Origin of the stabilization of the metastable tetragonal high-pressure phase in SrCuO2 thin films grown on SrTiO3 substrates by pulsed laser deposition
Introduction
SrCuO2 (SCO) is a ternary transition metal oxide that undergoes a structural phase transition as a function of pressure, with no change in the oxidation state or stoichiometry [1]. The thermodynamically stable phase of SCO, also known as low-pressure phase, is a one-dimensional Heisenberg antiferromagnet and has an orthorhombic crystal structure that consists of CuO zigzag spin-chains along the c-axis [2]. Experimental investigations on single crystals of orthorhombic SCO revealed a new highly efficient mode of thermal conduction, namely heat transport by magnetic excitations [3], [4]. In bulk form the orthorhombic low-pressure phase can be transformed into various metastable polymorphs by applying pressures above 7 GPa. Such structural transformations have been achieved both at high temperatures [5] and at room temperature [6]. At high temperatures and high pressures the orthorhombic low-pressure phase transforms into the tetragonal high-pressure (hp I) phase. This tetragonal structure is known as the infinite-layer structure and has been the subject of extensive theoretical and experimental studies concerning the appearance of high-temperature superconductivity in cuprates [5], [7], [8]. Recently, at room temperature and pressures above 7 GPa a new high-pressure tetragonal superstructure (hp II) has been observed for SCO [6], [9], which is stable up to 34.2 GPa. Above this pressure, another high-pressure phase with an orthorhombic crystal structure (hp III) is formed [6]. While the pressure-induced transformations occurring at room-temperature, i.e., orthorhombic to tetragonal (hp II) and tetragonal (hp II) to orthorhombic (hp III), are reversible structural phase transformations [6], the transformation at high temperatures, i.e., orthorhombic to tetragonal (hp I), is an irreversible one.
An extensive number of experimental and theoretical studies on the growth of SCO films have revealed a strong correlation between the stabilized structure and the film thickness [10], [11], the growing temperature [12] and the crystallographic [1] and electrostatic nature [10] of the substrate. All these parameters have been shown to have a significant influence on the structure [1], [11], [12], texturing [11], [12] and even the stoichiometry [1], [12], [13] of the grown films. In our previous work, we have reported that ∼30 nm thick SCO films deposited on SrTiO3 (STO) substrates can be stabilized in either the tetragonal (hp I) or the orthorhombic low-pressure phase depending on the substrate temperature [12]. Chang et al. [13] have also reported the growth by MOCVD of SCO films which exhibited the tetragonal (hp I) phase and the low-pressure orthorhombic phase for low and high deposition temperatures, respectively. In addition, they reported that for intermediate temperatures the obtained films exhibited an orthorhombic structure but with a different stoichiometry, i.e., Sr2CuO3. It has also been shown that the deposition of a polar film, such as tetragonal (hp I) phase of SCO, on a non-polar substrate, such as STO, could lead to the appearance of polar electrostatic instabilities [14], [15] that may trigger an atomic reconstruction of the film structure [16], [17], [18]. Such modifications at the atomic scale may result either in the formation of structures with completely different intrinsic properties or in stoichiometric changes [16], [18]. Using first-principles calculation methods, Zhong et al. [10] predicted that as the SCO/STO films are made thinner, the formation of chain-like type of structures becomes an energetically favorable way to resolve any electrostatic instability. Following this prediction, Samal et al. [11] experimentally verified that indeed SCO/STO films with thickness of t ≤ 5 unit cells exhibit a structural transformation from the tetragonal high-pressure (hp I) structure to a new chain-like cubic structure.
To our knowledge no systematic study has addressed the origin of the stabilization of the metastable tetragonal (hp I) structure in SCO thin films. In general metastable structural phases may be stabilized in thin film form either through epitaxial strain or through non-equilibrium film growth processes. To elucidate which mechanism applies in our case we have systematically investigated the evolution of structure and the modification in stoichiometry of SCO films grown by pulsed laser deposition on TiO2-terminated STO substrates as a function of substrate temperature.
Section snippets
Experimental considerations
SCO thin films were grown on TiO2 terminated (0 0 1) SrTiO3 (CrysTec GmbH) substrates by the pulsed laser deposition (PLD) method using a KrF* excimer laser source (λ = 248 nm, τFWHM = 25 ns) at a repetition rate of 1 Hz. The growth process was carried out in an oxygen ambient pressure of 0.2 mbar with a pulse energy density of 1.3 J cm−2. The target-to-substrate separation distance was 8 cm and the same number of laser pulses (500) has been applied for the deposition of all films. The following interesting
Results
The out-of-plane X-ray diffraction patterns shown in Fig. 1 reveal that for substrate temperatures 600 °C ≤ Ts ≤ 630 °C the as-grown SCO films exhibit the metastable tetragonal (hp I) phase with the c-axis oriented along the growth direction. For Ts > 630 °C, in addition to reflections of the tetragonal (hp I) phase new peaks corresponding to the low-pressure orthorhombic phase appear, i.e., the films possess a mixture of tetragonal and orthorhombic phases. At Ts = 690 °C, the tetragonal phase completely
Discussion
In order to understand both the formation of the mixed phase thin films as a function of the substrate temperature and the nature of the structural transformation from the tetragonal to the orthorhombic phase upon heating, it is useful to distinguish and review in detail the possible factors involved in these phenomena. We first refer to the intrinsic (thermodynamic) properties of bulk SCO. As reported by Xingjian et al. [32], when tetragonal SCO powder is heated above 400 °C it transforms to a
Conclusions
SrCuO2 films have been grown on TiO2 terminated (0 0 1) SrTiO3 substrates at various temperatures in the range of 600 °C to 700 °C. We have shown that by tuning the substrate temperature one can obtain SrCuO2 films that exhibit both structural phases in pure form and also films composed of a mixture of tetragonal and orthorhombic phases. These films exhibited a stoichiometry variation (Sr/Cu ratio varied from 0.8 to 1) across the temperature range of the structural transformation. We propose that
Acknowledgments
This work was supported by the European Commission through the ITN Marie Curie LOTHERM, Contract no.: PITN-GA-2009-238475.
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