Pam A. Thomas

Investigation of the structure and phase transitions in the novel A-site substituted distorted perovskite compound Na0.5Bi0.5TiO3

Rietveld neutron powder profile analysis of the compound Na0.5Bi0.5TiO3 (NBT) is reported over the temperature range 5–873 K. The sequence of phase transitions from the high-temperature prototypic cubic structure (above 813 K), to one of tetragonal (673–773 K) and then rhombohedral structures (5–528 K) has been established. Coexisting tetragonal/cubic (773–813 K) and rhombohedral/tetragonal (with an upper temperature limit of 145 K between 528 and 673 K) phases have also been observed. Refinements have revealed that the rhombohedral phase, space group R3c, with aH = 5.4887 (2), cH = 13.5048 (8) A, V = 352.33 (3) A3, Z = 6 and Dx = 5.99 Mg m−3, exhibits an antiphase, a−a−a− oxygen tilt system, ω = 8.24 (4)°, with parallel cation displacements at room temperature. The tetragonal phase, space group P4bm, with aT = 5.5179 (2), cT = 3.9073 (2) A, V = 118.96 (1) A3, Z = 2 and Dx = 5.91 Mg m−3, possesses an unusual combination of in-phase, a0a0c+ oxygen octahedra tilts, ω = 3.06 (2)°, and antiparallel cation displacements along the polar axis. General trends of cation displacements and the various deviations of the octahedral network from the prototypic cubic perovskite structure have been established and their systematic behaviour with temperature is reported. An investigation of phase transition behaviour using second harmonic generation (SHG) to establish the centrosymmetric or non-centrosymmetric nature of the various phases is also reported.

An x-ray diffraction and Raman spectroscopy investigation of A-site substituted perovskite compounds: the (Na1-xKx)0.5Bi0.5TiO3 (0le xle1) solid solution

The (Na1 - x Kx )0.5 Bi0.5 TiO3 perovskite solid solution is investigated using x-ray diffraction (XRD) and Raman spectroscopy in order to follow the structural evolution between the end members Na0.5 Bi0.5 TiO3 (rhombohedral at 300 K) and K0.5 Bi0.5 TiO3 (tetragonal at 300 K). The Raman spectra are analysed with special regard to the hard modes and suggest the existence of nano-sized Bi3+ TiO3 and (Na1 - 2x K2x )+ TiO3 clusters. The complementary use of XRD and Raman spectroscopy suggests, in contrast to previous reported results, that the rhombohedral tetragonal phase transition goes through an intermediate phase, located at 0.5 x 0.80. The structural character of the intermediate phase is discussed in the light of sub- and super-group relations.

Monoclinic crystal structure of polycrystalline Na0.5Bi0.5TiO3

Bismuth-based ferroelectric ceramics are currently under intense investigation for their potential as Pb-free alternatives to lead zirconate titanate-based piezoelectrics. Na0.5Bi0.5TiO3 (NBT), one of the widely studied compositions, has been assumed thus far to exhibit the rhombohedral space group R3c at room temperature. High-resolution powder x-ray diffraction patterns, however, reveal peak splitting in the room temperature phase that evidence the true structure as monoclinic with space group Cc. This peak splitting and Cc space group is only revealed in sintered powders; calcined powders are equally fit to an R3c model because microstructural contributions to peak broadening obscure the peak splitting.

Revised structural phase diagram of (Ba0.7Ca0.3TiO3)-(BaZr0.2Ti0.8O3)

The temperature-composition phase diagram of barium calcium titanate zirconate (x(Ba0.7Ca0.3TiO3)-(1 − x)(BaZr0.2Ti0.8O3); BCTZ) has been reinvestigated using high-resolution synchrotron x-ray powder diffraction. Contrary to previous reports of an unusual rhombohedral-tetragonal phase transition in this system, we have observed an intermediate orthorhombic phase, isostructural to that present in the parent phase, BaTiO3, and we identify the previously assigned T-R transition as a T-O transition. We also observe the O-R transition coalescing with the previously observed triple point, forming a phase convergence region. The implication of the orthorhombic phase in reconciling the exceptional piezoelectric properties with the surrounding phase diagram is discussed.The temperature-composition phase diagram of barium calcium titanate zirconate (x(Ba{sub 0.7}Ca{sub 0.3}TiO{sub 3})-(1 - x)(BaZr{sub 0.2}Ti{sub 0.8}O{sub 3}); BCTZ) has been reinvestigated using high-resolution synchrotron x-ray powder diffraction. Contrary to previous reports of an unusual rhombohedral-tetragonal phase transition in this system, we have observed an intermediate orthorhombic phase, isostructural to that present in the parent phase, BaTiO{sub 3}, and we identify the previously assigned T-R transition as a T-O transition. We also observe the O-R transition coalescing with the previously observed triple point, forming a phase convergence region. The implication of the orthorhombic phase in reconciling the exceptional piezoelectric properties with the surrounding phase diagram is discussed.

The tetragonal phase of Na0.5Bi0.5TiO3 – a new variant of the perovskite structure

The structure of the tetragonal phase of the A-site-substituted perovskite sodium bismuth titanate, Na(0.5)Bi(0.5)TiO3, has been determined by neutron powder diffraction at 698 K. The structure was refined in space group P4bm with a (= b) = 5.5191 (1), c = 3.9085 (1) A, V= 119.055 (5) A3, Z = 2 and Dx = 5.91 Mg m(-3). The structure exhibits an unusual combination of in-phase (a0a0c+) tilts and antiparallel cation displacements along the polar c axis, which results in a new variant of the perovskite structure.

Evidence for a non-rhombohedral average structure in the lead-free piezoelectric material Na0.5Bi0.5TiO3

The potential lead-free piezoelectric material sodium bismuth titanate, Na0.5Bi0.5TiO3, was investigated by means of high-resolution single-crystal X-ray diffractometry. The splitting of Bragg peaks observed in the high-resolution reciprocal-space maps suggests that the average structure of Na0.5Bi0.5TiO3 has lower than rhombohedral symmetry. This observation is contrary to the commonly adopted model, which has followed from many previous analyses of neutron and X-ray powder diffraction data.

A photoferroelectric material is more than the sum of its parts

To the Editor — The defining property of ferroelectrics is a reversible spontaneous electric polarization whose magnitude and direction can be sensitively tuned by varying temperature, pressure, electric field, strain or chemical composition1. What makes ferroelectrics interesting is the coupling of the electric polarization to other properties of the material. For instance, ferroelectric– ferroelastic materials present a coupling between electric polarization and mechanical deformation, which can lead to a remarkable piezoelectric response with numerous applications for actuators and sensors. Another widely pursued materials class is that of ferroelectric–ferroelastic–magnetic materials, termed multiferroics, which present interesting coupling phenomena with high potential for electronic and spintronic applications. In this Correspondence we focus on another fascinating property of ferroelectric materials that is due to their interaction with light. Materials that show both photosensitive and ferroelectric properties define a field that was termed photoferroelectrics a long time ago2, but which has been largely overlooked and is now deserving of renewed attention. The present revival of photoferroelectrics focuses on ferroelectric photovoltaic materials. Although photovoltaic effects in ferroelectrics have been known for 50 years2, they have remained an academic curiosity, mainly because of their reported low powerconversion efficiency. This view has recently changed following reports that the low conversion efficiencies can be overcome by large, above-bandgap photovoltages3, the possibility of tip-enhanced photovoltaic effects at the nanoscale4 or the fundamental role of domain walls, which present a much larger efficiency than the bulk3,5. All this indicates that ferroelectric photovoltaic materials potentially have a bright future in solar-energy generation. But how are we to separate fact from fiction, and hype from hope in discussing their potential? One of the selling points for ferroelectric photovoltaics is the extremely large, abovebandgap open-circuit voltage, which points to a fundamentally different, polarizationrelated charge-separation mechanism compared with classical semiconductor solar cells. In addition, the presence of ferroelectric domain boundaries further increases the photovoltage significantly because of the electrical fields existing within the narrow domain walls3,5. Therefore, advances in this field could arise from investigating materials with engineered domain boundaries6, materials with an intrinsically complex landscape of the local electric polarization such as relaxor ferroelectrics, or complex oxides with an engineered bandgap. Yet, how do photovoltaic ferroelectrics compare with other solar-cell technologies? To achieve a high power output a solar cell needs to show high photovoltage, high photocurrent, and of course high quantum efficiency. Unfortunately, in ferroelectric photovoltaics, quantum efficiencies remain at best on the order of 1% and bulk conductivities are also low. Similarly, the photovoltage arising from an individual domain wall, which is essentially an interface limited in width, is modest; high voltages will only originate cumulatively from a large series of domains. Significant efforts will be needed before ferroelectrics could reach similar performances to those of semiconductor solar cells. This may seem a daunting task, but we should not forget examples such as that of organic solar cells, which have increased their conversion efficiency from 1% to 10% in the past ten years. But, in our view, such a focus on only photovoltaics is too restrictive, and we believe that more attention should be paid to other photoinduced effects in ferroelectrics. It is important to realize that photoinduced effects can, and usually will, be coupled to and with other functional properties. A good example of this is photostriction — the deformation induced by irradiation of light — which can be described in ferroelectrics as the combination of photovoltaic and piezoelectric effects. The photovoltaic effect creates an internal electric field, which in turn leads to significant deformation by the inverse piezoelectric effect. Light-induced size changes as recently reported7 for BiFeO3 single crystals can thus be understood from their ferroelectric properties and photovoltaic effects. Highly strained BiFeO3 thin films8 with enhanced piezoelectricity are likely to show even stronger photostrictive effects. BiFeO3 is also antiferromagnetic and it has been shown that its magnetic properties can be modified by both electric field and strain deformation, which presents the opportunity for also tuning the magnetic properties by photovoltage and photostriction. This example illustrates the general principle and interest of having interactions between photoelectric effects and other (multi-)ferroic or correlated-electron effects such as charge order, metal–insulator phase transitions, electronic and magnetic phase separations and so on. The breadth of both possible photo-induced effects and correlated-electron physics in ferroelectrics is enormous, leaving us with a wide field of possible investigations into interesting physics and possible new applications, with the potential for remote (optical) control. Finally, most of the recent work in the field focuses on the multiferroic perovskite BiFeO3, which is probably only the tip of the iceberg in terms of other interesting and useful materials. Investigations of photo-induced effects in multiferroics where magnetism causes ferroelectricity offer new degrees of freedom and coupling mechanisms, also on the ultrafast timescale. Generally speaking, the search for new interesting systems, be it in bulk form, thin films, clever nanostructures or domain-engineered materials, is crucial for a deeper understanding of photo-induced effects in ferroelectrics or more generally polar materials. Perhaps, beyond any hype on photovoltaic materials, it is rather on the broader family of photoferroelectrics that we should place most of our hope. ❐

A comprehensive study of the phase diagram of KxNa1−xNbO3

The phase diagram of lead-free piezoelectric KxNa1−xNbO3 has been studied, with particular focus on the proposed morphotropic phase boundaries, by powder and single crystal x-ray diffraction. The tilt system and cation displacement has been mapped out as a function of temperature and composition, highlighting changes in the oxygen octahedra at x=0.2 and x=0.4 at room temperature. The orthorhombic to monoclinic boundary at x=0.5 has been investigated, with a subtle change in the structure observed. The conclusion is that KxNa1−xNbO3 does not display a morphotropic phase boundary comparable with that in lead zirconate titanate, and that the most significant structural change as a function of composition occurs at x=0.2 because of the change of the tilt system.

The missing boundary in the phase diagram of PbZr1−xTixO3

PbZr(1-x)Ti(x)O3 (PZT) is one of the most important and widely used piezoelectric materials. The study of its local and average structures is of fundamental importance in understanding the origin of its high-performance piezoelectricity. Pair distribution function analysis and Rietveld refinement have been carried out to study both the short- and long-range order in the Zr-rich rhombohedral region of the PZT phase diagram. The nature of the monoclinic phase across the Zr-rich and morphotropic phase boundary area of PZT is clarified. Evidence is found that long-range average rhombohedral and both long- and short-range monoclinic regions coexist at all compositions. In addition, a boundary between a monoclinic (M(A)) structure and another monoclinic (M(B)) structure has been found. The general advantage of a particular monoclinic distortion (M(A)) for high piezoactivity is discussed from a spatial structural model of susceptibility to stress and electric field, which is applicable across the wide field of perovskite materials science.

A structural study of the (Na 1− x K x ) 0.5 Bi 0.5 TiO 3 perovskite series as a function of substitution ( x ) and temperature

Rietveld neutron powder profile analysis of the (Na 1− x K x ) 0.5 Bi 0.5 TiO 3 (NKBT) series ( x =0, 0.2, 0.4, 0.5, 0.6, 0.8, 1.0) is reported over the temperature range 293–993 K. A detailed characterization of the structures and phase transitions occurring across this series as a function of temperature has been made. Room-temperature refinements have revealed a rhombohedral phase, space group R 3 c for x =0, 0.2, and 0.4, which exhibits an antiphase, a − a − a − oxygen tilt system with parallel cation displacements along [111] p . An intermediate zero-tilt rhombohedral phase, space group R 3 m possessing cation displacements along [111] p , has been established for x =0.5 and 0.6. At the potassium-rich end of the series at x =0.8 and 1.0, a tetragonal phase, space group P 4 mm is observed possessing cation displacements along [001]. At the sodium-rich end of the series for x =0.2, the unusual tetragonal structure with space group P 4 bm is seen for Na 0.5 Bi 0.5 TiO 3 which possesses a combination of in-phase a 0 a 0 c + tilts and antiparallel cation displacements along the polar axis. Temperature-induced phase transitions are reported and structural modifications are discussed.