Bulk and element-specific studies of some complex transition-metal Zintl phases.

摘要

The commercial development of spin-polarized electronic transport devices for use in magnetic information storage has perpetuated a great interest in developing new materials possessing unique magnetic and electronic properties for direct applications in devices. A series of new compounds belonging to a unique class of materials termed transition-metal Zintl phases have been synthesized, offering a unique opportunity to study the correlation between structure and properties on a bulk and element-specific scale.; Chapters 1 and 2 present the structure and properties of two new transition metal Zintl phases: Ca₂₁Mn₄Sb₁₈ and Eu ₁₀Mn₆Sb₁₃ respectively. The structure of Ca ₂₁Mn₄Sb₁₈ is described as containing four discrete units per formula unit: a [Mn₄Sb₁₀]22− anion, two [Sb₂]4− anions, four Sb 3− anions, and 21 Ca2+ cations. The magnetic interactions of this system are best described by a ferrimagnetic model, and temperature dependent resistivity shows this compound is a small band gap semiconductor. The structure of Eu₁₀Mn₆Sb₁₃ is composed of Eu2+ cations alternating with double-layers of edge- and corner-sharing Mn-centered tetrahedra, and [Sb₂] 4− dumbbells. 151Eu Mössbauer spectra confirm that the europium is divalent with an average isomer shift of −11.1 mm/s at 100 K. Bulk susceptibility data indicate paramagnetic behavior with a ferromagnetic component present below 60 K. Temperature dependent electrical resistivity measurements show semiconducting behavior above 60 K ( E_a = 0.115(2) eV), with metallic behavior below 40.; Chapter 3 presents X-ray magnetic circular dichroism measurements (XMCD) of Yb₁₄MnSb₁₁ that reveal strong dichroism in the Mn L₂₃ edge, the Sb M₄₅ edge shows a weak dichroism indicating antialignment to the Mn, and the Yb N₄₅ edge shows no dichroism. The bulk magnetization can be understood as Mn in a 2+ configuration, with cancellation of one spin by an antialigned moment from the Sb 5p band of the Sb₄ cage surrounding the Mn.; Chapter 4 presents X-ray photoelectron spectroscopy (XPS) measurements of Yb₁₄MnSb₁₁ and Yb₁₄ZnSb₁₁. Detailed analysis of clean Yb₁₄ZnSb₁₁ sample surfaces reveal an intermediate Yb valence consistent with single crystal X-ray diffraction studies, but the Mn analog shows no contribution from Yb3+ states. Further detailed analysis of the core and valence band structures of Yb₁₄MnSb₁₁ and Yb₁₄ZnSb₁₁ are presented.