Colossal and reversible barocaloric effect in liquid-solid-transition materials n-alkanes.

Emerging caloric cooling technology provides a green alternative to conventional vapor-compression technology which brings about serious environmental problems. However, the reported caloric materials are much inferior to their traditional counterparts in cooling capability. Here we report the barocaloric (BC) effect associated with the liquid-solid-transition (L-S-T) in n-alkanes. A low-pressure of ~50 MPa reversibly triggers an entropy change of ~700 J kg-1 K-1, comparable to those of the commercial refrigerants in vapor-based compression systems. The Raman study and theoretical calculations reveal that applying pressure to the liquid state suppresses the twisting and random thermal motions of molecular chains, resulting in a lower configurational entropy. When the pressure is strong enough to drive the L-S-T, the configurational entropy will be fully suppressed and induce the colossal BC effect. This work could open a new avenue for exploring the colossal BC effect by evoking L-S-T materials.

Low Thermal Expansion Modulated by Off-Stoichiometric Effect in Nonstoichiometric Laves Phase Hf0.87Ta0.13Fe2+x Compounds.

Materials with a low coefficient of thermal expansion (CTE) are extremely demanded in many fields, varying from microelectronics to space technology. Here we report a novel method to achieve low CTE, which differs essentially from the conventional way that uses additives with negative thermal expansion (NTE) to compensate for the positive CTE of the matrix. The stoichiometric Hf0.87Ta0.13Fe2+x (x = 0) shows a giant NTE, which is gradually suppressed with increasing x and finally changed to near-zero thermal expansion (ZTE) at x ≈ 0.4. The excess Fe was suggested to form anti-site defects by occupying the 4f sites. As revealed by electron spin resonance (ESR) spectra, the weakened NTE is closely related to a slower ferromagnetic (FM) ordering process than observed at x = 0. In addition, the CTE can be further tuned by introducing an extra α-Fe phase to achieve a low CTE (e.g., 3.3 ppm/K for x = 1.0) with markedly enhanced mechanical properties, beneficial to applications.

Isotropic Low Thermal Expansion over a Wide Temperature Range in Ti1- xZr xF3+ x (0.1 ≤ x ≤ 0.5) Solid Solutions.

TiF3 exhibits a rhombohedral to ReO3-type cubic phase transformation at ∼340 K. Here we report that, by introducing ZrF4 into TiF3, the cubic phase is stabilized at least down to 123 K in the Ti1- xZr xF3+ x compounds. All compounds exhibit low thermal expansion (LTE) between 123 and 623 K, and a nearly zero thermal expansion (ZTE) was obtained in Ti0.7Zr0.3F3.3 (αL = 0.91 ppm/K). The analysis of pair distribution function reveals that the cation-centered octahedra are partially changed to pentagonal bipyramids in Ti1- xZr xF3+ x due to the excess fluorines relative to the case of TiF3. Therefore, the cooperative rotation of the polyhedra tends to be restricted, and the cubic phase is thus stabilized. The restrained polyhedral rotations compete against the lattice softening caused by the introduction of Zr4+, giving rise to the LTE. Our present strategy is applicable to other rhombohedral metal trifluorides for the design of new isotropic ZTE materials.

Effects of Cr Substitution on Negative Thermal Expansion and Magnetic Properties of Antiperovskite Ga1-x Cr x N0.83Mn3 Compounds.

Negative thermal expansion (NTE) and magnetic properties were investigated for antiperovskite Ga1-x Cr x N0.83Mn3 compounds. As x increases, the temperature span (ΔT) of NTE related with Γ5g antiferromagnetic (AFM) order is expanded and shifted to lower temperatures. At x = 0.1, NTE happens between 256 and 318 K (ΔT = 62 K) with an average linear coefficient of thermal expansion, α L = -46 ppm/K. The ΔT is expanded to 81 K (151-232 K) in x = 0.2 with α L = -22.6 ppm/K. Finally, NTE is no longer visible for x ≥ 0.3. Ferromagnetic order is introduced by Cr doping and continuously strengthened with increasing x, which may impede the AFM ordering and thus account for the broadening of NTE temperature window. Moreover, our specific heat measurement suggests the electronic density of states at the Fermi level is enhanced upon Cr doping, which favors the FM order rather than the AFM one.

Observation of the Spin-Glass Behavior in Co-Based Antiperovskite Nitride GeNCo3.

Single-phase antiperovskite nitride GeNCo3 with space group Pm3̅m is successfully synthesized by a solid-gas reaction. The crystal structure, magnetism, specific heat at low temperatures, Hall effect, and electrical and thermal transport properties are widely investigated. Exhilaratingly, a canonical spin-glass (SG) behavior is observed in GeNCo3 with a freezing temperature T0 = 79.43 K, dynamical exponent zν = 6.156, and flipping time τ0 = 5.0 × 10(-12) s. The origin of the SG state in GeNCo3 is likely due to the atomic disorder introduced by the Ge vacancies. This is further proven by the measurements of Ge0.9NCo3 with more Ge deficiencies.

Unusual ferromagnetic critical behavior owing to short-range antiferromagnetic correlations in antiperovskite Cu(1-x)NMn(3+x) (0.1 ≤ x ≤ 0.4).

For ferromagnets, varying from simple metals to strongly correlated oxides,the critical behaviors near the Curie temperature (T(C)) can be grouped into several universal classes. In this paper, we report an unusual critical behavior in manganese nitrides Cu(1-x)NMn(3+x) (0.1 ≤ x ≤ 0.4). Although the critical behavior below T(C) can be well described by mean field (MF) theory, robust critical fluctuations beyond the expectations of any universal classes are observed above T(C) in x = 0.1. The critical fluctuations become weaker when x increases, and the MF-like critical behavior is finally restored at x = 0.4. In addition, the paramagnetic susceptibility of all the samples deviates from the Curie-Weiss (CW) law just above T(C). This deviation is gradually smeared as x increases. The short-range antiferromagnetic ordering above T(C) revealed by our electron spin resonance measurement explains both the unusual critical behavior and the breakdown of the CW law.

Good thermoelectric performance in strongly correlated system SnCCo3 with antiperovskite structure.

We report the magnetic, electrical, and thermoelectric properties of SnCCo3, where good thermoelectric performance [figure of merit ZT ∼ 0.035(2), 258 K] and strong electron correlation (Kadowaki-Woods ratio RKW ∼ 4a0) are observed. The thermoelectric properties of ACCo3 (A = Al, Ga, Ge) and SnCM3 (M = Mn, Fe) were also investigated for comparison. As a result, the ZT value of SnCCo3 is the largest among all of those samples, which is mainly attributed to the large Seebeck coefficient caused by the strong electron correlation and low carrier density. Moreover, the ZT value can be effectively enhanced by proper chemical doping in SnCCo3.