K₂Ni(SeO₃)₂: A Perfect S=1 Triangular-Lattice Antiferromagnet with Strong Geometric Frustration and Easy-Plane Anisotropy

Anew perfect triangular lattice antiferromagnet K2Ni(SeO3)(2) (S = 1) has beensynthesized via solid-state reaction method, in which the Ni2+ ions in the triangular-lattice layer and the nearest-neighborNi(2+) ions in the adjacent layer constitute magnetic pyramids,resulting in both in-plane and inter-plane geometric frustration. We synthesized a single crystal of K2Ni(SeO3)(2) (S = 1) with an equilateraltriangularlattice (space group R-3m). Themagnetic susceptibility & chi;(T) and heat capacity C (p)(T) show that the compoundis antiferromagnetically ordered at T (N) & AP; 7 K. The Curie-Weiss temperature is about & theta;(CW) = -72.41 and -71.83 K when the field is appliedalong the ab plane (H//ab) and parallel to the c axis (H//c), respectively, indicating a strong geometricfrustration (f & AP; 10). Both the & chi;(T) and M(H) point to thepresence of easy-plane anisotropy. Density functional theory (DFT)calculations estimate the intraplane exchange (J/k (B) = -19.04 K) and the interplane exchange(J & PRIME;/k (B) = -0.01K), proving the excellent two-dimensionality. The 50 T high-fieldmagnetization M(H) at 2 K indicatesa sign of 1/3 magnetization plateau. The high-field electron spinresonance demonstrates the antiferromagnetic resonance modes witheasy-plane anisotropy, yielding J highly consistentwith the DFT result. Thus, K2Ni(SeO3)(2) is a perfect triangular-lattice antiferromagnet with strong frustrationand easy-plane anisotropy.