Chemical Pressure Effect Research Articles

Physical Properties of a Frustrated Quasi-One-Dimensional NaCuFe2(VO4)3 Magnet and Effect of Chemical Pressure Induced by the Substitution of Sodium for Lithium

The structural, thermal, static magnetic, and resonance properties of the low-dimensional NaCuFe2(VO4)3 compound obtained by the solid-phase synthesis have been investigated. In the temperature range of 110–300 K, the electron spin resonance in the X band with a g factor of 2.008 has been detected. The magnetic properties of a sample with a high frustration level in the paramagnetic, antiferromagnetic, and disordered states have been examined. A shift of the Neel temperature to the high-temperature region in an external magnetic field has been observed. The origin of the disordered magnetism in NaCuFe2(VO4)3 are discussed. The features of substitution of sodium for lithium on the physical properties of the ACuFe2(VO4)3 (A = Na, Li) system have been established. It is shown that the chemical pressure changes the crystal lattice parameters, spacings between magnetic ions, and crystallite size, which is reflected in the physical properties of the material.

Effect of chemical and hydrostatic pressure on the coupled magnetostructural transition of Ni-Mn-In Heusler alloys

Ni-Mn-In magnetic shape-memory Heusler alloys exhibit generally a large thermal hysteresis at their first-order martensitic phase transition which hinder a technological application in magnetic refrigeration. By optimizing the Cu content in Ni$_2$Cu$_x$Mn$_{1.4-x}$In$_{0.6}$, we obtained a thermal hysteresis of the martensitic phase transition in Ni$_{2}$Cu$_{0.2}$Mn$_{1.2}$In$_{0.6}$ of only 6 K. We can explain this very small hysteresis by an almost perfect habit plane at the interface of martensite and austenite phases. Application of hydrostatic pressure does not reduce the hysteresis further, but shifts the martensitic transition close to room temperature. The isothermal entropy change does not depend on warming or cooling protocols and is pressure independent. Experiments in pulsed-magnetic fields on Ni$_{2}$Cu$_{0.2}$Mn$_{1.2}$In$_{0.6}$ find a reversible magnetocaloric effect with a maximum adiabatic temperature change of -13 K.

A substantial increase of Curie temperature in a new type of diluted magnetic semiconductors via effects of chemical pressure

Chemical pressure is an effective method to tune physical properties, particularly for diluted magnetic semiconductors (DMSs) of which ferromagnetic ordering is mediated by charge carriers. Via substitution of smaller Ca for larger Sr, we introduce chemical pressure on (Sr,Na)(Cd,Mn)2As2 to fabricate a new DMS material (Ca,Na)(Cd,Mn)2As2. Carriers and spins are introduced by substitutions of (Ca,Na) and (Cd,Mn), respectively. The unit cell volume reduces by 6.2% after complete substitution of Ca for Sr, suggesting a subsistent chemical pressure. Importantly, the local geometry of the [Cd/MnAs4] tetrahedron is optimized via chemical compression that increases the Mn–As hybridization leading to enhanced ferromagnetic interactions. As a result, the maximum Curie temperature (TC) is increased by about 50% while the maximum saturation moment increases by over 100% from (Sr,Na)(Cd,Mn)2As2 to (Ca,Na)(Cd,Mn)2As2. The chemical pressure estimated from the equation of state is equal to an external physical pressure of 3.6 GPa.

Effect of Indium doping on the superconductivity of layered oxychalcogenide La2O2Bi3Ag1-xInxS6

We report on the substitution effect of Indium (In) at the Ag site of layered oxychalcogenide La2O2Bi3Ag1−xInxS6. The Tc decreases with increasing In concentration. A hump in the normal state resistivity at an anomaly temperature (T*) near 180 K was observed for all the samples. The anomaly in the resistivity at T* is indicating the possible occurrence of a charge-density-wave (CDW) transition. The T* does not markedly change by In doping. The x dependence of Seebeck coefficient suggests that carrier concentration does not change by In doping. The EDX analysis indicates small amount of Bi deficiency, which suggests that the Bi site is slightly substituted by In. The CDW transition is robust against the In substitution at Ag site, while Tc is decreasing due to the Bi site substitution by In. On the basis of those analyses, we propose that the suppression of superconductivity in the In-doped La2O2Bi3Ag1-xInxS6 system is caused by negative in-plane chemical pressure effect and partial substitution of In for the inplane Bi site.

Universal relationship between low-energy antiferromagnetic fluctuations and superconductivity in BaFe2(As1−xPx)2

To identify the key parameter for optimal superconductivity in iron pnictides, we measured the $^{31}$P-NMR relaxation rate on BaFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$ ($x = 0.22$ and 0.28) under pressure and compared the effects of chemical substitution and physical pressure. For $x = 0.22$, structural and antiferromagnetic (AFM) transition temperatures both show minimal changes with pressure up to 2.4~GPa, whereas the superconducting transition temperature $T_{\rm c}$ increases to twice its former value. In contrast, for $x=0.28$ near the AFM quantum critical point (QCP), the structural phase transition is quickly suppressed by pressure and $T_{\rm c}$ reaches a maximum. The analysis of the temperature-dependent nuclear relaxation rate indicates that these contrasting behaviors can be quantitatively explained by a single curve of the $T_{\rm c}$ dome as a function of Weiss temperature $\theta$, which measures the distance to the QCP. Moreover, the $T_{\rm c}$-$\theta$ curve under pressure precisely coincides with that with chemical substitution, which is indicative of the existence of a universal relationship between low-energy AFM fluctuations and superconductivity on BaFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$.

Enhancement of magnetoresistance and magnetocaloric effect at room temperature in polycrystalline Pr0.8-xLaxSr0.2MnO3 (x = 0.2) compound

Abstract The effect of chemical pressure on the ferromagnetic insulator ground state and its effect on magnetotransport and magnetocaloric properties has been explored in this study. Magnetic and magnetotransport measurements reveal that the increased chemical pressure in Pr 0.6 La 0.2 Sr 0.2 MnO 3 due to the partial substitution by La in Pr 0.8 Sr 0.2 MnO 3 compound change the low-temperature insulator phase to the metallic phase. A room temperature enhancement of magnetoresistance (MR), as well as large relative cooling power (RCP), have been achieved with increasing chemical pressure in x = 0.2 compound. This enhancement of MR and magnetocaloric effect (MCE) in Pr 0.6 La 0.2 Sr 0.2 MnO 3 compound have been attributed to the short- range ferromagnetic cluster formation at higher temperature ( T > T C ) .

Effect of chemical pressure on the magnetocaloric effect of perovskite-like RCrO3 (R-Yb, Er, Sm and Y)

A comparative study of the magnetocaloric effect and refrigerant capacitance properties of orthorhombic perovskite-like isostructural compounds RCrO3 (R: Yb, Er, Y and Sm) was carried out (R cations with and without a magnetic moment and different cation size). Two scales of values can be observed at the Cr3+ and R3+ ordering temperatures. A higher magnetocaloric effect near the rare earth ion ordering temperature with values ranging from −ΔSM(TNCr) = [0.15 to 0.72] and a lower one at the Cr ordering temperature ranging from −ΔSM(TNR) = [-0.2 to 12.5]. A normal and an inverse magnetocaloric effect were observed in the low temperature regime for the case of Sm at and above the spin reorientation of the Cr ions. The evolution of −ΔSM with R3+ ion radius increase and with total paramagnetic system moment are also discussed.

Effects of chemical pressure on diluted magnetic semiconductor (Ba,K)(Zn,Mn)2As2**Project supported by the National Key R&D Program of China (Grant No. 2017YFB0405703), the Ministry of Science and Technology of China (Grant Nos. 2018YFA03057001 and 2015CB921000), and the National Natural Science Foundation of China through the Research Projects (Grant Nos. 11534016 and 61504166).

Chemical pressure induced by iso-valent doping has been widely employed to tune physical properties of materials. In this work, we report effects of chemical pressure by substitution of Sb or P into As on a recently discovered diluted magnetic semiconductor (Ba,K)(Zn,Mn)2As2, which has the record of reliable Curie temperature of 230 K due to independent charge and spin doping. Sb and P are substituted into As-site to produce negative and positive chemical pressures, respectively. X-ray diffraction results demonstrate the successful chemical solution of dopants. Magnetic properties of both K-under-doped and K-optimal-doped samples are effectively tuned by Sb- and P-doping. The Hall effect measurements do not show decrease in carrier concentrations upon Sb- and P-doping. Impressively, magnetoresistance is significantly improved from 7% to 27% by only 10% P-doping, successfully extending potential application of (Ba,K)(Zn,Mn)2As2.

Semimetal-to-semiconductor transition and charge-density-wave suppression in 1T−TiSe2−xSx single crystals

The transition metal dichalcogenide $1T$-TiSe$_2$ is a quasi-two-dimensional layered material with a phase transition towards a commensurate charge density wave (CDW) at a critical temperature T$_{c}\approx 200$K. The relationship between the origin of the CDW instability and the semimetallic or semiconducting character of the normal state, i.e., with the non-reconstructed Fermi surface topology, remains elusive. By combining angle-resolved photoemission spectroscopy (ARPES), scanning tunneling microscopy (STM), and density functional theory (DFT) calculations, we investigate $1T$-TiSe$_{2-x}$S$_x$ single crystals. Using STM, we first show that the long-range phase coherent CDW state is stable against S substitutions with concentrations at least up to $x=0.34$. The ARPES measurements then reveal a slow but continuous decrease of the overlap between the electron and hole ($e$-$h$) bands of the semimetallic normal-state well reproduced by DFT and related to slight reductions of both the CDW order parameter and $T_c$. Our DFT calculations further predict a semimetal-to-semiconductor transition of the normal state at a higher critical S concentration of $x_c$=0.9 $\pm$0.1, that coincides with a melted CDW state in TiSeS as measured with STM. Finally, we rationalize the $x$-dependence of the $e$-$h$ band overlap in terms of isovalent substitution-induced competing chemical pressure and charge localization effects. Our study highlights the key role of the $e$-$h$ band overlap for the CDW instability.

Transport Properties of Molecular Conductors (BSBS)2XF6 (X = As, Sb, and Ta): Investigation of Intermolecular Transfer Integrals in the Radical-Cationic State of Benzothienobenzothiophene-Type Semiconductors

[1]Benzoseleno[3,2-b]benzoselenophene (BSBS) derivatives are known as high-performance transistor materials. Novel and highly conductive charge-transfer (CT) complexes, (BSBS)2XF6 (X = As and Sb), have been prepared, and their transport properties are investigated. At room temperature, these complexes exhibit conductivities larger than 600 S cm–1. The chemical pressure effect originating from the counter anion size is estimated to be 0.3 kbar. The thermoelectric (TE) power is 15–17 μV K–1, demonstrating that the BSBS complexes have remarkably large transfer integrals of 310 meV, as observed in the highly conductive benzothienobenzothiophene (BTBT) complexes. From both molecular orbital calculations and experiments, it is demonstrated that the BTBT skeleton has the property of forming a very large energy bandwidth. The TE power of metallic CT complexes based on organic transistor molecules is useful for experimentally determining the intermolecular transfer integrals of charged semiconductor molecules.

Mechanical and chemical pressure effects on the AeFe2As2 (Ae = Ba, Sr, Ca) compounds: Density functional theory

We have studied the pressure-induced structural, magnetic and electronic properties of AeFe2As2 (Ae = Ba, Sr, Ca) compounds in the framework of density functional theory within the GGA-PBE method. The effects of chemical pressure generated by Sr and Ca substitutions in BaFe2As2 have been investigated. We have found a magnetic transition at the same primitive unit cell volume, around 81 Å3 for the (Ba⧹Ca)Fe2As2 compounds, which predicts a magnetic transition pressure of 12 GPa for SrFe2As2. The structural parameters of FeAs4 tetrahedra are obtained after ionic relaxation and compared with the existing experimental results. The change of these internal parameters is ascribed to pressure-induced charge transfer, i.e., as the As-Ae bonds absorb electrons, the Fe-As bonds are depleted, and thus the number of holes in the Fe-As layers increases. The changes of the density of states, the band structure and the hole- and electron- pockets of Fermi surface by pressure have been discussed.

Chemical and biological effects of low pressure N2-O2 plasma setup on polymeric materials

Abstract The effects of low-pressure RF N2-O2 plasma produced using a 13.56 MHz hollow cathode discharge system on the polymeric materials in terms of both biological and chemical sides are studied. A complete inactivation of most of the studied bacteria has occurred within 30 min. However, this treatment caused an increase of the surface hydrophilicity, as manifested by the contact angle measurements. This increase was explained by the formation of the N and O functionalities revealed by the X-ray photoelectron technique analysis. A careful analysis of the high-resolution C 1s peak has also revealed some variations on the C–C, C–O and O=C– bonds and the emergence of a new C-N bond.

Discrimination between two memory channels by molecular alloying in a doubly bistable spin crossover material.

A multistable spin crossover (SCO) molecular alloy system [Fe1-x M x (nBu-im)3(tren)](P1-y As y F6)2 (M = ZnII, NiII; (nBu-im)3(tren) = tris(n-butyl-imidazol(2-ethylamino))amine) has been synthesized and characterized. By controlling the composition of this isomorphous series, two cooperative thermally induced SCO events featuring distinct critical temperatures (T c) and hysteresis widths (ΔT c, memory) can be selected at will. The pristine derivative 100As (x = 0, y = 1) displays a strong cooperative two-step SCO and two reversible structural phase transitions (PTs). The low temperature PTLT and the SCO occur synchronously involving conformational changes of the ligand's n-butyl arms and two different arrangements of the AsF6 - anions [T1c = 174 K (ΔT1c = 17 K), T2c = 191 K (ΔT2c = 23 K) (scan rate 2 K min-1)]. The high-temperature PTHT takes place in the high-spin state domain and essentially involves rearrangement of the AsF6 - anions [TPTc = 275 K (ΔTPTc = 16 K)]. This behavior strongly contrasts with that of the homologous 100P [x = 0, y = 0] derivative where two separate cooperative one-step SCO can be selected by controlling the kinetics of the coupled PTLT at ambient pressure: (i) one at low temperatures, T c = 122 K (ΔT c = 9 K), for temperature scan rates (>1 K min-1) (memory channel A) where the structural modifications associated with PTLS are inhibited; (ii) the other centered at T c = 155 K (ΔT c = 41 K) for slower temperature scan rates ≤0.1 K min-1 (memory channel B). These two SCO regimes of the 100P derivative transform reversibly into the two-step SCO of 100As upon application of hydrostatic pressure (ca. 0.1 GPa) denoting the subtle effect of internal chemical pressure on the SCO behavior. Precise control of AsF6 - ↔ PF6 - substitution, and hence of the PTLT kinetics, selectively selects the memory channel B of 100P when x = 0 and y ≈ 0.7. Meanwhile, substitution of FeII with ZnII or NiII [x ≈ 0.2, y = 0] favors the low temperature memory channel A at any scan rate. This intriguing interplay between PT, SCO and isomorphous substitution was monitored by single crystal and powder X-ray diffractometries, and magnetic and calorimetric measurements.

Magnetic properties of RCoO3 cobaltites (R = La, Pr, Nd, Sm, Eu). Effects of hydrostatic and chemical pressure

We have investigated the temperature dependence of the magnetic susceptibility χ(T) of rare-earth cobaltites RCoO3 (R = La, Pr, Nd, Sm, Eu) in the temperature range 4.2–300 K and also the influence of hydrostatic pressure up to 2 kbar on their susceptibility at fixed temperatures T = 78 and 300 K. The specific dependence χ(T) observed in LaCoO3 and the anomalously large pressure effect (dln χ/dP ∼−100 Mbar−1 for T = 78 K) are analyzed in the framework of a two-level model with energy levels difference Δ. The ground state of the system is assumed to be nonmagnetic with the zero spin of Co3+ ions, and magnetism at a finite temperature is determined by the excited magnetic spin state. The results of the analysis, supplemented by theoretical calculations of the electronic structure of LaCoO3, indicate a significant increase in Δ with a decrease in the unit cell volume under the hydrostatic pressure. In the series of RCoO3 (R = Pr, Nd, Sm, Eu) compounds, the volume of crystal cell decreases monotonically due to a decrease in the radius of R3+ ions. This leads to an increase in the relative energy Δ of the excited state (the chemical pressure effect), which manifests itself in a decrease in the contribution of cobalt ions to the magnetic susceptibility at a fixed temperature, and also in a decrease in the hydrostatic pressure effect on the susceptibility of RCoO3 compounds, which we have observed at T = 300 K.

Atomistic simulation study of the ferroelectric and paraelectric phases of the hexagonal RMnO3 (R = Er, Tm, Yb, Lu, In and Sc) family

A set of interatomic potential parameters for modeling the hexagonal manganites h-RMnO3 (R = Er, Tm, Yb, In and Sc) was developed. The structural properties in the ferroelectric (FE) phase were studied, that is, the lattice parameters, unit cell volume and interatomic distances for all manganites. The difference between the calculated and experimental lattice parameters was less than 0.5% for all members of the h-RMnO3 family. An extensive study of the chemical and hydrostatic pressure effects for h-RMnO3 was performed via atomistic simulation and compared with the experimental results and those obtained from density functional theory. Chemical pressure produces different effects depending on the R atom of the hexagonal manganite family, and the lattice parameters decrease with an increase in the hydrostatic pressure. The effects on the ferroelectric distortion of these materials under chemical and hydrostatic pressure were analyzed, and the structural properties of the h-RMnO3 family in its paraelectric (PE) phase were determined on the basis of the interatomic potential derived for the ferroelectric phase.

Pressure effect in the Kondo semimetal CeRu4Sn6 with nontrivial topology**Project supported by the Ministry of Science and Technology of China (Grant Nos. 2015CB921303 and 2017YFA0303103), the National Natural Science Foundation of China (Grant Nos. 11474332 and 11774404), and the Chinese Academy of Sciences through the Strategic Priority Research Program (Grant No. XDB07020200).

Kondo semimetal CeRu4Sn6 is attracting renewed attention due to the theoretically predicted nontrivial topology in its electronic band structure. We report hydrostatic and chemical pressure effects on the transport properties of single- and poly-crystalline samples. The electrical resistivity ρ(T) is gradually enhanced by applying pressure over a wide temperature range from room temperature down to 25 mK. Two thermal activation gaps estimated from high- and low-temperature windows are found to increase with pressure. A flat ρ(T) observed at the lowest temperatures below 300 mK appears to be robust against both pressure and field. This feature as well as the increase of the energy gaps calls for more intensive investigations with respect to electron correlations and band topology.

Mixed A-Cation Perovskites for Solar Cells: Atomic-Scale Insights Into Structural Distortion, Hydrogen Bonding, and Electronic Properties

Hybrid lead halide perovskites containing a mixture of A-site cations such as the formamidinium (CH(NH2)2+, FA) and the smaller cesium (Cs+) cations have attracted considerable interest due to their improved stability and solar cell performance. However, the structural changes at the atomic scale and modifications to the optoelectronic properties of these mixed cation perovskites are not fully understood. Here, we investigate the FA1–xCsxPbI3 (x ≤ 0.25) system using a combination of static and dynamic ab initio computational methods. We find that the incorporation of Cs+ cations into the parent FAPbI3 structure induces a chemical pressure or lattice strain effect through Cs/FA ion size mismatch resulting in structural distortion and stronger FA-iodide (N–H···I) hydrogen bonding interactions. The dynamic tilting of PbI6 octahedra and the rotational motion of FA cations are also suppressed, which leads to symmetry-breaking of the lattice. Such symmetry-breaking distortions of the Pb/I lattice give rise to a...

Effect of chemical pressure on physical properties of antiferromagnetic Kondo lattice Ce2Ni3Ge5

We have studied the enhancement of Kondo scattering in the heavy fermion compounds Ce2Ni3Ge1-xSix5 by measuring electrical resistivity ρ(T), magnetic susceptibility χ(T), isothermal magnetization M(B), specific heat CP(T) and magnetoresistance. All the members of this series crystallize in U2Co3Si5-type orthorhombic structure with space group Ibam. Magnetic susceptibility and specific heat data show that two step antiferromagnetic transitions at x = 0 reduce to a single transition at x = 0.15 followed by a complete suppression of antiferromagnetic ordering at x = 0.20. Positive magnetoresistance below the transition temperature confirms that the transitions are antiferromagnetic up to x = 0.15 and the negative magnetoresistance in the paramagnetic region implies the freezing out of spin-flip scattering in Kondo compounds under the application of magnetic field. Kondo temperature, estimated from paramagnetic Curie temperature and magnetoresistance data, increases with increasing x in Ce2Ni3Ge1-xSix5 for x⩽0.80. The γ values also indicate that the Kondo temperature enhances with increasing x after the suppression of antiferromagnetic ordering. Maximum γ value (∼76 mJ mol−1 K−2 Ce−1) is observed at x = 0.20 which indicates that the heavy fermion behavior is more pronounced at the magnetic-nonmagnetic boundary. A significant resistivity increase at a particular magnetic field is observed for T = 3.5 K, 3.8 K and 4 K (x = 0 sample) and it is observed for T = 3 K (x = 0.15 sample).

Superconductivity in the 2-Dimensional Homologous Series AMm Bi3 Q5+m (m=1, 2) (A=Rb, Cs; M=Pb, Sn; Q=Se, Te).

Superconductivity in the two-dimensional AMm Bi3 Q5+m family of semimetals is reported. The AMBi3 Te6 (m=1) and AM2 Bi3 Te7 (m=2) members of the homologous series with A=Rb, Cs and M=Pb, Sn undergo a bulk superconducting transition ranging from 2.7 to 1.4 K depending on the composition. The estimated superconducting volume fraction is about 90 %. Superconducting phase diagrams as a function of chemical pressure are constructed for the solid solution products of each member of the homologous series, AMBi3-x Sbx Te6-y Sey and AM2 Bi3-x Sbx Te7-y Sey (0≤x≤3 or 0≤y≤2). The structural flexibility of the ternary AMm M'3 Te5+m semiconducting homology to form isostructural analogues with a variety of metals, M=Pb, Sn; M'=Bi, Sb, gives access to a large number of electronic configurations and superconductivity due to chemical pressure effects.

Thermodynamic Picture of Dimer-Mott Organic Superconductors Revealed by Heat Capacity Measurements with External and Chemical Pressure Control

This article reviews and discusses the thermodynamic properties of dimer-Mott-type molecular superconductive compounds with (BEDT-TTF)2X composition, where BEDT-TTF is bis(ethylenedithio)tetrathiafulvalene and X denotes counter-anions, respectively. We focus mainly on the features occurring in the κ-type structure in which the d-wave superconductive phase appears depending on the Coulomb repulsion U and the bandwidth W, which is tunable by external and chemical pressures. First, we report the high-pressure ac (alternating current) calorimetry technique and experimental system constructed to measure single-crystal samples of molecule-based compounds to derive low-temperature thermodynamic parameters. Using extremely small resistance chips as a heater and a thermometer allows four-terminal detection of an accurate temperature and its oscillation in the sample part with sufficient sensitivity. From the analyses of the temperature dependence of the ac heat capacity of κ-(BEDT-TTF)2Cu(NCS)2 under external pressures, we discuss the changes in the peak shape of the thermal anomaly at the superconductive transition temperature Tc at various external pressures p. The rather sharp peak in CpT−1 at Tc = 9.1 K with a strong coupling character at ambient pressure is gradually reduced to weaker coupling as the pressure increases to 0.45 GPa concomitant with suppression of the transition temperature. This feature is compared with the systematic argument of the chemical–pressure effect on the basis of thermal anomalies around the superconductive transition of κ-(BEDT-TTF)2X compounds and other previously studied typical dimer-Mott 2:1 compounds. Finally, the discussion is extended to the chemical pressure effect on the normal state electronic heat capacity coefficient γ obtained by applying magnetic fields higher than Hc2 and the residual γ*, which remains in the superconductive state due to the induced electron density of states around the node structure. From the overall arguments with respect to both chemical and external pressures, we suggest that a crossover of the electronic state inside the superconductive phase occurs and the coupling strength of electron pairs varies from the electron correlation region near the metal-insulator boundary to the band picture region.