Lattice Anharmonicity of Stereochemically Active Lone Pairs Controls Thermochromic Band Gap Reduction of PbVO\u3csub\u3e3\u3c/sub\u3eCl

Copyright © 2020 American Chemical Society. 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 6s2 lone pair of Pb on the reversible thermochromic transition in the mixed-anion inorganic compound, PbVO3Cl. This 6s2 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 6s2 lone pair and ultimately, a pronounced change between yellow and red at ∼200 °C. X-ray diffraction analyses of PbVO3Cl demonstrate two-dimensional features in contrast to the three-dimensional network in isostructural BaVO3Cl. 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 6s2 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

OBSERVATION AND ANALYSIS OF THE XeF21030cm−1XeF^{2} 1030 cm^{-1} BANDS

$^{1}$ S. Reichman and F. Schreiner, J. Chem. Phys., 51 (1969) 2355. $^{2}$ P. Isao, C. Cobb and H. Claassen, J. Chem. Phys., 54 (1971) 5247. $^{3}$ N. Brassington, H. Edwards and D. Long, J. Chem. Soc. Faraday 11, 74 (1978) 1208.Author Institution: Department of Chemistry, University of Northern Colorado; Department of Chemistry and Biochemistry, University of ColoradoThe intrared spectrum of $XeF_{2}$ is observed using a Fourier transform infrared spectrometer at $0.02 cm^{-1}$ resolution. At this resolution we are able to rotationally resolve the $0^{1}10\rightharpoonup 2^{1}10$ and either $0^{2}20$ or $0^{0}01\rightharpoonup 2^{0}01$ bands. The information obtaineo in this experiment is combined with previous $infrared^{1}$ and $Raman^{2,3}$ studies of lower vibrational states to determine rovibrational constants for the ground state of the $XeF_{2}$ molecule

CHEMILUMINESCENCE FROM REACTION OF Aℓ2,Ga2A\ell_{2}, Ga_{2} WITH HALOGEN-CONTAININC MOLECULES

Author Institution:Chemiluminescence from molecules of the type $M_{2}X$ has been observed in a molecular beam-scattering gas arrangement from reaction of the metal dimmer with $F_{2}, NF_{3}$, and $Cl_{2}$. The emission extends from approximately 400 nm to the red limit of our detection system (about 750 nm) and consists of two broad. Structureless features. We associate these features with emission from the two bound excited states. Our data also place a lower bound of 100 kcal $mole^{-1}$ on the bond strength of the $M_{2}-X$ bond in the ground state of $M_{2}$X, which may be unstable with respect to $M +$MX. Further attempts to obtain quantitative knowledge of the potentials using a metal nozzle source will be discussed