Structure and frustrated magnetism of the two-dimensional triangular lattice antiferromagnet Na2BaNi(PO4)(2) *

A new frustrated triangular lattice antiferromagnet Na2BaNi(PO4)(2) was synthesized by high temperature flux method. The two-dimensional triangular lattice is formed by the Ni2+ ions with S = 1. Its magnetism is highly anisotropic with the Weiss constants theta (CW) = -6.615 K (H perpendicular to c) and -43.979 K (H parallel to c). However, no magnetic ordering is present down to 0.3 K, reflecting strong geometric spin frustration. Our heat capacity measurements show substantial residual magnetic entropy existing below 0.3 K at zero field, implying the presence of low energy spin excitations. These results indicate that Na2BaNi(PO4)(2) is a potential spin liquid candidate with spin-1.