Triclinic Phase Research Articles

Unconventional Charge Density Wave Order in the Pnictide Superconductor Ba(Ni_{1-x}Co_{x})_{2}As_{2}.

Ba(Ni_{1-x}Co_{x})_{2}As_{2} is a structural homologue of the pnictide high temperature superconductor, Ba(Fe_{1-x}Co_{x})_{2}As_{2}, in which the Fe atoms are replaced by Ni. Superconductivity is highly suppressed in this system, reaching a maximum T_{c}=2.3 K, compared to 24K in its iron-based cousin, and the origin of this T_{c} suppression is not known. Using x-ray scattering, we show that Ba(Ni_{1-x}Co_{x})_{2}As_{2} exhibits a unidirectional charge density wave (CDW) at its triclinic phase transition. The CDW is incommensurate, exhibits a sizable lattice distortion, and is accompanied by the appearance of α Fermi surface pockets in photoemission [B. Zhou etal., Phys. Rev. B 83, 035110 (2011)PRBMDO1098-012110.1103/PhysRevB.83.035110], suggesting it forms by an unconventional mechanism. Co doping suppresses the CDW, paralleling the behavior of antiferromagnetism in iron-based superconductors. Our study demonstrates that pnictide superconductors can exhibit competing CDW order, which may be the origin of T_{c} suppression in this system.

Structural, electronic, optical and lattice dynamic properties of the different WO3 phases: First-principle calculation

First-principle calculations were performed to investigate the structural, electronic, optical, and lattice dynamic properties of the cubic (c), hexagonal (h), tetragonal (α), orthorhombic (β), monoclinic (γ) and triclinic (δ) WO3 phases. Based on the analysis of structures and relative energies, we noted that the asymmetric distortion of WO6 octahedron was mainly responsible for the enhanced crystal stability of α-, β-, γ- and δ-WO3, while the titling of WO6 octahedron played a secondary effect. The results of the calculated dipole moment indicated that the phase transformation from β-to γ-WO3 induced the change of antiferroelectricity-ferroelectricity, which was contributed to the octahedral distortion along x and y directions. The band gaps calculated by PBE0 hybrid function were 1.562, 1.943 and 2.244eV for c-, h- and α-WO3. The increasing of the band gaps were contributed to the decreasing valence band width due to the expanded octahedral volume. Importantly, the distortion of the WO6 octahedron lifted the level of 5dxy states at bottommost of the conduction band, which much enlarged the band gaps of β-, γ- and δ-WO3 to 2.974, 2.881 and 2.813eV. The calculated optical properties gave the decreasing static dielectric constants ε1(0) of 8.9, 7.6, 7.3, 5.6, 5.3 and 5.1 for c-, h-, α-, β-, γ-, and δ-WO3, respectively, which were agreement with the band gaps. The lattice dynamic properties presented unstable structures of c-, h- and α-WO3, and stable structures of β-, γ-, and δ-WO3. The instability mainly originated from the stretching vibrations of OWO bonds with few contributions by the blending vibrations of WO bonds. IR and Raman spectra were discussed and compared with experimental data, which confirmed the reasonability of the calculated results. The analyzed vibrational modes with different frequencies at Gamma point noted that the grid of WO3 presented “soft” characteristic and the WO6 octahedron was “rigid”.

Rapid sintering of ceramic solid electrolytes LiZr2(PO4)3 and Li1.2Ca0.1Zr1.9(PO4)3 using a microwave sintering process at low temperatures

NASICON-type LiZr2(PO4)3 (LZP) has a high bulk ionic conductivity (∼10−4 S cm−1) and a wide electrochemical stability window between 0 and 5.5 V. However, LZP suffers from a problem in which its desirable rhombohedral phase transforms to the triclinic phase (∼10−8 S cm−1) below 50 °C. In this work, we reported a rhombohedral-structured LiZr2(PO4)3 (α-LZP) at room temperature with a high total Li-ion conductivity (2.8 × 10−6 S cm−1) by a microwave sintering process. Moreover, this approach realized a drastic reduction in the sintering temperature from the typical 1200 °C–900 °C with only a 4 h holding time. Using this approach, we also demonstrated the successful synthesis of Ca-incorporated Li1.2Ca0.1Zr1.9(PO4)3 (LCZP) with an ionic conductivity of 1.7 × 10−5 S cm−1.

Peculiarities of Phase Formation in TbBO3 during Isothermal Annealing

Detailed X-ray studies of changes in the structure of terbium orthoborate TbBO3 in the course of successive high-temperature isothermal annealing of the initial mixture in the form of an amorphous precursor and in the form of a homogenized mixture of microcrystalline Tb7O12 and B2O3 powders have been carried out. It is shown that the formation of TbBO3 crystals in both cases occurs through the formation of intermediate two-phase states. Particularly, the triclinic high-temperature ν-TbBO3 phase is formed already at the first crystallization stages (about 600°C) almost simultaneously with the equilibrium vaterite modification of TbBO3 (sp. gr. P63/mmc) during low-temperature annealing of an amorphous precursor, which transforms into the vaterite modification at an annealing temperature of 850°C. The phase of monoclinic terbium trioxoborate TbB3O6 is formed at the first stages of phase formation (about 800°C) almost simultaneously with the vaterite phase when the feedstock is annealed in the form of a homogenized mixture of microcrystalline Tb7O12 and B2O3 powders. It also transforms into the vaterite modification at an annealing temperature of 950°, which is preserved up to the highest annealing temperatures in the experiment (1200°C). A hypothetical explanation of the formation of such two-phase states during low-temperature annealing of the feedstock and their disappearance at higher annealing temperatures is proposed.

Suppression of Electrochemically Driven Phase Transitions in Nanostructured MoS2 Pseudocapacitors Probed Using Operando X-ray Diffraction.

Pseudocapacitors with nondiffusion-limited charge storage mechanisms allow for fast kinetics that exceed conventional battery materials. It has been demonstrated that nanostructuring conventional battery materials can induce pseudocapacitive behavior. In our previous study, we found that assemblies of metallic 1T MoS2 nanocrystals show faster charge storage compared to the bulk material. Quantitative electrochemistry demonstrated that the current response is capacitive. In this work, we perform a series of operando X-ray diffraction studies upon electrochemical cycling to show that the high capacitive response of metallic 1T MoS2 nanocrystals is due to suppression of the standard first-order phase transition. In bulk MoS2, a phase transition between 1T and triclinic phases (Li xMoS2) is observed during lithiation and delithiation in both the galvanostatic traces (as distinctive plateaus) and the X-ray diffraction patterns with the appearance of the additional peaks. MoS2 nanocrystal assemblies, on the other hand, show none of these features. We hypothesize that the reduced MoS2 crystallite size suppresses the first-order phase transition and gives rise to solid solution-like behavior, potentially due to the unfavorable formation of nucleation sites in confined spaces. Overall, we find that nanostructuring MoS2 suppresses the 1T-triclinic phase transition and shortens Li-ion diffusion path lengths, allowing MoS2 nanocrystal assemblies to behave as nearly ideal pseudocapacitors.

High-Temperature Crystal Chemistry of α-, β-, and γ-BiNbO4 Polymorphs.

Thermal behavior of the orthorhombic (α) and triclinic (β) polymorphs of BiNbO4 was studied by the methods of high-temperature powder X-ray diffraction (HTPXRD) and differential scanning calorimetry (DCS) in the temperature range 25-1200 °C. The study revealed the sequence of thermal phase transformations and the new high-temperature modification, γ-BiNbO4, which was formed above 1001 °C and existed up to the melting temperature of BiNbO4. The incongruent melting of BiNbO4 was characterized by the formation of the cubic phase with the approximate composition Bi3NbO7. The HTPXRD method was used in this study to evaluate thermal deformations and to calculate thermal-expansion coefficients (TEC) of the three modifications of BiNbO4 (α, β, and γ). The average volumetric TECs of these three modifications were in the range 19-36 × 10-6 °C-1. The triclinic phase β-BiNbO4 demonstrated the highest anisotropy of thermal expansion. α-BiNbO4 was characterized by the minimal TEC and anisotropy, which indicated its greatest stability. The crystal structure of γ-BiNbO4 was determined at 1100 °C using powder data and was refined using the Rietveld method (the α-LaTaO4 structural type, the space group Cmc21, a = 3.95440(3) Å, b = 15.0899(1) Å, c = 5.65524(5) Å, V = 337.458(5) Å3, Rwp = 4.82, RBragg = 3.61%). The methods of thermal analysis and high-temperature powder X-ray diffraction revealed that, during the heating, bismuth orthoniobate underwent the following sequence of phase transitions: α-BiNbO4 → γ-BiNbO4 → β-BiNbO4 and β-BiNbO4 → γ-BiNbO4 → β-BiNbO4 or, at slow heating, β-BiNbO4 → (α-BiNbO4) → γ-BiNbO4 → β-BiNbO4, where γ-BiNbO4 is the high-temperature phase of bismuth orthoniobate.

Enhancement of vanadium oxide doped Eu+3 for gas sensor application

Thin films of vanadium oxide nanoparticles doped with different concentrations of europium oxide (2, 4, 6, and 8) wt % are deposited on glass and Si substrates with orientation (111) utilizing by pulsed laser deposition technique using Nd:YAG laser that has a wavelength of 1064 nm, average frequency of 6 Hz and pulse duration of 10 ns. The films were annealed in air at 300 °C for two hours, then the structural, morphological and optical properties are characterized using x-ray diffraction (XRD), Field emission scanning electron microscopy (FESEM) and UV-Vis spectroscopy respectively. The X-ray diffraction results of V2O5:Eu2O3 exhibit that the film has apolycrystalline monoclinic V2O5 and triclinic V4O7 phases. The FESEM image shows a homogeneous pattern and confirms the formation of uniform nanostructures on the glass substrate. The type of the particle found nanoparticles with different doping concentrations of Eu2O3. The optical energy gap increases with the increase of doping concentration and it varies from 2.67 eV to 2.71 eV. The prepared thin films are used to fabricate sensor against nitrogen dioxide gas. The dependence of sensitivity and response time on doping ratio and operation temperature of gas sensors has been studied, the maximum sensitivity was about 100%, the response time is equal to 24s and recovery time 16s for V2O5 doped 2% Eu2O3 at 50 °C.

On the structural stability of crystalline ceria phases in undoped and acceptor-doped ceria materials under in situ reduction conditions.

The reduction of pure and Sm-doped ceria in hydrogen has been studied by synchrotron-based in situ X-ray diffraction to eventually prove or disprove the presence of crystalline cerium hydride (CeH x ) phases and the succession of potential structural phase (trans)formations of reduced cerium oxide phases during heating-cooling cycles up to 1273 K. Despite a recent report on the existence of bulk and surface CeH x phases during reductive treatment of pure CeO2 in H2, structural analysis by Rietveld refinement as well as additional 1H-NMR spectroscopy did not reveal the presence of any crystalline CeH x phase. Rather, a sequence of phase transformations during the re-cooling process in H2 has been observed. In both samples, the reduced/defective fluorite lattice undergoes at first a transformation into a bixbyite-type lattice with a formal stoichiometry Ce0.58 3+Ce0.42 4+O1.71 and Sm0.15 3+Ce0.39 3+Ce0.46 4+O1.73, before a transformation into rhombohedral Ce7O12 takes place in pure CeO2. This phase is clearly absent for the Sm-doped material. Finally, a triclinic Ce11O20 phase appears for both materials, which can be recovered to room temperature, and on which a phase mixture of bixbyite Ce0.66 3+Ce0.34 4+O1.67, rh-Ce0.60 3+Ce0.40 4+O1.70 and tri-Ce0.48 3+Ce0.52 4+O1.76 (for pure CeO2) or bixbyite Sm0.15 3+Ce0.47 3+Ce0.38 4+O1.69 and tri-Sm0.15 3+Ce0.31 3+Ce0.54 4+O1.77 (for Sm-doped CeO2) prevails. The absence of the rhombohedral phase indicates that Sm doping leads to the stabilization of the bixbyite phase over the rhombohedral one at this particular oxygen vacancy concentration. It is worth noting that recent work proves that hydrogen is indeed incorporated within the structures during the heat treatments, but under the chosen experimental conditions it has apparently no effect on the salient structural principles during reduction.

TEM Microstructural Evolution and Formation Mechanism of Reaction Layer for 22MnB5 Steel Hot-Dipped in Al–10% Si

Microstructural evolution and formation mechanism of reaction layer for 22MnB5 steel hot-dipped in Al–10Si (in wt %) alloy was investigated. The microstructural identification of the reaction layer was characterized via transmission electron microscopy and electron backscatter diffraction. In addition, the formation mechanisms of the phases were discussed with vertical section (isopleth) of the (Al–Si–Fe) ternary system. The solidified Al–Si coating layer consisted of three phases of Al, Si, and τ5 (Al8Fe2Si). The reaction layer on the Al–Si coating layer side is a fine τ5 phase (Al8Fe2Si) of 5 μm thickness. The layer on the steel side consisted of an η phase (Fe2Al5) of thickness of 500 nm or less. τ1 (Al2Fe3Si3, triclinic) phase of 200-nm-thickness was formed in the η phase, and κ phase (Fe3AlC) of 40–50 nm thickness was formed between η phase and steel. The τ5 phase was formed by isothermal solidification at 690 °C in the liquid Al–10 wt % Si when 3.73–29.0 wt % of Fe was dissolved from the boron steel into the Al–Si liquid bath. It was considered that the η phase was formed by the diffusion reaction of Al, Si, and Fe between τ5 and ferrite steel. κ (Fe3AlC) phase was formed by the reaction of the carbon, which is barely employed in η and τ phases, and diffused Al.

Microstructure and phase transformations of silica glass and vanadium oxide by severe plastic deformation via high-pressure torsion straining

Plastic deformation of oxides is generally difficult due to their hard and brittle nature resulting from their covalent or ionic bondings. In this study, severe plastic deformation was successfully introduced in SiO2 glass and crystalline VO2 by high-pressure torsion (HPT). SiO2 glass transformed to a denser amorphous phase without showing any crystallization after processing in the temperature range of 300–723 K. Crystalline VO2 transformed from the coarse-grained monoclinic phase to a nanograined triclinic phase with insulating electrical properties by plastic strain effect. These findings indicate the potential of HPT process in tuning the crystal structure of optical and electrical oxides.

Characterization of Crystalline Cu<sub>2</sub>SnS<sub>3</sub> Synthesized via Low Temperature Solvothermal Method

Cu2SnS3 flower-like nanoparticles consisted of nanoflakes were successfully produced using solvothermal technique at 120°C for 12 h. The precursors used in this process were CuCl2.2H2O, SnCl2.2H2O, and thiourea with dimethyl sulfoxide (DMSO) as a solvent. The results showed that the as-obtained product was triclinic Cu2SnS3 with secondary phases CuS and SnS, and after annealing process, a pure triclinic was obtained. The crystal structure of the obtained samples was investigated by X-ray diffraction which assured the obtained data and the average crystallite size of as-prepared and annealed CTS was found to be 18.42 and 31.6 nm, respectively. Scanning electron microscopy (SEM) showed that the surface morphology has the shape of flower like consisted of nanoflakes. The transmission electron microscope (TEM) displayed the formation of sheets. The band gap was measured using UV-Vis absorption spectroscopy and found that the as-prepared sample has 1.97 eV band gap, which obtained for various phases CuS and SnS. However, this band was changed to 1.26 eV upon the heat treatment at 500°C for 10 minutes. This value of Eg is comparable with the value reported by other literatures.

Graphene-induced bi-tilted two-component smectic CG phase with bulk ferroelectricity in hydrogen-bonded dimer liquid crystals

Nanocomposites of the hydrogen-bonded dimeric liquid crystal heptyloxybenzoic acid (7OBA) with admixture of graphene flakes (GFs) are investigated with microtexture polarization analysis for new effects in their electro-optical behavior and characterized with Raman spectroscopy. In the nanocomposite with GF concentration of 3 × 10−4 wt% we establish as lowest-temperature LC state the triclinic smectic CG phase featuring chirality and ferroelectricity, previously found only in 7OBA nanocomposites with carbon nanotubes and in large banana-like bent-core molecules even though pure 7OBA is typically achiral. In the present study we find the CG phase manifested as two detached sub-structures with a smooth transition between them. For the appearance of the CG phase with its substructures denoted as CGcl and CGln we propose an explanation based on the π–π interaction of the 7OBA molecules with graphene and different coupling combinations of the clinic and leaning degree of freedom of the doubly tilted molecular geometry in the CG ordering. We examine the response of these structures to electric field, and demonstrate the presence in both of them of ferroelectric polarization with polar vector pointing out of the smectic layer planes. Molecular models are proposed for the 7OBA ordering in the substructures CGcl and CGln which are supported by the Raman spectroscopy results. The dispersion of the GFs in the nanocomposite bulk is confirmed by scanning electron microscopy in the solid state.

Photodegradation of 17α-Ethynylstradiol (EE2) on Nanostructured Material of Type WO3-SBA-15

The growing contamination from the use of endocrine deregulator (EDs) has made the scientific community come worrying, because of this, studies to eliminate these pollutants in the water resources have intensified. Among the processes used, ‘Advanced Oxidative Processes’ (AOP’s) stands out, by means of heterogeneous photocatalysis for the removal of these organic pollutants present in the environment. The paper’s main point is on the synthesis of photocatalysts WO3-SBA-15 in the molar ratios Si/W (10, 25, 50, and 75) and the photocatalytic activity in the degradation process of 17α-ethynylstradiol (EE2), present in several water contaminants. The catalysts prepared were characterized by XRD, XRF, Raman, FTIR, UV-vis, adsorption, and desorption of nitrogen, and to determine the photocatalytic activity, frequency of turnover (TOF) was used. Through the characterizations, it can be observed that insertion of nanoparticles in WO3 did not cause changes in the mesoporous structure of the SBA-15 and there is presence of monoclinic, triclinic, and orthorhombic phases of WO3. The photocatalytic activity showed to be dependent on the calcination temperature and the molar ratio of the catalysts WO3-SBA-15, being the sample WO3-SBA-15(50) at 600 °C which presented the greatest degradation potential in the degradation of 17α-ethynylstradiol, with 98%, using ultraviolet radiation, for 4 h. Moreover, reuse of the catalyst did not show significant losses after 4 cycles, with 82% in EE2 degradation.

Solvated and Ferroelectric Phases of the Charge Transfer Co-Crystal TMB-TCNQ

We have successfully determined the crystal structure of both the low temperature and solvated phases of 3,3′,5,5′-tetramethylbenzidine-tetracyanoquinodimethane (TMB-TCNQ). The triclinic solvate phase was erroneously believed for years to be a polymorph of TMB-TCNQ, while using X-rays diffraction, infrared spectroscopy, and differential scanning calorimetry, we have determined the presence of crystallization of acetonitrile molecules within the structure and observed the conversion to the room temperature monoclinic phase of TMB-TCNQ upon desolvation promoted both by heating and by grinding. The nonsolvated phase undergoes inversion symmetry breaking below 160 K, giving rise to stack dimerization. A combination of the X-ray crystal structure determination, Hirschfeld surface analysis, and density functional theory calculations suggest a rather complex scenario for the first order phase transition, which implies, besides stack dimerization, molecular inclination and small increase of the degree of charge t...

A novel solid solution (K1-xNax)2Mo2O7 (0.0 ≤ x ≤ 0.3) ceramics with ultra-low sintering temperatures

The (K1-xNax)2Mo2O7 (0.0 ≤ x ≤ 0.3) ceramics with ultra-low sintering temperatures were prepared by a modified solid-state reaction method. Polyvinyl ethanol (PVA) binder was replaced by ethanol and isostatic pressing technology was used, which improved the density of the ceramics effectively and avoided the side effect of the residual PVA. When x ≤ 0.3, the (K1-xNax)2Mo2O7 ceramics are with a triclinic phase and all components can be sintered well at 450 °C. The substitution of Na+ at the K-site modified the dielectric properties of K2Mo2O7 ceramic and in particular increased the Q × f value. Among all the components, the (K0.8Na0.2)2Mo2O7 ceramic exhibited the best microwave dielectric properties with a permittivity of 8.7, a Q × f value of 50 300 GHz and a temperature coefficient of −64 ppm/°C. The (K1-xNax)2Mo2O7 (0.0 ≤ x ≤ 0.3) ceramics are very potential candidates for ultra-LTCC devices and the modified solid-state reaction method is an excellent substitution for the conventional solid-state method.

Control of Crystallinity in PbPc:C60 Blend Film and Application for Inverted Near-Infrared Organic Photodetector.

Inverted near-infrared (NIR) organic photodetectors (OPDs) are required to combine the OPDs with an n-channel silicon-based integrated circuit. NIR absorption in the 930-960 nm range is important because the intensity of solar irradiation is low in this wavelength regime. Here, we controlled the crystallinity of lead(II) phthalocyanine (PbPc) in a PbPc:C60 blendfilmto obtain NIR absorption. To form a triclinic phase responsible for NIR light absorption, a substrate was heated during fabrication and C60 was used as a templating layer, as well as an electron extraction layer, for an inverted structure. NIR absorption near 950 nm was enhanced, and the structural properties of the film changed dramatically. The OPD with enhanced NIR absorption exhibited a responsivity of 244 mA/W and an external quantum efficiency of 31.1% at a reverse bias of -3 V and 970 nm. The OPD detectivity also increased to 9.01 × 1012 and 1.36 × 1011 cm Hz1/2/W under a zero bias and a reverse bias of -3 V, respectively.

Variation of crystal structure, magnetization, and dielectric properties of Nd and Ba co‐doped BiFeO3multiferroics

AbstractMultiferroics having composition Bi0.80Nd0.20‐xBaxFeO3were prepared to investigate the effect of doping on crystal structure, magnetic, and dielectric properties. The Rietveld refinement deduces the formation of mixed structural symmetry. With larger content of Nd, crystal structure consisting of major rhombohedral R3c and minor orthorhombic Pnma has been accomplished. The fraction of rhombohedral phase has been found to increase with doping of Ba up tox= 0.10. At compositionx= 0.15, the orthorhombic phase Pnma disappears, and there is evolution of triclinic phase P1 in place of it. The mixed structure now accomplished contains ≈61% rhombohedral R3c and rest 39% triclinic P1. In solely Ba‐doped sample (ie, atx= 0.20), the fraction of rhombohedral R3c phase again rises and attains ≈92% fraction of the structure along with rest triclinic P1 phase. The M‐H loops depict enormous enhancement in magnetic properties with increasing doping of Ba. Dielectric constant (ε′) and dielectric loss (tanδ) both were found to increase with doping of Ba. The anomalies present in the dielectric constant and dielectric loss with temperature may be regarded to the hopping conduction of e−between Fe3+and Fe2+and their interaction with oxygen vacancies.

Strategy to Enhance the Luminescence of Lanthanide Ions Doped MgWO4 Nanosheets through Incorporation of Carbon Dots.

Highly fluorescent nanomaterials have shown great potential application in optics area. However, further improving their fluorescence properties especially for nanomaterials still remains a challenge. Specifically, luminescence of lanthanide (Ln) ions doped two-dimensional (2D) nanosheets is seldom studied. Herein, MgWO4:Ln3+ (Ln = Eu, Tb) nanosheets were successfully synthesized via a simple hydrothermal method, and the luminescence properties of these nanosheets are obviously improved through incorporation of fluorescent carbon dots (CDs) onto the surface of MgWO4:Ln3+ (CDs@MgWO4:Ln3+) nanosheets. The obtained MgWO4:Ln3+ samples have a 2D nanosheet morphology with triclinic phase, and the morphology and phase structure can be maintained after incorporating CDs onto the nanosheets' surface. Under the excitation of UV light, the obtained MgWO4:Ln3+ nanosheets exhibit the characteristic emission of the doping ions, and the emission intensity of CDs@MgWO4:Eu3+ and CDs@MgWO4:Tb3+ nanosheets increases 2- and 7-fold compared to the corresponding samples without incorporation of CDs, respectively. This luminescent enhancement mechanism might be due to the capturing electrons by CDs and energy transfer from CDs to luminescent Ln3+. The fluorescence enhancement through incorporation of CDs provides a simple and environment-friendly strategy for further improving luminescence property of other lanthanide ions doped nanomaterials.

Pressure-induced dimerization and valence bond crystal formation in the Kitaev-Heisenberg magnet α−RuCl3

Magnetization and high-resolution x-ray diffraction measurements of the Kitaev-Heisenberg material alpha-RuCl3 reveal a pressure-induced crystallographic and magnetic phase transition at a hydrostatic pressure of p=0.2 GPa. This structural transition into a triclinic phase is characterized by a very strong dimerization of the Ru-Ru bonds, accompanied by a collapse of the magnetic susceptibility. Ab initio quantum-chemistry calculations disclose a pressure-induced enhancement of the direct 4d-4d bonding on particular Ru-Ru links, causing a sharp increase of the antiferromagnetic exchange interactions. These combined experimental and computational data show that the Kitaev spin liquid phase in alpha-RuCl3 strongly competes with the crystallization of spin singlets into a valence bond solid.

Evolution of structure and superconductivity in Ba(Ni1−xCox)2As2

The effects of Co-substitution on Ba(Ni$_{1-x}$Co$_x$)$_2$As$_2$ ($0\leq x\leq 0.251$) single crystals grown out of Pb flux are investigated via transport, magnetic, and thermodynamic measurements. BaNi$_2$As$_2$ exhibits a first order tetragonal to triclinic structural phase transition at $T_s=137 K$ upon cooling, and enters a superconducting phase below $T_c=0.7 K$. The structural phase transition is sensitive to cobalt content and is suppressed completely by $x\geq0.133$. The superconducting critical temperature, $T_c$, increases continuously with $x$, reaching a maximum of $T_c=2.3 K$ at the structural critical point $x=0.083$ and then decreases monotonically until superconductivity is no longer observable well into the tetragonal phase. In contrast to similar BaNi$_2$As$_2$ substitutional studies, which show an abrupt change in $T_c$ at the triclinic-tetragonal boundary that extends far into the tetragonal phase, Ba(Ni$_{1-x}$Co$_x$)$_2$As$_2$ exhibits a dome-like phase diagram centered around the first-order critical point. Together with an anomalously large heat capacity jump $\Delta C_e/\gamma T\sim 2.2$ at optimal doping, the smooth evolution of $T_c$ in the Ba(Ni$_{1-x}$Co$_x$)$_2$As$_2$ system suggests a mechanism for pairing enhancement other than phonon softening.