1.5 Intrinsic defects

Where does the missing lithium go? Not into missing oxygen: as-grown LiNbO₃ has essentially no oxygen vacancies, unusual for an oxide P-comp. Instead some niobium ions sit on lithium sites (antisites), and charge balance is restored by empty lithium sites (Li vacancies). In congruent crystal about 1 in 100 lithium sites holds a niobium and about 4 in 100 are empty P-comp.

These few percent of misplaced ions matter far beyond their number. An electron captured by a niobium antisite stays put, dressed by the lattice distortion it causes: a small polaron with its own near-infrared absorption band. Antisites trap the charge behind optical damage, set the built-in bias field, and raise the coercive field (1.6, 1.8).

centre    band peak
          --+----+-----+----+----+----+-----+----+
Nb_Li      no band listed
V_Li       no band listed
V_Nb       no band listed
Nb_Li4+                 |
Nb_Nb4+          |
bipolaron                        |
O- hole                          |
OH-        |
740 nm                  |
          --+----+-----+----+----+----+-----+----+
E (eV)    0.3   1.0        2.0       3.0       4.0
lambda nm 4133 1240        620       413       310

OH- stretch, wavenumber (cm-1)
          3440      3460      3480      3500
            +---------+---------+---------+-------
CLN                       |      |   3467 w 3482 s
FWHM                     [=======+=======] ~32
SLN                      |           3466

Nb_Li^4+  bound small polaron   1.60 eV  775 nm 
EPR 10-line 93Nb, g_perp 1.72, g_par 1.9        
KV  Nb_Li^•••     Nb4+ on Li+ site, +3          

Compensation models

Written per formula unit, x = antisite fraction P-comp:

Li-site vacancy model:  [Li(1-5x) Nb_Li(x) V_Li(4x)] Nb O3            (1 Nb_Li^4+ compensated by 4 V_Li^-)
Nb-site vacancy model:  [Li(1-5x) Nb_Li(5x)] [Nb(1-4x) V_Nb(4x)] O3   (5 Nb_Li^4+ compensated by 4 V_Nb^5-)

Both the Li-site and the Nb-site vacancy model give R₂ = (1 − 5x)/(2 − 4x), so x = (1 − 2R₂)/(5 − 4R₂) derived. Here the charges are relative to the host site (± convention, L3).

At 48.45 mol% the Li-site model gives 1.9 × 10²⁰ Nb_Li per cm³, with 1.886 × 10²² formula units per cm³ derived.

Where the Nb sits: observations

Electron and hole centres (polaron model)

Values in parentheses are hc/E or E = hc/λ of the sourced number derived.

Polaron stabilization energy about 0.3 eV and phonon energy about 0.1 eV are used in band-shape fits P-comp. Nb_Li⁴⁺ lifetime at room temperature: microseconds in congruent crystal, 10–100 ms near stoichiometry, up to seconds in Li-enriched Mg-doped crystal P-comp; it forms within 400 fs P-comp. GRIIRA (green-induced infrared absorption) shares the MgO threshold of photorefraction and is assigned to the Nb_Li polaron P(abs).

Hydrogen

Hydrogen enters from the atmosphere during or after growth and sits as OH⁻ on oxygen sites P-comp.

The model debate. Two diffraction studies of congruent crystal disagree on where the excess Nb compensation sits: Abrahams and Marsh (1986) and Iyi et al. (1992). Volk & Wöhlecke 2008 calls the Li-site vacancy model “commonly accepted” and notes a third reading, that the degree of filling depends on growth conditions so all three faults (Nb_Li, V_Li, V_Nb) coexist P-comp. The two models predict identical composition and differ in density and site occupancies, which is why the decision came from occupancy refinement plus NMR, not from chemistry P-comp.

Kroeger-Vink notation. A prime is negative and a bullet positive charge relative to the host site; many authors write − and + instead P-comp. Nb_Li^•••• denotes Nb⁵⁺ on an Li⁺ site (four excess positive charges). The polaron names use valence instead: Nb_Li⁴⁺ is Nb⁴⁺ on a Li site, three excess positive charges.

Reduction and the test for antisites. Annealing in vacuum or hydrogen at 400–600 °C creates bipolarons; low-temperature illumination converts them into Nb_Li⁴⁺ with the 1.6 eV band and the 10-line EPR, which serves as a spectroscopic test for antisites; above 600 °C oxygen vacancies do form and complicate the picture P-comp. An alternative reading of the visible band in terms of oxygen vacancies “has not been completely ruled out” P-comp. The polaron review of Schirmer et al. is the primary reference for band shapes P; the gallery shows peak positions only.

High temperature. The room-temperature defect picture does not hold at high temperature; Li disorder grows (1.2). A synchrotron study finds a small fraction of Li on the vacant octahedra even in stoichiometric crystal at room temperature P-comp.