Metastable γ-Li2TiTeO6: Negative Chemical Pressure Interception and Polymorph Tuning of SHG

Intercepting metastable phases by chemical approaches is an important solution to explore structural varieties of functional materials under positive/negative pressure, as paradigmatically exemplified by the polymorph modification in Li2TiTeO6. Here, we stabilized a novel metastable Li2TiTeO6 (denoted as γ-phase) in the ordered-ilmenite-type R3 via facile topotactic reaction from Na2TiTeO6, which was found to crystallize in R3 instead of the reported R3̅ structure. The calculated equilibrium volume of γ-Li2TiTeO6 is larger than that of the ground-state Pnn2-Li2TiTeO6 (denoted as α-phase), indicating that γ-Li2TiTeO6 can only be stabilized under "negative pressure" quantified to be around −6 GPa. The γ-phase irreversibly transforms into the α-phase around 560 °C under ambient pressure, accompanied by a steep increase (∼500 times) of the second harmonic generation (SHG), indicating a potential application of γ-Li2TiTeO6 as an optical thermometer. These findings elegantly show that chemical pressure as well as physical pressure is powerful to tune the polymorphs for metastable phases and exotic properties as paradigmatically exemplified by Li2TiTeO6, which undergoes consecutive polymorph tuning of γ (−6 GPa), α (0 GPa), β (6 GPa, R3-Ni3TeO6 type), and δ (40 GPa, predicted P21/n double perovskite) phases with densified atomic packing.