Anomalous thermal expansion of iron borate crystals FeBO3 near the Néel point

Study of the anomalous behavior of the linear thermal expansion coefficient near the antiferromagnetic phase transitions in iron borate crystals FeBO3 are reported. The lattice parameters and volume of FeBO3 have been measured in the temperature range 25–300˚C by the X-ray diffraction method. The thermal expansion is evidently sharply anisotropic and spike near the Néel temperature.


1.Introduction
Iron borate, FeBO3, represents a rare example of a magnetically ordered ferric oxide with a high transparency in the visible spectral region and with a Faraday rotation reaching 5000°/cm. From the magnetic point of view, FeBO3 is a two-sublattice easy-plane antiferromagnetic with a weak in-plane moment and the Néel temperature TN = 348 K [1]. The practical applications of the iron borate FeBO3 requires knowledge of the particularities of thermal behavior.
In [1] for the first time was estimated the difference between the thermal expansions of FeBO3 along the z and x axes to be determined from optical data. Previously, we have investigated anisotropy of the thermal expansion of FeBO3 for temperatures 25, 400, 500 и 600 °С [2]. However, correct evaluations of the thermal expansion coefficients could be done in the temperature range from 400 to 600˚C since below Néel point TN = 75°C, FeBO3 is an easy-plane weak antiferromagnetic with the anomalous behavior of physical properties near the region of phase transition into paramagnetic. Accordingly, it was study the characteristics of the thermal expansion of iron borate in the region of the Néel temperature, which was the aim of this study.

2.Object and method of research
The crystalline structure of iron borate, FeBO3, grown by the vapor transport crystallization method, was first determined in work [3] and refined in [4]. Iron borate, FeBO3, has rhombohedral calcite-type structure with point group symmetry 3 ̅ ( 3 ) and space group 3 ̅ ( 3 6 ). The space group has an Rtype Bravais lattice; thus, the unit cell parameters can be specified in both hexagonal and  (6)   For structural studies, samples of iron borate, FeBO3, were ground into powder by standard technology. X-ray powder diffraction measurements of iron borate was executed at the Center of collective use of Institute of Metal Physics of the Ural Branch of the Russian Academy of Sciences. The in-situ powder X-ray diffraction was carried out with a Shimadzu XRD-7000 Maxima diffractometer (Cu (Kα) radiation, graphite monochromator with a thermal attachment (Anton Paar) in the temperature range 18-300°C under pressure of 10 Pa. SiO2 were used as an external standard. The heating rate is 5 K / min, the exposure time before shooting at each point is 15 min. The temperature stability during the measurements was ±4°С. The diffraction angle 2θ was varied from 22-85˚ range with step recording was 0.02°.

3.Analysis and discussion
The lattice parameters determined at different temperatures are given in Table 1. It can be seen that both the parameters «a» and «c» increase with temperature. The room temperature lattice constants obtained in the present study are compared with those available in the literature. The temperature range from 18 to 300°C includes the antiferromagnetic transition temperature TN= 75°C, at which point our measurements show a kink in the temperature dependence curves of the cell parameters, figure 2.
The variation curves of parameters «a» and «c» in two ranges: from 18°С to 80°С and from 80°С to 300°С were approximated to the straight lines: were 0 -lattice constant at room temperature; 1 -characteristic constants; temperature in ˚C.
The values of characteristic constants determined by least-squares method are shown in table 2.
were dadifference of lattice parameters a for the interval dt, dt -difference of temperature, we obtain from (1) the thermal expansion coefficient, table 3. The three-dimensional thermal expansion diagrams of FeBO3 drawing according to [6] is shown in figure 3. The thermal expansion is evidently sharply anisotropic at room temperature (αmax/αmin = 2,9) and the anisotropy increases to αmax/αmin = 4,8 while heating above TN.
A calculation of the thermal expansion coefficients along the «a» and «c» axes of FeBO3 gave the following results, figure 4.

Conclusion
It was found that near the Néel temperature of the phase transition of a magnetic-paramagnetic is accompanied by a sharp jump in the coefficients of thermal expansion along the «a» axis is Δαa=3.39×10 -6 °C -1 and along the «c» axis is Δαc =5.78×10 -6 °C -1 as shown in Figure 4. In fact, there are exchange striction along the z(«c») and x(«a») axes as the difference between of the regular contribution and of the contribution associated with the magnetic order to the thermal expansion. The value of exchange striction along the z, which was evaluated of ≈0.53× 10 -3 at TN = 348 K in [1] are in good agreement with our results ≈0.41× 10 -3 .