Direct evidence of a near-ideal <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msub> <mml:mi>J</mml:mi> <mml:mi>eff</mml:mi> </mml:msub> <mml:mo>=</mml:mo> <mml:mn>1</mml:mn> <mml:mtext>/</mml:mtext> <mml:mn>2</mml:mn> </mml:mrow> </mml:math> ground state in triangular-lattice <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msub> <mml:mi>Na</mml:mi> <mml:mn>2</mml:mn> </mml:msub> <mml:mi>BaCo</mml:mi> <mml:msub> <mml:mrow> <mml:mo>(</mml:mo> <mml:msub> <mml:mi>PO</mml:mi> <mml:mn>4</mml:mn> </mml:msub> <mml:mo>)</mml:mo> </mml:mrow> <mml:mn>2</mml:mn> </mml:msub> </mml:mrow> </mml:math>

We investigated the local Co 3 d electronic structure of Na 2 BaCo ( PO 4 ) 2 using polarization-dependent x-ray absorption spectroscopy (XAS) in combination with full multiplet cluster calculations. We employed the line-fitting inverse partial fluorescence yield (IPFY) technique to obtain accurate XAS spectra from strong insulating materials. Our combined experimental and theoretical analysis reveals a very small effective trigonal distortion of only 11 meV in the CoO 6 octahedra, indicating a close to ideal condition to render a ground state with the J eff = 1 / 2 character. With our cluster model we were also able to simulate magnetic susceptibility measurements along different directions in the crystal. These findings highlight Na 2 BaCo ( PO 4 ) 2 as a promising platform for exploring exotic magnetic phenomena associated with J eff = 1 / 2 ground states on triangular lattices.