Phase Stability and Magnetic Properties of Compositionally Complex n = 2 Ruddlesden-Popper Perovskites.

Four new compositionally complex perovskites with multiple (four or more) cations on the B site of the perovskites have been studied. The materials have the general formula La0.5Sr2.5(M)2O7-δ (M = Ti, Mn, Fe, Co, and Ni) and have been synthesized via conventional solid-state synthesis. The compounds are the first reported examples of compositionally complex n = 2 Ruddlesden-Popper perovskites. The structure and properties of the materials have been determined using powder X-ray diffraction, neutron diffraction, energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and magnetometry. The materials are isostructural and adopt the archetypal I4/mmm space group with the following unit cell parameters: a ∼ 3.84 Å, and c ∼ 20.1 Å. The measured compositions from energy dispersive X-ray spectroscopy were La0.51(2)Sr2.57(7)Ti0.41(2)Mn0.41(2)Fe0.39(2)Co0.38(1)Ni0.34(1)O7-δ, La0.59(4)Sr2.29(23)Mn0.58(5)Fe0.56(6)Co0.55(6)Ni0.42(4)O7-δ, La0.54(2)Sr2.49(13)Mn0.41(2)Fe0.81(5)Co0.39(3)Ni0.36(3)O7-δ, and La0.53(4)Sr2.55(19)Mn0.67(6)Fe0.64(5)Co0.31(2)Ni0.30(3)O7-δ. No magnetic contribution is observed in the neutron diffraction data, and magnetometry indicates a spin glass transition at low temperatures.

Magnetic field-temperature phase diagram, exchange constants and specific heat exponents of the antiferromagnet MnNb2O6.

This work presents the magnetic field-temperature (H-T) phase diagram, exchange constants, specific heat (CP) exponents and magnetic ground state of the antiferromagnetic MnNb2O6polycrystals. Temperature dependence of the magnetic susceptibilityχ(=M/H) yields the Néel temperatureTN= 4.33 K determined from the peak in the computed ∂(χT)/∂TvsTplot in agreement with the transition in theCPvsTdata atTN= 4.36 K. The experimental data ofCPvsTnearTNis fitted toCP=A|T-TN|-αyielding the critical exponentα= 0.12 (0.15) forT>TN(T 50 K toχ=χ0+C/(T-θ) withχ0= -1.85 × 10-4emu mol-1Oe-1yieldsθ= -17 K, andC= 4.385 emu K mol-1Oe-1, the latter giving magnetic momentµ= 5.920µBper Mn2+ion. This confirms the effective spinS= 5/2 andg= 2.001 for Mn2+and the dominant exchange interaction being antiferromagnetic in nature. Using the magnitudes ofθandTNand molecular field theory (MFT), the exchange constantsJ0/kB= -1.08 K for Mn2+ions along the chainc-axis andJ⊥/kB= -0.61 K as the interchain coupling perpendicular toc-axis are determined. These exchange constants are consistent with the expectedχvsTvariation for the Heisenberg linear chain. TheH-Tphase diagram, mapped using theM-Hisotherms andM-Tdata at differentHcombined with the reported data of Nielsenet al, yields a triple-pointTTP(H,T) = (18 kOe, 4.06 K). The spin-flopped state aboveTTPand the forced ferromagnetism forH> 192 kOe are used to estimate the anisotropy energyHA≈ 0.8 kOe.

Modulation of Peptide Based Nano-Assemblies with Electric and Magnetic Fields.

Peptide based nano-assemblies with their self-organizing ability has shown lot of promise due to their high degree of thermal and chemical stability, for biomaterial fabrication. Developing an effective way to control the organization of these structures is important for fabricating application-oriented materials at the molecular level. The present study reports the impact of electric and magnetic field-mediated perturbation of the self-assembly phenomenon, upon the chemical and structural properties of diphenylalanine assembly. Our studies show that, electric field effectively arrests aggregation and self-assembly formation, while the molecule is allowed to anneal in the presence of applied electric fields of varying magnitudes, both AC and DC. The electric field exposure also modulated the morphology of the self-assembled structures without affecting the overall chemical constitution of the material. Our results on the modulatory effect of the electric field are in good agreement with theoretical studies based on molecular dynamics reported earlier on amyloid forming molecular systems. Furthermore, we demonstrate that the self-assemblies formed post electric-field exposure, showed difference in their crystal habit. Modulation of nano-level architecture of peptide based model systems with external stimulus, points to a potentially rewarding strategy to re-work proven nano-materials to expand their application spectrum.