Mixed
metal phenylphosphonates of composition Mn x Co 1؊x (O 3 PC 6 H 5 ) ' H 2 O were prepared with 04x41. Atomic absorption, X-ray powder di4raction, and electron paramagnetic resonance measurements indicate that the mixedmetal solid solutions are homogeneous and isostructural with the single-metal-parent compounds over the entire concentration range, with a small, systematic evolution of the a and c in-plane unit cell parameters. The temperature dependence of the magnetic data for the pure Mn 2؉ (x ؍ 1) and pure Co 2؉ (x ؍ 0) samples was 5tted by standard 2D Heisenberg and 2D Ising models, respectively, yielding nearest-neighbor exchange interaction energies of J ؍ ؊2.27؎0.02 K for Mn(O 3 PC 6 H 5 ) ' H 2 O and J ؍ ؊2.43؎0.05 K for Co(O 3 PC 6 H 5 ) ' H 2 O. The magnetic phase diagram, down to 2 K, was constructed over the entire composition range. Both dc and ac magnetic susceptibilities were used to identify the transitions to low temperature, long-rangeordered antiferromagnetic states. In the Mn 2؉ -and Co 2؉ -rich regions, the ordering temperature, T N , decreases relative to the pure materials, as expected for magnetic ion impurity doping. For intermediate values of x, Mn 2؉ +Mn 2؉ interactions dominate, resulting in a minimum in T N near x ؍ 0.25. A weak negative magnetization was observed for x(0.25. No evidence of spin glass behavior was observed for any concentration at any temperature.