Electronegativity Adaption Approach for Solar-Blind UV Birefringent Crystals With Ultrawide Bandgaps.

Area of Science:

• Optics and Materials Science
• Solid-State Chemistry

Background:

• Birefringent crystals are crucial for optics, modulating light polarization.
• Steretochemically active lone-pair (SCALP) effect enhances birefringence but causes UV red-shift.
• Existing SCALP crystals have limited UV transparency, hindering solar-blind applications.

Purpose of the Study:

• To develop an electronegativity-adaption approach to widen the UV transparency of lone-pair birefringent crystals.
• To investigate ternary tellurite fluorides (ATeF5) for improved UV optical properties.
• To engineer bandgaps in SCALP-based materials for solar-blind UV applications.

Main Methods:

• Synthesis and characterization of ternary tellurite fluorides, ATeF5 (A = K, Rb, Cs, NH4).
• Measurement of UV-Vis absorption spectra to determine cut-off edges and bandgaps.
• Calculation of birefringence values at 550 nm.
• Electronic structure analysis using density functional theory (DFT).

Main Results:

• ATeF5 compounds exhibit significantly blue-shifted UV cut-off edges and bandgaps exceeding 5 eV.
• NH4TeF5 shows a UV cut-off edge of 218 nm (5.69 eV bandgap) and birefringence of 0.130@550 nm.
• NH4TeF5 possesses the largest bandgap among SCALP-based materials with birefringence > 0.1.
• Electronic structure analysis confirms adaptable electronegativity widens bandgaps by lowering HOMO more than LUMO.

Conclusions:

• The electronegativity-adaption approach successfully widens the UV transparency of SCALP-based birefringent crystals.
• Ternary tellurite fluorides, particularly NH4TeF5, are promising materials for solar-blind UV optical applications.
• This research provides insights into bandgap engineering for designing advanced optical crystals.