Crystal structures of Eu(II) substituted barium magnesium fluorides: Ba_0.78(3)Eu_0.22(3)MgF₄ and Ba_5.20(6)Eu_0.80(6)Mg₇F₂₆

Article

Reference

Crystal structures of Eu(II) substituted barium magnesium fluorides:

Ba

_0.78(3)

Eu

_0.22(3)

MgF

₄

and Ba

_5.20(6)

Eu

_0.80(6)

Mg

₇

F

₂₆

KUBEL, Frank, HAGEMANN, Hans-Rudolf, BILL, Hans

Abstract

Barium in BaMgF[4] and in Ba[6]Mg[7]F[26] can be partially replaced by Sr or Eu. The single crystal structural analysis of the title compounds (a = 409.06 pm, b = 1452.7 pm, c = 579.02 pm, space group Cmc2[1] for Ba[0.78(3)]Eu[0.22(3)]MgF[4] [Z = 2, R[w] = 0.023 for 998 reflections] and a = 583.47 pm, b = 1209.09 pm and c = 1506.56 pm, space group Immm for Ba[5.20(6)]Eu[0.80(6)]Mg[7]F[26]) MgF[4] [Z = 2, R[w] = 0.019 for 2694 reflections] confirms the substitution of barium by europium. A split refinement of the substituted barium site in both crystals reveals a trend of europium to get closer to the nearest fluoride neighbors by simultaneously reducing the coordination sphere.

KUBEL, Frank, HAGEMANN, Hans-Rudolf, BILL, Hans. Crystal structures of Eu(II) substituted barium magnesium fluorides: Ba

_0.78(3)

Eu

_0.22(3)

MgF

₄

and Ba

_5.20(6)

Eu

_0.80(6)

Mg

₇

F

₂₆

. Zeitschrift für Kristallographie , 1999, vol. 214, p. 139-142

DOI : 10.1524/zkri.1999.214.3.139

Available at:

http://archive-ouverte.unige.ch/unige:3710

Disclaimer: layout of this document may differ from the published version.

©

by

^R. Oldenbourg Verlag.^München

Crystal ^{structures of Eu(II)} substituted ^barium magnesium ^fluorides:

Ba0.78(3)Euo.22(3)MgF4 ^and Ba5.20(6)Euo.80(6)Mg7F26

F. Kübel*. ^H.

Hagemann

and H. Bill

Université^deGenève. Departement^deChimie

Physique,

³⁰q. E. Ansermet.CH-1211 Genève4, Switzerland Received June 2, 1998;

accepted

^{October 14. 1998}

Abstract. Barium ⁱⁿ

BaMgF4

^{and in}

Ba6Mg7F26

^{can be}

partially replaced ^by

^{Sror Eu. The}

single crystal

^structural

analysis

^{of the title}

compounds ^(a

^{=409.06 pm.}

b= 1452.7_pm, ^c= 579.02pm, space group

Cmc2\

^for

Ba,,.7S(3,Eu,).22(3,MgF4 ^[Z

⁼

^{2, Rw}

^{=0.023 for} 998 reflec-

tions]

^{and a =}583.47 pm, ^b= 1209.09pm and ^c= 1506.56 pm, space ^group Immm for

Ba5.20(6)Euo.go(6)

^•

Mg7F26) ^MgF4 [Z=2, ^Rw

^{= 0.019 for 2694}

reflections]

confirms the substitution of ^barium

by europium.

^A

split

refinement of the substituted barium site ⁱⁿ both

crystals

reveals a trend of

europium

^to

^get

^{closer to the nearest}

fluoride

neighbors by simultaneously reducing

^{the coordi-}

nation

sphere.

Introduction

Alkaline earth fluorides are excellent

optical

^host

crystals

which _are

transparent

^{well below 200nm. Certain rare-} earth

doped

^{hosts of this}

family

^exhibit

particularly

^inter-

esting phosphor

^{behaviour. We chose}

^Eu(Il) doped

^hosts

as model

systems

^to

investigate

^{in more detail} the relevant

properties ^by spectroscopic ^{techniques [1]}

^and

^X-ray

^dif-

fraction.

Compounds

^ofthe

^{type BavMvF-}

^{whereM=Mn—Zn}

and

Mg

^{have been studied in the}

^past [e.g. 2-4].

However,

the

single crystal ^X-ray

^{structures of the}

three bariurn

magnesium

^fluorides

BaMgF4, Ba2Mg3F|o [5]

^and

Ba6Mg7F26 ^[6]

^have

only

^been

reported

^{very re-}

cently.

^{It has been shown}

previously

^that

^BaMgF4

^can

form solid solutions without

structural change

^ofthe

type

Ba1_.vM.ïMgF4

^{with M=Eu}

^[7]

^{or M=Sr}

^[8]

^{with x ran-}

ging

^{from 0 to ca 0.55.}

EuMgF4

^and

SrMgF4

^{do not ap-}

pear^to

crystallize

^{in the same} space group ^as

BaMgF4 ^[9].

The

compound ^Ba6Mg7F26

^{does not melt}

congruently.

^It

has been

synthesized

either from a

non-equilibrium

^cooled

barium-magnesium

^{fluoride melt or}

using

^{a LiF flux}

^[6].

This latter method ^has allowed ^{us to} prepare and ^charac-

*

Correspondence

^author

(e-mail^: frank.kubel+el71@tuwien.ac.at)

Present address: Prof. Dr. Frank Kubel. Institut für

Mineralogie.

Kristallographie

^und Strukturchemie, Getreidemarkt ^{9. A-1060} Wien, Austria

terize

Ba5.24(4)Sr().76(4)Mg7F26 ^[10].

^{We will}

^present

^below

the structures of ^{the two title}

compounds

^{and compare}

them with ^the

parent

^{structures as well as with the struc-}

tures of

Sr-substituted analogs.

Experimental

Single crystals

^of

Bai_vEuAMgF4

with nominal jc-values of 0.25 were

prepared by

^slow

cooling

^{of a melt}

prepared

from

high purity ^{BaF2, EuF2}

^and

MgF2.

^{The melt was}

contained

ⁱⁿ a

graphite

crucible and

kept

^{under an inert}

atmosphere. Special

^{care was taken to} avoid contamination

by

oxygen ^{and water.}

Using

^a

^microscope operating

^under

polarized light,

^we

inspected

^the

crystalline

^{mass. We}

^found,

^{besides a}

major- ity

^of

BaMgF4-type crystals (optically ^biaxial),

^several

crystal

^{needles with a similar}

^shape

^{as observed pre-}

viously [6]

^for

Ba(,Mg7F26 (optically ^uniaxial).

^{This pre-}

paration provided

^the

single ^crystals

used for the

crystal

structure determinations

reported

^below.

Table ¹ summarizes ^the

experimental

^{conditions for the}

single crystal ^X-ray

^structure determinations ^of both crys- tals.

Results and discussion

A.

Ba„.78Euo.22MgF4

As observedfor theSr substituted

samples

ⁱⁿ

BavSri_vMgF4 [8],

^our

synthesis

^conditions

^{(cooled melt)}

^{led to ferrroe-}

lectrically single

^domain

crystals,

whereas the

BaMgF4 sample

^obtained

^by

solid reaction

[5] presented

^{~50% of}

ferroelectric

^domains.

The

crystal

^lattice

parameters

^{of this}

compound

^are

slightly

smaller than those

reported

^{for a}

crystalline powder

^{of nominal}

composition Bao.8.sEuo.i5MgF4 ^[7]

(a

⁼

409.06(2)

^pm,

^b

⁼

^1452.7(

¹

⁾

pm, ^c=

579.02(3)

pm and ^{V =}

344.08(3) 106 pm3

^{vs a =}

^411.2(2)

^{pm, b=}

1452.8(4)

pm and c⁼

581.2(2)

pm and

V⁼

347.2(2) 106pm3).

^These reduced unit cell param-

eters confirm ^a further substitution of

Ba2+ by ^Eu2+.

Using

^the

reported

^{unit cell volumes of}

^EuMgF4 (320.41 106pm3) ^[9]

^{and of}

^{BaMgF4 (348.7 106pm3) [5],}

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140 F. Kübel, H.

Hagemann

^{and H. Bill}

Table1.

Crystal

^{data(e.s.d.'s inbrackets)} and conditions of data col- lection and refinement for

Ba5.20(6)Euo.8o(6)Mg7F26

^and

Bao.îsai

Hu

0.22U)MgF4

Formula

Ba5.2()(6)Euo.sO(6|Mg7F2(, Bao.78(3)EuU.22(3)MgF4

Formulamass

(g mor1) Space

group

a

(pm)

b(pm)

c(pm)

V(106pm3)

Z

Calc.

density

(g

cm"')

Crystal ^{size (mm')}

Diffractometer type

Wavelength.

radiation Method

Temperature

0 range(°)

hkl range No. of refl.

mesured No. of

independent

^refl.

No. of refl.

with />3(7(7) Program^used Absorption

coefficient Abs. corr.

anal. Tmin/Tmax

R(Rwbased

on

I/o2)

Number of parameters Goodness of fit Extinction

coefficient Absolute

structurefactor e"

densitymin/max

(KT6 pm"3)

1499.7 Immm(No. 71)

583.47(1) 1209.09(10) 1506.56(14) 1062.83(13) 2

4.695

240.9

Cmc2, ^{(No. 36)} 409.06(2) 1452.70(10)

579.02(3) 344.08(3) 2 4.658

0.082 x0.095 ^x0.230 0.032^x0.072^x0.096 STOE

Mo/C„ 71.073 pm

2.2-50.0 -8 ^-> 8, -31 -12^-» 12 11355 3106 0.035 2694

12.45 0.055/0.085 0.024 (0.019)

69 1.75

815(17)

-3.9/1.4

(u/0-scan 300 K

2.8-50.0

31, -12 ^-> 12.

-26 ^->26.0 ^-> 32 7798

1038 0.039 998 XTAL 3.2 13.32 0.031/0.066 0.024(0.023) 40

1.93 177(8) 0.00(3) -1.5/2.0

Table 2.

Partially

standardized (metals and fluoridesare

separated

^for clarity) ^{[I4J atomic}

positional

parameters, atomic

displacement

^fac-

tors and site

occupation

parameters of Bao

78(3)Euo.22(3)MgF4

with e.s.d.'s in

parenthesis.

Atom U ^PP

Ba Eu

Mg

F(l) F(2) F(3) F(4)

0.3523(3) 0.3411(5) 0.0850(¹) 0.0263(2) 0.1646(2) 0.1961(2) 0.5802(3)

0.36655"

0.36655"

0.3282(2) 0.0143(5) 0.5962(6) 0.1335(5) 0.3181(6)

0.0146(2) 0.0146(2) 0.0088(3) 0.0174(6) 0.0177(7) 0.0133(6) 0.0226(8)

0.78(3) 0.22(3)

a: fixed value

Atomicdisplacement parameters

[106pm2]

Atom U\ ^I 2 Ur Un U\. Uis

Ba Eu

Mg

F(l) F(2) F(3) F(4)

0.657(5) 0.657(5) 0.94(4) 2.5(1) 1.8(1) 1.5(1) 0.62(7)

2.11(6) 2.11(6) 0.83(4) 1.35(9) 2.0(1)

1.02(9) 3.6(2)

1.601(9) 1.601(9) 0.85(6)

1.4(1) 1.5(1) 1.4(1) 2.6(2)

-1.101(9) -1.101(9) 0.09(3) -0.59(8) -0.6(1)

0.3(8) 0.7(1)

|The

anisotropic displacement

factor in the ^structure factor

expression

is:

Uij

⁼exp

-

2jr(c/|,/iV2

⁺

U22k2b"2

⁺

(,.;/r

^;+

2Unhka"b*

⁺

+

lUnhlac*

⁺

2U2iklb'c)].

The

comparison

^{of the} structural data ^of

BaMgF4 [5|. ^BaZnF4 [llj

^and

Ba^Mg^,, [6],

^{as well as the}

Eu-substituted

crystals

^{reveals a}

significant ^anisotropy

of the atomic

displacement parameters

ⁱⁿ

BaMgF4

and

Bao.7S(3)Eu().22(3)MgF4 ^{(see Fig. 2)}

^{as well as in}

Bai_xSrvMgF4 ^[7|

^{which is not seen in}

Ba6Mg7F26

^and

Ba5.20(6)Euo.80(6)Mg7F26-

A detailed

investigation

^on

^{BaZnF4 [11-13]}

^{revealed a}

structural

instability

^{related to a} rotation of the

ZnF<-,

^octa-

hedra. A similar behavior ^is

probably

^also

present

ⁱⁿ

BaMgF4

and its solid solutions.

one obtains

by

^linear

interpolation

^an estimated Eu mole fraction of 0.16 in ^the

crystal,

^{which is somewhat less}

than the nominal

composition

^{of the melt}

^(X(Eu)

⁼

^0.25).

The refined ^Eu

population

^{was found to be}

^0.22(3).

Table 2 collects the atomic

positional

^and

displacement parameters

^for

Bao.78Euo.22MgF4.

^{Table3 collects some se-}

lected bond

lengths.

The

comparison

^{of therefined}

Mg—F

^bonddistances for this

compound

with those for

BaMgF4

^and

Bai_vSr,MgF4 [8]

^{shows a trend towards a more}

symmetrical ^Mg—F

^en-

vironment with

increasing

substitution of Ba in the

crystal

(see Fig. ^1).

^The

^Mg—F

distances found in

Bao.^Sro 27MgF4 [8]

^areverysimilar^tothosein

Bao.78Euo.22MgF4. ^(see Fig. ^1).

1 Additional material ^to thispaper^{canbe ordered}

referring

^tothe

no. CSD 410308 (for

Bao.78Euo.22MgF4)

and CSD 410309 (for Ba.Y2Euo.sMg7F2(,), ^names of the authors and citation of the paper^at theFachinformationszentrum Karlsruhe. Gesellschaft für wissenschaft- lich-technische Information mbH, D-76344

Eggenstein-Leopolds-

hafen.

Germany.

The list offy/vdatais available fromtheauthorup

to^oneyear after thepublication ^hasappeared.

Table 3. Selected metal fluoride distances for

Bao.78(3)Eu().22(.?)MgF4

(pm).

Ba Eu

Eu 16.3(8)

F2 x2 258.71(22) F3 250.1(6)

F3 264.0(5) F2 x2 257.68(21)

F3 x2 265.84(23) ^{F3 x2} 262.00(24)

F4 279.2(4) ^F4 285.4(5)

Fl x2 283.3(4) ^F2 288.8(7)

F2 303.4(6) ^{Fl x2 293.7(5)}

F4 332.3(6) ^F4 337.5(4)

F4 332.3(4) ^{F4 348.5(8)}

Mg

F2 193.6(4)

Fl 194.3(3)

F3 196.9(3)

Fl 200.8(3)

F4 x2 204.73(2)

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203

206

Ü

²⁰⁴

O)202i

-o 21

o LL6)^198A

'- 196 a

S. 0

1194

_CD

£

H W 192-

190 188

«I T

MgF2 O MgF1

A MgF3

V MgF1b O MgF4

0.0 0.1 0.2 0.3 0.4 0.5 0.6 xin

Ba1_xMxMgF4

M=Sr, x=0.27andx=0.55, M=Eu,x=0.22

Fig.

^1.

Magnesium

fluoride distances in BaMgF4

[5],

and in its solid solutions with Sr and Eu.

Fig.

^{2. ORTEP}

plot

^{of the structure of}

Bao.78<3)Euo.22<3)MgF4 along

the a-axis. The

magnesium

-

fluoride bonds^are drawn for

clarity.

B.

Ba5.20(6)EUo.80(6)Mg7F26

Table 4 collects the atomic

positional

^and

displacement

parameters

^for

Bas.20(6)Eu0.80(6)Mg7F26.

Table 5 collects

some selected bond

lengths.

^{It is}

important

^{to note that}

this

compound

^{as well as}

Ba5.25Srn.75Mg7F26 ^[10] crystal-

lizes in the same orthorhombic ^{structure as}

Ba6Mg7F26 [6].

There is no indication of ^{a monoclinic}

symmetry

^which had been

reported

^for

^{Ba6Zn7F26 [4].}

In

Ba5.20(6)Euo.80(6)Mg7F26, mainly

^{one of the two Ba}

sites is

subject

^to substitution

by

^{Eu. This}

preferential

^sub-

stitution ^{takes also}

place

ⁱⁿ

Bas.25Sro.75Mg7F26 [10].

In a first refinement of the reflection intensities of

Ba5.20(6)Eun.80(6)Mg7F26,

^no

split position

^{of Ba2/Eu2 was}

assumed ^{and a}

position

^{of z=0.3500 was} refined. The electron

density

^{map revealed a small} residual electron

density

^{close to Ba2/Eu2 and the atomic}

displacement

^fac-

tor of ^Ba2/Eu2 was calculated to

U\\

⁼

0.890(5) l/22

⁼

0.735(4)

^and

U33

⁼

1.324(5) 106pm2.

^{The residual}

density

^{vanished when}

separation

of both elements was

Table 4.

Partially

standardized [14] ^atomic

positional

parameters, atomic

displacement

factors and site

occupation

parameters ^of

Ba5.2Eu().siMg7F26

with e.s.d.'s in

parenthesis.

Atom PP

7: 72

Ba(l)

Eu(l) Ba(2)

Eu(2)

Mg(l)

⁰

Mg(2)

⁰ Mg(3) ^0.2530(2)

0.2329(2)

72

0.2512(3) 0.2544(2) 0

72

o o

Mg(4)

F(l) F(2) F(3)F(4) F(5) F(6) F(7)

0.26006(¹)

0.26006(1)

0 0 0 0.26957(8) 0 0.11289(6)

72

0.3883(1) 0.16383(9) 0.2837(1) 0 0 0

0.35126(1) 0.35126(1) 0.3531(2) 0.3427(2) 0.22056(6)

0 0 0 0.24214(6) 0 0

0.12692(9) 0.0880(1) 0.0888(1) 0.3704(1)

1.047(2) 1.047(2) 0.79(1) 0.79(1) 0.90(2) 0.91(2) 0.78(2) 0.67(2) 1.32(2) 2.64(4) 1.26(2) 2.12(4) 1.28(4) 1.12(3) 2.50(6)

0.94(1) 0.06(1) 0.72(1) 0.28(1)

Atomic

displacement

parameters

[106 pm2]

Atom Ui Ur U33 <Vi: U Ur

Bad) 1.032(4)0.979(4)1.131(4) 0 0 0.008(3) Eu(l) 1.032(4) 0.979(4)1.131(4) 0 0 0.008(3) Ba(2) 0.888(5) 0.732(4) 0.76(3) ⁰ 0 0

Eu(2) 0.888(5) 0.732(4) 0.76(3) ^{0 0 0}

Mg(l) 0.83(4) 0.90(3) 0.97(3) 0 0 0

Mg(2) 0.68(3) 0.77(3) 1.29(3) 0 0 0

Mg(3)

0.59(3) 0.71(3) 1.04(3) ⁰ 0 0

Mg(4) 0.62(5) 0.55(4) 0.83(4) 0 0 0

F(ï)

1.15(3) 1.16(3) ^1.65(4) -0.26(3) -0.11(2) 0.05(2) F(2) 2.10(8) 1.74(6) 4.07(9) -1.34(6) ^{0 0} F(3) 1.02(4) 0.72(4) ^2.05(5) 0.15(3) ^{0 0} F(4) 3.24(9) ^{1.74(5) 1.38(5) 0 0} -0.32(4)

F(5) 0.87(6) 1.95(7) ^1.02(5) 0 0 0

F(6)

0.87(6) 1.62(6) 0.88(5) ^{0 0 0}

F(7) 2.6(1) 4.0(1) 0.90(6) ^{0 0 0}

assumed and the atomic

displacement ^{parameter changed}

^to

Uu

⁼

0.888(5) U22

⁼

0.732(4)

^and

U33

⁼

0.76(3) 106 pm2

and was foundto be more

isotropic.

Further the

agreement

values decreased

slightly ^(Rw

^{2.0 ->}

^{Rw 1.9%).}

The distance between both

fractionally occupied

^sites

was found^to be

15.7(3)

pm

The

split

refinement of the atomic

positions

^{of Eu and}

Ba on this site revealed that the

Eu(II)

^{ion forms shorter}

bonds

(c/(Eu-Fl)

⁼

256.6(2) pm)

^{with the four closest sur-}

Fig.

^3. Local environmentaround Ba2/Eu2 in

Ba5.20(6)Euo.80(6)Mg7F26.

The shortest Eu-Fl bonds ^aredrawn.

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142 ^F.Kübel, ^H.Hagemannand H. Bill

Bal/Eul Ba2 Eu2 Table 5. Selected metal fluoride distances for

Ba5.2()(6)Euo.80(6)Mg7F26 (pm).

Fl x2 Fl x2 F3 x2 F4 x2 F5 F7 F2 x2 F4

260.16(9) 277.78(8) 281.46(8) 298.30(3) 304.22(5) 315.76(3) 321.63(11) 339.19(14)

Eu2 F4 x2 Fl x4 F7 x2 F2 x4

15.7(3) 263.28(16) 266.19(17) 292.90(3) 297.85(20)

Fl x4 F4 x2 F7 x2 F2 x4

256.62(16) 265.54(16) 294.72(4) 309.70(20)

Mgl

Fl x4 195.35(9) F6 198.47(18)

F7 225.81(21) Mg2

F4 x2 191.98(14) F3 x2 195.90(14)

F2 x2 205.15(16)

Mg3

F5 x2 195.80(13)

F3 x2 198.08(11)

F6 x2 199.22(12) Mg4

F7 x2 195.18(19)

F2 x2 198.28(15)

F2 x2 198.28(15)

rounding

fluoride ions than the Ba ion

(d(Ba-Fl)

⁼

266.2(2) pm).This

is illustrated in

Fig.

^3.

A similar

approach

^{was made for}

B%7g(3)Euo.22(3)MgF4,

where a distance of

16.3(8)

pm between the

fractionnally occupied

^{Ba and Eu sites was} obtained. As a conse-

quence, the interatomic distances between Ba to F and Eu

to F show some differences. The values of the shortest Eu-F distances are smaller than the Ba-F distances with

250.1(6)[Eu-F3]

pm and

257.68(21 )[Eu-F2]

pm

compared

to

264.0(5)[Ba-F3]

pm and 258.71

(22)[Ba-F2]

pm in

Bao.7S(3)EU().22(3)MgF4.

Conclusions

In both

compounds (BaMgF4

^and

Ba(,Mg7F26)

^{the substitu-}

tion of ^Ba

by

^{Sr or Eu shows a} similar behaviour which

can be associated ^{with the} similar ionic radius of

Sr(II)

and

Eu(II).

The

split

refinement of Ba and Eu in both title

crystals

reveals the formation of shorter bonds between

Eu(II)

^and

the nearest fluoride

neighbors.

The concomitant increase of the

longest

^{Eu-F bond}

length

illustrates the trend in- duced

by

the smaller ion ^{to reduce the} coordination ^num- ber ofthe local metal-fluoride cluster. The fact that the Ba site in

Ba6Mg7F26 preferentially

substituted

by

^{Sror Eu is}

the one with the smaller coordination number underlines this

tendency.

Further

spectroscopic

measurements ^on rare earth

doped (or substituted)

^barium

magnesium

^{fluorides are in}

progress.

Acknowledgment.

^{This workwas}

supported by

^{the Swiss}

priority

pro-

ject "Optique".

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