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LINE BROADENING IN THE FERROELECTRIC
K4Fe(CN)6
×3H2O
F. Placido
To cite this version:
F. Placido. LINE BROADENING IN THE FERROELECTRIC K4Fe(CN)6×3H2O. Journal de
JOURNAL DE PHYSIQUE Colloque C6, supplbment au no 12, Tome 37, de'cembre 1976, page C6-31
LINE
BROADENING
IN THE FERROELECTRIC
K4Pe(CN), . 3
H,O
F. PLACID0
Department of Physics, University of Edinburgh, Edinburgh, EH9 352, Scotland
RBsum6. - On a confirm6 l'observation des maxima de I'aire de la raie de KFCT dans la rkgion de TO mais on attribue cet effet B l'elargissement de la raie. Cet Clargissement de la raie est attribu6 ii I'hkmitropie microscopique qui se prksente facilement en cristaux et disparaft au cours
de plusieurs cycles de tempkrature.
Abstract. - The observation of maxima of the line area of KFCT near T O has been confirmed but these are shown to be due to marked line broadening which also occurs near TO. This line broadening is shown to disappear with thermal cycling and appears to be produced by micros- copic twinning which occurs readily in KFCT.
1. Introduction. - Although Potassium Ferrocya- nide Trihydrate (KFCT) was first shown to be ferreolec- tric in 1959 [I] the mechanism of the transition is still in dispute. A number of Mossbauer effect measure- ments have been reported, with apparently contra- dictory results and with very different interpreta- tions [2, 3, 4, 51. In this paper it is suggested that previously reported anomalies in the Mossbauer line area near To, the transition temperature [2, 51 are a consequence of severe microscopic twinning between tetragonal and monoclinic modifications of KFCT which is known to occur readily [6] and to be respons- ible for multiple peaks in the dielectric constant near To. Simultaneous measurements of Mossbauer spectra and dielectric constant on the same crystal slice are shown to establish that both the secondary peaks in the dielectric constant and the maxima in the Mossbauer line area can be removed by slow ther- mal cycling. A striking feature of the present results is the marked line broadening which is shown to accom- pany the increases in line area and also to be removed by thermal cycling.
2. Experimental. - KFCT has the chemical for- mula K,Fe(CN), .3 H,O and grows readily as yellow transparent crystals from aqueous solutions. The samples used in the present experiments were grown by slow cooling of a saturated aqueous solution. Tempe- rature control was achieved using a precision three- term programmable controller manufactured by West Instruments Limited, Brighton, Sussex. The two structural modifications which co-exist at room tempe- rature have been examined by Toyoda et al. [6] and the monoclinic form has been shown to have space group C 2/c and a = 9.41
A,
b = 16.8 A, c = 9.39A,
p
= 900 3' while the tetragonal form has a = 9.41A,
c = 33.67 A and space group 14,la. The crystal struc-
tures are very similar, consisting of K,Fe(CN), layers separated by water molecules and differ mainly in the stacking arrangement of these heavy atom layers. The pseudotetragonal nature of the monoclinic layers makes twinning very probable and large single crystals of either monoclinic or tetragonal are very dif- ficult to grow. Of most importance is the fact that twinning can occur on a microscopic scale, when the crystal structure can be thought of as the parent struc- ture with some stacking faults in the layered arrange- ment. The first dielectric measurements were performed on such a crystal by Waku et al. [I] and showed the now characteristic presence of secondary peaks below a main peak occuring near
-
22 OC. The later work of Toyoda et a1 [6] showed that a single monocliniccrystal exhibited only one peak in the dielectric cons- tant in the [101] direction and no anomalies in other directions. Tetragonal crystals showed no anomalies on first cooling but transformed irreversibly into the monoclinic form at around - 60 OC and on subse- quent warming showed dielectric peaks in both [10i] and [loll directions. Peaks were observed in both directions because the tetragonal-monoclinic trans- formation produced not a single monoclinic but a twinned monoclinic-monoclinic crystal with some layers apparently rotated by 90° around the mono- clinic b-axis. Toyoda et al. [6] also showed that twinned monoclinic-tetragonal crystals produced mul- tiple peaks in the dielectric constant in both the monoclinic [lor] and [loll directions. Most authors have assumed from Toyoda's work that cooling a twinned tetragonal-monoclinic crystal below
-
60 OC will ensure a crystal which is completely monoclinic but this is not so, and multiple peaks in the dielectric constant can be observed through many temperature cycles with such crystals. In performing such tempera- ture cycling it is noticeable however that the structureof the secondary peaks progressively changes, with their position varying and their intensity decreasing. This suggests that it is possible to obtain a large monoclinic crystal eventually. Since the object of the present experiments was in the first instant to try and settle the question of why anomalous maxima were obtained in the Mossbauer line area, simultaneous Mossbauer and dielectric constant measurements were made over several very slow thermal cycles of the same crystal slice (each of the full scans, i. e. figures 1,
T e m p e r a t u r e ( K )
FIG. 1. -First cooling of slice oriented in 45" direction. Capacitance measured along [10i].
2, 4, 5 took in the region of 60 days to complete). The Mossbauer measurements were made using a conven- tional constant acceleration spectrometer with a 10 mC 57Co : Pd source. The crystal was mounted in a gas-flow nitrogen cryostat with a long term tempera- ture stability of better than 0.1 K. KFCT crystals cleave very readily perpendicular to the monoclinic b-axis which is not very convenient for measurements along [lor] since this direction lies in the cleavage plane. Therefore, as a compromise between counting statistics and the [10'1] direction the crystal cleavage plane was oriented so that the gamma ray direction was at 450 to [lo71 by mounting on suitably cut brass holders. The dielectric constant was continually moni- tored by painting silver paste electrodes onto the crystal and measuring the capacitance on a self- balancing bridge.
3. Results and discussion. - Results are first pre- sented of three temperature scans on one crystal
slice, of thickness 0.16 _f 0.02 mm, in the 450 orienta- tion and one further scan on the same crystal with the gamma ray direction perpendicular to the cleavage plane and therefore perpendicular to the direction of spontaneous polarization,
P,.
In all cases the Moss- bauer parameters were obtained from a least squares fit to a single Lorentzian lineshape. The Voigt lineshape and also full integral fits were tried but led to no significant improvement in the goodness of fit.In the following figures the capacitance (arbitrary units), line area, isomer shift (I. S.) and full width at half maximum ( T ) are shown for each temperature scan. Figure 1 shows the results obtained in the 450 orientation on the first cooling of the crystal. A large number of secondary peaks in the capacitance are observed in the region 220-250 K and the line area shows several maxima which are very similar to those observed by Hazony et al. [Z] and Montano et al. [5]. However, the most striking features are the very large changes in linewidth from 0.28 mm/s at 263 K to 0.61 mm/s at 255 K and 0.94 mm/s at 233.6 K. Figure 2
Temperature ( K )
FIG. 2. -First heating of slice oriented in 45" direction. Capacitance measured along [lor].
LINE BROADENING IN THE FERROELECTRIC K4Fe(CN)6.3 H z 0 C6-33
Temperature ( K )
FIG. 3.
-
Second cooling of slice oriented in 4S0 direction. Capacitance measured along [lo?].there may be small broader peaks in the low tempera- ture tail. The Mossbauer results were taken over a restricted range of temperature concentrating on the phase transition region. It can be seen that no signi- ficant anomalies remain in either the line area or line- width. This crystal was then brought back to room temperature and re-oriented so that the gamma ray direction was along the monoclinic [OlO] direction, perpendicular to
P,,
New silver paste electrodes were applied so that the capacitance was also measured in this direction. Figure 4 shows the results obtained in this orientation on cooling from rbom temperature. As observed by previous authors no anomalies are apparent in any of the measured parameters. While this last run served as an excellent check on the equip- ment it was not felt to be conclusive since the anomalies in the line area and linewidth for the 4 5 O direction had been removed by the temperature cycling and any anomalies in the [010] direction might well have been removed in a similar fashion. For this reason another temperature scan was performed on a different clea- vage plane, of thickness 0.13 f 0.02 mm, which had not been subjected to any thermal cycling. This slice was again oriented with the gamma ray direction along [010] but silver paste electrodes were applied so that the capacitance was measured along [10i], as in the first three scans, to determine whether anomalies were present in the capacitance. Figure 5 shows the results obtained for this arrangement on cooling from room temperature. The capacitance again shows someTemperature ( K )
FIG. 4.
-
Third cooling. Gamma ray direction along [010] and capacitance measured along [OlO].Temperoture I K )
FIG. 5.
-
First cooling of second slice. Gamma ray _direction along [010] and capacitance measured along [loll.secondary peaks but no anomalies are observed in the line area or linewidth.
are apparent when only a single peak is seen in the capacitance it seems clear that the explanation of these results is connected with the microscopic twinning of monoclinic and tetragonal layers in the crystal. The simplest explanation is that the same mechanism which produces the secondary peaks in the dielectric constant is responsible for the observed increases in Mossbauer linewidth and area. It has been suggested [7] that the secondary peaks in the dielectric constant arise from changes in the translation period of the mono- clinic and tetragonal layer stacking. This does not give rise to peaks in the [OlO] direction since the tetra- gonal-monoclinic transition involves motion of the heavy atom layers in the (010) plane only. It is proposed that the increases in linewidth are produced by the same means, line broadening only occuring when the
iron atoms have a component of veIocity in the gamma ray direction. The line area anomalies can then be explained as following from the line broadening through extinction effects. The Mossbauer anomalies and secondary peaks in the capacitance are not reproducible in the present experiments presumably because the very slow cooling and heating leads to the removal of strains produced in the crystal during growth, resulting finally in a completely monoclinic sample. Since the crystal then shows no anomalies in the line area it can be concluded that no evidence for a soft mode accompanying the ferroelectric transition in monoclinic KFCT can be obtained from the Mossbauer effect. This is in agreement with the results on powdered KFCT [3,4] where no anomalies at To were observed.
References
[I] WAKU, S., HIRABAYASKI, H., IWASAKI, H. and KIKIYAMA, R., 151 MONTANO, P. A., SCHECHTER, H. and SHIMONY, U., Phys.
J. Phys. Soc. Japan 14 (1959) 973. Rev. 3 (1970) 858.
[2] HAZONY, Y., EARLS, D. E. and LEFKOWITZ, I., Phys. Rev. [6] TOYODA, H., NUZEKI, N. and WAKU, S., J. Phys. Soc. Japan
166 (1968) 507. 15 (1960) 1831.
[3] GLEASON, T. G. and WALKER, J. C., Phys. Rev. 188 (1969) 893. [71 KRASNIKOVA, A. Y. and KOPTSX, V. A., SOV. Phys. Solid
