Structural and physical properties of the chiral antiferromagnet CeRhC2

We report a study of the structural, magnetic, transport, and thermodynamic properties of polycrystalline samples of ${\mathrm{CeRhC}}_{2}$ and ${\mathrm{LaRhC}}_{2}$. ${\mathrm{CeRhC}}_{2}$ crystallizes in a tetragonal structure with space group $P{4}_{1}$, and it orders antiferromagnetically below ${T}_{\text{N1}}\ensuremath{\approx}1.8\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. Powder neutron diffraction measurements reveal a chiral magnetic structure with a single propagation vector ${Q}_{m}=(1/2,1/2,0.228(5))$, indicating an antiferromagnetic arrangement of Ce magnetic moments in the $ab$ plane and incommensurate order along the $c$ axis with a root-mean-square ordered moment of ${m}_{\text{ord}}=0.68\phantom{\rule{0.16em}{0ex}}{\textmu{}}_{\text{B}}$. A second antiferromagnetic phase $({T}_{\text{N2}})$ becomes apparent in electrical resistivity, Hall, and heat capacity measurements in fields above 0.3 T. Electrical resistivity measurements reveal that ${\mathrm{LaRhC}}_{2}$ is a semiconductor with a band gap of ${E}_{\text{g}}\ensuremath{\sim}24$ meV, whereas resistivity and Hall measurements indicate that ${\mathrm{CeRhC}}_{2}$ is a semimetal with a carrier concentration of $n\ensuremath{\sim}{10}^{20}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$. With applied hydrostatic pressure, the zero-field antiferromagnetic transition of ${\mathrm{CeRhC}}_{2}$ is slightly enhanced, and ${\mathrm{CeRhC}}_{2}$ becomes notably more metallic up to 1.36 GPa. The trend toward metallicity is in line with density-functional calculations that indicate that both ${\mathrm{LaRhC}}_{2}$ and ${\mathrm{CeRhC}}_{2}$ are semimetals, but the band overlap is larger for ${\mathrm{CeRhC}}_{2}$, which has a smaller unit cell volume than that of its La counterpart.