field parameters, were

in a good agreement with the observed spectra.

The splitting of the energy levels and emission

spectra of the Tm3þ ion were calculated from the

structural models with fixed coordination polyhe-

dra of the Tm3þ ion. It was found that the splittingof the 3F4 and 1G4 levels represented the coordi-

nation number of the Tm3þ ion, and the splitting

of the 3F4 level could be evaluated from the

emission spectrum of the 1G4–3F4 transition. The

observed emission spectrum of the 1G4–3F4 tran-

sition at 20 K resembled the emission spectrum

calculated from the models of 8-fold coordinated

Tm3þ ions. It was tentatively concluded that theTm3þ ion in the fluoride glass based on AlF3 was

mainly coordinated by 8 F� ions.

Acknowledgements

This study was financially supported by a

Grant-in-Aid from the Ministry of Education withthe contract number #09450239. The authors

would like to thank Morita Chemical Industries

Co. and Central Glass Co. for the supply of fluo-

rides.

References

[1] S. Sudo, Optical Fiber Amplifiers: Materials, Devices, and

Applications, Artech House, Boston, MA, 1997, and

references therein.

[2] C. Brecher, L.A. Riseberg, Phys. Rev. B 13 (1976) 81.

[3] C. Brecher, L.A. Riseberg, Phys. Rev. B 21 (1980) 2607.


[4] V.K. Tikhomirov, S.A. Tikhomirova, J. Non-Cryst. Solids

274 (2000) 50.

[5] S.A. Brawer, M.J. Weber, Phys. Rev. Lett. 45 (1980) 460.

[6] G. Comier, J.A. Capobianco, Europhys. Lett. 24 (1993) 743.

[7] M.T. Harrison, R.G. Denning, J. Lumin. 69 (1996) 265.

[8] K. Soga, H. Inoue, A. Makishima, S. Inoue, Phys. Chem.

Glasses 36 (1995) 253.

[9] H. Inoue, K. Soga, A. Makishima, J. Non-Cryst. Solids

222 (1997) 212.

[10] K. Soga, H. Inoue, A. Makishima, J. Non-Cryst. Solids

274 (2000) 69.


298 (2002) 270.


306 (2002) 17.

[13] T. Nanba, H. Inoue, Y. Arai, H. Hagihara, I. Yasui,

Mater. Sci. Forum 32&33 (1988) 385.

[14] Y. Akasaka, T. Nanba, H. Inoue, T. Osuka, I. Yasui, J.

Non-Cryst. Solids 140 (1992) 249.

[15] R.D. Shannon, C.T. Prewitt, Acta Cryst. B 25 (1969) 925.

[16] W.-C. Wang, Y. Chen, T.-D. Hu, J. Appl. Phys. 79 (1996)

3477.

Documents

Structure around the Tm3+ ion in a glass based on AlF3