J. K. Liang

Toward more realistic projections of soil carbon dynamics by Earth system models

Soil carbon (C) is a critical component of Earth system models (ESMs), and its diverse representations are a major source of the large spread across models in the terrestrial C sink from the third to fifth assessment reports of the Intergovernmental Panel on Climate Change (IPCC). Improving soil C projections is of a high priority for Earth system modeling in the future IPCC and other assessments. To achieve this goal, we suggest that (1) model structures should reflect real-world processes, (2) parameters should be calibrated to match model outputs with observations, and (3) external forcing variables should accurately prescribe the environmental conditions that soils experience. First, most soil C cycle models simulate C input from litter production and C release through decomposition. The latter process has traditionally been represented by first-order decay functions, regulated primarily by temperature, moisture, litter quality, and soil texture. While this formulation well captures macroscopic soil organic C (SOC) dynamics, better understanding is needed of their underlying mechanisms as related to microbial processes, depth-dependent environmental controls, and other processes that strongly affect soil C dynamics. Second, incomplete use of observations in model parameterization is a major cause of bias in soil C projections from ESMs. Optimal parameter calibration with both pool- and flux-based data sets through data assimilation is among the highest priorities for near-term research to reduce biases among ESMs. Third, external variables are represented inconsistently among ESMs, leading to differences in modeled soil C dynamics. We recommend the implementation of traceability analyses to identify how external variables and model parameterizations influence SOC dynamics in different ESMs. Overall, projections of the terrestrial C sink can be substantially improved when reliable data sets are available to select the most representative model structure, constrain parameters, and prescribe forcing fields.

Structure and magnetic properties of Mn-doped ZnO nanoparticles

We report the crystal structure and magnetic properties of the Zn1-xMnxO compounds synthesized by a combustion method. The Zn1-xMnxO compounds with x=0-0.1 crystallize in the wurtzite ZnO structure. The lattice parameters a and c of Zn1-xMnxO increase linearly with the Mn content, indicating that Mn2+ ions substitute for Zn2+ ions. Scanning electron microscopy shows that the average particle radius of Zn0.95Mn0.05O is about 40 nm. From the Curie-Weiss behavior of susceptibility at high temperature, it was found that the Mn-Mn interaction is dominated by antiferromagnetic coupling with effective nearest-neighbor exchange constant J about -32 K and the large negative value of the Curie-Weiss temperature

Ferroelectric transition of Aurivillius compounds Bi5Ti3FeO15 and Bi6Ti3Fe2O18

Single-phase Bi5Ti3FeO15 and Bi6Ti3Fe2O18 ceramics have been synthesized by solid state reaction. The ferroelectric transition of the compounds was studied by differential scanning calorimetry, high-temperature x-ray diffraction, and temperature-dependent dielectric measurements. Two solid-state structural transitions were observed in both compounds, one is the orthorhombic↔tetragonal transition (ferroelectric transition) at 1021 K for Bi5Ti3FeO15 and 973 K for Bi6Ti3Fe2O18, and the other is accompanied by an abrupt lattice expansion of the tetragonal phase at about 1110 K for Bi5Ti3FeO15 and about 1090 K for Bi6Ti3Fe2O18.

Magnetic properties of Bi(Fe1−xCrx)O3 synthesized by a combustion method

Single-phase samples of Bi(Fe1−xCrx)O3 with x=0, 0.1, and 0.2 were synthesized by a combustion method. X-ray diffraction reveals that the lattice parameters of Bi(Fe1−xCrx)O3 perovskites decrease linearly with the Cr content, indicating that Cr ions substitute for Fe ions to form a solid solution. X-ray photoelectron spectroscopy investigation shows that Cr ions have the Cr3+ valence state in Bi(Fe1−xCrx)O3. The frequency dependence of dielectric constants was investigated at room temperature. Magnetic measurements show hysteresis loops at both 5 and 300K and the substitution of Cr for Fe enhances the magnetization.

Low-temperature synthesis and photoluminescence of AlN

High-quality aluminum nitride powder was obtained through the reaction of aluminum metal with ammonia at 450 degrees C in an autoclave. The synthesized powder was characterized by X-ray powder diffraction, high-resolution transmission electron microscopy (HRTEM) and photoluminescence spectrum. X-ray diffraction and HRTEM indicated that the powder consisted of wurtzite AIN with an average size of about 32 nm. Photoluminescence spectrum confirmed the existence of a blue luminescence band in AIN due to nitrogen vacancies. A possible low-temperature synthesis mechanism was discussed

Blue emission and Raman scattering spectrum from AlN nanocrystalline powders

Photoluminescence and Raman scattering spectra from AIN nanocrystalline powders are studied. A blue emission band centered at 420 nm (2.95 eV) is observed. This band may be ascribed to the transition from the shallow level of V-N to the ground state of the deep level of the V-Al(3-)-3 x O-N(+) defect complexes. A phonon structure resulting fi om the transition from the shallow level to the excited states of the deep level is also observed. The broadening of peaks and the low-wave-number-shift of the phonon frequency observed on the Raman scattering spectra are the result of nano-sized effects

Structural transition and atomic ordering in the non-stoichiometric double perovskite Sr2FexMo2-xO6

The structural transition and atomic ordering of a new series of ordered double perovskite oxides Sr2FexMo2-xO6 (0.8 less than or equal to x less than or equal to 1.5) have been studied by X-ray powder diffraction (XRD). Rietveld refinement of the powder diffraction profiles indicates that the crystal structure of the compounds Sr2FexMo2-xO6 changes from a tetragonal I4/mmm lattice to a cubic Fm(3)over-bar-m lattice around x = 1.2 and the lattice parameters decrease slightly as Fe content increases. The degree of ordering in Sr2FexMo2-xO6 exhibits a maximal at x = 0.95 and decreases as x deviates from 0.95

Structure and Heat Capacity of Wurtzite GaN from 113 to 1073 K

High pure wurtzite structure GaN has been synthesized by gas reaction method. Its structure was determined by powder x-ray diffraction using the Rietveld technique. The heat capacity C-p was measured from 113 to 1073 K, which can be represented by C-p = 0.362 + 3.010 x 10(-4)T - 3.411 x 10(3)T(-2) - 7.791 x 10(-8)T(2). NO measurable phase transition was observed in this temperature range.

Phase relations in the system La2O3-CaO-B2O3

The subsolidus phase relations of the system La2O3-CaO-B2O3 are investigated by means of X-ray diffraction (XRD) analyses. There are seven binary compounds and five ternary compounds in this system. The phase diagram comprises eighteen three-phase regions. A new ternary compound Ca4LaO(BO3)(3) (CLOB) is identified. It has a monoclinic structure with cell parameters a=8.1732(1) Angstrom, b=16.0860(3) Angstrom, c=3.6268(1) Angstrom, beta =101.40(1)degrees and space group Cm. At low temperature, the XRD patterns of the CLOB compound indicate that a phase transition in the low temperature range is absent. The expansion coefficients in the three major directions are different, the value of alpha (c) being the largest

Effects of Cu on crystallographic and magnetic properties of Sm(Co,Cu)7

We have investigated the structural stability and magnetic properties of SMCo7-xCu, compounds with the TbCu7-type structure using x-ray powder diffraction and magnetic measurement. A large solid solution with 0.8 less than or equal to x less than or equal to 4.0 in SMCo7-xCux compounds has been observed. Both the lattice parameters and unit cell volume increase with increasing Cu content. SMCo7-xCux compounds exhibit ferromagnetic order. A strong uniaxial magnetocrystalline anisotropy with an anisotropy field as high as 20 T is obtained with x = 0.8 at 5 K. However, the saturation magnetization and Curie temperature decrease with increasing Cu content.