Udumula Subbarao

Crystal structure of YbCu6In6 and mixed valence behavior of Yb in YbCu(6-x)In(6+x) (x = 0, 1, and 2) solid solution.

High quality single crystals of YbCu(6)In(6) have been grown using the flux method and characterized by means of single crystal X-ray diffraction data. YbCu(6)In(6) crystallizes in the CeMn(4)Al(8) structure type, tetragonal space group I4/mmm, and the lattice constants are a = b = 9.2200(13) Å and c = 5.3976(11) Å. The crystal structure of YbCu(6)In(6) is composed of pseudo-Frank-Kasper cages filled with one ytterbium atom in each ring. The neighboring cages share corners along [100] and [010] to build the three-dimensional network. YbCu(6-x)In(6+x) (x = 0, 1, and 2) solid solution compounds were obtained from high frequency induction heating and characterized using powder X-ray diffraction. The magnetic susceptibilities of YbCu(6-x)In(6+x) (x = 0, 1, and 2) were investigated in the temperature range 2-300 K and showed Curie-Weiss law behavior above 50 K, and the experimentally measured magnetic moment indicates mixed valent ytterbium. A deviation in inverse susceptibility data at 200 K suggests a valence transition from Yb(2+) to Yb(3+) as the temperature decreases. An increase in doping of Cu at the Al2 position enhances the disorder in the system and enhancement in the trivalent nature of Yb. Electrical conductivity measurements show that all compounds are of a metallic nature.

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.

New structure type in the mixed-valent compound YbCu4Ga8.

The new compound YbCu(4)Ga(8) was obtained as large single crystals in high yield from reactions run in liquid gallium. Preliminary investigations suggest that YbCu(4)Ga(8) crystallizes in the CeMn(4)Al(8) structure type, tetragonal space group I4/mmm, and lattice constants are a = b = 8.6529(4) Å and c = 5.3976(11) Å. However, a detailed single-crystal XRD revealed a tripling of the c axis and crystallizing in a new structure type with lattice constants of a = b = 8.6529(4) Å and c = 15.465(1) Å. The structural model was further confirmed by neutron diffraction measurements on high-quality single crystal. The crystal structure of YbCu(4)Ga(8) is composed of pseudo-Frank-Kasper cages occupying one ytterbium atom in each ring which are shared through the corner along the ab plane, resulting in a three-dimensional network. The magnetic susceptibility of YbCu(4)Ga(8) investigated in the temperature range 2-300 K showed Curie-Weiss law behavior above 100 K, and the experimentally measured magnetic moment indicates mixed-valent ytterbium. Electrical resistivity measurements show the metallic nature of the compound. At low temperatures, variation of ρ as a function of T indicates a possible Fermi-liquid state at low temperatures.

Pressure-induced electronic topological transition in Sb2S3

We report the high-pressure vibrational properties and a pressure-induced electronic topological transition in the wide bandgap semiconductor Sb2S3 (E g  =  1.7-1.8 eV) using Raman spectroscopy, resistivity and x-ray diffraction (XRD) studies. In this report, high-pressure Raman spectroscopy and resistivity studies of Sb2S3 have been carried out to 22 GPa and 11 GPa, respectively. We observed the softening of phonon modes [Formula: see text], [Formula: see text] and B 2g and a sharp anomaly in their line widths at 4 GPa. The resistivity studies corroborate this anomaly around similar pressures. The changes in resistivity as well as Raman line widths can be ascribed to the strong phonon-phonon coupling, indicating clear evidence of isostructural electronic topological transition in Sb2S3. The previously reported pressure dependence of a/c ratio plot obtained also showed a minimum at ~5 GPa consistent with our high-pressure Raman and resistivity results.

Flux growth of Yb6.6Ir6Sn16 having mixed-valent ytterbium

The compound Yb6.6Ir6Sn16 was obtained as single crystals in high yield from the reaction of Yb with Ir and Sn run in excess indium. Single-crystal X-ray diffraction analysis shows that Yb6.6Ir6Sn16 crystallizes in the tetragonal space group P42/nmc with a = b = 9.7105(7) Å and c = 13.7183(11) Å. The crystal structure is composed of a [Ir6Sn16] polyanionic network with cages in which the Yb atoms are embedded. The Yb sublattice features extensive vacancies on one crystallographic site. Magnetic susceptibility measurements on single crystals indicate Curie-Weiss law behavior <100 K with no magnetic ordering down to 2 K. The magnetic moment within the linear region (<100 K) is 3.21 μB/Yb, which is ∼70% of the expected value for a free Yb(3+) ion suggesting the presence of mixed-valent ytterbium atoms. X-ray absorption near edge spectroscopy confirms that Yb6.6Ir6Sn16 exhibits mixed valence. Resistivity and heat capacity measurements for Yb6.6Ir6Sn16 indicate non-Fermi liquid metallic behavior.

Structure and properties of SmCu6 − xIn6 + x (x = 0, 1, 2)

AbstractHigh quality single crystals of SmCu6In6 were obtained from the reactions run in excess liquid indium and characterized by means of single crystal X-ray diffraction data. SmCu6In6 crystallizes in the YCu6In6 structure type, tetragonal space group I4/mmm and lattice constants are a = b = 9.2036(3) Å and c = 5.4232(3) Å. SmCu6 − xIn6 + x (x = 0, 1, 2) compounds were obtained using arc melting and characterized using powder X-ray diffraction. Magnetic susceptibility data follow modified Curie–Weiss law behaviour above 50 K and the experimentally measured magnetic moment values in SmCu4In8, SmCu5In7 and SmCu6In6 are 0.83 μB, 1.45 μB and 0.90 μB per Sm atom, respectively. SmCu5In7 and SmCu6In6 compounds show antiferromagnetic ordering at TN = 7.8 K, and no magnetic ordering was observed for SmCu4In8 down to 2 K. Electrical resistivity measurements show that all compounds are of metallic nature. Graphical AbstractSingle phase SmCu6–xIn6–x (x = 0, 1 and 2) samples were synthesized using high frequency heat treatment. The compounds were crystallized in the YCu6In6 structure type. Physical properties of the SmCu6–xIn6+x (x = 0, 1 and 2) compounds have shown strong dependence on the copper to indium ratio.

Evolution of dealloyed PdBi2 nanoparticles as electrocatalysts with enhanced activity and remarkable durability in hydrogen evolution reactions

The development of non-Pt based electrocatalysts for the hydrogen evolution reaction (HER) is a pre-requisite for the generation of hydrogen, a feasible and cost-effective source of hydrogen. Structural transitions and generating metal deficiency are the effective ways of manipulating the d-band centre of a metal surface which enhances the catalytic activity of metal nanoparticles towards the HER. Charge-transfer from in situ generated oxide species to the metal centre also leads to an enhancement in catalytic activity towards the HER. In the present work, we report a facile colloidal synthesis of PdBi2 nanoparticles using sodium borohydride as the reducing agent. Upon annealing the as-synthesized nanoparticles, a phase transition from the lower symmetry monoclinic phase to the higher symmetry tetragonal phase has been observed, and hence a change in morphology from agglomerated to core–shell nanoparticles. Potential electrochemical cycling of both monoclinic and tetragonal PdBi2 leads to the formation of a Pd-rich PdBi2−x alloy with enhanced catalytic activity (onset potential: −11 mV and −18 mV vs. the RHE; 20 mA cm−2 current density at an overpotential of ∼140 mV and ∼207 mV for monoclinic and tetragonal phases, respectively). The low co-ordination number of Pd active sites formed by the dissolution of Bi alters the d-band centre of Pd, and hence the optimal energy required for hydrogen adsorption leading to enhanced activity. Although the obtained composition after potential cycling is almost similar for both phases it is seen that the dealloyed monoclinic phase shows higher activity as compared to the dealloyed tetragonal one. Cyclic voltammetry of the monoclinic PdBi2 shows the formation of Bi–O species after potential cycling. Electron transfer from the Bi–O species to the Pd centre enhances the charge-transfer kinetics of the HER on the catalyst surface and hence an increased catalytic activity of the monoclinic phase as compared to the tetragonal one. Thus, in situ generated oxide species facilitate charge-transfer from oxide to the metal surface which in turn enhances catalytic activity towards the HER.

Are we underrating rare earths as an electrocatalyst? The effect of their substitution in palladium nanoparticles enhances the activity towards ethanol oxidation reaction

Since the advent of catalysis, transition metal-based materials have continually been exploited as efficient electrocatalysts, whereas rare-earths (REs) have been neglected due to their misleading name, i.e. rare-earth, and cost. In fact, most REs are abundant and less expensive than the most explored transition metals. In view of this, we attempted to study the chemical effects of small amounts of RE (10%) substitution in the Pd lattice (REPd) for comparison with transition metal-substituted Pd (TMPd) towards the ethanol oxidation reaction (EOR). The electrochemical activities of REPd (RE = Eu and Yb) towards EOR were found to show many fold increases in both specific and mass activities as compared to those of TMPd (TM = Cr and Ni) and commercial Pd/C. Theoretical investigations assisted by DFT calculations support the experimental observations with a perfect synergy between the adsorption energies of –OH and –COCH3 to the catalyst surface, which provides unprecedented catalytic activity for REPd as compared to the case of other catalysts. The promotional effects of RE were well exploited in enhancing the activity and stability of Pd towards EOR, as observed by electrochemical studies, X-ray absorption near edge spectroscopy and DFT calculations.

Yb7Ni4InGe12: a quaternary compound having mixed valent Yb atoms grown from indium flux

The new intermetallic compound Yb7Ni4InGe12 was obtained as large silver needle shaped single crystals from reactive indium flux. Single crystal X-ray diffraction suggests that Yb7Ni4InGe12 crystallizes in the Yb7Co4InGe12 structure type, and tetragonal space group P4/m and lattice constants are a = b = 10.291(2) Å and c = 4.1460(8) Å. The crystal structure of Yb7Ni4InGe12 consists of columnar units of three different types of channels filled with the Yb atoms. The crystal structure of Yb7Ni4InGe12 is closely related to Yb5Ni4Ge10. The effective magnetic moment obtained from the magnetic susceptibility measurements in the temperature range 200-300 K is 3.66μB/Yb suggests mixed/intermediate valence behavior of ytterbium atoms. X-ray absorption near edge spectroscopy (XANES) confirms that Yb7Ni4InGe12 exhibits mixed valence.

Swinging Symmetry, Multiple Structural Phase Transitions, and Versatile Physical Properties in RECuGa3 (RE = La–Nd, Sm–Gd)

The compounds RECuGa3 (RE = La-Nd, Sm-Gd) were synthesized by various techniques. Preliminary X-ray diffraction (XRD) analyses at room temperature suggested that the compounds crystallize in the tetragonal system with either the centrosymmetric space group I4/mmm (BaAl4 type) or the non-centrosymmetric space group I4mm (BaNiSn3 type). Detailed single-crystal XRD, neutron diffraction, and synchrotron XRD studies of selected compounds confirmed the non-centrosymmetric BaNiSn3 structure type at room temperature with space group I4mm. Temperature-dependent single-crystal XRD, powder XRD, and synchrotron beamline measurements showed a structural transition between centro- and non-centrosymmetry followed by a phase transition to the Rb5Hg19 type (space group I4/m) above 400 K and another transition to the Cu3Au structure type (space group Pm3̅m) above 700 K. Combined single-crystal and synchrotron powder XRD studies of PrCuGa3 at high temperatures revealed structural transitions at higher temperatures, highlighting the closeness of the BaNiSn3 structure to other structure types not known to the RECuGa3 family. The crystal structure of RECuGa3 is composed of eight capped hexagonal prism cages [RE4Cu4Ga12] occupying one rare-earth atom in each ring, which are shared through the edge of Cu and Ga atoms along the ab plane, resulting in a three-dimensional network. Resistivity and magnetization measurements demonstrated that all of these compounds undergo magnetic ordering at temperatures between 1.8 and 80 K, apart from the Pr and La compounds: the former remains paramagnetic down to 0.3 K, while superconductivity was observed in the La compound at T = 1 K. It is not clear whether this is intrinsic or due to filamentary Ga present in the sample. The divalent nature of Eu in EuCuGa3 was confirmed by magnetization measurements and X-ray absorption near edge spectroscopy and is further supported by the crystal structure analysis.