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ABO₃-type perovskite relaxor ferroelectrics (RFEs) have emerged as the preferred option for dielectric capacitive energy storage. However, the compositional design of RFEs with high energy density and efficiency poses significant challenges owing to the vast compositional space and the absence of general rules. Here, we present an atomic-level chemical framework that captures inherent characteristics in terms of radius and ferroelectric activity of ions. By categorizing A/B-site ions as host framework, rattling, ferroelectrically active, and blocking ions and assembling these four types of ions with specific criteria, linear-like relaxors with weak locally correlated and highly extendable unit-cell polarization vectors can bemore » constructed. As example, we demonstrate two new compositions of Bi_0.5K_0.5TiO_3-based and BaTiO_3-based relaxors, showing extremely high recoverable energy densities of 17.3 and 12.1 J cm^–3, respectively, both with a high efficiency of about 90%. Further, the role of different types of ions in forming heterogeneous polar structures is identified through element-specific local structure analysis using neutron total scattering combined with reverse Monte Carlo modeling. Further, our work not only opens up new avenues toward rational compositional design of high energy storage performance lead-free RFEs but also sheds light on atomic-level manipulation of functional properties in compositionally complex ferroelectrics.« less

Negative thermal expansion (NTE) alloys possess great practical merit as thermal offsets for positive thermal expansion due to its metallic properties. However, achieving a large NTE with a wide temperature range remains a great challenge. Herein, a metallic framework-like material FeZr₂ is found to exhibit a giant uniaxial (1D) NTE with a wide temperature range (93-1078 K, $$\overline{α}_{l}$$=–34.01×10^–6K^–1 ). Such uniaxial NTE is the strongest in all metal-based NTE materials. The direct experimental evidence and DFT calculations reveal that the origin of giant NTE is the couple with phonons, flexible framework-like structure, and soft bonds. Interestingly, the present metallic FeZr₂more » excites giant 1D NTE mainly driven by high-frequency optical branches. It is unlike the NTE in traditional framework materials, which are generally dominated by low energy acoustic branches. In the present study, a giant uniaxial NTE alloy is reported, and the complex mechanism has been revealed. It is of great significance for understanding the nature of thermal expansion and guiding the regulation of thermal expansion.« less

Understanding the negative thermal expansion (NTE) mechanism is of great importance. In this work, we consider the new NTE compound GdFe(CN)₆ (αv = -34.2×10^-6 K^-1) as a case study to investigate the NTE mechanism from the perspective of the lattice vibrational dynamics. The atomic mean-square displacements suggest that the NTE of GdFe(CN)₆ comes from the strong tension effect induced by the transverse vibrations of the atomic –Fe–Ctriple bondN–Gd– linkages, with the largest contribution given by N atoms. Lattice dynamics calculations show that three low-frequency optical modes at about 50 cm^-1 show the largest negative Grüneisen parameters thus providing the largestmore » contribution to the NTE. In conclusion, the existence of these unusual low-frequency vibrational modes can be ascribed to the presence of GdN₆ trigonal prisms in the framework structure of GdFe(CN)₆.« less

The presence of high crystallographic symmetry and nanoscale defects are favorable for thermoelectrics. With proper electronic structures, a highly symmetric crystal tends to possess multiple carrier channels and promote electrical conductivity without sacrificing Seebeck coefficient. In addition, nanoscale defects can effectively scatter acoustic phonons to suppress thermal conductivity. Here, we report that the triple doping of Cu₂SnSe₃ leads to a high ZT value of 1.6 at 823 K for Cu_1.85Ag_0.15(Sn_0.88Ga_0.1Na_0.02)Se₃, and a decent average ZT (ZT_ave) value of 0.7 is also achieved for Cu_1.85Ag_0.15(Sn_0.93Mg_0.06Na_0.01)Se₃ from 475 to 823 K. Our study reveals: (1) Ag doping on Cu sites generates numerousmore » point defects and greatly decreases lattice thermal conductivity. (2) Doping Mg or Ga converts the monoclinic Cu₂SnSe₃ into a cubic structure. This symmetry enhancing leads to increase in the effective mass from 0.8 m_e to 2.6 m_e (m_e, free electron mass) and the power factor from 4.3 μW/cm^–1K^–2 for Cu₂SnSe₃ to 11.6 μW/cm^–1K^–2. (3) Na doping creates dense dislocation arrays and nanoprecipitates, which strengthens the phonon scattering. (4) Pair distribution function analysis shows localized symmetry breakdwon in the cubic Cu_1.85Ag_0.15(Sn_0.88Ga_0.1Na_0.02)Se₃. Furthermore, the present work provides a standpoint to design promising thermoelectric materials by synergistically manipulating crystal symmetry and nanoscale defects.« less

Negative or zero thermal expansion (NTE or ZTE) of materials is intriguing for controllable thermal expansion. In this work, we report a series of orthorhombic α-Cu_2-xZn_xV₂O₇ (x = 0, 0.1, 0.2), in which the volumetric coefficients of thermal expansion are successfully tuned from -10.19 × 10^-6 K^-1 to -1.58 × 10^-6 K^-1 in the temperature range of 100–475 K by increasing the content of Zn²⁺. It has been revealed that the transverse vibrations of oxygen bonded with vanadium are dominant in the contraction of the bc plane, leading to the overall volume NTE in α-Cu₂V₂O₇. The introduction of Zn²⁺ densifiesmore » the crystal structure, which is presumed to suppress the space of transverse vibrations and results in the ZTE in α-Cu_1.8Zn_0.2V₂O₇. This work presents an effective method to realize ZTE in anisotropic framework systems.« less

Negative thermal expansion (NTE) behaviors have been observed in various types of compounds. The achievement in the merits of promising low-cost and facile NTE oxides remains challenging. In the present work, a simple and low-cost Cu₂P₂O₇ has been found to exhibit the strongest NTE among the oxides (alpha(v) similar to -27.69 X 10^-6 K^-1, 5-375 K). The complex NTE mechanism has been investigated by the combined methods of high-resolution synchrotron X-ray diffraction, neutron powder diffraction, X-ray pair distribution function, extended X-ray absorption fine structure spectroscopy, and density functional theory calculations. Interesting, the direct experimental evidence reveals that the coupling twistmore » and rotation of PO₄ and CuO₅ polyhedra are the inherent factors for the NTE nature of Cu₂P₂O₇, which is triggered by the transverse vibrations of oxygen atoms. The present new NTE material of Cu₂P₂O₇ also has been verified for the practical application.« less

Negative thermal expansion (NTE) behavior is an interesting physical phenomenon, but the number of NTE materials is limited. In this study, a new NTE compound has been found, FeFe(CN)₆ Prussian blue analogue, where the average linear coefficient of thermal expansion (α_l) is –4.260 × 10^–6 K^–1 between 100 and 450 K. The NTE properties and local vibration dynamics have been investigated by joint experiments of synchrotron X-ray diffraction, X-ray pair distribution function, and extended X-ray absorption fine structure spectroscopy. It has been observed that the Fe–C/Fe–N bonds expand with increasing temperature, while the unit cell shrinks in FeFe(CN)₆. The vibrationmore » directions of both Fe–C and Fe–N prefer to be perpendicular to the linkage of Fe–C[≡]N–Fe rather than being parallel. More pieces of evidence indicate that the transverse vibrations of N atoms dominate the NTE behavior of FeFe(CN)₆. In conclusion, the present results prove directly that the transverse thermal vibrations of C and N atoms are crucial for the occurrence of the NTE of Prussian blue analogues.« less

The control of thermal expansion of open framework Prussian blue analogues is vital but remains challenging. The present work proposes an effective method to control the thermal expansion, in which guest ions (Na⁺) and molecules (H₂O) can adjust the coeffecient of thermal expansion from strong negative, to near zero, to positive in GaFe(CN)₆ Prussian blue analogues. Direct experimental evidence by high resolution synchrotron X-ray diffraction and X-ray absorption fine structure shows that the guest ions or molecules have intense dampening effect on the transverse vibrations of -Ga–N≡C–Fe– linkage responsible for NTE, especially for N atoms. The role of guests inmore » controlling thermal expansion is attributed to the strong interaction of local environment – steric dampening. Furthermore, the present study demonstrates that electrochemical or redox intercalation of guest ions can be a general and effective method for controlling thermal expansion for those open framework materials with negative thermal expansion driven by low Frequency phonons.« less