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

Osteo-cise: Strong Bones for Life: protocol for a community-based randomised controlled trial of a multi-modal exercise and osteoporosis education program for older adults at risk of falls and fractures

Background : Osteoporosis affects over 220 million people worldwide, and currently there is no \u27cure\u27 for the disease. Thus, there is a need to develop evidence-based, safe and acceptable prevention strategies at the population level that target multiple risk factors for fragility fractures to reduce the health and economic burden of the condition. Methods : The \u27Osteo-cise: Strong Bones for Life\u27 study will investigate the effectiveness and feasibility of a multi-component targeted exercise, osteoporosis education/awareness and behavioural change program for improving bone health and muscle function, and reducing falls risk in community-dwelling older adults at an increased risk of fracture. Men and women aged 60 years or above will participate in an 18-month randomised controlled trial comprising a 12-month structured and supervised community-based program and a 6-month \u27research to practise\u27 translational phase. Participants will be randomly assigned to either the \u27Osteo-cise\u27 intervention or a self-management control group. The intervention will comprise a multi-modal exercise program incorporating high velocity progressive resistance training, moderate impact weight-bearing exercise and high challenging balance exercises performed three times weekly at local community-based fitness centres. A behavioural change program will be used to enhance exercise adoption and adherence to the program. Community-based osteoporosis education seminars will be conducted to improve participant knowledge and understanding of the risk factors and preventative measures for osteoporosis, falls and fractures. The primary outcomes measures, to be collected at baseline, 6, 12, and 18 months, will include DXA-derived hip and spine bone mineral density measurements and functional muscle power (timed stair-climb test). Secondary outcomes measures include: MRI-assessed distal femur and proximal tibia trabecular bone micro-architecture, lower limb and back maximal muscle strength, balance and function (four square step test, functional reach test, timed up-and-go test and 30-second sit-to-stand), falls incidence and health-related quality of life. Cost-effectiveness will also be assessed. Discussion : The findings from the Osteo-cise: Strong Bones for Life study will provide new information on the efficacy of a targeted multi-modal community-based exercise program incorporating high velocity resistance training, together with an osteoporosis education and behavioural change program for improving multiple risk factors for falls and fracture in older adults at risk of fragility fracture.<br /

Rapid one-pot synthesis and photoelectrochemical properties of copper vanadates

Solution combustion synthesis (SCS) is shown to be versatile for the rapid-one-pot synthesis of three compounds and four polymorphs in the Cu-V-0 ternary family: alpha-CuV2O6, alpha- and beta-Cu2V2O7, and gamma-Cu3V2O8. These compounds feature copper/vanadium stoichiometric ratios ranging from 1:1 to 3:1; their structural, electronic, optoelectronic, and photoelectrochemical attributes were comprehensively characterized by a combination of theoretical and experimental techniques. The main contribution of the present study is the demonstration that a range of stoichiometries in this compound family can be derived simply by tuning the precursor mole ratio in the SCS procedure. The Cu-V-O family of samples, derived by SCS, is shown to exemplify the strong effect of compound stoichiometry on the optoelectronic and photoelectrochemical properties. Overall, alpha-CuV2O6 showed the best performance, rooted in the direct nature of the optical transition in this material. Finally, SCS is very time-efficient and the various compositions can be obtained in a matter of minutes, as opposed to hours or even days in classical solution-based or ceramic synthesis routes2428372847COORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL DE NÍVEL SUPERIOR - CAPES88881.131530/2016-0C.J. and A.K. acknowledge the support of the“Széchenyi 2020”program within the framework of the GINOP-2.3.2-15-2016-00013 project. M.T.G. and C.L. acknowledge CAPES-PDSE(Process#88881.131530/2016-0) for partial financial supportunder the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior Program. M.N.H. was supported by the National Science Foundation (Award No. DMR-1609811).M.N.H. and H.P.S. thank the Texas Advanced ComputingCenter (Austin, TX) facility for their computational needs.The authors thank the three anonymous reviewers forperceptive comments on an earlier version of this manuscrip

Structural Diversity of Rare-Earth Oxychalcogenides

Mixed-anion systems have garnered much attention in the past decade with attractive properties for diverse applications such as energy conversion, electronics, and catalysis. The discovery of new materials through mixed-cation and single-anion systems proved highly successful in the previous century, but solid-state chemists are now embracing an exciting design opportunity by incorporating multiple anions in compounds such as oxychalcogenides. Materials containing rare-earth ions are arguably a cornerstone of modern technology, and herein, we review recent advances in rare-earth oxychalcogenides. We discuss ternary rare-earth oxychalcogenides whose layered structures illustrate the characters and bonding preferences of oxide and chalcogenide anions. We then review quaternary compounds which combine anionic and cationic design strategies toward materials discovery and describe their structural diversity. Finally, we emphasize the progression from layered two-dimensional compounds to three-dimensional networks and the unique synthetic approaches which enable this advancement

Stereochemically Active Lone Pairs: Unraveling the Electronic Origin of Dielectric Response in Nonlinear Optical TeO₂

Tellurite glasses have emerged as a promising candidate for nonlinear optics (NLO) due to their high refractive index and third-order optical susceptibility (χ3). In this work, we focus on the electronic structure of crystalline TeO2, a precursor to numerous TeO2-based glasses. All three crystalline TeO2 polymorphs are wide-gap semiconductors and have electronic contributions from the Te4+ 5s2 lone pairs near the Fermi level, which affect optical properties. Based on the formation energies, the formation of oxygen vacancies is strongly favorable during synthesis. Introduction of an oxygen vacancy induces notable changes in the chemical environment in the pristine α-TeO2 structure and the response of the Te 5s2 lone pairs. Comparative analysis to the measured refractive indices and static dielectric constant (a second-rank tensor) shows that the calculated values of α-TeO2 with the presence of an oxygen vacancy (α-TeO2:Ov) agree well with experimental data. The high distortion index and stereochemical activity ratio of α-TeO2:Ov contribute to its polarizable asymmetric electron density. High polarization and faster orientation of the dipoles in α-TeO2 with the presence of an oxygen vacancy make it an excellent NLO material for ultrafast optical switch applications. This study can help to guide design principles and improve our understanding of the electronic origin of NLO materials containing lone pairs

Structure and optical properties of a new AgBiW2O8 polymorph

In this work, we report a novel polymorph of AgBiW2O8, which was synthesized by a simple arc melting method. The structural details of this compound were determined by X-ray powder diffraction, and elemental composition was confirmed with ICP-AES. beta-AgBiW2O8 adopts the scheelite structure type with tetragonal space group I4(1)/a and lattice parameters a = b = 5.3019(3) and c= 11.678(2) angstrom. Structural details and optical properties are discussed in detail. Diffuse reflectance spectroscopy (DRS) measurements of beta-AgBiW2O8 showed light absorption profile extended to the visible region, which suggests the material has a smaller band gap energy than Bi2WO6, as expected due to contribution of Ag d-orbitals to the electronic structure170486489COORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL DE NÍVEL SUPERIOR - CAPES88881.131530/2016-0