Open Access

Structural Behaviour of Solid Solutions in the NdAlO3-SrTiO3 System

  • Natalia Ohon1,
  • Roman Stepchuk1,
  • Kostiantyn Blazhivskyi1 and
  • Leonid Vasylechko1Email author
Nanoscale Research Letters201712:148

https://doi.org/10.1186/s11671-017-1937-8

Received: 30 December 2016

Accepted: 20 February 2017

Published: 23 February 2017

Abstract

Single-phase mixed aluminates-titanates Nd1-x Sr x Al1-x Ti x O3 (x = 0.3 ÷ 0.9) were prepared from stoichiometric amounts of constituent oxides Nd2O3, Al2O3, TiO2 and strontium carbonate SrCO3 by solid-state reaction technique in air at 1773 K. Crystal structure parameters of Nd1-x Sr x Al1-x Ti x O3 were refined by full-profile Rietveld refinement in space groups R \( \overline{3} \) c (x = 0.3, 0.5, 0.7 and 0.8) and Pm \( \overline{3} \) m (x = 0.9). Comparison of the obtained structural parameters with the literature data for the end members of the system NdAlO3 and SrTiO3 revealed formation of two kinds of solid solutions Nd1-xSrxAl1-xTixO3 with the cubic and rhombohedral perovskite structure. Morphotropic rhombohedral-to-cubic phase transition in Nd1-xSrxAl1-xTixO3 series occurs at x = 0.84. Based on the results obtained as well as the literature data for the parent compounds, the tentative phase diagram of the NdAlO3–SrTiO3 pseudo-binary system have been constructed.

Keywords

Perovskite aluminates and titanatesCrystal structureSolid solutionPhase transition

Background

Mixed aluminates-titanates with perovskite structure formed in the RAlO3ATiO3 pseudo-binary systems (R = rare earths, A = Sr, Ca) are prospective functional materials. In conjunction with alkaline-earth titanates, rare earth aluminates reveal excellent temperature-stable high-Q microwave dielectric properties and are widely used as radio-frequency ceramics in modern electronic devices ([16] and references herein). The highest Q-values among RAlO3- and ATiO3-based microwave ceramics were reported for LaAlO3–SrTiO3 system, which exhibits solid solubility across the entire compositional range. It was shown that dielectric properties of mixed aluminates-titanates ceramics do not significantly depend on the nature of the rare earth and the value of resonant frequency (t f) can be tuned by changing the concentration of solid solution. Thus potentially useful ceramics with temperature-stable relative permittivity can be obtained in other RAlO3ATiO3 perovskite series.

The interest to the RAlO3–SrTiO3 systems has been increased considerably during the last decade after discovering of the intrigue phenomena of two-dimensional electron gas at the interface between two insulators LaAlO3 and SrTiO3 [7]. The interface effects occurred in the RAlO3–SrTiO3 perovskite systems are in the focus of active research in the field of tunable metal-insulator transition, 2D superconductivity, coexistence of superconductivity and ferromagnetism, etc. [811].

The aim of the present work is the study of the phase and structural behaviour of the mixed aluminates-titanates formed in the NdAlO3–SrTiO3 pseudo-binary system. At room temperature, the end members of the system—NdAlO3 and SrTiO3—adopt different variants of the perovskite structure: rhombohedral R \( \overline{3} \) c and cubic Pm \( \overline{3} \) m, respectively. Rhombohedral NdAlO3 transforms into the cubic perovskite structure near 2100 K ([6] and references herein), whereas strontium titanate SrTiO3 undergoes a low-temperature (LT) phase transition from the cubic to tetragonal I4/mcm perovskite structure below 105 K [12, 13]. Owing to the abovementioned peculiarities of NdAlO3 and SrTiO3 crystal structures, complex phase and structural behaviour is expected in the mixed neodymium-strontium aluminate-titanate system.

Methods

Mixed aluminates-titanates of nominal compositions Nd1−x Sr x Al1−x Ti x O3 (x = 0.3, 0.5, 0.7, 0.8, 0.9) were prepared by solid-state reaction technique. Stoichiometric amounts of the constituent oxides Nd2O3, Al2O3, TiO2 and strontium carbonate SrCO3 were ball-milled in ethanol for 5 h, dried, pressed into pellets and annealed in air at 1673 K for 9 h. After cooling the product, it was regrinded and repeatedly annealed at 1773 K for 9 h. X-ray powder diffraction technique (Huber imaging plate Guinier camera G670, Cu Kα1 radiation, λ = 1.54056 Å) was used for the phase and structural characterization of the samples at room temperature. All crystallographic calculations including full-profile Rietveld refinement were performed by using WinCSD program package [14].

Results and Discussion

Analysis of X-ray diffraction (XRD) data collected at room temperature (RT) showed that all samples synthesized adopt pure perovskite structure. No traces of foreign phases were detected (Fig. 1). Close examination of diffraction maxima revealed detectable rhombohedral deformation of the Nd1−x Sr x Al1−x Ti x O3 samples with x = 0.3 and 0.5, whereas no visible reflections splitting or deformation was observed for the specimens with higher x values (Fig. 1, inset). However, a presence of minor superstructure (113) reflection, which is indicative for rhombohedral distortion of perovskite structure, testifies that the rhombohedral structure in Nd1−x Sr x Al1−x Ti x O3 series persists at least up to x = 0.8.
Fig. 1

Experimental XRD patterns of the Nd1-xSrxAl1-xTixO3 series. Stars indicate a superstructure (113) reflection. Inset shows enlarger part of patterns

Full-profile Rietveld refinement of Nd1-x Sr x Al1-x Ti x O3 structures, performed in space groups R \( \overline{3} \) c and Pm \( \overline{3} \) m for the samples with x ≤ 0.8 and x = 0.9, respectively, entirely confirms suggested crystal structures of the specimens. Examples of graphical results of Rietveld refinement, showing excellent fits between experimental and calculated profiles of rhombohedral Nd0.5Sr0.5Al0.5Ti0.5O3 and cubic Nd0.1Sr0.9Al0.1Ti0.9O3 structures are presented on Fig. 2. Refined structural parameters of all synthesized Nd1-x Sr x Al1-x Ti x O3 samples and corresponding residuals are presented in Table 1.
Fig. 2

Graphical results of Rietveld refinement of Nd0.1Sr0.9Al0.1Ti0.9O3 and Nd0.5Sr0.5Al0.5Ti0.5O3 structures. The experimental X-ray powder diffraction patterns (dots) are shown in comparison with the calculated patterns (blue lines). The difference curves between measured and calculated profiles are shown below the diagrams. Inset shows the view of the cubic and rhombohedral structures as corner-shared Al/TiO6 octahedra with Nd/Sr species located between them

Table 1

Unit cell parameters, coordinates and isotropic displacement parameters of atoms in Nd1−x Sr x Al1−x Ti x O3 structures at RT

Atoms,

sites

Parameter,

residuals

x in Nd1−x Sr x Al1−x Ti x O3, space group

0.3,

R \( \overline{3} \) c

0.5,

R \( \overline{3} \) c

0.7,

R \( \overline{3} \) c

0.8,

R \( \overline{3} \) c

0.9,

Pm \( \overline{3} \) m

 

a, Å

5.3836(4)

5.4281(4)

5.4674(5)

5.4849(8)

3.8911(1)

 

c, Å

13.1180(2)

13.251(1)

13.368(2)

13.428(3)

Nd/Sr,

6c (0, 0, ¼)

B iso , Å 2

0.73(2)

0.91(1)

0.67(2)

0.83(1)

0.90(4)

Al/Ti,

6b (0, 0, 0)

B iso , Å 2

0.54(4)

0.41(2)

0.44(3)

0.53(2)

0.48(5)

O,

18e (x, 0, ¼)

x

0.5395(8)

0.5358(6)

0.5277(6)

0.5213(7)

 

B iso , Å 2

1.48(8)

1.25(5)

1.72(6)

1.57(5)

1.36(11)

 

R I

0.021

0.026

0.030

0.028

0.033

 

R P

0.076

0.091

0.083

0.086

0.092

Concentration dependencies of the obtained lattice parameters and unit cell volumes of Nd1-x Sr x Al1-x Ti x O3 series in comparison with the literature data for NdAlO3 [6] and SrTiO3 [12] (Fig. 3) prove a formation of two kinds of solid solutions in the NdAlO3–SrTiO3 pseudo-binary system. Simultaneous aliovalent substitution of Sr2+ and Ti4+ species for Nd3+ and Al3+ sites reduces rhombohedral deformation in Nd1-x Sr x Al1-x Ti x O3 series and led to morphotropic phase transition to the cubic perovskite structure at x = 0.84 (Fig. 3). In the related systems LaAlO3–SrTiO3 and PrAlO3–SrTiO3, the phase boundary between two perovskite structures takes place above x = 0.8 and at x = 0.88, respectively [15, 16].
Fig. 3

Unit cell dimensions of Nd1-xSrxAl1-xTixO3 series. The rhombohedral lattice parameters are normalized to the perovskite cell as follow: a p = a r /√2,c p = c r /√12,V p = V r /6. The dotted line marks the phase boundary between the Rh and the C phases

A decreasing structural deformation in Nd1−x Sr x Al1−x Ti x O3 series as a consequence of increasing Goldschmidt tolerance factor with increasing x should significantly effect on the temperature of structural phase transition R \( \overline{3} \) c − Pm \( \overline{3} \) m, which occurs in NdAlO3 at about 2100 K [6]. According to structural phase diagram of the related LaAlO3–SrTiO3 system [15], the temperature of rhombohedra-to-cubic transition decreases almost linearly from 850 K in “pure” LaAlO3 to 350 K in the sample with nominal composition La0.2Sr0.8Al0.2Ti0.8O3. Our recent in situ X-ray synchrotron powder diffraction investigations of the PrAlO3–SrTiO3 series [16] showed that the R \( \overline{3} \) c − Pm \( \overline{3} \) m transition temperature decreases gradually from 1770 K in PrAlO3 to 930 K in Pr0.5Sr0.5Al0.5Ti0.5O3 sample. Similar structure and phase behaviour at the elevated temperatures are also expected in the studied NdAlO3–SrTiO3 system, tentative phase diagram of which is shown on Fig. 4. However, extrapolation of the cubic phase boundary from high-temperature region to the higher SrTiO3 concentrations would be speculative because of the different low-temperature structures of the parent compounds NdAlO3 and SrTiO3 (R \( \overline{3} \) c and I4/mcm, respectively). Evidently, phase boundary between the R \( \overline{3} \) c and I4/mcm structural modifications of Nd1-x Sr x Al1-x Ti x O3 solid solution has to be present in the SrTiO3-rich part of the phase diagram at low temperatures (Fig. 4). In addition, appearance of intermediate orthorhombic phase between rhombohedral R \( \overline{3} \) c and tetragonal I4/mcm phase fields, as it occurs in the related PrAlO3–SrTiO3 [16] and NdAlO3–CeAlO3 [17] systems, could not be neglected.
Fig. 4

Tentative phase diagram of the PrAlO3–SrTiO3 pseudo-binary system. The letters L, C, Rh and Te designate liquid, cubic, rhombohedral and tetragonal phase fields, respectively. The solid symbols designate the rhombohedral (triangles) and cubic (squares) perovskite structures experimentally observed in Nd1-xSrxAl1-xTixO3 series at RT

Comprehensive analysis of A/B-cation substitution on the antiferrodistortive phase transition Pm \( \overline{3} \) m − I4/mcm in SrTiO3 recently performed in [18] revealed that transition temperature increases in nonlinear manner with decreasing tolerance factor, depending on substituent concentration. Based on this observation, one can predict that SrTiO3-richest samples in the NdAlO3–SrTiO3 system will transform to the tetragonal I4/mcm structure at the temperatures higher than the pure SrTiO3 (105 K). To shad light on the low-temperature phase behaviour in the NdAlO3–SrTiO3 system, thorough structural, calorimetric and spectroscopic investigations are required.

Conclusions

Continuous solid solution Nd1-xSrxAl1-xTixO3 with perovskite structure is formed in the NdAlO3–SrTiO3 pseudo-binary system at 1773 K. Comparison of the obtained structural parameters with corresponding data for the parent compounds NdAlO3 and SrTiO3 proves a decrease of perovskite structure deformation as a consequence of increasing Goldschmidt tolerance factor with increasing x in Nd1-xSrxAl1-xTixO3 series. As a result, concentration-induced phase transition from a rhombohedral R \( \overline{3} \) c to the cubic perovskite structure occurs in the Nd1-xSrxAl1-xTixO3 system at x = 0.84. Experimental X-ray powder diffraction patterns and crystal structure parameters of rhombohedral Nd0.7Sr0.3Al0.7Ti0.3O3 and cubic Nd0.1Sr0.9Al0.1Ti0.9O3 phases are published by the International Centre of Diffraction Data (ICDD) in the last release of the Powder Diffraction Files (PDF cards NN 00-066-0395 and 00-066-0396, respectively) [19].

Declarations

Acknowledgements

The work was supported in parts by the Ukrainian Ministry of Education and Sciences (Project “RZE”) and ICDD Grant-in-Aid program.

Authors’ Contributions

NO contributed to the data evaluation and manuscript writing. RS synthesized the samples and contributed to the data evaluation. KB contributed to the sample preparation and to the discussion of the results. LV performed structural characterization of the samples and contributed to the manuscript writing. All authors read and approved the final manuscript.

Competing Interests

The authors declare that they have no competing interests.

Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

Authors’ Affiliations

(1)
Lviv Polytechnic National University

References

  1. Jancar B, Suvorov D, Valant M, Drazic G (2003) Characterization of CaTiO3-NdAlO3 dielectric ceramics. J Eur Ceram Soc 23:1391–1400View ArticleGoogle Scholar
  2. Zheng H, de Gyorgyfalva GDC C, Quimby R, Bagshaw H, Ubic R, Reaney IM, Yarwood J (2003) Raman spectroscopy of B-site order–disorder in CaTiO3 – based microwave ceramics. J Eur Ceram Soc 23:2653–2659View ArticleGoogle Scholar
  3. Nenasheva E, Mudroliubova L, Kartenko N (2003) Microwave dielectric properties of ceramics based on CaTiO3 − LnMO3 system (Ln − La, Nd M − Al, Ga). J Eur Ceram Soc 23:2443–2448View ArticleGoogle Scholar
  4. Inagaki Y, Suzuki S, Kagomija I et al (2007) Crystal structure and microwave dielectric properties of SrTiO3 doped LaAlO3 single crystal grown by FZ. J Eur Ceram Soc 27:2861–2864View ArticleGoogle Scholar
  5. Shimada T, Kura K, Ohtsuki S (2006) Dielectric properties and far infrared reflectivity of lanthanum aluminate-strontium titanate ceramics. J Eur Ceram Soc 26:2017–2021View ArticleGoogle Scholar
  6. Vasylechko L, Senyshyn A, Bismayer U. Perovskite-type aluminates and gallates. In: Gschneidner KA, Jr, Bünzli J-CG, Pecharsky VK, editors. Handbook on the physics and chemistry of rare earths. vol. 39. North-Holland, Netherlands, 2009. p. 113–295. ISBN: 978-0-444-53221-3.Google Scholar
  7. Ohtomo A, Hwang HY (2004) A high-mobility electron gas at the LaAlO3/SrTiO3 heterointerface. Nature 427:423–426View ArticleGoogle Scholar
  8. Thiel S, Hammerl G, Schmehl A, Schneider CW, Mannhart J (2006) Tunable quasi-two-dimensional electron gases in oxide heterostructures. Science 313:1942–1945View ArticleGoogle Scholar
  9. Reyren N, Thiel S, Caviglia AD, Fitting Kourkoutis L, Hammerl G, Richter C, Schneider CW, Kopp T, Rüetschi A-S, Jaccard D, Gabay M, Muller DA, Triscone J-M, Mannhart J (2007) Superconducting interfaces between insulating oxides. Science 317:1196–1199View ArticleGoogle Scholar
  10. Dikin DA, Mehta M, Bark CW, Folkman CM, Eom CB, Chandrasekhar V (2011) Coexistence of superconductivity and ferromagnetism in two dimensions. Phys Rev Lett 107:056802View ArticleGoogle Scholar
  11. Xiang X, Qiao L, Xiao HY, Gao F, Zu XT, Li S, Zhou WL (2014) Effects of surface defects on two-dimensional electron gas at NdAlO3/SrTiO3 interface. Sci Rep 4:5477Google Scholar
  12. Kiat JM, Roisnel T (1996) Rietveld analysis of strontium titanate in the Müller state. J Phys Condens Matter 8:3471–3415View ArticleGoogle Scholar
  13. Hayward SA, Salje EKH (1999) Cubic-tetragonal phase transition in SrTiO3 revisited: Landau theory and transition mechanism. Phase Transitions 68:501–522View ArticleGoogle Scholar
  14. Akselrud L, Grin Y (2014) WinCSD: software package for crystallographic calculations (version 4). J Appl Crystallogr 47:803–805View ArticleGoogle Scholar
  15. Bednorz JG, Müller KA, Arend H, Gränicher H (1983) Phase diagram of the (LaAlO3)l-x (SrTiO3)x solid-solution system, for x ≤ 0.8. Mat Res Bull 18:181–187View ArticleGoogle Scholar
  16. Vasylechko L, Stepchuk R, Yu P, Rosner H (2016) Concentration and temperature induced phase transitions in PrAlO3-SrTiO3 system. Nanoscale Res Lett 11:17View ArticleGoogle Scholar
  17. Vasylechko L, Senyshyn A, Trots D, Niewa R, Schnelle W, Knapp M (2007) CeAlO3 and Ce1-x R x AlO3 (R = La, Nd) solid solutions: crystal structure, thermal expansion and phase transitions. J Solid State Chem 180:1277–1290View ArticleGoogle Scholar
  18. McCalla E, Walter J, Leighton C (2016) A unified view of the substitution-dependent antiferrodistortive phase transition in SrTiO3. Chem Mater 28:7973–7981View ArticleGoogle Scholar
  19. Vasylechko L, ICDD Grant-in-Aid (2014) Semiconductor Electronics Dept. Lviv Polytechnic National UniversityGoogle Scholar

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© The Author(s). 2017

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