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The layered delafossite PdCoO2 has been predicted to be one of very few materials with a thermopower that is highly anisotropic and switches sign between different crystallographic directions. These properties are of interest for various applications, but have been difficult to verify because sufficiently large crystals have not been available. We report measurements of the high-temperature electrical resistivity, thermal conductivity, and thermopower of phase-pure PdCoO2 powder compacts prepared by a highly Pd-efficient synthesis route. While the electronic transport of the polycrystalline samples is dominated by that of the Pd planes, the thermopower exhibits a well-defined deviation from the in-plane character at temperatures above 600K, which is indicative of opposing trends in the Seebeck coefficients within and perpendicular to the delafossite layers. The experimental data are consistently described by a combination of effective-medium models based on the main axes transport quantities. The results support the predicted ambipolar thermopower anisotropy in PdCoO2.

We report the crystal structures and magnetic properties of two pseudopolymorphs of the $S=1/2 {\mathrm{Ti}}^{3+}$ coordination framework, $\mathrm{K}\mathrm{Ti}{({\mathrm{C}}_{2}{\mathrm{O}}_{4})}_{2}\ifmmode\cdot\else\textperiodcentered\fi{}x{\mathrm{H}}_{2}\mathrm{O}$. Single-crystal x-ray and powder neutron diffraction measurements on $\ensuremath{\alpha}\ensuremath{-}\mathrm{K}\mathrm{Ti}{({\mathrm{C}}_{2}{\mathrm{O}}_{4})}_{2}\ifmmode\cdot\else\textperiodcentered\fi{}x{\mathrm{H}}_{2}\mathrm{O}$ confirm its structure in the tetragonal $I4/mcm$ space group with a square planar arrangement of ${\mathrm{Ti}}^{3+}$ ions. Magnetometry and specific heat measurements reveal weak antiferromagnetic interactions, with ${J}_{1}\ensuremath{\approx}7$ K and ${J}_{2}/{J}_{1}=0.11$ indicating a slight frustration of nearest- and next-nearest-neighbor interactions. Below 1.8 K, $\ensuremath{\alpha}\ensuremath{-}\mathrm{K}\mathrm{Ti}{({\mathrm{C}}_{2}{\mathrm{O}}_{4})}_{2}\ifmmode\cdot\else\textperiodcentered\fi{}x{\mathrm{H}}_{2}\mathrm{O}$ undergoes a transition to G-type antiferromagnetic order with magnetic moments aligned along the $c$ axis of the tetragonal structure. The estimated ordered moment of ${\mathrm{Ti}}^{3+}$ in $\ensuremath{\alpha}\ensuremath{-}\mathrm{K}\mathrm{Ti}{({\mathrm{C}}_{2}{\mathrm{O}}_{4})}_{2}\ifmmode\cdot\else\textperiodcentered\fi{}x{\mathrm{H}}_{2}\mathrm{O}$ is suppressed from its spin-only value to $0.62(3)\phantom{\rule{0.28em}{0ex}}{\ensuremath{\mu}}_{B}$, thus verifying the two-dimensional nature of the magnetic interactions within the system. $\ensuremath{\beta}\ensuremath{-}\mathrm{K}\mathrm{Ti}{({\mathrm{C}}_{2}{\mathrm{O}}_{4})}_{2}\ifmmode\cdot\else\textperiodcentered\fi{}2{\mathrm{H}}_{2}\mathrm{O}$, on the other hand, realizes a three-dimensional diamondlike magnetic network of ${\mathrm{Ti}}^{3+}$ moments within a hexagonal $P{6}_{2}22$ structure. An antiferromagnetic exchange coupling of $J\ensuremath{\approx}54$ K---an order of magnitude larger than in $\ensuremath{\alpha}\ensuremath{-}\mathrm{K}\mathrm{Ti}{({\mathrm{C}}_{2}{\mathrm{O}}_{4})}_{2}\ifmmode\cdot\else\textperiodcentered\fi{}x{\mathrm{H}}_{2}\mathrm{O}$---is extracted from magnetometry and specific heat data. $\ensuremath{\beta}\ensuremath{-}\mathrm{K}\mathrm{Ti}{({\mathrm{C}}_{2}{\mathrm{O}}_{4})}_{2}\ifmmode\cdot\else\textperiodcentered\fi{}2{\mathrm{H}}_{2}\mathrm{O}$ undergoes N\'eel ordering at ${T}_{N}=28$ K, with the magnetic moments aligned within the $ab$ plane and a slightly reduced ordered moment of $0.79\phantom{\rule{0.28em}{0ex}}{\ensuremath{\mu}}_{B}$ per ${\mathrm{Ti}}^{3+}$. Through density-functional theory calculations, we address the origin of the large difference in the exchange parameters between the $\ensuremath{\alpha}$ and $\ensuremath{\beta}$ pseudopolymorphs. Given their observed magnetic behaviors, we propose $\ensuremath{\alpha}\ensuremath{-}\mathrm{K}\mathrm{Ti}{({\mathrm{C}}_{2}{\mathrm{O}}_{4})}_{2}\ifmmode\cdot\else\textperiodcentered\fi{}x{\mathrm{H}}_{2}\mathrm{O}$ and $\ensuremath{\beta}\ensuremath{-}\mathrm{K}\mathrm{Ti}{({\mathrm{C}}_{2}{\mathrm{O}}_{4})}_{2}\ifmmode\cdot\else\textperiodcentered\fi{}2{\mathrm{H}}_{2}\mathrm{O}$ as close to ideal model $S=1/2$ Heisenberg square and diamond lattice antiferromagnets, respectively.

A hyperkagome iridate ${\mathrm{Li}}_{3}{\mathrm{Ir}}_{3}{\mathrm{O}}_{8}$ is synthesized by an ion-exchange reaction from ${\mathrm{Na}}_{4}{\mathrm{Ir}}_{3}{\mathrm{O}}_{8}$. The transport, magnetic, and thermodynamic measurements suggest a metallic state. The electronic structure calculation shows that ${\mathrm{Li}}_{3}{\mathrm{Ir}}_{3}{\mathrm{O}}_{8}$ hosts a semimetallic electronic structure produced by a competition between the formation of a localized molecular orbital of Ir $d$ electrons on the ${\mathrm{Ir}}_{3}$ triangles and the strong spin-orbit coupling as in the sister compound ${\mathrm{Na}}_{3}{\mathrm{Ir}}_{3}{\mathrm{O}}_{8}$. The semimetallic state induced by spin-orbit coupling is quite robust against the moderate change of lattice distortion associated with the different ionic radii of ${\mathrm{Li}}^{+}$ and ${\mathrm{Na}}^{+}$.

Magnetic materials are a cornerstone for developing spintronic devices for the transport of information via magnetic excitations. To date, relatively few materials have been investigated for the purpose of spin transport, mostly due to the paucity of suitable candidates as these materials are often chemically complex and difficult to synthesize. We present the crystal growth and a structure solution on the high-temperature crystal structure of the layered, polar ferrimagnet ${\mathrm{CaBaFe}}_{4}{\mathrm{O}}_{7}$, which is a possible new contender for spintronics research. The space group is identified as $P3$ by refinement of single crystal and powder neutron diffraction data. At 400 K, the trigonal lattice parameters are $a=11.0114(11)\phantom{\rule{4pt}{0ex}}\AA{}$ and $c=10.330(3)\phantom{\rule{4pt}{0ex}}\AA{}$. The structure is similar to the low-temperature phase with alternating layers of triangular and Kagome-arranged Fe-O tetrahedra. We also present details of the crystal growth by traveling solvent method.