Negative thermal expansion (NTE) is an intriguing physical property of solids, which is a consequence of a complex interplay among the lattice, phonons, and electrons. Interestingly, a large number of NTE materials have been found in various types of functional materials. In the last two decades good progress has been achieved to discover new phenomena and mechanisms of NTE. In the present review article, NTE is reviewed in functional materials of ferroelectrics, magnetics, multiferroics, superconductors, temperature-induced electron configuration change and so on. Zero thermal expansion (ZTE) of functional materials is emphasized due to the importance for practical applications. The NTE functional materials present a general physical picture to reveal a strong coupling role between physical properties and NTE. There is a general nature of NTE for both ferroelectrics and magnetics, in which NTE is determined by either ferroelectric order or magnetic one. In NTE functional materials, a multi-way to control thermal expansion can be established through the coupling roles of ferroelectricity-NTE, magnetism-NTE, change of electron configuration-NTE, open-framework-NTE, and so on. Chemical modification has been proved to be an effective method to control thermal expansion. Finally, challenges and questions are discussed for the development of NTE materials. There remains a challenge to discover a "perfect" NTE material for each specific application for chemists. The future studies on NTE functional materials will definitely promote the development of NTE materials.

Figure S1. XRD patterns of the CeO 2 hydrothermal treated at 170 o C for (a) 12h, (b) 24h, (c) 48h, (d) 144h.

Next-generation advanced high/pulsed power capacitors rely heavily on dielectric ceramics with high energy storage performance. However, thus far, the huge challenge of realizing ultrahigh recoverable energy storage density (Wrec) accompanied by ultrahigh efficiency (η) still existed and has become a key bottleneck restricting the development of dielectric materials in cutting-edge energy storage applications. Here, we propose a high-entropy strategy to design “local polymorphic distortion” including rhombohedral-orthorhombic-tetragonal-cubic multiphase nanoclusters and random oxygen octahedral tilt, resulting in ultrasmall polar nanoregions, an enhanced breakdown electric field, and delayed polarization saturation. A giant Wrec ~10.06 J cm−3 is realized in lead-free relaxor ferroelectrics, especially with an ultrahigh η ~90.8%, showing breakthrough progress in the comprehensive energy storage performance for lead-free bulk ceramics. This work opens up an effective avenue to design dielectric materials with ultrahigh comprehensive energy storage performance to meet the demanding requirements of advanced energy storage applications.

Most materials expand on heating, known as positive thermal expansion. There are some instances most of which have been discovered in the past decade to exhibit a negative thermal expansion (NTE). [1][2][3][4][5][6][7][8][9] The nature of NTE behavior originates from the effect of atomic vibrations, (e.g., the low-energy transverse mode (ice), 2 the coupled rotation of rigid polyhedra (ZrW 2 O 8 , Fe[Co-(CN) 6 ]), 1,4 and active vibration modes of carbon fullerenes and nanotubes), 7 from the effect of magnetic transition (Invar alloy), 3 or from the changes in electron configuration (Sm 2.72 C 60 , YbCuAl). 8 The occurrence of NTE materials immediately found their important technical applications in many fields, because the overall thermal expansion coefficient (TEC) could be tailored by introduction of NTE materials. 1,2 In particular, zero thermal expansion (ZTE) is very interesting, where the volume neither expands nor contracts with the temperature fluctuation. 3-6 The ZTE could be achieved to form composite by combining the materials with positive thermal expansion with NTE materials. However, the fabrication of ZTE composite is hampered by the poor thermal stability of NTE compounds. For example, ZrW 2 O 8 will be decomposed at a relatively low temperature (777°C). 1 The requirement of ZTE will be satisfied if the ZTE is available in a single phase. Up to now, rare materials exhibit the novel ZTE, such as Invar alloys and Fe-[Co(CN) 6 ]. 3,4 Moreover, the ZTE generally appears in a low temperature (below room temperature). The ZTE over a wider temperature range would be very useful for the applications.PbTiO 3 (PT) as an important perovskite-type multifunctional material exhibits a unique NTE in the perovskite family. 9,10 The unit cell volume of PT contracts over a wide temperature range in the ferroelectric phase (25-490°C) with an average intrinsic volumetric TEC (-1.99 × 10 -5°C-1 ). 9b The NTE of PT-based compounds can be controlled over a large range from -0.11 × 10 -5 to -3.92 × 10 -5°C-1 , which covers the range found in almost all other known NTE oxides. 9 However, a low or ZTE could only be achieved by sacrificing the temperature range, that is, reducing the Curie point (T C ), such as for Pb 0.80 La 0.20 TiO 3 (-0.11 × 10 -5°C -1 , 25-130°C). 9b It is a challenge to expand ZTE to the hightemperature range. On the basis of our previously studied PbTiO 3 -based compounds, we could only access a low expansion or ZTE by reducing the tetragonality (c/a), resulting in the decrease in the ZTE temperature range (region II in Figure 1). To obtain the ZTE in a wider temperature range, a kind of PbTiO 3 -based compound should be found in the region I where c/a is large and the absolute value of TEC is low (Figure 1). Recently, in the PbTiO 3 -BiMeO 3 (Me is cations with an average valence +3), the Bi substitution plays an unusual role in which both T C and c/a are considerably enhanced, owing to the strong coupling between the Pb/Bi cations and the B-site cations with strong ferroelectricity activity, suc...

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.