In this contribution, we report the initial observation of neutral muonium defect centers in ZnGeP_2. The spin-precession frequencies in a 4.0 T field yield a zero-temperature hyperfine constant of ～1965 MHz for the promptly formed Mu° state. Subsequently, we performed T_1~(-1) longitudinal depolarization measurements in low magnetic fields. Decoupling curves show a different anisotropic Mu~0 with A_2 = 3185 MHz and D = 374 MHz, where D is the dipolar contribution. The relaxation rates suggest rapid diffusion of Mu~0, most likely via quantum tunneling below 80 K and by thermally activated hops above 100 K. We have determined the ionization energy to be roughly 200 meV. We present our preliminary evaluation of the motion or site transition dynamics in addition to hyperfine characterization of the neutral muonium centers observed in ZnGeP_2.
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