Concentration- and Temperature-Induced Phase Transitions in PrAlO₃–SrTiO₃ System

Abstract

Single-phase mixed aluminates-titanates Pr_1−x Sr _x Al_1−x Ti _x O₃ (x = 0.1, 0.2, 0.3, 0.5, 0.7) with rhombohedral perovskite structure were prepared by solid-state reaction technique at 1823–1873 K. Morphotropic rhombohedral-to-cubic phase transition in Pr_1−x Sr _x Al_1−x Ti _x O₃ series is predicted to occur at x = 0.88. The temperature-induced structural phase transition R  $\overline{3}$  с − Pm $\overline{3}$  m in Pr_0.5Sr_0.5Al_0.5Ti_0.5O₃, detected at 930 K by in situ high-temperature X-ray synchrotron powder diffraction, occurs at considerably lower temperature as the corresponding transformation in the parent compound PrAlO₃ (1770 K). Such remarkable drop of the transition temperature is explained by gradual decrease of the perovskite structure deformation in the Pr_1−x Sr _x Al_1−x Ti _x O₃ series with increasing Sr and Ti contents as a consequence of the increasing Goldschmidt tolerance factor. For Pr_0.3Sr_0.7Al_0.3Ti_0.7O₃ phase, a sequence of the low-temperature phase transformation R  $\overline{3}$  с − Immb(C2/m) − I4/mcm was detected.

Background

Rare earth aluminates RAlO₃ with perovskite structure and SrTiO₃-based materials show diverse technological application. In particular, they are used as solid electrolytes and anode materials in solid oxide fuel cells, as substrates for thin film epitaxy, materials for laser hosts, scintillates and phosphors, high-temperature ceramics and refractory materials ([1–5] and references herein). Due to the opposite signs of the temperature coefficient of the resonant frequency (τ _f ) of the RAlO₃ and SrTiO₃ compounds, mixed aluminates-titanates formed in the RAlO₃–SrTiO₃ systems are considered as prospective microwave materials with a high dielectric constant, moderate quality factor and a near zero value of τ _f [5–7].

During the last decade, RAlO₃–SrTiO₃ systems are of considerable interest in the physics of materials used in modern engineering. The two-dimensional electron gas at the interface between two insulators LaAlO₃ and SrTiO₃ [8] has been an active research area in the field-tunable metal-insulator transition, 2D superconductivity, coexistence of superconductivity and ferromagnetism, etc. [9–11]. Just recently, a similar effect was reported on the interfaces of SrTiO₃ and RAlO₃ (R = La, Pr, Nd) and RGaO₃ compounds (R = La, Nd) in both crystalline and amorphous forms [12].

The aim of the present work is the study of the phase and structural behaviour of the mixed aluminates-titanates formed in the PrAlO₃–SrTiO₃ pseudo-binary system. At room temperature, the end members of the system—PrAlO₃ and SrTiO₃—adopt different variants of perovskite structure—rhombohedral R $\overline{3}$  с and cubic Pm  $\overline{3}$  m, respectively. Rhombohedral PrAlO₃ transforms into the cubic perovskite structure at about 1770 K ([4], and references herein). In addition, PrAlO₃ undergoes a sequence of low-temperature (LT) structural phase transformations from the rhombohedral to an orthorhombic Imma structure at 205 K and from orthorhombic to a monoclinic C2/m structure at 151 K ([4], and references herein). Strontium titanate SrTiO₃ undergoes a low-temperature structural phase transition from the cubic to the tetragonal I4/mcm perovskite structure below 105 K [13, 14]. Owing to the abovementioned peculiarities of the crystal structures PrAlO₃ and SrTiO₃ and their structural instabilities, extremely complex phase and structural behaviour are expected in the mixed praseodymium-strontium aluminate-titanate system.

Methods

Mixed aluminates-titanates Pr_1−x Sr _x Al_1−x Ti _x O₃ (x = 0.1, 0.2, 0.3, 0.5, 0.7) were prepared from stoichiometric amounts of the constituent oxides Pr₆O₁₁, Al₂O₃, TiO₂ and strontium carbonate SrCO₃ by solid-state reaction technique. The precursor powders were ball milled in ethanol for 3–6 h, dried, pressed in the pellets and sintered in air at 1673–1773 K for 18 h (the samples with x = 0.1 and 0.2) and at 1593 K for 24 h (the samples with x = 0.3, 0.5 and 0.7). After regrinding and powdering, the obtained products were pressed in the pellets and repeatedly fired in air at 1873 K (x = 0.1 and 0.2) and 1823 K (x = 0.3, 0.5 and 0.7) for 10 h.

X-ray powder diffraction technique (Huber imaging plate Guinier camera G670, Cu Kα₁ radiation) was used for the phase and structural characterization of the samples at room temperature. Thermal behaviour of the mixed aluminates-titanates has been studied exemplarily on Pr_0.5Sr_0.5Al_0.5Ti_0.5O₃ and Pr_0.3Sr_0.7Al_0.3Ti_0.7O₃ samples in the temperature ranges of 298–1173 K and 20–298 K, respectively. Corresponding in situ high-resolution X-ray synchrotron powder diffraction experiments were performed at beamlines B2 at HASYLAB/DESY (Hamburg, Germany) and ID22 at ESRF (Grenoble, France) during beamtimes allocated to the experiments I-20110214 and hc2044, respectively.

All crystallographic calculations including full-profile Rietveld refinement were performed by using WinCSD program package [15].

Results and Discussion

Examination of X-ray powder diffraction patterns revealed a formation of the single-phase perovskite structures in all samples synthesised (Fig. 1). No traces of parasitic phases were detected. An analysis of the splitting of diffraction maxima in the Pr_1−x Sr _x Al_1−x Ti _x O₃ patterns with x = 0.1, 0.2, 0.3 and 0.5 proves rhombohedral deformation of the perovskite structure. The rhombohedral splitting decreases with the increase in the content of strontium and titanium in the Pr_1−x Sr _x Al_1−x Ti _x O₃ series, completely vanishing in the Pr_0.3Sr_0.7Al_0.3Ti_0.7O₃ sample. However, the presence of a weak (113) reflection at the diffraction pattern of this sample (Fig. 1) indicates that its structure still remains rhombohedral.

Fig. 1

Experimental XRD patterns (Cu Kα₁ radiation, λ = 1.54056 Å) of the Pr_1-x Sr _x Al_1-x Ti _x O₃ series

From the experimental XRD patterns of Pr_1−x Sr _x Al_1−x Ti _x O₃ samples, the crystal structure parameters of the mixed aluminates-titanates were derived. Full-profile Rietveld refinement performed in space group R  $\overline{3}$  с resulted in excellent agreement between calculated and experimental profiles (see, for example, Fig. 2) and led to the final structural parameters and residuals presented in Table 1.

Fig. 2

Graphical results of Rietveld refinement of Pr_0.8Sr_0.2Al_0.8Ti_0.2O₃. The experimental XRD pattern (blue dots) is shown in comparison with the calculated one. Vertical bars indicate positions of diffraction maxima in space group R  $\overline{3}$  с. The inset shows the view of the structure as corner-shared Al/TiO₆ octahedra with Pr/Sr species located between them

Table 1 Unit cell parameters, coordinates and isotropic displacement parameters of atoms in Pr_1-x Sr _x Al_1-x Ti _x O₃ structures at RT

Comparison of the obtained structural parameters of the praseodymium-strontium mixed aluminates-titanates with the literature data for the “pure” PrAlO₃ and SrTiO₃ (Fig. 3) clearly proves the formation of the extended solid solution Pr_1−x Sr _x Al_1−x Ti _x O₃ with rhombohedral perovskite structure. A morphotropic phase transition from rhombohedral to the cubic perovskite structure in the Pr_1−x Sr _x Al_1−x Ti _x O₃ series can be expected at x = 0.88, as it follows from the analysis of the concentration dependence of the unit cell dimensions of the rhombohedral lattice (Fig. 3). In the related LaAlO₃–SrTiO₃ system, the rhombohedral solid solution exists up to 60 mole % of LaAlO₃; after that, the transition to the cubic perovskite structure takes place [16].

Fig. 3

Unit cell dimensions of Pr_1-x Sr _x Al_1-x Ti _x O₃ series. The rhombohedral lattice parameters are normalized to the perovskite cell as follows: a _p   = a _r /√2, c _p   = c _r /√12, V _p   = V _r /6. The dashed line marks the phase boundary between the Rh and the C phases

In situ high-temperature X-ray synchrotron powder diffraction investigation of the Pr_0.5Sr_0.5Al_0.5Ti_0.5O₃ sample revealed a continuous phase transition from rhombohedral to the cubic perovskite structure at elevated temperatures. As it was established from the temperature-resolved X-ray synchrotron powder diffraction measurements, the rhombohedral lattice parameters a and c increase anisotropically with temperature and merge together at 930 K, when the transition to the ideal perovskite structure occurs (Fig. 4).

Fig. 4

Temperature evolution of unit cell dimensions of Pr_0.5Sr_0.5Al_0.5Ti_0.5O₃. The rhombohedral lattice parameters are normalized to the perovskite cell as follows: a _p   = a _r /√2, c _p   = c _r /√12, V _p   = V _r /6. The dashed line marks the phase boundary between the Rh and the C phases

In comparison with the parent PrAlO₃ compound, in which transformation to the cubic perovskite structure occurs around 1770 K [4], the R  $\overline{3}$  с − Pm  $\overline{3}$  m transition in Pr_0.5Sr_0.5Al_0.5Ti_0.5O₃ takes place at considerably lower temperature of 930 K. Such remarkable drop of the phase transition temperature can be explained by gradual decrease of the perovskite structure deformation in the Pr_1−x Sr _x Al_1−x Ti _x O₃ series with increasing of Sr and Ti content. According to the phase diagram of the RAlO₃-based perovskite systems [4], the temperature of the R  $\overline{3}$  с − Pm  $\overline{3}$  m phase transition decreases linearly with increasing radii of R cation as a consequence of the increasing Goldschmidt tolerance factor.

Graphical results of Rietveld refinement of the high-temperature modifications of the Pr_0.5Sr_0.5Al_0.5Ti_0.5O₃ structure and refined structural parameters at selected temperatures are presented in Fig. 5 and Table 2, respectively.

Fig. 5

X-ray synchrotron powder diffraction patterns of Pr_0.5Sr_0.5Al_0.5Ti_0.5O₃ (HASYLAB B2 data, λ = 0.53820 Å). Full-profile Rietveld refinement was performed in space groups Pm  $\overline{3}$  m (T = 1173 K) and R  $\overline{3}$  с (T = 373 K). Experimental (dots) and calculated patterns, difference profiles and positions of the diffraction maxima are given

Table 2 Refined structural parameters of rhombohedral and cubic modifications of Pr_0.5Sr_0.5Al_0.5Ti_0.5O₃

Spot-check examination of low-temperature structural behaviour of the Pr_1−x Sr _x Al_1−x Ti _x O₃ system was performed on the example of a Pr_0.3Sr_0.7Al_0.3Ti_0.3O₃ sample at temperatures of 20, 80, 160 and 220 K. Extremely high resolution of the beamline ID22 at ESRF allows to detect subtle changes in the reflections splitting at different temperature measurements (Fig. 6), which clearly prove a sequence of LT phase transformations in this sample.

Fig. 6

Fragments of LT synchrotron powder diffraction patterns of Pr_0.3Sr_0.7Al_0.3Ti_0.3O₃ (ESRF ID22 data, λ = 0.40003 Å). Splitting of (110) _c and (200) _c reflections in rhombohedral (rh), orthorhombic (o) and tetragonal (t) modifications of Pr_0.3Sr_0.7Al_0.3Ti_0.3O₃ is shown

Crystal structures of Pr_0.3Sr_0.7Al_0.3Ti_0.3O₃ at RT and at 220 K were refined in the space group R  $\overline{3}$  с, thus confirming the results derived from the convenient XRD data (Table 1). Taking into account the character of the reflection splitting, crystal structure parameters of the low-temperature modification of Pr_0.3Sr_0.7Al_0.3Ti_0.3O₃ at 20 and 80 K were successfully refined in space group I4/mcm. X-ray synchrotron diffraction features of Pr_0.3Sr_0.7Al_0.3Ti_0.3O₃ at 160 K could be successfully modelled either in the orthorhombic Immb or in the monoclinic I2/m (C2/m) perovskite structure. Since in both cases during the refinement procedures, practically the same residuals were obtained, a preference should be given to the more symmetric orthorhombic structure. Taking into account that the end members of the system show different sequences of LT phase transformations R  $\overline{3}$  с − Immb − C2/m (PrAlO₃) and Pm  $\overline{3}$  m − I4/mcm (SrTiO₃), additional investigations are required in order to shade light on the complex phase and structural behaviour of the mixed aluminates-titanates below RT.

Conclusions

The formation of extended solid solution Pr_1−x Sr _x Al_1−x Ti _x O₃ with rhombohedral perovskite structure has been revealed in the PrAlO₃–SrTiO₃ pseudo-binary system based on X-ray powder diffraction data. An analysis of the obtained structural parameters in comparison with the data for the parent compounds PrAlO₃ and SrTiO₃ revealed a decrease of perovskite structure deformation in Pr_1−x Sr _x Al_1−x Ti _x O₃ series with increasing Sr and Ti content as a consequence of the increasing Goldschmidt tolerance factor. As a result, concentration-induced phase transition from a rhombohedral to the cubic perovskite structure takes place in the Pr_1−x Sr _x Al_1−x Ti _x O₃ system at x = 0.88. A decreasing structure deformation in Pr_1−x Sr _x Al_1−x Ti _x O₃ series leads to the significant decrease of the temperature-induced phase transition R  $\overline{3}$  с − Pm  $\overline{3}$  m from 1770 to 930 K in PrAlO₃ and Pr_0.5Sr_0.5Al_0.5Ti_0.5O₃, respectively. The sequence of the low-temperature phase transition R  $\overline{3}$  с − Immb(I2/m) − I4/mcm in Pr_0.3Sr_0.7Al_0.3Ti_0.3O₃ revealed during the spot-check X-ray synchrotron powder diffraction examination promises an extremely complex picture of the phase and structural relations in PrAlO₃–SrTiO₃ system below room temperature. Further structural and calorimetric investigations of the system are in progress.