Signatures for Berezinskii-Kosterlitz-Thouless critical behavior in the planar antiferromagnet <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>BaNi</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mi mathvariant="normal">V</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mn>8</mml:mn></mml:msub></mml:mrow></mml:math>

We investigate the critical properties of the spin-1honeycomb antiferromagnet BaNi_2_2_8 both below and above the ordering temperature T_Nusing neutron diffraction and muon spin rotation measurements. Our results characterize BaNi_2_2_8as a two-dimensional (2D) antiferromagnet across the entire temperature range, displaying a series of crossovers from 2D Ising-like to 2D XY and then to 2D Heisenberg behavior with...