Influence of A-site cations on germanium iodates as mid-IR nonlinear optical materials: A2Ge(IO3)6 (A = Li, K, Rb and Cs) and BaGe(IO3)6·H2O

In this work, a series of germanium iodates with different A-site cations A2Ge(IO3)6 (A = Li, K, Rb and Cs) and BaGe(IO3)6·H2O have been synthesized and further explored as potential mid-IR nonlinear optical (NLO) materials. It is found that the change of the alkali metal ionic radius from Li+ to K+–Cs+ greatly affects the coordination environment that alters the arrangement of polar [IO3]− anionic groups, thus leading to the crystal structural shift from the noncentrosymmetric (NCS) structure of P63 for Li2Ge(IO3)6 to the centrosymmetric (CS) structure of R for A2Ge(IO3)6 (A = K, Rb and Cs). The substitution of two alkali ions by one alkali-earth metal ion of Ba2+ with different ionic radius and valence produces another germanium iodate, BaGe(IO3)6·H2O, which contains a crystal water molecule to participate in coordination and crystallizes in another polar space group R3. Li2Ge(IO3)6 and BaGe(IO3)6·H2O with the NCS structure possess second-order NLO properties, in which Li2Ge(IO3)6 shows a very strong second harmonic generation (SHG) response of about 20 × KDP (KH2PO4) under laser irradiation at 1064 nm and 1.95 × KTP (KTiOPO4) under laser irradiation at 1950 nm with type-I phase-matching characteristics, while BaGe(IO3)6·H2O only exhibits a moderate SHG response of 1.5 × KDP with non-phase-matching characteristics. Especially, Li2Ge(IO3)6 shows a large bandgap of 3.60 eV with a laser damage threshold over 12 times that of AgGaS2 under the same measured conditions, and it also shows a relatively wide mid-IR transparent window up to 10 μm and a good crystal growth habit with a high decomposition temperature over 360 °C, suggesting that it will be a promising mid-IR NLO crystal.