  Single crystals of SrMn2P2 and CaMn2P2 were grown using Sn flux and
characterized by single-crystal x-ray diffraction, electrical resistivity rho,
heat capacity Cp, and magnetic susceptibility chi = M/H measurements versus
temperature T and magnetization M versus applied magnetic field H isotherm
measurements. The x-ray diffraction results show that both compounds adopt the
trigonal CaAl2Si2-type structure. The rho(T) measurements demonstrate
insulating ground states for both compounds. The chi(T) and Cp(T) data reveal a
weak first-order antiferromagnetic (AFM) transition at the Neel temperature TN
= 53(1) K for SrMn2P2 and a strong first-order AFM transition at TN = 69.8(3) K
for CaMn2P2. Both compounds show an isotropic and nearly T-independent chi(T <
TN). {31}P NMR measurements confirm the strong first-order transition in
CaMn2P2 but show critical slowing down near TN for SrMn2P2 thus evidencing
second-order character. The NMR measurements also indicate that the AFM
structure of CaMn2P2 is commensurate with the lattice whereas that of SrMn2P2
is incommensurate. These first-order AFM transitions are unique among the class
of trigonal (Ca, Sr, Ba)Mn2(P, As, Sb, Bi)2 compounds which otherwise exhibit
second-order AFM transitions. This result presents a challenge to understand
the systematics of magnetic ordering in this class of materials in which
magnetically-frustrated antiferromagnetism is quasi-two-dimensional.
