Regulation of the phase transition temperature and hysteresis width by changing the composition of Eu1–xLaxFe3(BO3)4 solid solution

A detailed study of the structural phase transition in ${\mathrm{Eu}}_{1\text{--}x}{\mathrm{La}}_{x}{\mathrm{Fe}}_{3}{(\mathrm{B}{\mathrm{O}}_{3})}_{4}$ mixed crystals as a function of composition is reported. By analyzing a frequency shift of an electronic f-f transition in high-resolution optical spectra of ${\mathrm{Eu}}^{3+}$ ions, we detected a decrease in the phase transition temperature ${T}_{\mathrm{s}}$ from 87.05 to 12.2 K (upon cooling) and a simultaneous increase in thermal hysteresis $\mathrm{\ensuremath{\Delta}}{T}_{\mathrm{s}}$ from 0.29 to 4.7 K with increasing $x$ from $x=0$ to $x=0.12$. A rectangular hysteresis loop was observed. The experimental ${T}_{\mathrm{s}}(x)$ and $\mathrm{\ensuremath{\Delta}}{T}_{\mathrm{s}}(x)$ dependences are described within the developed analytical model utilizing linear decrease in ${T}_{\mathrm{s}}$ with $x$ and treating the increase in $\mathrm{\ensuremath{\Delta}}{T}_{\mathrm{s}}$ in terms of the impurity-related decrease in the interaction between some local order parameters. We argue that ${R}_{1\ensuremath{-}x}{R}_{x}^{\ensuremath{'}}{\mathrm{Fe}}_{3}{(\mathrm{B}{\mathrm{O}}_{3})}_{4}$ solid solutions, where $R$ and $R$\ensuremath{'} are different rare-earth elements, can be used to implement optical storage devices and switches operating at any chosen temperature between 0 and 450 K. It is found that the changes in the composition and, correspondingly, structural phase transition parameters do not affect the magnetic phase transformation. ${\mathrm{Eu}}_{0.88}{\mathrm{La}}_{0.12}{\mathrm{Fe}}_{3}{(\mathrm{B}{\mathrm{O}}_{3})}_{4}$ demonstrates the structural phase transition at about 12 K, well below the N\'eel temperature ${T}_{\mathrm{N}}=32\phantom{\rule{0.16em}{0ex}}\mathrm{K}$.