The magnetic lanthanide $\left({\mathrm{Ln}}^{3+}\right)$ ions in the fergusonite and scheelite crystal structures form a distorted or stretched diamond lattice which is predicted to host exotic magnetic ground states. In this study, polycrystalline samples of the fergusonite orthotantalates $M\text{−}{\mathrm{LnTaO}}_4$ (Ln = Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er) were synthesized and then characterized using powder diffraction and bulk magnetometry and heat capacity. ${\mathrm{TbTaO}}_4$ orders antiferromagnetically at 2.25 K into a commensurate magnetic cell with $\vec{k}=0$, magnetic space group 14.77 $(P{2}_1^{\prime }/c)$, and Tb moments parallel to the $a$ axis. No magnetic order was observed in the other materials studied, leaving open the possibility of exotic magnetic states at $T<2$ K.

• Received 4 February 2022
• Accepted 6 April 2022

DOI:https://doi.org/10.1103/PhysRevMaterials.6.044410