Mössbauer-Effect Measurements in Antiferromagnetic Fe<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">Cl</mml:mi></mml:mrow><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>

M\"ossbauer-effect data for single-crystal specimens of anhydrous ferric chloride are presented which cover the region in the $H\ensuremath{-}T$ plane for $H$ less than kOe and the temperatures less than 300 K. The magnetic phase boundaries between the paramagnetic, antiferromagnetic, and "spin-flop" phases are observed. The zero-field data show antiferromagnetic order with ${T}_{N}=8.76$ K, and the hyperfine field is found to obey a power law of the form ${H}_{\mathrm{hf}}(T)=H(0) B{(1\ensuremath{-}\frac{T}{{T}_{N}})}^{\ensuremath{\beta}}$, where $\ensuremath{\beta}=0.156$, $B=1.035$, and $H(0)=495$ kOe. This power law holds for the full range of temperatures less than ${T}_{N}$. The data just below ${T}_{N}$ show extreme broadening in the M\"ossbauer spectrum, reminiscent of relaxation effects in paramagnets, most likely due to critical slowing down. The data in zero field are consistent with the unusual spiral-spin structure found by neutron diffraction. However, for fields larger than 15 kOe applied in the c direction, the M\"ossbauer data show two distinct hyperfine fields as if there were two sublattices instead of a spiral structure. When the applied field in the crystalline $c$ direction exceeds 40 kOe (at $T=4.2$ K), a spin reorientation takes place similar to what one finds in a "spin flop." The field necessary to "flop" these spins increases with temperature to about 48 kOe at 8.4 K where a triple point exists, and the N\'eel temperature decreases with increasing applied field. The data in applied fields also show the extreme broadening of the resonance lines as the critical temperature is approached. The data in the paramagnetic region show the influence of the magnetic interactions. The results suggest that Fe${\mathrm{Cl}}_{3}$ is possibly near a tricritical point.