Realising square and diamond lattice S=1/2 Heisenberg antiferromagnet models in the α and β phases of the coordination framework, KTi(C2O4)2·xH2O
Abstract
We report the crystal structures and magnetic properties of two psuedo-polymorphs of the S=1/2 Ti3+ coordination framework, KTi(C2O4)2·xH2O. Single-crystal X-ray and powder neutron diffraction measurements on α-KTi(C2O4)2·xH2O confirm its structure in the tetragonal I4/mcm space group with a square planar arrangement of Ti3+ ions. Magnetometry and specific heat measurements reveal weak antiferromagnetic interactions, with J1≈7 K and J2/J1=0.11 indicating a slight frustration of nearest- and next-nearest-neighbor interactions. Below 1.8 K, α undergoes a transition to G-type antiferromagnetic order with magnetic moments aligned along the c axis of the tetragonal structure. The estimated ordered moment of Ti3+ in α is suppressed from its spin-only value to 0.62(3)~μB, thus verifying the two-dimensional nature of the magnetic interactions within the system. β-KTi(C2O4)2·2H2O, on the other hand, realises a three-dimensional diamond-like magnetic network of Ti3+ moments within a hexagonal P6222 structure. An antiferromagnetic exchange coupling of J≈54 K -- an order of magnitude larger than in α -- is extracted from magnetometry and specific heat data. β undergoes N\'eel ordering at TN=28 K, with the magnetic moments aligned within the ab plane and a slightly reduced ordered moment of 0.79~μB per Ti3+. Through density-functional theory calculations, we address the origin of the large difference in the exchange parameters between the α and β psuedo-polymorphs. Given their observed magnetic behaviors, we propose α-KTi(C2O4)2·xH2O and β-KTi(C2O4)2·2H2O as close to ideal model S=1/2 Heisenberg square and diamond lattice antiferromagnets, respectively.