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Some ferroelectric and dielectric properties of triglycine fluoberyllate
J. Phys. Chem. Solids
Pergamon Press 1966. Vol. 27, pp. 247-252.
SOME FERROELECTRIC
Printed in Great Britain.
AND DIELECTRIC
PROPERTIES OF TRIGLYCINE
FLUOBERYLLATE
H. H. WIEDER and C. R. PARKJZRSON U.S.
Naval Ordnance Laboratory, Corona, California
(Received 26 May 1965 ; in revised form 27 August 196.5)
Abstract-Large area tabular TGFB crystals with inhibited growth along the [OOl] direction were grown by doping the growth solution with selected monovalent cations. Irrespective of crystalline habit, the Curie temperature of TGFB was found to be Tc = 73”+0*2”C, the Curie constant C = 2512” + 80’ and the ratio of the slope of the dielectric stiffness in the ferroelectric to that of the the spontaneous polarization P8 = 3.7 x 10ds C/cm2 paraelectric region is p = - 3.99. At +2X!, and the 50 c/s “coercivity” is 1.1 kV/cm. Crystalline imperfections introduced during growth are distributed anisotropically and have a marked effect upon the dielectric and ferroelectric polarization process in TGFB.
1.
INTRODUCTION THE physical properties of triglycine sulfate (TGS) have been investigated extensively ever since the discovery of ferroelectricity in this compound. (1) Considerably less attention has been devoted to the triglycine fluoberyllate isomorphous crystal, (TGFB). The original dielectric and ferroelectric measurements made by HOSHINO et aZ.@) have remained unchallenged to this date, although they reported some significant differences between the thermal and dielectric parameters of TGS and those of TGFB. In particular, the ratio of the slopes of the dielectric stiffness vs. temperature determined in the ferroelectric and paraelectric regions was given as -2-l 1 for TGFB ; hence the adiabatic correction to the isothermal dielectric constant may be considered to be negligible. This is in contrast to TGS which@) requires an adiabatic correction of the order of 20% and where the experimentally determined ratio of the slopes varies from -2.3 to -3 *O. It is significant that crystals such as BaTiOs or KNbOs which undergo a first-order thermodynamic transition at the onset of the ferroelectric state do not require an adiabatic correction term. Yet for TGFB as for TGS, the transition appears to be of second order.@) 247
The Curie temperature of TGFB determined by HOSHINO et al. (2) from dielectric and ferroelectric measurements was given as +7O”C. On the other hand, GUREVICHand ZHELUDEV@)determined the phase-transition to occur between +73”C and + 75°C from their measurements of the anisotropic electrical conductivity. The spontaneous polarization of P8 appears to be independent of temperature below approximately 245°K; this is in contrast, however, with TGS which still appears to have a significantcsls) pyroelectric coefficient (dP,/dT) at 100°K. If (dPs/dT)0 at room temperature for TGFB, then it might be used to test the first order phenomenological and statistical theory for ferroelectricity proposed by GONZALO and LOPEZ-ALONSO@) specifically for TGS.Thereasonsoutlined aboveledustoan experimental investigation of the dielectric and ferroelectric properties of TGFB. We were also interested in attempting to change the normal habit of TGFB by means of a chemically induced habit modification such as applied(7) to TGS. The advantage of a tabular habit with the ferroelectric axis parallel to the main crystal surface is that sectioning of identical laminae from a crystal specimen is simplified, and mechanical strains around localized crystalline defects introduced
248
H. H. WIEDER
and C. R. PARKERSON
during growth may be traced by their effect upon the local polarization and the polarization reversal process.
Solutions No. 3 and No. 4 were prepared respectively with 12.8 g and 68 g TlsBeFd per 100 g TGFB in solution. A turbidity developed on addition of the TlsBeFd and it was found necessary 2. PREPARATION OF HABIT MODIFIED to dilute both the No. 3 and No. 4 solutions with TRIGLYCINE FLUOBERYLLATE approximately 300 ml HsO each to reduce the saturation temperature to +25”C. Each of these The synthesis and growth of TGFB crystals solutions were then filtered and placed in open started with the preparation of fluoberyllic acid plastic beakers maintained at +8O”C; they were which is formed by the following reaction in concentrated to approximatelytheir originalvolume aqueous solution : before dilution. BeFs f2HF +HsBeF4 After saturating each of the four solutions at +40*5”C, the excess crystals were filtered out Beryllium fluoride is very soluble in water; howrepeatedly. The solutions were returned thereafter ever, it is very slow to go into solution. A quantity to their plastic containers, maintained at a temperaof 74*41g BeFs was placed in a sealed flask with ture of + 55°C for about 2 hr. The temperaturewas 400 ml Ha0 and the solution was agitated conallowed to drop to + 47°C at which time seed crystinuously by means of a magnetic stirrer. A period tals mounted on plastic tubing were planted in of 10 to 14 days was required to dissolve all the BeFs. The BeFs solution was then reacted in a the solution. The containers were placed in a water bath maintained at +40*5X’ and a programmed plastic beaker with 63.4 g HF (132.0 g 48% HF) temperature reduction was started at 0*2”C!per day. which produced 1378 g HsBeF4 in 468 g I&O. Numerous growth. runs using these solutions in The HzBeF4 solution was then reacted with 357-O the range of 40*5”-30°C confirm the following g glycine (3 moles glycine per mole HsBeF4) to conclusions : produce 494*8 g triglycine fluoberyllate in 468 g 1. Solutions No. 1 and No.2 containing Csf addiHz0 : tive yield crystals with the modified, tabular habit 3 NHsCHsCOOH + HsBeF4 shown in Fig. l(a). 2. Solutions No. 1 and No. 2 containing Csf +(NH2CH2COOH)3. HzBcF4 additive are extremely unstable as regards extraneThe solution was heated to complete the solution ous nucleation and surface twinning, particularly as the seed crystal begins to grow in size. Changing process then allowed to cool to room temperature; the temperature reduction to as low as O*OS”Cper the excess TGFB crystallized out. day did not improve this instability. In order to constrain growth in the [OOl] direc3. There does not appear to be any large differtion, monovalent cations(T) need be introduced ence in stability of a TGFB solution containing into the mother solution. It was necessary, there5.6 g CssBeFa additive (No. 2) compared to that fore, to prepare a quantity of CssBeF4 and containing 13~2 g CssBeF4 (No. 1) per 100 g TlsBeF4. The CssBeF4 was formed as a concenTGFB in solution. trated solution in water by the reaction of CSOH 4. Solutions of TGFB containing TlsBeF4 addiwith an equivalent amount of HzBeF4 solution. tive are more stable than those containing Cs+ and The TlsBeF4 was formed by reacting Tl3CO3 with good single crystals such as shown in Fig. l(b) can an equivalent amount of HsBeF4 solution, heating be grown consistently in the temperature range of to remove the COs, and then crystallizing out the +40~5”c-+30”c. TlsBeF4 with an excess of absolute ethyl alcohol. 5. A concentration of 10.6 g Tlf per 100 g Two Csf doped solutions were prepared by adding the CssBeF4 solution to two TGFB SO~U- TGFB in solution (solution No. 3) has a smaller effect upon the crystalline habit of TGFB comtions. Solution No. 1 with 13.2 g CszBeF4 per pared to TGS for the same Tlf concentration, 100 g TGFB in solution and solution No. 2 with Growth in the (001) plane for TGS is less than 1% 5.6 g CssBeF4 per 100 g TGFB in solution. These that of the normal, uninhibited growth; for TGFB, solutions were then clarified of sedimentation by the corresponding growth is approximately 12%. filtering.
FIG. 1. (a) Crystal of habit modified TGFB grown in Csf doped solution; longest dimension is 9.61 cm, thickness is 0.18 cm.
FIG. 1. (b) TGFB crystal grown in Tl+ doped solution sectioned into laminae normal to the ferroelectric (010) axis, after reassembly.
FIG. 2. Static pattern of polarization reversal at 400 cs; specimen was reconstituted showing pattern on each face of a lamina and ita relation to position within the crystal.
SOME FERROELECTRIC
AND DIELECTRIC
PROPERTIES
Decreasing the Tl+ concentration to 5.6 g per 100 g TGFB (solution #4) decreases the growth inhibition along the (001) plane which is then 35% of normal. We presume that the decrease in the interaction cross-section between the Tl anions and the BeF4 cations in solution is due to the larger effective mass of the ions in TGFB compared to TGS. A variant of the method described above was used for growing well defined, untwinned TGFB crystals from a Cs+ doped solution. Sufficient additional TGFB was added to 320 ml of TGFB solution No. 2 (Cs+) to increase the saturation temperature to 64.0% and the excess crystals removed by rapid filtration through a plasticized filter fabric roughly equivalent to a 100 mesh screen. The solution was maintained at 75°C for several hr and the temperature was then reduced to 70°C. A seed crystal was introduced and the container placed in a +64mO”C bath; the temperature was further reduced at the rate of 0.15”C per day in the range of +64-O-+ 59.7”C. In this temperature region, no difficulties due to twinning or nucleation were experienced such as found earlier for the range of 3O”C-+O”C. 3. STRUCTURE OF HABIT MODIFIED TGFB
In order to examine the structural features of a tabular crystal such as shown in Fig. l(b), it was cut into identical laminae perpendicular to the ferroelectric axis and along the (010) cleavage plane. A slurry of electroluminescent powder in silicone oil was applied to both crystal surfaces normal to the ferroelectric axis. The experimental procedure for providing a static pattern of regions within a laminae in which the polarization vector is either free to follow the applied alternating electric field or is locked by local constraints, was described earlier.(Tls) Figure 2 shows such a pattern reconstituted from both of the major surfaces of a habit modified specimen. The consistency between the two surfaces of each lamina indicates that the dark, fan-shaped regions which represent the locked polarization, penetrate the entire volume of each lamina. The light regions undergo a reversal in sign of polarization during each half cycle of the 400 c/s electric field. The locked polarization regions are symmetrical about the seed crystal; they appear as fan-
OF TRIGLYCINE
FLUOBERYLLATE
249
shaped regions connecting the seed with the [llO] edges of the crystal. The delineation of these regions is sharper than in similar observations made on TGS. Etching experiments similar to those made by TOYODA et al.(Q) on TGS indicate that the locked polarization region also contains a higher density of etch pits than the rest of the crystal. As in the case of TGS, it appears reasonable to attribute the locked polarization to a flattened spiral dislocation network radiating outwardly from the seed crystal. Similar results were also obtained with crystals grown from a Cs+ doped solution and a much less symmetrical, but analogous pattern was obtained from a crystal with normal habit grown, from an undoped solution. Figure 2 also shows a secondary complex pattern of gray segments which had a random orientation. Their density and distribution is a function of the amplitude of the sinusoidal field. We presume that these regions consist of domains locked upon random imperfections distributed through the crystal during growth or alternatively, introduced into the laminae during the cleaving and sectioning process. We believe that large differential growth rates along different crystallographic directions might cause the high dislocation densities in the cleavage plane in TGFB and its isomorphs. Thus far, we have been unable to alter this process by changes in thermal regime during growth, introduction of chemical impurities, repeated purification of the mother solution or choice of seeds or seeding techniques. 4. DIELECTRIC AND FERROELECTRIC PROPERTIES
A total of fourteen specimens of TGFB were selected from the “free” regions of crystal laminae such as shown in Fig. 2. The specimens were diced into parallelepipeds with square surface areas of 0.05 cm2 and variable thickness in the range between 0.89 cm and 0.02 cm. Figure 3 shows the measured dielectric constant along the ferroelectric axis, eb as a function of temperature for a representative specimen. From the data, it is evident that the peak value, emaXis more than three times that of HOSHINOet aA;@) the Curie temperature is Tc = 73 kO.2 C in contrast to their data of Tc = 70°C. From graphs of dielectric stiffness (l/x) vs. temp. we determined the Curie constant C = 2512” + 80”
H. H. WIEDER
250
and C. R. PARKERSON
which is comparable to the previously determined value(s) of C = 2340”. The second coefficient E of the free energy expansion(ls) in terms of P and T, was calculated from the slope of plots of Pt vs. T in the immediate
I
y)60
I
I 65
I 70 TEMPERATURE
= 0.96
KV/
CU
1. = 1.96 0 ‘3.52
KV/CM KV/CM
*
KV/
* 5.03
I 75
CM
I 60 OC
FIG. 4. ebvs. temp. for the same specimen used in Fig. 3 with d.c. biasing field Eb, as parameters shows clamping effect of &. 60 TEMPERATURE
70
*C
FIG. 3. Spontaneous polarization as a function of temperature for a specimen taken from “free” polarization region of a lamina. Circles represent data taken with temperature increasing, crosses with temperature decreasing from Tc. Curve was calculated by using three terms of free energy expansion in P. and T. vicinity of T,; it was also determined, as shown in Fig. 4, from the shift(llJs) of e,,,, to temperature above T,, as a function of biasing fields Eb. The value C;= 3.7 x lo-10 (e.s.u./cms)-2, thus calculated is considerably smaller than t = 13.5 x lo-10 (e.s.u./cms)-s determined by HOSHINOet al.@) We also calculated the third coefficient of the free energy expansion, 4 = 1-l x lo-17 (e.s.u./cms)-4, by using three terms of the seriesos) and fitting their sum by means of a least-square approximation to the experimentally measured spontaneous polarization
Ps vs.
temp.
shown
in Fig.
5.
These
data
were
loops in the conventional manner; in contrast with the measurements of Hoshino et al.;(s) the slope (dP8/dT) in the vicinity of room temperature is not zero and continues to increase as the temperature is lowered; at +25”C, the spontaneous polarization is Pa = determined
from
hysteresis
0 TEMPERATURE
lC
FIG. 5. Real component eb, of complex dielectric con&nut along ferroelectric axis measured as a function of temperature at a frequency of 10 kc s for “free” TFGB specimen.
SOME FERROELECTIUC AND DIELECTRIC PROPERTIES OF TRIGLYCINE FLUOBERYLLATE 251
3 -7 x 10-s C/cma compared
to their Pa = 3 -2 x 10-s C/cm? The extrapolated data for Pz vs. T aswellas other data such as shown in Fig. 5, indicate the Curie temperature to be 73’ + 0*2”C. The ratio of slopes of the linear (l/x) vs. T ferroelectric to paraelectric regions is - 3 -99. This indicates a large, rather than a negligible, adiabatic correction for eb, The ratio p, of the slopes, including the adiabatic correction should be:
specimen containing a large number of lattice the domains in imperfections which “lock-in” their immediate vicinity by an electrical or el~~ornech~i~l interaction. Other investigations made upon TGFB specimens cut from a crystal of normal habit grown by conventional means, gave entirely analogous results to those shown in Figs. 2 and 5 ; the Curie temperature Tc z 73”C!, e,,, E 3 x 10s and Ps = 3.6 x 1tP C/cm2 at +WC. Ferroelectric specimens taken from the “locked @TC P = -2 *+c2cpt polarization” regions of laminae such as shown in [ Fig. 2, have an apparent internal biasing field in cp is the specific heat per unit volume at constant excess of 20 kV/cm. They are therefore unsuitable polarization. The molal specific heat(s) is 20 cal/mol for investigations of the polarization reversal “C and TGFB has 312.22 g/mole; the density was process. The intermediate, gray regions shown in determined expe~en~y as 166 g~cms, hence Fig. 2 are partially “locked”; two specimens were cp = O-1062 cal/(cms-“C). The equivalent energy selected from this area for further investigations. per unit volume per degree is cp = 444 x 10s ergs/ Upon being subjected to a peak electric field E = 6 (ems-“C). From the parameters C, Tc and 5, the kV/cm at 50 c/s, both samples were found to have adiabatic correction is calculated as 1.12 hence slightly sheared, double hysteresis loops similar to In view of the uncertainty in cp, thedataof Stankowska and StankowskiforTGS.@s) P = -4.24. this appears to be in good agreement with the Raising the amplitude of the applied field to 12 experimentally determined p = -3 -99. kV/cm caused an apparent annealing; the double Figure 6 shows the experimentally measured loop pattern gradually merged into a normal single “coercivity” determined from hysteresis loops loop with P8 N 3 m7x 10-s C/cm2 for both speciobtained at a frequency of 50 cjs upon the same mens and respective “coercivities” of EC = 4 specimen described above. At +2S”C, the 50 cs kV/cm for sample FL-l and EC = 6 kV/cm for “coercivity” is E$ = 1.1 kV/cm, a value considFL-2. Similar annealing effects were found by erably smaller than that of Hoshino et aZ.,@f ~~YNo~~(~~) on radiation damaged TGS. & = 5 kV/cm. It may have been obtained on a The temperature dependence of the spontaneous polarization of these specimens is shown in Fig. 7. It was measured in a 50 c/s alternating field with a peak amplitude of 15.4 kV[cm. In such a field, the hysteresis loops seem to be normal and symmetrical; no further annealing effects appeared above a peak applied field of 14 kV/cm. As the temperature was raised towards the Curie temperature, both specimens are shown to have a Ps vs. T dependence in good agreement with that of Fig. 5. The temperature was decreased thereafter in steps, each step being held for 5 min at ~0~05°C while the a.c. field was held constant. Both specimens appear to have a large thermal hysteresis with (dPs/dT) N 0 below +35”C; for sample FL-l the measured P, upon return to +25”C is Ps = 3.15 x Ii I I I I I I I k 20 30 40 50 10-s Cjcms in fair agreement with the data of 60 70 80 TEMPERATURE *C Hoshino et lal. ; (2) however, the Curie temperature Eta 6. Temperature dependence of “coercivity”of same is still +73”C in contrast to their data. The higher Curie temperature of our specimens was also specimendescribedby Fig. 5 measuredat 50 c/s.
I
H.
252
H. WIEDER
and C. R. PARKERSON
confirmed by additional e vs. T measurements with a compensatory biasing fie1d.W The internal bias of each specimen was evaluated as 1Ebl = 0.2 kV/cm for FL-l I&l, = 3.1 kV/cm for FL-2 at +25”C. At this temperature, a prolonged (in excess of 1 hr) application of an alternating field with a peak value of 15 -4 kV/cm was insufficient to anneal these specimens back to their original P8 values. A period of 6-8 h.r is required at +25”C in order to restore the original polarization. The applied field
these respects, the data conform closely to similar observations made on TGS. It may be assumed that these defects are electrically charged since their diffusion affects eb and ECas well as P8 causing a redistribution of a volume space charge and the compensation at domain walls of the polarization associated with individual domains. Thus the net available polarization which may be reversed by an applied field appears smaller in consequence of these “locked” domains. In the vicinity of room temperature, the diffusion of these defects is qualitatively slow, but increases with temperature; the relaxation time of this process being temperature dependent leads to a “freezing-in” of defects in a specimen thermally cycled to the Curie temperature and back to ambient while subjected to an applied field. Consequently, the selection of a specimen of TGFB or similar ferroelectric crystals must be made with due regard to its past thermal history and the density and distribution of imperfections within it. Acknotuledgement-We are indebted to the Information Systems Branch, Office of Naval Research for continued support of these investigations. It is a pleasure to acknowledge the expert assistance of D. A. COLLINS and D. GADDIE.
0 20
50
60 40 50 TEMPERATURE =C
70
60
FIG. 7. Spontaneous polarization vs. temp. for two specimens taken from a “partially-locked” polarization sector of one lamina. Initially, temperature increases as shown by arrow, sample FL-l (squares) and FL-2 (circles) have same P, vs. T; as temperature decreases, each specimen passes through a different, time dependent relaxation of P,.
does not alter the rate of this process to any significant extent nor is there any consistency with respect to the relaxation time of the thermal hysteresis; it varies between individual specimens as well as in repeated runs made upon the same specimen within the indicated time range. The experimental data described above, confirms the second-order character of the thermodynamic transition of TGFB and illustrates the strong interaction between “built-in” lattice defects and the polarization reversal process. In
REFERENCES 1. PEPINSKY R., OKAYAS. and JONA F., Bull. Am. Phys. Sot. Ser. II, 2,220 (1957). 2. HOSHINO S., MITSUI T., JONA F. and PEPINSKY R., Phys. Rev. 107,1255 (1957). 3. TRIEBWASSERS., IBM J. Res. 2, 212 (1958). 4. GUREVICHV. M. and ZHELUDEVI. S., Kristallografiya 6, 778 (1961). 5. CHYNOWETH A. G.. Phvs. Rev. 117. 1235 11960). 6. GONZALOJ. A. and LOPEZ-ALONG J. R.; J. ihys.
Chem. solids 25, 303 (1964). 7. WIFJ)ER H. H. and PARKERSONC. R.. , J. Phvs. _ Chem. Solids 25. 241 (1964). 8. ZHELUDEVi. S., I%LIM~NOVA. A., YURIN V. A. and POMANYUKN. A.. KristalloPrafiva 6. 676 (1961). 9. TOYODA H., TANAL Y. and &;o’KAwA W., ‘Elect;. Comm. Lab., Nippon Tel and Tel Corp. 9, 485 (1961). 10. JONA F. and SHIFUNE G., Ferroelectric Crystals pp. 33-36. Macmillan, Pergamon Press, New York (1962). Il. WIEDER H. H., J. Appl. Phys. 30,lOlO (1959). 12. TAUREL L. and CHAPELLE J., C.R. Acad. Sci., Paris
249,378 (1959). 13. STANKOW~KAJ. and STANKOWSKI J., Phys. Sot. Proc. 75, 455 (1960). 14. CHYNOWETH A., Phys. REV. 113, 159 (1959). 15. WIEDER H. H., CLAWSONA. R. and PARKERSONC. R., J. Appl. Phys. 33,172O (1962).
Pergamon Press 1966. Vol. 27, pp. 247-252.
SOME FERROELECTRIC
Printed in Great Britain.
AND DIELECTRIC
PROPERTIES OF TRIGLYCINE
FLUOBERYLLATE
H. H. WIEDER and C. R. PARKJZRSON U.S.
Naval Ordnance Laboratory, Corona, California
(Received 26 May 1965 ; in revised form 27 August 196.5)
Abstract-Large area tabular TGFB crystals with inhibited growth along the [OOl] direction were grown by doping the growth solution with selected monovalent cations. Irrespective of crystalline habit, the Curie temperature of TGFB was found to be Tc = 73”+0*2”C, the Curie constant C = 2512” + 80’ and the ratio of the slope of the dielectric stiffness in the ferroelectric to that of the the spontaneous polarization P8 = 3.7 x 10ds C/cm2 paraelectric region is p = - 3.99. At +2X!, and the 50 c/s “coercivity” is 1.1 kV/cm. Crystalline imperfections introduced during growth are distributed anisotropically and have a marked effect upon the dielectric and ferroelectric polarization process in TGFB.
1.
INTRODUCTION THE physical properties of triglycine sulfate (TGS) have been investigated extensively ever since the discovery of ferroelectricity in this compound. (1) Considerably less attention has been devoted to the triglycine fluoberyllate isomorphous crystal, (TGFB). The original dielectric and ferroelectric measurements made by HOSHINO et aZ.@) have remained unchallenged to this date, although they reported some significant differences between the thermal and dielectric parameters of TGS and those of TGFB. In particular, the ratio of the slopes of the dielectric stiffness vs. temperature determined in the ferroelectric and paraelectric regions was given as -2-l 1 for TGFB ; hence the adiabatic correction to the isothermal dielectric constant may be considered to be negligible. This is in contrast to TGS which@) requires an adiabatic correction of the order of 20% and where the experimentally determined ratio of the slopes varies from -2.3 to -3 *O. It is significant that crystals such as BaTiOs or KNbOs which undergo a first-order thermodynamic transition at the onset of the ferroelectric state do not require an adiabatic correction term. Yet for TGFB as for TGS, the transition appears to be of second order.@) 247
The Curie temperature of TGFB determined by HOSHINO et al. (2) from dielectric and ferroelectric measurements was given as +7O”C. On the other hand, GUREVICHand ZHELUDEV@)determined the phase-transition to occur between +73”C and + 75°C from their measurements of the anisotropic electrical conductivity. The spontaneous polarization of P8 appears to be independent of temperature below approximately 245°K; this is in contrast, however, with TGS which still appears to have a significantcsls) pyroelectric coefficient (dP,/dT) at 100°K. If (dPs/dT)0 at room temperature for TGFB, then it might be used to test the first order phenomenological and statistical theory for ferroelectricity proposed by GONZALO and LOPEZ-ALONSO@) specifically for TGS.Thereasonsoutlined aboveledustoan experimental investigation of the dielectric and ferroelectric properties of TGFB. We were also interested in attempting to change the normal habit of TGFB by means of a chemically induced habit modification such as applied(7) to TGS. The advantage of a tabular habit with the ferroelectric axis parallel to the main crystal surface is that sectioning of identical laminae from a crystal specimen is simplified, and mechanical strains around localized crystalline defects introduced
248
H. H. WIEDER
and C. R. PARKERSON
during growth may be traced by their effect upon the local polarization and the polarization reversal process.
Solutions No. 3 and No. 4 were prepared respectively with 12.8 g and 68 g TlsBeFd per 100 g TGFB in solution. A turbidity developed on addition of the TlsBeFd and it was found necessary 2. PREPARATION OF HABIT MODIFIED to dilute both the No. 3 and No. 4 solutions with TRIGLYCINE FLUOBERYLLATE approximately 300 ml HsO each to reduce the saturation temperature to +25”C. Each of these The synthesis and growth of TGFB crystals solutions were then filtered and placed in open started with the preparation of fluoberyllic acid plastic beakers maintained at +8O”C; they were which is formed by the following reaction in concentrated to approximatelytheir originalvolume aqueous solution : before dilution. BeFs f2HF +HsBeF4 After saturating each of the four solutions at +40*5”C, the excess crystals were filtered out Beryllium fluoride is very soluble in water; howrepeatedly. The solutions were returned thereafter ever, it is very slow to go into solution. A quantity to their plastic containers, maintained at a temperaof 74*41g BeFs was placed in a sealed flask with ture of + 55°C for about 2 hr. The temperaturewas 400 ml Ha0 and the solution was agitated conallowed to drop to + 47°C at which time seed crystinuously by means of a magnetic stirrer. A period tals mounted on plastic tubing were planted in of 10 to 14 days was required to dissolve all the BeFs. The BeFs solution was then reacted in a the solution. The containers were placed in a water bath maintained at +40*5X’ and a programmed plastic beaker with 63.4 g HF (132.0 g 48% HF) temperature reduction was started at 0*2”C!per day. which produced 1378 g HsBeF4 in 468 g I&O. Numerous growth. runs using these solutions in The HzBeF4 solution was then reacted with 357-O the range of 40*5”-30°C confirm the following g glycine (3 moles glycine per mole HsBeF4) to conclusions : produce 494*8 g triglycine fluoberyllate in 468 g 1. Solutions No. 1 and No.2 containing Csf addiHz0 : tive yield crystals with the modified, tabular habit 3 NHsCHsCOOH + HsBeF4 shown in Fig. l(a). 2. Solutions No. 1 and No. 2 containing Csf +(NH2CH2COOH)3. HzBcF4 additive are extremely unstable as regards extraneThe solution was heated to complete the solution ous nucleation and surface twinning, particularly as the seed crystal begins to grow in size. Changing process then allowed to cool to room temperature; the temperature reduction to as low as O*OS”Cper the excess TGFB crystallized out. day did not improve this instability. In order to constrain growth in the [OOl] direc3. There does not appear to be any large differtion, monovalent cations(T) need be introduced ence in stability of a TGFB solution containing into the mother solution. It was necessary, there5.6 g CssBeFa additive (No. 2) compared to that fore, to prepare a quantity of CssBeF4 and containing 13~2 g CssBeF4 (No. 1) per 100 g TlsBeF4. The CssBeF4 was formed as a concenTGFB in solution. trated solution in water by the reaction of CSOH 4. Solutions of TGFB containing TlsBeF4 addiwith an equivalent amount of HzBeF4 solution. tive are more stable than those containing Cs+ and The TlsBeF4 was formed by reacting Tl3CO3 with good single crystals such as shown in Fig. l(b) can an equivalent amount of HsBeF4 solution, heating be grown consistently in the temperature range of to remove the COs, and then crystallizing out the +40~5”c-+30”c. TlsBeF4 with an excess of absolute ethyl alcohol. 5. A concentration of 10.6 g Tlf per 100 g Two Csf doped solutions were prepared by adding the CssBeF4 solution to two TGFB SO~U- TGFB in solution (solution No. 3) has a smaller effect upon the crystalline habit of TGFB comtions. Solution No. 1 with 13.2 g CszBeF4 per pared to TGS for the same Tlf concentration, 100 g TGFB in solution and solution No. 2 with Growth in the (001) plane for TGS is less than 1% 5.6 g CssBeF4 per 100 g TGFB in solution. These that of the normal, uninhibited growth; for TGFB, solutions were then clarified of sedimentation by the corresponding growth is approximately 12%. filtering.
FIG. 1. (a) Crystal of habit modified TGFB grown in Csf doped solution; longest dimension is 9.61 cm, thickness is 0.18 cm.
FIG. 1. (b) TGFB crystal grown in Tl+ doped solution sectioned into laminae normal to the ferroelectric (010) axis, after reassembly.
FIG. 2. Static pattern of polarization reversal at 400 cs; specimen was reconstituted showing pattern on each face of a lamina and ita relation to position within the crystal.
SOME FERROELECTRIC
AND DIELECTRIC
PROPERTIES
Decreasing the Tl+ concentration to 5.6 g per 100 g TGFB (solution #4) decreases the growth inhibition along the (001) plane which is then 35% of normal. We presume that the decrease in the interaction cross-section between the Tl anions and the BeF4 cations in solution is due to the larger effective mass of the ions in TGFB compared to TGS. A variant of the method described above was used for growing well defined, untwinned TGFB crystals from a Cs+ doped solution. Sufficient additional TGFB was added to 320 ml of TGFB solution No. 2 (Cs+) to increase the saturation temperature to 64.0% and the excess crystals removed by rapid filtration through a plasticized filter fabric roughly equivalent to a 100 mesh screen. The solution was maintained at 75°C for several hr and the temperature was then reduced to 70°C. A seed crystal was introduced and the container placed in a +64mO”C bath; the temperature was further reduced at the rate of 0.15”C per day in the range of +64-O-+ 59.7”C. In this temperature region, no difficulties due to twinning or nucleation were experienced such as found earlier for the range of 3O”C-+O”C. 3. STRUCTURE OF HABIT MODIFIED TGFB
In order to examine the structural features of a tabular crystal such as shown in Fig. l(b), it was cut into identical laminae perpendicular to the ferroelectric axis and along the (010) cleavage plane. A slurry of electroluminescent powder in silicone oil was applied to both crystal surfaces normal to the ferroelectric axis. The experimental procedure for providing a static pattern of regions within a laminae in which the polarization vector is either free to follow the applied alternating electric field or is locked by local constraints, was described earlier.(Tls) Figure 2 shows such a pattern reconstituted from both of the major surfaces of a habit modified specimen. The consistency between the two surfaces of each lamina indicates that the dark, fan-shaped regions which represent the locked polarization, penetrate the entire volume of each lamina. The light regions undergo a reversal in sign of polarization during each half cycle of the 400 c/s electric field. The locked polarization regions are symmetrical about the seed crystal; they appear as fan-
OF TRIGLYCINE
FLUOBERYLLATE
249
shaped regions connecting the seed with the [llO] edges of the crystal. The delineation of these regions is sharper than in similar observations made on TGS. Etching experiments similar to those made by TOYODA et al.(Q) on TGS indicate that the locked polarization region also contains a higher density of etch pits than the rest of the crystal. As in the case of TGS, it appears reasonable to attribute the locked polarization to a flattened spiral dislocation network radiating outwardly from the seed crystal. Similar results were also obtained with crystals grown from a Cs+ doped solution and a much less symmetrical, but analogous pattern was obtained from a crystal with normal habit grown, from an undoped solution. Figure 2 also shows a secondary complex pattern of gray segments which had a random orientation. Their density and distribution is a function of the amplitude of the sinusoidal field. We presume that these regions consist of domains locked upon random imperfections distributed through the crystal during growth or alternatively, introduced into the laminae during the cleaving and sectioning process. We believe that large differential growth rates along different crystallographic directions might cause the high dislocation densities in the cleavage plane in TGFB and its isomorphs. Thus far, we have been unable to alter this process by changes in thermal regime during growth, introduction of chemical impurities, repeated purification of the mother solution or choice of seeds or seeding techniques. 4. DIELECTRIC AND FERROELECTRIC PROPERTIES
A total of fourteen specimens of TGFB were selected from the “free” regions of crystal laminae such as shown in Fig. 2. The specimens were diced into parallelepipeds with square surface areas of 0.05 cm2 and variable thickness in the range between 0.89 cm and 0.02 cm. Figure 3 shows the measured dielectric constant along the ferroelectric axis, eb as a function of temperature for a representative specimen. From the data, it is evident that the peak value, emaXis more than three times that of HOSHINOet aA;@) the Curie temperature is Tc = 73 kO.2 C in contrast to their data of Tc = 70°C. From graphs of dielectric stiffness (l/x) vs. temp. we determined the Curie constant C = 2512” + 80”
H. H. WIEDER
250
and C. R. PARKERSON
which is comparable to the previously determined value(s) of C = 2340”. The second coefficient E of the free energy expansion(ls) in terms of P and T, was calculated from the slope of plots of Pt vs. T in the immediate
I
y)60
I
I 65
I 70 TEMPERATURE
= 0.96
KV/
CU
1. = 1.96 0 ‘3.52
KV/CM KV/CM
*
KV/
* 5.03
I 75
CM
I 60 OC
FIG. 4. ebvs. temp. for the same specimen used in Fig. 3 with d.c. biasing field Eb, as parameters shows clamping effect of &. 60 TEMPERATURE
70
*C
FIG. 3. Spontaneous polarization as a function of temperature for a specimen taken from “free” polarization region of a lamina. Circles represent data taken with temperature increasing, crosses with temperature decreasing from Tc. Curve was calculated by using three terms of free energy expansion in P. and T. vicinity of T,; it was also determined, as shown in Fig. 4, from the shift(llJs) of e,,,, to temperature above T,, as a function of biasing fields Eb. The value C;= 3.7 x lo-10 (e.s.u./cms)-2, thus calculated is considerably smaller than t = 13.5 x lo-10 (e.s.u./cms)-s determined by HOSHINOet al.@) We also calculated the third coefficient of the free energy expansion, 4 = 1-l x lo-17 (e.s.u./cms)-4, by using three terms of the seriesos) and fitting their sum by means of a least-square approximation to the experimentally measured spontaneous polarization
Ps vs.
temp.
shown
in Fig.
5.
These
data
were
loops in the conventional manner; in contrast with the measurements of Hoshino et al.;(s) the slope (dP8/dT) in the vicinity of room temperature is not zero and continues to increase as the temperature is lowered; at +25”C, the spontaneous polarization is Pa = determined
from
hysteresis
0 TEMPERATURE
lC
FIG. 5. Real component eb, of complex dielectric con&nut along ferroelectric axis measured as a function of temperature at a frequency of 10 kc s for “free” TFGB specimen.
SOME FERROELECTIUC AND DIELECTRIC PROPERTIES OF TRIGLYCINE FLUOBERYLLATE 251
3 -7 x 10-s C/cma compared
to their Pa = 3 -2 x 10-s C/cm? The extrapolated data for Pz vs. T aswellas other data such as shown in Fig. 5, indicate the Curie temperature to be 73’ + 0*2”C. The ratio of slopes of the linear (l/x) vs. T ferroelectric to paraelectric regions is - 3 -99. This indicates a large, rather than a negligible, adiabatic correction for eb, The ratio p, of the slopes, including the adiabatic correction should be:
specimen containing a large number of lattice the domains in imperfections which “lock-in” their immediate vicinity by an electrical or el~~ornech~i~l interaction. Other investigations made upon TGFB specimens cut from a crystal of normal habit grown by conventional means, gave entirely analogous results to those shown in Figs. 2 and 5 ; the Curie temperature Tc z 73”C!, e,,, E 3 x 10s and Ps = 3.6 x 1tP C/cm2 at +WC. Ferroelectric specimens taken from the “locked @TC P = -2 *+c2cpt polarization” regions of laminae such as shown in [ Fig. 2, have an apparent internal biasing field in cp is the specific heat per unit volume at constant excess of 20 kV/cm. They are therefore unsuitable polarization. The molal specific heat(s) is 20 cal/mol for investigations of the polarization reversal “C and TGFB has 312.22 g/mole; the density was process. The intermediate, gray regions shown in determined expe~en~y as 166 g~cms, hence Fig. 2 are partially “locked”; two specimens were cp = O-1062 cal/(cms-“C). The equivalent energy selected from this area for further investigations. per unit volume per degree is cp = 444 x 10s ergs/ Upon being subjected to a peak electric field E = 6 (ems-“C). From the parameters C, Tc and 5, the kV/cm at 50 c/s, both samples were found to have adiabatic correction is calculated as 1.12 hence slightly sheared, double hysteresis loops similar to In view of the uncertainty in cp, thedataof Stankowska and StankowskiforTGS.@s) P = -4.24. this appears to be in good agreement with the Raising the amplitude of the applied field to 12 experimentally determined p = -3 -99. kV/cm caused an apparent annealing; the double Figure 6 shows the experimentally measured loop pattern gradually merged into a normal single “coercivity” determined from hysteresis loops loop with P8 N 3 m7x 10-s C/cm2 for both speciobtained at a frequency of 50 cjs upon the same mens and respective “coercivities” of EC = 4 specimen described above. At +2S”C, the 50 cs kV/cm for sample FL-l and EC = 6 kV/cm for “coercivity” is E$ = 1.1 kV/cm, a value considFL-2. Similar annealing effects were found by erably smaller than that of Hoshino et aZ.,@f ~~YNo~~(~~) on radiation damaged TGS. & = 5 kV/cm. It may have been obtained on a The temperature dependence of the spontaneous polarization of these specimens is shown in Fig. 7. It was measured in a 50 c/s alternating field with a peak amplitude of 15.4 kV[cm. In such a field, the hysteresis loops seem to be normal and symmetrical; no further annealing effects appeared above a peak applied field of 14 kV/cm. As the temperature was raised towards the Curie temperature, both specimens are shown to have a Ps vs. T dependence in good agreement with that of Fig. 5. The temperature was decreased thereafter in steps, each step being held for 5 min at ~0~05°C while the a.c. field was held constant. Both specimens appear to have a large thermal hysteresis with (dPs/dT) N 0 below +35”C; for sample FL-l the measured P, upon return to +25”C is Ps = 3.15 x Ii I I I I I I I k 20 30 40 50 10-s Cjcms in fair agreement with the data of 60 70 80 TEMPERATURE *C Hoshino et lal. ; (2) however, the Curie temperature Eta 6. Temperature dependence of “coercivity”of same is still +73”C in contrast to their data. The higher Curie temperature of our specimens was also specimendescribedby Fig. 5 measuredat 50 c/s.
I
H.
252
H. WIEDER
and C. R. PARKERSON
confirmed by additional e vs. T measurements with a compensatory biasing fie1d.W The internal bias of each specimen was evaluated as 1Ebl = 0.2 kV/cm for FL-l I&l, = 3.1 kV/cm for FL-2 at +25”C. At this temperature, a prolonged (in excess of 1 hr) application of an alternating field with a peak value of 15 -4 kV/cm was insufficient to anneal these specimens back to their original P8 values. A period of 6-8 h.r is required at +25”C in order to restore the original polarization. The applied field
these respects, the data conform closely to similar observations made on TGS. It may be assumed that these defects are electrically charged since their diffusion affects eb and ECas well as P8 causing a redistribution of a volume space charge and the compensation at domain walls of the polarization associated with individual domains. Thus the net available polarization which may be reversed by an applied field appears smaller in consequence of these “locked” domains. In the vicinity of room temperature, the diffusion of these defects is qualitatively slow, but increases with temperature; the relaxation time of this process being temperature dependent leads to a “freezing-in” of defects in a specimen thermally cycled to the Curie temperature and back to ambient while subjected to an applied field. Consequently, the selection of a specimen of TGFB or similar ferroelectric crystals must be made with due regard to its past thermal history and the density and distribution of imperfections within it. Acknotuledgement-We are indebted to the Information Systems Branch, Office of Naval Research for continued support of these investigations. It is a pleasure to acknowledge the expert assistance of D. A. COLLINS and D. GADDIE.
0 20
50
60 40 50 TEMPERATURE =C
70
60
FIG. 7. Spontaneous polarization vs. temp. for two specimens taken from a “partially-locked” polarization sector of one lamina. Initially, temperature increases as shown by arrow, sample FL-l (squares) and FL-2 (circles) have same P, vs. T; as temperature decreases, each specimen passes through a different, time dependent relaxation of P,.
does not alter the rate of this process to any significant extent nor is there any consistency with respect to the relaxation time of the thermal hysteresis; it varies between individual specimens as well as in repeated runs made upon the same specimen within the indicated time range. The experimental data described above, confirms the second-order character of the thermodynamic transition of TGFB and illustrates the strong interaction between “built-in” lattice defects and the polarization reversal process. In
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Chem. solids 25, 303 (1964). 7. WIFJ)ER H. H. and PARKERSONC. R.. , J. Phvs. _ Chem. Solids 25. 241 (1964). 8. ZHELUDEVi. S., I%LIM~NOVA. A., YURIN V. A. and POMANYUKN. A.. KristalloPrafiva 6. 676 (1961). 9. TOYODA H., TANAL Y. and &;o’KAwA W., ‘Elect;. Comm. Lab., Nippon Tel and Tel Corp. 9, 485 (1961). 10. JONA F. and SHIFUNE G., Ferroelectric Crystals pp. 33-36. Macmillan, Pergamon Press, New York (1962). Il. WIEDER H. H., J. Appl. Phys. 30,lOlO (1959). 12. TAUREL L. and CHAPELLE J., C.R. Acad. Sci., Paris
249,378 (1959). 13. STANKOW~KAJ. and STANKOWSKI J., Phys. Sot. Proc. 75, 455 (1960). 14. CHYNOWETH A., Phys. REV. 113, 159 (1959). 15. WIEDER H. H., CLAWSONA. R. and PARKERSONC. R., J. Appl. Phys. 33,172O (1962).