Xin
Peng
a,
Yangchun
Rong
a,
Longlong
Fan
a,
Kun
Lin
a,
He
Zhu
a,
Jinxia
Deng
b,
Jun
Chen
a and
Xianran
Xing
*a
aDepartment of Physical Chemistry, University of Science and Technology Beijing, Beijing, 100083, P. R. China
bDepartment of Chemistry, University of Science and Technology Beijing, Beijing, 100083, P. R. China. E-mail: xing@ustb.edu.cn; Fax: +86 10 6233 2525; Tel: +86 10 62334200
First published on 6th October 2015
It is known that negative thermal expansion (NTE) in PbTiO3 (PT) ferroelectrics can be controlled by chemical substitutions. In the present work, however, we report a method to change the NTE of PT by introducing cation deficiency at both the A (Pb2+) and B sites (Ti4+). We investigated the spontaneous polarization, tetragonality c/a, coefficient of thermal expansion, and Curie temperature TC with 8% Pb2+ deficient PT (P92T), 2% Ti4+ deficient PT (PT98), and pure PT. We found that while Pb2+ deficiency distinctively weakens the NTE, the effect of B-site deficiency could be ignored. These phenomena are ascribed to the NTE mechanism of spontaneous volume ferroelectrostriction. The present study provides a possible way to control the NTE of PT-based materials.
To date, the influence of A/B-site deficiency on NTE in PbTiO3 has not been reported. In the present work, we prepared nonstoichiometric PT samples to introduce Pb2+/Ti4+ deficiencies. The Pb2+/Ti4+ deficiencies can cause a spontaneous polarization displacement change, which in turn causes a distortion of the primitive cell, and thus controls its CTE. The experimental results show that the CTE of PT with 2% Ti4+ deficiency (PT98) is nearly equal to that of pure PT, while the NTE of an 8% Pb2+ deficient compound (P92T) is significantly weakened.
PT | P92T | PT98 | |
---|---|---|---|
Composition (Pb:Ti) | 0.9979:1 | 0.9395:1 | 1:0.9843 |
a/Å | 3.8990(1) | 3.9019(2) | 3.8994(1) |
c/Å | 4.1542(1) | 4.1384(3) | 4.1540(1) |
c/a | 1.065 | 1.060 | 1.065 |
V/Å3 | 63.153(2) | 63.006(7) | 63.163(2) |
δz Ti/Å | 0.395(7) | 0.368(8) | 0.400(9) |
δz Pb/Å | 0.523(4) | 0.497(4) | 0.524(4) |
P s/μC cm−2 | 66.6(9) | 62.7(10) | 67.1(11) |
T C/°C | 490.6 | 484.3 | 491.3 |
CTE/°C−1 | −1.99 × 10−5 | −1.68 × 10−5 | −2.00 × 10−5 |
As shown in Fig. 1, PT98 and P92T samples are in pure tetragonal phases, and the crystal structure can be well refined using the same structure model with PT (P4mm). In Table 1, we compare the c/a, cell volume, Ps at room temperature and TC of P92T, PT98 and PT. It is found that PT and PT98 are very similar in their structural properties, whereas P92T is greatly different from PT. The c axis of P92T is also smaller than that of PT, while the a axis is bigger than that of PT. Thus, the c/a of P92T (1.060) is smaller than that of PT which has a value of 1.065. The unit cell volume of P92T at room temperature is also smaller than that of PT. It is known that the ferroelectric dipole is aligned with the direction of the c axis, which strongly correlates with the lattice and the Ps displacement. As shown in Table 1, the A-site Ps displacement of Pb2+(δzPb) decreases from 0.524 Å of PT to 0.497 Å of P92T, while the B-site Ps displacement of Ti4+(δzTi) decreases from 0.400 Å to 0.368 Å. This indicates that the reduction in the c axis is due to its close relation with the decrease in the Ps displacement. Due to the more covalent bonding of Ti with the four adjacent oxygens (O2) in the ab plane (dsp hybridization), the linkage of O1–Ti–O1 along the c axis becomes more flexible than the stiff Ti–O2 bonds24,25 during compression and elongation. On the other hand, for the B-site deficiency, the crystal structure properties of PT are rarely affected due to the fact that only a small concentration of B-site vacancies can be introduced. Therefore, the ferroelectric properties cannot be significantly affected, and thus the NTE does not change apparently.
Fig. 1 XRD patterns of PT98 and P92T. Observed (point), calculated (line) and difference profiles at room temperature after Rietveld refinement using the P4mm space group for PT98 and P92T. |
High temperature X-ray diffraction measurements from RT to 700 °C were performed to determine the evolution of the cell parameters of PT, P92T and PT98. As shown in Fig. 2(a), the a axis in all three compositions increases upon heating, while the c axis decreases with increasing temperature below TC. The c axis of P92T is also smaller than that of the other two compositions, which reduces the unit cell volume (Fig. 2b). As a result, the NTE of P92T is weakened significantly, while the NTE of PT98 is nearly identical to that of PT. The CTE of P92T is −1.58 × 10−5 per °C, which is smaller than that for PT (−1.99 × 10−5 per °C). As shown in Fig. 2b, the difference in the NTE is mainly due to the change in the c-axis. It has been known that there is coupling between the c-axis and Ps for PT-based ferroelectrics. In the P92T sample, the Ps displacements are reduced at both A and B-sites compared with PT, but not for PT98 (Table 1), thus indicating that ferroelectricity is reduced significantly in P92T. It is therefore possible to conclude that the weakened NTE of P92T is a result of the reduction in ferroelectricity. The present study is in good agreement with previous studies.1
Fig. 2 Temperature dependence of the (a) lattice constant, and (b) unit cell volume of PT, P92T and PT98. The error bar is too small to view as it is smaller than the experimental data icon. |
Additionally, the phase transition temperatures from tetragonal to cubic have been measured using DSC measurements as shown in Fig. 3. The TC values of PT, P92T and PT98 are determined to be 490.6 °C, 491.3 °C and 484.3 °C, respectively. The reduction in TC indicates a reduced Ps, which is consistent with the structure refinement results.
The Raman spectra of PT, PT98 and P92T are shown in Fig. 4. PT has a tetragonal space group symmetry C4v1 with an ABO3 formula unit cell, and is composed of 12 optical modes that can be divided into three categories: three A1-symmetry modes, eight E-symmetry modes, and one B1-symmetry mode. The three transverse optical (TO) modes of A1-symmetry (A1(1TO), A1(2TO), and A1(3TO)) are important for PbTiO3-based ferroelectric compounds because the vibrations are along the direction of the Ps.26 Specifically, the A1(1TO) soft mode is composed of the displacement of the TiO6 octahedron relative to the lead atoms, while the A1(2TO) soft mode is composed of the displacements of the titanium ion relative to the oxygen and lead ions. In A1(3TO) soft mode, titanium ions, with oxygen ions lying inbetween, move in the c-axis direction.26,27 The change in these A1-symmetry modes are highly correlative with the Ps. The results show that the Raman active modes of A1(1TO), A1(2TO) and A1(3TO) soften from PT to P92T, but remain relatively similar from PT to PT98 (Fig. 4). This indicates that the Ps displacements are reduced at both the A- and B-sites in P92T, but not in PT98. Furthermore, the ferroelectricity is reduced in P92T, but not in PT98, which confirms the results described above.
In view of these findings, we can thus conclude that the decrease in the Ps displacement is caused by the defects in PT. To clarify, the c axis is closely related with Ps displacement such that the decrease of Ps displacement triggers the decrease in the c axis. As shown in Table 1, both δzTi and δzPb of P92T are smaller than those for PT at room temperature. This is why the c axis of P92T is much smaller than that of PT; on the other hand, the a axis is related with the TiO6 octahedron which is stable in a single phase, so the change in the a axis is small. This explains why the unit cell volume of P92T is smaller than that of PT in the ferroelectric phase below TC (Fig. 2(b)). Furthermore, at TC, the ferroelectric phase transforms to a paraelectric cubic one and the Ps displacements disappear, making the unit cell volume at TC similar for both PT and P92T, thus causing the NTE of P92T to clearly decrease. In addition, the TC of P92T is found to be smaller than that of PT due to the small Ps displacement of P92T and less energy is needed for the phase transition, which can be described by the Landau theory.28
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5qi00154d |
This journal is © the Partner Organisations 2015 |