single crystals containing corrugated-honeycomb Mn sublattices*

CaMn₂P₂ and SrMn₂P₂ are insulators that crystallize in the trigonal CaAl₂Si₂-type structure containing corrugated honeycomb layers of Mn²⁺ spins 5/2. Magnetic susceptibility χ and heat capacity measurements versus temperature T reveal a strong first-order antiferromagnetic (AFM) transition in CaMn₂P₂ at T_N = 69.8(3) K and a weak first-order AFM transition in SrMn₂P₂ at T_N = 53(1)  K. The χ(T) data for both compounds exhibit nearly isotropic and T-independent behavior below T_N. NMR measurements indicate a commensurate AFM structure for CaMn₂P₂ and an incommensurate one for SrMn₂P₂. CaMn₂P₂ and SrMn₂P₂ are rare examples where an AFM transition in zero-magnetic field is thermodynamically of first order. These first-order AFM transitions are unique among the (Ca, Sr, Ba)Mn₂(P, As, Sb, Bi)₂ compounds that exhibit second-order AFM transitions. Determining the mechanism for the first-order transitions may lead to the development of new materials of technological interest.