  The Eu{+2} spins-7/2 in EuNi2As2 with the body-centered tetragonal ThCr2Si2
structure order antiferromagnetically below the Neel temperature TN =15 K into
a helical antiferromagnetic (AFM) structure with the helix axis aligned along
the tetragonal c axis and the Eu ordered moments aligned ferromagnetically
within the ab plane as previously reported from neutron diffraction
measurements [T. Jin, et al., Phys. Rev. B 99, 014425 (2019)]. Here we study
the crystallographic, magnetic, thermal, and electronic transport properties of
Bi-flux-grown single crystals using single-crystal x-ray diffraction,
anisotropic magnetic susceptibility chi, isothermal magnetization M, heat
capacity Cp, and electrical resistivity rho measurements versus applied
magnetic field H and temperature T. Vacancies are found on the Ni sites
corresponding to the composition EuNi{1.95}As2. The chi{ab}(T) data below TN
are fitted well by molecular field theory (MFT), and the helix turn angle kd
and the Eu-Eu Heisenberg exchange constants are extracted from the fit
parameters. The kd value is in good agreement with the neutron-diffraction
result. The magnetic contribution to the zero-field heat capacity below TN is
also fitted by MFT. The isothermal in-plane magnetization Mab exhibits two
metamagnetic transitions versus H, whereas Mc(T = 2 K) is nearly linear up to H
=14 T, both behaviors being consistent with MFT. The Mc(H,T), rho(Hc,T), and
Cp(Hc,T) data yielded a Hc-T phase diagram separating the AFM and paramagnetic
phases in good agreement with MFT. Anisotropic chi(T) literature data for the
ThCr2Si2-type helical antiferromagnet EuRh2As2 are also fitted well by MFT. A
comparison is made between the crystallographic and magnetic properties of
ThCr2Si2- type EuM2Pn2 compounds with M = Fe, Co, Ni, Cu, or Rh, and Pn = P or
As, where only ferromagnetic and c-axis helical AFM structures are found.
