RESUMO
Series of Ca1-x Pr x Co2As2 (x = 0, 0.10, 0.25, 0.4, 0.6, 0.75, 0.85, 1) single crystals have been synthesized in order to clarify the variation of magnetic order from antiferromagnetic (AFM) in CaCo2As2 to ferromagnetic (FM) in PrCo2As2. It is found that the lattice constant of c-axis are contracted with the introduction of Pr into Ca sites in CaCo2As2. Electronic transport measurements reveal the metallicity in this system. Systematic magnetic measurements and analysis show that substituting only 10% of Pr for Ca changes the magnetic ground state from A-type AFM ordering of Co magnetic moment in CaCo2As2 to FM ordering in Ca1-x Pr x Co2As2 (0.1 ⩽ x ⩽ 1). Most importantly, the abrupt drop of low temperature magnetic susceptibility below T FiM with x ⩾ 0.25 and the observed magnetic pole reversal with x ⩾ 0.4 suggests an AFM coupling between Co 3d and Pr 4f magnetic sublattice. Finally, a detailed magnetic phase diagram in this system has been obtained.
RESUMO
The intentional production and degradation of plastic debris may result in the formation of nanoplastics. Currently, the scarce information on the environmental behaviors of nanoplastics hinders accurate assessment of their potential risks. Herein, the aggregation kinetics of different surface-modified polystyrene nanoparticles in monovalent and divalent electrolytes was investigated to shed some light on the fate of nanoplastics in the aquatic environment. Three monodisperse nanoparticles including unmodified nanoparticles (PS-Bare), carboxylated nanoparticles (PS-COOH) and amino modified nanoparticles (PS-NH2), as well as one polydisperse nanoparticles that formed by laser ablation of polystyrene films (PS-Laser) were used as models to understand the effects of surface groups and morphology. Results showed that aggregation kinetics of negatively charged PS-Bare and PS-COOH obeyed the DLVO theory in NaCl and CaCl2 solutions. The presence of Suwannee river natural organic matters (SRNOM) suppressed the aggregation of PS-Bare and PS-COOH in monovalent electrolytes by steric hindrance. However, in divalent electrolytes, their stability was enhanced at low concentrations of SRNOM (below 5â¯mgâ¯C L-1), while became worse at high concentrations of SRNOM (above 5â¯mgâ¯C L-1) due to the interparticle bridging effect caused by Ca2+ and carboxyl groups of SRNOM. The cation bridging effect was also observed for PS-laser in the presence of high concentrations of divalent electrolytes and SRNOM. The adsorption of SRNOM could neutralize or even reverse surface charges of positively charged PS-NH2 at high concentrations, thus enhanced or inhibited the aggregation of PS-NH2. No synergistic effect of Ca2+ and SRNOM was observed on the aggregation of PS-NH2, probably due to the steric repulsion imparted by the surface modification. Our results highlight that surface charge and surface modification significantly influence aggregation behaviors of nanoplastics in aquatic systems.