Susan M. Kauzlarich

Light Element Group 13–14 Clathrate Phases

There has been a renewed interest in clathrate compounds composed of light elements as promising thermoelectric materials due to their potential for chemical tuning. Clathrate structures are ideal frameworks for investigating the phonon glass electron crystal (PGEC) model for efficient band engineering. In this model, the guest atom provides for phonon scattering (phonon glass) to reduce thermal conductivity while tuning the chemical composition of the framework allows for control over electronic transport (electron crystal). This chapter provides an overview of the synthesis, structure, and properties of light element group 13-Si compounds with the clathrate structure. The primary focus will be on alkali and alkaline earth metal containing clathrates, A8ExSi46−x (A = Sr, Ba, Eu, Na, K; E = B, Al, Ga). Additionally, hydrogen capacity in Si clathrate structures will be presented. By reviewing the current status of the field, we will demonstrate the potential of these materials for electronic and thermoelectric applications and new avenues for research.

Synthesis and Thermoelectric Properties of the YbTe-YbSb System

The syntheses of YbTe1−xSbx (xxa0=xa00, 0.05, 0.2, 0.5, 0.8, 1) were investigated by solid state reactions and formed into dense pellets by spark plasma sintering. X-ray powder diffraction and microprobe analysis indicated no solubility of Sb in YbTe, and these phases are better described as composite phases (YbTe)1−x(YbSb)x (xxa0=xa00, 0.05, 0.2, 0.5, 0.8, 1). Thermal conductivity, electrical resistivity, and Seebeck coefficients were acquired for the larger values of x (xxa0=xa00.2, 0.5, 0.8, 1) from room temperature to 773xa0K, and the figure of merit was calculated. Thermal conductivities for xxa0=xa00, 0.05 are also reported; however, measurements of Seebeck coefficients and electrical resistivity were not possible due to large resistivity. The figure of merit for all samples was low, and the maximum zT measured was zT791Kxa0=xa00.018 for YbSb. Low figures of merit were primarily the result of very high resistivity in YbTe rich samples, and high thermal conductivity, and a small Seebeck coefficient in all samples.

Monte Carlo Simulations of Frustrated Classical Spin Systems

We present preliminary results of Monte Carlo simulations of classical bilayer Heisenberg models with frustrating interlayer couplings. We studied the low temperature magnetic structure of three geometrically different systems. In systems with identical square lattices in the two layers, we can reproduce the basic features of the ground state phase diagram of the classical J 1 — J 2 model if the relative displacement of two layers is along a diagonal; when the two layers are off-set by half a lattice constant in only one direction, we find that a twisted spin phase can be set up in the direction of frustration. We also studied a system with two different square lattices arranged to resemble the geometry found in the mixed layer pnictide oxides (Sr 2 Mn 3 Pn 2 O 2, Pn = As, Sb) of Brock et al. Within a restricted region of phase space, this model does exhibit the orthogonal order observed in experiments.

Perpendicular Order in Frustrated Magnetic Layers

We report preliminary results of Monte Carlo simulations of coupled two dimensional, square lattice, classical, antiferromagnetic Heisenberg systems. We find that when the two layers are offset so that frustrating interactions are present, the magnetic order in the planes can be made orthogonal, even in the absence of any explicit symmetry breaking terms on the Hamiltonian. Connections are made to the magnetic behavior of recently sythesized pnictide-oxide materials.