M'-LnTaO4: a model for the 2D square lattice of magnetic ions

The lanthanide tantalates LnTaO4 are a family of materials which have recently been investigated for their dielectric, luminescent and ion-conducting properties. In the M' phase, which can be stabilised for the heavier lanthanides Ln = Tb-Lu, the magnetic Ln3+ ions occupy a slightly distorted 2D square lattice. Magnetism in M'-YbTaO4 has recently been investigated as a model material for the Heisenberg J1-J2 square lattice model, which is geometrically frustrated because of competition between the exchange interactions J1 (sides of squares) and J2 (diagonals). Our initial study of four monoclinic M'-LnTaO4 compounds (Ln = Tb, Dy, Ho, Er) reveals Curie-Weiss behaviour with antiferromagnetic ordering in TbTaO4 and DyTaO4 at 2.7 K, and no ordering above 1.8 K in the Ho and Er compounds. We propose a nuclear and magnetic neutron diffraction study at 1.5, 30 and 300 K in order to confirm the crystal structure and determine the magnetic structure of M'-TbTaO4.