An energy band engineering design to enlarge the band gap of KTiOPO4 (KTP)-type sulfates via aliovalent substitution

Abstract

Herein, two fluoride sulfates, i.e. RbSnFSO₄ and RbSbF₂SO₄, were successfully designed via aliovalent substitution using KTiOPO₄ (KTP) as a matrix and replacing both [TiO₆]⁸⁻ and [PO₄]³⁻ functional groups in KTP with [SnO₄F₂]⁸⁻, [SbO₄F₂]⁷⁻ and [SO₄]²⁻ units. Compared to the parent compound KTP (3.52 eV), both sulfates exhibited a sharply enlarged band gap, i.e. 4.41 eV for RbSnFSO₄ and 4.75 eV for RbSbF₂SO₄, due to the introduction of F⁻ and the displacement of Ti⁴⁺ cations. Laser damage threshold (LDT) measurements verified the experimental trends. RbSnFSO₄ crystallizes in the centrosymmetric (CS) monoclinic space group P2₁/m (no. 11), and RbSbF₂SO₄ crystallizes in the orthorhombic crystal system with the noncentrosymmetric (NCS) polar space group Pna2₁ (no. 33). Second-harmonic generation (SHG) measurements manifest that RbSbF₂SO₄ is phase-matchable with the SHG response being 0.96 times that of KH₂PO₄ (KDP). Both fluoride sulfates reported herein are promising short-wave ultraviolet optical materials.