Analysis Of Low-temperature Data Research Articles

Magnetic properties of nickel halide hydrates including deuteration effects

Magnetic measurements on variously hydrated nickel chlorides and bromides, including deuterated forms, are reported. Results include locations and sizes of susceptibility maxima, Tmax and χmax, ordering temperatures Tc, Curie constants and Weiss theta in the paramagnetic regime, and primary and secondary exchange interactions from analysis of low temperature data. For the latter a 2D Heisenberg model augmented by interlayer exchange in a mean-field approximation is applied. Magnetization data to 16kG as a function of temperature show curvature and hysteresis characteristics quite system dependent. For four materials high field magnetization data to 70kG at 2.00K are also obtained. Comparison is made with theoretical relations for spin-1 models. Trends are apparent, primarily that Tmax of each bromide hydrate is less than for the corresponding chloride, and that for a given halide nD2O (n=1 or 2) deuterates exhibit lesser Tmax than do nH2O hydrates. A monoclinic unit cell determined from powder X-ray diffraction data on NiBr2·2D2O is different from and slightly larger than that of NiBr2·2H2O. This provides some rationale for the difference in magnetic properties between these.

Investigation on magnetic, electrical and thermoelectric power of Bi-substituted La0.8Ca0.2MnO3 manganites

In the present work, the structural, magnetic, magneto-transport and thermoelectric properties of polycrystalline compounds of La0.8−xBixCa0.2MnO3(0≤x≤0.1) compounds are reported. Structure refinement using Rietveld method confirms that the samples are single phased and crystallize in rhombohedral structure with R-3C space group. Magnetic studies reveal that the pristine sample exhibits TC at 221K and it shift towards lower temperature with Bi doping. Deviation of the temperature dependent of inverse susceptibility curves from the Curie–Weiss law confirms the existence of Griffiths-like phase. All the samples exhibit metal to insulator transition at temperature TMI, which is found to decrease with increase in Bi doping which is consistent with the magnetic studies. Magneto-resistance MR% data shows that its peak value increases with Bi-doping. The analysis of electrical resistivity data in the metallic region (T<TMI) suggests that the electron–electron scattering is predominant in this region whereas the insulating region (T>TMI) is well described using Small polaron hopping (SPH) and Mott's variable range hopping (VRH) models. Analysis of thermoelectric power data in the high temperature region suggests that the SPH mechanism is operative in this regime, while the analysis of low temperature data reveals that the electron–magnon scattering is dominant in the region.

Temperature Dependent Rh.cntdot..cntdot..cntdot.Rh EXAFS in Dinuclear and Adsorbed Rhodium Species

In a recent study of metal adsorbates, we have observed in several model complexes that EXAFS absorber-backscatterer features for metal-metal interactions may be highly temperature dependent. This in turn can make metal-metal EXAFS undetectable in data collected at room temperature. The importance of observing these features in order to correctly identify adsorbate species in heterogeneous catalyst systems cannot be overemphasized and prompts us to give a preliminary account of our results on crystallographically characterized Rh complexes. The preliminary results of our study on a series of dinuclear rhodium model complexes and resulting adsorbed species indicate that the observation of metal-metal EXAFS can be highly temperature dependent and missing in room temperature data. For metal adsorbates, where bridged polynuclear species are possible surface species, observation of metal-metal EXAFS is critical in correctly identifying adsorbed species, and thus the collection and analysis of low temperature data is imperative. 16 refs., 1 fig.

Water II is a "strong" liquid

Rationalization of anomalous devitrification relationships between hyperquenched vitreous water (and related ASW forms) and vitrified aqueous solutions requires the assumption that the viscosity-temperature relationship of water near but above its glass transition temperature is that of a «strong» liquid, like SiO 2 . This would support Speedy's claim that water II (ASW above T g ) is a separate and distinct state of liquid water. The conclusion is supported by analysis of low-temperature data on water-swollen polymers (hydrogels)