Ferroelectric-like Transition Research Articles

Hydrogen bond as a trigger of ferroelectric-like phase transition in lithium-thallium tartrate monohydrate

The dielectric and pyroelectric properties have been investigated in lithium-thallium tartrate monohydrate crystals at 2–100 K. It is shown that ferroelectric-like transition at 10–12 K occurs only in the proton subsystem without changes in the symmetry of the unit cell, which is confirmed by polarized Raman spectroscopy. Similar behavior of dielectric properties was detected earlier in non-polar phase of a number of ferroelectrics after a sharp cooling below the Curie point, which created anisotropic thermoelastic stresses. In this case, the temperature anomalies of dielectric permittivity coincide with the specific points of the proton subsystem of OH…O hydrogen bonds.

DC bias electric field effects on ac electrical conductivity of MAPbI3 suggesting intrinsic changes on structure and charge carrier dynamics

Methylammonium lead iodide (MAPbI3) emerges as a promising halide perovskite material for the next generation of solar cells due to its high efficiency and flexibility in material growth. Despite intensive studies of their optical and electronic properties in the past ten years, there are no reports on dc bias electric field effects on conductivity in a wide temperature range. In this work, we report the combined effects of frequency, temperature, and dc bias electric field on the ac conductivity of MAPbI3. We found that the results of dc bias electric fields are very contrasting in the tetragonal and cubic phases. In the tetragonal phase, sufficiently high dc bias electric fields induce a conductivity peak appearance ∼290 K well evidenced at frequencies higher than 100 kHz. Excluding possible degradation and extrinsic factors, we propose that this peak suggests a ferroelectric-like transition. In the absence of a dc bias electric field, the ac conductivity in the tetragonal phase increases with temperature while decreases with temperature in the cubic phase. Also, ac activation energies for tetragonal and cubic phases were found to be inversely and directly proportional to the dc bias electric field, respectively. This behavior was attributed to the ionic conduction, possibly of MA+ and I− ions, for the tetragonal phase. As for the cubic phase, the ac conduction dynamics appear to be metallic-like, which seems to change to a polaronic-controlled charge transport to increased dc bias electric fields.

Free-Carrier-Induced Ferroelectricity in Layered Perovskites.

Doping ferroelectrics with carriers is often detrimental to polarization. This makes the design and discovery of metals that undergo a ferroelectriclike transition challenging. In this Letter, we show from first principles that the oxygen octahedral rotations in perovskites are often enhanced by electron doping, and this can be used as a means to strengthen the structural polarization in certain hybrid-improper ferroelectrics-compounds in which the polarization is not stabilized by the long-range Coulomb interactions but is instead induced by a trilinear coupling to octahedral rotations. We use this design strategy to predict a cation ordered Ruddlesden-Popper compound that can be driven into a metallic ferroelectriclike phase via electrolyte gating.

Pressure-induced ferroelectric-like transition creates a polar metal in defect antiperovskites Hg3Te2X2 (X\u2009=\u2009Cl, Br)

Ferroelectricity is typically suppressed under hydrostatic compression because the short-range repulsions, which favor the nonpolar phase, increase more rapidly than the long-range interactions, which prefer the ferroelectric phase. Here, based on single-crystal X-ray diffraction and density-functional theory, we provide evidence of a ferroelectric-like transition from phase I213 to R3 induced by pressure in two isostructural defect antiperovskites Hg3Te2Cl2 (15.5 GPa) and Hg3Te2Br2 (17.5 GPa). First-principles calculations show that this transition is attributed to pressure-induced softening of the infrared phonon mode Γ4, similar to the archetypal ferroelectric material BaTiO3 at ambient pressure. Additionally, we observe a gradual band-gap closing from ~2.5 eV to metallic-like state of Hg3Te2Br2 with an unexpectedly stable R3 phase even after semiconductor-to-metal transition. This study demonstrates the possibility of emergence of polar metal under pressure in this class of materials and establishes the possibility of pressure-induced ferroelectric-like transition in perovskite-related systems.

Electrical detection of ferroelectriclike metals through the nonlinear Hall effect

Ferroelectric-like metals are a relatively rare class of materials that have ferroelectric-like distortion and metallic conductivity. LiOsO$_3$ is the first demonstrated and the most investigated ferroelectric-like metal. The presence of free carriers makes them difficult to be studied by traditional ferroelectric techniques. In this paper, using the symmetry analysis and first-principles calculations, we demonstrate that the ferroelectric-like transition of LiOsO$_3$ can be probed by a kind of electrical transport method based on nonlinear Hall effect. The Berry curvature dipole exists in the ferroelectric-like phase, and it can lead to a measurable nonlinear Hall conductance with a conventional experimental setup. However, the symmetry of the paraelectric-like phase LiOsO$_3$ vanishes the Berry curvature dipole. The Berry curvature dipole shows a strong dependence on the polar displacement, which might be helpful for the detection of polar order. The nonlinear Hall effect provides an effective method for the detection of phase transition in the study of the ferroelectric-like metals and promotes them to be applied in the ferroelectric-like electronic devices.

Ferroelectric order versus metallicity in Sr1−xCaxTiO3−δ ( x=0.009 )

We report on a thermal-expansion study of the ferroelectric phase transition in insulating Sr$_{1-x}$Ca$_x$TiO$_3$ ($x=0.009$) and its evolution upon increasing charge-carrier concentration up to $n\simeq 60 \times 10^{19}$cm$^{-3}$. Although electric polarization is screened by mobile charge carriers, we find clear signatures of the ferroelectric phase transition in the thermal-expansion coefficient $\alpha$ of the weakly doped metallic samples. Upon increasing $n$, the transition temperature $T_\mathrm{C}(n)$ and the magnitude of the anomalies in $\alpha$ rapidly decrease up to a threshold carrier density $n^\star$ above which broadened anomalies remain present. There is no indication for a sign change of $\alpha$ as is expected for a pressure-dependent quantum phase transition with $n$ as the control parameter. Thus, the ferroelectriclike transition is either continuously fading away or it transforms to another low-temperature phase above $n^\star$, but this change hardly affects the temperature-dependent $\alpha(T)$ data.

Parity-breaking in single-element phases: ferroelectric-like elemental polar metals

Polar metals based on binary and ternary compounds have been demonstrated in literature. Here, we propose a design principle for ferroelectric-like elemental polar metals and relate it to real materials. The design principle is that, to be an elemental polar metal, atoms should occupy at least two inequivalent Wyckoff positions in a crystal with a polar space group, where inversion symmetry is spontaneously broken. According to this rule, we propose the first class of potential ferroelectric-like elemental polar metals in a distorted α-La-like structure with a polar space group P63mc in which two inequivalent Wyckoff positions 2a (0, 0, z) and 2b (1/3, 2/3, z) are occupied by group-V elements (phosphorus, arsenic, antimony, and bismuth). Analyses based on first-principles calculations indicate that the dynamically stable polar phase results from a lone pair driven polar distortion of the nonploar phase in P63/mmc symmetry where two inequivalent Wyckoff positions 2a (0, 0, 0) and 2c (1/3, 2/3, 1/4) are occupied. This ferroelectric-like transition involves a transition from a metallic state to a semimetallic state. These predicted polar phases are metastable with respect to their corresponding ground phases. Moreover, ionic bonding characters are found due to the inequivalence in Wyckoff positions between group-V atoms. Our work opens a route to single-element parity-breaking phases.

Short review of high-pressure crystal growth and magnetic and electrical properties of solid-state osmium oxides

High-pressure crystal growth and synthesis of selected solid-state osmium oxides, many of which are perovskite-related types, are briefly reviewed, and their magnetic and electrical properties are introduced. Crystals of the osmium oxides, including NaOsO3, LiOsO3, and Na2OsO4, were successfully grown under high-pressure and high-temperature conditions at 6GPa in the presence of an appropriate amount of flux in a belt-type apparatus. The unexpected discovery of a magnetic metal–insulator transition in NaOsO3, a ferroelectric-like transition in LiOsO3, and high-temperature ferrimagnetism driven by a local structural distortion in Ca2FeOsO6 may represent unique features of the osmium oxides. The high-pressure and high-temperature synthesis and crystal growth has played a central role in the development of solid-state osmium oxides and the elucidation of their magnetic and electronic properties toward possible use in multifunctional devices.

Thermodynamic and neutron diffraction studies on multiferroic NdMn2O5

Magnetically frustrated RMn2O5 oxides have attracted considerable attention in recent years, because most of the members of this family show spin ordering induced dielectric polarization along with strong magneto-electric coupling. Although the true origin of the ferroelectricity is still a matter of debate, it has been observed that the magneto-electric phase diagram can be substantially tuned with the variation of rare earth elements. In this work, we have chosen NdMn2O5 as the compound of our interest since it lies exactly in between the ferroelectric and non-ferroelectric members of this family and also, because there are few investigations performed on RMn2O5 systems with large rare earth atoms . With the combination of heat capacity, magnetic susceptibility, dielectric permittivity, powder X-ray diffraction, and powder neutron diffraction measurements, it has been found that NdMn2O5 undergoes an incommensurate magnetic ordering around 30K followed by a possible ferroelectric-like transition at ∼26K. Another lock-in kind of magnetic transition appears when the temperature is decreased to ∼15K. With further lowering of temperature, an antiferromagnetic ordering, which is presumably associated with the Nd3+, is achieved near 4K. This study thus sheds light on a new compound of the RMn2O5 series presenting different multiferroic properties.

Ferroelectric-like structural transition in metallic LiOsO3

It is assumed that ferroelectric behaviour only appears in the materials with an energy gap. However, a ferroelectric-like structural transition has been experimentally explored in metallic LiOsO3 [Nature Materials 12, 1024 (2013)]. In this paper, we investigate the possible origin of a ferroelectric-like transition and its effect on the electronic structure through theoretical simulations. By performing first-principles calculations, we found that the topologically compact structures formed by strong Os–O bonds forces Li ions to be displaced, which is responsible for the ferroelectric-like structural transition. Our calculations also indicate that the chemical valence should be Lin+[OsO3]n−, where n < 1. Furthermore, the intrinsic electric dipoles induced by the ion displacement are spontaneously screened by the remanent electrons of Li ions. Thus, the explored mechanism of the ferroelectric-like transition in metallic LiOsO3 may help develop other ferroelectric materials.

Multiferroicity and magnetoelectric coupling in half-doped manganite La0.5Ca0.5MnO3

We present experimental evidences to verify the peculiar multiferroics predicted in the half-doped manganite La0.5Ca0.5MnO3. The dielectric constant exhibits a frequency-independent peak just at the antiferromagnetic charge ordering transition temperature TCO, indicating a close correlation between the ferroelectriclike transition and charge ordering. In addition to a large magnetodielectric effect, the dynamic magnetoelectric coupling measurements provide solid evidences to an enhanced magnetoelectric coupling below TCO. Moreover, the magnetic field dependence of the coupling coefficient shows a close correlation with the magnetization hysteresis loop, which reveals that a large magnetoelectric coupling only persists in the charge ordering state.

Slowing down of the relaxational dynamics at the ferroelectric phase transition in one-dimensional (TMTTF) 2AsF 6

We present measurements of the dielectric response of quasi one-dimensional system (TMTTF) 2AsF 6 in a wide temperature and frequency range. We provide a thorough characterization of the relaxational dynamics observed close to the ferroelectric-like transition at T c=100 K. Our measurements, extending up to 100 MHz, reveal a continuous slowing down of the mean relaxation time when approaching T c from high as well as from low temperatures. The simultaneous critical rise of the dielectric constant and relaxation time point to an explanation of the transition in terms of a classic ferroelectric scenario.

Specific heat studies for λ-(BEDT-TSF) 2FeCl 4

We have measured the specific heat on λ-(BEDT-TSF) 2FeCl 4 as a function of temperature by a thermal relaxation method to study the metal-to-insulator (MI) transition around 8 K and anomalous ferroelectric-like state revealed by microwave measurements for the metallic state. The specific heat exhibits a sharp peak due to the antiferromagnetic ordering at T MI =8.3 K . We have found a remarkable jump at 70 K labeled as T FM. Together with the ferroelectric-like response emerging around T FM, this result confirms thermodynamically a ferroelectric-like transition at T FM in the metallic states. Furthermore, the thermal conductance of the sample may exhibit an anomaly at T FM.

Metal–insulator transitions in magnetic conductor λ-(BEDT-TSF) 2FeCl 4 studied by [formula omitted] and [formula omitted] NMR

We have performed 1 H and 77 Se NMR measurements on a π–d organic metal λ-(BEDT-TSF) 2FeCl 4 containing paramagnetic ions Fe 3+. It is found from 1 H NMR measurements that the resonance line starts to split around T FM =70 K , just a temperature where a ferroelectric-like transition occurs, and with further decreasing temperature, the spectrum becomes significantly broad near the metal–insulator transition T MI =8.3 K . From 77 Se NMR measurements, near the phase boundary between the forced-ferromagnetic metallic state and the canted antiferromagnetic insulating state at 1.6 K and 11.67 T, a single peak becomes split with two new peaks emerging at the both sides of the zero-shift.