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 $\alpha$-KTi(C$_2$O$_4$)$_2\cdot$xH$_2$O confirm its structure in the tetragonal $I4/mcm$ space group with a square planar arrangement of Ti$^{3+}$ ions. Magnetometry and specific heat measurements reveal weak antiferromagnetic interactions, with $J_1\approx7$ K and $J_2/J_1=0.11$ indicating a slight frustration of nearest- and next-nearest-neighbor interactions. Below $1.8$ K, $\alpha$ 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 $\alpha$ is suppressed from its spin-only value to $0.62(3)~\mu_B$, thus verifying the two-dimensional nature of the magnetic interactions within the system. $\beta$-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 $P6_222$ structure. An antiferromagnetic exchange coupling of $J\approx54$ K -- an order of magnitude larger than in $\alpha$ -- is extracted from magnetometry and specific heat data. $\beta$ undergoes N\'eel ordering at $T_N=28$ K, with the magnetic moments aligned within the $ab$ plane and a slightly reduced ordered moment of $0.79~\mu_B$ per Ti$^{3+}$. Through density-functional theory calculations, we address the origin of the large difference in the exchange parameters between the $\alpha$ and $\beta$ psuedo-polymorphs. Given their observed magnetic behaviors, we propose $\alpha$-KTi(C$_2$O$_4$)$_2\cdot$xH$_2$O and $\beta$-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.
Highlights
The discovery of new magnetic materials allows for the realization of theoretical ground state predictions as well as the identification of novel emergent phenomena
Systems based on the S = 1/2 Heisenberg frustrated square lattice (FSL) model, for example, are extensively studied as they provide a rich magnetic phase diagram depending on the degree of frustration between exchange interactions along the sides, J1, and across the diagonal, J2, of the square net
A multi-bank refinement of this model to Powder neutron diffraction (PND) data collected on High-Resolution Powder Diffractometer (HRPD) further confirms this structure at all measured temperatures [Fig. 4(a)] and gives an overall χ2 = 3.06 and Rp = 1.81% at 1.8 K with the model shown in Table
Summary
The discovery of new magnetic materials allows for the realization of theoretical ground state predictions as well as the identification of novel emergent phenomena. Recent investigations of A-site spinels have revived interest in the Heisenberg frustrated diamond lattice (FDL) model following the experimental observation of spin-spiral structures[22] and spin-liquid regimes[23, 24]. As such structures indicate exchange competition, this behavior can be ascribed to the presence of a frustrated further-neighbor coupling, J2, beyond the nearest-neighbor interaction, J1. Perturbations further enrich the magnetism of such FDL systems, and spin-orbit coupling [27,28,29], further-neighbor couplings, and structural distortions[22] can play a significant role in determining the magnetic ground state
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