New Zintl phases, a class of compounds containing elements that are all considered metals, but exhibit properties of a semiconductor, were synthesized and characterized. Compounds with unique magnetic properties and/or thermoelectric properties (for the direct conversion of heat into electricity) were studied. Several new compounds showed ultra-low thermal conductivity with low electrical resistivity, possible applications include thermal barriers in electronic applications.

Single crystals of Yb_{14- x}RE_xMnSb₁₁ (0 < x < 0.6, RE = Ce, Pr, Nd, Sm, and Gd) and (x = 0.1, 0.4; RE = Tb, Dy, Ho) were synthesized by Sn flux. Yb₁₄MnSb₁₁ is described as a partially screened d-metal Kondo system with the isolated MnSb₄ tetrahedral cluster having a d⁵⁺ hole configuration that results in four unpaired electrons measured in the ferromagnetically ordered phase. All of the Yb atoms in Yb₁₄MnSb₁₁ are present as Yb²⁺, and the additional RE in Yb_14-xRE_xMnSb₁₁ is trivalent, contributing one additional electron to the structure and altering the magnetic properties. The RE preferentially substitutes the four Yb sites according to size. All compounds show ferromagnetic ordering in the range of 39−53 K attributed to the MnSb₄ cluster. The effective moments, total unpaired electrons, measured above the ferromagnetic ordering temperature agree well with the calculated moments assuming the RE substitutes as a trivalent cation. In Ce-, Pr-, Nd-, and Sm-substituted crystals, the saturation moments are consistent with ~ 4 unpaired electrons attributed to the MnSb₄ cluster, indicating that local moments of Pr, Nd, and Sm do not contribute to the magnetic order. In the case of RE = Pr, this is confirmed by neutron diffraction. In contrast, the magnetic measurements of RE = Gd show that the moments of Gd ferromagnetically order with the moments of MnSb₄, and reduced screening of moments on Mn²⁺ is evident. The small RE's, Tb, Dy, and Ho, show significant coercive fields with the largest coercive field observed for RE = Dy (1000 Oe). For the maximum x of Yb_14-xRE_xMnSb₁₁ there are enough carriers for the Ruderman-Kittel-Kasuya-Yosida (RKKY) mechanism of magnetic coupling via conduction electrons to still be valid in describing the ferromagnetic ordering. The sensitive variation of magnetic behavior is attributed to the various RE substitutions resulting in different interactions of the 4f-orbitals with the 3d-orbitals of Mn in the MnSb₄ cluster conducted through 5p-orbitals of Sb.

New Eu_{11− x}Yb_xCd₆Sb₁₂ Zintl solid solutions with the Sr₁₁Cd₆Sb₁₂ structure type were prepared by tin flux reaction. Efforts to make the Yb compositions for x exceeding ~  3 resulted in other structure types. The influence of the rare earth (RE) metal sites on thermal and electronic properties of RE₁₁Cd₆Sb₁₂ solid solutions was studied by measuring their thermoelectric properties from 5 to 300 K after consolidation by either spark plasma sintering (SPS) or hot pressing (HP). Defects in the phases made by SPS resulted in lower thermoelectric efficiencies. 

A series of alkali metal containing compounds with type I clathrate structure, A₈Ga₈Si₃₈ (A = K, Rb, Cs) and K₈Al₈Si₃₈, were synthesized and characterized. Surface photovoltage spectroscopy on films shows that these compounds are semiconductors with band gaps in the range 1.14 to 1.40 eV. Theoretical investigation of the excited state processes suggested that these phases might be useful for photovoltaic applications.

A number of other new Zintl phases were synthesized and their structure, electronic and magnetic properties characterized, presented at national and international meetings and published in peer reviewed scientific publications.

Graduate and undergraduate students learned techniques in solid state materials syntheses such as metallurgical routes to thermodynamic phases and metal flux routes to prepare high-quality single crystals. Magnetic properties were measured and spark plasma sintering of various compounds was optimized to press fully dense pellets for thermoelectric property measurements. Students learn how to perform each measurement independently and interpret the results.  Graduate students participate in outreach by mentoring undergraduate students from MURPPS, Mentorships for Undergraduate Research Participants in the Physical and Mathematical Sciences, and high school students sponsored by the American Chemical Society, project SEED, summer experience for the economically disadvantaged. Undergraduate students presented research at the UC Davis Undergraduate Research Symposium and the LaRock Chemistry Conference, along with the American Chemical Society meeting. Graduate and undergraduate students benefitted from interactions with national labs such as Argonne National Laboratory and the Jet Propulsion Laboratory. Research was presented at national and international meetings such as the American Chemical Society National meetings and the International Conference on Thermoelectrics (ITC). This proposal funded 7 graduate students (4 women and 2 underrepresented minority) and 9 undergraduates (3 women and 4 underrepresented minorities) and 3 high schools students (all underrepresented minorities). 3 graduate students completed their Ph.D. and obtained research positions in industry or postdoctoral positions and 1 researcher obtained as assistant professor position. This proposal utilized national facilities such as the synchrotron for X-ray powder diffraction at Argonne National Laboratory.

Last Modified: 04/15/2018
Modified by: Susan M Kauzlarich