[en] This paper reports on the notion of the dead or reference state that has been introduced for systematic evaluation of the specific chemical enthalpy per equivalent of available electrons, ρ⁰_β, and the specific chemical exergy per equivalent of available electrons, ρ⁰_ε, of an organic compound, for which structural information is available. It has been determined that the values of ρ⁰_β and ρ⁰_ε for organic compounds in each homolog approach asymptotically to a constant value as their molecular sizes increase. However, all the values of ρ⁰_β are nearly identical, as are the values of ρ⁰_ε except for compounds with relatively small molecular sizes

[en] Introduction of the concepts of the availability (or exergy), datum level materials, and the dead state has been regarded as some of the most significant recent developments in classical thermodynamics. Not only the available energy balance but also the material and energy balances of a biological system may be established in reference to the datum level materials in the dead state or environment. In this paper these concepts are illustrated with two examples of fermentation and are shown to be useful in identifying sources of thermodynamic inefficiency, thereby leading naturally to the rational definition of thermodynamic efficiency of a biochemical process

[en] Magnetic and calorimetric measurements have been made to reveal a Pr ordering in the (Pb_0.5Cu_0.5)₂(Ba_0.5Sr_0.5)₂PrCu₂O₈ compound which has the (Pb,Cu)-2212-type tetragonal structure with lattice parameters a=3.878(3) A and c=27.594(9) A. The transition occurs at a relatively high T_N(Pr) of 9 K. In comparison, isostructural (Pb_0.5Cu_0.5)₂(Ba_0.5Sr_0.5)₂GdCu₂O₈ exhibits a T_N(Gd) of only 2.2 K. The magnetic entropy associated with the Pr ordering has a lower limit value equivalent to about 20% of Rln3 expected for Pr³⁺ with a quasitriplet ground state, suggesting a more three-dimensional-like process. These results represent the observation of anomalous Pr ordering in the 2212-type cuprate and a complete demonstration of such effects in all two-CuO₂-layer m212 (m=1,2,3) systems

[en] A superconducting transition temperature T_c of 1.8 K with a transition width of 1.4--2.2 K and a diamagnetic onset as high as 4.6 K are reported for a metastable LaPd₂B₂C compound with the LuNi₂B₂C-type structure. The low-temperature normal-state specific-heat data yield an electronic-term coefficient γ of 7.1 mJ/mol K², a Debye temperature θ_D of 310 K, and a bulk superconducting heat-capacity-jump ratio ΔC/γT_c of 1.4. The systematic variation of T_c for the metastable compounds in the RPd₂B₂C system (R=Y, Th, Th_1-xLa_x, La) is correlated with the Pd-Pd nearest-neighbor in-plane distance d. A longer d value of 2.797 A was observed for 1.8-K LaPd₂B₂C superconductor while an optimum d value of 2.652 A was reported for 23-K YPd₂B₂C. Small Pd-Pd wave function overlapping, along with the observed small specific-heat γ value and normal-state Pauli paramagnetic susceptibility of LaPd₂B₂C, suggest that the variation of the Pd(4d)-dominated conduction band and thus the Fermi-level density of states N(E_F) are the determining factors for the T_c variation of the present system

[en] Structural, magnetic, and calorimetric studies have been made to reveal an anomalous Pr antiferromagnetic ordering at T_N(Pr) of 6 K in HgSr₂PrCu₂O_6+δ. This compound has the Hg-1212-type tetragonal structure with lattice parameters a=3.8532 A and c=12.1199 A. X-ray Rietveld refinement yields a Pr-O bond length d(Pr-O) of 2.492 A between Pr and oxygen in the adjacent CuO₂ layers and an O-Pr-O bond angle of 101.3 degree. The low-temperature magnetic entropy associated with the Pr ordering is only about 10% of Rln3 expected for Pr³⁺ with a quasitriplet ground state, suggesting a more two-dimensional-like ordering process. This result supports the earlier observation of a systematic dependence of T_N(Pr) on Pr-O bond length in various M(Ba,Sr)₂PrCu₂O_y (M=Cu, Hg, Tl, Pb) 1212-type Pr compounds. This in turn points to the importance of a superexchange magnetic coupling mechanism. copyright 1996 The American Physical Society

[en] Superconductivity as well as magnetic ordering of the Pr spin were observed in polycrystalline Pb₂Sr₂PrCu₃O_8.01. High resolution neutron profile refinement analysis shows that the sample is single phase with a nominal oxygen concentration of 8.01(2). The AC susceptibility measurements reveal diamagnetic responses, that indicate the onset of superconductivity, at temperatures below 60K. And a DC applied magnetic field of 10kG destroys the diamagnetic screening of the superconducting state. As the temperature is further reduced, the AC susceptibility shows an upturn at T=20K below which χ_AC increases with decreasing temperature, and then a cusp that is typical of antiferromagnetic ordering is evident at T=5K. The upturn in AC susceptibility corresponding to the magnetic ordering of Pr spins is clearly demonstrated by neutron diffraction measurements. The Pr spins are aligned antiparallel along all three crystallographic directions with T_N∼7K, and the ordering is two-dimensional in nature. ((orig.))

[en] Magnetic ordering among Pr ions occurs at a relatively high temperature of 17 K in PrBa₂Cu₃O₇, but is absent in Pr₂CuO₄ and Bi₂Sr₂PrCu₂O₈ above 1.6 K. This work extends the study of magnetic properties to other Pr-containing cuprates including TlBa₂PrCu₂O₇, (Pb_.6Cu_.4)Sr₂PrCu₂O₇ and Pb₂Sr₂PrCu₃O₈. Low temperature calorimetric and SQUID measurements show broad peaks in C and dχ/dT in all cases. The onset temperatures are near 10 K, while the maxima in C as well as minima in dχ/dT prevail at about 8 to 5 K. The heat capacity data provides additional information concerning the entropy associated with the magnetic anomalies and large contributions seemingly linear with temperature. (orig.)