Crystal electric field levels of TbTaO4 and TbNbO4

TbTaO4 and TbNbO4 are examples of magnetic Tb3+ ions on a "stretched diamond lattice", which displays magnetic frustration. These compounds are isostructural, since Nb5+ and Ta5+ have almost identical ionic radii, and both compounds order antiferromagnetically at low temperatures (T_N = 2.25 K and 1.9 K respectively). However, dielectric measurements show clear differences between the tantalate and niobate which might relate to crystal electric field (CEF) considerations and/or a second-order Jahn-Teller effect. This proposal aims to characterise the CEF levels of TbTaO4 and TbNbO4 in order to support and better understand our data regarding magnetoelectric coupling.