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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
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[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.
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[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).
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[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.
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