Structure, magnetism, and colossal magnetoresistance of the ternary rare earth transition metal Zintl compounds

In order to better understand the intriguing magnetic behavior and negative colossal magnetoresistive effect in the Eu₁₄MnSb₁₁ compound, a series of pseudo-ternary rare earth transition metal Zintl compounds has been synthesized by doping onto the Eu sites. The influence of chemical substitution on the structural and physical property of Eu_14−xA_x MnSb₁₁ (A = Ca, Sr, Ba, and Yb) will be discussed in terms of the site preferences of the dipositive metal dopants onto the four crystallographically inequivalent Eu sites.; Chapter 1 presents a new structural Zintl phase, Sr₂₁Mn ₄Sb₁₈ which has been synthesized while investigating the feasibility of Sn-flux technique for making transition metal Zintl compounds. The structure can be described in the framework of the Zintl-Klemn concept and the uniqueness of the structure originates from the heteronuclear tetrahedral cluster in the compound, [Mn₄Sb ₁₁]−24, which is the largest isolated unit among the heteronuclear tetrahedral anions of Zintl phases reported to date. In Chapter 2, a pseudo-binary phase of the transition metal (Ti) with the main group elements (Sn and Sb) is reported. The compound shows mixed occupancies of main group elements in the anion sites and the site preferences of the Sb and Sn atoms were explained by the recently developed differential fractional site occupancy (DFSO) concept.; Chapter 3 and 4 reports the synthesis and characterization of the physical properties of the Eu_14−xA_xMnSb₁₁ (A = Ca, Sr, Ba, and Yb; x < 3) compounds. A small amount of dopants has been substituted into the Eu sites of Eu₁₄MnSb ₁₁ to probe the complex magnetic exchange interactions of the compound: the R-R interaction (4f-4f), the R-T interaction (4f-3d), and the T-T interaction (3 d-3d). Magnetic susceptibility measurement reveals different magnetization processes for the four doped systems corresponding to the site preference of each dopant in Eu_14−xA_xMnSb ₁₁.; The solid solution of the ternary transition metal Zintl phase, Ca _14−xEu_xMnSb₁₁ (0 < x < 14), has been prepared and various physical properties are discussed as a function of Eu doping concentration in Chapter 5. The paramagnetic Curie temperature of the Ca_14−xEu_xMnSb₁₁ compounds shows an interesting change by varying _x and the trend is interpreted by the interaction between Mn 3d and Eu 4 f moment based on the preferential replacement of Eu onto the four crystallographically inequivalent Ca sites; Lastly, Chapter 6 reports the magnetic ordering of the Ca₁₄MnSb ₁₁ compound determined by high resolution neutron diffraction on powder samples. The preliminary determination of Ca₁₄MnSb₁₁ magnetic structure would provide useful information to understand many other interesting magnetic orderings in the same family compound, such as antiferromagnetic ordering of Ba₁₄MnBi₁₁ and ferrimagnetic behaviors of Eu₁₄MnSb₁₁.