Pbam Symmetry Research Articles

Softening of antiferroelectricity in PbZrO3-Pb(Mn1/2W1/2)O3 complex perovskite solid solution

A new solid solution of complex perovksite structure, (1-x)PbZrO3-xPb(Mn1/2W1/2)O3 (PZ-xPMnW, with x = 0–0.1), is prepared by solid state reaction method. Its crystal structure, dielectric properties, and antiferroelectricity are investigated. It is found that the crystal structure of the solid solution remains in the orthorhombic Pbam symmetry with x ≤ 0.1. The induced ferroelectric polarization (PInd) is enhanced, whereas the critical field (ECr) is decreased, with increasing PMnW concentration, indicating the softening of antiferroelectricity in PZ. The Curie temperature (TC) is also decreased with the substitution of PMnW for PZ. The grain size is significantly enlarged, which could be attributed to the presence of a transient liquid phase during the sintering process. Measurements of the magnetization by means of SQUID confirm that the Mn ion exists in a high spin state with a 2+ oxidation state. The softening of antiferroelectric order and the improvement in induced polarization make the PZ-PMnW ceramics an interesting material system for such applications as energy storage devices.

Electrochemical properties of a new nanocrystalline NaMn2O4 cathode for rechargeable sodium ion batteries

Nanocrystalline NaMn2O4 with a crystallite size of ∼8–10 nm exhibiting a new close packed hexagonal crystalline form, different from the known stable orthorhombic (Pbam or Pmnm symmetry) or monoclinic structures common to the Na–Mn–O system, has been synthesized by a high energy mechano-chemical milling process (HEMM) using Na2O2 and Mn2O3 as starting materials. The newly synthesized structure of NaMn2O4 has been studied as a cathode for sodium ion rechargeable batteries. The HEMM derived NaMn2O4 shows a 1st cycle discharge capacity ∼75 mAh/g, ∼86 mAh/g and ∼95 mAh/g when cycled at a rate of ∼40 mA/g in the potential window ∼2.0–4.0 V, ∼2–4.2 V and ∼2–4.5 V, respectively. The nanostructured NaMn2O4 shows a fade in capacity of 0.3% per cycle and a moderate rate capability when cycled in the potential window 2–4 V. However, electrolyte decomposition occurring during charging of the electrode above ∼3.8 V needs to be resolved in order utilize the full capacity of NaMn2O4 as well as improve the stability of the electrode.

Study of Antiferroelectric PbZrO3-Pb(M1/2W1/2)O3 (M = Mg, Zn & Mn) Complex Perovskite Solid Solutions

Ceramics of the perovskite solid solution of 0.99PbZrO3-0.01Pb(M1/2W1/2)O3 (M = Mg, Zn & Mn) have been prepared by the solid state reaction method. Their crystal structures and dielectric and antiferroelectric properties have been characterized. It is found that the crystal structure of the solid solutions remains in the orthorhombic Pbam symmetry with the addition of Pb(M1/2W1/2)O3 up to 1 % while the induced polarization (PInd) is improved and the critical field (ECr) is decreased. The Curie temperature (TC) is also decreased with the substitution of Pb(M1/2W1/2)O3 for PbZrO3 whereas the maximum permittivity (ɛ’max) is increased significantly. The grains of the ceramics are significantly grown, which could be attributed to the presence of a transient liquid phase in the solid solutions.

Effect of Size on Magnetic Properties of ${\hbox{NdMn}}_2{\hbox{O}}_5$ Nanorods

The effects of size on the magnetic properties of NdMn2O5 nanorods were studied by X-ray diffractometry, transmission electron microscopy, and measurements of magnetic susceptibility and hysteresis. Five types of nanorods with different axial lengths (〈LC〉)×diameters(〈LD〉)-68(36) nm×31(14) nm, 78(30) nm×33(8) nm, 95(36) nm×40(12) nm, 122(48) nm×54(16) nm, 249(46) nm×142(36) nm were made. The axial directions of the five types of nanorod were parallel to the c axis of the crystal. No particular radial direction is found. All samples crystallize with orthorhombic Pbam symmetry. Upon cooling, only the sample with 〈LC〉 = 249 nm occurs in ferrimagnetic and antiferromagnetic ordering at 80 K and 9 K, respectively. Hysteresis testing reveals a ferrimagnetic domain between 10 K and 50 K only in the 〈LC〉 = 249 nm sample. These tests reveal that the magnetic critical size of NdMn2O5 is between 〈LC〉 = 249 nm and 122 nm. For memory storage applications, the estimated maximal capacity of NdMn2O5 nanorods as upstanding pillars is about 32 Gbit/in2.

High Energy Density Mixed Polymeric Phase from Carbon Monoxide and Nitrogen

Carbon monoxide and nitrogen are among the potentially interesting high-energy density materials. However, in spite of the physical similarities of the molecules, they behave very differently at high pressures. Using density functional theory and structural prediction methods, we examine the ability of these systems to combine their respective properties and form novel mixed crystalline phases under pressures of up to 100 GPa. Interestingly, we find that CO catalyzes the molecular dissociation of N2, which means mixed structures are favored at a relatively low pressure (below 18 GPa), and that a three-dimensional framework with Pbam symmetry becomes the most stable phase above 52 GPa, i.e., at much milder conditions than in pure solid nitrogen. This structure is dynamically stable at ambient pressure and has an energy density of approximately 2.2 kJ g(-1), making it a candidate for a high-energy density material, and one that could be achieved at less prohibitive experimental conditions.

Layer stacking and twinning in HoB 2C

The crystal structure of HoB 2C has been refined using powder X-ray diffraction data. The structure has B 2C layers stacked directly above each other in the c-direction with Ho atoms sandwiched between layers. There is no evidence for a doubling of the c-axis, as was previously reported, from the X-ray or powder neutron data. The refined structure is close to having orthorhombic Pbam symmetry but with a slight monoclinic distortion (space group P112/ m, Z=4, a=6.77454(8) Å, b=6.78618(9) Å, c=3.69604(3) Å and γ=90.091(1)°). Large anisotropic strain broadenings of the diffraction peaks are consistent with a model for [110] twinning.

An electron microscopy study of the atomic structure of a mullite in a reaction-sintered composite

The mullite matrix of a reaction-sintered mullite/zirconia composite has been characterized by various electron microscopy techniques. The mullite was determined to be stoichiometrically Al4.76Si1.23O9.61 (1.7:1 mullite) with a disordered vacancy structure and Pbam symmetry. The atomic structure of the mullite has been described in terms of an average unit cell.