Structure of (001)-, (110)-, and (111)-oriented Pb(Fe 1/2 Nb 1/2 )O 3 epitaxial thin films on SrRuO 3 -buffered SrTiO 3

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We have studied the lattice structure of variously oriented lead iron niobate (PFN) thin films with thicknesses of 50 < t < 500 nm that were deposited by pulsed laser deposition (PLD). We have identified that (001)-, (110)-, and (111)-oriented PFN thin films have tetragonal, orthorhombic, and rhombohedral phases at room temperature, respectively. The change in phase stability, when deposited on substrates of different orientations, is discussed with respect to the influence of epitaxial stress.

I. INTRODUCTION

Lead iron niobate, Pb(Fe1/2Nb1/2)O3 or PFN, is a relaxor ferroelectric, with a Curie temperature range (or temperature of dielectric maximum, Tmax) of Tc ≈ 376 to 393 K.1–3 The structure of PFN has been an interesting topic since the material was first reported by Smolenskii et al. in 1958.1 It has been reported that the structure of PFN ceramics is rhombohedral (R) with lattice parameters of ar ⳱ 4.0123 Å and ␣r ⳱ 89.89° in the ferroelectric phase at room temperature, and cubic (C) above Tc in the paraelectric phase.1,4–6 Later, PFN single crystals were grown, and a ferroelectric → ferroelectric phase transformation was reported at 353K.7 Using highresolution x-ray diffraction (XRD), Ehses and Schmid showed that the ferroelectric → ferroelectric transformation was T → R,8 where the T phase is sandwiched between C and R ones over a narrow temperature region (40 K) on cooling below Tc. The C and T phases are commonly accepted as established; however, in recent years, the existence of the R phase has become more controversial. Bonny et al.3 and Lampis et al.9 reported PFN crystals to be monoclinic (M) at room temperature, whereas other neutron10 and single-crystal7,8 studies again reported it to be R. Strontium ruthenate, SrRuO3 (SRO), which is often used as a bottom electrode because of its low resistivity, was first reported by Randall and Ward in 1959.11–13 At room temperature, bulk SRO has a GdFeO3-type14 orthorhombic structure with lattice constants of ao ⳱ 5.5670 Å, bo ⳱ 5.5306 Å, and co ⳱ 7.8446 Å.15 It can also be seen as a slightly distorted pseudocubic perovskite with single unit-cell lattice constants of apc ⳱ a)

Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0081 J. Mater. Res., Vol. 23, No. 3, Mar 2008

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bpc ⳱ cpc ⳱ 3.923 Å, ␣ ⳱ ␤ ⳱ 90°, and ␥ ⳱ 89.6°; or as a pseudo-orthorhombic double unit cell with apo ⳱ 5.57 Å, bpo ⳱ 5.53 Å, cpo ⳱ 3.92 Å, and ␣ ⳱ ␤ ⳱ ␥ ⳱ 90°. Strontium titanate, SrTiO3 (STO), is a good choice for a substrate because of its cubic perovskite structure with lattice constant of ac ⳱ 3.905 Å at room temperature. Thin films are known to sometimes have different lattice structures and/or constants, relative to corresponding bulk material forms, due to constraints imposed by substrates. Accordingly, epitaxial films might provide the chance to study the properties of a material in a different crystal structure (or lattice constant) than that available to crystals/ceramics.16–19 For example, B

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