Analysis of the defect clusters in congruent lithium tantalate

Abstract

A wide range of technological applications of lithium tantalate (LT) is closely related to the defect chemistry. In literature, several intrinsic defect models have been proposed. Here, using a combinational approach based on DFT and solid-state NMR, we demonstrate that distribution of electric field gradients (EFGs) can be employed as a fingerprint of a specific defect configuration. Analyzing the distribution of 7 Li EFGs, the FT-IR and electron spin resonance (ESR) spectra, and the 7 Li spin-lattice relaxation behavior, we have found that the congruent LT samples provided by two manufacturers show rather different defect concentrations and distributions although both were grown by the Czochralski method. After thermal treatment hydrogen out-diffusion and homogeneous distribution of other defects have been observed by ESR, NMR, and FT-IR. The defect structure in one of two congruent LT crystals after annealing has been identified and proved by defect formation energy considerations, whereas the more complex defect configuration, including the presence of extrinsic defects, has been suggested for the other LT sample. The approach of searching the EFG fingerprints from DFT calculations in NMR spectra can be applied for identifying the defect clusters in other complex oxides.

Lithium Niobate Single Crystals and Powders Reviewed—Part II

“…As will be discussed in the next section, this framework is usually adopted to describe the substitution mechanisms in doped LN-concerning at least the type of intrinsic defects. However, the main result presented by Vyalikh et al must not be overlooked [9], mostly because of the overall confidence recently put on the results obtained with ever more advanced simulations [6,8,47,48].…”

“…Electrical charge compensation occurs through an adequate accommodation of both cation species and their respective vacancies (V Nb and V Li ). Since the latter are defects with a net electrical charge, other defects with countercharges are required to guarantee overall charge neutrality [9]. The occurrence of a stacking fault envisioned as the relatively excess Nb partially occupying the available V Li s is proposed.…”

“…Several defect models have been constructed, and debate still prevails on which of these indeed represents a "solution of the off-ST LN crystal structure with a stable charge equilibrium" [7]. The discrepancies observed over time might be addressed to intrinsic factors persistent on the different crystal growth technologies or suppliers, as recently reported for isostructural lithium tantalate (LiTaO 3 , LT) [9], a situation that also leads to the irreproducibility of results in at least two cases of doped LN crystals [5,11]. The main characteristics of the three most popular models ever devised are now described in the lines that follow.…”

“…For example, the specific configuration of extrinsic defect clusters. Apropos Kong et al have made a relevant statement regarding the evolution of simulation methods over time [6]: Recently, Vyalikh et al (2018) have performed a detailed study based on Density Functional Theory (DFT) electronic structure calculations and measurement of the spectral characteristics under three different techniques, and, among six proposed cluster models (often found in literature), they found out that two are the most probable to describe cLT [9]; these are therein labeled as models M1 and M3. The first-the least probable among the two according to this study, model M1, with calculated energy formation of 1.94 eV-was initially proposed by Kim et al (2001), and it assumes that a Nb Li antisite (or Ta Li ) is surrounded by three V Li s in the nearest neighborhood and fourth V Li located along the polar c-axis [24], as shown on the left side of Figure 3.…”

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

“…Due to the lithium deficiency of congruent lithium niobate crystals, v Li have been considered as possible charge compensators for Nb 5+ Li + or Ta 5+ Li + antisites for a long time. Models with plane and space configurations of v Li for non-stoichiometric defects were proposed [96,97] and used for calculations of stability of intrinsic defects and defect clusters in LN [250][251][252][253] 233 U (I = 5/2) was described with A = 0.0145 and A ⊥ = 0.0128 cm −1 . g = 0.71, g ⊥ = 0.724 were determined for the line of natural U 5+ (even isotopes with I = 0 have total natural abundance about 99%).…”