Investigation of nonlinear optical properties in -A₂BBO₆ (A = Li, Na, K; B = Ti, Zr, Hf; B = Se, Te) by first-principles calculations

Nonlinear optical (NLO) crystals based on oxides typically have wide bandgaps and large laser damage thresholds (LDTs), which are important for generating high-power and continuous terahertz radiation. Recently, a new family of NLO materials alpha-A(2)BB ' O-6 including Li2TiTeO6 (LTTO) with a strong second harmonic generation (SHG) efficiency of 26 x KH2PO4 (KDP) and a large LDT of 550 MW cm(-2) were reported. Herein, we systematically study the electronic structures and NLO properties of alpha-A(2)BB ' O-6 (A = Li, Na, K; B = Ti, Zr, Hf; B ' = Se, Te) to explore the relationship between the structure and SHG coefficient. First, 15 members of the A(2)BB ' O-6 family are demonstrated to be highly stable and NLO materials, excluding K2TiTeO6, K2TiSeO6 and K2ZrSeO6. Then, the electronic band structure, dipole moment and distortion of BO6/B ' O-6 octahedrons, SHG coefficient and terahertz absorption spectrum are calculated comprehensively with the element variation of A-site, B-site and B '-site. Finally, the magnitude of the SHG coefficient is found to be directly proportional to the value of total dipole moment and distortion, and inversely proportional to the bandgap value. Most importantly, among the A(2)BB ' O-6 materials, K2HfSeO6 shows the smallest direct bandgap of 2.99 eV, the largest SHG coefficient d(33) of about 5 x LTTO and low terahertz absorbance from 0.1 to 9 THz. Our results provide new NLO crystals that may have potential application in terahertz radiation sources and other nonlinear electronics.