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Evidence for a new phase transition in Rb2ZnCl4 by Raman scattering
Solid State Communications, Vol. 33, PP. 155—156. Pergamon Press Ltd. 1980. Printed in Great Britain. EVIDENCE FOR A NEW PHASE TRANSITION IN Rb2ZnC14 BY RAMAN SCATTERING E. Francke, M. Le Postollec, J.P. Mathieu and H. Poulet Département de Recherches Physiques, associé au CNRS, Université P. et M. Curie, 4 place Jussieu, 75230, Paris, France (Received 16 July 1979 by M. Balkanski) The Raman spectra of an oriented single crystal of Rb2ZnC14 give evidence for the existence of a phase mode at low temperature, as predicted by Wada et al. A new soft mode whose frequency decreases as the temperature T3 72 K is approached from below, was observed in the a(cc)b scattering orientation, strongly supporting the onset of a new phase transition.
1. INTRODUCTION CONSIDERABLE INTEREST arose recently on the successive phase transformations of Rb2ZnC14 from a parent paraelectric Fincn phase to an incommensurate phase at T1 = 302 K, followed by a lock in into a cornmensurate improper ferroelectric phase at T2 = 189 K with P3 1 a and a tripled primitive cell along c [1, 21. It is generally acceptedthat these successive phase transitions are very similar to those of K2SeO4. Wada et al. [3] reported the observation of two soft modes in K2SeO4 by Raman scattering measurements. One of them observed below the prototype— incommensurate transition point corresponds to the amplitude mode; the other, being observed only in the low temperature commensurate phase, corresponds to the phase mode. In an extention of their work [4] to Rb2ZnCL~, they observed the amplitude mode, but could not detect the phase mode above 77 K, the lowest temperature of their measurements. We have extended the study of the Raman spectrum of this crystal to lower temperatures in an attempt to record the phase mode. 2. EXPERIMENTAL
producible within 0.1 K. The sample being illuminated by the laser beam, its true temperature is known only within 1 or 2 K.
3. RESULTS AND DISCUSSION In the a(ca)b scattering geometry in which the q = 0 phase mode is Raman active [3, 5] we observe, as shown in Fig. 1, below 90K, a very weak but defmite Rarnan feature on the flank of the Rayleigh line. Its frequency increases as the temperature decreases and is well defined at 80K (7.7 cm’); below 69 K there is some ambiguities in assignments because something happens in the crystal as seen below. In spite of the restricted range of observation, we believe that this mode is likely to be the phase mode because of its strong temperature dependence. In the a(cc)b scattering geometry the results of Wada et al. [4] above 77 K are confirmed and need not to be reported anew. As shown in Fig. 2, lowering the temperature leads to the unexpected observation of a new strongly temperature dependent mode whose frequency varies between 15 cm~at 9 K and 5 cm’ at 64K. In the range 47—64 K the frequency ofcritical this 2with the mode obeys exponent (3=the law T P = A(T3 T)’~’ 1 K”2. This 3 = 72K and A = 2.2 cm mode remains underdamped in approaching the temperature T 3. This is an unusual behavior for a soft mode. As far as we know, only in a molecular crystal, chioranil —
~,
Single crystals of about 1 cm3 were grown by slow evaporation at 18°Cof aqueous solution containing stochiometric mixtures of RbC1 and ZnC12. The crystalline samples were cut into the shape of parallelepipeds, the edges of which were parallel to a(Y), b(X) and c(Z). The Raman spectra were excited by the 514.5 nm radiation of an Ar~laser with a power from 0.5 to 1 W. They were recorded with a T 800 Coderg spectrometer. The sample was cooled in a helium gas cryostat. The temperature was measured with a thermocouple Cu—constantan placed in contact with the sample. The comparative temperatures of different measurements were estimated to be re-
[6], was reported an underdamped zone boundary soft mode. We n~ticethat this mode does not appear above 72 K, so it is likely a zone boundary mode of the ferroelectric phase, which is believed [7] to belong to the space group P21cn in which all q = 0 modes are Raman active.
Acknowledgement preparation. 155
—
We thank Mme Lenain for crystal
156
EVIDENCE FOR A NEW PHASE TRANSITION IN Rb2ZnC14 a(ca)b
Vol. 33, No. 1
a(cc)b
IJ.L~. T~1~
T~/L\
7
10
0
10
20
Fig. 1. Raman spectrum of Rb2ZnCI4 in the a(ca)b geometry showing the q = 0 phase mode in the commensurate low temperature phase.
V~~~ç1
2. 3.
K. Gesi & M. lizumi, J. Phys. Soc. Japan 46, 697 (1979). S. Sawada, Y. Shiroishi, A. Yamamoto, M. Takashige & M. MatsuO, J. Phys. Soc. Japan 43, 2099 (1977). M. Wada, H. Uwe, A. Sawada, Y. Ishibashi,
0
Fig. 2. Raman spectrum of Rb2ZnC14 in the a(cc) b geometry at low temperatures. In addition to the 26 cm~’ soft mode and its accompanying mode at 19 cm’ already observed above 77 K by Wada etal., it is seen a new strongly temperature dependent mode (indicated by an arrow).
REFERENCES 1.
50
4. S. 6. 7.
Y. Takagi & T. Sakudo, J. Phys. Soc. Japan 43, 544 (1977). M. Wada, A. Sawada & Y. Ishibashi, J. Phys. Soc. Japan 45, 1429 (1978). V. Dvorak & J. Petzelt, J. Phys. CII, 4827 (1978). DM. Hanson, J. Chem. Phys. 63, 5046 (1975). A.K. Moskalev, l.A. Belobrova & I.P. Aleksandrova Soy. Phys. Solid State 20,(1 1), 1896 (1978).
1. INTRODUCTION CONSIDERABLE INTEREST arose recently on the successive phase transformations of Rb2ZnC14 from a parent paraelectric Fincn phase to an incommensurate phase at T1 = 302 K, followed by a lock in into a cornmensurate improper ferroelectric phase at T2 = 189 K with P3 1 a and a tripled primitive cell along c [1, 21. It is generally acceptedthat these successive phase transitions are very similar to those of K2SeO4. Wada et al. [3] reported the observation of two soft modes in K2SeO4 by Raman scattering measurements. One of them observed below the prototype— incommensurate transition point corresponds to the amplitude mode; the other, being observed only in the low temperature commensurate phase, corresponds to the phase mode. In an extention of their work [4] to Rb2ZnCL~, they observed the amplitude mode, but could not detect the phase mode above 77 K, the lowest temperature of their measurements. We have extended the study of the Raman spectrum of this crystal to lower temperatures in an attempt to record the phase mode. 2. EXPERIMENTAL
producible within 0.1 K. The sample being illuminated by the laser beam, its true temperature is known only within 1 or 2 K.
3. RESULTS AND DISCUSSION In the a(ca)b scattering geometry in which the q = 0 phase mode is Raman active [3, 5] we observe, as shown in Fig. 1, below 90K, a very weak but defmite Rarnan feature on the flank of the Rayleigh line. Its frequency increases as the temperature decreases and is well defined at 80K (7.7 cm’); below 69 K there is some ambiguities in assignments because something happens in the crystal as seen below. In spite of the restricted range of observation, we believe that this mode is likely to be the phase mode because of its strong temperature dependence. In the a(cc)b scattering geometry the results of Wada et al. [4] above 77 K are confirmed and need not to be reported anew. As shown in Fig. 2, lowering the temperature leads to the unexpected observation of a new strongly temperature dependent mode whose frequency varies between 15 cm~at 9 K and 5 cm’ at 64K. In the range 47—64 K the frequency ofcritical this 2with the mode obeys exponent (3=the law T P = A(T3 T)’~’ 1 K”2. This 3 = 72K and A = 2.2 cm mode remains underdamped in approaching the temperature T 3. This is an unusual behavior for a soft mode. As far as we know, only in a molecular crystal, chioranil —
~,
Single crystals of about 1 cm3 were grown by slow evaporation at 18°Cof aqueous solution containing stochiometric mixtures of RbC1 and ZnC12. The crystalline samples were cut into the shape of parallelepipeds, the edges of which were parallel to a(Y), b(X) and c(Z). The Raman spectra were excited by the 514.5 nm radiation of an Ar~laser with a power from 0.5 to 1 W. They were recorded with a T 800 Coderg spectrometer. The sample was cooled in a helium gas cryostat. The temperature was measured with a thermocouple Cu—constantan placed in contact with the sample. The comparative temperatures of different measurements were estimated to be re-
[6], was reported an underdamped zone boundary soft mode. We n~ticethat this mode does not appear above 72 K, so it is likely a zone boundary mode of the ferroelectric phase, which is believed [7] to belong to the space group P21cn in which all q = 0 modes are Raman active.
Acknowledgement preparation. 155
—
We thank Mme Lenain for crystal
156
EVIDENCE FOR A NEW PHASE TRANSITION IN Rb2ZnC14 a(ca)b
Vol. 33, No. 1
a(cc)b
IJ.L~. T~1~
T~/L\
7
10
0
10
20
Fig. 1. Raman spectrum of Rb2ZnCI4 in the a(ca)b geometry showing the q = 0 phase mode in the commensurate low temperature phase.
V~~~ç1
2. 3.
K. Gesi & M. lizumi, J. Phys. Soc. Japan 46, 697 (1979). S. Sawada, Y. Shiroishi, A. Yamamoto, M. Takashige & M. MatsuO, J. Phys. Soc. Japan 43, 2099 (1977). M. Wada, H. Uwe, A. Sawada, Y. Ishibashi,
0
Fig. 2. Raman spectrum of Rb2ZnC14 in the a(cc) b geometry at low temperatures. In addition to the 26 cm~’ soft mode and its accompanying mode at 19 cm’ already observed above 77 K by Wada etal., it is seen a new strongly temperature dependent mode (indicated by an arrow).
REFERENCES 1.
50
4. S. 6. 7.
Y. Takagi & T. Sakudo, J. Phys. Soc. Japan 43, 544 (1977). M. Wada, A. Sawada & Y. Ishibashi, J. Phys. Soc. Japan 45, 1429 (1978). V. Dvorak & J. Petzelt, J. Phys. CII, 4827 (1978). DM. Hanson, J. Chem. Phys. 63, 5046 (1975). A.K. Moskalev, l.A. Belobrova & I.P. Aleksandrova Soy. Phys. Solid State 20,(1 1), 1896 (1978).