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Phase transition at 211 K in crystalline barium nitrite hemihydrate
J. Phys. Chew. Solids Vol. 47, No. ii, Printed in Great Britain.
pp. IOSS-t088,
1986
0022-3697X6 %3.00 + 0.00 Q 1986 Fwgamon Journals Ltd.
PHASE TRANSITION AT 211 K IN CRYSTALLINE BARrU~ NITRITE HE~I~YDRATE HITOSHI KAWAJI, KAZUYA SAITO, TOORU ATAKE and YASUTOSHI SAITO Research Laboratory of Engineering Materials, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama 227, Japan (Received 14 April 1986; accepted 15 May 1986)
Abstract-A phase transition has been observed at 211 K in crystalline barium nitrite hemihydmte by RTA and dielectric measurements. A dielectric relaxation phenomenon was also found and the activation energies governing the relaxation were determined to be 23 kJ mol-’ and 41 kJ mol-’ below and above the transition temperature, respectively. Some discussions have been made on the nature of the phase transition. K~ywordr: Ea(NO,), . frH20, phase transition, DTA, dielectric constant. dielectric relaxation.
INTRODUCTION
It is well known that barium nitrite monohydrate [Ba(NO& *H,O] is pyroelectric [l-7] below the phase t~nsition at 350 K 181.No phase transition has been reported below room temperature [6]. As the temperature increases, the monohydrate crystal dehydrates at about 370 K and forms hemihydrate [Ba(NO,),*iH,O] [S, 93. On further heating, the hemihydrate changes into the anhydrate [Ba(NO,),] at about 450 K [8,9]. The anhydrate crystal undergoes two phase transitions at 469 K and 497 K before melting at 542 K. The crystal structure of the highest temperature phase is monoclinic [lo]. The entropies of the two transitions and the entropy of fusion are 13.6, 12.2 and 17.6 J K-’ mol-‘, respectively, as de-
termined by DSC [lo]. Since the value of the entropy of fusion is very small, the highest tem~rature solid phase is expected to be a kind of “ionic plastic crystal” [ 11, 121in which the orientational motions of the ions are highly excited. Any other studies other than those described above have so far been very few, and little is known about accurate phase relations and the~~ynamic properties of the compounds of barium nitrite. This situation prompted us to start thermodynamic studies of this series of substances. In the course of the studies, a phase transition of crystalline barium nitrite hemihydrate was discovered. In this paper, the experimental details will be given of the DTA and the dieiectric measurements, and the determination of the transition point will be reported. A dielectric relaxation phenomenon observed in barium nitrite hemihydrate will be also described. EXPERIMENT
Sample preparation Barium nitrite monohydrate was purchased from Kanto Kagaku Co. Ltd. and purified by repeated
recrystallization from aqueous solution. The purified specimen was identified as the expected monohydrate by powder X-ray diffractometry. The X-ray diffraction patterns were obtained with a diffractometer using N&filtered CuKcl radiation, and no detectable impurity was found in the specimen. The mass percentage of Ba was 55.51 (calculated as 55.52) as determined by gravimetric analysis. The dehydration behavior of a sample of monohydrate was preliminarily studied by TG-DTGDTA (TG-20, Seiko I & E Ltd.). On heating, dehydration proceeded and the rate became greater when the transition was traversed at about 350K, and the hemihydrate was formed after an abrupt loss of weight at about 370 K. The dehydration was also seen in the DTA trace. As the single crystal of monohydrate broke down during the dehydration, a single crystal of hemihydrate was not obtained. The preliminary experiments provided useful information for the preparation of the hemihydrate sample used in the DTA and the dielectric measurements.
On the basis of the preliminary experiments described above, experiments of the highly-sensitive classical type of DTA were carried out between 100 and 480 K on the hemihydrate sample; the details of the apparatus will be described elsewhere [13]. The powdered specimen of monohyd~te (weighed about 300 mg) was put into a sample vesse1 which was made of glass. The sample vessel was not sealed off. It was then loaded into the DTA apparatus. As the apparatus was vacuum-tight, the atmosphere surrounding the sample could be controlled. By repeated replacements of the gas inside the apparatus at about 380 K, the partial pressure of the water vapor was conditioned to be at the equilibrium pressure of hemihydrate. The performance of the conditioning of the atmosphere inside the apparatus was confirmed by 1085
1086
HITOSHIKAWAJI er al
Fig. 1. Sectional view of the cryostat for dielectric measurements. A, sample tablet; B, electrodes; C, plates supporting electrodes; D, nuts; E, electrical leads; F, thermocouple; G, screwed stainless steel rod; H, spring; I, copper mantle; J, copper flange; K, stainless steel tubing; L, copper rod; M, brass rod; N, copper rod; 0, copper wires; P, manganin heater.
the stainless steel tubing K. The heater P (manganin wire, 0.29 mm in diameter and 25 R in total resistance) was wound closely with an adhesive (LARCTPI, Mitsui Toatsu Chemicals Inc.) on the copper rod L (8 mm in diameter, 50mm in length). The assembly was placed in a Dewar vessel which was used as empty or filled with liquid nitrogen depending on the operating temperature. The dielectric measurements were performed by using a vector impedance meter (HP-4800A, Hewlett-Packard Co.). Data acquisition and manipulation were automated; the system was composed of a universal scanner (TR7200, Takeda Riken Industry Co. Ltd.), a digital multimeter (TR6851) and a microcomputer (PC-9801 M2, NEC Corp.). Taking the shape of the sample tablet and the stray capacity of the apparatus (ca 30pF) into account, complex dielectric constants were calculated and printed out. The sample tablet of hemihydrate used for dielectric measurements was prepared in situ as follows. The purified powder of monohydrate was pressed into a tablet (16 mm in diameter and 0.5 mm in thickness) at 300 MPa. The tablet was loaded into the cryostat, and then heated up to 375 K. The dehydration from monohydrate to hemihydrate was clearly detected as an abnormal increase in the complex dielectric constant. The water vapor generated in the course of dehydration was absorbed on silica gel. As the cryostat was airtight, the atmosphere surrounding the tablet could be maintained at the water vapor pressure of hemihydrate during the experiments. RESULTS AND DISCUSSION
the fact that no anomaly due to the dehydration from monohydrate to hemihydrate was detected at about 370 K in the DTA on heating.
Dielectric measurements A simple conventional apparatus was constructed for the dielectric measurements of the hemihydrate in the temperature range of about lo&370 K. The major portion of the cryostat is shown in Fig. 1. The sample tablet A was interposed between the pair of electrodes B (14 mm in diameter) which was pressed moderately by the spring H. The electrical leads E were connected to a coaxial cable (6mm in outer diameter) which ran through the spring H and the stainless steel tubing K (10 mm in diameter, 0.2 mm in wall-thickness and 25 cm in length), and the cable was taken out of the cryostat through epoxy-resin sealing at the top flange. The thermocouple F (chromelconstantan, 0.13 mm in diameter, double silk-insulation, Driver-Harris Co. Ltd.) was attached to the upper plate of the electrode. The copper mantle I (35 mm in diameter, 2 mm in wall-thickness and 85 mm in length) was soldered vacuum-tight with Wood’s alloy to the copper flange J (35 mm in diameter and 3 mm in thickness) which was fixed to
In the DTA experiments on the powdered sample of barium nitrite hemihydrate, an exothermic peak appeared on cooling, and reheating gave an endothermic peak at the same temperature. The DTA traces are shown in Fig. 2. The anomaly is small and very broad extending from about 190 K to about 215 K for both cooling and heating. It is worth noting that the anomaly has a long tail on the low temperature side. The results suggest a higher order mechanism of phase transition in barium nitrite hemihydrate. The transition temperature is obtained as the onset temperature for the phase transition upon cooling. Upon heating, on the other hand, the peak of the anomaly should be taken as the transition point. The same temperature of 211 K was obtained in both determinations, which was almost independent of the rates of cooling and heating in the DTA experiments (0.02-0.05 K ). Thus, it is concluded that a phase transition occurs at (211.0 &-0.3) K in crystalline barium nitrite hemihydrate. The existence of the phase transition was demonstrated also by dielectric measurements. The temperature dependences of the complex dielectric constants (c’ and’e”) obtained at 1.0 kHz are shown in Fig. 3. The transition is clearly detected as a small hump at
ssr
1087
Phase transition at 211 K Cooling ( -0.040
K se’ 1
I
I
0
100 Fig.
Heating (0.026 KS-‘)
Fig. 2. DTA traces of Ba(NO,)*.i H,O.
211 K in the curve of the imaginary part, 6 “. A small anomaly due to the phase transition exists also in the t’ curve, although it is hardly observable in Fig. 3. These anomalies are independent of the measuring frequencies from 320 Hz-320 kHz, and are always observed at 211 K. On the contrary, there exists a large anomaly in the 6” with a maximum at about 180 K, at which the E’ curve shows large variation. These phenomena depend on the measuring frequency. Such behavior implies the existence of a kind of relaxation mechanism. Thus, the 6” curves measured at various frequencies are shown in Fig. 4. As the frequency increases, the maximum temperature increases and exceeds the transition point when the measurements
10 ,
I
.
I
200 r/K
300
4. Imaginary parts of dielectric constants Ba(NO,), .f H,O measured at various frequencies.
of
are made at 32 kHz. The maximum value of E” also increases as the measu~ng frequency increases, This is probably due to asymmetry in the potential hindering reorientation of some atomic groups (I-&O and/or NO;) in the crystal [14]. From the results, an Arrhenius plot is obtained as shown in Fig. 5, assuming that the average relaxation frequency coincides with the measming frequency at the maximum point. Two straight lines can be drawn and are clearly divided at the transition point. From the slopes, the activation energies are determined as 23 kJ mol-l and 41 kJ mol-’ below and above the transition temperature, respectively. These values of activation energy are reasonable for reorientation of HZ0 and/or NO; in the crystal. There remains a problem to be considered, that is, the nature of the phase transition at 211 K in barium nitrite hemihydrate. Although the DTA traces suggest a higher order mechanism of the phase tranT/K
250
200
150
c
Z ::4 4
d -. 5 2
3
:
I
200 T/
I
300
:, *
K
Fig. 3. Complex dielectric constants of Ba(NO,),*f II,0 measured at 1.0 kHz.
5
6
T-l/ kK-’
Fig. 5. Arrhenius
plot of the relaxation Ba(NO,), *f H,O.
frequency
of
HITOSHIKAWAJI et at.
1088
sition, the jump which appeared in the Arrhenius plot (Fig. 5) is characteristic of the first order transition. A possible explanation is that the transition is of the first order with a very small latent heat and a rather large tail on the low temperature side. In order to clarify the mechanism of the phase transition and molecular motion in the crystal, further extensive studies will be needed. More detailed thermodynamic investigation
is under way.
REFERENCE I. Gladkii V. V. and Zheludev I. S., ~~~s~ai~ogra~y~lo,63 (1965). 2. Gladkii V. V. and Zheludev I. S., Kristailograjiya 12,
905 (1967).
3. Haussiihl S., Acta Crysr. A 34, 547 (1978).
4.
Abrahams
S. C., Bernstein J. L. and Liminga R.,
J. them. Phys. 72, 58.57 (1980). 5. Liminga R., Abrahams S. C. and Bernstein J. L.. J. appl. Crysr. 13, 516 (1980). 6. Kvick A., Liminga R. and Abrahams S. C., J. &WI. P&s. 76, 5508 (1982). 7. Liminga R., Abrahams S. C., Glass A. M. and Kvick A., Phys. Rev. B 26, 6896 (1982). 8. Gallagher P. K., Abrahams S. C., Wood D. L., Schrey F. and Liminga R., .I. &em. Whys. 75, 1903 (1981). 9. Gallagher P. K., Thermochim. Acta 51. 233 (19811. 10. Abrahams S. C., Gallagher P. K., O’Bryan H. M.‘and Liminga R., J. a&. Whys. 52, 2837 (1981). i L. Moriya K., Matsuo T., Suga H. and Seki S., L*ht~nr. Left. 1427 (1977). 12. The PlasticaNy Crystalline State (Edited by J. N. Sherwood). John Wiley & Sons Ltd., Chichester (1979). 13. Atake T., Hamano A. and Saito Y., Thermochim. Acta (in press). 14. Meakins R. J., Prog. Dielecrrics 2, 153 (1960).
pp. IOSS-t088,
1986
0022-3697X6 %3.00 + 0.00 Q 1986 Fwgamon Journals Ltd.
PHASE TRANSITION AT 211 K IN CRYSTALLINE BARrU~ NITRITE HE~I~YDRATE HITOSHI KAWAJI, KAZUYA SAITO, TOORU ATAKE and YASUTOSHI SAITO Research Laboratory of Engineering Materials, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama 227, Japan (Received 14 April 1986; accepted 15 May 1986)
Abstract-A phase transition has been observed at 211 K in crystalline barium nitrite hemihydmte by RTA and dielectric measurements. A dielectric relaxation phenomenon was also found and the activation energies governing the relaxation were determined to be 23 kJ mol-’ and 41 kJ mol-’ below and above the transition temperature, respectively. Some discussions have been made on the nature of the phase transition. K~ywordr: Ea(NO,), . frH20, phase transition, DTA, dielectric constant. dielectric relaxation.
INTRODUCTION
It is well known that barium nitrite monohydrate [Ba(NO& *H,O] is pyroelectric [l-7] below the phase t~nsition at 350 K 181.No phase transition has been reported below room temperature [6]. As the temperature increases, the monohydrate crystal dehydrates at about 370 K and forms hemihydrate [Ba(NO,),*iH,O] [S, 93. On further heating, the hemihydrate changes into the anhydrate [Ba(NO,),] at about 450 K [8,9]. The anhydrate crystal undergoes two phase transitions at 469 K and 497 K before melting at 542 K. The crystal structure of the highest temperature phase is monoclinic [lo]. The entropies of the two transitions and the entropy of fusion are 13.6, 12.2 and 17.6 J K-’ mol-‘, respectively, as de-
termined by DSC [lo]. Since the value of the entropy of fusion is very small, the highest tem~rature solid phase is expected to be a kind of “ionic plastic crystal” [ 11, 121in which the orientational motions of the ions are highly excited. Any other studies other than those described above have so far been very few, and little is known about accurate phase relations and the~~ynamic properties of the compounds of barium nitrite. This situation prompted us to start thermodynamic studies of this series of substances. In the course of the studies, a phase transition of crystalline barium nitrite hemihydrate was discovered. In this paper, the experimental details will be given of the DTA and the dieiectric measurements, and the determination of the transition point will be reported. A dielectric relaxation phenomenon observed in barium nitrite hemihydrate will be also described. EXPERIMENT
Sample preparation Barium nitrite monohydrate was purchased from Kanto Kagaku Co. Ltd. and purified by repeated
recrystallization from aqueous solution. The purified specimen was identified as the expected monohydrate by powder X-ray diffractometry. The X-ray diffraction patterns were obtained with a diffractometer using N&filtered CuKcl radiation, and no detectable impurity was found in the specimen. The mass percentage of Ba was 55.51 (calculated as 55.52) as determined by gravimetric analysis. The dehydration behavior of a sample of monohydrate was preliminarily studied by TG-DTGDTA (TG-20, Seiko I & E Ltd.). On heating, dehydration proceeded and the rate became greater when the transition was traversed at about 350K, and the hemihydrate was formed after an abrupt loss of weight at about 370 K. The dehydration was also seen in the DTA trace. As the single crystal of monohydrate broke down during the dehydration, a single crystal of hemihydrate was not obtained. The preliminary experiments provided useful information for the preparation of the hemihydrate sample used in the DTA and the dielectric measurements.
On the basis of the preliminary experiments described above, experiments of the highly-sensitive classical type of DTA were carried out between 100 and 480 K on the hemihydrate sample; the details of the apparatus will be described elsewhere [13]. The powdered specimen of monohyd~te (weighed about 300 mg) was put into a sample vesse1 which was made of glass. The sample vessel was not sealed off. It was then loaded into the DTA apparatus. As the apparatus was vacuum-tight, the atmosphere surrounding the sample could be controlled. By repeated replacements of the gas inside the apparatus at about 380 K, the partial pressure of the water vapor was conditioned to be at the equilibrium pressure of hemihydrate. The performance of the conditioning of the atmosphere inside the apparatus was confirmed by 1085
1086
HITOSHIKAWAJI er al
Fig. 1. Sectional view of the cryostat for dielectric measurements. A, sample tablet; B, electrodes; C, plates supporting electrodes; D, nuts; E, electrical leads; F, thermocouple; G, screwed stainless steel rod; H, spring; I, copper mantle; J, copper flange; K, stainless steel tubing; L, copper rod; M, brass rod; N, copper rod; 0, copper wires; P, manganin heater.
the stainless steel tubing K. The heater P (manganin wire, 0.29 mm in diameter and 25 R in total resistance) was wound closely with an adhesive (LARCTPI, Mitsui Toatsu Chemicals Inc.) on the copper rod L (8 mm in diameter, 50mm in length). The assembly was placed in a Dewar vessel which was used as empty or filled with liquid nitrogen depending on the operating temperature. The dielectric measurements were performed by using a vector impedance meter (HP-4800A, Hewlett-Packard Co.). Data acquisition and manipulation were automated; the system was composed of a universal scanner (TR7200, Takeda Riken Industry Co. Ltd.), a digital multimeter (TR6851) and a microcomputer (PC-9801 M2, NEC Corp.). Taking the shape of the sample tablet and the stray capacity of the apparatus (ca 30pF) into account, complex dielectric constants were calculated and printed out. The sample tablet of hemihydrate used for dielectric measurements was prepared in situ as follows. The purified powder of monohydrate was pressed into a tablet (16 mm in diameter and 0.5 mm in thickness) at 300 MPa. The tablet was loaded into the cryostat, and then heated up to 375 K. The dehydration from monohydrate to hemihydrate was clearly detected as an abnormal increase in the complex dielectric constant. The water vapor generated in the course of dehydration was absorbed on silica gel. As the cryostat was airtight, the atmosphere surrounding the tablet could be maintained at the water vapor pressure of hemihydrate during the experiments. RESULTS AND DISCUSSION
the fact that no anomaly due to the dehydration from monohydrate to hemihydrate was detected at about 370 K in the DTA on heating.
Dielectric measurements A simple conventional apparatus was constructed for the dielectric measurements of the hemihydrate in the temperature range of about lo&370 K. The major portion of the cryostat is shown in Fig. 1. The sample tablet A was interposed between the pair of electrodes B (14 mm in diameter) which was pressed moderately by the spring H. The electrical leads E were connected to a coaxial cable (6mm in outer diameter) which ran through the spring H and the stainless steel tubing K (10 mm in diameter, 0.2 mm in wall-thickness and 25 cm in length), and the cable was taken out of the cryostat through epoxy-resin sealing at the top flange. The thermocouple F (chromelconstantan, 0.13 mm in diameter, double silk-insulation, Driver-Harris Co. Ltd.) was attached to the upper plate of the electrode. The copper mantle I (35 mm in diameter, 2 mm in wall-thickness and 85 mm in length) was soldered vacuum-tight with Wood’s alloy to the copper flange J (35 mm in diameter and 3 mm in thickness) which was fixed to
In the DTA experiments on the powdered sample of barium nitrite hemihydrate, an exothermic peak appeared on cooling, and reheating gave an endothermic peak at the same temperature. The DTA traces are shown in Fig. 2. The anomaly is small and very broad extending from about 190 K to about 215 K for both cooling and heating. It is worth noting that the anomaly has a long tail on the low temperature side. The results suggest a higher order mechanism of phase transition in barium nitrite hemihydrate. The transition temperature is obtained as the onset temperature for the phase transition upon cooling. Upon heating, on the other hand, the peak of the anomaly should be taken as the transition point. The same temperature of 211 K was obtained in both determinations, which was almost independent of the rates of cooling and heating in the DTA experiments (0.02-0.05 K ). Thus, it is concluded that a phase transition occurs at (211.0 &-0.3) K in crystalline barium nitrite hemihydrate. The existence of the phase transition was demonstrated also by dielectric measurements. The temperature dependences of the complex dielectric constants (c’ and’e”) obtained at 1.0 kHz are shown in Fig. 3. The transition is clearly detected as a small hump at
ssr
1087
Phase transition at 211 K Cooling ( -0.040
K se’ 1
I
I
0
100 Fig.
Heating (0.026 KS-‘)
Fig. 2. DTA traces of Ba(NO,)*.i H,O.
211 K in the curve of the imaginary part, 6 “. A small anomaly due to the phase transition exists also in the t’ curve, although it is hardly observable in Fig. 3. These anomalies are independent of the measuring frequencies from 320 Hz-320 kHz, and are always observed at 211 K. On the contrary, there exists a large anomaly in the 6” with a maximum at about 180 K, at which the E’ curve shows large variation. These phenomena depend on the measuring frequency. Such behavior implies the existence of a kind of relaxation mechanism. Thus, the 6” curves measured at various frequencies are shown in Fig. 4. As the frequency increases, the maximum temperature increases and exceeds the transition point when the measurements
10 ,
I
.
I
200 r/K
300
4. Imaginary parts of dielectric constants Ba(NO,), .f H,O measured at various frequencies.
of
are made at 32 kHz. The maximum value of E” also increases as the measu~ng frequency increases, This is probably due to asymmetry in the potential hindering reorientation of some atomic groups (I-&O and/or NO;) in the crystal [14]. From the results, an Arrhenius plot is obtained as shown in Fig. 5, assuming that the average relaxation frequency coincides with the measming frequency at the maximum point. Two straight lines can be drawn and are clearly divided at the transition point. From the slopes, the activation energies are determined as 23 kJ mol-l and 41 kJ mol-’ below and above the transition temperature, respectively. These values of activation energy are reasonable for reorientation of HZ0 and/or NO; in the crystal. There remains a problem to be considered, that is, the nature of the phase transition at 211 K in barium nitrite hemihydrate. Although the DTA traces suggest a higher order mechanism of the phase tranT/K
250
200
150
c
Z ::4 4
d -. 5 2
3
:
I
200 T/
I
300
:, *
K
Fig. 3. Complex dielectric constants of Ba(NO,),*f II,0 measured at 1.0 kHz.
5
6
T-l/ kK-’
Fig. 5. Arrhenius
plot of the relaxation Ba(NO,), *f H,O.
frequency
of
HITOSHIKAWAJI et at.
1088
sition, the jump which appeared in the Arrhenius plot (Fig. 5) is characteristic of the first order transition. A possible explanation is that the transition is of the first order with a very small latent heat and a rather large tail on the low temperature side. In order to clarify the mechanism of the phase transition and molecular motion in the crystal, further extensive studies will be needed. More detailed thermodynamic investigation
is under way.
REFERENCE I. Gladkii V. V. and Zheludev I. S., ~~~s~ai~ogra~y~lo,63 (1965). 2. Gladkii V. V. and Zheludev I. S., Kristailograjiya 12,
905 (1967).
3. Haussiihl S., Acta Crysr. A 34, 547 (1978).
4.
Abrahams
S. C., Bernstein J. L. and Liminga R.,
J. them. Phys. 72, 58.57 (1980). 5. Liminga R., Abrahams S. C. and Bernstein J. L.. J. appl. Crysr. 13, 516 (1980). 6. Kvick A., Liminga R. and Abrahams S. C., J. &WI. P&s. 76, 5508 (1982). 7. Liminga R., Abrahams S. C., Glass A. M. and Kvick A., Phys. Rev. B 26, 6896 (1982). 8. Gallagher P. K., Abrahams S. C., Wood D. L., Schrey F. and Liminga R., .I. &em. Whys. 75, 1903 (1981). 9. Gallagher P. K., Thermochim. Acta 51. 233 (19811. 10. Abrahams S. C., Gallagher P. K., O’Bryan H. M.‘and Liminga R., J. a&. Whys. 52, 2837 (1981). i L. Moriya K., Matsuo T., Suga H. and Seki S., L*ht~nr. Left. 1427 (1977). 12. The PlasticaNy Crystalline State (Edited by J. N. Sherwood). John Wiley & Sons Ltd., Chichester (1979). 13. Atake T., Hamano A. and Saito Y., Thermochim. Acta (in press). 14. Meakins R. J., Prog. Dielecrrics 2, 153 (1960).