Ba₂NaClP₂O₇: Unprecedented Phase Matchability Induced by Symmetry Breaking and Its Unique Fresnoite-Type Structure

To combine noncentrosymmetry, large second order nonlinear optical (NLO) susceptibility, deep UV transparency, and phase matchability simultaneously into a single structure is a great challenge. Especially, the relationship between the macrophase matchability and microstructure has been less investigated. Herein, we report a unique Ba₂NaClP₂O₇ representing the first phase-matchable example of the fresnoite family, which exhibits excellent NLO properties, including short absorption edge (<176 nm, ca. 7.0 eV), strong second-harmonic generations at the incident lasers of 850-1110 nm (obv. 0.9-1.3 × KDP; cal. dave, 1.11 vs 0.96 pm/V of KDP), and large laser-induced damage threshold (1.4 × KDP). More interestingly, Ba₂NaClP₂O₇ illustrates an unprecedented mechanism of phase matchability that is promoted by crystallographic disorder. The uniaxial symmetry breaking caused by the disorder of [P₂O₇]⁴⁻ group results in a structural anisotropy that changes the band dispersions along k-paths of G-Y and G-X, resulting in an increase of refractive index n x and a n y decrease, eventually enlarging the birefringence.