Significance: Halide perovskite (HaP) semiconductors are revolutionizing the field of photovoltaic (PV) solar energy conversion by showing remarkable performance of solar cells made with HaPs. “Ferroelectrics” is one frequently suggested reason because it may allow the spatial separation of the flow of electrons from where they were generated (holes). Unlike common, electrically insulating, ferroelectric materials, HaPs [especially tetragonal methylammonium lead triiodide (MAPbI ₃ )] are semiconducting, and to find out whether they are ferroelectric requires an approach that is different from what is done customarily. Using such an approach, we prove that tetragonal MAPbI ₃ is definitely ferroelectric. What still remains to be seen is whether this ferroelectric nature is important for how MAPbI ₃ -based solar cells operate around room temperature.

Electrostrictors, materials developing mechanical strain proportional to the square of the applied electric field, present many advantages for mechanical actuation as they convert electrical energy into mechanical, but not vice versa. Both high relative permittivity and reliance on Pb as the key component in commercial electrostrictors pose serious practical and health problems. Here we describe a low relative permittivity (<250) ceramic, Zr_xCe_1-xO₂ (x < 0.2), that displays electromechanical properties rivaling those of the best performing electrostrictors: longitudinal electrostriction strain coefficient ~10⁻¹⁶ m²/V²; relaxation frequency ≈ a few kHz; and strain ≥0.02%. Combining X-ray absorption spectroscopy, atomic-level modeling and electromechanical measurements, here we show that electrostriction in Zr_xCe_1-xO₂ is enabled by elastic dipoles produced by anharmonic motion of the smaller isovalent dopant (Zr). Unlike the elastic dipoles in aliovalent doped ceria, which are present even in the absence of an applied elastic or electric field, the elastic dipoles in Zr_xCe_1-xO₂ are formed only under applied anisotropic field. The local descriptors of electrostrictive strain, namely, the cation size mismatch and dynamic anharmonicity, are sufficiently versatile to guide future searches in other polycrystalline solids.

A key conundrum of biomolecular electronics is efficient electron transport (ETp) through solid-state junctions up to 10 nm, often without temperature activation. Such behavior challenges known charge transport mechanisms, especially via nonconjugated molecules such as proteins. Single-step, coherent quantum-mechanical tunneling proposed for ETp across small protein, 2-3 nm wide junctions, but it is problematic for larger proteins. Here we exploit the ability of bacteriorhodopsin (bR), a well-studied, 4-5 nm long membrane protein, to assemble into well-defined single and multiple bilayers, from ∼9 to 60 nm thick, to investigate ETp limits as a function of junction width. To ensure sufficient signal/noise, we use large area (∼10^-3 cm²) Au-protein-Si junctions. Photoemission spectra indicate a wide energy separation between electrode Fermi and the nearest protein-energy levels, as expected for a polymer of mostly saturated components. Junction currents decreased exponentially with increasing junction width, with uniquely low length-decay constants (0.05-0.5 nm^-1). Remarkably, even for the widest junctions, currents are nearly temperature-independent, completely so below 160 K. While, among other things, the lack of temperature-dependence excludes, hopping as a plausible mechanism, coherent quantum-mechanical tunneling over 60 nm is physically implausible. The results may be understood if ETp is limited by injection into one of the contacts, followed by more efficient charge propagation across the protein. Still, the electrostatics of the protein films further limit the number of charge carriers injected into the protein film. How electron transport across dozens of nanometers of protein layers is more efficient than injection defines a riddle, requiring further study.

Surface pyroelectricity and piezoelectricity induced by water incorporation during growth in α-glycine were investigated. Using the periodic temperature change technique, we have determined the thickness (~280 µm) of the near surface layer (NSL) and its pyroelectric coefficient (160 pC/(K × cm²) at 23 °C) independently. The thickness of NSL remains nearly constant till 60 °C and the pyroelectric effect vanishes abruptly by 70 °C. The piezoelectric effect, 0.1 pm/V at 23 °C measured with an interferometer, followed the same temperature dependence as the pyroelectric effect. Abrupt disappearance of both effects at 70 °C is irreversible and suggests that water incorporation to α-glycine forms a well defined near surface phase, which is different form α-glycine because it is polar but it too close to α-glycine to be distinguished by X-ray diffraction (XRD). The secondary pyroelectric effect was found to be <14% of the total, which is unexpectedly small for a material with a large thermal expansion coefficient. This implies that water incorporation infers minimal distortions in the host lattice. This finding suggests a path for the control of the piezoelectric and pyroelectric effects of the crystals using stereospecific incorporation of the guest molecules.

This work reportsthat the octahedral hydrated Al3+ andMg(2+) ions operate within electrolytic cells as kosmotropic(long-range order-making) "ice makers" of supercooledwater (SCW). 10(-5) M solutions of hydrated Al3+ and Mg2+ ions each trigger, near the cathode(-20 & PLUSMN; 5 V), electro-freezing of SCW at -4 & DEG;C.The hydrated Al3+ ions do so with 100% efficiency, whereasthe Mg2+ ions induce icing with 40% efficiency. In contrast,hydrated Na+ ions, under the same experimental conditions,do not induce icing differently than pure water. As such, our studyshows that the role played by Al3+ and Mg2+ ionsin water electro-freezing is impacted by two synchronous effects:(1) a geometric effect due to the octahedral packing of the coordinatedwater molecules around the metallic ions, and (2) the degree of polarizationwhich these two ions induce and thereby acidify the coordinated watermolecules, which in turn imparts them with an ice-like structure.Long-duration molecular dynamics (MD) simulations of the Al3+ and Mg2+ indeed reveal the formation of "ice-like"hexagons in the vicinity of these ions. Furthermore, the MD showsthat these hexagons and the electric fields of the coordinate watermolecules give rise to ultimate icing. As such, the MD simulationsprovide a rational explanation for the order-making properties ofthese ions during electro-freezing.