19 th INTERNATIONAL CONGRESS ON ACOUSTICS MADRID, 2-7 SEPTEMBER 2007

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1 9 th INTERNATIONAL CONGRESS ON ACOUSTICS MADRID, -7 SEPTEMBER 007 ULTRASONIC ATTENUATION AND PIEZOELECTRIC PROPERTIES AT PHASE TRANSITIONS OF LAYERED POLAR CRYSTALS OF CuInP S 6 FAMILY PACS:.5.Cg Samulionis, Vytautas, Banys, Juras and Vysochanskii, Yulian Physics Faculty of Vilnius University, Sauletekio al. 9/, LT-0, Vilnius, Lithuania vytautas.samulionis@ff.vu.lt Institute of Solid State Physics and Chemistry, Uzhgorod University, Pidgirna 6, Uzhgorod, Ukraine, yulian.vysochanskii@univ.uzhgorod.ua ABSTRACT The paper reviews recent results of ultrasonic and piezoelectric investigation in CuInP S 6, CuInP Se 6, CuCrP S 6 layered crystals and their solid solutions in temperature range 00-0 K. The ultrasonic velocity dispersion and attenuation peaks were observed at phase transitions (PT). The critical ultrasonic behaviour is strongly anisotropic and depends on composition of solid solution. The largest ultrasonic anomalies appeared for ultrasonic wave propagating normally to layers: i.e. along polarization direction. In solid solutions, approaching phase boundaries, the widths of critical attenuation peaks increase what we attribute to the increase of order parameter relaxation time. Investigation of the second ultrasonic harmonic revealed extremely high elastic nonlinearity for longitudinal ultrasound propagating across layers in CuInP S 6 family crystals. The existence of piezoelectric sensitivity in low temperature phases was confirmed by direct ultrasonic and resonance methods. The influence of DC electric field on critical ultrasonic anomalies and piezoelectric parameters also is discussed. It was shown that at room temperature, above PT, piezoelectricity due to electrostriction occurred under DC bias electric field applied along c-axis of the CuInP S 6 family crystals. INTRODUCTION The Sn P S 6 family crystals are photoconductors, exhibiting strong piezoelectric effect and can be used in acoustoelectronic and nonlinear acoustic devices []. The variety of ultrasonic phenomena such as acoustoelectric interaction, acoustoelectric voltage, relaxing ultrasonic attenuation oscillations and photostimulated shift of phase transitions down to lower temperature were observed and investigated in these crystals [-]. Recently the new, crystals of this family were grown: i.e. CuInP S 6, CuInP Se 6 and CuCrP S 6 and their solid solutions, which also are promising materials for functional electronics. They all are highly anizotropic and crystallize in a layered two-dimensional structure of the Cu I M III P S 6 (M=In, Cr) type []. Double sheets of sulphur atoms sandwiching the metal cations and P-P groups, which occupy the octahedral voids, determined by the sulphur atoms form such lattice. At elevated temperatures, the smeared out copper electronic distribution perpendicular to the layer can be satisfactorily modelled by two vertically disposed positions, one distinctly (Cu) and the other (Cu) slightly shifted from the octahedral centre where the crystal structure has a centrosymmetric space group C/c. The incomplete occupancy of these sites may be interpreted as a static or dynamic kind of disorder. A very interesting feature of these crystals is that they involve antiparallel shifts of Cu and In, Cr cations away from the midplane of a layer along c-axis. The Cu I sublattice is polar in CuInP S 6 below phase transition (PT) at T c = K and coexists with an In III sublattice of unequal and opposite polarity. The structure of the paraelectric phase is C/c, and at low temperature the space group changes to Pc [5]. The Cu I sublattice is antipolar in CuCrP S 6 below 50 K; it coexists with an In III sublattice of equal and opposite polarity []. So the CuInP S 6 and CuCrP S 6 crystals can be defined as ferrielectric and antiferroelectric systems respectively. At low temperature the space group changes to Cc or Pc for both materials. In CuCrP S 6 the intermediate phase has been observed by neutron powder diffraction, calorimetric

2 [6], ultrasonic and piezoelectric studies [7]. The phase transition sequence is very close to that of Sn P Se 6 where the existence intermediate-incommensurate phase has been confirmed. From calorimetric studies it was suggested that in CuCrP S 6 the incommensurate phase exists also [6]. This intermediate quasi-antipolar phase can be interpreted in terms of occasionally flipping dipoles in an otherwise antipolar phase, or static clusters of up and down dipoles that form glassy precursor to long-range order. In CuInP Se 6 compound the ferroelectric PT at T c 6 K (the symmetry reduction P?c? Pc at the PT) was observed [5]. The nonpolar phase in CuInP S 6, CuInP Se 6 and CuCrP S 6 also was shown to be not of the usual paraelectric type, but, because of relatively strong dipole-dipole interactions, has polar clusters even at temperatures far above the transition []. Therefore, it is of interest to study this polar-layered CuInP S 6 family system in order to obtain information about ultrasonic and piezoelectric behaviour near phase transitions. In this contribution, we summarize the results of extended experimental investigation piezoelectric sensitivity, linear and nonlinear elastic properties in this new family of CuInP S 6 crystals. The investigations of temperature dependencies of ultrasonic attenuation, velocity, second harmonic and piezoelectric properties revealed the anomalies at phase transitions. The large anisotropy of ultrasonic attenuation and harmonic generation was observed. The DC electric field induced piezoelectric effect had been found in the paraelectric phase of layered crystalline plates. EXPERIMENTAL The crystalline samples of CuInP (S x Se -x ) 6 and CuCr x In -x P S 6 were grown by solid chemical transport reactions and had the form of thin plates with c-axis normal to the plate surfaces. Large CuInP S 6 crystals were grown by Bridgeman method. The ultrasonic and piezoelectric measurements were carried out by the methods, which we previously used for such measurements in pure CuInP S 6 and CuCrP S 6 crystals [,7]. Longitudinal ultrasonic velocity and attenuation measurements were carried out by automatic computer controlled pulse-echo ultrasonic system. This equipment allowed us to measure time delay changes less than 0. ns; therefore the relative ultrasonic velocity measurements on very thin samples were possible. In the same pulse-echo ultrasonic system simple piezoelectric measurements can be also performed. The experimental set up is shown in Fig.. RFP U DC RFP C QB T Cu(In,Cr)P S 6 plate Figure. Experimental set up for piezoelectric test using pulse-echo ultrasonic system In this case radio frequency pulse excites lithium niobate ultrasonic transducer T. Ultrasonic wave passes fussed quartz buffer (QB) and excites CuInP S 6 plate. The electric signal appears only if plate is piezoelectric or in paraelectric phase piezoelectricity can be induced by applied DC voltage (U DC ) due to electrostriction. For calibration, the electric admitance frequency measurements of the same CuInP S 6 plate were also performed on homemade automatic resonance-antiresonance apparatus [7]. Therefore the values of absolute sound velocity and electromechanical coupling factor at required stabilized temperature were obtained by measurements of electromechanical antiresonance frequencies. Silicone oil was the material for making acoustic bonds. Silver paint electrodes were used for electric measurements. The temperature stabilization and measurement accuracy was better than 0.0 K. RESULTS AND DISCUSSION The phase diagram of CuInP (Se x S -x ) 6 crystals is complicated. Pure CuInP S 6 exhibits the first order ferrielectric PT near K. At this PT the large critical slowing down in ultrasonic velocity is observed for all longitudinal ultrasonic modes. With substitution S by Se the PT temperature 9 th INTERNATIONAL CONGRESS ON ACOUSTICS ICA007MADRID

3 decreases and velocity anomalies decrease also (Fig. ). First order PT anomaly is smeared. In this figure the temperature dependencies of ultrasonic velocity measured at 0 MHz are shown for compounds with Se content: - 0, - 0.0, , v / v, % T, K Figure. The temperature dependencies of 0 MHz longitudinal ultrasonic velocity of mixed CuInP S 6 layered crystals. Se content: 0, 0.0, 0.05, Corresponding to velocity minima, longitudinal ultrasonic attenuation peaks along c-axis were observed at the similar temperatures (Fig. ). Also broadening of attenuation anomalies is clearly seen.,,,0 α, c m - 0,8 0,6 0, 0, Figure. The temperature dependencies of 0 MHz longitudinal ultrasonic attenuation of sulphur rich CuInP S 6 layered crystals. Se content: 0, - 0.0, , Due to the small thickness of samples (d 0. mm) the data of ultrasonic attenuation and velocity for solid solutions were not very perfect. As usually, the ultrasonic behaviour at the ferroelectric phase transitions is described by interaction of the order parameter (polarization) with the ultrasonic waves according to the relaxation theory of Landau-Chalatnikov [8], which implies piezoelectric coupling of elastic wave with order parameter and a critical increase of the polarization relaxation time. Such explanation is correct only for temperatures below phase transitions. In the paraelectric phase ultrasonic behaviour is described by fluctuation theory [9]. The temperature dependences of longitudinal ultrasonic velocity and attenuation show that polarization relaxation time not only increase at T c, but also with the increase of Se concentration. The steep increase of velocity in polar phases is similar to that which was observed in DDSP, DMAAS [9] single crystals, where the relaxation time of the order parameter was comparatively long. In this case, the temperature dependence of ultrasonic velocity in the low temperature phase must follow the behaviour of the order parameter square. The smearing of anomalies is the indication of appearance of relaxor or glassy state in accordance with previous dielectric investigations where typical dipolar glass dielectric dispersion was found [0]. The increase of attenuation in solid solutions far above PT is attributed to the interaction of ultrasonic wave with mobile copper ions, because Cu+ ionic conductivity is high enough []. As it was mentioned in introduction, CuInP S 6 crystals are highly anizotropic, therefore ultrasonic attenuation peaks is much less when measured along layers. In Fig. the temperature dependencies of longitudinal ultrasonic attenuation coefficient are shown for different propagation directions. 9 th INTERNATIONAL CONGRESS ON ACOUSTICS ICA007MADRID

4 α, cm -,,,0 0,8 0,6 0, 0, Figure. Temperature dependencies of ultrasonic attenuation along c-axis (across layers) -(), and along layers () at 0 MHz frequencies for pure CuInP S 6 crystals. The same orientation dependence was obtained for the second longitudinal ultrasonic harmonic amplitude. Across layers the second harmonic amplitude is very large because of extremely large acoustic nonlinearity [], whereas along layers the second harmonic amplitude was much smaller (Fig.5). The ultrasonic input displacement at 0 MHz frequency was u 0 = cm in both cases. It could be seen that second harmonic amplitude increases at phase transition temperature, due to the increase of acoustic nonlinearity. Minimum in curve is associated with the increase of ultrasonic attenuation at PT (see [] for details. The second harmonic reaches maximum when the first harmonic attenuation coefficient a satisfies saturation condition: a = ln/l, where L sample length. Sample length was 0.9 cm for mode across layers and 0.5 cm - along layers.,5,0,5 U II, 0 - cm,0,5,0 0, Figure 5. The temperature dependencies of second longitudinal ultrasonic harmonic amplitudes at 0 MHz frequencies for propagating directions along c-axis (across layers), and along layers in pure CuInP S 6 crystals. The ultrasonic input intensity was the same in both cases. The anisotropy of ultrasonic attenuation and nonlinear elastic properties to our opinion arises from large anisotropy of bonding forces, determined by anharmonicity of appropriate longitudinal phonon modes. The elastic nonlinearity for longitudinal mode propagating normal to layers is very large and exceeds the nonlinear parameters of other known nonlinear crystals, what is important for applications in nonlinear acoustic devices. In order to prove existence of piezoelectricity in CuInP S 6 family crystals we used direct measurement of electric signal arising on a thin plate working as ultrasonic transducer in usual pulse-echo ultrasonic experiment (see Fig.). The exciting lithium niobate transducer was attached to one end of quartz buffer and to another end a thin plate under investigation was glued. It is necessary to note that this method is very convenient for CuInP S 6 family layered crystals, because they usually grow as thin plates and could be easily cleaved. In polar phase the crystalline plate works as ultrasonic transducer. The temperature dependence of signal detected by such transducer roughly represents the dependence of piezoelectric coefficient. Using this simple test the piezoelectric sensitivity was checked in pure CuInP S 6, CuInP Se6, CuCrP S 6 layered crystals and their solid solutions. In Fig. 5 the examples of such electric signals detected by transducers of various compositions are shown. Indeed in all samples, the piezoelectric signal appeared at low temperatures (below PT) and increased with temperature 9 th INTERNATIONAL CONGRESS ON ACOUSTICS ICA007MADRID

5 decreasing. In pure CuInP Se 6, CuCrP S 6 crystals and CuCr 0. In 0.7 P S 6 solid solution another anomaly appears below PT. Such behaviour we associate with existence of intermediate quasipolar phase. It should be noted that the results were obtained for polarized samples. The polarized samples were obtained in cooling from room temperature to below 00 K in 0 kv/cm DC bias field. Then DC field was removed and measurements were performed in heating cycle. Piezoelectric RF signals are presented in arbitrary units in order to show distinct anomalies which appeared at temperatures, where phase transitions exist U, a.u 5 CuInP S 6 CuInP Se 6 CuCrP S 6 0 CuInP (S 0.9 Se 0. ) 6 5 CuCr 0, In 0,7 P S Figure 5. The temperature dependencies of 0 MHz longitudinal ultrasonic signals detected by c-cut plates of pure CuInP S 6 (), CuInP Se 6 (), CuCrP S 6 () crystals and solid solutions (,5). For more detail discussion regarding piezoelectric properties of CuInP S 6 family crystals and solid solutions of various contents we refer to original papers [-]. Above PT, in paraelectric phase CuInP S 6 family crystals belong to the centrosymmetric point group /m [and there is no electromechanical conversion. With DC electric field applied along the polar c-axis, c cut plates worked as ultrasonic transducer and an RF signal appeared above transition temperature and increased with increasing DC field (Fig 6). 0, 0, 0, U, V 0, -0, -0, - CuIn 0.8 Cr 0. P S 6 - CuIn 0.7 Cr 0. P S 6-0, E, kv / cm Figure 6. The detected ultrasonic signal in CuIn 0.8 Cr 0. P S 6 () and CuIn 0.7 Cr 0. P S 6 () plates as a function of DC electric field applied along c-axis. 0,9 0,8 CuIn 0.8 Cr 0. P S 6 Y, a.u. 0,7 0,6 - E = 0 - E = kv/cm - E = 0 kv/cm - E = 5 kv/cm 0,5 T = 95 K Figure 7. The frequency dependences of electric admittance of the CuIn 0.8 Cr 0. P S 6 plate at room temperature, measured in various DC electric fields 9 th INTERNATIONAL CONGRESS ON ACOUSTICS ICA007MADRID 5 f, khz

6 Consequently, piezoelectricity due to electrostriction was observed in these mixed crystals. This was also confirmed by the resonance-antiresonance method. In the electric admittance Y frequency dependencies resonance-antiresonance character was observed when DC bias field increased (Fig. 7). In this case, it was possible to obtain the elastic and electromechanical parameters of such electrostrictive ultrasonic transducers by measuring the resonance and antirerezonance frequencies. The calculated values of electromechanical coupling coefficient were of order in DC electric field of order 5 kv/cm. This suggests possible electroacoustic applications for DC electric field controlled ultrasonic transducers. In our experiment we used thin Cu(In,Cr)P S 6 samples without any mechanical cutting and polishing. It is an advantage of such layered crystals that they can be obtained as thin plates with quite parallel surfaces and comparatively large DC electric field can be applied without breakdown. CONCLUSIONS The temperature dependencies of piezoelectric response, longitudinal ultrasonic velocity, attenuation and second harmonic generation near phase transitions in layered polar crystals of CuInP S 6 family are measured. For longitudinal mode the critical slowing down in ultrasonic velocity and attenuation peaks have been observed in the vicinity of phase transitions. The large anisotropy of linear and nonlinear elastic properties was observed. Piezoelectric effect was observed in the low temperature phases of these polar-layered crystals. Above transitions in the paraelectric phase, the piezoelectricity due to electrostriction occurred under DC bias electric field applied along c-axis of the CuInP S 6 family crystals. ACKNOWLEDGEMENTS Support of Lithuanian State Science and Studies Foundation is gratefully acknowledged. References: [] V. Samulionis, V. Valevichius, J. Grigas and Yu. Vysochanskii: Investigation of the second ultrasonic harmonic generation in Sn P S 6 and Sn P Se 6 crystals. Ferroelectrics 05 (990) 97-0 [] V. Samulionis, J. Banys, Yu. Vysochanskii and N. Cajipe: Elastic and electromechanical properties of new ferroelectric-semiconductor materials of Sn P S 6 family. Ferroelectrics 57 (00) - [] V. Samulionis, J. Banys and Yu. Vysochanskii: Ultrasonic investigation of photo-stimulated phenomena in ferroelectric semiconductors. Ferroelectrics 57 (00) 5 7 [] V.B. Cajipe, J. Ravez, V. Maisonnneuve, A, Simon, C. Pyen, R. Von Der Muhl and J.E. Fisher: Copper ordering in lamellar CuMP S 6 (M=Cr,In): transition to an antiferroelectric or ferroelectric phase. Ferroelectrics 85 (996) 5-8 [5] V. Maisonneuve,M. Evain, C. Payen, V.B. Cajipe and P. Molinie: Room temperature crystal structure of the layered phase CuInP S 6. J. Alloys and Compounds 8 (995) [6] K. Moriya, N. Kariya, A. Inaba, T. Matsuo, I. Pritz and Yu. Vysochanskii: Low temperature calorimetric study of phase transitions in CuCrP S 6. Solid State commun. 6 (005) 7-76 [7] V. Samulionis, Yu. Vysochanskii and V. Cajipe: Ultrasonic and piezoelectric investigation of Sn P S 6 type photosensitive ferroelectric-semiconductor crystals. Ferroelectrics 95 (00) -0 [8] L.D. Landau, I.M. Khalatnikov: About anomalous sound attenuation near the phase transition of second order (in Russian). Sov.Phys.(Doklady) 96 (95) [9] V. Valevicius, V. Samulionis and J. Banys: Ultrasonic dispersion in the phase transition region of ferroelectric materials. J. Alloys and Compounds, / (99) 69-7 [0] J. Banys, R. Grigalaitis, J. Macutkevic, A. Brilingas, V. Samulionis, J. Grigas and Yu. Vysochanskii: Dipolar glass behaviour in mixed CuInP (S 0.7Se 0.) 6 crystals Ferroelectrics 8 (005) 6-68 [] V. Maisonneuve, J.M. Reau, MinDong, V.B. Cajipe, C. Payen, and J. Ravez: Ionic conductivity in ferroic CuInCrP S 6 and CuInP S 6. Ferroelectrics 96 (997) [] J. Banys, J. Macutkevic, V. Samulionis, A. Brilingas and Yu. Vysochanskii: Dielectric and ultrasonic investigation of phase transitions in CuInP S 6 crystals. Phase Transitions 77 (00) 5-58 [] V. Samulionis, J. Banys, Yu. Vysochanskii and I. Studeniak: Investigation of Ultrasonic and Acoustoelectric Properties of Ferroelectric - Semiconductor Crystals. Ferroelectrics 6 (006) 9-8 [] V. Samulionis, J. Banys and Yu. Vysochanskii: Ultrasonic and Piezoelectric Investigation of Phase Transitions in Layered CuIn -XCr XP S 6 Crystals, Feroelectrics 8 (007) -0 9 th INTERNATIONAL CONGRESS ON ACOUSTICS ICA007MADRID 6