β ￿ γ-SnF2 phase transition : neutron diffraction and NMR study

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β � γ-SnF2 phase transition : neutron diffraction and NMR study

J. Pannetier, G. Denes, M. Durand, J.L. Buevoz

To cite this version:

J. Pannetier, G. Denes, M. Durand, J.L. Buevoz.

β

�

γ-SnF2 phase transition : neutron diffraction and

NMR study. Journal de Physique, 1980, 41 (9), pp.1019-1024. �10.1051/jphys:019800041090101900�.

�jpa-00208915�

1019

03B2

^~

03B3-SnF2 phase transition : neutron diffraction and NMR study

J.

Pannetier (~),

^{G. Denes}

(~~),

^M.

Durand (~~)

^{and J. L.}

Buevoz (~) (*)

(~) ^Institut Laue-Langevin, 156 X, 38042 Grenoble Cedex, France

(~~) Laboratoire de Chimie Minérale D (**), Université de ^Rennes I, 35042 Rennes Cedex, ^France (Reçu ^le 27 février 1980, accepté ^{le 6 mai} 1980)

Résumé. 2014 Les paramètres ^{de maille et les} positions atomiques ^ont été déterminés _par diffraction des neutrons pour les deux variétés orthorhombique (03B2) ^et quadratique (03B3) ^de SnF2. Les résultats de diffraction et de RMN

(19F)

indiquent ^une transition continue du 2e ordre à 66 °C. La

position

^{des atomes de} fluor varie avec la tempé-

rature mais les

bipyramides

(SnF4) ^{ne sont} pas modifiées lors de la transition.

Abstract. ²⁰¹⁴ Lattice and

atomic-position

parameters ^of SnF2 have been measured by ^neutron diffraction for both orthorhombic (03B2) ^and tetragonal (03B3) phases. ^Both structural and 19F NMR results indicate a continuous second order transition at 66 °C. The fluorine atoms undergo large displacements ^{as a} function of temperature ^{but there} is no change of size of the (SnF4) bipyramids.

J. Physique ⁴¹ (1980) ^{1019-1024 SEPTEMBRE 1980,}

Classification Physics ^Abstracts 61.12 ^- 76.60 ^- 64.00

Introduction. ^- Stannous fluoride

SnF2

^{is known}

to exhibit three different modifications : the ^stable

room-temperature phase

^which

crystallizes

^from

aqueous solutions is monoclinic

a-SnF2

with space group

C2/c [1, 2].

^{In the} range 140 ^to 180 °C it under- goes ^a first order

phase

transition

[3]

^to

y-SnF2 (P41212,

^{Z =}

4) ;

upon

cooling y-SnF2

^transforms

to

fi-SnF2 (P212121,

^{Z =}

4,

ferroelastic

phase) through

a second order

phase

transition. The structure of these two new modifications

(fl-

^and

y-SnF2)

^{has been}

previously

determined

[4]

^from

X-ray powder

^data.

The purpose of this paper is ^to

present

^{a more}

precise study

^{of the}

P:;±

y

phase

transition. We have made neutron diffraction measurements at different

temperatures

^{in order to determine the}

temperature dependence

^{of the cell}

parameters

and atomic

posi-

tions ; ^{it is} concluded that the motion of fluorine atoms in the

(a, b) plane

^is

responsible

^{for the}

phase

transition.

Changes

^{are observed in the}

19F

^NMR

line

shape

^when

going through

the transition but motional

narrowing

^occurs

only

^above 110 °C. This

P y-SnF2 phase

transition is rather similar to the

pressure-induced

transition in

Te02 [5, 6].

1.

Experimental.

^{- The}

sample

^{material was} pow- dered monoclinic

a-SnF2

obtained from OSI.

Samples

(*) Present ^{address :} CNET, ^BP 42, 38240 Meylan, ^France.

(**) Laboratoire associé au C.N.R.S., ^n° 254.

were sealed under vacuum in

high-purity

quartz

ampoules. Tetragonal y-SnF2

^was

prepared in

^situ

by heating

^an

a-SnF2 sample

up ^to 190 °C for half an

hour and further

cooling

^{down to the} measurement

temperature :

orthorhombic

p-SnF 2

^{was obtained}

by heating

^an

a-SnF2 sample (in

^a quartz

ampoule)

up ^to 190 °C in a mufHe furnace and

rapidly quenching

^to

room temperature. ^No

a-SnF2

^was observed in the

p-SnF 2 samples,

^{even at the}

highest temperature

^we used

(62 °C)

^{but a}

y-SnF2 sample

^{at 80 °C}

partly

transformed back ^to

oc-SnF2.

The neutron diffraction

experiments

^were

perform-

ed at the Institut

Laue-Langevin

^{on two} spectro-

meters :

- D lA

(multiple

^{counter bank}

[7]) :

^{Â = 1.908}

À,

T =

24, 42, 51,

^{62 and 177 °C.}

- D1B

(position

sensitive detector

[8]) :

^{:/L = 1.28}

A,

T ⁼ 125 °C.

The

heating

^{device was a} conventional furnace with vanadium heater and shields

working

^under

low-pres-

sure

helium,

^the temperature ^was measured with a

thermocouple

^{in contact with the}

sample.

^{D lA data}

were collected from 160 to 150° in steps ^{of 0.100}

(2 0), taking

^{about 20 h for each} temperature. ^{The data} from the 10 counters were summed

using

^{the ILL}

POWDER _program

[9] ;

^the quartz pattern

(measured

on an empty quartz

tube)

^was substracted to

produce

the final diffraction

pattern.

^{DIB data were collected}

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:019800041090101900

1020

Figs. 1a,16. ^{- Neutron} diffraction pattern of p-SnF 2 (24 °C) ^and y-SnF2 (177 °C). ^{Crosses are} experimental points ; ^{the solid curve is the} least-squares ^refined profile ; ^the difference profile ^{is shown at the bottom.}

1021

from 2° to 820

(2 0)

^{in 5 h} and reduced

using

^ILL

programs [ 10] .

The Rietveld _program

[11]

^{as modified}

by

^Hewat

[12]

was used for all refinements.

Scattering lengths

^of

0.62

(Sn)

^{and 0.56}

(F),

^{all x}

10-12

cm, ^were used.

Refinements used

only isotropic

^thermal parameters ; three half-width parameters ^were included in the refinement

together

^{with an} overall scale

factor,

^a

zero

point correction,

^an asymmetry parameter ^and the cell parameters which led to

21(fi-SnF2)

^or

14(y-SnF2)

^refined

parameters. R

^factors

(nuclear)

range from 6.2 to 10

%

except ^{for the} pattern ^{close to} the transition

(T

^{= 62}

°C)

for which R ⁼ 13.3

%.

All

the

19F NMR spectra ^{were taken on a BRUKER} SXP

pulse

spectrometer

operating

^{at 84.67 MHz.}

Ti

^{was measured}

using

^{a n - i -}

n/2 pulse

sequence.

Spectra

^were recorded from room temperature ^to

190 °C,

^the

temperature being

stabilized to better than ± 0.5 °C. All

samples

^were sealed under vacuum

in

glass tubes ; they

^were

relatively

^{free of} paramagne- tic

impurities

^{as evidenced}

by

^the

long spin-lattice

relaxation times

(Tl -

^{250 s at}

room-temperature).

2. Cell déformation. ^- The

room-temperature

pat-

tern of

fi-SnF2 (Fig.1) clearly

^{shows the}

splitting

^{of the}

Figs. 2a, ^{2b. - Lattice} parameters of fl- ^and y-SnF2 ^vs. temperature (neutron ^and X-ray data). Typical standard deviations are given ^{for a}

few X-ray points; ^{for neutron} results standard deviations are

smaller than the data points. For sake of clarity ^some X-ray points

have been omitted. Solid and broken lines are least-squares ^fitted

to the experimental ^data.

hkl and hkO lines

corresponding

^{to the} orthorhombic

distortion ;

^as

previously

^observed

by high

tempera-

ture

X-ray

diffraction

[4],

^these

splittings disappear

^at

66 OC and the

corresponding

transition is a pure ferro-

paraelastic phase

transition

(422F222

^{in Aizu’s nota-}

tion).

The refined lattice parameters ^are

plotted

ⁱⁿ

figures

^{2a and 2b.}

They

compare well with

previous X-ray

^values

[3].

^{The main} difference is the

higher precision

^{of the}

parameters

^{from neutron}

profile refinement, especially

^{in the}

fl-phase

^{below the}

transition temperature

(i.e.

^{when a -}

b).

The transition temperature ^can be obtained accu-

rately

^{from a}

plot

^of

(b - alb

⁺

a)2

^vs. temperature.

A

least-squares

^{fit to} the four data

yields

which _goes to zero at T ⁼

6b(1)

^{°C in}

good

agreement with

previous

^results.

3. Atomic

displacements.

- Previous studies of this

phase

transition led us to assume space groups

P212121

^and

P41212 (or P43212) respectively

^for

fi-

and

y-SnF2.

^Profile

fitting

^structure refinement from neutron diffraction data agree well with these space groups ^and

refinements, starting

^from

X-ray

^deter-

mined

coordinates, quickly converged

^to

meaningful

values of atomic coordinates and

isotropic

^thermal

parameters.

Figures

^{3 and 4} show the atomic coordi-

Figs. 3a, ^{3b. -} Temperature dependence ^{of the x} and y ^fluorine position coordinates in fi- ^and y-SnF2. Error bars on the data points

indicate the standard deviations as obtained from the profile fitting

structure refinement. Solid lines are only guides ^{for the} eye.

1022

Fig. ^{4. -} Temperature dependence ^{of the z fluorine} position

coordinates in fl- ^and y-SnF2.

nates of fluorine atoms vs. temperature. ^These

posi-

tion parameters ^have been calculated in the same

coordinate system

(P41212)

^{in order to}

display

^their

splitting

^{in the}

(a, b) plane

^{and their}

equivalence

ⁱⁿ

the

tetragonal phase.

^These

plots clearly

^{show that}

the main feature of

the P -+

y

phase

transition is the motion of fluorine ^atoms in the

(a, b) plane.

^Fluorine

displacements along

^{the c axis are} very small : as

profile

refinement is known to underestimate standard deviations

[13],

^these

displacements

^{as well as the dif-}

ference between

z(Fl)

^and

z(F2)

^are

hardly significant.

This also holds for the Sn

displacements (Fig. 5)

which are not

larger

than three standard deviations.

Fig. ^{5. -} Temperature dependence of the tin position coordinates in fi- ^and y-SnF2.

Worlton and

Beyerlein [5] pointed

^out the increase of the

Debye-Waller

factors of the anions in the

vicinity

^{of the}

Te02 phase transition ;

^no such effect is observed for

SnF2

^{and the}

isotropic

^thermal para- meters are almost constant in the _range 24 OC ^to 62 OC with values

(averaged

^{over four}

temperatures) :

They

^increase up ^to

in the

y-phase

^{at 177 °C.}

4. 19F NMR results. ^-

Figure

⁶ illustrates the

change

^{in the NMR} spectra ^{as a} function

of tempera-

Fig. ^{6. - 19F} powder spectra ^of fi- ^and y-SnF2. The measunng

frequency ^{is 84.67 MHz.}

ture. The low

temperature (fi-SnF2) spectrum

^is

strongly asymmetric

^{and can be}

interpreted

^{as dou-}

blets

coming

^{from two} kinds of fluorine atoms

(FI

and

F2)

with different chemical

shifts ; high

tempera-

ture spectra

(y-SnF2)

suggest

only

^{one kind} of fluorine atom with

anisotropic

chemical shift.

The width of the resonance line was measured for the three

phases

^a-,

13-

^and

y-SnF2 (Fig. 7).

^{For mono-}

clinic

a-SnF2,

^motional

narrowing

^occurs

just

^above

room temperature ^{whereas it} is observed

only

^above

110 °C for

y-SnF2.

^The

y-SnF2

lattice is thus

effectively rigid

^{and no}

change

^or

discontinuity

is observed at

the fi

^± y

phase

transition.

Fig. ^{7. -} Temperature dependence of the width at half-maximum of the resonance line for _a-, fl- ^and y-SnF2.

The results of the

T1

measurements are

presented

in

figure

^{8. No}

change

is observed at the

fi

^± y

phase transition ;

^{this was}

expected since,

^{in this} range of temperatures, ^the

spin-lattice

relaxation is

governed by paramagnetic impurities (extrinsic regime).

^The

fit of

y-SnF2

data above 140 °C

(intrinsic regime) yields

^an activation _energy for the relaxation _process

ofE.(y)

⁼

0.52(1)

^eV much smaller than the activation energy obtained from

conductivity

measurements

(0.74

^eV

[14])

which indicates that the relaxation

1023

process which is

responsible

^for

Tl

^is digèrent than that for

conductivity.

^For

a-SnF2

^{one obtains}

in close agreement ^{with the}

conductivity

^data

(0.52 eV [14]).

Fig. ^{8. -} Spin-lattice (IIT,) relaxation rates as a function of

reciprocal temperature ⁱⁿ a-, fl- ^and y-SnF2.

5. Discussion. - All the above observations indi-

cate that

the fi

^+:t

y-SnF2

transition is a continuous pure strain transition whose main structural feature is the motion of fluorine atoms in the

(a, b) plane.

Moreover,

calculation of interatomic distances and

angles

^{leads to the}

following

^{results :}

- The shortest Sn-F distances in the

(SnF4)

^tri-

gonal bipyramid

^{are constant} in the range 240 ^to 177 OC with the

following

^values

(Fig. 9) :

Fig. ^{9. -} (SnF4) trigonal bipyramid ; ^black sphere ^{is tin.}

in

good

agreement ^with the values

usually

^observed

for this coordination

(A configuration according

^to

Brown

[15]).

- The Sn-F-Sn

angle

^{is constant in}

y-SnF2

^but

two different temperature

dependent angles

^{are observ-}

ed in the

low-temperature phase (Fig. 10).

Fig. ^{10. -} Temperature dependence ^{of the Sn-F-Sn} angles.

Then we can conclude that the

(SnF4) trigonal bipyramids

retain their

shape

^when the solid _goes

through

^the transition and it is

only

^the orientation of one

bipyramid

^with respect ^{to the}

neighbouring bipyramids

^{which is}

modified ;

^{in other}

words, fl-

^and

y-SnF2

^can be considered as

being

^built up from

rigid (SnF4)

^{units and the} soft feature

[16]

^{of the structure is}

the Sn-F-Sn

angle.

^{This is confirmed}

by looking

^{at the}

orientation of the cone of

zero-expansion (i.e.

^the

locus of the directions

along

which the dilatation is

zero

[17]);

^{as the thermal}

expansion

^is

negative along

the b axis in

fi-SnF2,

the axis of the cone coincides with b; ^{the cone can be defined}

by

^two

semi-opening angles 0(b, c)

^and

0(b, a) [3].

^{These two}

angles

^which

define the directions of

null-expansion

^{in the}

planes (b, c)

^and

(b, a)

^{can be} calculated from the thermal

expansion

coefficients

[17] :

^{as shown in}

figure 11, they

^{are not constant}

(solid line, Fig. 11)

^{but their}

values at the transition temperature

provide

^informa-

tion about the mechanism of the transition. Indeed,

Fig. 11. - p-SnF 2 : semi-opening angles ^{of the} zero-expansion

cone (angles ^{between the} directions of null-expansion ^{and the}

b axis). Solid lines : calculated from thermal expansion coefficients.

Broken line : calculated from the orientation of (SnF4) bipyramids

in the cell (F (axial)-F (axial) ^{and F} (equat.)-F (equat.) orientation within the cell). ^{The values at} the transition temperature ^{are cal-} culated from the y-SnF2 ^structure.

1024

the thermal contraction _upon

cooling

^{in the}

tetragonal phase

^takes

place

^{until some}

critically

^{short distance}

is reached and one can assume that the directions of

zero-expansion

^{at the} transition coincide with the directions of the shortest

bonds,

^{i.e. the structure is} locked

along

^{these bonds.}

Figure

¹¹

clearly

^shows

that,

^{at the} transition, ^the directions

of null-expansion

are

along

^{the F}

(axial)-F (axial) (e(a - b))

^{and F}

(equatorial)-F (equatorial) (O(b, c)) axis;

^{at lower}

temperatures

(

⁴⁰

°C)

^{the cell} distortion becomes

large

^{and other} interactions

(F-F contacts)

^{occur in}

the determination of the

zero-expansion

^directions

whose variation with temperature ^{becomes more}

complex.

This

description

^of

the P

y transition

by

^a

locking

of the structure

along

^{the axis of the}

(SnF4) bipyra-

mids is

actually

ⁱⁿ agreement with both the NMR results and the

Debye-Waller

^{factors which do not}

show any increase of the fluorine thermal motion at the transition. A recent

reinvestigation [6]

^{of the}

pressure induced ferroelastic

phase

transition in

Te02 using

^{the Landau}

theory

^{led to similar conclu-}

sions about the structural mechanism of that transi- tion.

References

[1] MCDONALD, R. C., HO-KUEN HAU, H., ERIKS, K., Inorg.

Chem. 15 (1976) ^762.

[2] DENES, G., PANNETIER, J., LUCAS, J., ^LE MAROUILLE, ^J. Y., J. Solid State Chem. 30 (1979) ^335.

[3] DENES, G., Thèse d’Etat, ^Rennes (1978), unpublished.

[4] DENES, G., PANNETIER, J., LUCAS, J., ^J. Solid State Chem., in _press.

[5] WORLTON, ^T. G., BEYERLEIN, ^R. A., Phys. ^{Rev. B 12} (1975) ^1899.

[6] UWE, H., TOKUMOTO, H., Phys. ^{Rev. B 19} (1979) ^3700.

[7] HEWAT, ^A. W., BAILEY, I., ^Nucl. Instrum. Methods 137 (1976) 463.

[8] ALLEMAND, R., BOURDEL, J., ROUDAUT, E., CONVERT, P., IBEL, K., JACOBE, J., COTTON, ^J. P., FARNOUX, B., ^Nucl.

Instrum. Methods 126 (1975) ^29.

[9] HEWAT, ^A. W., Powder Retrieval and Refinement System, ILL Report (1978).

[10] WOLFERS, P., unpublished computer programs.

[11] RIETVELD, ^H. M., J. Appl. Crystallogr. ² (1969) ^65.

[12] HEWAT, ^A. W., ^{UKAERE Harwell} Report ^RRL 73/897 (1973).

[13] SAKATA, M., COOPER, ^M. J., ^J. Appl. Crystallogr. 12 (1979) ^554.

[14] ANSEL, D., DEBUIGNE, J., DENES, G., PANNETIER, J., LUCAS, J., Ber. Bunsenges. Phys. ^{Chem. 82} (1978) ^376.

[15] BROWN, ^I. D., ^{J. Solid State Chem. 11} (1974) ^214.

[16] MEGAW, ^H. D., Crystal Structures : a Working Approach (Saunders Co., Philadelphia) ^1973.

[17] WOOSTER, W. A., ^{Tensors and} Group Theory for Physical Properties of Crystals (Clarendon Press, London) ^1973.