Understanding the mechanism that correlates phonon transport
with
chemical bonding and solid-state structure is the key to envisage
and develop materials with ultralow thermal conductivity, which are
essential for efficient thermoelectrics and thermal barrier coatings.
We synthesized thallium selenide (TlSe), which is comprised of intertwined
stiff and weakly bonded substructures and exhibits intrinsically ultralow
lattice thermal conductivity (κL) of 0.62–0.4
W/mK in the range 295–525 K. Ultralow κL of
TlSe is a result of its low energy optical phonon modes which strongly
interact with the heat carrying acoustic phonons. Low energy optical
phonons of TlSe are associated with the intrinsic rattler-like vibration
of Tl+ cations in the cage constructed by the chains of
(TlSe2)
n
n–, as evident in low
temperature heat capacity, terahertz time-domain spectroscopy, and
temperature dependent Raman spectroscopy. Density functional theoretical
analysis reveals the bonding hierarchy in TlSe which involves ionic
interaction in Tl+–Se while Tl3+–Se
bonds are covalent, which causes significant lattice anharmonicity
and intrinsic rattler-like low energy vibrations of Tl+, resulting in ultralow κL.
We report the origin of room temperature weak ferromagnetic behavior of polycrystalline Pb(Fe 2/3 W 1/3 )O 3 (PFW) powder. The structure and magnetic properties of the ceramic powder prepared by a Columbite method were characterized by X-ray and neutron diffraction, Mössbauer spectroscopy and magnetization measurements. Rietveld analysis of diffraction data confirm the formation of single phase PFW, without traces of any parasitic pyrochlore phase. PFW was found to crystallize in the cubic structure at room temperature. The Rietveld refinement of neutron diffraction data measured at room temperature confirmed the G-type antiferromagnetic structure of PFW in our sample. However, along with the antiferromagnetic (AFM) ordering of the Fe spins, we have observed the existence of weak ferromagnetism at room temperature through: (i) a clear opening of hysteresis (M-H) loop, (ii) bifurcation of the field cooled and zero-field cooled susceptibility; supported by Mössbauer spectroscopy results. The P-E loop measurements showed a non-linear slim hysteresis loop at room temperature due to the electronic conduction through the local inhomogeneities in the PFW crystallites and the inter-particle regions. By corroborating all the magnetic measurements, especially the spin glass nature of the sample, with the conduction behavior of the sample, we report here that the observed ferromagnetism originates at these local inhomogeneous regions in the sample, where the Fe-spins are not perfectly aligned antiferromagnetically due to the compositional disordering.
Abstract. We report on the single phase synthesis and room temperature structural characterization of PbFe 0.67 W 0.33 O 3 (PFW) multiferroic. The PFW was synthesized by low temperature sintering, Columbite method. Analysis of powder XRD pattern exhibits single phase formation of PFW with no traces of pyrochlore phase. Detailed analysis of room temperature neutron diffraction (ND) reveals cubic phase at room temperature, space group Pm-3m. The ND pattern clearly reveals magnetic Bragg peak at 2 = 18.51 o (Q = 1.36Å -1 ). The refinement of magnetic structure reveals G-type antiferromagnetic structure in PFW at room temperature. The dielectric constant and loss tangent decreases with increasing frequency. The room temperature P-E measurements shows a non-linear slim hysteresis, typical nature of relaxor multiferroics, with saturation and remnant polarizations of P s = 1.50 µC/cm 2 and P r = 0.40 µC/cm 2 , respectively.
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