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

Single-phase mixed aluminates-titanates Pr1−xSrxAl1−xTixO3 (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 Pr1−xSrxAl1−xTixO3 series is predicted to occur at x = 0.88. The temperature-induced structural phase transition R  \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ \overline{3} $$\end{document}3¯  с − Pm\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ \overline{3} $$\end{document}3¯  m in Pr0.5Sr0.5Al0.5Ti0.5O3, 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 Pr1−xSrxAl1−xTixO3 series with increasing Sr and Ti contents as a consequence of the increasing Goldschmidt tolerance factor. For Pr0.3Sr0.7Al0.3Ti0.7O3 phase, a sequence of the low-temperature phase transformation R  \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ \overline{3} $$\end{document}3¯  с − Immb(C2/m) − I4/mcm was detected.


Background
Rare earth aluminates RAlO 3 with perovskite structure and SrTiO 3 -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][2][3][4][5] and references herein). Due to the opposite signs of the temperature coefficient of the resonant frequency (τ f ) of the RAlO 3 and SrTiO 3 compounds, mixed aluminates-titanates formed in the RAlO 3 -SrTiO 3 systems are considered as prospective microwave materials with a high dielectric constant, moderate quality factor and a near zero value of τ f [5][6][7].
During the last decade, RAlO 3 -SrTiO 3 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 LaAlO 3 and SrTiO 3 [8] has been an active research area in the fieldtunable metal-insulator transition, 2D superconductivity, coexistence of superconductivity and ferromagnetism, etc. [9][10][11]. Just recently, a similar effect was reported on the interfaces of SrTiO 3 and RAlO 3 (R = La, Pr, Nd) and RGaO 3 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 aluminatestitanates formed in the PrAlO 3 -SrTiO 3 pseudo-binary system. At room temperature, the end members of the system-PrAlO 3 and SrTiO 3 -adopt different variants of perovskite structure-rhombohedral R 3 с and cubic Pm 3 m, respectively. Rhombohedral PrAlO 3 transforms into the cubic perovskite structure at about 1770 K ( [4], and references herein). In addition, PrAlO 3 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 SrTiO 3 undergoes a lowtemperature 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 PrAlO 3 and SrTiO 3 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 O 3 (x = 0.1, 0.2, 0.3, 0.5, 0.7) were prepared from stoichiometric amounts of the constituent oxides Pr 6 O 11 , Al 2 O 3 , TiO 2 and strontium carbonate SrCO 3 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 Kα 1 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 0.5 Sr 0.5 Al 0.5 Ti 0.5 O 3 and Pr 0.3 Sr 0.7 Al 0.3 Ti 0.7 O 3 samples in the temperature ranges of 298-1173 K and 20-298 K, respectively. Corresponding in situ highresolution X-ray synchrotron powder diffraction experiments were performed at beamlines B2 at HASY-LAB/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 (   Table 1. Comparison of the obtained structural parameters of the praseodymium-strontium mixed aluminates-titanates with the literature data for the "pure" PrAlO 3 and SrTiO 3 (Fig. 3) clearly proves the formation of the extended solid solution Pr 1−x Sr x Al 1−x Ti x O 3 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 O 3 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 LaAlO 3 -SrTiO 3 system, the rhombohedral solid solution exists up to 60 mole % of LaAlO 3 ; after that, the transition to the cubic perovskite structure takes place [16].
In situ high-temperature X-ray synchrotron powder diffraction investigation of the Pr 0.5 Sr 0.5 Al 0.5 Ti 0.5 O 3 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).
In comparison with the parent PrAlO 3 compound, in which transformation to the cubic perovskite structure occurs around 1770 K [4], the R 3 с − Pm 3 m transition in Pr 0.5 Sr 0.5 Al 0.5 Ti 0.5 O 3 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 O 3 series with   increasing of Sr and Ti content. According to the phase diagram of the RAlO 3 -based perovskite systems [4], the temperature of the R 3 с − Pm 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 hightemperature modifications of the Pr 0.5 Sr 0.5 Al 0.5 Ti 0.5 O 3 structure and refined structural parameters at selected temperatures are presented in Fig. 5 and Table 2, respectively.
Spot-check examination of low-temperature structural behaviour of the Pr 1−x Sr x Al 1−x Ti x O 3 system was performed on the example of a Pr 0.3 Sr 0.7 Al 0.3 Ti 0.3 O 3 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.
Crystal structures of Pr 0.3 Sr 0.7 Al 0.3 Ti 0.3 O 3 at RT and at 220 K were refined in the space group R 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 0.3 Sr 0.7 Al 0.3 Ti 0.3 O 3 at 20 and 80 K were successfully refined in space group I4/mcm. X-ray synchrotron diffraction features of Pr 0.3 Sr 0.7 Al 0.3 Ti 0.3 O 3 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 3 с − Immb − C2/m (PrAlO 3 ) and Pm 3 m −

Competing Interests
The authors declare that they have no competing interests.
Authors' Contributions LV synthesised the samples, performed the HT synchrotron powder diffraction measurements at HASYLAB and structural characterization of the samples and wrote the manuscript. RS contributed to the data evaluation, figure and table preparation and manuscript writing. YP and HR performed the laboratory X-ray and LT synchrotron diffraction measurements at ESRF. All authors read and approved the final manuscript.  (5) 3.8600 (2) 3.8667 (2) 3.8714 (2) c, Å 13.2708 (7) 13.3041 (9) 13.350 (2) ---Pr/Sr, 6c in R 3 с; 1b in Pm 3 m