Dimer paramagnetic centers in lead germanate crystals doped with iron and halogen (Cl$^-$, Br$^-$, F$^-$) ions

The dimer complexes Fe$^{3+}$–Cl$^-$, Fe$^{3+}$–Br$^-$, and Fe$^{3+}$–О$^{2-}$ in ferroelectric lead germanate crystals doped with iron and annealed in chlorine-, bromine-, and fluorine-containing atmospheres have been studied using the electron paramagnetic resonance method. These complexes are formed by Fe$^{3+}$ ions in the trigonal position of lead and their associated anions located in the interstitial channel of the structure. The positions of the charge-compensating anions in the channel have been discussed based on the analysis of the parameters of the spin Hamiltonian and their temperature dependence.