Effect of Pr-Ca substitution on the transport and magnetic behavior of LaMnO3 perovskite

The effect of simultaneous substitution of a fluctuating cation and a divalent cation in LaMnO₃ perovskite modifies the properties of the material to exhibit large valence colossal magnetoresistance (CMR) effect. A good example of these properties is (La_1−2x Pr _x Ca _x )MnO₃ (LPCMO) type CMR material. In this communication it is reported that, with the increase in x (for x=0.1, 0.15, 0.2), the T _c varies between 100 and 120 K with improvisation in metal-insulator transition. Interestingly, resistance increases with x from few hundred ohms to few kilo ohms with corresponding decrease in the unit cell volume. The results of the studies using X-ray diffraction (XRD), electrical resistivity, magnetoresistance and ac susceptibility measurements on LPCMO samples for understanding the structural, transport and magnetic properties are discussed in detail.     

关于K个复相关矩阵相等的检验

§ 1.Ｉｎｔｒｏｄｕｃｔｉｏｎ　ＷｅａｒｅｇｉｖｅｎｋｉｎｄｅｐｅｎｄｅｎｔＷｉｓｈａｒｔｄｅｎｓｉｔｉｅｓｏｆｔｈｅ (ｐ +ｑ)× (ｐ +ｑ)ｒａｎｄｏｍｓｙｍｍｅｔｒｉｃｐｏｓｉｔｉｖｅｄｅｆｉｎｉｔｅｍａｔｒｉｃｅｓＧ1,… ,Ｇｋｔｏｂｅｇ(Ｇｉ) =Ｋｅｘｐ -12 ｔｒＲ- 1ｉ Ｇｉ Ｇｉ12 (ｎｉ- ｑ-ｐ- 1) ,(1 )ｗｈｅｒｅｉ=1 ,… ,ｋ,ａｎｄＲｉｄｅｎｏｔｅｓｔｈｅｐｏｐｕｌａｔｉｏｎｃｏｒｒｅｌａｔｉｏｎｍａｔｒｉｘｏｆｔｈｅｉ ｔｈｐｏｐｕｌａｔｉｏｎａｎｄＫａｓａｇｅｎｅｒｉｃｌｅｔｔｅｒｄｅｎｏｔｅ…     

Structural studies and T c dependence in La2−x Dy x Ca y Ba2Cu4+y O z type mixed oxide superconductors

A new series of mixed oxide superconductors with the stoichiometric composition La_2−x Dy _x Ca _y Ba₂Cu_4+y O _z (x=0.0 − 0.5, y=2x) has been studied for structural and superconductiong properties. Our earlier studies on La_2−x (Y/Er) _x Ca _y Ba₂Cu_4+y O _z series, show a strong dependence of T _c on hole concentration (p _sh). In the present work, the results of the analysis of the neutron diffraction measurements at room temprerature on x=0.3 and 0.5 samples are reported. It is interesting to know that Ca substitutes for both La and Ba site with concomitant displacement of La onto Ba site. Superconductivity studies show that maximum T _c is obtained for x=0.5, y=1.0 sample (T _c ∼ 75 K), for La_1.5Dy_0.5Ca₁Ba₂Cu₅O _z (La-2125).     

Synthesis and structure of polymeric networks of silver hexafluoroacetylacetonate complexes of THF, toluene, and vinyltrimethylsilane

The reaction of 1,1,1,5,5,5-hexafluoroacetylacetone (Hhfac) with Ag2O in the presence of L=THF, toluene, and Me3SiCH=CH2 was studied to obtain [Ag(hfac)L]x complexes for use as chemical vapor deposition precursors. The structures and volatilities of these three complexes were compared to those of the previously synthesized Ag(hfac)(Me3SiC triple bond CSiMe3), 1, which was also crystallographically characterized for comparison. The reaction of Ag2O with Hhfac in THF forms the polymeric complex [Ag4(hfac)4(THF)2]infinity, 2, which has tetrametallic subunits with hfac ligands that bridge via oxygen and carbon. Both 4- and 5-coordinate silver metal centers are found in 2. Ag2O reacts with Hhfac in toluene to form a complex with a similar tetrametallic unit [Ag4(hfac)4(toluene)2]infinity, 3. In this case, the tetrametallic subunits are assembled via bridging toluene molecules, and each silver is 6-coordinate. In the presence of excess vinyltrimethylsilane (vtms), Ag2O and Hhfac form [Ag3(hfac)3(vtms)]infinity, 4, which contains trimetallic subunits assembled via oxygen atoms of bridging hfac ligands and 5- and 6-coordinate silver.     

Utility of anhydrous neodymium nitrate as a precursor to extended organoneodymium nitrate networks

Hydrated neodymium nitrates can be readily transformed to anhydrous ether solvates which react with cyclopentadienyl reagents to make organometallic nitrate complexes with variable degrees of oligomerization. Heating Nd(NO(3))(3)(H(2)O)(6) in tetrahydrofuran at reflux, removal of solvent, drying at 100 degrees C under high vacuum, and addition of hot THF generates Nd(NO(3))(3)(THF)(3), 1. Using dimethoxyethane, Nd(NO(3))(3)(DME)(2), 2, can be obtained similarly. Addition of NaC(5)Me(5) to 1 generates (C(5)Me(5))Nd(NO(3))(3)(THF)Na(THF)(x)complexes which crystallize as ([(C(5)Me(5))(NO(3))(2)Nd(THF)(micro-NO(3))](2)Na(THF)(4))[Na(THF)(6)], 3, or [(C(5)Me(5))Nd(THF)(mu-NO(3))(3)Na(THF)(2)](n), 4, depending on the conditions. The trimetallic Nd(2)Na unit in 3 forms an extended system in 4 via bridging nitrates. Addition of KC(5)Me(5) and 18-crown-6 to 1 generates another extended complex [(C(5)Me(5))Nd(THF)(NO(3))(mu-NO(3))(2)K(18-crown-6)](n), 5, in which an 18-crown-6 ligated potassium links neodymium centers via two bridging nitrates and an agostic interaction between a C(5)Me(5) methyl group and potassium.