The electrical, magnetic and transport properties of Zn doped polycrystalline samples of Sr2Fe1−xZnxMoO6 (x = 0,0.05,0.15 and 0.25) with the double perovskite structure have been investigated. The subtle replacement of Fe3+ ions by Zn2+ ions facilitates the formation of a more ordered structure, while further substitution leads to disordered structure because of the presence of a striped phase. Analysis of the x-ray powder diffraction patterns based on Rietveld analysis indicates that the replacement of Fe3+ by Zn2+ ions favours the formation of Mo6+ ions. The spin-glass behaviour can be explained on the basis of the competition between the antiferromagnetic superexchange and the ferromagnetic double-exchange interaction. The low-field magnetoresistance was moderately enhanced at x = 0.05, and its origin was found to be the competition between the decrease of the concentration of the itinerant electrons and the weaker antiferromagnetic superexchange in the antiphase boundaries. An almost linear negative magnetoresistance in moderate field has been observed for x = 0.25. A possible double-exchange mechanism is proposed for elucidating the observations; it also suggests a coexistence of (Fe3+,Mo5+) and (Zn2+,Mo6+) valence pairs.
Ferroelectrics were often modified by dopants for corresponding applied fields. The LiNbO3 (LN) crystal, one of the most attractive ferroelectric functional materials, was often doped by magnesium or some rare-earth ions for corresponding application. Though many kinds of dopants for the LN crystal have been researched, the mechanism of the dopant to regulate the ferroelectricity was never reported. Herein, the origin of ferroelectric modification was investigated from the thermal distortion of the fine lattice structure. It was proved by using the state-of-the-art first-principles approach based on density functional theory that the rare-earth ions and antisite NbLi defects are poisonous to the LN lattice structure, which generates the lattice distortion. Furthermore, the lattice distortion evolved into a lattice-plane split with the increasing temperature. On the contrary, the magnesium doped LN crystal keeps the complete LN structure even at 1275 K. Finally, the mechanism of ferroelectric modification was analyzed from the thermal behavior of dopant cations.
Abstract Precise point positioning (PPP) using global navigation satellite system (GNSS) phase and code measurements has recently been the primary technique for time and frequency comparisons. Several scholars have studied multi-GNSS PPP clock comparison, but the inconsistent pseudorange bias from receivers with different correlator spaces and front-end designs in pseudorange observations has not been considered. In this work, we analyze the characteristics of inconsistent pseudorange biases of the Global Positioning System (GPS), BeiDou Navigation Satellite System (BDS) and their effects on PPP frequency transfer. The biases are confirmed to exist in receivers from manufacturers Septentrio, Trimble and Leica and differ by manufacturers. To explicitly investigate the pseudorange biases effects of GPS and BDS on PPP time and frequency comparisons, nine stations with receivers from three different manufacturers that can track BDS-2 and BDS-3 signals are selected. Regarding PPP frequency transfer with inhomogeneous receivers, the modified Allan variances (MDEVs) of GPS and BDS PPP frequency stability are significantly optimized. According to the receiver-type classification strategy, the overall improvements of frequency transfer with Trimble-Septentrio and Trimble-Leica are 33%, 7% and 39% and 23%, 14% and 23% for GPS, BDS-2 and BDS-3, respectively. Moreover, the convergence time of clock comparisons is obviously shortened after using the bias corrections. For GOPE-BRUX and GOPE-MATE links, the corrected cases yield average clock difference stability gains of 38%, 35% and 53% and 35%, 35% and 23% for GPS, BDS-2, and BDS-3, respectively. The results show that the bias corrections are vital and allow more stable time links for PPP frequency transfer.
The effects of F doping on the structural and electronic properties of the (5, 5) single-walled boron nitride nanotube (BNNT) are investigated by using the density functional theory method. The chemiadsorption of F maintains the hexagonal BN network, increases the lattice constant, and introduces acceptor impurity states. On the other hand, substitutional doping of F destroys the hexagonal BN network, decreases the lattice constant, but does not alter the insulating feature of the BNNT. The observed insulator-to-semiconducting transition, a lattice contraction, and a highly disordered atom arrangement in the sidewall of BNNTs upon F doping appear to be most reasonably attributed to a codoping of dominating substitutional F over chemiabsorbed F, which can induce deep donor impurity states, a lattice contraction, and a destruction of the hexagonal BN network simultaneously.
A ferromagnet-metal-type composite, La0.7Ca0.3MnO3 (LCMO)–Ag, was fabricated by a two-step chemical route which can avoid the doping of Ag into the lattice of LCMO. The grain size of Ag can be reduced by increasing calcination temperature, which favors the penetration of Ag into LCMO matrices. A large enhancement in magnetoresistance (MR) near room temperature and a dramatic decrease in resistivity are observed for the samples calcined at above the melting temperature of Ag. We suggest that the shift of metal–insulator transition up to Curie temperature in melted-Ag-added LCMO and magnetic inhomogeneity are responsible for the enhanced MR.
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