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We report the enthalpy of oxygen vacancy formation in thin films of electron-doped SrTiO 3 , under different degrees of epitaxial stress. We demonstrate that both compressive and tensile strain decrease this energy at a very similar rate and promote the formation of stable doubly ionized oxygen vacancies. Moreover, we also show that unintentional cationic vacancies introduced under typical growth conditions, produce a characteristic rotation pattern of TiO6 octahedra. The local concentration of oxygen vacancies can be modulated by an electric field with an AFM tip, changing not only the local electrical potential but also producing a nonvolatile mechanical response whose sign (up/down) can be reversed by the electric field ; This work was supported the Ministerio de Economía y Competitividad of Spain (Project No. MAT2016-80762-R), and Xunta de Galicia (Centro singular de investigación de Galicia accreditation 2016-2019) and the European Union (European Regional Development Fund - ERDF). L.I. also acknowledges the Ministerio de Economía y Competitividad of Spain for an FPI grant ; SI

Bulk SrTiO3 is a quantum paraelectric in which an antiferrodistortive distortion below ≈105 K and quantum fluctuations at low temperature preclude the stabilization of a long-range ferroelectric state. However, biaxial mechanical stress, impurity doping, and Sr nonstoichiometry, among other mechanisms, are able to stabilize a ferroelectric or relaxor ferroelectric state at room temperature, which develops into a longer-range ferroelectric state below 250 K. In this paper, we show that epitaxial SrTiO3 thin films grown under tensile strain on DyScO3 exhibit a large reduction of thermal conductivity, of ≈60% at room temperature, with respect to identical strain-free or compressed films. The thermal conductivity shows a further reduction below 250 K, a temperature concurrent with the peak in the dielectric constant [J. H. Haeni et al., Nature (London) 430, 758 (2004)]. These results suggest that strain gradients in the relaxor and ferroelectric phase of SrTiO3 are very effective phonon scatterers, limiting the thermal transport in this material. ; F.R. acknowledges financial support of the Ministerio de Economía y Competitividad of Spain (Project No. MAT2016- 80762-R), and Xunta de Galicia (Centro Singular de Investigación de Galicia accreditation 2016-2019) and the European Union (European Regional Development Fund ERDF), and the European Commission through the project 734187 - SPICOLOST (H2020-MSCA-RISE-2016). J.A.M. would like to acknowledge the National Science Foundation, Division of Electrical, Communication and Cyber Systems, Award No. 1901972. This work was supported by the National Natural Science Foundation of China (Grant No. 51876186), Natural Science Foundation of Zhejiang Province (Grant No. LZ19E060002), the Fundamental Research Funds for the Central Universities (K20200145), and ZJUI with W.-L. Ong as one of the Principal Supervisors. ; Peer reviewed