Longlong Fan

Zero Thermal Expansion and Ferromagnetism in Cubic Sc1–xMxF3 (M = Ga, Fe) over a Wide Temperature Range

The rare physical property of zero thermal expansion (ZTE) is intriguing because neither expansion nor contraction occurs with temperature fluctuations. Most ZTE, however, occurs below room temperature. It is a great challenge to achieve isotropic ZTE at high temperatures. Here we report the unconventional isotropic ZTE in the cubic (Sc1-xMx)F3 (M = Ga, Fe) over a wide temperature range (linear coefficient of thermal expansion (CTE), αl = 2.34 × 10(-7) K(-1), 300-900 K). Such a broad temperature range with a considerably negligible CTE has rarely been documented. The present ZTE property has been designed using the introduction of local distortions in the macroscopic cubic lattice by heterogeneous cation substitution for the Sc site. Even though the macroscopic crystallographic structure of (Sc0.85Ga0.05Fe0.1)F3 adheres to the cubic system (Pm3̅m) according to the results of X-ray diffraction, the local structure exhibits a slight rhombohedral distortion. This is confirmed by pair distribution function analysis of synchrotron radiation X-ray total scattering. This local distortion may weaken the contribution from the transverse thermal vibration of fluorine atoms to negative thermal expansion, and thus may presumably be responsible for the ZTE. In addition, the present ZTE compounds of (Sc1-xMx)F3 can be functionalized to exhibit high-Tc ferromagnetism and a narrow-gap semiconductor feature. The present study shows the possibility of obtaining ZTE materials with multifunctionality in future work.

Enhanced piezoelectric and ferroelectric properties in the BaZrO3 substituted BiFeO3-PbTiO3

BiFeO3-PbTiO3 (BF-PT) compounds possess very high Curie temperature and tetragonality compared to other PbTiO3-based piezoceramics. The BaZrO3 (BZ), with weakly ferroelectric active cations, was introduced into the BiFeO3-PbTiO3 to reduce the tetragonality (c/a) and improve the piezoelectric property. For the (0.8-x)BiFeO3-0.2BaZrO3-xPbTiO3, the BaZrO3 substitution can effectively decrease the tetragonality (c/a) from 1.18 to 1.02 for those compositions near the morphotropic phase boundary. The piezoelectric property of BiFeO3-PbTiO3 can be much enhanced with an optimal piezoelectric constant ∼270 pC/N with a reduced TC of 270 °C. Both the temperature dependent dielectric properties and polarization loops verified the existence of antiferroelectric relaxor, which was not observed in previous reported BiFeO3-PbTiO3 based materials.

Electrostrictive and relaxor ferroelectric behavior in BiAlO3-modified BaTiO3 lead-free ceramics

The crystal structure of (1−x)BaTiO3-xBiAlO3 (x = 0, 0.02, 0.05, 0.08, and 0.1) ceramics was determined using X-ray diffraction and Raman spectroscopy at room temperature, which revealed a phase transition from tetragonal to rhombohedral with increasing x. The dielectric properties were studied as a function of temperature at different frequencies, which indicated that the phase transition temperature (Tm) decreased with increasing x. The relaxor behavior was observed by frequency and temperature dependent dielectric permittivity. The Lorenz-type quadratic law was used to characterize the dielectric permittivity peaks near Tm of high-temperature slopes at 1 MHz. The temperatures Tm of dielectric permittivity peaks fit very well with the Vogel-Fulcher law in x = 0.05 and x = 0.1. The polarization hysteresis loops and electrostrictive were displayed at room temperature. The sample for x = 0.1 exhibits a slim loop with negligible hysteresis and a subtle linear feature, which is a promising transducer materia...

Ordered Structure and Thermal Expansion in Tungsten Bronze Pb2K0.5Li0.5Nb5O15

The crystal structure and thermal expansion behaviors of a new tetragonal tungsten bronze (TTB) ferroelectric, Pb2K(0.5)Li(0.5)Nb5O15, were systematically investigated by selected-area electron diffraction (SAED), neutron powder diffraction, synchrotron X-ray diffraction (XRD), and high-temperature XRD. SAED and Rietveld refinement reveal that Pb2K(0.5)Li(0.5)Nb5O15 displays a commensurate superstructure of simple orthorhombic TTB structure at room temperature. The structure can be described with space group Bb2₁m. The transition to a paraelectric phase (P4/mbm) occurs at 500 °C. Compared with Pb2KNb5O15 (PKN), the substitution of 0.5K(+) with small 0.5Li(+) into PKN causes the tilting of NbO6 octahedra away from the c axis with Δθ ≈ 10° and raises the Curie temperature by 40 °C, and the negative thermal expansion coefficient along the polar b axis increases more than 50% in the temperature range 25-500 °C. We present that, by introduction of Li(+), the enhanced spontaneous polarization is responsible for the enhanced negative thermal expansion along the b axis, which may be caused by more Pb(2+) in the pentagonal caves.

High piezoelectric performance in a new Bi-based perovskite of (1−x)Bi(Ni1/2Hf1/2)O3−xPbTiO3

Preparation, piezoelectric, and dielectric properties were investigated in a new Bi-based piezoelectric material of (1−x)Bi(Ni1/2Hf1/2)O3-xPbTiO3. The system can form a pure perovskite structure with the morphotropic phase boundary locating at x = 0.62, separating the rhombohedral and tetragonal phases. It is interesting to observe that the morphotropic phase boundary composition shows a very high piezoelectric coefficient of d33 (446 pC/N), which is comparable to BiScO3-PbTiO3 (460 pC/N). The Curie temperature of the morphotropic phase boundary is around 290 °C. Furthermore, the system has a relatively low coercive field, which makes the poling easily. Temperature dependence of dielectric properties also shows that the Bi(Ni1/2Hf1/2)O3-PbTiO3 system has a strong relaxor feature. Present new Bi-based perovskite of Bi(Ni1/2Hf1/2)O3-PbTiO3 is a competitive piezoelectric material with high piezoelectric performance.

Enhanced piezoelectric and antiferroelectric properties of high-TC perovskite of Zr-substituted Bi(Mg1/2Ti1/2)O3-PbTiO3

Bi(Mg1/2Ti1/2)O3-PbTiO3 is a promising high-TC piezoelectrics in the Bi-based perovskite family of BiMeO3-PbTiO3. In this study, zirconium is utilized to further improve the high temperature piezoelectric properties of Bi(Mg1/2Ti1/2)O3-PbTiO3. Substitution of Zr for Ti is observed to decrease the tetragonality (c/a) near the morphotropic phase boundary, while TC can be well maintained by the substitution of smaller and ferroelectrically active Ti by a larger and ferroelectrically weaker Zr cation. A softer coercive field and enhanced domain mobility is observed, ultimately leading to a strong ferroelectric activity. The piezoelectric property of Zr-substituted Bi(Mg1/2Ti1/2)O3-PbTiO3 is enhanced to 260 pC/N, when compared with Bi(Mg1/2Ti1/2)O3-PbTiO3 (225 pC/N). Good high temperature piezoelectric property was found in the tetragonal phase of Zr-substituted Bi(Mg1/2Ti1/2)O3-PbTiO3. Thermal depoling of aligned domains for this composition occurs at approximately 300 °C. Thus, Zr-substituted Bi(Mg1/2Ti1/2)O3-PbTiO3 could be used for high temperature actuator applications. Furthermore, an apparent ferroelectric-antiferroelectric phase transition was observed as a function of both the composition in the rhombohedral phase and the temperature. An antiferroelectric relaxor exists in the Zr-substituted Bi(Mg1/2Ti1/2)O3-PbTiO3.

Structure, piezoelectric, and ferroelectric properties of BaZrO3 substituted Bi(Mg1/2Ti1/2)O3-PbTiO3 perovskite

The structure and electric properties of (0.9-x)Bi(Mg1/2Ti1/2)O3-xPbTiO3-0.1BaZrO3 (0.45 ≤ x ≤ 0.53) ceramics were investigated. The morphotropic phase boundary between tetragonal ferroelectric and pseudo-cubic relaxor phases is ascertained at x = 0.50. The BaZrO3 substitution can much reduce the coercive field of Bi(Mg1/2Ti1/2)O3-PbTiO3. The studies on temperature dependence of both ferroelectric and dielectric constant indicate a direct evidence for the antiferroelectric relaxor phase, which was ever suggested in the binary system of Bi(Mg1/2Ti1/2)O3-PbTiO3. The phase transition of ferroelectric to antiferroelectric relaxor produces the thermal depoling below the Curie temperature. The ceramic of BMT-0.47PT-0.1BZ exhibits a high strain 0.37% and a large-signal d33 (530 pm/V) in the antiferroelectric-relaxor phase. BaZrO3 substituted Bi(Mg1/2Ti1/2)O3-PbTiO3 shows an analogous phase diagram to that of lead-free (Bi, Na)TiO3-BaTiO3.

High‐Curie‐Temperature Ferromagnetism in (Sc,Fe)F3 Fluorides and its Dependence on Chemical Valence

A magnetic metal-fluoride system is shown for the first time to have a high Curie temperature (≈545 K). The magnetism correlates intimately with the Fe(2+)/Fe(3+) ratio. As the ratio increases, the weak magnetism displayed by unordered magnetic moments intensifies, and these magnetic moments align in parallel. Simultaneously, a magneto-volume effect is also shown to increase the lattice volume.

Extensive domain wall motion and deaging resistance in morphotropic 0.55Bi(Ni1/2Ti1/2)O3–0.45PbTiO3 polycrystalline ferroelectrics

Domain wall motion is known as a major source of extrinsic contributions to the dielectric and piezoelectric properties of ferroelectric materials. In the present work, we report the extent of non-180° domain wall motion during strong and weak electric field amplitudes in situ using time-resolved, high-energy X-ray diffraction in the ferroelectric morphotropic phase boundary composition 0.55Bi(Ni1/2Ti1/2)O3–0.45PbTiO3 (BNT-45PT). After application of strong electric fields, two phases are shown to coexist. In the tetragonal phase of this material, the extent of 90° domain wall motion is significant and the domain alignment is nearly saturated. Weak (subswitching) cyclic electric fields are then also shown to induce domain wall motion. Deaging, or the progressive loss of preferred domain orientation during sequentially increasing field amplitudes, is notably low in these materials, showing that the initial domain alignment is strongly stabilized. Overall, the in situ measurements reveal that domain wall mo...

Hydration and Thermal Expansion in Anatase Nanoparticles

A tunable thermal expansion is reported in nanosized anatase by taking advantage of surface hydration. The coefficient of thermal expansion of 4 nm TiO2 along a-axis is negative with a hydrated surface and is positive without a hydrated surface. High-energy synchrotron X-ray pair distribution function analysis combined with ab initio calculations on the specific hydrated surface are carried out to reveal the local structure distortion that is responsible for the unusual negative thermal expansion.