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

Abstract

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 Li 2 TiTeO 6 . Here, we stabilized a novel metastable Li 2 TiTeO 6 (denoted as γ-phase) in the ordered-ilmenite-type R3 via facile topotactic reaction from Na 2 TiTeO 6 , which was found to crystallize in R3 instead of the reported R3̅ structure. The calculated equilibrium volume of γ-Li 2 TiTeO 6 is larger than that of the ground-state Pnn2-Li 2 TiTeO 6 (denoted as α-phase), indicating that γ-Li 2 TiTeO 6 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-Li 2 TiTeO 6 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 Li 2 TiTeO 6 , which undergoes consecutive polymorph tuning of γ (−6 GPa), α (0 GPa), β (6 GPa, R3-Ni 3 TeO 6 type), and δ (40 GPa, predicted P21/n double perovskite) phases with densified atomic packing.

Investigation of nonlinear optical properties in α-A2BB′O6 (A = Li, Na, K; B = Ti, Zr, Hf; B′ = Se, Te) by first-principles calculations

“…In the ABB′O material family, when smaller Li + ions at the A site are replaced with Na + ions, it results in Na 2 TiTeO 6 exhibiting an approximately twofold increase in SHG intensity compared to β-LTTO. 15 Moreover, flexible substitution at the B site of ABB′O has also been reported. The β-Li 2 ZrTeO 6 crystal exhibits a SHG response of 2.5 × KDP with a band gap of 4.06 eV and a LDT larger than 1300 MW cm −2 .…”

“…[15][16][17] Among these phases, a-Li 2 TiTeO 6 (LTTO) shows a band gap of 3.67 eV and a large SHG coefficient of 26 Â KDP, 18 which is larger than that of b-LTTO. 15 The spontaneous polarization constant of a-Li 2 GeTeO 6 is larger than that of b-Li 2 GeTeO 6 , 16 which is closely related to SHG. Moreover, centrosymmetric g-A 2 BB 0 O 6 do not possess SHG.…”

“…11 In recent years, a new family of NLO materials with a general formula of A 2 BB′O 6 (ABB′O; A = Li, Na, K; B = Ti, Zr, Hf; B′ = Se, Te) were investigated, because they usually contain both octahedrally coordinated d 0 transition metal cations and main-group cations with lone-pair electrons as mentioned above. 12–14 A 2 BB′O 6 crystals generally possess four phases, including the α-phase with the orthorhombic space group Pnn 2, β-phase with the trigonal space group R 3, γ-phase with the space group R 3̄ and the theoretically predicted δ -phase with the monoclinic space group P 21/ n . 15–17 Among these phases, α-Li 2 TiTeO 6 (LTTO) shows a band gap of 3.67 eV and a large SHG coefficient of 26 × KDP, 18 which is larger than that of β-LTTO.…”

“…12–14 A 2 BB′O 6 crystals generally possess four phases, including the α-phase with the orthorhombic space group Pnn 2, β-phase with the trigonal space group R 3, γ-phase with the space group R 3̄ and the theoretically predicted δ -phase with the monoclinic space group P 21/ n . 15–17 Among these phases, α-Li 2 TiTeO 6 (LTTO) shows a band gap of 3.67 eV and a large SHG coefficient of 26 × KDP, 18 which is larger than that of β-LTTO. 15 The spontaneous polarization constant of α-Li 2 GeTeO 6 is larger than that of β-Li 2 GeTeO 6 , 16 which is closely related to SHG.…”

Data-Driven High-Throughput Screening and Experimental Realization of Ag2B(IV)B′(VI)O6 under Negative Chemical-Pressure

“…It has been manifested in the study of γ-LTTO, where the calculations of energy−volume (E−V) curves are proven to be an effective to guide NP phases exploration. 17 The Ag + cation has similar chemical characteristics with alkali metal cations like Li + and Na + . This resemblance often facilitates their interchangeable modification as seen in as the preparation of Ag 3 LiIr 2 O 6 23,24 and AgCoO 2 25 from their alkalimetal precursors.…”