Vibrational fingerprints of LiNbO3-LiTaO3 mixed crystals

The structural and vibrational properties of lithium niobate (LN) - lithium tantalate (LT) mixed crystals (LNT, LiNb
_1-x
Ta
_x
O
₃
) are investigated over the whole composition range by first-principles simulations. The crystal volume grows roughly linearly from LT to LN, whereby the lattice parameters a and c show minor deviations from the Vegard behavior between the end-compounds, LiNbO
₃
and LiTaO
₃
. Our calculations in the framework of the density functional theory show the TO
₁
, TO
₂
and TO
₄
-modes to become harder with increasing Nb concentration. TO
₃
becomes softer with increasing Nb content, instead. The frequency shifts of the zone center A
₁
-TO phonon modes for crystals with different compositions are found to be as large as 30 cm
^-1
. Raman spectroscopy, which is sensitive to the A
₁
modes, can be therefore employed to determine the crystal composition.