Experimental results are reported on S-state ions incorporated into PbFCl-type tetragonal single crystals. The spin Hamiltonian parameters of these ions are determined and correlated with structural and physico-chemical properties of the studied compounds. The experimental b20 spin Hamiltonian parameters vary linearly with the lattice constant a of the host unit cell. Further, the dependence of the spin Hamiltonian parameters on temperature has been investigated and parametrised. The results of a Raman investigation of the hosts are presented.