Sumanta Sarkar

Metallic Yb2AuGe3: An Ordered Superstructure in the AlB2-Type Family with Mixed-Valent Yb and a High-Temperature Phase Transition

The intermetallic compound Yb(2)AuGe(3) was synthesized from indium flux. Yb(2)AuGe(3) crystallizes in the orthorhombic Ca(2)AgSi(3)-type structure which is an ordered superstructure of the AlB(2) structure type. The structure was refined in the Fmmm space group with lattice parameters a = 8.5124(17) Å, b = 14.730(3) Å, c = 8.4995(17) Å. Temperature-dependent powder X-ray diffraction studies show that Yb(2)AuGe(3) undergoes a phase transition from orthorhombic to hexagonal upon heating above 773 K. The compound shows weak paramagnetism that derives from a combination of Curie and Pauli paramagnetism with a magnetic moment value of 0.33(2) μ(B)/Yb atom. Magnetic ordering was not observed down to 2 K. Yb(2)AuGe(3) is metallic, and at low temperature the resistivity varies as T(2), indicating possible Fermi liquid behavior. Heat capacity measurements suggest that Yb(2)AuGe(3) is possibly a moderate heavy fermion system.

Yb5Ga2Sb6: A Mixed Valent and Narrow-Band Gap Material in the RE5M2X6 Family

A new compound Yb5Ga2Sb6 was synthesized by the metal flux technique as well as high frequency induction heating. Yb5Ga2Sb6 crystallizes in the orthorhombic space group Pbam (no. 55), in the Ba5Al2Bi6 structure type, with a unit cell of a = 7.2769(2) Å, b = 22.9102(5) Å, c = 4.3984(14) Å, and Z = 2. Yb5Ga2Sb6 has an anisotropic structure with infinite anionic double chains (Ga2Sb6)(10-) cross-linked by Yb(2+) and Yb(3+) ions. Each single chain is made of corner-sharing GaSb4 tetrahedra. Two such chains are bridged by Sb2 groups to form double chains of 1/∞ [Ga2Sb6(10-)]. The compound satisfies the classical Zintl-Klemm concept and is a narrow band gap semiconductor with an energy gap of around 0.36 eV calculated from the electrical resistivity data corroborating with the experimental absorption studies in the IR region (0.3 eV). Magnetic measurements suggest Yb atoms in Yb5Ga2Sb6 exist in the mixed valent state. Temperature dependent magnetic susceptibility data follows the Curie-Weiss behavior above 100 K and no magnetic ordering was observed down to 2 K. Experiments are accompanied by all electron full-potential linear augmented plane wave (FP-LAPW) calculations based on density functional theory to calculate the electronic structure and density of states. The calculated band structure shows a weak overlap of valence band and conduction band resulting in a pseudo gap in the density of states revealing semimetallic character.

Structural phase transitions in a new compound Eu2AgGe3.

A new intermetallic compound Eu2AgGe3 has been synthesized using high-frequency induction heating method. Single-crystal X-ray diffraction data showed that Eu2AgGe3 crystallizes in the orthorhombic Ba2LiSi3 structure type, with Fddd space group and lattice parameters a = 8.7069(17) Å, b = 15.011(3) Å, c = 17.761(4) Å. Eu2AgGe3 is composed of infinite arrays of hexagonal [Ag3Ge3] units stacked along the [001] direction, and the Eu sites are sandwiched between these parallel hexagonal networks. Temperature-dependent powder XRD data and DTA hint toward a structural phase transition from orthorhombic to hexagonal above 477 K and an unusual reversible transition to the original phase, i.e., orthorhombic phase at around 718 K. Magnetic measurements on Eu2AgGe3 sample show paramagnetic behavior above 100 K and weak ferromagnetic interactions below 80 K. Mössbauer spectroscopy and X-ray absorption near-edge spectroscopic (XANES) studies reveal that Eu atoms in Eu2AgGe3 exist in the divalent oxidation state.

Single crystal growth of europium and ytterbium based intermetallic compounds using metal flux technique

AbstractThis article covers the use of indium as a potential metal solvent for the crystal growth of europium and ytterbium-based intermetallic compounds. A brief view about the advantage of metal flux technique and the use of indium as reactive and non-reactive flux are outlined. Large single crystals of EuGe2, EuCoGe3 and Yb2AuGe3 compounds were obtained in high yield from the reactions of the elements in liquid indium. The results presented here demonstrate that considerable advances in the discovery of single crystal growth of complex phases are achievable utilizing molten metals as solvents. Graphical AbstractMetal flux is an important synthetic tool in solid-state chemistry for both crystal-growth studies and exploratory studies of europium and ytterbium-based intermetallic compounds. Large single crystals of EuGe2, EuCoGe3 and Yb2AuGe3 compounds were obtained in high yield from the reactions of the elements in liquid indium.

Layer specific optical band gap measurement at nanoscale in MoS2 and ReS2 van der Waals compounds by high resolution electron energy loss spectroscopy

Layer specific direct measurement of optical band gaps of two important van der Waals compounds, MoS2 and ReS2, is performed at nanoscale by high resolution electron energy loss spectroscopy. For monolayer MoS2, the twin excitons (1.8 and 1.95 eV) originating at the K point of the Brillouin zone are observed. An indirect band gap of 1.27 eV is obtained from the multilayer regions. Indirect to direct band gap crossover is observed which is consistent with the previously reported strong photoluminescence from the monolayer MoS2. For ReS2, the band gap is direct, and a value of 1.52 and 1.42 eV is obtained for the monolayer and multilayer, respectively. The energy loss function is dominated by features due to high density of states at both the valence and conduction band edges, and the difference in analyzing band gap with respect to ZnO is highlighted. Crystalline 1T ReS2 forms two dimensional chains like superstructure due to the clustering between four Re atoms. The results demonstrate the power of HREELS...

Crystal structure and physical properties of indium flux grown RE2AuSi3 (RE = Eu, Yb)

Two new intermetallic compounds Eu2AuSi3 and Yb2AuSi3 having prismatic and rod like shapes, respectively, were grown using indium as inert metal flux. The crystal structure of both compounds was refined using X-ray diffraction data on selected single crystals. Both compounds are ordered variants of the AlB2 structure type and crystallize in the orthorhombic system. Eu2AuSi3 adopts the Ca2AgSi3 structure type, Fmmm space group and lattice parameters a = 8.3060(17) A, b = 9.0369(18) A, c = 14.377(3) A. Yb2AuSi3, on the other hand, crystallizes in the Ba2LiSi3 structure type, Fddd space group and lattice parameters a = 8.2003(16) A, b = 14.187(3) A, c = 16.869(3) A. Both Eu2AuSi3 and Yb2AuSi3 consist of two dimensional hexagonal units built up of [Au3Si3] mixed sites and the europium atoms are sandwiched between two adjacent hexagonal layers. Magnetic measurements suggest both compounds have divalent rare earth atoms. The temperature dependent magnetic susceptibility data of Eu2AuSi3 suggests the Curie–Weiss behavior above 30 K and an antiferromagnetic ordering around 26 K. Yb2AuSi3 is diamagnetic in the higher temperature range (>175 K) and is weakly paramagnetic below 100 K indicating a probable valence fluctuation in Yb.

Crystal Structure and Band Gap Engineering in Polyoxometalate-Based Inorganic-Organic Hybrids.

We have demonstrated engineering of the electronic band gap of the hybrid materials based on POMs (polyoxometalates), by controlling its structural complexity through variation in the conditions of synthesis. The pH- and temperature-dependent studies give a clear insight into how these experimental factors affect the overall hybrid structure and its properties. Our structural manipulations have been successful in effectively tuning the optical band gap and electronic band structure of this kind of hybrids, which can find many applications in the field of photovoltaic and semiconducting devices. We have also addressed a common crystallographic disorder observed in Keggin-ion (one type of heteropolyoxometalate [POMs])-based hybrid materials. Through a combination of crystallographic, spectroscopic, and theoretical analysis of four new POM-based hybrids synthesized with tactically varied reaction conditions, we trace the origin and nature of the disorder associated with it and the subtle local structural coordination involved in its core picture. While the crystallography yields a centrosymmetric structure with planar coordination of Si, our analysis with XPS, IR, and Raman spectroscopy reveals a tetrahedral coordination with broken inversion symmetry, corroborated by first-principles calculations.

Chemically designed CeO2 nanoboxes boost the catalytic activity of Pt nanoparticles toward electro-oxidation of formic acid

A comprehensive study was carried out on platinum nanoparticle decorated CeO2 nanoboxes as an efficient catalyst towards electro-oxidation of formic acid. We herein report the effect of different parameters such as the size and shape of CeO2 nanoboxes, thickness of the CeO2 shell, mode of attachment of Pt nanoparticles on the CeO2 surface and screening of Pt loading on the electro-oxidation of formic acid. A facile low temperature aqueous phase method was utilized for the synthesis of CeO2 nanoboxes using Cu2O nanocubes as a soft sacrificial template. Owing to the exposed active surface area and optimum shell thickness, the smallest CeO2 boxes loaded with Pt (50 nm) showed excellent activity for formic acid oxidation with a current density of 4.52 mA cm−2. The highly porous hollow nanoboxes act as a bifunctional support for Pt nanoparticles. The catalyst also exhibited significant activity in the oxidation of other small molecules such as ethanol and methanol.

Selective colorimetric detection of Cu2+ by lanthanide based hybrid complexes associated with a single crystal growth mediated transformation

Two rare earth (Dy3+ and Nd3+) based complexes were synthesized using the ligands bipyridine, p-amino benzoic acid and nitro benzoic acid. The complexes were characterized by single crystal and powder XRD, IR spectroscopy and thermogravimetric analysis. Both complexes showed characteristic luminescence properties of the lanthanides and weak metamagnetic transitions at low temperature. Both the Dy and Nd compounds were found to detect Cu2+ selectively in very low concentrations of the transition metal salt in the mixture of other transition metals like Cu2+, Co2+, Ni2+, Zn2+, Cd2+, Ag+ and Au3+. This selective copper uptake is associated with a fast distinct colour change as a result of which the detection of copper can be realized visually. The Nd compound was more selective and detected Cu2+ selectively till 10−5 M concentration of the Cu salt. The Cu2+ encapsulation in the Nd complex was characterized through a rare single crystal growth mediated transformation involving an irreversible exchange of the metal ion of the complex. This single crystal transformation facilitates the easy understanding of the copper encapsulation mechanism and also provides information on the detector complex, which is rarely studied.

Nature of low dimensional structural modulations and relative phase stability in RexMo(W)1-xS2 transition metal dichalcogenide alloys

We report on the various types of Peierls like two dimensional structural modulations and relative phase stability of 2H and 1T poly-types in the RexMo1-xS2 and RexW1-xS2 alloy system. Theoretical calculation predicts a polytype phase transition cross over at ∼50 at. % of Mo and W in ReS2 in both monolayer and bulk form, respectively. Experimentally, two different types of structural modulations at 50% and a modulation corresponding to trimerization at 75% alloy composition are observed for RexMo1-xS2 and only one type of modulation is observed at the 50% RexW1-xS2 alloy system. The 50% alloy system is found to be a suitable monolithic candidate for metal semiconductor transition with minute external perturbation. ReS2 is known to be in the 2D Peierls distorted 1Td structure and forms a chain like superstructure. Incorporation of Mo and W atoms into the ReS2 lattice modifies the metal-metal hybridization between the cations and influences the structural modulation and electronic properties of the system. ...