Structures of Impurity Defects in Lithium Niobate and Tantalate Derived from Electron Paramagnetic and Electron Nuclear Double Resonance Data

Abstract

Point intrinsic and extrinsic defects, especially paramagnetic ions of transition metals and rare-earth elements, have essential influence on properties of lithium niobate, LN and tantalate, LT, and often determine their suitability for numerous applications. Discussions about structures of the defects in LN/LT have lasted for decades. Many experimental methods facilitate progress in determining the structures of impurity centers. This paper gives current bird’s eye view on contributions of Electron Paramagnetic Resonance (EPR), and Electron Nuclear Double Resonance (ENDOR) studies to the determination of impurity defect structures in LN and LT crystals for a broad audience of researchers and students. Symmetry and charge compensation considerations restrict a number of possible structures. Comparison of measured angular dependences of ENDOR frequencies with calculated ones for Li and Nb substitution using dipole–dipole approximation allows unambiguously to determine the exact location of paramagnetic impurities. Models with two lithium vacancies explain angular dependencies of EPR spectra for Me3+ ions substituting for Li+ like Cr, Er, Fe, Gd, Nd, and Yb. Self-compensation of excessive charges through equalization of concentrations of Me3+(Li+) and Me3+(Nb5+) and appearance of interstitial Li+ in the structural vacancy near Me3+(Nb5+) take place in stoichiometric LN/LT due to lack of intrinsic defects.

Chemical environment and occupation sites of hydrogen in LiMO3

“…In addition to the intrinsic defect clusters, the extrinsic point defects of transition metal ions and their charge balancing play an important role because of their influence on the material properties. 22,[37][38][39] Various methods have been applied to obtain a detailed understanding of the chemical environment of the extrinsic species. [39][40][41][42][43] Nevertheless, the real picture of the extrinsic defect incorporation on Li and M lattice sites and their clustering remains quite complex.…”

“…22,[37][38][39] Various methods have been applied to obtain a detailed understanding of the chemical environment of the extrinsic species. [39][40][41][42][43] Nevertheless, the real picture of the extrinsic defect incorporation on Li and M lattice sites and their clustering remains quite complex. Since previous literature 44 suggests that the defect clusters are decorated with hydrogen, it can be assumed that the differences between stoichiometric and congruent structure, i.e.…”

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

“…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.…”

Bispiral approach for calculation of powder spectra of electron paramagnetic and nuclear magnetic resonance

“…The spectrum 𝑆 1 with the 𝜀 value similar to the value for the 𝑆 0 spectrum was selected from the set (𝜏 1 is its calculation time). The following parameters were used: 𝑔 𝑥𝑥 =2.0, 𝑔 𝑦𝑦 =2.1, 𝑔 𝑧𝑧 =2.2, and 𝑊=0.2mT for the spin ½; 𝑔=2.0, zero-field splitting parameters 𝑏 2 0 =0.4, 𝑏 2 2 =0.02 cm -1 , and 𝑊=2mT for spins 3/2, 5/2, and 7/2 (see [ 46 ] for notations).…”