Cl*

Pyrochlore antiferromagnets have attracted interests in terms of unconventional ground states and spin excitations owing to competing interactions. When its magnetism is dominated by classical spins coupled by Heisenberg interactions, a spin liquid state is expected as its ground state because of infinite degeneracy in the ground state (R. Moessner and J. T. Chalker, Phys. Rev. Lett. 80, 2929 (1998); Phys. Rev. B 58, 12049 (1998).). In this research, we report structural and magnetic properties together with low-temperature neutron diffraction patterns on a new pyrochlore antiferromagnet Na₃Mn(CO₃)₂Cl. The structure of Na₃Mn(CO₃)₂Cl is isotypic with that of the Co-analogue Na₃Co(CO₃)₂Cl, which exhibits all-in-all-out magnetic order below 1.5 K (Z. Fu et al., Phys. Rev. B 87, 214406 (2013)). On the other hand, no magnetic Bragg peak indicating a magnetic order was detected down to 0.05 K in Na₃Mn(CO₃)₂Cl. A high degeneracy near the ground state is suggested by a magnetic entropy estimated from heat capacity experiments and enhancement of diffuse scattering from neutron diffraction experiments.