*2.2. X-Ray Di*ff*raction*

These patterns were measured in air at room temperature using a Bruker D-8 Advance diffractometer with the Bragg-Brentano <sup>θ</sup>-<sup>θ</sup> geometry, a source of CuK<sup>α</sup> radiation (<sup>λ</sup> <sup>=</sup> 1.5406 . *A*), a Ni 0.5% CuKβ filter in the secondary beam, and a 1-dimensional position sensitive silicon strip detector (Bruker, Linxeye, Karlsruhe, Germany). The diffraction intensity, as a function of the 2θ angle, was measured between 5.00◦ and 110.00◦, with a step of 0.02◦ every 38.4 s. Sample LN-STm displays a pure ferroelectric lithium niobate (LN) phase, with Bragg peaks resembling those of the COD-2101175 card previously deposited with the Crystallographic Open Database; supplementary crystallographic data can be obtained free of charge from the Web page of the database [23].

Rietveld refinement was performed using computational package X'Pert HighScore Plus from PANalytical, version 2.2b (2.2.2), released in 2006 [24]. Instructions in the section named *Automatic Rietveld Refinement* from the HighScore Online Plus Help document were first followed and then adapted for phase quantification of the samples. In short, an archive with information about the atomic coordinates of LN ("2101175.cif") was downloaded from the Crystallopgraphic Open Database [23]. For the secondary phases LiNb3O8 and Li3NbO4, ICSD-2921 and ICSD-75264 from The Inorganic Crystal Structure Database were used, respectively [25]. The archives were then inserted, along with the experimental data, and Rietveld analysis in "Automatic Mode" was executed, followed by iterative executions in "Semi-automatic Mode," in which different "Profile Parameters" were allowed to vary until satisfactory indexes of agreement were obtained. The averaged crystallite size was also calculated by Rietveld refinement, following instructions from the *Size*/*Strain Analysis* section; a single lanthanum hexaboride (LaB6) crystal was used in this case as the standard sample, analyzed with the ICSD-194636 card.
