Crystal structure and magnetic properties of the Ba₃TeCo ₃P₂O₁₄, Pb₃TeCo₃P ₂O₁₄, and Pb₃TeCo₃V ₂O₁₄ langasites

We report the structural and magnetic characterizations of Ba ₃TeCo₃P₂O₁₄, Pb₃TeCo ₃P₂O₁₄, and Pb₃TeCo ₃V₂O₁₄, compounds that are based on the mineral dugganite, which is isostructural to langasites. The magnetic part of the structure consists of layers of Co²⁺ triangles. Nuclear and magnetic structures were determined through a co-refinement of synchrotron and neutron powder diffraction data. In contrast to the undistorted P321 langasite structure of Ba₃TeCo₃P₂O₁₄, a complex structural distortion yielding a large supercell is found for both Pb ₃TeCo₃P₂O₁₄ and Pb ₃TeCo₃V₂O₁₄. Comparison of the three compounds studied along with the zinc analog Pb₃TeZn ₃P₂O₁₄, also characterized here, suggests that the distortion is driven by Pb²⁺ lone pairs; as such, the Pb compounds crystallize in a pyroelectric space group, P2. Magnetic susceptibility, magnetization, and heat capacity measurements were performed to characterize the magnetic behavior. All three compounds become antiferromagnetic with Néel temperatures T_n≈21 K (Ba₃TeCo ₃P₂O₁₄), ≈13 K (Pb₃TeCo ₃P₂O₁₄), and ≈8 K (Pb₃TeCo ₃V₂O₁₄), and they exhibit magnetic transitions at high applied magnetic fields, suggesting intrinsically complex magnetic behavior for tetrahedrally coordinated d⁷ Co²⁺ in this structure type.