- Email: [email protected]
Paraelastic behavior of potassium cyanide
~
Solid State Communications, Vol.59,No.]], pp.717-719, ]986. Printed in Great Britain.
0038-]098/86 $3.00 + .00 Pergamon Journals Ltd.
PARAELASTIC BEHAVIOR OF POTASSIUM CYANIDE t Sidnei Paciornik, Lucia H. de Salvo Souza and Luiz C. Scavarda do Carmo Departamento de F{sica, Pontif{cia Universidade CatSlica Cx.P. 38071, Rio de Janeiro, RJ, Brasil
(Received on 12th May 1986 by R.A. Cowley) The low temperature Curie-type paraelastic behavior of low concentration anionic impurities dissolved in simple cubic alkali halide crystals has already been analysed for a number of cases. The CN- radical in particular can substitute the halide ion in any concentration (up to 100% forming pure cyanide crystals), producing room temperature alkali halide-like crystals that above about 85% concentration show low temperature ferroelastic phases of crystallization. This paper presents the experimental evidence of room temperature paraelastic alignment exploring some analogies between elastic and magnetic (or electric) properties in the high concentration impurity range and above the phase transition (Curie) temperature.
The analysis of electric and magnetic properties induced in low concentration electric and magnetic dipole doped alkali halides have been subject to extensive studies I and led to a full comprehension of the paraelectric and paromagnetic properties of such added impurities in the otherwise dielectric and diamagnetic hosts. These studies have used mainly optical (light absorption of electronic or vibrational transitions) or magnetic (EPR or sometimes ENDOR) techniques due to the large (7 to I0 eV) energy gap of the alkali halides. The recent interest on elastic and specifically photoelastic materials has increased with the development of new materials for integrated optics 2 and this should lead to some work on the elastic ÷ magnetic or electric analogy. The extension of this type of work to paraelasticity was firstly done through the use of low-concentration O~ doped KC~ crystals3; the O~ molecules, being non-spherical, have the degeneracy of the position states defined by their internuclear axes lifted by an external elastic field (uniaxial stress),and,consequentl~ populate more densely certain directions producing, at suitably low temperatures, a reversible and macroscopic bulk alignment of the molecules. The facility to dissolve any concentration of CN- ions (an anionic substitution impurity with an internal elastic and electric dipole) allowed the study of firstly the low concentration paraelastic properties of such diatomic ions 4 and secondly the high concentration
ferroelastic properties of the materials ranging from about 85% of CN- anionic substitution to halide ions up to pure cyanide crystals 5. The potassium cyanide crystals are pseudo alkali halides with a cubic structure at room temperature6; in this phase the CN- ion reorients almost freely in a shallow orientational potential. Below a critical temperature T c the cyanide crystal undertakes a disorder ÷ order crystal lattice phase transition followed by an elastic alignment of the CN- radicals (heads and tails are freely exchanged but the molecular axis is practically restricted to a given direction). Full electric order of the finally frozen molecules is only achieved at lower temperatures in a second phase transition 5. While the room temperature structure is cubic (prototype crystal), the low temperature lattice has a domain structure of crystallites of orthorhombic symmetry6,the number of possible domain orientations being equal to the ratio of the number of symmetry operations in the prototype to the number of operations in the low temperature latticeT.The spontaneous deformation (and domain formation) caused by the cooling proccess below T c is the origin of the so-called ferroelastic property, in analogy to the ferromagnetic property responsible for the spontaneous magnetization (and domain formation) found in magnets cooled through the Curie temperature. The analogy extends to the domain alignment in the presence of an external stimulus; magnets align their domains in an external magnetic field showing a consequent macroscopic magnetization while KCN aligns its domains (in this case reduced in first approximation to a total of 6 species) in an external uniaxial stress field showing a consequent macroscopic deformation 8. In this paper we explore the experimental evidence of another theoretically expected
t - Work partially supported by CNPq and FINEP (Brazilian agencies) * - Present address: Dipartmnento di Fisica, Universita Degli Studi di Milano, Via Celoria 16, 20133 Milano, Italy. 717
718
PARAELASTIC BEHAVIOR OF POTASSIUM CYANIDE
elastic ÷ magnetic (or electric) analogy: as it is known, the magnets (or electrets) are paramagnetic (or paraelectric) if heated above T c (they align in a reversible way in an external magnetic or electric field) while the low temperature ferroelastic KCN crystals are paraelastic above the critical temperature (Tc=I68K). In this sense we expect that the elastic response (measured through the stress induced alignment of CN- radicals) of KCN in the high temperature phase should increase as the temperature decreases towards T c (the phase transition temperature) as well as it should show a linear dependance on the external stress. To detect stress-induced alignment of CNradicals we used the induced birefringencemethod. This method is based on the optical anisotropy of the CN- radical polarizability tensor which presents a difference in polarizability A~=0.32~ 3 between its major and minor axes 9. This anisotropy leads to a macroscopic birefringence when a significant number of radicals is aligned in the same direction, thus becoming an efficient tool to monitor stress induced alignment. The measurements were made using a dewar suitable for temperature variations between 77K (liquid nitrogen) and 300K with a simple mechanical system for the application of uniaxial mechanical stress. As probe light source we used a He-Ne laser (ImW).We also used two polarizers and a photodiode as detector.The first polarizer sets the direction of polarization of the incident light that proceeds towards a crystal that acts as a stress induced retarder; finally, the emerging light is analysed through the second polarizer. This system has already been described before 10. The induced phase shift 6 and the birefringence An can be obtained from simple light intensity measurements using parallel and crossed polarizers always forming 45 degrees with the direction of application of the external stress through the expressions tg 2 6 = 11/1// An = (~)/(360d) where I// and I~ are the intensities of light emerging from the system respectively with parallel and crossed polarizers~ o % is the wavelength of laser light (6328A), d is the optical path (the crystal thickess), 6 is the induced phase shift in degrees and An is the birefringence. Figure one shows the results obtained for birefringence measurements in pure KCN stressed along the (001) direction and analysed with light propagating in the (I00) direction. It can be seen that the stress-induced birefringence increases strongly towards the transition temperature in a completely different behavior from the situation of zero applied stress (the small value of 6 in this latter case is constant through the temperature range studied and is caused by internal strains that appear in the crystal due either to the process of growth or cleaving procedures). Moreover, the induced
Vol. 59, No. II
KCN o =30x
106dyn/cm 2
400
Q;
300
L /L
5
,# i
o 4X c-
~D
o,o
3
2OO
I00
50
$00 T-Tc(K)
150
Figure 1 - Variation of the stress-induced phase shift (or birefringence) with temperature for KCN. Stress value o=30×106dyne/om 2 . Crystal thickness d=l,2mm.
alignment is reversible (it disappears after stress removal). Figure two shows that ~ has a linear dependence with the applied stress, allowing 70 Brewster value (i Brewster = 10 -13 cm2/dyne) for the stress optical coefficient C at room temperature. The paraelastic behavior is thus put in evidence in agreement with the predictions made with the aid of the elastic-magnetic analogy discussed above. The results shown in figure one should also be compared with those obtained for stress induced birefringence of pure alkali-halides (C values in the i to 4 Brewster range) 11, that show a temperature independent elastic response. The above comparison shows the fundamental role played by the non-spherical CN- ion when substituting the spherical halide ions in this type of lattice. The elastic dipole associated with the molecule in this high concentration limit partially aligns in a rather small elastic field showing a higher degree of alignment close (and just above) T c. This behavior is radically distinct from the low concentration and low temperature paraelasticity discussed in connection with O~ , CN- doped alkali halides; in this case, the low concentration precludes elastic dipole - elastic dipole interactions. On the other hand, in the high concentration limit, the collective behavior of the elastic radicals - that ultimately leads to the ferroelastic first order phase transition (thus defining a critical
Vol. 59, No. II
PARAELASTIC BEHAVIOR OF POTASSIUM CYANIDE
3oo
KCN T= 3 0 0 K
f,n
~-'~ 2 0 0 "u
I00 /
•
I
I00
I
I
300
200
I
400
O (I 0 e dyn ) cm 2
Figure 2 - Variation of the stress-induced phase shift with stress for KCN. Temperature T=300K. Crystal thickness d=2,5rmn.
temperature) - brings a measurable elastic behavior to higher temperatures. The tensorial character of the elastic dipole 12 should make the KCN elastic response highly dependent on the external stress direction relatively to the cubic axes. Experiments with distinct symmetries and with other crystals of the alkali halide-cyanide family are under way.
Acknowledgements - We would llke to thank Dr. Fritz Luty (Physics Department, The University of Utah) and Dr. Milton de Souza (Physics and Chemistry Institute at Sao Carlos, The University of Sao Paulo) for kindly providing the samples used in this experiment.
REFERENCES I. 2.
3. 4. 5.
6.
W. Beall Fowler, Physics of Color Centers, Academic Press (1968). L. Kuhn, M.L. Dakss, P.F. Heidrich and B.A. Scott, Applied Physics Letters 17, 265 (1970). W. Kanzig, Journal of Physics and Chemistry of Solids 23, 479 (1962). F. Luty, Physical Reveiw B iO, 3677 (1974). D. Durand, L.C. Scavarda do Carmo, A. Anderson and F. Luty, Physical Review B 22, 4005 (1980). A. Cimino and G.S. Parry, II Nuovo Cimento XIX, 971 (1961).
7. 8.
K.Aizu, Physical Review B 2, 754 (1970). L.C.Scavarda do Carmo and F. Luty, Gatlinburg 1978. 9. M.D. Julian, PhD Thesis, The University of Utah (unpublished). I0. A.J. Michael, Journal of the Optical Socie of America 58, 889 (1968). ii. C.S. Chen, J.P. Szczesniak and J.C. Corell: Journal of Applied Physics 46, 303 (1975). 12. A.S. Nowick and W.R. Heller, Advances in Physics (GB) 12, 251 (1963).
Solid State Communications, Vol.59,No.]], pp.717-719, ]986. Printed in Great Britain.
0038-]098/86 $3.00 + .00 Pergamon Journals Ltd.
PARAELASTIC BEHAVIOR OF POTASSIUM CYANIDE t Sidnei Paciornik, Lucia H. de Salvo Souza and Luiz C. Scavarda do Carmo Departamento de F{sica, Pontif{cia Universidade CatSlica Cx.P. 38071, Rio de Janeiro, RJ, Brasil
(Received on 12th May 1986 by R.A. Cowley) The low temperature Curie-type paraelastic behavior of low concentration anionic impurities dissolved in simple cubic alkali halide crystals has already been analysed for a number of cases. The CN- radical in particular can substitute the halide ion in any concentration (up to 100% forming pure cyanide crystals), producing room temperature alkali halide-like crystals that above about 85% concentration show low temperature ferroelastic phases of crystallization. This paper presents the experimental evidence of room temperature paraelastic alignment exploring some analogies between elastic and magnetic (or electric) properties in the high concentration impurity range and above the phase transition (Curie) temperature.
The analysis of electric and magnetic properties induced in low concentration electric and magnetic dipole doped alkali halides have been subject to extensive studies I and led to a full comprehension of the paraelectric and paromagnetic properties of such added impurities in the otherwise dielectric and diamagnetic hosts. These studies have used mainly optical (light absorption of electronic or vibrational transitions) or magnetic (EPR or sometimes ENDOR) techniques due to the large (7 to I0 eV) energy gap of the alkali halides. The recent interest on elastic and specifically photoelastic materials has increased with the development of new materials for integrated optics 2 and this should lead to some work on the elastic ÷ magnetic or electric analogy. The extension of this type of work to paraelasticity was firstly done through the use of low-concentration O~ doped KC~ crystals3; the O~ molecules, being non-spherical, have the degeneracy of the position states defined by their internuclear axes lifted by an external elastic field (uniaxial stress),and,consequentl~ populate more densely certain directions producing, at suitably low temperatures, a reversible and macroscopic bulk alignment of the molecules. The facility to dissolve any concentration of CN- ions (an anionic substitution impurity with an internal elastic and electric dipole) allowed the study of firstly the low concentration paraelastic properties of such diatomic ions 4 and secondly the high concentration
ferroelastic properties of the materials ranging from about 85% of CN- anionic substitution to halide ions up to pure cyanide crystals 5. The potassium cyanide crystals are pseudo alkali halides with a cubic structure at room temperature6; in this phase the CN- ion reorients almost freely in a shallow orientational potential. Below a critical temperature T c the cyanide crystal undertakes a disorder ÷ order crystal lattice phase transition followed by an elastic alignment of the CN- radicals (heads and tails are freely exchanged but the molecular axis is practically restricted to a given direction). Full electric order of the finally frozen molecules is only achieved at lower temperatures in a second phase transition 5. While the room temperature structure is cubic (prototype crystal), the low temperature lattice has a domain structure of crystallites of orthorhombic symmetry6,the number of possible domain orientations being equal to the ratio of the number of symmetry operations in the prototype to the number of operations in the low temperature latticeT.The spontaneous deformation (and domain formation) caused by the cooling proccess below T c is the origin of the so-called ferroelastic property, in analogy to the ferromagnetic property responsible for the spontaneous magnetization (and domain formation) found in magnets cooled through the Curie temperature. The analogy extends to the domain alignment in the presence of an external stimulus; magnets align their domains in an external magnetic field showing a consequent macroscopic magnetization while KCN aligns its domains (in this case reduced in first approximation to a total of 6 species) in an external uniaxial stress field showing a consequent macroscopic deformation 8. In this paper we explore the experimental evidence of another theoretically expected
t - Work partially supported by CNPq and FINEP (Brazilian agencies) * - Present address: Dipartmnento di Fisica, Universita Degli Studi di Milano, Via Celoria 16, 20133 Milano, Italy. 717
718
PARAELASTIC BEHAVIOR OF POTASSIUM CYANIDE
elastic ÷ magnetic (or electric) analogy: as it is known, the magnets (or electrets) are paramagnetic (or paraelectric) if heated above T c (they align in a reversible way in an external magnetic or electric field) while the low temperature ferroelastic KCN crystals are paraelastic above the critical temperature (Tc=I68K). In this sense we expect that the elastic response (measured through the stress induced alignment of CN- radicals) of KCN in the high temperature phase should increase as the temperature decreases towards T c (the phase transition temperature) as well as it should show a linear dependance on the external stress. To detect stress-induced alignment of CNradicals we used the induced birefringencemethod. This method is based on the optical anisotropy of the CN- radical polarizability tensor which presents a difference in polarizability A~=0.32~ 3 between its major and minor axes 9. This anisotropy leads to a macroscopic birefringence when a significant number of radicals is aligned in the same direction, thus becoming an efficient tool to monitor stress induced alignment. The measurements were made using a dewar suitable for temperature variations between 77K (liquid nitrogen) and 300K with a simple mechanical system for the application of uniaxial mechanical stress. As probe light source we used a He-Ne laser (ImW).We also used two polarizers and a photodiode as detector.The first polarizer sets the direction of polarization of the incident light that proceeds towards a crystal that acts as a stress induced retarder; finally, the emerging light is analysed through the second polarizer. This system has already been described before 10. The induced phase shift 6 and the birefringence An can be obtained from simple light intensity measurements using parallel and crossed polarizers always forming 45 degrees with the direction of application of the external stress through the expressions tg 2 6 = 11/1// An = (~)/(360d) where I// and I~ are the intensities of light emerging from the system respectively with parallel and crossed polarizers~ o % is the wavelength of laser light (6328A), d is the optical path (the crystal thickess), 6 is the induced phase shift in degrees and An is the birefringence. Figure one shows the results obtained for birefringence measurements in pure KCN stressed along the (001) direction and analysed with light propagating in the (I00) direction. It can be seen that the stress-induced birefringence increases strongly towards the transition temperature in a completely different behavior from the situation of zero applied stress (the small value of 6 in this latter case is constant through the temperature range studied and is caused by internal strains that appear in the crystal due either to the process of growth or cleaving procedures). Moreover, the induced
Vol. 59, No. II
KCN o =30x
106dyn/cm 2
400
Q;
300
L /L
5
,# i
o 4X c-
~D
o,o
3
2OO
I00
50
$00 T-Tc(K)
150
Figure 1 - Variation of the stress-induced phase shift (or birefringence) with temperature for KCN. Stress value o=30×106dyne/om 2 . Crystal thickness d=l,2mm.
alignment is reversible (it disappears after stress removal). Figure two shows that ~ has a linear dependence with the applied stress, allowing 70 Brewster value (i Brewster = 10 -13 cm2/dyne) for the stress optical coefficient C at room temperature. The paraelastic behavior is thus put in evidence in agreement with the predictions made with the aid of the elastic-magnetic analogy discussed above. The results shown in figure one should also be compared with those obtained for stress induced birefringence of pure alkali-halides (C values in the i to 4 Brewster range) 11, that show a temperature independent elastic response. The above comparison shows the fundamental role played by the non-spherical CN- ion when substituting the spherical halide ions in this type of lattice. The elastic dipole associated with the molecule in this high concentration limit partially aligns in a rather small elastic field showing a higher degree of alignment close (and just above) T c. This behavior is radically distinct from the low concentration and low temperature paraelasticity discussed in connection with O~ , CN- doped alkali halides; in this case, the low concentration precludes elastic dipole - elastic dipole interactions. On the other hand, in the high concentration limit, the collective behavior of the elastic radicals - that ultimately leads to the ferroelastic first order phase transition (thus defining a critical
Vol. 59, No. II
PARAELASTIC BEHAVIOR OF POTASSIUM CYANIDE
3oo
KCN T= 3 0 0 K
f,n
~-'~ 2 0 0 "u
I00 /
•
I
I00
I
I
300
200
I
400
O (I 0 e dyn ) cm 2
Figure 2 - Variation of the stress-induced phase shift with stress for KCN. Temperature T=300K. Crystal thickness d=2,5rmn.
temperature) - brings a measurable elastic behavior to higher temperatures. The tensorial character of the elastic dipole 12 should make the KCN elastic response highly dependent on the external stress direction relatively to the cubic axes. Experiments with distinct symmetries and with other crystals of the alkali halide-cyanide family are under way.
Acknowledgements - We would llke to thank Dr. Fritz Luty (Physics Department, The University of Utah) and Dr. Milton de Souza (Physics and Chemistry Institute at Sao Carlos, The University of Sao Paulo) for kindly providing the samples used in this experiment.
REFERENCES I. 2.
3. 4. 5.
6.
W. Beall Fowler, Physics of Color Centers, Academic Press (1968). L. Kuhn, M.L. Dakss, P.F. Heidrich and B.A. Scott, Applied Physics Letters 17, 265 (1970). W. Kanzig, Journal of Physics and Chemistry of Solids 23, 479 (1962). F. Luty, Physical Reveiw B iO, 3677 (1974). D. Durand, L.C. Scavarda do Carmo, A. Anderson and F. Luty, Physical Review B 22, 4005 (1980). A. Cimino and G.S. Parry, II Nuovo Cimento XIX, 971 (1961).
7. 8.
K.Aizu, Physical Review B 2, 754 (1970). L.C.Scavarda do Carmo and F. Luty, Gatlinburg 1978. 9. M.D. Julian, PhD Thesis, The University of Utah (unpublished). I0. A.J. Michael, Journal of the Optical Socie of America 58, 889 (1968). ii. C.S. Chen, J.P. Szczesniak and J.C. Corell: Journal of Applied Physics 46, 303 (1975). 12. A.S. Nowick and W.R. Heller, Advances in Physics (GB) 12, 251 (1963).