Lattice Anharmonicity of Stereochemically Active Lone Pairs Controls Thermochromic Band Gap Reduction of PbVO₃Cl

Abstract

Stereochemically active lone pairs of electrons play an important role in a diverse range of physical phenomena in many materials, ranging from semiconducting halide perovskites to thermochromic inorganic−organic hybrids. In this paper, we demonstrate the importance of the 6s 2 lone pair of Pb on the reversible thermochromic transition in the mixed-anion inorganic compound, PbVO 3 Cl. This 6s 2 stereochemically active lone pair results in subtle structural distortions upon heating while maintaining its overall orthorhombic structure. These distortions result in competing interactions with the Pb 6s 2 lone pair and ultimately, a pronounced change between yellow and red at ∼200 °C. X-ray diffraction analyses of PbVO 3 Cl demonstrate two-dimensional features in contrast to the three-dimensional network in isostructural BaVO 3 Cl. X-ray and neutron pair distribution function experiments reveal that Pb−O interatomic distances decrease upon heating, while Pb−Cl distances are only affected by thermal motion. X-ray photoelectron spectroscopy measurements provide experimental evidence of the presence of the 6s 2 lone pair at the valence band maximum, which are corroborated by first-principles calculations. The results demonstrate a broadly generalizable mechanism for using repulsions between lone-pair electrons of p-block cations to drive discontinuous changes of local symmetry and electronic structure.

Chemistry, Structure, and Function of Lone Pairs in Extended Solids

“…65,66 Another example of the influence of lone pair behavior on optical properties is in the thermochromic material PbVO 3 Cl. 63 This material exhibits a color change from yellow to red above 573 K (shown in figure 7c), which is not accompanied by a crystallographic phase transition. A combined approach of total scattering, HAXPES, and DFT revealed behavior in PbVO 3 Cl that was not present in the s 0 analog BaVO 3 Cl (figure 7d).…”

Structural Properties of NdTiO2+xN1–x and Its Application as Photoanode

“…Beyond metal oxides, mixed-anion inorganic compounds, such as oxyfluorides, oxynitrides, oxide–carbodiimides, oxysulfides, oxyhalides, and oxyhydrides, could offer diverse functionalities originating from the different properties of the secondary anion. − Within the past two decades, transition-metal oxynitrides have emerged as promising photoactive materials because of their frequently higher theoretical solar-to-hydrogen efficiency. This is in particular promising for the quaternary metal oxynitrides because they can exhibit a smaller band gap than the ternary oxides and even than the binary nitride Ta 3 N 5 ( E g = 2.1 eV).…”

Structural Properties of NdTiO2N and Its Application as Photoanode

“…Beyond metal oxides, mixed-anion inorganic compounds, such as oxyfluorides, oxynitrides, oxide-carbodiimides, oxysulfides, oxyhalides, and oxyhydrides could offer diverse functionalities originating from the different properties of the secondary anion. [5][6][7][8] Within the recent two decades, transition-metal oxynitrides have emerged as promising photoactive materials due to their frequently higher theoretical solar-tohydrogen efficiency. This is in particular promising for the quaternary metal oxynitrides, because they can exhibit a smaller band gap than the binary Ta3N5 (Eg = 2.1 eV).…”