Showing papers by "Santiranjan Shannigrahi published in 2013"

Facile Self-Assembly of Intermetallic [Ni2Gd2] Cubane Aggregate for Magnetic Refrigeration

Abstract: Molecular nanomagnets used as magnet refrigerants at low temperatures are important advanced materials for cryogenic sensors of astronomical instruments and possible alternatives to ultra-low temperature He technologies. This application is based on the magnetocaloric effect (MCE), which follows an adiabatic demagnetization process in which the change of an applied magnetic field (DB0) leads to a change of the magnetic entropy (DSm) and temperature (DTad). [1c] Molecular nanomagnets that exhibit a large MCE usually have a large ground state spin with a large metal-to-ligand ratio and negligible magnetic anisotropy. The isotropic Gd ion, which has the largest number of unpaired electron spins in the 4f orbital with a weak super-exchange interaction between metal centers, is the metal of choice for high-nuclearity metal (Gd) or intermetallic (3d-Gd) metal aggregates or coordination polymers. The most desirable attribute of these materials is their large ground spin states with an associated high DSm, which is a key parameter that determines the MCE of a magnetic system. Many of these complexes showing promising MCE properties are usually obtained from solvothermal methods under harsh conditions, such as [Ni6Gd6P6], [4a] [Gd24], [4e] [Co8Gd8], [4b] [Co6Gd8], [4d] etc. , or slow evaporation of reaction mixtures over a prolonged period, such as [Ln42M10], [4h] [Mn4Ln4], [6] [Gd ACHTUNGTRENNUNG(HCOO) ACHTUNGTRENNUNG(OAc)2ACHTUNGTRENNUNG(H2O)2],[7] etc. g, i, j] This limits the materials to those that are inert and stable in solution. Another common problem is that the large ground state spins of these polynuclear metal aggregates are offset by complicated magnetic exchange interactions within the metal aggregates which could lead to small DSm, [2,6, 8] thus ultimately reducing the atom efficiency in MCE applications. Our objective is therefore to seek a facile synthetic method for structurally distinctive intermetallic molecular materials that can emulate Gd alloys as molecular magnetic coolants and show satisfactory magnetic entropy. A variety of homoand heterometallic metal aggregates using 2-(hydroxymethyl)pyridine (hmpH) as a ligand have been reported, such as [Mn2Mn III 2] , [9] [Mn3Ni], [10] [Mn2Ln2], [11] [Mn4], [12] [Gd4M8], [13] etc. By using hmpH as a ligand, a series of polynuclear metal aggregates such as the [NaNi6] Anderson-type framework, [14] and [Ni4] [15] and [Co3Ln] cubanes [16] have been prepared. Herein, we report a simple one-pot self-assembly of a [Ni2Gd2] heterometallic cubane at room temperature and its X-ray crystallographic analysis. Different from the reported [Ni6Gd6], [4a] [Ni12Gd36], [4f] and [Ni10Gd42] [4h] systems, this molecular aggregate is stable in CH2Cl2 solution and exhibits a pronounced cryogenic MCE with -DSm=34.4 Jkg K 1 at 4.5 K for a field change of DH=7 T. Its isolation fuels the promise of the use of small intermetallic aggregates and clusters as molecular magnetic refrigerants and paves a chemical pathway to engineering devices. A mixture of Ni ACHTUNGTRENNUNG(OAc)2, GdACHTUNGTRENNUNG(OAc)3, and hmpH (1:1:2) in THF at room temperature self-assembles over 12 hours into a neutral product formulated as [Ni2Gd2ACHTUNGTRENNUNG(hmp)4ACHTUNGTRENNUNG(OAc)6] (1) (Scheme 1). The electrospray ionization (ESI) mass spec-

Collective vibrational modes, x-ray photoemission spectra, and dielectric susceptibility of Ba2YMO6 (M = Nb,Ta)

Abstract: The density functional theory under the generalized gradient approximation has been used to investigate the electronic structure of double perovskite oxides Ba2YNbO6 (BYN) and Ba2YTaO6 (BYT) synthesized by the solid-state reaction technique. The band gaps of the systems are measured from the uv-visible spectra and compared with the theoretically calculated values. The x-ray photoemission spectra (XPS) of the systems are obtained. The valence-band XPS spectra are analyzed from calculated density of states. The binding energy of Ba 3d5/2 is found to be less in BYT than in BYN, which has been correlated to the degree of delocalization of d electrons as well as electronegativity of Nb and Ta in the systems. The real and imaginary parts of the optical dielectric function and the refractive index are calculated. The calculated spectra are compared with the experimental results. The interband contribution to the optical properties of these perovskite oxides has been analyzed. The collective vibrational modes of the atoms of the materials are calculated to examine the various vibrational excitations of the systems.