Low Energy Phase Transition Research Articles

Low-energy ferroelectric–paraelectric phase transitions of three-dimensional A(II)B(I)X3-type perovskite ferroelectrics: How to realize high phase transition temperatures

Organohalide lead perovskites are widely used in memory components, sensors, and photocells, owing to their excellent (opto)electronic or ferroelectric properties, especially light weight, and easy processing compared to pure inorganic perovskites. For their commercialization, the exploration of environmentally friendly lead-free perovskites has become a hot issue. Recently, a class of metal-free three-dimensional (3D) halide perovskites has been synthesized, and these perovskites exhibit superior ferroelectricity. With reference to the experiment-synthesized (MDABCO)(NH4)I3 (MDABCO = N-methyl-N′-diazabicyclo[2.2.2]octonium), two new perovskites (MDABCO)KI3 and (MDABCO)RbI3 are designed through replacing NH4+ by metal ions. By using the first-principles calculations, we find that these (MDABCO)BI3 (B = NH4+, K+, and Rb+) perovskites all have a similar phase transition mechanism with ignorable barriers but prominent ferroelectricity (large spontaneous polarizations and high phase transition temperatures). Our design strategy can have important implications for the development of novel 3D perovskite ferroelectrics following this specific low-energy phase transition mechanism.

Valid Geometric Solutions for Indentations with Algebraic Calculations

This paper analyzes the force vs depth loading curves of conical, pyramidal, wedged and for spherical indentations on a strict mathematical basis by explicit use of the indenter geometries rather than on still world-wide used iterated “contact depths” with elastic theory and violation of the energy law. The now correctly analyzed loading curves provide as yet undetectable phase-transition. For the spherical indentations, this includes an obvious correction for the varying depth/radius ratio, which had previously been disregarded. Only algebraic formulas are now used for the calculation of material’s properties without data-fittings, or simplifications, or false simulations. Penetration resistance differences of materials’ polymorphs provide precise intersection values as kink unsteadiness by equalization of linear regression lines from mathematically linearized loading curves. These intersections indicate phase transition onset values for depth and force. The precise and correct determination of phase-transition onsets allows for energy and phase-transition energy calculations. The unprecedented algebraic equations are most simply and mathematically reproducibly deduced. There are no restrictions for elastic and/or plastic behavior and no use of different formulas for different force ranges. The novel indentation formulas reveal unprecedented access to the onset, energy and transition energy of phase-transitions. This is now also achieved for spherical indentations. Their formula as deduced for plotting is reformulated for integrations. The distinction of applied work (Wapplied) and indentation work (Windent) allows now for comparing spherical with pyramidal indentation phase-transitions. Only low energy phase-transitions from pyramidal indentation may be missed in spherical indentations. The rather low penetration depths of sphere calottes calculate very close for cap and flat area values. This allows for the calculation of the indentation phase-transition onset pressure and thus the successful comparison with hydrostatic anvil pressurizing results. This is very helpful for their interpretations, as low energy phase-transitions are often missed under the anvil, and it further strengthens the unparalleled ease of the indentation techniques. Exemplification is reported for pyramidal, spherical, and hydrostatic anvil stressing by the numerical analysis of published germanium data. The previous widely accepted historical indentation theories and standards are challenged. Falsely simulated and even published so-called “experimental” indentation data from the literature can most easily be checked. They are mathematically unsound and their correction is urgently necessary for scientific reasons and daily safety with stressed materials. The motivation for this paper is the challenge of worldwide incorrect ISO 14577 standards for false and incomplete characterization of materials. The minimization of catastrophic failures e.g. in aviation requires the strengthening and the advancements of the mathematical truth by using our closed formulas that are based on undeniable geometric and algebraic calculation rules.

Phase Transition Pathway Sampling via Swarm Intelligence and Graph Theory.

The prediction of reaction pathways for solid-solid transformations remains a key challenge. Here, we develop a pathway sampling method via swarm intelligence and graph theory and demonstrate that our pallas method is an effective tool to help understand phase transformations in solid-state systems. The method is capable of finding low-energy transition pathways between two minima without having to specify any details of the transition mechanism a priori. We benchmarked our pallas method against known phase transitions in cadmium selenide (CdSe) and silicon (Si). pallas readily identifies previously reported, low-energy phase transition pathways for the wurtzite to rock-salt transition in CdSe and reveals a novel lower-energy pathway that has not yet been observed. In addition, pallas provides detailed information that explains the complex phase transition sequence observed during the decompression of Si from high pressure. Given the efficiency to identify low-barrier-energy reaction pathways, the pallas methodology represents a promising tool for materials by design with valuable insights for novel synthesis.

Prediction of low energy phase transition in metal doped MoTe2 from first principle calculations

Metal–insulator transitions in two dimensional materials represent a great opportunity for fast, low energy, and ultradense switching devices. Due to the small energy difference between its semimetallic and semiconducting crystal phases, phase transition in MoTe2 can occur with an unprecedented small amount of external perturbations. In this work, we used the density functional theory to predict critical strain and electrostatic voltage required to control the phase transition of 3d and 4d metal doped MoTe2. We found that small doping contents dramatically affect the relative energies of MoTe2 crystal phases and can largely reduce the energy input to trigger the transition compared to the pristine case. Moreover, the kinetics corresponding to the phase transition in the proposed doped materials are several orders of magnitude faster than in MoTe2. For example, we predict 6.3% Mn doped MoTe2 to switch phase under 1.19 V gate voltage in less than 1μs with an input energy of 0.048aJ/nm3. Due to the presence of the dopant, the controlled change of phase is often complemented with a change in magnetic moment leading to multifunctional phase transition.

One-particle spectroscopic intensities as a signature of shape phase transition: The γ-unstable case

We investigate the evolution of one-particle spectroscopic intensities as a possible signature of shape phase transitions. The study describes the odd systems in terms of the interacting boson--fermion model. We consider the particular case of an odd $j=3/2$ particle coupled to an even-even boson core that undergoes a phase transition from spherical U(5) to \ensuremath{\gamma}-unstable O(6) situation. At the critical point, our findings are compared with the one-particle spectroscopic intensities that can be obtained within the E(5/4) model proposed by[F. Iachello, Phys. Rev. Lett. 95, 052503 (2005); F. Iachello, in Symmetries and Low-Energy Phase Transitions in Nuclear Structure Physics, edited by G. Lo Bianco (University of Camerino Press, Camerino, Italy, in press)].

Monopole-antimonopole bound states as a source of ultrahigh-energy cosmic rays

The electromagnetic decay and final annihilation of magnetic monopole-antimonopole pairs formed in the early universe has been proposed as a possible mechanism to produce the highest energy cosmic rays. We show that for a monopole abundance saturating the Parker limit, the density of magnetic monopolonium formed is many orders of magnitude less than that required to explain the observed cosmic ray flux. We then propose a different scenario in which the monopoles and antimonopoles are connected by strings formed at a low energy phase transition $(\ensuremath{\sim}100 \mathrm{GeV}).$ The bound states decay by gravitational radiation, with lifetimes comparable to the age of the Universe. This mechanism avoids the problems of the standard monopolonium scenario, since the binding of monopoles and antimonopoles is perfectly efficient.

Low Energy Phase Transitions in Molecular Crystals and Raman Spectroscopy: The Case of Benzil

Abstract The low-energy phase transition discovered by Esherick and Kohler in solid benzil has been studied by Raman spectroscopy as a function of temperature and pressure. Some of the frequency shift curves exhibit a discontinuity in their slopes at the transition point, but the most striking feature is the continuous increase of the intensity of a Raman line lying at 47 cm−1 when decreasing temperature or increasing pressure, followed by an intensity jump of the same line at the transition. Starting from these observations, a mechanism for the phase transition is proposed.