High temperature phase transitions in NaNbO 3 epitaxial films grown under tensile lattice strain

NaNbO 3 based materials have attracted significant interest due to their promising ferro- and piezoelectric properties 1 and are thus considered as possible candidates to replace hazardous lead-based materials in piezo-/ferroelectric technological devices. Temperature dependent phase transitions of bulk NaNbO 3 have been frequently studied 2,3 and a complex sequence has been observed in the temperature range from 640 °C down to -100 °C 3-5 . Above 640 °C, NaNbO 3 is paraelectric and cubic, while a tetragonal, four orthorhombic, and finally a rhombohedral phase emerges at 640, 575, 520, 480, 360, and 100 °C during field-free cooling, We have investigated high temperature phase transitions in NaNbO 3 thin films epitaxially grown under tensile lattice strain on (110) DyScO 3 substrates using metal-organic vapor phase epitaxy (MOVPE). At room temperature, a very regular stripe domain pattern consisting of the monoclinic a 1 a 2 ferroelectric phase was observed. Temperature-dependent studies of the refractive index and the optical band gap as well as in situ high-resolution X-ray diffraction measurements prove a ferroelectric-ferroelectric phase transition in the range between 250 °C and 300 °C. The experimental results strongly suggest that the high-temperature phase exhibits a distorted orthorhombic a 1 /a 2 crystal symmetry, with the electric polarization vector lying exclusively in the plane. A second phase transition was observed at about 500 °C, which presumably signifies the transition to the paraelectric phase. Both phase transitions show a pronounced temperature-dependent hysteresis, indicating first-order phase transitions. 3 substrate exhibit an orthorhombic c phase with exclusive vertical electrical polarization which transforms into the inclined monoclinic M A phase when the epitaxial strain is partially relaxed. On the other hand, the introduction of tensile strain through epitaxial growth on rare-earth scandates (ReScO 3 ) leads to the formation of the monoclinic a 1 a 2 -phase with pure in-plane electrical polarization 14,17 . In this study, tensile strained epitaxial NaNbO 3 films have been grown on (110) DyScO 3 (DSO) substrate. In situ high-resolution X-ray diffraction (HRXRD) and spectroscopic ellipsometry (SE) reveal a first-T tensile strained NaNbO 3 thin films grown by MOVPE on (110) DyScO 3 substrate. At room temperature we observe a very regular stripe domain pattern consisting of the monoclinic a 1 a 2 ferroelectric phase with exclusive in-plane electrical polarization. Temperature-dependent refractive index and optical band gap measurements as well as in situ HR-XRD measurements show a ferroelectric-to-ferroelectric phase transition between 250 °C and 300 °C. Our experimental data strongly suggest that the high temperature phase exhibits a distorted orthorhombic a 1 /a 2 symmetry with pure in-plane polarization presumably along [11 0] DSO and [001] DSO . In addition, a subsequent phase transition presumably to the paraelectric phase was observed at about 500 °C. For both transitions, a pronounced hysteretic behavior was observed, suggesting first-order phase transitions. Our observations on T s

order phase transition at high temperatures between 250 °C and 300 °C, with a pronounced hysteresis observed between the heating and cooling processes. Although more detailed investigations are still needed, our data at this stage indicate that the high-temperature phase exhibits a distorted a1/a2 orthorhombic symmetry with pure in-plane polarization.
NaNbO3 films with a nominal thickness of about 40 nm were grown on 0.1° off-oriented (110) DyScO3 substrates (CrysTec GmbH Berlin, Source material purity 4 N) by liquid delivery spin MOVPE. Details of the deposition method are given in Ref. 18 , however, in the present study we use slightly different parameters (Flash evaporation temperature of 230 °C for the Na(thd) (99.999 %) solution and 190 °C for the Nb(EtO)5 (99.999 %) solution, a Na-to-Nb concentration ratio in the source liquids of 4 and an O2-to-Ar ratio of 0.6). Fig. 1a shows an atomic force micrograph (AFM; Bruker Dimension Icon) of such a 40 nm NaNbO3 film. The surface morphology mimics the step structure of the substrate prior to growth due to 0.1° offorientation, i.e., it is formed of regularly spaced surface steps, indicating a step-flow growth mode with a very low root mean square (rms) surface roughness of about 0.14 nm. A piezoresponse force microscope (PFM) equipped with a dual AC resonance tracking (DART) mode (Asylum Research MFP-3D stand-alone instrument) was applied to image the ferroelectric domains. Lateral PFM measurements presented in Fig. 1b 22 . In contrast, the corresponding vertical PFM image (not shown here) exhibits no significant signal, proving that the electric polarization vector is aligned exclusively in the film plane. These results are consistent with the formation of ferroelectric a1a2 domains as described in our previous publications 14,17,20 .
At room temperature the (110) DyScO3 substrate exhibits an orthorhombic symmetry (a = 5.4424 Å, b = 5.7194 Å, c = 7.9043 Å, 21 ), with a nearly quadratic surface unit cell with in-plane lattice parameters of 3.948 Å and 3.952 Å along the [11 ̅ 0]DSO and [001]DSO directions, respectively. In Pbcm symmetry, the orthorhombic lattice parameters of NaNbO3 are given by aNNO = 5.5047 Å, bNNO = 5.5687 Å and cNNO = 15.523 Å 23 . For reasons of simplicity, we use the pseudocubic (pc) notation 24 with lattice parameters apc = 3.881 Å and bpc = cpc = 3.915 Å and αpc = 89.34° being the angle between the bpc and cpc directions (Fig. 1c). The main pseudocubic axes of the epitaxial NaNbO3 film are parallel to the in-plane substrate directions This is the author's peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset. However, it has been successfully applied to crystalline material 27,28 . To meet our main objective of detecting the phase transition from temperature dependent SE measurements, we found that the use of TL dispersion is adequate to fit the ellipsometric data of NaNbO3 film.
The obtained film thickness of t = (43.5 ± 0.4) nm is in good agreement with the X-ray data (t = 42.5 ± 0.5 nm) and a surface roughness of about (0.40 ± 0.04) nm could be determined. Room temperature SE measurements performed at several angles of incidence of 60°, 65° and 70° and This is the author's peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset. PLEASE CITE THIS ARTICLE AS DOI: 10.1063/5.0087959 5 sample orientations (supplementary material, Fig. S1), revealed only a weak dependence of the refractive index (n) and the extinction coefficient (k) (supplementary material Fig. S2), further details are provided in supplementary. Therefore, in the following we focus on results obtained at an angle of incidence of 70°, and an isotropic model was throughout used for all temperaturedependent SE measurements. This is the author's peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.

PLEASE CITE THIS ARTICLE AS DOI: 10.1063/5.0087959
electronic properties of orthorhombic NaNbO3 revealed that NaNbO3 has an indirect band gap 32 . Therefore, we will discuss the estimated indirect band gap of NaNbO3 film with the reported indirect band gaps. From Table 2  (ii) Increasing the temperature to 320 °C the film peak exhibits a pronounced horizontal splitting. This is attributed to a change of the thin film crystal symmetry and indicates a structural phase transition between 300 °C and 320 °C, which is in good agreement with This is the author's peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.

PLEASE CITE THIS ARTICLE AS DOI: 10.1063/5.0087959
the SE measurements (Fig. 4). The center of mass of the doubled film peaks is slightly shifted towards larger Qx values as compared to the substrate peak, indicating a slightly reduced horizontal lattice spacing.
(iii) The horizontal splitting persists up to 500 °C, while above 500 °C it disappears, leaving a single film peak that appears at identical Qx value as the substrate peak. This is a fingerprint of a second structural phase transition at about 500 °C, which fits well to the SE measurements described above in Fig. 4.
(iv) During the cooling cycle (Fig.5 g-k), the phase transition occurs at lower temperatures.
This hysteretic behavior implies a first order phase transition and corresponds well to the behavior of n(T) and Eg(T) in Fig. 4.
The horizontal peak splitting displayed in Fig. 5  This is the author's peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset. PLEASE CITE THIS ARTICLE AS DOI: 10.1063/5.0087959