Yangchun Rong

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.

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.

Effects of oxygen vacancy on the electronic structure and multiferroics in sol–gel derived Pb0.8Co0.2TiO3 thin films

The single phase Pb(0.8)Co(0.2)TiO3 thin films were synthesized on a Pt/Ti/SiO2/Si substrate by the sol-gel route. The present films exhibited homogeneous microstructure with low porosity. O 1s X-ray photoelectron spectroscopy (XPS) was used to detect the amount of oxygen vacancies. The ferroelectric measurements showed that the ferroelectricity deteriorates with the increase in the number of oxygen vacancies. X-ray absorption spectroscopy (XAS) and XPS were used to study the electronic structure. The results indicated that the decreased ferroelectricity might be ascribed to the weakened hybridization between O 2p and Pb 6s and Ti 3d orbitals. The ferromagnetic behaviors were also observed in the thin films and saturated magnetization raised monotonously with the oxygen vacancy rising due to the enhanced F-center exchange interaction. Magnetoelectric coupling of the films weakened with oxygen vacancy increase.

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.

Structure, phase transition and negative thermal expansion in ammoniated ZrW2O8

In the present work, we have developed an effective way to control the thermal expansion of ZrW2O8 by the insertion of NH3 into the void of the framework structure. Neutron powder diffraction (NPD) and XPS studies reveal that N is bonding to W in the ammoniated ZrW2O8 and retains the original structure with the space group P213. This kind of ammoniation improves the thermal stability, raised the phase transition temperature about 50 K, and weakens the negative thermal expansion (NTE) from −7.8 × 10−6 K−1 to −2.1 × 10−6 K−1. These behaviors account for bonding of NH3 to the neighboring atoms of ZrW2O8, and hinder the rocking motion of the corner shared polyhedral structure.

Controllable negative thermal expansion, ferroelectric and semiconducting properties in PbTiO3–Bi(Co2/3Nb1/3)O3 solid solutions

Semiconductor functional materials have been widely applied in electronic devices. However, the reliability and lifetime of the devices suffer from the undesirable mismatch of the coefficients of thermal expansion. It is important to develop semiconductor materials with controllable thermal expansion. Here, we develop a remarkable group of ferroelectric semiconductor materials (1 − x)PbTiO3–xBi(Co2/3Nb1/3)O3 (PT–100xBCN) that have not only tunable electronic and optical properties, but also controllable negative, zero, and positive thermal expansion. Through the BCN chemical substitution, the bandgap (Eg) is reduced from 2.60 eV to 2.26 eV to achieve semiconducting properties in PbTiO3-based ferroelectrics. Meanwhile, we have achieved a gradual transition from negative, to zero, and then to positive thermal expansion (αV: −12.3 × 10−6 to 7.4 × 10−6 K−1). Besides, PT–BCN exhibits ferroelectric and piezoelectric properties. The relatively high piezoelectric coefficient d33 ∼ 200 pC N−1 was achieved in the composition of PT–42BCN near the morphotropic phase boundary. PT–BCN shows good negative temperature coefficient (NTC) thermistor characteristics. The coexistence of light absorption, thermal expansion, and electronic properties in PT–BCN makes it a promising material for future applications. The present study would offer an approach to developing and exploring new multifunctional semiconducting materials with controllable thermal expansion.

Temperature-independent ferroelectric property and characterization of high-TC 0.2Bi(Mg1/2Ti1/2)O3-0.8PbTiO3 thin films

Ferroelectric property stability against elevated temperature is significant for ferroelectric film applications, such as non-volatile ferroelectric random access memories. The high-T C 0.2Bi(Mg1/2Ti1/2)O3-0.8PbTiO3 thin films show the temperature-independent ferroelectric properties, which were fabricated on Pt(111)/Ti/SiO2/Si substrates via sol-gel method. The present thin films were well crystallized in a phase-pure perovskite structure with a high (100) orientation and uniform texture. A remanent polarization (2P r) of 77 μC cm−2 and a local effective piezoelectric coefficient d 33 * of 60 pm/V were observed in the 0.2Bi(Mg1/2Ti1/2)O3-0.8PbTiO3 thin films. It is interesting to observe a behavior of temperature-independent ferroelectric property in the temperature range of room temperature to 125 °C. The remanent polarization, coercive field, and polarization at the maximum field are almost constant in the investigated temperature range. Furthermore, the dielectric loss and fatigue properties of 0.2Bi(Mg1/2Ti1/2)O3-0.8PbTiO3 thin films have been effectively improved by the Mn-doping.

Structure and electrical properties of tetragonal tungsten bronze Ba2CeFeNb4O15

The crystal structure and electrical microstructure of a tetragonal tungsten bronze (TTB) ceramic, BaCeFeNb4O15 (BCFN), were investigated by high-resolution synchrotron X-ray powder diffraction (SPD), selected area electron diffraction (SAED), and AC impedance spectroscopy. SPD and SAED reveal that the BCFN has a tetragonal structure with space group P4/mbm, and includes an incommensurate modulated behavior. Impedance and AC conductivity tests in the range of 200–360 °C suggest thermally activated electrical behavior which originates from both the bulk and the grain boundary elements of the ceramics. The dielectric relaxation in the grain boundaries is due to the trap-controlled ac conduction around doubly ionized oxygen vacancies while the relaxation of the bulk may be associated with the localized electron hopping between the transition-metal ions. These results could be helpful in understanding the electrical conduction and relaxation processes in Fe-containing TTB-type oxides.

Cation deficiency effect on negative thermal expansion of ferroelectric PbTiO3

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.

Enhanced high-temperature piezoelectric properties of traditional Pb(Zr,Ti)O3 ceramics by a small amount substitution of KNbO3

Crystal structure, piezoelectric, and dielectric properties were investigated on the (1-x)Pb(Zr0.54Ti0.46)O3-xKNbO3 system. The piezoelectric properties have been significantly improved by substituting a small amount of KNbO3. In the morphotropic phase boundary (x = 0.015), the compound not only shows enhanced piezoelectric coefficient d33 = 450 pC/N, which is two times larger than that of unmodified Pb(Zr,Ti)O3 (d33 = 223 pC/N), but also the Curie temperature (TC = ~380 °C) is still well maintained at a high level. This phenomenon challenges our general knowledge that in piezoelectric materials the Curie temperature and piezoelectric properties are mutually contradictory. It should be noted that a giant total strain as high as 0.73% is also observed. The high thermal depoling temperature more than 300 °C combined with the excellent piezoelectric properties suggest it as a potential candidate for high temperature actuators and sensors applications.