X-ray diffraction analysis and specific heat capacity of (Bi_1−xLa_x)FeO₃ perovskites

Abstract

Phase relations of BiFeO₃–LaFeO₃ system at room temperature were investigated by X-ray powder diffraction. A rhombohedral (R3c) phase exists in the composition range of 0 ≤ x ≤ 0.1 and an orthorhombic (Pbnm) phase in the range of 0.6 < x ≤ 1.0. Thermal expansion of single-phase samples with x = 0, 0.1, 0.7, 0.9 and 1.0 was studied by temperature-dependent X-ray diffraction. The temperature dependences of expansion coefficients of the samples were obtained. Specific heat capacity of the samples with x = 0, 0.1, 0.8 and 1.0 was measured in the temperature range of 200–760 K by a modulated-temperature differential scanning calorimeter. The phase boundaries of the antiferromagnetic–paramagnetic transition were determined.

Introduction

Recently, materials that simultaneously exhibit ferroelectric and magnetic orders are attracting more and more attention. This is not only due to the expectation that multiferroics would offer potential applications for new devices taking advantage of the multiferroic coupling, but also because of the interesting physics in this class of materials. BiFeO₃ (BFO) that exhibits a rhombohedrally distorted perovskite structure (space group: R3c) is an interesting candidate as a magnetoelectric material because the ferroelectricity and antiferromagnetic order coexist at room temperature. G-type antiferromagnetic ordering takes place at 640 K [1], [2], while ferroelectric order appears at a high temperature of 1100 K [3]. LaFeO₃ (LFO) also has the perovskite structure (space group: Pbnm) [4] with an antiferromagnetic ordering (T_N ≈ 738 K) [5]. The effect of the La substitution for Bi in BFO on the physical properties is interesting, and the BFO–LFO system has been studied by several authors [6], [7], [8], [9]. However, the reported phase relations of the system are not consistent with each other, and the thermodynamic data are scarce.

Roginskaya et al. [6] have investigated the phase relations by X-ray powder diffraction (XRD) and the phase transition from the antimagnetic ordering to the paramagnetic state by the temperature-dependent magnetic measurement in the system (1 − x)BFO–xLFO. They reported that a continuous series of solid solutions existed in four modifications: the rhombohedral modification in 0 ≤ x < 0.188, pseudomonoclinic I in 0.188 < x < 0.55, pseudomonoclinic II in 0.55 < x < 0.73 and pseudomonoclinic III in the range of x > 0.75, and T_N linearly increased with the LFO content in the whole composition range [6]. Polomska et al. [7] reported the similar results as those in Ref. [6]. However, Gabbasova et al. [8] gave a different phase relation at room temperature that R3c existed in 0 < x < 0.06, P1 in 0.06 < x < 0.24, C222 in 0.24 < x < 0.40, C2₁22 in 0.40 < x < 0.55 and Pbn2₁ in 0.55 < x < 0.70. Recently, Zhang et al. [9] investigated the system in the composition range of 0 ≤ x ≤ 0.4 by XRD and found that the phase transition from rhombohedral to orthorhombic took place near x = 0.3 and no low symmetric phase existed. In this work, the samples of (1 − x)BFO–xLFO with 0 ≤ x ≤ 1 were prepared, the phase relations and thermal expansion were investigated by XRD, and the antiferromagnetic transition and specific heat capacity were determined by differential scanning calorimeter (DSC) and modulated-temperature DSC (MDSC).