Crystal structure, phase transition and structural deformations in iron borate (Y_0.95Bi_0.05)Fe₃(BO₃)₄in the temperature range 90–500 K

Abstract

An accurate X-ray diffraction study of (YBi)Fe(BO) single crystals in the temperature range 90-500 K was performed on a laboratory diffractometer and used synchrotron radiation. It was established that the crystal undergoes a diffuse structural phase transition in the temperature range 350-380 K. The complexity of localization of such a transition over temperature was overcome by means of special analysis of systematic extinction reflections by symmetry. The transition temperature can be considered to be T ≃ 370 K. The crystal has a trigonal structure in the space group P321 at temperatures of 90-370 K, and it has a trigonal structure in the space group R32 at 375-500 K. There is one type of chain formed by the FeO octahedra along the c axis in the R32 phase. When going into the P321 phase, two types of nonequivalent chains arise, in which Fe atoms are separated from the Y atoms by a different distance. Upon lowering the temperature from 500 to 90 K, a distortion of the Y(Bi)O, FeO, B(2,3)O coordination polyhedra is observed. The distances between atoms in helical Fe chains and Fe-O-Fe angles change non-uniformly. A sharp jump in the equivalent isotropic displacement parameters of O1 and O2 atoms within the Fe-Fe chains and fluctuations of the equivalent isotropic displacement parameters of B2 and B3 atoms were observed in the region of structural transition as well as noticeable elongation of O1, O2, B2, B3, Fe1, Fe2 atomic displacement ellipsoids. It was established that the helices of electron density formed by Fe, O1 and O2 atoms may be structural elements determining chirality, optical activity and multiferroicity of rare-earth iron borates. Compression and stretching of these helices account for the symmetry change and for the manifestation of a number of properties, whose geometry is controlled by an indirect exchange interaction between iron cations that compete with the thermal motion of atoms in the structure. Structural analysis detected these changes as variations of a number of structural characteristics in the c unit-cell direction, that is, the direction of the helices. Structural results for the local surrounding of the atoms in (YBi)Fe(BO) were confirmed by EXAFS and Mössbauer spectroscopies.

Crystal structure and structural phase transition in bismuth-containing HoFe3(BO3)4in the temperature range 11–500 K

“…However, it is lower than T str = 174 K, found by Hinatsu et al (2002). An analysis of systematic extinction determined that a diffuse phase transition in (Y 0.95 Bi 0.05 )-Fe 3 (BO 3 ) 4 occurs at about 370 K (Frolov et al, 2018;Smirnova et al, 2018). This is close to T str = 350 K found by Fausti et al (2006), but lower than T str = 445 K for powder samples (Hinatsu et al, 2002).…”

“…To study the structure of the single crystal above and below the phase transition temperature T str , we obtained complete sets of X-ray diffraction data at 90,293,350,360,365,370,380,390 and 500 K using the Rigaku Oxford CCD Xcalibur EOS S2 X-ray diffractometer, equipped with a Cobra Plus (Oxford Cryosystems) temperature device with a laminar flow of nitrogen gas. The temperature of the transition T str was assumed to be 360 K (Erofeev et al, 2016), 427 K (Hinatsu et al, 2002) or close to the temperature of 370 K found for (Y 0.95 Bi 0.05 )Fe 3 (BO 3 ) 4 (Smirnova et al, 2018).…”

“…The data on the temperature value of the structural phase transition in holmium iron borate are contradictory, similar to the case for yttrium iron borate (Smirnova et al, 2018). Hinatsu et al (2002) performed a differential thermal analysis (DTA) on the HoFe 3 (BO 3 ) 4 powder samples prepared in the oxide system without Bi.…”

“…Such reflections begin to appear at 380-365 K. The number of observed systematic extinctions sharply increases from 194 at 365 K to 4734 at 360 K, which indicates a structural phase transition [ Fig. 3(a) Temperature dependence of the number of systematic extinctions in space group R32 (hexagonal setting) with I > 3(I), observed in the diffraction pattern of (a) (Ho 0.96 Bi 0.04 )Fe 3 (BO 3 ) 4 and (b) (Y 0.95 Bi 0.05 )Fe 3 (BO 3 ) 4 (Smirnova et al, 2018), and (c) temperature dependence of the intensity of systematic extinctions in space group R32 (hexagonal setting) observed in the diffraction pattern of (Ho 0.96 Bi 0.04 )-Fe 3 (BO 3 ) 4 in the temperature range 90-500 K. General view of the (Ho 0.96 Bi 0.04 )Fe 3 (BO 3 ) 4 structure: (a) at low temperatures (90-360 K) in the space group P3 1 21 and (b) at high temperatures (365-500 K) in the space group R32. The upper images show a top view of the unit cells.…”

“…So far, using energy-dispersive X-ray spectroscopy (EDX), we observed the entry of Bi dopant atoms into the chemical composition of the gadolinium (Smirnova et al, 2016) and yttrium (Smirnova et al, 2018) iron borates grown using Bi 2 Mo 3 O 12 -based flux. The bismuth atoms share a crystallographic site with rare-earth atoms.…”