Formation of New Crystalline Phases upon High-Temperature Annealing of Iron Borate FeBO 3 in Different Gas Media
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Formation of New Crystalline Phases upon High-Temperature Annealing of Iron Borate FeBO3 in Different Gas Media N. I. Snegireva, *, I. S. Lyubutina, S. V. Yagupovb, A. G. Kulikova, V. V. Artemova, Yu. A. Mogilenecb, and M. B. Strugatskyb a Shubnikov
Institute of Crystallography, Federal Scientific Research Centre Crystallography and Photonics, Russian Academy of Sciences, Moscow, 119333 Russia b Physics and Technology Institute, Vernadsky Crimean Federal University, Simferopol, 295007 Russia *e-mail: [email protected] Received August 11, 2020; revised August 14, 2020; accepted August 14, 2020
Iron borate FeBO3 crystals are annealed in neutral, oxidizing, and reducing gas media on a specially designed setup. The effect of the chemical medium and annealing modes on the surface morphology and phase composition of the samples is established. It is shown that annealing in a neutral or oxidizing medium leads to the recrystallization of FeBO3 into the hematite α-Fe2O3 phase, whereas annealing in a reducing medium results in the formation of the pyroborate Fe2B2O5 phase and the metal α-Fe iron phase. The difference in the nature of structural transformations is associated with the change in the valence state of iron ions upon annealing in the reducing atmosphere. This result is of interest as a method for the transformation of crystalline phases. DOI: 10.1134/S0021364020180113
1. INTRODUCTION Iron borate FeBO3 is isostructural to the mineral calcite CaCO3 and belongs to the space group R 3c [1]. Although this compound was first synthesized back in 1963, the interest of researchers in it is increasing owing to the possibility of using FeBO3 crystals in various fields, including optoelectronics and synchrotron technologies [1–16]. An insulator–semiconductor transition with electron spin crossover [2–6], magnetic birefringence of sound [7], and magnetoacoustic phenomena induced by femtosecond laser pulses [8] were studied in FeBO3 crystals. It was shown in a number of theoretical works that FeBO3 nanoparticles can be used as efficient optical converters [9, 10], and the introduction of iron borate in the electrolyte composition of lithium-ion batteries significantly improves their characteristics [11]. In addition, iron borate single crystals, where Fe is enriched in the 57Fe isotope, are used in spectroscopic studies to separate the resonance range from “white” synchrotron radiation [12–14, 17, 18]. It is known that the FeBO3 compound without contact with the atmosphere melts incongruently at a temperature above 900°C [19, 20]. Beyond the limits of stability, irreversible changes in the structure of iron borate occur because of the formation of new crystalline phases. The effect of high temperatures on the phase composition of iron borate single and polycrystals was previously studied in our works [21, 22] and in
[19, 20]. It was found that FeBO3 is transformed to hematite α-Fe2O3 upon annealing and the Fe3BO6 phase is also detected under certain temperature conditions in annealed samples [19, 21]. In addition,
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