Neutron diffraction study of magnetic ordering in PrCu2Si2, PrCu2Ge2, PrFe2Ge2 and NdFe2Ge2

A neutron diffraction study of polycrystalline PrCu₂Si₂ [1], PrCu₂Ge₂ [2], PrFe₂Ge₂ [3] and NdFe₂Ge₂ [4] intermetallics carried out at liquid helium temperature shows the presence of a collinear antiferromagnetic order below T_N = 19 ± 1 K [1], T_N = 16 ± 1 K [2], T_N = 9 ± 1 K [3] and 13 ± 1 K [4]. Magnetic moment, parallel to the c-axis is localized on RE ions only. The magnetic structure of these compounds consists of ferromagnetic layers perpendicular to the c-axis coupled antiferromagnetically with sequence +-+- for PrCu₂Si₂ and PrCu₂Ge₂ and +--+ for PrFe₂Ge₂ and NdFe₂Ge₂. The RE moments amount close to the free ion values for Fe containing compounds but are smaller in those containing Cu suggesting a fairly strong influence of crystal field.     

Crystal structure and magnetic properties of RCu5−xPdx (R=Pr, Nd, Sm and Eu) alloys

We report the effect of replacing Cu by Pd in RCu₅ (R=Pr, Nd, Sm and Eu). The parent RCu₅ compounds crystallize in the hexagonal CaCu₅-type structure. The hexagonal symmetry is retained in PrCu₄Pd and EuCu_5−xPd_x (x=1 and 2) but the crystal structure changes to cubic AuBe₅-type in PrCu₃Pd₂, NdCu_5−xPd_x (x=1 and 2) and SmCu₄Pd. Substitution with Pd leads to lattice expansion and modifies the magnetic behavior. While PrCu₅ is known to be a van-Vleck paramagnet with a singlet ground state, PrCu₄Pd and PrCu₃Pd₂ show ferromagnetic-like behavior at low temperatures. SmCu₄Pd orders ferromagnetically near 28 K in contrast to the antiferromagnetic nature of the parent SmCu₅. The divalent nature of the Eu ions in EuCu₅ is retained in the ternary alloys, but the Curie temperature is reduced from 57 to 24.5 and 14.5 K in EuCu₄Pd and EuCu₃Pd₂, respectively, inferred from the location of peak in the heat capacity of these two compounds. The magnetic hyperfine field at the Eu nucleus measured with ¹⁵¹Eu Mössbauer spectroscopy in the ternary Eu-alloys is comparable to that in EuCu₅. The magnetic behavior of NdCu₄Pd is similar to that reported in NdCu₅. The zero-field-cooled, low-field magnetization of NdCu₃Pd₂ shows a region of diamagnetic behavior roughly between 21 and 4 K, but the field-cooled response is positive.     

141Pr Mössbauer spectroscopy using the current integrating technique

¹⁴¹Pr Mössbauer spectroscopy has been realized with use of the current integrating technique. As an example, we show the¹⁴¹Pr Mössbauer spectrum of PrCu₂Si₂ at 4.2 K.     

Magnetic properties of PrCu2 at high pressure

We report a study of the low-temperature high-pressure phase diagram of the intermetallic compound PrCu₂, by means of molecular-field calculations and ^63,65Cu nuclear-quadrupole-resonance (NQR) measurements under pressure. The pressure-induced magnetically-ordered phase can be accounted for by considering the influence of the crystal electric field on the 4f electron orbitals of the Pr³⁺ ions and by introducing a pressure-dependent exchange interaction between the corresponding local magnetic moments. Our experimental data suggest that the order in the induced antiferromagnetic phase is incommensurate. The role of magnetic fluctuations both at high and low pressures is also discussed.     

The superconductive properties and crystal structure of Ba2(Y1−x Pr x )Cu3O9−y solid solutions

A series of Ba₂(Y_1–x Pr _x )Cu₃O_9–y solid solutions were prepared with the orthorhombic distorted perovskite structure. The compound Ba₂PrCu₃O_9–y belonging to tetragonal system is a good semiconductor. The superconductivity is observed for samples withx<0.6. With the increase of Y content, the distortion of crystal structure is greater and the superconductive transition temperature of zero resistanceTc(0) raises.     

Crystal and magnetic structure of Ho2NiGe6

The crystal and magnetic structure of Ho₂NiGe₆ was studied by powder neutron diffraction. The paramagnetic neutron diffraction data confirmed the Ce₂CuGe₆-type crystal structure reported earlier for this compound. Below the Néel temperature equal to 11 K the Ho magnetic moments form a uniaxial antiferromagnetic ordering. The Ho magnetic moments equal to 8.16(7)μ_B at 1.5 K are parallel to the b-axis. The data are compared with those published for HoNi_0.46(6)Ge₂.     

Phase transition and triplet exciton mobility in anthracene—tetracyanobenzene charge-transfer single crystals

Triplet-Excitons in a mixed 1 : 1 anthracene—tetracyanobenzene single crystal are established by ESR-spectrocopy. These excitons (lifetime 0.5 msec) are characterized by a hopping frequency of v = 1.3 × 10⁶ K at 300K. The temperature dependence of the ESR line shape indicates a phase transition of the crystal at T_t = 205K. Above and below the phase transition the exciton mobility is nearly temperature independent.     

Unusual Pr antiferromagnetic ordering in high-Tc cuprates

The anomalous Pr antiferromagnetic order with high Néel temperature T_N(Pr) are reported throughout the whole Pr_1+xBa_2−xCu₃O_7−y or 1212-type Cu(Ba_2−xPr_x)PrCu₂O_7−y system (−0.2<x<1; −0.4<y<1), where three distinct crystal structures were observed: orthorhombic 1212-chain O(I) (space group Pmmm), tetragonal T (P4/mmm), and orthorhombic O(II) (Cmmm). Systematic variation of T_N(Pr) in this system as well as in other 1212 and 2212 cuprates M_nA₂PrCu₂O₇ (n=1, 2, M=Hg, Tl, Pb/Cu, Bi, Nb, Cu; A=Sr, Ba, Ba/Pr) was discussed through the correlation of T_N(Pr) with Pr–O bond length. The importance of quasi-two-dimensional Pr–O–Pr superexchange magnetic coupling through strong wave function overlap between the overextended Pr-4f orbital with eight O-2p_π orbitals in the adjacent CuO₂ bi-layer is discussed. No superconductivity was observed in the present study.     

Magnetic susceptibility and electrical resistivity of electrotransport purified scandium single crystals from ≈ 1 to 300 k

The magnetic susceptibility of electrotransport purified Sc single crystals was measured from 0.7 to 295 K. Both the easy (χ_a) and hard (χ_c) directions were measured using a microcomputer controlled Faraday apparatus. Above 50 K the present results agree with earlier work on a less pure sample, but below 50 K the present results are lower by as much as 4%. In addition the maximum at ≈ 30 K and the minimum at ≈ 6 K are much more pronounced in the purer material, and the susceptibility increases rapidly as the temperature approaches 0 K. Electrical resistivity measurements were made on the same single crystals from 1.6 to 300 K. The resistance ratios were 365 for the a-axis crystal and 274 for the c-axis crystal. At 300 K, ρ_a ≈ 2.5 ρ_c.     

Magnetic properties of the ternary hydrides of Nd6Mn23 and Sm6Mn23

The intermetallic compounds Nd₆Mn₂₃ and Sm₆Mn₂₃ and their ternary hydrides were studied by X-ray diffraction. The crystal structure (Th₆Mn₂₃ type) is preserved after hydrogen absorption. From the change in lattice constants a volume increase of about 14% was deduced. The temperature dependence of the magnetization of Nd₆Mn₂₃, Sm₆Mn₂₃ and their hydrides was studied in the range 4–500 K. The uncharged compounds have magnetic ordering temperatures above 400 K while in the hydrides magnetic ordering occurs close to 200 K. Indications were obtained of a substantial weakening of the magnetic coupling between the rare earth and manganese sublattice magnetization.     

On the ground state of the Kondo lattice system CeCu6

Magnetization measurements have been performed on a single crystal of CeCu₆ along the three main crystallographic directions between 1.5 and 300 K. The results are interpreted in terms of a crystal field calculation slightly modified by a spin fluctuation contribution of Kondo origin. Anisotropic magnetic properties of the Kondo lattice are discussed.     

Visible spectra and flourescence decay rates of a crystal and frozen solutions of uranyl nitrate

The absorption and fluorescence spectra of a UO₂(NO₃)₂.6H₂O single crystal at 77 K agree with those reported previously. Analysis of the absorption spectrum yielded four electronic levels at 20590, 20625, 21474 and 23240 cm^-1. The doublet at 20590 and 20625 cm^?1 converged to form a singlet when the crystal was dissolved in water. This doublet was then interpreted as the result of Davydov splitting. The fluorescence decay rate of the doublet was measured between 77 and 300 K and its small thermal dependence explained by a theoretical model. The fluorescence and excitation spectra of frozen uranyl nitrate solutions at 77 K were recorded, and found to depend on molarity. This dependence was explained by assuming that there were three uranyl-nitrate-water species. The existence of these species is consistent with other measurements.     

Mössbauer study of a single crystal of RbFeF4

Mössbauer measurements taken between 1.4 and 300 K on a single crystal of RbFeF₄ show that below the Néel temperature of 133.8 K the hyperfine field is at an angle of 16.5 ± 0.5° to the principal axis of the electric field gradient tensor. The hyperfine field is seen to be along the [001] axis allowing the identification of the principal axis of the EFG with the shortest ligand of the tilted FeF₆ octahedra that are constituents of the crystal structure. At 300 K the ratio of recoil-free fractions (?_⊥/?_:) is measured as 0.96 ± 0.05 but at 77 K the measured ratio is 1.7 ± 0.2.     

γ-LiAlO2晶体生长、改性和热学性质研究

采用快速提拉法生长出了透明、完整的γ-LiAlO₂晶体，但是晶体的高熔点和易 挥发性限制了γ-LiAlO₂晶体质量.采用气相传输平衡法（vapor transport equilibra tion technique,VTE）工艺对晶体改性，半高宽（FWHM）值从1169arcsec降至442arcsec，继续升高VTE处理温度至1300℃，FWHM值反而升高至552arcsec.快速提拉法生长出来晶体，100］方向和［001］方向的热膨胀系数分别为172398×10^-6/K，107 664×10^-6/K.经过三步VTE处理后［100］和［001］方向热膨胀系数降至16 6539×10^-6/K和101784×10^-6/K. 关键词： 2')" href="#">γ-LiAlO₂ 气相传输平衡法 热膨胀系数     

Low-temperature phase transitions in LiKSO4: A neutron study

A single crystal neutron diffraction study of the low temperature structural phase transitions in LiKSO₄ has been carried out. Detailed temperature dependence of the Bragg peak profiles and integrated intensities for a group of about 20 reflections has been investigated upon cooling and heating in the temperature range 300 K to 96 K. The crystal undergoes three clear phase transitions at 205 K (from sp. gr. P6₃ to sp. gr. P31c), at 189 K and at about 135 K on cooling. The corresponding temperatures are 250 K, 190 K and 138 K on the heating cycle. These transitions are extremely sluggish, the kinetics depending on the thermal treatment of the crystal.     

Internal friction in Cu6PS5Br superionic crystals and related composites

Mechanical properties of Cu₆PS₅Br single crystals and composites based on them have been investigated by the internal friction method. The measurements of the internal friction and the shear modulus have been performed in the temperature range of 80–300 K at deformation frequencies of 10–100 mHz in a mode of forced torsional vibrations. The maxima caused by the superionic and ferroelastic phase transitions have been found in temperature dependences of the internal friction. It has been shown that a more than two-fold decrease in the shear modulus with increasing temperature in the range of 150–230 K is caused by mobility unfreezing in the cation sublattice of the Cu₆PS₅Br single crystal during the superionic phase transition. An abrupt (more than threefold) increase in the shear modulus upon heating in the range of 260–270 K is caused by the ferroelastic phase transition of the Cu₆PS₅Br single crystal. Parameters of the internal friction of this single crystal in the course of mentioned phase transitions have been determined.     

Original magnetic behaviour observed in RNi5−xCux alloys (R = Pr,Gd or Y)

The Curie temperatures, spontaneous magnetizations and initial susceptibilities were investigated in the temperature interval from 1.5 to 40 K for the pseudobinary alloys RNi_5−xCu_x, where R = Pr, Gd and Y, x ≤ 2.6. It was discovered that the solid solutions PrNi_5−xCu_x between the PrNi₅ and PrCu₅ Van Vleck paramagnets are ferromagnets for intermediate Cu concentrations. It was shown that the ferromagnetic ordering in PrNi_5−xCu_x alloys can be explained by concentrational enhancement of 3d band susceptibility which in its turn leads to the enhancement of f-f exchange interactions. The maxima on the concentration dependences of both Curie temperatures for the RNi_5−xCu_x ferromagnets with R = Pr, Gd and susceptibility for the YNi_5−xCu_x Pauli paramagnets are manifestations of 3d band magnetism.     

双钙钛矿SrKFeWO6的电子结构与磁性

基于密度泛函理论框架下的第一性原理,采用考虑在位库仑作用的广义梯度近似（GGA＋U）下的投影缀加波（PAW）方法,研究了具有双钙钛矿结构的Sr₂FeWO₆和SrKFeWO₆材料的晶体结构、电子结构以及磁性性质.结构优化表明,K空穴掺杂稳定了FeO₆及WO₆八面体结构,Fe-O-W键角更加接近180°,有利于Fe-O-W-O-Fe超交换作用;对电子结构分析发现掺杂元素本身对总态密度贡献很小,空穴（p 关键词： 电子结构 磁性 6')" href="#">SrKFeWO₆ 双钙钛矿     

Thermo- and photostimulated processes of polarization and depolarization in CdI<Subscript>2</Subscript>: Ag

Currents of thermogradient polarization and depolarization of an electret state that arises in a photochromic crystal CdI₂: Ag during one-side cooling of the sample in the dark from 325 to 90 K in the presence of a temperature gradient directed along the crystallographic axis C ₆ have been found. At 90 K, the crystal polarized in the thermogradient electromotive force field is characterized by the photosensitivity in the near-edge, impurity, and infrared spectral regions. It is revealed that the electret state in the CdI₂: Ag crystal is also formed at room temperature during photolysis under irradiation of the samples by integrated light from a xenon lamp. Models of photosensitive centers formed upon doping of the CdI₂ crystal from the melt by the Ag⁺ impurity and during the occurrence of thermo- and photostimulated chemical reactions are proposed. The mechanism of the photochromic effect, including the change in the charge state of silver impurity ions, is considered.