Realising square and diamond lattice $S=1/2$ Heisenberg antiferromagnet models in the $Î±$ and $Î²$ phases of the coordination framework, KTi(C$_2$O$_4$)$_2\cdot$\textit{x}H$_2$O

Aly H. Abdeldaim; Teng Li; Lewis Farrar; Alexander A. Tsirlin; Wenjiao Yao; Alexandra S. Gibbs; Pascal Manuel; Philip Lightfoot; GÃ¸ran J. Nilsen; Lucy Clark

doi:10.1103/PhysRevMaterials.4.104414

paper

Realising square and diamond lattice $S = 1/2$ Heisenberg antiferromagnet models in the $I^\pm$ and $I^^{2}$ phases of the coordination framework, KTi(C $_{2}$ O $_{4}$ ) $_{2} \cdot$ \textit{x}H $_{2}$ O

arXiv:2009.03214 · doi:10.1103/PhysRevMaterials.4.104414

Abstract

We report the crystal structures and magnetic properties of two psuedo-polymorphs of the $S = 1/2$ Ti $^{3 +}$ coordination framework, KTi(C $_{2}$ O $_{4}$ ) $_{2} \cdot$ xH $_{2}$ O. Single-crystal X-ray and powder neutron diffraction measurements on $I^\pm$ -KTi(C $_{2}$ O $_{4}$ ) $_{2} \cdot$ xH $_{2}$ O confirm its structure in the tetragonal $I 4/ m c m$ space group with a square planar arrangement of Ti $^{3 +}$ ions. Magnetometry and specific heat measurements reveal weak antiferromagnetic interactions, with $J_{1} \approx 7$ K and $J_{2} / J_{1} = 0.11$ indicating a slight frustration of nearest- and next-nearest-neighbor interactions. Below $1.8$ K, $I^\pm$ 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 Ti $^{3 +}$ in $I^\pm$ is suppressed from its spin-only value to $0.62 (3) I^¼_{B}$ , thus verifying the two-dimensional nature of the magnetic interactions within the system. $I^^{2}$ -KTi(C $_{2}$ O $_{4}$ ) $_{2} \cdot$ 2H $_{2}$ O, on the other hand, realises a three-dimensional diamond-like magnetic network of Ti $^{3 +}$ moments within a hexagonal $P 6_{2} 22$ structure. An antiferromagnetic exchange coupling of $J \approx 54$ K -- an order of magnitude larger than in $I^\pm$ -- is extracted from magnetometry and specific heat data. $I^^{2}$ undergoes NÃ©el ordering at $T_{N} = 28$ K, with the magnetic moments aligned within the $ab$ plane and a slightly reduced ordered moment of $0.79 I^¼_{B}$ per Ti $^{3 +}$ . Through density-functional theory calculations, we address the origin of the large difference in the exchange parameters between the $I^\pm$ and $I^^{2}$ psuedo-polymorphs. Given their observed magnetic behaviors, we propose $I^\pm$ -KTi(C $_{2}$ O $_{4}$ ) $_{2} \cdot$ xH $_{2}$ O and $I^^{2}$ -KTi(C $_{2}$ O $_{4}$ ) $_{2} \cdot$ 2H $_{2}$ O as close to ideal model $S = 1/2$ Heisenberg square and diamond lattice antiferromagnets, respectively.

12 pages, 7 figures

Realising square and diamond lattice S=1/2 Heisenberg antiferromagnet models in the I^± and I^2 phases of the coordination framework, KTi(C2​O4​)2​⋅\textit{x}H2​O

Abstract

Realising square and diamond lattice $S = 1/2$ Heisenberg antiferromagnet models in the $I^\pm$ and $I^^{2}$ phases of the coordination framework, KTi(C $_{2}$ O $_{4}$ ) $_{2} \cdot$ \textit{x}H $_{2}$ O