Orthorhombic and monoclinic ferroelectric phases investigated by Raman spectroscopy in PZN-4.5%PT and PZN-9%PT crystals

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Abstract

Polarized Raman spectra measurements have been performed on orientated single crystals of Pb[(Zn1/3Nb2/3)0.955Ti0.045]O3 (PZN-4.5%PT) and Pb[(Zn1/3Nb2/3)0.91Ti0.09]O3 (PZN-9%PT) poled by an electric field along the [1̄01] pseudo-cubic direction in order to determine the nature of the induced ferroelectric phases. Under field-cooled (FC) conditions the ferrolectric phase transition sequence, tetragonal–orthorhombic–rhombohedral, was observed in PZN-4.5%PT. Comparison between the phase transitions induced in PZN-9%PT and PZN-4.5%PT crystals in FC shows that the low- temperature ferroelectric phase in PZN-9%PT cannot be orthorhombic and it can be attributed to the monoclinic phase. The characteristic temperatures found by Raman spectroscopy correspond to those obtained by dielectric measurements and by X-ray diffraction reported previously.

Introduction

Relaxor-based ferroelectric materials such as Pb[(Mg1/3Nb2/3)1−xTix]O3 (PMN-x%PT) and Pb[(Zn1/3Nb2/3)1−xTix]O3 (PZN-x%PT) have attracted a lot of interest in the last decade because of their substantial piezoelectric and electromechanical properties. These properties are especially enhanced near the morphotropic phase boundary (MPB) that separates the rhombohedral (R) and the tetragonal (T) phases. Such behaviour is a common feature of the well-known Pb(ZrxTi1−x)O3 (PZT) systems. This region is located approximately at 35% of PT for PMN-PT [1] and at 9% of PT for PZN-PT [2]. The exceptional electromechanical properties of PZT, PMN-PT, and PZN-PT have recently been attributed to the polarization rotation mechanism in the intermediate monoclinic (called MA, MB or MC: see [3] for significations) phase occurring close to the MPB [4], [5], [6], [7], [8]. The transition sequence RMBOMCT predicted by Vanderbilt and Cohen [3] is well evidenced [9], [10] in PMN-PT but continues to be debated in PZN-PT. Studies by X-ray and neutron diffraction have shown that the low temperature phase for PZN-PT around the MPB composition should be monoclinic (MC) with space group Pm [11], [12], [13], [14]. In previous works, this low temperature phase has been attributed to the orthorhombic phase (O) [2], [15]. In addition, it was shown to be possible to obtain a stable quasi-single monoclinic (MC) domain state by poling a PZN-9%PT single crystal along the pseudo-cubic [1̄01] direction [11], [16]. (In all the paper crystallographic directions and planes were indexed using the pseudo-cubic axes of the perovskite cell.) Recently, using a micro-Brillouin scattering technique, Ko et al. [17] suggested that this induced phase can be attributed to an orthorhombic phase. At the same time, in a PZN-4.5%PT single crystal, Liu and Lynch [18] demonstrated by optical observations and hysteresis loop measurements that when an electric field larger than 11 kV/cm is applied along [1̄01] direction, a rhombohedral to orthorhombic phase transition takes place at room temperature. Renault et al. [19] demonstrated by X-ray diffraction that the O phase with C2v symmetry in PZN-4.5%PT can also be induced under field-cooled (FC) conditions between the R and T phases in the 365–390 K temperature range.

The characterization of the phase symmetry of these compounds by Raman scattering is known to be complicated [20], [21], [22], [23], [24], [25], [26]. For example, Raman scattering bands were observed in the paraelectric phase having an average cubic (Pm3̄m) symmetry where Raman activity is forbidden, and in going from the paraelectric to the ferroelectric phase, no narrowing of the Raman lines was observed. This behaviour was attributed to the translational symmetry breaking induced by the chemical disorder on the B site and to atomic displacement disorder as confirmed recently by lattice dynamics calculations and spectra simulations [27].

In spite of the complexity of the scattering processes in relaxors and relaxor-based ferroelectrics and of the difficulties of mode assignment in the ferroelectric phase [28], it has been shown that the temperature variation of the Raman spectra and the intensity of some modes can be associated with the structural phase transitions [20], [23], [25], [26], [29].

Up to now, most of the Raman studies reported on PZN-PT and PMN-PT were made on crystals oriented and/or poled along the [001] pseudo-cubic axes [20], [21], [22], [23], which is not the appropriate direction to characterize rhombohedral, orthorhombic or the monoclinic ferroelectric phases. In addition, multi-domain states were obtained after poling along this direction. In the present work, we report for the first time polarized Raman spectra of single crystals of PZN-4.5%PT and PZN-9%PT poled by an electric field along the [1̄01] direction. The purpose is to compare the Raman scattering behaviour of the induced orthorhombic single domain state in PZN-4.5%PT and the monoclinic quasi-single domain state of PZN-9%PT. Since we cannot attribute any phase symmetry using the zone-centre mode assignment, we compare in FC the TM phase transition in PZN-9%PT and the TO phase transition in PZN-4.5%PT and demonstrate that these transitions are different. Contrary to what was reported previously [17] we show that the low-temperature phase in PZN-9%PT crystal can be monoclinic.

Section snippets

Experiment

The single crystals PZN-4.5%PT and PZN-9%PT were grown by the flux method [11]. They were orientated and cut with the faces perpendicular to the orthorhombic axes using the Laüe technique. The samples have parallelepiped shape with sizes 1.9×1.4×1.3 mm3 for PZN-4.5%PT crystal and 1.7×1.2×0.7 mm3 for PZN-9%PT crystal. The large faces of both the crystals are perpendicular to the [101] (X-direction) and the smaller faces are perpendicular to [010] (Y-direction) and to [1̄01] (Z-direction). Both

Results and discussion

When an E-field is applied along an off-polar direction of a crystal, it leads to the setting up of a multidomain configuration which depends on the E-field direction. The domain configurations of the R, T and O phases were described in previous papers [11], [19], [30]. For the E-field direction [1̄01], they were designated as 2R,2T, and 1O. The digit stands for the number of equivalent ferroelectric domains. The monoclinic phase for which the polar direction makes an angle of a few degrees

Conclusions

Raman scattering was used to characterize the ferroelectric phases in poled PZN-4.5%PT and PZN-9%PT single crystals orientated along the orthorhombic axes. Since we cannot attribute any phase symmetry using the zone-centre mode assignment due to the breaking of the translation symmetry, the macrodomain ferroelectric phases in PZN-4.5%PT and PZN-9%PT crystals could not be described by the Raman mode assignment. However, a comparison of the phase transitions induced in PZN-4.5%PT and in PZN-9%PT

Acknowledgements

The authors are grateful to M. Pham Thi and A.É. Renault for crystals they have provided us and gratefully acknowledge helpful discussions with I. Lukyanchuk, M.G. Karkut and G. Calvarin. This work was partially supported by the Regional European Development Funds.

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