Effect of electron delocalization on the “recoil-free” absorption of $\gamma$-ray photons in Fe$_{1.75}$V$_{0.25}$BO$_4$ warwickite

Mössbauer spectroscopy is used to study the characteristic features of the crystal lattice dynamics in powdered single crystals of Fe$_{1.75}$V$_{0.25}$BO$_4$ warwickite in the temperature range of $4.2$–$505$ K. The Debye temperature ($\Theta_{\text{D}} = 260$ K) is determined from the temperature dependence of the probability of the Mössbauer effect in the thin absorber approximation. It is found that the electron delocalization related to the fast electronic transfer between neighboring Fe$^{3+}$ and Fe$^{2+}$ cations takes place in the temperature range of $260$–$505$ K. As a result, iron cations exhibiting the mixed valence (Fe$^{2.5+}$) arise. This process correlates with a change in the elastic properties of the lattice. Such correlation leads to a sharp decrease in the recoil-free absorption of $\gamma$-ray photons by the crystal lattice in the range of $260$–$400$ K.