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Barocaloric effects─solid-state thermal changes induced by the application and removal of hydrostatic pressure─offer the potential for energy-efficient heating and cooling without relying on volatile refrigerants. Here, we report that dialkylammonium halides─organic salts featuring bilayers of alkyl chains templated through hydrogen bonds to halide anions─display large, reversible, and tunable barocaloric effects near ambient temperature. The conformational flexibility and soft nature of the weakly confined hydrocarbons give rise to order–disorder phase transitions in the solid state that are associated with substantial entropy changes (>200 J kg–1 K–1) and high sensitivity to pressure (>24 K kbar–1), the combination of which drives strong barocaloric effects at relatively low pressures. Through high-pressure calorimetry, X-ray diffraction, and Raman spectroscopy, we investigate the structural factors that influence pressure-induced phase transitions of select dialkylammonium halides and evaluate the magnitude and reversibility of their barocaloric effects. Furthermore, we characterize the cyclability of thin-film samples under aggressive conditions (heating rate of 3500 K s–1 and over 11,000 cycles) using nanocalorimetry. Taken together, these results establish dialkylammonium halides as a promising class of pressure-responsive thermal materials. 

Abstract The layered square-planar nickelates, Nd n +1 Ni n O 2 n +2 , are an appealing system to tune the electronic properties of square-planar nickelates via dimensionality; indeed, superconductivity was recently observed in Nd 6 Ni 5 O 12 thin films. Here, we investigate the role of epitaxial strain in the competing requirements for the synthesis of the n  = 3 Ruddlesden-Popper compound, Nd 4 Ni 3 O 10 , and subsequent reduction to the square-planar phase, Nd 4 Ni 3 O 8 . We synthesize our highest quality Nd 4 Ni 3 O 10 films under compressive strain on LaAlO 3 (001), while Nd 4 Ni 3 O 10 on NdGaO 3 (110) exhibits tensile strain-induced rock salt faults but retains bulk-like transport properties. A high density of extended defects forms in Nd 4 Ni 3 O 10 on SrTiO 3 (001). Films reduced on LaAlO 3 become insulating and form compressive strain-induced c -axis canting defects, while Nd 4 Ni 3 O 8 films on NdGaO 3 are metallic. This work provides a pathway to the synthesis of Nd n +1 Ni n O 2 n +2 thin films and sets limits on the ability to strain engineer these compounds via epitaxy. 

Barocaloric effectsthermal changes in a material induced by applied hydrostatic pressureoffer promise for creating solid-state refrigerants as alternatives to conventional volatile refrigerants. To enable efficient and scalable barocaloric cooling, materials that undergo high-entropy, reversible phase transitions in the solid state in response to a small change in pressure are needed. Here, we report that pressure-induced spin-crossover (SCO) transitions in the molecular iron(II) complex Fe[HB(tz)3]2 (HB(tz)3− = bis[hydrotris(1,2,4-triazol-1-yl)borate]) drive giant and reversible barocaloric effects at easily accessible pressures. Specifically, high-pressure calorimetry and powder X-ray diffraction studies reveal that pressure shifts as low as 10 bar reversibly induce nonzero isothermal entropy changes, and a pressure shift of 150 bar reversibly induces a large isothermal entropy change (>90 J kg−1 K−1) and adiabatic temperature change (>2 K). Moreover, we demonstrate that the thermodynamics of the SCO transition can be fine-tuned through systematic deuteration of the tris(triazolyl)borate ligand. These results provide new insights into pressure-induced SCO transitions and further establish SCO materials as promising barocaloric materials. 

Template‐based strategies are becoming increasingly important for controlling the position of nanoparticle‐based (NP‐based) structures on surfaces for a wide variety of encoding and device fabrication strategies. Thus, there is an increasing need to understand the behavior of NPs in confined spaces. Herein, a systematic investigation of the diffusion and adsorption properties of DNA‐modified NPs is presented in lithographically defined, high‐aspect‐ratio pores using a template‐confined, DNA‐mediated assembly. Leveraging the sequence‐specific binding affinity of DNA, it is discovered that although NP adsorption in deep polymer pores follows a traditional Langmuir adsorption model when under thermodynamic control, such NPs kinetically follow Fick's classical law of diffusion. Importantly, these observations allow one to establish design rules for template‐confined, DNA‐mediated NP assembly on substrates based on pore dimensions, NP size and shape, NP concentration, temperature, and time. As a proof‐of‐concept example, these design rules are used to engineer a vertical, four‐layer assembly consisting of individual octahedral NPs stacked on top of one another, with in‐plane positioning defined by pores generated by e‐beam lithography.

 

Due to their well‐defined 3D architectures, permanent porosity, and diverse chemical functionalities, metal–organic framework nanoparticles (MOF NPs) are an emerging class of modular nanomaterials. Herein, recent developments in the synthesis and postsynthetic surface functionalization of MOF NPs that strengthen the fundamental understanding of how such structures form and grow are highlighted; the internal structure and external surface properties of these novel nanomaterials are highlighted as well. These fundamental advances have resulted in MOF NPs being used as components in chemical sensors, biological probes, and membrane separation materials, as well as building blocks for colloidal crystal engineering.

 

A chromium(II)‐based metal–organic framework Cr₃[(Cr₄Cl)₃(BTT)₈]₂(Cr‐BTT; BTT³⁻=1,3,5‐benzenetristetrazolate), featuring coordinatively unsaturated, redox‐active Cr²⁺cation sites, was synthesized and investigated for potential applications in H₂storage and O₂production. Low‐pressure H₂adsorption and neutron powder diffraction experiments reveal moderately strong Cr–H₂interactions, in line with results from previously reported M‐BTT frameworks. Notably, gas adsorption measurements also reveal excellent O₂/N₂selectivity with substantial O₂reversibility at room temperature, based on selective electron transfer to form Cr^IIIsuperoxide moieties. Infrared spectroscopy and powder neutron diffraction experiments were used to confirm this mechanism of selective O₂binding.