Low-temperature Crystal Research Articles

Vapour-liquid equilibrium and low-temperature liquid-crystal phase diagram of discotic colloids

Discotic colloids give rise to a paradigmatic family of liquid crystals with sound applications in Materials Science. In this paper, Monte Carlo simulations are employed to characterise the low-temperature liquid crystal phase diagram and the vapour-liquid coexistence of discotic colloids interacting via a Kihara potential. Discoidal particles with thickness-diameter aspect ratios L ∗ ≡ L / D = 0.5 , 0.3, 0.2 and 0.1 are considered. For the less anisotropic particles ( L ∗ ≥ 0.2 ), coexistence of a vapour phase with the isotropic fluid and with the columnar liquid crystal phase is observed. As the particle anisotropy increases, the vapour-liquid coexistence shifts to lower temperatures and its density range diminishes, eventually merging with coexistences involving the liquid crystal phases. The L ∗ = 0.1 fluid displays a rich sequence of mesophases, including a nematic phase and a novel lamellar phase in which particles arrange in layers perpendicular to the nematic director.

Phonon-mediated quantum efficiency measurement in semiconductors

Accurate quantum efficiency measurement not only provides crucial information for the photovoltaic cell industry but also supports experiments aimed at directly detecting dark matter and elastic neutrino interactions. The dark matter direct searches paradigm has recently expanded to include particles with masses below 1,MeV/c2, where the expected signal in an electron–recoil interaction is approximately in the eV range, just above the energy gap for silicon and germanium. A robust calibration method for ionization signals in this lower energy region is essential. This paper presents a method for measuring quantum efficiency and yield (q/E) in semiconductors using phonon-mediated calorimetry. The Neganov–Trofimov–Luke phonon amplification method in low-temperature semiconductor crystals has been employed to indirectly measure ionization down to single-electron accuracy. Specifically, at zero bias, the phonon readout directly quantifies the total energy deposited within the detector, independent of the ionization yield. This eliminates a significant source of systematic uncertainty in quantum efficiency estimates associated with total energy uncertainty. The paper includes results from an updated ionization efficiency measurement in a germanium detector.

Phase transitions of choline dihydrogen phosphate: A vibrational spectroscopy and periodic DFT study.

Choline dihydrogen phosphate, [Chol][H2PO4], is a proton-conducting ionic plastic crystal exhibiting a complicated sequence of phase transitions. Here, we address the argument in the literature around the thermal properties of [Chol][H2PO4] using Raman and infrared microspectroscopy. The known structure of the low-temperature crystal, which contains the anti-conformer of [Chol]+ and hydrogen-bonded dimers of anions, was used to do periodic density functional theory calculations of the vibrational frequencies. Raman spectra indicate that the solid-solid transition at 20 °C is linked to a conformational change to the gauche [Chol] conformer with a concurrent local rearrangement of the anions. The distinct bands of lattice modes in the low-frequency range of the Raman spectra vanish at the 20 °C transition. Given the ease with which metastable crystals can be produced, Raman mappings demonstrate that a sample of [Chol][H2PO4] at ambient temperature can contain a combination of anti- and gauche conformers. Heating to 120 °C causes continuous changes in the local environment of anions rather than melting as suggested by a recent calorimetric investigation of [Chol][H2PO4]. The monotonic change in vibrational spectra is consistent with earlier observations of a very small entropy of fusion and no abrupt jump in the temperature dependence of ionic conductivity along the phase transitions of [Chol][H2PO4].

Four-Dimensional Printing of Temperature-Responsive Liquid Crystal Elastomers with Programmable Shape-Changing Behavior.

Liquid crystal elastomers (LCEs) are polymer networks that exhibit anisotropic liquid crystalline properties while maintaining the properties of elastomers, presenting reversible high-speed and large-scale actuation in response to external stimuli. Herein, we formulated a non-toxic, low-temperature liquid crystal (LC) ink for temperature-controlled direct ink writing 3D printing. The rheological properties of the LC ink were verified under different temperatures given the phase transition temperature of 63 °C measured by the DSC test. Afterwards, the effects of printing speed, printing temperature, and actuation temperature on the actuation strain of printed LCEs structures were investigated within adjustable ranges. In addition, it was demonstrated that the printing direction can modulate the LCEs to exhibit different actuation behaviors. Finally, by sequentially conforming structures and programming the printing parameters, it showed the deformation behavior of a variety of complex structures. By integrating with 4D printing and digital device architectures, this unique reversible deformation property will help LCEs presented here apply to mechanical actuators, smart surfaces, micro-robots, etc.

Conversion of stable crystals to metastable crystals in a solution by periodic change of temperature.

Using a Becker-Döring-type model including cluster incorporation, we study the possibility of conversion of stable crystals to metastable crystals in a solution by a periodic change of temperature. At low temperature, both stable and metastable crystals are assumed to grow by coalescence with monomers and corresponding small clusters. At high temperature, a large amount of small clusters produced by the dissolution of crystals inhibits the dissolution of crystals, and the imbalance in the amount of crystals increases. By repeating this process, the periodic temperature change can convert stable crystals into metastable crystals.

Phase behaviour and relaxation dynamics of the asymmetric azoxybenzene

ABSTRACT The phase behaviour and molecular dynamics of the liquid crystal 4-decyl-4’-heptylazoxybenzene (abbreviated as 10AB7) are described by differential scanning calorimetry, polarized optical microscopy, X-ray diffraction and broadband dielectric spectroscopy. Upon fast cooling/heating rates, the 10AB7 forms two liquid crystal phases, nematic and smectic A, as well as two conformationally disordered crystal phases (one of them does not appear on cooling). Upon slow cooling, there is the multiplicity of crystal phases. The dielectric spectra reveal the complex dynamics in the individual thermodynamic states of 10AB7. The Arrhenius type high-frequency relaxation, related with the rotations around the long axes of molecules, is detected for each liquid-like phases and for the high-temperature crystal phase. In the crystal phases there is also the Arrhenius type relaxation associated with the collective motions of molecules combined due to the mutual interactions through NO molecular groups. Additionally, the Vogel–Fulcher–Tammann type process is visible in the low-temperature crystal phase.

Observation of Distinct Structural Phase Transition in Bulk CrI3 Magnet

Exploring structural phase is not only vital for fundamental physical properties, but also crucial for creating innovative devices. The emerging two-dimensional (2D) ferromagnet CrI3 has been widely studied because of its novel magnetic order, stacking order and related properties. Conventionally, it is predicted that the high-temperature monoclinic crystal phase will convert into low-temperature rhombohedral crystal phase in bulk CrI3 at a critical temperature of ~ 220 K. Strikingly, here, a high-temperature monoclinic phase in bulk CrI3 flake is completely preserved upon cooling from 295 to 10 K beyond the conventional phase changes, breaking out the regular cognition. This work indicates that complex structural phases coexist in bulk CrI3 ferromagnet, which are attributed to the difference between the surface and inner layers. These findings not only deepen the understanding of 2D ferromagnetic materials, but also provide opportunities for constructing future 2D-based magneto-optic and magneto-mechanical devices.

Fluorosubstitution of the Molecular Core in Chiral Esters with Short Terminal Carbon Chains: Influence on Physical Properties

Comparative study of chiral liquid crystalline (S)-(1)-4’-(1-methylheptylcarbonyl) biphenyl-4-yl 4-[4-(2,2,3,3,4,4,4-heptafluorobutoxy)butyl-1-oxy] benzoate (4HH) and (S)-4’-(1-methylheptyloxycarbonyl)biphenyl-4-yl 4-[4-(2,2,3,3,4,4,4-heptafluorobutoxy) butyl-1-oxy]-2,3-difluorobenzoate (4FF) is performed by complementary methods. For 4HH melting of the low-temperature crystal phase and subsequent cold crystallization (from antiferroelectric smectic CA* phase to the high-temperature crystal phase) are reported, crystallization kinetics is examined and a monotropic hexatic SmXA* phase is observed on cooling. For 4FF rich polymorphism in the solid state is investigated mainly by simultaneous X-ray diffraction and differential scanning calorimetry measurements. Influence of fluorosubstitution on structural, electro-optic and dielectric properties of the smectic phases is reported. Unit cell parameters of crystal phases of 4HH and 4FF are determined. The reported results show that the double fluorosubstitution slows down the Goldstone mode and PH phason in the smectic phases and facilitates crystallization.

High Thermoelectric Figure of Merit of FeSb2−x Thin Films via Defect Engineering for Low-Temperature Cooling Applications

In general, both the thermal and electrical conductivity of metal/metalloid/semimetal materials decrease with increasing temperature; that is, these two parameters are in lockstep. Defect engineering has recently been proven to have tremendous potential for improving the thermoelectric performance of topological materials with two-dimensional layered structures. We have explored the effect of Sb point-defect engineering on the thermoelectric performance of FeSb2−x (x = 0, 0.1, 0.2, and 0.3) thin films at low temperatures. Thin-film surface and grain-boundary phonon scattering contribute to the overall decrease in the thermal conductivity (κ) compared with single crystals. The current results indicate that FeSb1.8 shows the lowest κ value, the main reason being that its nanostructure scatters more phonons over a wider wavelength range. In addition, the presence of Sb point defects enables an enhancement of the Seebeck coefficient (S) to 357 μV/K at 45 K for FeSb1.8 thin films, coupled with an imperceptible decrease in the corresponding electrical conductivity (σ). Despite these results, the power factor (PF) is much lower than that of single crystals. The optimal peak thermoelectric figure of merit (ZT) of ~ 0.071 at 55 K is one order of magnitude higher than the ZTmax of low-temperature FeSb2 single crystals. These characteristics indicate that FeSb2 thin films are promising materials for use in low-temperature thermoelectric devices.

Feasibility of Molten Salt Reactor Heat Exchanger Online Monitoring

Abstract Online structural health corrosion monitoring in advanced molten salt reactor heat exchangers is desirable for detecting tube degradation prior to leaks that would either cause mixing of heat exchanger fluids or release of radiologically contaminated fluids beyond the design containment boundary. This program seeks to demonstrate the feasibility for a torsional wave mode sensor to attach to the outside of a long (30-m) heat exchanger tube in the stagnant flow area where the tube joins the heat exchanger plenum and where it is possible to protect a sensor cable from high-force flow connecting through a heat exchanger shell to a monitoring instrument. The envisioned sensor and cable management approach will be impractical to implement on existing heat exchangers; rather, sensors must be installed in conjunction with the heat exchanger fabrication. Initially, flaw surrogates of interest (50% notch and 50% flat-bottom hole) have been detected in a 3-m tube using low-temperature PZT piezoelectric crystals. The transducer consisted of multiple shear elements placed circumferentially around a tube. The program will continue to investigate higher temperature piezoelectric ceramics, long-term performance of high-temperature adhesives, and flaw sensitivity on long (30-m +) tubes.

Characterization of temperature-dependent superlattice in Cu:KTN crystal

In this study, we grew CuO doped potassium tantalum niobate (Cu:KTN) crystals with a uniform superlattice structure by the off-center top-seeded solution growth (TSSG) method. The process of crystal superlattice structure formation was observed under a polarizing microscope at variable temperatures. It was found that the formation of the superlattice structure in the crystal was closely related to the formation process of the domain structure in the crystal. The 90° domain structure in the crystal promoted the formation of the superlattice structure in the crystal. The purpose of the formation of the superlattice structure is to enable the crystal to reach a more stable state. The clear diffraction effect of the crystal superlattice structure is similar to the x-ray diffraction phenomenon of low-temperature crystals, and it exists in the crystal in a three-dimensional structure.

Characterization of low temperature crystal growth parameters of the growth-dominated phase change materials GeSb6Te

Phase change memory materials are semiconducting alloys used in optical- and resistivity-based memory devices that have large differences in optoelectronic properties between the amorphous and crystalline states. Alloys for memory applications must have a low crystallization rate at service temperatures so the written memory remains stable for long times. GeSb6Te may be attractive for use because of its rapid crystallization at high temperatures, but its low temperature crystallization behavior has not been fully assessed. Here, the crystallization parameters of amorphous films of GeSb6Te are characterized using in-situ hot stage optical microscopy under isothermal conditions for temperatures ranging from 185 °C to 195 °C. Individual grain sizes were tracked as well as the percent of material crystallized, from which the direct measurement of the crystal growth rate and Johnson-Mehl-Avrami-Kolmogorov parameters were calculated. GeSb6Te was found to have comparable growth and crystallization rates in the low temperature region as other growth-dominated phase change materials.

Multistep Crystallization and Melting Pathways in the Free-Energy Landscape of a Au-Si Eutectic Alloy.

Crystals do eventually melt if they are heated to their characteristic melting point. However, this is practically only the case for high‐temperature stable crystals, whereas low‐temperature metastable crystals generally transform, before melting, into a more stable solid during heating. Here, it is illustrated that low‐temperature crystals can, however, be melted via fast differential scanning calorimetry (FDSC), even in metallic systems where nucleation and growth kinetics are rapid. For a Au–Si eutectic alloy, various metastable and stable solid states, i.e., (Au–α), (Au–β), γ, and (Au–Si), which form under well‐controlled conditions and melt at high heating rates by preventing the metastable‐to‐stable solid phase transition, are isolated. It is demonstrated that Au81.4Si18.6 can fully melt at various temperatures, i.e., 294 °C, 312 °C, 352 °C, and 363 °C, with differing melting enthalpies ranging from 6.52 to 9.83 kJ mol−1. The melting and crystallization paths of the metastable solids are determined by constructing an energy−temperature diagram. This approach advances the general understanding of nucleation in metallic and other systems, and is expected to contribute to the detailed understanding of thermophysical phenomena that occur at spatially reduced dimensions and/or short time scales, for example in thin‐film deposition, nanomaterials production, or additive manufacturing.

Magnetism in NdMn0.1Fe0.9O3 compound

The low temperature crystal and magnetic structures of NdMn0.1Fe0.9O3 were determined on the basis of X-ray and neutron powder diffraction experiments as well as from specific heat and magnetization measurements. The compound crystallizes in the orthorhombic crystal structure, space group Pnma, in the entire temperature interval 20 mK < T < 300 K. The lattice parameters exhibit standard thermal expansion effects for T > 20 K and at lower temperatures the anomalies due to magnetostriction effect were observed. The iron sublattice orders magnetically into 3 different magnetic structures, namely into Γ5 = (Ax, Fy, Gz) for 130 K < T < TN (Néel temperature); Γ1 = (Gx, Cy, Az) for 15 K < T < 130 K and Γ3 = (Cx, Gy, Fz) for 20 mK < T < 15 K. The Nd ions order at temperatures below 1.6–1.75 K into the same Γ3 phase as the iron sublattice at these temperatures. The obtained magnetic structures fit perfectly in between the magnetic phases of closely related NdFeO3 and NdMn0.5Fe0.5O3 compounds. Our study, together with all previously published data, completes the entire magnetic phase diagram of NdMn1−xFexO3 (0 ≤ x ≤ 1) solid solution substitutional system.

Low-Temperature Phase Transitions of the Ionic Liquid 1-Ethyl-3-methylimidazolium Dicyanamide.

Several calorimetric measurements have shown that 1-ethyl-3-methylimidazolium dicyanamide, [C2C1im][N(CN)2], is a glass-forming liquid, even though it is a low-viscous liquid at room temperature. Here, we found slow crystallization during cooling of [C2C1im][N(CN)2] along Raman spectroscopy measurements. The low-frequency range of the Raman spectrum shows that the same crystalline phase is obtained at 210 K either by cooling or by reheating the glass (cold-crystallization). Another crystalline phase is formed at ca. 260 K just prior the melting at 270 K. X-ray diffraction and calorimetric measurements confirm that there are two crystalline phases of [C2C1im][N(CN)2]. The Raman spectra indicate that polymorphism is related to [C2C1im]+ with the ethyl chain on the plane of the imidazolium ring (the low-temperature crystal) or nonplanar (the high-temperature crystal). The structural reason for the glass-forming ability of [C2C1im][N(CN)2], despite the relatively simple molecular structures of the ions, was pursued by quantum chemistry calculations and molecular dynamics (MD) simulations. Density functional theory calculations were performed for ionic pairs in order to draw free-energy surfaces of the anion around the cation. The MD simulations using a polarizable model provided maps of occurrence of anions around cations. Both the quantum and classical calculations suggest that the delocalization of preferred positions of the anion around the cation, which adopts different conformations of the ethyl chain, is on the origin of the crystallization being hampered during cooling and the resulting glass-forming ability of [C2C1im][N(CN)2].

Influence of partial substitution of Cu atoms by Zn and Cd atoms on polymorphic transformation in the Cu1.75Te crystal

The [Formula: see text], [Formula: see text] and [Formula: see text] compounds have been synthesized and low-temperature modification single crystals obtained from the high temperature modification by polymorphic transformation. The method of high temperature X-ray diffractometer has been used to study [Formula: see text] (x = 0.05 Zn and Cd) layered single crystal. It was established that such a substitution has a significant impact on the number and temperature of polymorphic transformations. Crystallgraphic parameters were determined for each phase. Temperature dependence of lattice parameters were obtained and determined change mechanism of lattice parameters by influence of temperature.

Using powder XRD and pair distribution function to determine anisotropic atomic displacement parameters of orthorhombic tridymite and tetragonal cristobalite.

Determination of the crystal structures of low-temperature tridymite and cristobalite using single-crystal XRD has been challenging because they generally occur as metastable fine-grained crystals in the geological environment. Therefore, using powder diffraction and scattering techniques is critical to study the low-temperature crystals. Synchrotron powder X-ray diffraction (XRD), pair distribution function (PDF) and transmission electron microscopy were used to investigate the structure of orthorhombic tridymite with C2221 symmetry and tetragonal cristobalite with P41212 symmetry, including their anisotropic atomic displacement parameters (ADPs). Rietveld refinement was used to determine the unit-cell parameters, fractional coordinates and isotropic atomic displacement parameters (Uiso) of the tridymite and cristobalite. The PDF method was used to determine ADPs for each atom. The results suggest that the crystal structure with high quality ADP values can be obtained using the combined methods of XRD and PDF analyses. The method can be used for characterizing crystals that are not suitable for single-crystal XRD.

Long Periodic Structure of a Room-Temperature Ionic Liquid by High-Pressure Small-Angle X-Ray Scattering and Wide-Angle X-Ray Scattering: 1-Decyl-3-Methylimidazolium Chloride.

The Bragg reflections of 1-decyl-3-methylimidazolium chloride ([C10 mim][Cl]), a room-temperature ionic liquid, are observed in a lowly scattered wavevector (q) region using high-pressure (HP) small-angle X-ray scattering methods. The HP crystal of [C10 mim][Cl] was characterized by an extremely long periodic structure. The peak position at the lowest q (1.4 nm-1 ) was different from that of the prepeak observed in the liquid state (2.3 nm-1 ). Simultaneously, Bragg reflections at high-q were detected using HP wide-angle X-ray scattering. The longest lattice constant was estimated to be 4.3 nm using structural analysis. The crystal structure of HP differed from that of the low-temperature (LT) crystal and the LT liquid crystal. With increasing pressure, Bragg reflections in the high-q component became much broader, and were accompanied by phase transition, although those in the low-q component were observed to be relatively sharp.

Phase Transitions of the Ionic Liquid [C2C1im][NTf2] under High Pressure: A Synchrotron X-ray Diffraction and Raman Microscopy Study

The interplay between crystallization and glass transition in the archetypal ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [C2C1im][NTf2], has been studied as a function of pressure up to ca. 12 GPa. Besides heterogeneous crystal nucleation, homogeneous nucleation in the sample inside the diamond anvil cell was also observed depending on compression/decompression rate. Amorphization of the crystal and glass formation under pressure has been followed by synchrotron X-ray diffraction. The characteristic Raman bands of the [NTf2]− anion provide a microscopic probe of the different phases. The crystalline phase is composed of the [NTf2]− cisoid conformer, but moisture implies formation of crystal with the transoid conformer. Raman spectra show that crystalline phases might become microscopically heterogeneous because of [NTf2]− conformational disorder. Raman mapping reveals the order–disorder evolution from crystal to glass. Crystals of [C2C1im][NTf2] formed under high pressure and room temperature are similar to previously reported low temperature and atmospheric pressure crystals. Thus, it is concluded that density is the main factor controlling crystallization and glass formation under high pressure of [NTf2]− based ionic liquids due to hindrance of efficient ion packing. The results highlight that ionic liquids are good models to understand fundamental questions related to the mechanism of crystallization and glass transition.

Silicification of Hydrothermal Gangue Minerals In Pb-Zn-Cu-Fluorite-Quartz-Baryte Veins

Abstract Late-stage pseudomorphous and perimorphous replacement of euhedral baryte and, to a lesser extent, fluorite and calcite by quartz is a common phenomenon in hydrothermal vein-type deposits. As a consequence of silicification, the primary mineral assemblage might be substantially altered, and therefore this process has a severe negative impact on the economic potential of mineral resources. Although these replacement textures are often reported and have a significant economic importance in mines producing baryte or fluorite of chemical grade, the process that causes this silicification is surprisingly poorly understood. In the present contribution, more than 40 Jurassic–Cretaceous and post-Cretaceous hydrothermal veins from the Schwarzwald mining district, including replacement textures of primary euhedral baryte, fluorite, and calcite, were investigated with respect to their macro- and microscopic textures. It appears that baryte is favorably replaced by pseudomorphs (bladed quartz), while fluorite and calcite are typically replaced by perimorphs. The textures indicate that the mode of replacement of the primary minerals happens continuously and after the initial vein formation. By combining these textural observations with calculated mineral solubilities, a detailed geochemical model has been developed. Existing fluid inclusion data indicate that substantial cooling of the hydrothermal solutions occurs after primary mineral formation. The calculated cooling path reveals opposing solubilities of quartz and the other gangue minerals (baryte, fluorite, and calcite) with decreasing temperature and explains the observed dissolution and precipitation textures. Furthermore, differences in temperature-solubility systematics between baryte on the one hand and fluorite and calcite on the other are responsible for the differences observed in the textures. This agrees with the occurrence of late-stage, low-temperature baryte crystals overgrowing primary baryte assemblages. Conversely, analogous late-stage calcite and fluorite assemblages are only rarely observed. In summary, silicification is a typical cooling effect in various hydrothermal vein-type deposits, but affects different gangue minerals in different ways depending on their temperature-dependent solubility.