Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userHydrogen tunneling in the perovskite ionic conductor BaCe(1-x)Y(x)O(3-d)
92
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
We present low-temperature anelastic and dielectric spectroscopy measurements on the perovskite ionic conductor BaCe(1-x)Y(x)O(3-x/2) in the protonated, deuterated and outgassed states. Three main relaxation processes are ascribed to proton migration, reorientation about an Y dopant and tunneling around a same O atom. An additional relaxation maximum appears only in the dielectric spectrum around 60 K, and does not involve H motion, but may be of electronic origin, e.g. small polaron hopping. The peak at the lowest temperature, assigned to H tunneling, has been fitted with a relaxation rate presenting crossovers from one-phonon transitions, nearly independent of temperature, to two-phonon processes, varying as T^7, to Arrhenius-like. Substituting H with D lowers the overall rate by 8 times. The corresponding peak in the dielectric loss has an intensity nearly 40 times smaller than expected from the classical reorientation of the electric dipole associated with the OH complex. This fact is discussed in terms of coherent tunneling states of H in a cubic and orthorhombically distorted lattice, possibly indicating that only H in the symmetric regions of twin boundaries exhibit tunneling, and in terms of reduction of the effective dipole due to lattice polarization.
rate research
Read More
Phonons are produced when an excited vacancy in cuprous oxide (Cu$_2$O) relaxes. Time resolved luminescence was used to find the excited copper vacancy (acceptor) and oxygen vacancy (donor) trap levels and lifetimes. It was also used to determine the typical energy and number of phonons in the phonon pulses emitted by vacancies. The vacancy properties of cuprous oxide are controlled by several synthesis parameters and by the temperature. We directly demonstrate the absorption of light by oxygen vacancies with transient absorption. Copper and oxygen vacancies behave differently, in part because the two kinds of traps capture carriers from different states. For example, the copper vacancy luminescence lifetime is around 25 times greater at low temperature. However, both kinds of vacancy luminescence are consistent with a Poissonian multiple phonon emission model.
Excited configurations of hydrogen in the oxyhydride BaTiO$_{3-x}$H$_x$ ($x=0.1-0.5$), which are considered to be involved in its hydrogen transport and exchange processes, were investigated by positive muon spin relaxation ($mu^+$SR) spectroscopy using muonium (Mu) as a pseudoisotope of hydrogen. Muons implanted into the BaTiO$_{3-x}$H$_x$ perovskite lattice were mainly found in two qualitatively different metastable states. One was assigned to a highly mobile interstitial protonic state, which is commonly observed in perovskite oxides. The other was found to form an entangled two spin-$frac{1}{2}$ system with the nuclear spin of an H$^-$ ion at the anion site. The structure of the (H,Mu) complex agrees well with that of a neutralized center containing two H$^-$ ions at a doubly charged oxygen vacancy, which was predicted to form in the SrTiO$_{3-delta}$ perovskite lattice by a computational study [Y. Iwazaki $et$ $al$., APL Materials 2, 012103 (2014)]. Above 100 K, interstitial Mu$^+$ diffusion and retrapping to a deep defect were observed, which could be a rate-limiting step of macroscopic Mu/H transport in the BaTiO$_{3-x}$H$_x$ lattice.
In a systematic study we investigate the effect of dopant level and hydration on the short-range structure of the proton conducting perovskite-type oxide BaIn_{x}Zr_{1-x}O_{3-x/2} (x = 0-0.75), using infrared and Raman spectroscopy. The results show that doping leads to significant local distortions of the average cubic structure of these materials. By increasing the In concentration from x = 0 to x = 0.75 new bands appear and grow in intensity in both the IR and Raman spectra, showing that the local distortions become successively more and more pronounced. The structural distortions are largely uncorrelated to the presence of oxygen vacancies, but instead are mainly driven by the size difference between the In^{3+} and Zr^{4+} ions, which leads to displacements of the cations and to tilting of the (In/Zr)O_{6} octahedra. Based on our results, we suggest that there is a threshold between x = 0.10 and x = 0.25 where the local structural distortions propagate throughout the whole perovskite structure. Comparison of our spectroscopic data with the proton conductivity reported for the same materials indicates that the presence of extended structural distortions are favorable for fast proton transport.
The rhodium doping in the LaCo$_{1-x}$Rh$_{x}$O$_3$ perovskite series ($x=0.02-0.5$) has been studied by X-ray diffraction, electric transport and magnetization measurements, complemented by electronic structure GGA+U calculations in supercell for different concentration regimes. No charge transfer between Co$^{3+}$ and Rh$^{3+}$ is evidenced. The diamagnetic ground state of LaCoO$_3$, based on Co$^{3+}$ in low-spin (LS) state, is disturbed even by a small doping of Rh. The driving force is the elastic energy connected with incorporation of a large Rh$^{3+}$ cation into the matrix of small LS Co$^{3+}$ cations, which is relaxed by formation of large Co$^{3+}$ in high-spin (HS) state in the next-nearest sites to the inserted Rh atom. With increasing temperature, the population of Co$^{3+}$ in HS state increases through thermal excitation, and a saturated phase is obtained close to room temperature, consisting of a nearest-neighbor correlation of small (LS Co$^{3+}$) and large (HS Co$^{3+}$ and LS Rh$^{3+}$) cations in a kind of double perovskite structure. The stabilizing role of elastic and electronic energy contributions is demonstrated in supercell calculations for dilute Rh concentration compared to other dopants with various trivalent ionic radius.
The purpose of this study was to investigate the magnetotransport properties of the Ge(0.743)Pb(0.183)Mn(0.074)Te mixed crystal. The results of magnetization measurements indicated that the compound is a spin-glass-like diluted magnetic semiconductor with critical temperature TSG = 97.5 K. Nanoclusters in the sample are observed. Both, matrix and clusters are magnetically active. Resistivity as a function of temperature has a minimum at 30 K. Below the minimum a variable-range hopping is observed, while above the minimum a metallic-like behavior occurs. The crystal has high hole concentration, p = 6.6E20 cm-3, temperature-independent. Magnetoresistance amplitude changes from -0.78 to 1.18% with increase of temperature. In the magnetotransport measurements we observed the anomalous Hall effect (AHE) with hysteresis loops. Calculated AHE coefficient, RS = 2.0E6 m3/C, is temperature independent. The analysis indicates the extrinsic skew scattering mechanism to be the main physical mechanism responsible for AHE in Ge(0.743)Pb(0.183)Mn(0.074)Te alloy.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
