Mid-infrared
(mid-IR) nonlinear optical (NLO) crystals with excellent
comprehensive performances are urgently demanded in the laser technology
field. Herein, a series of new langasite members, A3Te(Zn2Ge)Ge2O14 (A = Sr, Ba, and Pb), were
designed and synthesized through a rational chemical substitution
strategy with classic A3BC3D2O14 langasites as ideal prototypes. These crystals exhibited
excellent mid-IR NLO properties including a large second harmonic
generation (SHG) response (2.7, 3.2, 13.0 × KDP under 1064 nm
and 0.11, 0.12, 0.54 × AGS under 2090 nm), a large band gap (4.03,
3.98, and 3.43 eV), and a wide transparency region from the ultraviolet
(UV) to mid-IR region. Compared to those langasites with light D-site
cations, the titled crystals highlighted the reduction of vibration
frequency by the introduction of heavier Ge4+ cations,
which helps extend the transparency range in the mid-IR region. Additionally,
theoretical calculation results revealed the significant enhancement
of the SHG effect by Pb2+ cations on the A-site. This work
confirmed the feasibility of our chemical co-substitution strategy
and enriched the langasites.