STRUCTURE OF FLUOROPHOSPHATE GLASSES

STRUCTURE OF FLUOROPHOSPHATE GLASSES

Computer Aided Innovation of New Materials M. Doyama, T. Suzuki, J. Kihara and R. Yamamoto © Elsevier Science Publishers B.V. (North-Holland), 907 (...

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Computer Aided Innovation of New Materials M. Doyama, T. Suzuki, J. Kihara and R. Yamamoto © Elsevier Science Publishers B.V. (North-Holland),

907

(Editors) 1991

STRUCTURE OF FLUOROPHOSPHATE GLASSES H. Inoue and A. Makishima

Department of Materials Science, Faculty of Engineering, University of Tokyo, Bunkyo-ku, Tokyo 113, Japan

Molecular dynamics simulation have been made on fluorophosphate glasses in order to investigate the structure around P and O atoms. It was found that O atoms grouped around P atoms and PO4, PO3F, PO2F2 and POF^ tetrahedra were formed. Average number of O atoms in PX4 (X = 0,F) tetrahedra depended on oxygen con­ tent in a basic cell. The vibrational frequencies of A l F and PX4 polyhedra estimated are compared to the experimental ones. n

1. INTRODUCTION Fluorophosphate glass can be use for high power lasers as a host glass.* The nonlinear index of this glass is lower than those of silicates or phosphates glasses because the hyperpolarizability of the fluorine anion is smaller than that of oxygen.^ Schott(LG812), Owens Iilinois(E-309) and Hoya(LHG-lO) are known as typical fluorophosphate laser glasses. These glasses contain AlF^, Cap2 and a little P205 compo­ nent. The optical and physical properties of the fluorophosphate glass have been investigated and the spectroscopic properties of Nd in this glass have 1 1 "i 1 been reported. ' The effective linewidth of Nd for this glass is larger than for phosphate or BeF2~ based glasses. This broadened spectrum is attributed to a distribution of oxygen and fluorine coordination around Nd . An understanding of local Fields and interactions at an activator ion site in this glass is important for the laser applications. We have investigated the structure of AlFybased

glasses by means of x-ray and neutron diffraction analyses, molecular dynamics (MD) simulation and Raman s p e c t r a . ^ Good agreement has been ob­ tained between observed radial distribution functions and those calculated from the structural models simulated using MD method. In this study, we have estimated the vibrational frequencies of Al-F, P - 0 and P-F bonds in AlF^-based glass and fluorophos­ phate glasses using MD simulation to verify the atomic potentials. Furthermore, the structure around a P atom in the mixed anion ( oxygen and fluorine) glasses will be discussed. 2. MOLECULAR DYNAMICS SIMULATION Our simulations consisted of about 200 atoms in a cubic basic cell. The volume of a basic cell at 5000 K was 2 times larger than that calculated from the ob­ served density at room temperature and was reduced with temperature. Initial coordinates of atoms were given at random and the temperature was lowered by

Table 1 Number of atoms in the basic cells and sizes of cells at 300 K. system 40AlFv40CaF '20BaF fluorides + P 0 fluorides 4- A 1 P 0 flourodes + A l ( P 0 ) 3 2

4

4

3

2

Mg Ca Sr Ba Al P F

O total

size(A)

5 5 5

4 36 27

14.13 13.67 13.94 14.12

24 18 18 18

5 5 5

12 5 5 5

24 25 25 25

1 9 9

144 138 114 132

204 201 217 226

908

reducing the average kinetic energy of the atoms finally to 300 K in 1 . 8 x l 0 " s ( 18000 steps ) . The inter atomic potential energy included a Coulomb term and a Born-Mayer repulsion for atoms i and j of the form B^exp(-r/ p ) . The Coulomb potential was evaluated as the Ewald method. Time interval,At, in Verlet's algorithm took a value of 1x10"^ s. Potential parameters were taken as the same values as those used in the simulations of AIFybased '** and N a 0 A i 2 ^ 3 ~ ^ 2 ^ 5 glasses. The parameters for atomic pairs excluded in these studies were estimated from the ionic radii. The values of potential parameters, compositions and sizes of cells at 300 K were listed in Tables 1 and 2. The vibrational spectra were estimated by the Fourier transformation of the time-dependent auto­ correlation functions of the atomic distances in the structural units, such as A l F and P X (X = 0 , F ) . The autocorrelation function were obtained by the simulation for 10000 steps at 300 K. To avoid distort­ ing the spectra as the result of the Fourier transfor­ mation of the finite time histories, the four-term 74 dB Blackman-Harris window^was used. Computations were made with HITAC S-820 computer in the Computer Center, Institute for Molecular Science, Okazaki National Research Insti­ tute. n

4

2

Q

4

3. RESULTS AND DISCUSSION 3.1 40AlF -40CaF '20BaF GLASS Firstly, the vibrational spectra of A l F polyhedra in 40AlF '40CaF -20BaF were calculated from MD simulation for 10000 steps at 300 K. The structure feature of this glass have been described previously.^ 3

2

The parameters of the pair potential, which used in MD simulations for the diffraction and vibrational analysis, are listed in Table 2. In Fig.l, the spectrum was calculated using the equation I(CJ)=

S_5 dt exp(-i a; t) f(t)

(0

T

f(t)=Z

R ,_ A

F

in A1 F

polyhera ( 2 )

n

where l(oS) is the intensity, CD is frequency, R is the AlF distance in A l F polyhedra. In this calculation the only symmetric stretching vibrational mode of AlFn polyhedron is active. The strong band located in the range of 550 to 680 cm~* was assigned to the stretching vibration of A1F polyhedra. Kawamoto et al. have been reported that the stretching vibrational band with high depolarization locate at 550 cm-1 in Raman spectra of 4 0 A l F * 3 9 C a F ' 2 1 B a F glass. Frequency of stretching mode shifted to a little higher than the observed one. 3.2 A P 0 UNIT IN AlF^BASED GLASS Secondly, the simulations of AlF ~based glass with a PO4 unit were carried out. In this case, the initial coordinates of a PO^ tetrahedron were given at the center of a basic cell and other atoms were given at random. Though a P-O bond was the strongest bond in a basic cell, a PO4 tetrahedron did not always retain its P-O bonds on quenching. When all 4 O atoms in the tetrahedron were coordinated with Al atoms by chance, t h e P 0 tetrahedron retained its n

Q

3

2

2

4

3

4

2

n

3

2

2

Table 2 Repulsive constants B . . ( x l 0 " J ) iO

Ca Ba Al P F O

Ca 3.86

Ba 7.95 18.43

Al 2.25 4.34 1.95

P 2.34 4.84 1.63 1.33

p ; empirical constant = 0.3 ( A ) , a) = 0.25, b ) = 0.18, c) = 0.32, d) = 0.35

F 2.42 5.59 1.30 1.86 ) 0.84 a

O 13.87 ) 41.03^ 5.27 [ 18.02°) 1.06°' 1.18 ) a

a

0

100

200

300

400

500

600

700

800

900

1000 1100

W a v e n u m b e r (cm* )

d

Fig.1 The vibrational spectrum of A l F polyhedra in the structure model for 40AlF '40CaF '20BaF glass. 3

2

2

n

909

structure. In other cases, a F atom displaced an O atom which was not coordinated with an Al atom and P O F tetrahedron was formed. The dissociated O atom was coordinated by two AI atoms, such as Al-O-Al bond. These results were suggested that P-O-Al and Al-O-Al bonds were stable and Alkaline earth atoms did not coordinated an O atom in lowoxygen-content fluorophosphate glasses. In Fig.2(a) and (b), the vibrational spectra of PO4 and PO^F tetrahedra calculated using equation (1) and (2') were shown with the vibrational modes of 8 x

4 x

Rp-

X

(a)

F,

I

T

A

i

F

1

1.0 0.8 0.4 0.2

in PX t e t r a h e d r a ( 2 ' ) 4

0.0

As the vibrational modes belonging to A l were ac­ tive, the band at 1200 cm" in Fig.2(a) corresponded to the A l mode at 950 cm" , which is assigned to the stretching vibration of PO^ . And in Fig.2(b), the bands at 1250 and 650 cm" corresponded to the A l modes at 1008 and 705 cm" , respectively. Further­ more, the vibrational spectra of P-F and P-O bonds in this PO^F tetrahedra were shown in Fig.2(c) and (d), respectively. It was found that the higher band at 1250 cm" was due to PO3 and the lower band at 650 cm" to P-F bond. Our results corresponded to the Buhler's assignments. 3.3 FLUOROPHOSPHATE GLASSES It was expected in the structure of fluorophos phate glasses that the number of F atoms in the PX4 tetrahedron depended on the atomic ratio of oxygen to phosphorus ( O/P ). The MD simulations were made on two systems: fluorides + AIPO4 ( O/P = 4 ) and fluorides + A \ ( ? 0 ^ 2 ( O/P = 3 ). The composi­ tions were made by replacing AlF^ and aluminum phosphates and listed in Table 1. All O atoms were coordinated with P or Al atoms, as shown in Fig.3. 67% and 60% O atoms in basic cells belonged to PX4 tetrahedra in fluorides + AIPO4 and fluorides + A ^ P O ^ systems, respectively. The proportions of PX4 tetrahedra, which were classified by the number of O atoms, were listed in Table 3 with those of PX4 tetrahedra coordinated by O and F atoms randomly. It was concluded that O atoms grouped around P atoms. Average number of O atoms in PX4 tetrahe­ dra were 2.7 and 1.8 in fluorides + AIPO4 and fluo­

I

W E

1

1

1 A

1

1

E

0.6

these tetrahedra reported by Buhler et al. f(t)=Z

rides + A\(FO^)^ systems, respectively. These values depended on the O/P ratio. In Table 4, the propor­ tion of P-O-AI and Al-O-Al bonds were higher and there was no R-O-R bond (R = alkaline earth atom). 30% O atoms coordinated with alkaline-earth atoms. It is essential for detailed analysis to examine the

E A, II

1 1.0 0.8 0.6 0.4 0.2

Aj E

1

1 .

lL .

0.0 1

1

—L

(c)

1

l

1

1 1

1 1

1 1

1 1 —

.. .A

1 1



1



1

1

1.0 0.8 0.6 0.4 0.2 0.0 1

1

1

400

600

1 1

1

1

(d)

1.0 01 0.6 0.4 0.2 0.0

1 200

800 1000

Wavenumber

1

1

1200 1400 1600

(cm" ) 1

FigJZ The vibrational spectra of (a) P 0 ,

(b) P0 F, (c) P-F and (d) P-O in PO3F 3

tetrahedron in the structure models for fluorophosphate glasses.

4

910

F i g 3 The structural model the

glass

for

on fluorides

and

A 1 P 0 system. 4

Mg, CA, Sr and Ba,

0,O ,OAlandP p

Table 3 T h e p r o p o r t i o n s fo P X t e t r a h e d r a .

Table 4 C o o r d i n a t i o n a r o u n d O atoms.

4

+ A1P0 system 4

PO

4

P03F P0 F POF PF 2

3

4

2

(A)

(B)

11.2 44.4 44.4

0.3 (%) 4.0 20.0 42.8 32.9

-

+ A L ( P 0 ) system (A) (B) 3

77.8 22.2

-

+ A1P0 system

+ A1(P0 ) system

5.6 (%) 41.7 16.7 19.4 16.7 0.0

3.7 40.7 7.4 25.9 22.2 0.0

4

3

3

3

0.1 (%) 1.5 11.8 39.5 47.1

P-O-P P-O-Al P-O-R Al-O-Al Al-O-R R-O-R

(%)

R is an alkaline e a r t h a t o m .

influence of quenching rates and cell sizes on these structural units. The structures around O and P atoms in the models with initial coordinates given at random were consistent with those in the model

2) MJ.Weber, D. Milam and W.L.Smith, Opt. Eng. 17 (1978) 463. 3) O.Deutschbein, M.Faulstich, WJahn, G.Krolla and N.Neuroth, Appl. Opt. 17 (1978) 2228.

with a PO^ unit.

4) T.Nanba, H.Inoue, Y.Arai, H.Hasegawa, M. Misawa and I.Yasui, Mat. Sci. Forum, 32&33 (1988) 385. 5) H.Inoue, T.Nanba, H.Hasegawa and I.Yasui, Mat. Sci. Forum, 32&33, (1988) 403. 6) FJ.Harris, Proc. IEEE 56 (1978) 51. 7) Y.Kawamoto and A.Kono, J. Non-Cryst. Solids, 85 (1986) 335. 8) V.K.Buhler and W.Bues, Z. Anorg. Allgem. Chem., 308 (1961) 62.

ACKNOWLEDGEMENTS The authors would like to express their thanks IKETANI SCIENCE and TECHNOLOGY FOUN­ DATION for financial support of this work. REFERENCES 1) S.E.Stokowski, W.E.Martin and S.M.Yarema, J. Non-Cryst. Solids, 40 (1980) 481.