Structural and magnetic characterization of CeTa7O19 and YbTa7O19

Triangular lattice antiferromagnets are prototypes for frustrated magnetism and may potentially realize novel quantum magnetic states such as a quantum spin-liquid ground state. A recent work suggests ${\mathrm{NdTa}}_{7}{\mathrm{O}}_{19}$ with rare-earth triangular lattice is a quantum spin-liquid candidate and highlights the large family of rare-earth heptatantalates as a framework for quantum magnetism investigation. In this paper, we report the structural and magnetic characterization of ${\mathrm{CeTa}}_{7}{\mathrm{O}}_{19}$ and ${\mathrm{YbTa}}_{7}{\mathrm{O}}_{19}$. Both compounds are isostructural to ${\mathrm{NdTa}}_{7}{\mathrm{O}}_{19}$ with no detectable structural disorder. For ${\mathrm{CeTa}}_{7}{\mathrm{O}}_{19}$, the crystal field energy levels and parameters are determined by inelastic neutron scattering measurements. Based on the crystal field result, the magnetic susceptibility data could be well fitted and explained, which reveals that ${\mathrm{CeTa}}_{7}{\mathrm{O}}_{19}$ is a highly anisotropic Ising triangular-lattice antiferromagnet (${g}_{z}/{g}_{xy}\ensuremath{\sim}3$) with very weak exchange interaction ($\mathrm{J}\ensuremath{\sim}0.22$ K). For ${\mathrm{YbTa}}_{7}{\mathrm{O}}_{19}$, millimeter-sized single crystals could be grown. The anisotropic magnetization and electron spin resonance data show that ${\mathrm{YbTa}}_{7}{\mathrm{O}}_{19}$ has a contrasting in-plane magnetic anisotropy with ${g}_{z}/{g}_{xy}\ensuremath{\sim}0.67$ similar as that of ${\mathrm{YbMgGaO}}_{4}$. The above results indicate that ${\mathrm{CeTa}}_{7}{\mathrm{O}}_{19}$ and ${\mathrm{YbTa}}_{7}{\mathrm{O}}_{19}$ with pseudospin-1/2 ground states might either be quantum spin-liquid candidate materials or find applications in adiabatic demagnetization refrigeration due to the weak exchange interaction.