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Phase transition of RbCN
Physica B 180 & 181 (1992) North-Holland
Phase
transition
P. Bourson”,
PHYSICA 1
351-353
of RbCN
A. Ndtoungou”,
J. Bouillo$‘,
J.L.
Soubeyroux’
and D. Duranda
“Centre Lorrain d’Optique et Electronique des Solides, Universify of Metz - SUPELEC 2. rue E. Belin, 57078 Metz Cedex 3, France hLaboratoire de la Structure de la Matihe, University of Savoie, 9 rue de 1’Arc en Ciel, 74942 Annecy le vieux, France ‘Institut Laue-Langevin, 156 X, 38042 Grenoble Cedex, France
The phase transition of RbCN has been studied by a neutron therm0 diffractometer. The temperature evolution monoclinic lattice parameters and the effect of thermal cycles and annealing are pointed out and measured.
1. Introduction At
RbCN, NaCN and KCN in which the CN- elastic dipoles are rapidly reorientating among their possible orientations (pseudocubic phase). When the temperature decreases, an order-disorder phase transition occurs mainly due to a CN- dumbbell freezing along the ( 1 1 0) direction for NaCN or KCN (ferroelastic order with orthorhombic structure) [l], and close to the (0 3 1) direction for RbCN (antiferroelastic order with monoclinic structure [2]). In order to explain this different behaviour between KCN and RbCN phase transition mechanisms, we have already developed a lattice potential model [l] which gives good results for KCN. Such a calculation needs a good knowledge of the temperature dependence of the structural lattice parameters. In order to apply this model to RbCN, the kinetics of transition has been measured on this compound using a position sensitive detector installed on thermal neutron diffractometer DlB at Institut Laue-Langevin. In this paper experimental results are given and discussed. Moreover, the effect of thermal cycles and annealing has been studied. exhibit
room
temperature,
a rock
salt
structure
of the
from Rowe et al.‘s results [2]. In fig. 1, the unit cell of the elastically ordered monoclinic phase in an original cubic cell is given. The monoclinic lattice parameters a, b and c suffer no special anomaly when decreasing the temperature while the /3 angle (fig. 2) exhibits a much more significant change when compared to its value taken from the cubic symmetry (p, = 125.26”). The decreasing of /?<-p is indicative of an increasing of the CN- orientational disorder around the fitted positions. In addition, below about 40 K (TZ) a change in the slope is observed which seems to be correlated with a sudden increase of the reliability factor of the refinement. This observation is perhaps to be considered as an other evidence of a freezing-in dipole reorientation already described by Kondo et al. [4]. The distortion due to this monoclinic structure of initial cubic cell (E,: -1.7%, T,,: 90.8” and 88.4” at T,) (figs. 3 and 4) is much smaller than the one between the orthorhombic structure and the cubic phase in the case of KCN (Es: -8.4%, Tzg: 75” at T<)
[51. From this result the antiferroelastic ordered monoclinic phase can be seen as an intermediate phase between the disordered cubic and the ferroelastic ordered orthorhombic phases.
2. Sample preparation All the samples were grown from, the melt under an inert atmosphere at the University of Utah by the Crystal Growth Laboratory. Great care has been taken to reduce by annealing the residual stress produced during the grinding process [3]. As we will show later, stress application is able to modify the structural parameters below T, (transition temperature, unit cell distortions, etc.). 3. Experimental
results
0
1992 - Elsevier
,.
Tzg ( deg. 1
The pseudocubic phase transforms into a monoclinic phase below T, = 131.4 K. This paper deals only with the monoclinic phase whose structure at 4 K is taken 0921-4526/92/$05.00
irz! ‘..,
E’
Science
Publishers
Fig. 1. Unit cell monoclinic phase.
B.V. All rights
reserved
representation
of the
elastically
ordered
352
P. Bourson et al.
b lonoclinic
123.6
I Phase transition of RbCN
Tc
angle I3 (deg.)
Deformation
r
angle (T2g)
fP
~___
jBc-125,26” 123,4
86 1
Third cooling
. -RbCN
123,2
First cooling
90
84
+(A)
23,0 89
(B)08 22,6 76 12204
1
122,2
0
20
40
60
100
80
Temperature
contraction
10
RbCN
(B)
-6
RbCN
-8
-10
10
30
50
70
90
Temperature
110
130
150
170
(K)
Fig. 3. Temperature evolution of the uniaxial contraction (Eg) of the cubic cell in the monoclinic phase with (A) and without (B) annealing in RbCN and comparison with KCN (-).
70
90
110
130
150
170
(K)
Fig. 4. Temperature evolution of the deformation angles (T?,) of the cubic cell in the monoclinic phase with (A) and without (B) annealing for RbCN and comparison with KCN (- + -). 4.
EG (9/o)
-4
-
50
Temperature for
4.1. -g
30
(K)
Fig. 2. /3 monoclinic angle evolution with temperature the first cooling down (points) and the third one (line).
Uniaxial
t
86
140
120
Influence of the residual stresses Thermal
cycling
effects
The pseudocubic crystal breaks up at T, by the ordering process into a multi-domain monoclinic structure. Then the transition monoclinic - cubic induces defects in the cubic phase which in turn, if these defects have no time enough to relax, will transform into a less distorted monoclinic phase (smaller PC-/3 in fig. 2) when decreasing temperature again. Moreover, these residual stresses introduced by the thermal cycling delay the transition and a decrease of the transition temperature (about 1 K after three toolings) is observed. 4.2.
Annealing
effects
Grinding introduces defects in crystals and may even modify the nature of the ordered phase at the transition [3]. Thus, in a sample without annealing, a decrease of the transition temperature (about 2 K) is observed as well. Figures 3 and 4 show also the increase of the E, and T,, deformation of the cubic cell in the monoclinic phase if an annealing process has been performed. These results confirm that the reduction of residual stresses in these samples is of premium importance if structural parameters have to be determined.
P. Bourson et al. I Phase transition of RbCN
Acknowledgements We would like to thank Professor versity of Utah (USA) for providing
F. Liity of Unius the samples.
References [1] P. Bourson, Thesis, University of Metz (1990).
353
[2] J.M. Rowe, J.J. Rush and F. Luty, Phys. Rev. B 29 (1984) 2168. [3] P. Bourson, D. Durand, J. Bouillot and J.L. Soubeyroux, Phase Transition 31 (1991) 277. [4] Y. Kondo, D. Schoemaker and F. Liity, Phys. Rev. B 19 (1989) 4210. [5] P. Bourson, G. Gorzyca and D. Durand, Cryst. Lattice Defects and Amorphous Mat. 16 (1987) 311.
Phase
transition
P. Bourson”,
PHYSICA 1
351-353
of RbCN
A. Ndtoungou”,
J. Bouillo$‘,
J.L.
Soubeyroux’
and D. Duranda
“Centre Lorrain d’Optique et Electronique des Solides, Universify of Metz - SUPELEC 2. rue E. Belin, 57078 Metz Cedex 3, France hLaboratoire de la Structure de la Matihe, University of Savoie, 9 rue de 1’Arc en Ciel, 74942 Annecy le vieux, France ‘Institut Laue-Langevin, 156 X, 38042 Grenoble Cedex, France
The phase transition of RbCN has been studied by a neutron therm0 diffractometer. The temperature evolution monoclinic lattice parameters and the effect of thermal cycles and annealing are pointed out and measured.
1. Introduction At
RbCN, NaCN and KCN in which the CN- elastic dipoles are rapidly reorientating among their possible orientations (pseudocubic phase). When the temperature decreases, an order-disorder phase transition occurs mainly due to a CN- dumbbell freezing along the ( 1 1 0) direction for NaCN or KCN (ferroelastic order with orthorhombic structure) [l], and close to the (0 3 1) direction for RbCN (antiferroelastic order with monoclinic structure [2]). In order to explain this different behaviour between KCN and RbCN phase transition mechanisms, we have already developed a lattice potential model [l] which gives good results for KCN. Such a calculation needs a good knowledge of the temperature dependence of the structural lattice parameters. In order to apply this model to RbCN, the kinetics of transition has been measured on this compound using a position sensitive detector installed on thermal neutron diffractometer DlB at Institut Laue-Langevin. In this paper experimental results are given and discussed. Moreover, the effect of thermal cycles and annealing has been studied. exhibit
room
temperature,
a rock
salt
structure
of the
from Rowe et al.‘s results [2]. In fig. 1, the unit cell of the elastically ordered monoclinic phase in an original cubic cell is given. The monoclinic lattice parameters a, b and c suffer no special anomaly when decreasing the temperature while the /3 angle (fig. 2) exhibits a much more significant change when compared to its value taken from the cubic symmetry (p, = 125.26”). The decreasing of /?<-p is indicative of an increasing of the CN- orientational disorder around the fitted positions. In addition, below about 40 K (TZ) a change in the slope is observed which seems to be correlated with a sudden increase of the reliability factor of the refinement. This observation is perhaps to be considered as an other evidence of a freezing-in dipole reorientation already described by Kondo et al. [4]. The distortion due to this monoclinic structure of initial cubic cell (E,: -1.7%, T,,: 90.8” and 88.4” at T,) (figs. 3 and 4) is much smaller than the one between the orthorhombic structure and the cubic phase in the case of KCN (Es: -8.4%, Tzg: 75” at T<)
[51. From this result the antiferroelastic ordered monoclinic phase can be seen as an intermediate phase between the disordered cubic and the ferroelastic ordered orthorhombic phases.
2. Sample preparation All the samples were grown from, the melt under an inert atmosphere at the University of Utah by the Crystal Growth Laboratory. Great care has been taken to reduce by annealing the residual stress produced during the grinding process [3]. As we will show later, stress application is able to modify the structural parameters below T, (transition temperature, unit cell distortions, etc.). 3. Experimental
results
0
1992 - Elsevier
,.
Tzg ( deg. 1
The pseudocubic phase transforms into a monoclinic phase below T, = 131.4 K. This paper deals only with the monoclinic phase whose structure at 4 K is taken 0921-4526/92/$05.00
irz! ‘..,
E’
Science
Publishers
Fig. 1. Unit cell monoclinic phase.
B.V. All rights
reserved
representation
of the
elastically
ordered
352
P. Bourson et al.
b lonoclinic
123.6
I Phase transition of RbCN
Tc
angle I3 (deg.)
Deformation
r
angle (T2g)
fP
~___
jBc-125,26” 123,4
86 1
Third cooling
. -RbCN
123,2
First cooling
90
84
+(A)
23,0 89
(B)08 22,6 76 12204
1
122,2
0
20
40
60
100
80
Temperature
contraction
10
RbCN
(B)
-6
RbCN
-8
-10
10
30
50
70
90
Temperature
110
130
150
170
(K)
Fig. 3. Temperature evolution of the uniaxial contraction (Eg) of the cubic cell in the monoclinic phase with (A) and without (B) annealing in RbCN and comparison with KCN (-).
70
90
110
130
150
170
(K)
Fig. 4. Temperature evolution of the deformation angles (T?,) of the cubic cell in the monoclinic phase with (A) and without (B) annealing for RbCN and comparison with KCN (- + -). 4.
EG (9/o)
-4
-
50
Temperature for
4.1. -g
30
(K)
Fig. 2. /3 monoclinic angle evolution with temperature the first cooling down (points) and the third one (line).
Uniaxial
t
86
140
120
Influence of the residual stresses Thermal
cycling
effects
The pseudocubic crystal breaks up at T, by the ordering process into a multi-domain monoclinic structure. Then the transition monoclinic - cubic induces defects in the cubic phase which in turn, if these defects have no time enough to relax, will transform into a less distorted monoclinic phase (smaller PC-/3 in fig. 2) when decreasing temperature again. Moreover, these residual stresses introduced by the thermal cycling delay the transition and a decrease of the transition temperature (about 1 K after three toolings) is observed. 4.2.
Annealing
effects
Grinding introduces defects in crystals and may even modify the nature of the ordered phase at the transition [3]. Thus, in a sample without annealing, a decrease of the transition temperature (about 2 K) is observed as well. Figures 3 and 4 show also the increase of the E, and T,, deformation of the cubic cell in the monoclinic phase if an annealing process has been performed. These results confirm that the reduction of residual stresses in these samples is of premium importance if structural parameters have to be determined.
P. Bourson et al. I Phase transition of RbCN
Acknowledgements We would like to thank Professor versity of Utah (USA) for providing
F. Liity of Unius the samples.
References [1] P. Bourson, Thesis, University of Metz (1990).
353
[2] J.M. Rowe, J.J. Rush and F. Luty, Phys. Rev. B 29 (1984) 2168. [3] P. Bourson, D. Durand, J. Bouillot and J.L. Soubeyroux, Phase Transition 31 (1991) 277. [4] Y. Kondo, D. Schoemaker and F. Liity, Phys. Rev. B 19 (1989) 4210. [5] P. Bourson, G. Gorzyca and D. Durand, Cryst. Lattice Defects and Amorphous Mat. 16 (1987) 311.