In-plane Chemical Pressure Research Articles

Stabilization of Bi vibration along c-axis in BiS2-based layered compounds La(O,F)Bi(S,Se)2 by Se substitution

BiCh2-based layered compounds have been extensively studied as potential thermoelectric and unconventional superconducting materials. For both functionalities, in-plane chemical pressure effects improve their thermoelectric or superconducting properties. In this study, we investigate the effects of in-plane chemical pressure on atomic vibrations of Bi by analyzing lattice specific heat measured at T = 1.9–300 K with multiple Debye and Einstein models for thermoelectric LaOBi(S,Se)2 and superconducting LaO0.5F0.5Bi(S,Se)2. We reveal that in-plane chemical pressure enhances the oscillator number of the Einstein mode corresponding to large-amplitude Bi vibration along the c-axis in both the systems.

Lattice Anharmonicity in BiS2-Based Layered Superconductor RE(O,F)BiS2 (RE = La, Ce, Pr, Nd)

We studied Gr\"uneisen parameter ({\gamma}G) to investigate lattice anharmonicity in a layered BiS2-based superconductor system REO1-xFxBiS2 (RE = La, Ce, Pr, Nd), where in-plane chemical pressure was tuned by substituting the RE elements. With increasing in-plane chemical pressure, bulk modulus remarkably increases, and a high {\gamma}G is observed for RE = Nd. The dependence of {\gamma}G on in-plane chemical pressure exhibits a good correlation with Tc, and a higher Tc is achieved when {\gamma}G is large for RE = Nd. In addition, {\gamma}G shows a slight decrease by a decrease of F concentration (x) in REO1-xFxBiS2. Our results show that the anharmonic vibration of Bi along the c-axis is present in REO1-xFxBiS2, and the enhancement of the anharmonicity is positively linked to superconducting Tc and pairing mechanisms.

High-pressure effects on superconducting properties and crystal structure of Bi-based layered superconductor La2O2Bi3Ag0.6Sn0.4S6

The effects of pressure on the superconducting properties of a Bi-based layered superconductor La2O2Bi3Ag0.6Sn0.4S6, which possesses a four-layer-type conducting layer, have been studied through the electrical resistance and magnetic susceptibility measurements. The crystal structure under pressure was examined using synchrotron x-ray diffraction at SPring-8. In the low-pressure regime, bulk superconductivity with a transition temperature T c of ∼4.5 K was induced by pressure, which was achieved by in-plane chemical pressure effect owing to the compression of the tetragonal structure. In the high-pressure regime above 6.4 GPa, a structural symmetry lowering was observed, and superconducting transitions with a T c ∼ 8 K were observed. Our results suggest the possible commonality on the factor essential for T c in Bi-based superconductors with two-layer-type and four-layer-type conducting layers.

Bulk Superconductivity Induced by Se Substitution in Self-Doped BiCh2-Based Compound CeOBiS2−xSex

We report the Se substitution effects on the crystal structure, superconducting properties, and valence states of self-doped BiCh2-based compound CeOBiS2-xSex. Polycrystalline CeOBiS2-xSex samples with x = 0-1.0 were synthesized. For x = 0.4 and 0.6, bulk superconducting transitions with a large shielding volume fraction were observed in magnetic susceptibility measurements; the highest transition temperature (Tc) was 3.0 K for x = 0.6. A superconductivity phase diagram of CeOBiS2-xSex was established based on Tc estimated from the electrical resistivity and magnetization measurements. The emergence of superconductivity in CeOBiS2-xSex was explained with two essential parameters of in-plane chemical pressure and carrier concentration, which systematically changed with increasing Se concentration.

Superconductivity in La2O2M4S6 -Type Bi-based Compounds: A Review on Element Substitution Effects

Since 2012, layered compounds containing Bi-Ch (Ch: S and Se) layers have been extensively studied in the field of superconductivity. The most-studied system is BiS2-based superconductors with two-layer-type conducting layers. Recently, superconductivity was observed in La2O2M2S6 (M = metals), which contains four-layer-type conducting layers. The four-layer-type Bi-based superconductors are new systems in the family of Bi-based superconductors; we can expect further development of Bi-based layered superconductors. In this review article, we summarize the progress of synthesis, structural analysis, investigations on superconducting properties, and material design of the four-layer-type Bi-based superconductors. In-plane chemical pressure is the factor essential for the emergence of bulk superconductivity in the system. The highest Tc of 4.1 K was observed in Rare Earth elements (RE) substituted La2-xRExO2Bi3Ag0.6Sn0.4S6.

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.

Material Development and Physical Properties of BiS2-Based Layered Compounds

In 2012, we discovered superconductivity in Bi4O4S3 and LaO1-xFxBiS2, whose crystal structures are composed of alternate stacks of a BiS2-type conduction layer and a blocking layer. Since the discovery, many related superconductors and functional materials have been synthesized. Furthermore, the possibility of unconventional superconductivity has been proposed in recent theoretical and experimental studies. In addition, notable correlations between local structure and physical properties have been revealed in the BiS2-based systems. In this review article, we summarize the material development (probably all the compounds) and physical properties of BiS2-related materials developed in the last six years. The key parameters for the emergence of bulk superconductivity in the BiS2-based compounds are carrier doping to the parent phase (band insulator), pressure-induced structural transition, in-plane chemical pressure, and the suppression of in-plane disorder. We systematically review those parameters and the method of designing new superconductors with BiS2 layers. In addition, studies of BiS2-based compounds as thermoelectric materials are briefly reviewed.

Suppression of the magnetic order in CeFeAsO: Nonequivalence of hydrostatic and in-plane chemical pressure

We present a detailed investigation of the electronic properties of CeFeAsO under chemical (As by P substitution) and hydrostatic pressure by means of in-house and synchrotron M\"ossbauer spectroscopy. The Fe magnetism is suppressed due to both pressures and no magnetic order was observed above a P-substitution level of 40% or 5.2 GPa hydrostatic pressure. We compared both pressures and found that the isovalent As by P substitution change the crystallographic and electronic properties differently than hydrostatic pressure.

Single Crystal Growth and Superconducting Properties of Antimony-Substituted NdO0.7F0.3BiS2

Antimony (Sb) substitution of less than 8% was examined on a single crystal of a layered superconductor NdO0.7F0.3BiS2. The superconducting transition temperature of the substituted samples decreased as Sb concentration increased. A lattice constant along the c-axis showed a large decrease compared with that along the a-axis. Since in-plane chemical pressure monotonically decreased as Sb concentration increased, the suppression of the superconductivity is attributed to the decrease in the in-plane chemical pressure.

Bulk Superconductivity Induced by Se Substitution in BiCh2-Based Layered Compounds Eu0.5Ce0.5FBiS2−xSex

We report the effect of Se substitution on the crystal structure and superconductivity of BiCh2-based (Ch: S, Se) layered compounds Eu0.5Ce0.5FBiS2-xSex (x = 0-1). Crystal structure analysis showed that both lattice constants, a and c, increased with increasing x, which is different from the related La-doped system, such as Eu0.5La0.5FBiS2-xSex. This is due to Se substitution for both in-plane and out-of-plane Ch sites in the present Ce-doped system. Zero resistivity was observed for x = 0.2-1 above 2 K. Superconducting properties of Eu0.5Ce0.5FBiS2-xSex was investigated by magnetic susceptibility, and the highest superconducting transition temperature of 3.5 K was obtained for x = 0.6 with a large shielding volume fraction. The emergence of bulk superconductivity and metallic conductivity can be qualitatively described in terms of increased in-plane chemical pressure effect. A magnetic anomaly below 8 K, probably because of ferromagnetic order of magnetic moment of Ce3+ ions, coexists with bulk superconductivity in BiCh2 layer. Since the effect of Se substitution on the magnetic transition temperature is ignorable, we suggest that the coupling between the magnetic order at the (Eu,Ce)F layer and superconductivity at the Bi(S,Se)2 layer is weak.

Synchrotron powder X-ray diffraction and structural analysis of Eu0.5La0.5FBiS2-xSex

Eu0.5La0.5FBiS2-xSex is a new BiS2-based superconductor system. In Eu0.5La0.5FBiS2-xSex, electron carriers are doped to the BiS2 layer by the substitution of Eu by La. Bulk superconductivity in this system is induced by increasing the in-plane chemical pressure, which is controlled by the Se concentration (x). In this study, we have analysed the crystal structure of Eu0.5La0.5FBiS2-xSex using synchrotron powder diffraction and the Rietveld refinement. The precise determination of the structural parameters and thermal factors suggest that the emergence of bulk superconductivity in Eu0.5La0.5FBiS2-xSex is achieved by the enhanced in-plane chemical pressure and the decrease in in-plane disorder.

Bulk Superconductivity Induced by In-Plane Chemical Pressure Effect in Eu0.5La0.5FBiS2−xSex

We have investigated Se substitution effect to superconductivity of an optimally-doped BiS2-based superconductor Eu0.5La0.5FBiS2. Eu0.5La0.5FBiS2-xSex samples with x = 0-1 were synthesized. With increasing x, in-plane chemical pressure is enhanced. For x = 0.6, 0.8, and 1, superconducting transitions with a large shielding volume fraction are observed in magnetic susceptibility measurements, and the highest Tc is 3.8 K for x = 0.8. From low-temperature electrical resistivity measurements, a zero-resistivity state is observed for all the samples, and the highest Tc is observed for x = 0.8. With increasing Se concentration, characteristics of electrical resistivity changes from semiconducting-like to metallic, suggesting that the emergence of bulk superconductivity is linked with the enhanced metallicity. A superconductivity phase diagram of the Eu0.5La0.5FBiS2-xSex superconductor is established. Temperature dependences of electrical resistivity show an anomalous two-step transition under high magnetic fields. Hence, the resistivity data are analyzed with assuming in-plane anisotropy of upper critical field.

ChemInform Abstract: Recent Advances in Layered Metal Chalcogenides as Superconductors and Thermoelectric Materials: Fe‐Based and Bi‐Based Chalcogenides

AbstractReview: 97 refs.

Compositional and temperature evolution of crystal structure of new thermoelectric compound LaOBiS2−xSex

We examined the crystal structure of the new thermoelectric material LaOBiS2-xSex, whose thermoelectric performance is enhanced by Se substitution, by using powder synchrotron X-ray diffraction and Rietveld refinement. The emergence of metallic conductivity and enhancement of the thermoelectric power factor of LaOBiS2-xSex can be explained with the higher in-plane chemical pressure caused by the increase of Se concentration at the in-plane Ch1 site (Ch = S, Se). High-temperature X-ray diffraction measurements for optimally substituted LaOBiSSe revealed anomalously large atomic displacement parameters (Uiso) for Bi and Ch atoms in the BiCh2 conduction layers. The anisotropic analysis of the atomic displacement parameters (U11 and U33) for the in-plane Bi and Ch1 sites suggested that Bi atoms exhibit large atomic displacement along the c-axis direction above 300 K, which could be the origin of the low thermal conductivity in LaOBiSSe. The large Bi vibration along the c-axis direction could be related to in-plane rattling, which is a new strategy for attaining low thermal conductivity and phonon-glass-electron-crystal states.

Recent Advances in Layered Metal Chalcogenides as Superconductors and Thermoelectric Materials: Fe-Based and Bi-Based Chalcogenides.

Recent advances in layered (Fe-based and Bi-based) chalcogenides as superconductors or functional materials are reviewed. The Fe-chalcogenide (FeCh) family are the simplest Fe-based high-Tc superconductors. The superconductivity in the FeCh family is sensitive to external or chemical pressure, and high Tc is attained when the local structure (anion height) is optimized. The Bi-chalcogenide (BiCh2) family are a new group of layered superconductors with a wide variety of stacking structures. Their physical properties are also sensitive to external or chemical pressure. Recently, we revealed that the emergence of superconductivity and the Tc in this family correlate with the in-plane chemical pressure. Since the flexibility of crystal structure and electronic states are an advantage of the BiCh2 family for designing functionalities, I briefly review recent developments in this family as not only superconductors but also other functional materials.

In-plane chemical pressure essential for superconductivity in BiCh2-based (Ch: S, Se) layered structure.

BiCh2-based compounds (Ch: S, Se) are a new series of layered superconductors, and the mechanisms for the emergence of superconductivity in these materials have not yet been elucidated. In this study, we investigate the relationship between crystal structure and superconducting properties of the BiCh2-based superconductor family, specifically, optimally doped Ce1−xNdxO0.5F0.5BiS2 and LaO0.5F0.5Bi(S1−ySey)2. We use powder synchrotron X-ray diffraction to determine the crystal structures. We show that the structure parameter essential for the emergence of bulk superconductivity in both systems is the in-plane chemical pressure, rather than Bi-Ch bond lengths or in-plane Ch-Bi-Ch bond angle. Furthermore, we show that the superconducting transition temperature for all REO0.5F0.5BiCh2 superconductors can be determined from the in-plane chemical pressure.

In-plane chemical pressure in (U1−yCey)O2 studied by Raman spectroscopy

We investigate the nature of bonding and charge states in (U1−yCey)O2 (y=0.0, 0.2, 0.4, 0.6, 0.8 and 1.0) by Raman spectroscopy. Raman spectrum of UO2 exhibits two prominent bands below 1000cm−1, a F2g mode at 446cm−1 and a F1u LO mode at 578cm−1. As y is increased from 0 to 0.6, the F1u exhibits a large blue shift of 90cm−1, and from y=0.6 to 1.0, a red shift of 54cm−1. We show that our results can be interpreted as arising from anisotropic compression/relaxation of the lattice under Ce substitution and this can give an indication of its charge states. Alternate interpretations have been given in the literature on the effect of substituents and dopants to the Raman spectra of UO2 and CeO2. The present interpretation of chemical stress effects can be taken as another plausible explanation.