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Magnetocaloric properties of TbCrO3 and TmCrO3 are reported and compared with those of the previously reported rare-earth chromites RCrO3 (R = Gd, Dy, Ho, and Er) and other perovskite-type oxides. The samples of TbCrO3 and TmCrO3 in this work were synthesized using a citrate gel combustion technique, and their magnetic properties were investigated and compared with those reported previously on RCrO3 (R = Gd, Dy, Ho, and Er). The Cr3+–Cr3+ ordering temperatures were found to strongly depend on the ionic radii of the rare-earth. By fitting the dc magnetization data with modified Curie–Weiss law including the Dzyaloshinsky–Moriya antisymmetric exchange interaction (D) and the symmetric exchange constant Je, spin canting angles (α) were obtained. In general, α was found to increase with the decreasing ionic radii of R3+ in RCrO3. The magnetocaloric properties investigated included the magnetic entropy change (−ΔS) for a given change in magnetic field (ΔH), the corresponding adiabatic temperature change (ΔTad), and their relative variations (ΔTad/ΔH) and (−ΔS/ΔH). It is observed that for RCrO3, (−ΔS) measured in the vicinity of the ordering temperature of R3+–R3+, varies almost as G2/3 where G is the de Gennes factor. Among RCrO3, GdCrO3 shows the largest value of (−ΔS/ΔH), because of its largest G factor and its magnitudes of (ΔTad/ΔH) and (−ΔS/ΔH) compare well with the reported values for the perovskites GdFeO3 and EuTiO3. These comparisons presented here provide useful information on the potential use of these materials in magneto-refrigeration technology. [ABSTRACT FROM AUTHOR]

In order to acknowledge the enrichment mechanism of organic matter in the black shale of the Middle Ordovician Yanxi Formation in central and southern Hunan area, the shale samples of the Yanxi Formation in the Maduqiao, Jinweixiang, Hejiadong and Mianhuaping are selected to test abundance of organic matter, major, trace and rare earth elements. The results show that the average total organic carbon (TOC) of black shale in study area is 2.03%, indicating the high abundance of organic matter in the Yanxi Formation. The CIA values and w(Sr)/w(Cu) ratios show that the Yanxi Formation had a warm and humid climate. The concentrations of Zn, V, Ni and Cu imply a medium primary productivity in the Yanxi Formation, whereas they have weak correlations with TOC. The element concentrations of V, Cr, Ni, and Mo reveal that the black shales were deposited in anoxic-euxinic conditions. The accumulation of organic matter is jointly affected by paleoclimate, primary productivity, redox conditions, sea level change and terrigenous debris. The organic matter enrichment model of the Yanxi Formation shales is a 'deepwater retention basin' controlleded by anoxic conditions. [ABSTRACT FROM AUTHOR]