Unveiling the Defect Structure of Lithium Niobate with Nuclear Methods

Abstract

X-ray and neutron diffraction studies succeeded in the 1960s to determine the principal structural properties of congruent lithium niobate. However, the nature of the intrinsic defects related to the non-stoichiometry of this material remained an object of controversial discussion. In addition, the incorporation mechanism for dopants in the crystal lattice, showing a solubility range from about 0.1 mol% for rare earths to 9 mol% for some elements (e.g., Ti and Mg), stayed unresolved. Various different models for the formation of these defect structures were developed and required experimental verification. In this paper, we review the outstanding role of nuclear physics based methods in the process of unveiling the kind of intrinsic defects formed in congruent lithium niobate and the rules governing the incorporation of dopants. Complementary results in the isostructural compound lithium tantalate are reviewed for the case of the ferroelectric-paraelectric phase transition. We focus especially on the use of ion beam analysis under channeling conditions for the direct determination of dopant lattice sites and intrinsic defects and on Perturbed Angular Correlation measurements probing the local environment of dopants in the host lattice yielding independent and complementary information.

‘Horror Vacui’ in the Oxygen Sublattice of Lithium Niobate Made Affordable by Cationic Flexibility

“…Their structure and behavior upon heating/cooling strongly resembled those obtained by thermal reduction, showing broad features at ~1.6 eV, ~2.6 eV and ~3.2 eV; however, the main band showed trapped-electron character in contrast to the case of low-temperature irradiation where it was rather of trapped-hole type; upon heating there was again a reversible transformation of the main band at ~2.6 eV to the band at ~1.6 eV, whereby the same isosbestic point near 600 nm was observed as for reduced crystals [36,52]. The particle energy threshold found for increased defect formation was 0.3 MeV, corresponding to an oxygen displacement energy of 53 eV, in reasonable agreement with oxygen displacement thresholds in other oxides (see references in [6,52]). No saturation after five over-threshold doses of 2 × 10 18 e/cm 2 could be observed.…”

“…Originally, the same was also assumed for LiNbO 3 (LN), and most authors did not (and many still do not) have any doubts about simply postulating vacant oxygen sites (V O ) capable of trapping one or two electrons (F + and F-centers, respectively) in discussions of their experiments and proposed applications (see, e.g., [1,2]). The controversy about the availability of oxygen vacancies manifests in the reviews of Sánchez-Dena et al [3,4], is touched upon by some topical reviews [5,6], and is also closely related to the topics of other papers [7][8][9][10][11] of the present Special Issue, and deserves a clarifying discussion.…”

Correlation between Infrared Absorption and Lithium Sublattice Disorder in Magnesium-Doped Lithium Niobate

“…The minimum yields ( ), i. e., the ratio of counts in aligned and random incidence cases, are calculated in this study for a depth of 1 m corresponding to the alpha particle energy range of 7.357 to 7.566 MeV. In lithium niobate, the guiding effect for ions is mainly established by the repelling Coulomb potential of the Nb (Z = 41) atomic chains in all axial directions [ 26 ]. This leaves the chains consisting of Li atoms (Z = 3)—and to a lesser extent those of O (Z = 8)—very sensitive to disturbances in their sub-lattices, which manifest themselves in an increase of the minimum yield with respect to a perfect lattice.…”

“…Ion channeling is an important method for the study of lattice defects and dopant lattice location determination and has been intensively applied to the case of LiNbO (for an extensive review and details on the technique see [ 26 ]). While the Rutherford Backscattering Spectroscopy under channeling conditions is the most widely applied method, the investigation of light elements relies on Nuclear Reaction Analysis (NRA).…”

New Trends in Lithium Niobate: From Bulk to Nanocrystals

“…A similar detailed review on defects in LN and LT crystals studied by nuclear methods has been presented by Kling and Marques [4]. They focus on ion beam methods under channeling conditions for the direct determination of the lattice site of dopants and intrinsic defects.…”