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Crystal growth and phase transitions of CsPbCl3
Volume
28A, number
11
CRYSTAL
PHYSICS
GROWTH
AND
LETTERS
PHASE
TO March 1969
TRANSITIONS
OF CsPbCl3
S. HIROTSU and S. SAWADA Department
of Physics, Faculty of Science, Oh-okayama, Megwo-ku, Received
Three phase transitions were confirmed of conoscopic figures with temperature. and specific heat were also performed.
4 February
I
37 41.5 TEMPERATURE (‘C ) Fig. 1. Twin boundaries and extinction directions. Each cube face is a pseudo cubic (100) plane. Crosses indicate the extinction directions and arrows indicate directions of acute bisect&c or tetragonal c-axis.
762
of Technology,
1969
in the perovskite-type crystal CsPbCl2 by observing changes Measurements of the temperature dependence of birefringence
Previously it has been reported that CsPbCl3 undergoes a cubic to tetragonal phase transition at 470C and below this temperature the compound exhibits superstructure [1,2]. From the reults of structure analysis and transition entropy measurement, Mdller reported that this phase transition is of an order-disorder type [3]. On the other hand, the existence of another phase transition at about 40°C was confirmed by Sakudo et al. by means of X-ray and sound velocity measurements. These authors reported that the super-structure is developed below this phase transition [4]. Recently, we have prepared large single crystals of this substance by the Bridgman method. Our optical, X-ray and thermal studies have brought out several new facts about the phase transitions in this substance. The compound was prepared by fusing 99.5% PbC12 and CsCl together in stoichiometric proportions. The compound so prepared was zonerefined and only the purer part of the zone-refined ingot was used in single crystal growth. This procedure is effective in removing a small amount of other phases such as Cs4PbCl6 and unreacted reagents which arise as a result of deviations from stoichiometry. A fused quartz
I
Tokyo Institute Tokyo, Japan
ampoule was used as a crucible and the ampoule was sealed with HCl gas to prevent vaporization. The high and low temperature parts of the Bridgman furnace were kept at 610°C and 58OOC respectively. (The melting point is about 6000C). The pulling speed was %mm/hr. Owing to vaporization, crystal growth by the Czochralski technique was unsuccessful. The crystal so obtained was light yellow in colour and cleaves along the pseudo-cubic (100) plane. Under the polarizing microscope twins with the pseudo-cubic (110) as composition planes are frequently observed. Conoscopic observations were made on untwinned regions of single crystals. The conoscopic figure at room temperature corresponds to that of biaxial crystals with the acute bisectrix as one of the pseudo-cubic axis and other two axes (optic elasticity axes or principal vibration directions) along the pseudo-cubic [llO] direction. At 37OC a sudden change of the conoscopic figure occurs, that is, two axes directed
m
TEMFEI?AlU?E(.C 1
Fig. 2. Temperature dependence of birefringence na- xb. Suffixes a and b refer to principal vibration directions in a plane perpendicular to acute bisectrix. Circles indicate symmetrical extinction and dots indicate parallel extinction.
PHYSICS
Volume 28A, number 11
along pseudo-cubic [llO] change their direction to pseudo-cubic [llO]. Although the crystal remains biaxial above 37OC, the optic angle decreases gradually as the temperature rises and at 41.5OC the crystal becomes tetragonal. (The acute bisectrix becomes the tetragonal c-axis). Twins present at room temperature remain unchanged up to 470C at which temperature the crystal becomes cubic. Twin configurations and extinction directions are shown in fig. 1. The temperature dependence of birefringence n, - a,, was measured using a Berek compensator on cleavage planes and is shown in fig. 2. From this figure we see that the phase transition at 41.5oC should be of second order. On the other hand, a similar measurement on na - n, reveals that this quantity drops abruptly to zero around 47OC which shows the phase transition at 47oC to be of first order. The temperature dependence of the specific heat was measured with an adiabatic-type calorimeter. Two anomalies each centered at 4’l°C and at about 400C respectively were observed. A more careful measurement would reveal the separation of the latter into two peaks. The
MAGNETIC
ANISOTROPY
10 March 1969
LETTERS
transition entropy was estimated for each of the peaks. The results are: AS N 0.25 cal/mol. deg. for the peak at 47OC and AS N 0.35 cal/mol. deg. for the peak at 40°C. It should be noted that these values are much smaller than the value obtained by tiller from electrochemical measure, ments which amounts to 4.0 cal/mol. deg. for the cubic to tetragonal transition [3]. The authors are grateful to Dr. T. Sakudo, Mr. H. Unoki and Prof. J. Kobayashi for valuable discussions.
References C. K. Mfiler, Mat. Fys. Medd. Dan. Vid. Selsk. 32, no. 2 (1959). F. W.Ainger, C. C. Clark, A. Marsh and P. Waterworth, Proc. Intern. Meeting on Ferroelectricity (Prague, 1968) Vol. 1, p. 295. C.K.M&ler, Mat. Fys. Medd. Dan. Vid. Selsk. 32, no. 15 (1960). T. Sakudo, H. Unoki, Y. Fujii, J. Kobayashi and M. Yamada, to be published.
INDUCED
BY AN ELECTRIC
FIELD
A. A. HIRSCH, E. AHILEA and N. FRIEDMAN Department
of Physics,
Technion, Israel Institute
of Technology,
Haifa, Israel
Received 4 February 1969
A new process for inducing magnetic anisotropy in thin films is described. The discontinuous films are deposited in a high vacuum in the presence of an electric field. The magnetization loops are related to an anisotropy arising from field effected changes in particle shape.
In the last few years several investigators [l-3] have shown that an electric field applied during or after the deposition of thin metallic films in a high vacuum causes a significant irreversible increase in their electric conductivity. It is well known that such films are normally discontinuous, and one expects that the electric field will be concentrated at the narrowest gaps between neighboring metal islands. A stretching of these islands in the direction of the field across the corresponding gaps may reduce the shortest separation from island to island and therefore materially alter the conductivity. It
seems plausible to assume that the local electric fields will be in zigzag directions, and the elongations of the different metal islands are expected to be randomly oriented. Recently [4] when studying in our laboratory the conductivity of platinum films down to liquid helium temperatures, it was concluded that the activation energy decreased if a field of about 40 V/cm was applied while the film was being formed. Such an activation energy dependence may be attributed to a process of particle elongation and eventual coalescence, causing reduction in the intergranular gaps. 763
28A, number
11
CRYSTAL
PHYSICS
GROWTH
AND
LETTERS
PHASE
TO March 1969
TRANSITIONS
OF CsPbCl3
S. HIROTSU and S. SAWADA Department
of Physics, Faculty of Science, Oh-okayama, Megwo-ku, Received
Three phase transitions were confirmed of conoscopic figures with temperature. and specific heat were also performed.
4 February
I
37 41.5 TEMPERATURE (‘C ) Fig. 1. Twin boundaries and extinction directions. Each cube face is a pseudo cubic (100) plane. Crosses indicate the extinction directions and arrows indicate directions of acute bisect&c or tetragonal c-axis.
762
of Technology,
1969
in the perovskite-type crystal CsPbCl2 by observing changes Measurements of the temperature dependence of birefringence
Previously it has been reported that CsPbCl3 undergoes a cubic to tetragonal phase transition at 470C and below this temperature the compound exhibits superstructure [1,2]. From the reults of structure analysis and transition entropy measurement, Mdller reported that this phase transition is of an order-disorder type [3]. On the other hand, the existence of another phase transition at about 40°C was confirmed by Sakudo et al. by means of X-ray and sound velocity measurements. These authors reported that the super-structure is developed below this phase transition [4]. Recently, we have prepared large single crystals of this substance by the Bridgman method. Our optical, X-ray and thermal studies have brought out several new facts about the phase transitions in this substance. The compound was prepared by fusing 99.5% PbC12 and CsCl together in stoichiometric proportions. The compound so prepared was zonerefined and only the purer part of the zone-refined ingot was used in single crystal growth. This procedure is effective in removing a small amount of other phases such as Cs4PbCl6 and unreacted reagents which arise as a result of deviations from stoichiometry. A fused quartz
I
Tokyo Institute Tokyo, Japan
ampoule was used as a crucible and the ampoule was sealed with HCl gas to prevent vaporization. The high and low temperature parts of the Bridgman furnace were kept at 610°C and 58OOC respectively. (The melting point is about 6000C). The pulling speed was %mm/hr. Owing to vaporization, crystal growth by the Czochralski technique was unsuccessful. The crystal so obtained was light yellow in colour and cleaves along the pseudo-cubic (100) plane. Under the polarizing microscope twins with the pseudo-cubic (110) as composition planes are frequently observed. Conoscopic observations were made on untwinned regions of single crystals. The conoscopic figure at room temperature corresponds to that of biaxial crystals with the acute bisectrix as one of the pseudo-cubic axis and other two axes (optic elasticity axes or principal vibration directions) along the pseudo-cubic [llO] direction. At 37OC a sudden change of the conoscopic figure occurs, that is, two axes directed
m
TEMFEI?AlU?E(.C 1
Fig. 2. Temperature dependence of birefringence na- xb. Suffixes a and b refer to principal vibration directions in a plane perpendicular to acute bisectrix. Circles indicate symmetrical extinction and dots indicate parallel extinction.
PHYSICS
Volume 28A, number 11
along pseudo-cubic [llO] change their direction to pseudo-cubic [llO]. Although the crystal remains biaxial above 37OC, the optic angle decreases gradually as the temperature rises and at 41.5OC the crystal becomes tetragonal. (The acute bisectrix becomes the tetragonal c-axis). Twins present at room temperature remain unchanged up to 470C at which temperature the crystal becomes cubic. Twin configurations and extinction directions are shown in fig. 1. The temperature dependence of birefringence n, - a,, was measured using a Berek compensator on cleavage planes and is shown in fig. 2. From this figure we see that the phase transition at 41.5oC should be of second order. On the other hand, a similar measurement on na - n, reveals that this quantity drops abruptly to zero around 47OC which shows the phase transition at 47oC to be of first order. The temperature dependence of the specific heat was measured with an adiabatic-type calorimeter. Two anomalies each centered at 4’l°C and at about 400C respectively were observed. A more careful measurement would reveal the separation of the latter into two peaks. The
MAGNETIC
ANISOTROPY
10 March 1969
LETTERS
transition entropy was estimated for each of the peaks. The results are: AS N 0.25 cal/mol. deg. for the peak at 47OC and AS N 0.35 cal/mol. deg. for the peak at 40°C. It should be noted that these values are much smaller than the value obtained by tiller from electrochemical measure, ments which amounts to 4.0 cal/mol. deg. for the cubic to tetragonal transition [3]. The authors are grateful to Dr. T. Sakudo, Mr. H. Unoki and Prof. J. Kobayashi for valuable discussions.
References C. K. Mfiler, Mat. Fys. Medd. Dan. Vid. Selsk. 32, no. 2 (1959). F. W.Ainger, C. C. Clark, A. Marsh and P. Waterworth, Proc. Intern. Meeting on Ferroelectricity (Prague, 1968) Vol. 1, p. 295. C.K.M&ler, Mat. Fys. Medd. Dan. Vid. Selsk. 32, no. 15 (1960). T. Sakudo, H. Unoki, Y. Fujii, J. Kobayashi and M. Yamada, to be published.
INDUCED
BY AN ELECTRIC
FIELD
A. A. HIRSCH, E. AHILEA and N. FRIEDMAN Department
of Physics,
Technion, Israel Institute
of Technology,
Haifa, Israel
Received 4 February 1969
A new process for inducing magnetic anisotropy in thin films is described. The discontinuous films are deposited in a high vacuum in the presence of an electric field. The magnetization loops are related to an anisotropy arising from field effected changes in particle shape.
In the last few years several investigators [l-3] have shown that an electric field applied during or after the deposition of thin metallic films in a high vacuum causes a significant irreversible increase in their electric conductivity. It is well known that such films are normally discontinuous, and one expects that the electric field will be concentrated at the narrowest gaps between neighboring metal islands. A stretching of these islands in the direction of the field across the corresponding gaps may reduce the shortest separation from island to island and therefore materially alter the conductivity. It
seems plausible to assume that the local electric fields will be in zigzag directions, and the elongations of the different metal islands are expected to be randomly oriented. Recently [4] when studying in our laboratory the conductivity of platinum films down to liquid helium temperatures, it was concluded that the activation energy decreased if a field of about 40 V/cm was applied while the film was being formed. Such an activation energy dependence may be attributed to a process of particle elongation and eventual coalescence, causing reduction in the intergranular gaps. 763