Pyroelectric, ferroelectric, dielectric and thermal properties of Mn3B7O13Br single crystals

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Solid State Communications, Vol. 89, No. I1, pp. 963-969, 1994 Elsevier Science Ltd Printed in Great Britain. An rights reserved 0038 1098/94 $6.00+.00

Pergamon

0038-109S(93)E0012-M PYROELECTRIC, FERROELECTRIC, DIELECTRIC AND THERMAL PROPERTIES OF Mn3BTOI3Br SINGLE CRYSTALS J. Campa-Molina and A.G. Castellanos-Guzm~n* Facultad de Ciencias Fisico Matem~tticas, Universidad de Guadalajara, Apdo. Postal 2-638, 44281 Guadalajara Jal, Mexico M. Bfircena-Soto Facultad de Ciencias Quimicas, Universidad de Guadalajara, Apdo. Postal 2-638, 44281 Guadalajara Jal, Mexico and J. Reyes-G6mez CICBAS. Universidad de Colima, Apdo. Postal 2-1694, 28000 Colima Col, Mexico

(Received 28 January 1993; in revised form 19 July 1993 by A.A. Maradudin) Single crystals of Mn3B7Ol3Br (Mn-Br) boracite were studied between 300 and 550 K by means of thermal, dielectric, pyroelectric and ferroelectric characterization, under simultaneous visual control of the ferroelectric/ferroelastic domain state of the samples. We observe an unusual change of sign of the pyroelectric coefficient in Mn-Br at about 440 K which can be an indication of the existence of a new phase transition in this compound. A preliminary DSC study seems to confirm the presence of such a transition. 1. INTRODUCTION THE FAMILY of compounds known as boracites (general formula Me3BTOI3X, where Me is a divalent metal such as Mg, Cr, Mn, Fe, Co, Ni, Cu, Zn or Cd, and X is, usually, a halogen F, CI, Br, or I) has continued to attract the attention of researchers due to its unusual and somehow controversial properties [14]. In what follows we will refer to boracites by giving the symbols of the metal and halogen only, i.e. Mn3BTOI3Br= M n - B r and so on. All halogen boracites synthesized so far have been shown to have a high temperature cubic phase (space group F43c, point group 43m) which changes to an orthorhombic phase (space group Pca21 point group mm2) on cooling, with the exception of Cr-Br, Cr-I and Cu-I which remain cubic down to 4K [1]. The special character of this phase transition is reflected by the unusual and different dielectric anomaly displayed by

boracites: In Cu-CI [5, 6], Mn-I [6, 8] and Ni-I [7] the dielectric constant, er, jumps abruptly upward upon cooling through the transition from the high temperature phase, whereas in most boracites the jump of er at the transition is downwards on cooling [1]. The purpose of this paper is to report the results of a recent experimental study of pyroelectric, dielectric, ferroelectric and thermal properties of Mn3B7013Br (Mn-Br) single crystals in the entire range of temperature from 300 K to 550 K. These results seem to indicate the presence of a new phase transition in this compound between the cubic and orthorhombic phases and also confirms that the dielectric behavior of M n Br follows that of Cu-CI, Mn-I and Ni-I boracites. The matter is of interest because, previous to our work, Ye et al. [9] have found in Cr-C1 boracite a new phase transition from cubic (43m) to a fully ferroelastic/ antiferroelectric tetragonal ~,2m phase, so far thought to be unique among these compounds.

* Present address: Direcci6n de Vinculaci6n y Transferencia de Tecnologia. U. de G. Apdo. Postal 2-638, 44281 Guadalajara, Jal. Mexico. 963

2. EXPERIMENTAL Single crystals of Mn-Br were grown in closed

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P R O P E R T I E S OF Mn3B7013Br S I N G L E CRYSTALS

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30 Mn-Br 25

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-

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I

I

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I

300

350

400

450

500

550

600

Temperature (K) Fig. 1. Temperature and frequency dependence of the permittivity of Mn3B7Ol3Br boracite. quartz ampoules by the chemical vapor method of Schmid [10]. Platelets cut parallel to cubic (100) planes and ranging in thickness from 80 to 200 #m were polished with diamond paste of varying grain size (from 25 to 7#m) down to optical quality. Typical sample areas were 0.02-0.04cm 2. Semitransparent gold on chromium electrodes were evaporated in several samples in order not only to provide electrical contacts to the samples but to permit us to control visually, under the polarizing microscope between crossed polars, the ferroelectric/ ferroelastic domain state of the sample during physical evaluation. Except for thermal measurements, M n - B r samples were lodged in an especially modified hot stage (Leitz 350) provided with isotropic quartz windows suitable for polarizing microscopy in transmitted light. The temperature of the specimen was measured with a chromel-alumel (type K) thermocouple set in the hot stage chamber, as close to the sample as possible. Electrical connections to external circuitry was provided via platinum wires (4~ = 40#m) fixed by silver/epoxy at the center of both M n - B r crystal faces. The host hot stage was, in turn, mounted on the mechanical stage of a polarizing microscope Leitz Orthoplan Pol. for optical observations of the sample. Previous to measurements an electric poling process was applied to M n - B r in order to obtain a single domain state. This process basically consisted in heating up the samples a few degrees above the 43m-mm2 phase transition (Tc = 547K) then an electric field (d.c.) as high as 45 KV cm -I was applied to the samples and the temperature was lowered down to room temperature under the applied field. The single domain state of all M n - B r samples was controlled, throughout the entire temperature range of the electrical measurements here reported, by continuous observation of the birefringence.

3. RESULTS A N D DISCUSSION Dielectric behavior: The dielectric constant of M n - B r was derived from direct measurements of the electrical capacitance of the samples by means of an impedance analyzer HP-4192A with an accuracy better than 1%. Figure 1 shows the temperature dependence and frequency response between 100KHz and 5 M H z of the dielectric constant of M n - B r boracite measured in a single domain state with the spontaneous polarization, Ps, perpendicular to the (1 0 0) plane, i.e. e33. At frequencies of 100 kHz, and 1 MHz upon heating, the dielectric constant rises from a value of about 8 at room temperature to a peak value of 25 at a temperature of 547 K, whereas at higher frequencies 0c= 3 and 5 MHz) the permittivity reaches a lower peak of about 13. Above the transition 5~3m-rnm2 and up to temperatures as high as 600K, er remains essentially constant. These results are in agreement with those reported in a previous study for M n - B r [8] at a frequency of 1 MHz. However, the maximum value of er reported in that work at the transition temperature was 13, i.e. the value observed in this work at 3 MHz and 5 MHz. This difference, as well as the strong peaks at 100kHz, and 1MHz shown in Fig. 1 are artifacts connected with the appearance, in the vicinity of the 43m-mrn2 phase transition, of a complicated spikelike domain structure which is discussed below. It is clear, however, that for all four frequencies as well as in the previous report [8], the dielectric constant of M n - B r jumps downward at the transition to the nonpolar phase and remains essentially constant above T~, thus M n - B r is the fourth example of an unusual permittivity behavior observed in boracites [1]. Ferroelectric polarization: The temperature dependence of the spontaoeous polarization, P,, was measured continuously in M n - B r single domain samples by the charge-integration technique [11] using a digital electrometer Keithley 616, and with

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P R O P E R T I E S OF Mn3B7OI3Br S I N G L E CRYSTALS Mn-Br 20

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Fig. 2. Temperature dependence of the spontaneous polarization and pyroelectric coefficient of Mn3B7013Br boracite. a heating rate of 10 K min -l. The results are given in Fig. 2. A spontaneous polarization of about 0.49 × 10-2cm -2 was found at room temperature. In the temperature range from 263 to about 440 K an unusual increase of Ps with increasing temperature is observed. This behavior of Ps gives as a result, a sign reversal of the pyroelectric coefficient, P3, in M n - B r at about 440 K. This effect was present in all samples measured and was also reported in a previous study on Mn halogen boracites [8]. When changing the sense of polarization by an external d.c. field in a given sample, the signs of polarity changed accordingly. Above 440K the value of ps falls continuously to zero at the onset of the 43m-rnm2 phase transition. It is noteworthy to mention that in Fig. 2 the sign of P~ and p has been arbitrarily chosen in order to display both parameters on the same scale and graph. The magnitude of the spontaneous polarization in M n - B r is one order of magnitude smaller than that reported for F e - I [12], C u - B r [13] or Cr-C1 [14] boracites but higher than in Mg-C1 [15] or N i - I [16] and compares well with M n - I and Mn-C1 [8]. It is, however, in variance with the value reported initially for M n - B r [8] (Ps = 0.262 x 10-2cm -2 at room temperature). The origin of this difference, as well as the strong peaks observed in the permittivity at 100kHz and 1 M H z (see Fig. 1), is thought to be a complicated spike-like domain structure occurring beneath the single domain of cubic (100) facets which is illustrated in Plate I. This structure, already observed before in M n - B r (unpublished) is visible only near the phase transition, or in the unpoled state of the sample. During the evolvement of this work we poled several times our samples confirming through

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observation between crossed polars, that we had a homogeneous single domain sample. However, a careful observation in the vicinity of the phase transition of some samples (Plate I) revealed that as temperature is slowly increased the spike-like structure sets in whilst the single domain is fading out. As it is seen in Plate I, there is a difference of about 11 degrees between the temperature at which the single domain starts to fade out and the temperature at which the sample enters the cubic phase, as evidenced by the black region where the crystal starts to become optically isotropic. It is not sure whether this spikelike domain structure emerges in the vicinity of the phase transition or it is always there beneath the single domain. Unfortunately, we did not have means to tilt our samples in the hot stage in order to check this, but our higher spontaneous polarization values could be an indication that this is the case. Schmid and Tipmann [13] have described several types of "forbidden" and "allowed" 180 degrees head-head (tail-tail) domains which could be the origin of the structure observed in M n - B r . Further work is required in order to explain such a domain pattern as well as its influence. What we did, however, was to measure the dielectric constant in samples with this type of domains after we obtained an apparent single domain state (see Plate I, T = 271.2 Celsius) as a function of frequency and temperature, confirming that the strong peaks observed in Fig. 1 at f = 100kHz and f = 1 M H z were related with the fading out of the single domain state and the setting of the spikes. Such a strong peaks did not occur in crystals in which the aforementioned domain pattern was absent (see for example Fig. 1 of [8]). On the other hand, sign reversal of the pyroelectric coefficient has been reported for few materials [17], e.g. lithium sulphate, barium nitrate and in barium titanate ceramic (in which two successive sign reversals occurred). Unusual increase of Ps with increasing temperature have also been reported for Mg-CI [15] (but an independent report by Bochkov et al., Inorg. Matls. 11, 1301, 1976, did not reconfirm this behavior), Cr-C1 [14] and Co-CI [18] boracites. Among these, a new phase transition has been confirmed to occur in Cr-C1 [9]. To our knowledge the only specific study made on pyroelectric coefficient sign reversal is that of Lang [17], apart from the theoretical work of Born [19] predicting the functional form of the temperature dependence of such a coefficient. Among the possible explanations given by Lang: (1) a metastable phase transformation, (2) a structural phase transition and (3) a cancellation of the primary and secondary pyroelectric coefficients, we suggest that a plausible one for M n - B r is that

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P R O P E R T I E S OF Mn3BTO13Br S I N G L E C R Y S T A L S

related with a phase transition, not only because we did not observe a dependence o f this coefficient on heating rates but because we did observe visually a change of birefringence in M n - B r at the range of

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temperature involved is the sign reversal. Although a probable cancellation of the primary and secondary pyroelectric coefficients in M n - B r cannot be discarded a priori, unfortunately there is no data

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T=282.2°C Plate I. A cubic (1 0 0) platelet of M n - B r boracite observed under crossed polars in the vicinity of the cubicorthorhombic phase transition, illustrating the merging of a spike-like domain structure near Tc (see text).

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P R O P E R T I E S O F Mn3B7013Br S I N G L E CRYSTALS

available so far on the temperature dependence of elastic, thermal and piezoelectric coefficients for this boracite, which would allow us to discuss such a possibility. Another distinctive feature observed in Fig. 2 is the continuity of Ps at the bent of P~ in M n - B r . This is typical of a second order transition and has also been reported for Co-C1 [18] and N i - I [7] boracites. The order of the cubic-orthorhombic phase transition in boracites has not been determined unambiguously and it is still a matter of controversy: On one hand, according to some phenomenological theories [2, 3] such a transition should be first order, and a detailed calorimetric investigation in Cr-C1, F e - I , Cu-C1 and N i - B r by Delfino et al. [20] and in Mn-C1 and M n Br [21] showed that in these compounds such a transition is strongly first order, but on the other hand, experimental studies also indicate that this transition is closer to a second order in boracites such as M n - I [21], Cu-CI [22] and Co-C1 [18]. A thermodynamical model proposed by Sannikov [23] can explain the magnitude and the sign of the jump of

Fig. 3. Reorientation of spontaneous polarization in Mn3BTO13Br boracite by a d.c. electric field. Crystal in the diagonal position observed between crossed polars. (a) Spike-like domain pattern in M n - B r (see text). (b) After applying a d.c. electric field ( E = 9 . 9 K V c m - l ) the spikes are reoriented from the original direction.

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the permittivity observed in both types of dielectric behavior of boracites, considering that the m m m 2 3,3m phase transition in these compounds is the result of the merging of two: a second order and first order transitions in the region of intersection of their lines in the phase diagram. This only reflects the complexity and variety of situations observed in these compounds. The ferroelectric character of M n - B r was observed by two methods: In the first method, by using the circuit described by Sawyer and Tower [24] we obtained an unsaturated hysteresis loop of M n Br single crystals after applying an a.c. field as large as 3.75KVcm -l. Unfortunately our simple S - T circuit does not compensate for the phase effects due to circuit components and to that due to the finite conductivity and linear capacitance of the samples, hence the spontaneous polarization could not be read directly from the oscilloscope. In the second method, however, the reorientation of Ps in M n - B r is readily obtained by applying a d.c. field of 9 . 9 K V c m -1 to the spike-like domain pattern displayed by this sample. In Fig. 3(a), a cubic (1 00) M n - B r platelet, is observed in the diagonal position between crossed polars; the sample shows a large number of spike-like domains oriented parallel to the microscope polarizer vibration direction (i.e. along E - W according to DIN Norm 58874). After applying a d.c. field of 9 . 9 K V c m -1 we obtained the reorientation of this pattern which is now parallel to the microscope analyzer vibration direction (i.e. N-S). The sample is obviously in a highly unsaturated state, what normally follows, before measurements, is to pole the sample near its Tc with d.c. fields as high as 45 KV cm -1 . Incidentally, these spikes are identical to those forming the domain structure shown in Plate I. Thermal behavior: A detailed study of thermal properties of M n - X boracites was published elsewhere [21]. In that work, as well as in that of Delfino's [20], measurements were concentrated in the vicinity of the cubic-orthorhombic phase transition, thus in order to look for a presumed phase transition in the interval of temperature where we noticed a sign reversal of the pyroelectric coefficient, the specific heat, Cp, of a as-grown M n - B r crystal (weight of the sample = 9.67mg) was measured with a PerkinElmer DSC-7 calorimeter between 300 and 580K at heating rates of 2-10 K min -1. The Perkin-Elmer system was calibrated relative to the melting point, Tin, and transition enthalpy, AHt, of indium metal (T m 429.75K and AHt = 3266.63Jmole-l). For the Cp evaluation the reference substance used as A1203. The weight of this standard was made as close to that of the sample as possible. Closed aluminum =

P R O P E R T I E S OF Mn3B7OI3Br S I N G L E CRYSTALS

968

& <

5

I

I 430

J

I

480 TEMPERATURE

530

, 580

( K )

Fig. 4. Temperature dependence of the specific heat, Cp, of Mn3B7Ol3Br boracite. pans with lids were used as crucibles, to ensure good thermal contact between sample and crucible the flattest and largest facet of M n - B r sample was put parallel to the bottom of the crucible. The temperature dependence of the specific heat in M n - B r is shown in Fig. 4. It appears as a jump of Cp(T) function at 547 K. It is preceded, however, by an increase of Cp in the region of about 450 to 505 K reaching a maximum of 480K and then going down to previous values before displaying its anomaly at the 7~3m-mm2 phase transition. Cp values obtained in this work show a discrepancy with previous reports [8, 21] not only in magnitude but in the shape of the specific heat peak at the cubic-orthorhombic phase transition. Although these discrepancies are not readily explained, they could be related to the presence of growth sectors in the as-grown M n - B r crystal used for thermal characterization. These sectors are typical in crystals like boracites which are grown by the process permitting the free development of equilibrium growth facets [4, 25, 26]. Growth sectors have been proved to cause huge differences in Tc [8, 26] in boracites which, for example in M n - B r , is as high as 30 ° [10], multiple peaking in Cp [20, 21], and even the annhilitation of the dielectric anomaly in cubic (1 1 1) platelets of N i - I boracite [26]. Most probably they also were responsible of the serious discrepancy concerning the magnitude of the enthalpy of the transition reported for F e - I boracite [20]. 4. CONCLUSIONS Mn3B7OL3Br boracite presents interesting dielectric and pyroelectric properties: The dielectric anomaly of M n - B r at the 43m-ram2 phase follows that of Cu-C1, M n - I and Ni-I, as opposed to that observed in most boracites. It also shows an unusual

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sign reversal of the pyroelectric coefficient at about 440 K as well as an increase of the specific heat* of M n - B r between 450 to 505K although it is much more diffuse than the anomaly at the 543m-mm2 transition. This in spite of the fact that a discrepancy exists in the magnitude of the specific heat of M n - B r . We also noticed, visually, a change of birefringence in this temperature range, however, we could not evaluate quantitatively the temperature dependence of spontaneous birefringence in M n - B r . Our supposition of the presence of a new phase transition in M n - B r between the mm2-43m phases; as evidenced experimentally by the afore-mentioned changes, needs, however, a further verification since we can not exclude the possibility that the unusual increase of Ps could be related to other causes and lead us to an erroneous indication of additional phase transition. A detailed ferroelectric/ferroelastic domain study, high resolution measurements of birefringence, as well as a more precise thermal evaluation, in crystals of good quality, due to small amplitude of the calorimetric signal involved in the presumed transition of this interesting compound are now in progress, results of which will be reported in the near future.

Acknowledgements - Work partially supported by DGICSA-SESIC-SEP, M6xico. We thank M. Roland Boutellier of University of Geneva for his design and modification of our hot stage 350. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.

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Applications of Ferroelectrics and Related Materials, p. 143. Clarendon Press Oxford 12.

(1979). H. Schmid, P. Chan, L.A. P&ermann, F. Taufel & M. M~indly, Ferroelectrics 13, 351 (1976).

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13. 14. 15. 16. 17. 18. 19.

PROPERTIES OF Mn3B7013Br SINGLE CRYSTALS

H. Schmid & H. Tipmann, Ferroelectrics 20, 21 (1978). Z.G. Ye, J.P. Rivera, E. Burkhardt & H. Schmid, University of Geneva, private communication (1991). L.P. Torre, S.C. Abrahams & R.L. Barns, Ferroelectrics 4, 291 (1972). T. Miyashita & T. Murakami, J. Phys. Soc. Jap. 29, 1092 (1970). S.B. Lang, Phys. Rev. I!4, 3603 (1971). M.E. Mendoza-Alvarez, J.P. Rivera & H. Schmid, Jap. J. Appl. Phys. Suppl. 24-2, 1057 (1985). M. Born, Rev. Modern Phys. 17, 245 (1945).

20. 21. 22. 23. 24. 25. 26.

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M. Delfino, G.M. Loiacono & W.A. Smith, J. Solid State Chem. 33, 107 (1980). A.G. Castellanos-Guzman, J. Solid State Chem. 54, 78 (1984). P. Felix, M. Lambert & H. Schmid, Ferroelectrics 7, 131 (1974). D.G. Sannikov, Sov. Phys. JETP Lett. 31, 313 (1980). C.B. Sawyer & C.H. Tower, Phys. Rev. 35, 269 (1930). J-F. Rossignol, J-P. Rivera & H. Schmid, Jap. J. Appl. Phys. Suppl. 24-2, 574 (1985). H. Schmid, Rost. Kristallov. 7, 32 (1967) [Growth of Crystals 7, 25 (1969)].