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.     

The heat capacity of the layer compounds (CH3CH2CH2NH3)2MnCl4 and (CH3CH2CH2NH3)2CdCl4 from 10 K TO 300 K

The heat capacity of the layer compounds tetrachlorobis (n-propylammonium) manganese II and tetrachlorobis (n-propylammonium) cadmium II, (CH₃CH₂CH₂NH₃)₂MnCl₄ and (CH₃CH₂CH₂NH₃)₂CdCl₄ respectively, has been measured over the temperature range 10 K ?T ? 300 K.Two known structural phase transitions were observed for the Mn compound in this temperature region: at T = 112.8 ± 0.1 K (ΔH_t= 586 ± 2 J mol^?1; ΔS_t = 5.47 ± 0.02 J K^?1mol^?1) and at T =164.3 ± (ΔH_t = 496 ± 7 J mol^?1; ΔS_t =3.29 ± 0.05 J K^?1mol^?1). The lower transition is known to be from a monoclinic structure to a tetragonal structure, while the upper is from the tetragonal phase to an orthorhombic one. From comparison with the results for the corresponding methyl Mn compound it is deduced that the lower transition primarily involves changes in H-bonding while the upper transition involves motion in the propyl chain.A new structural phase transition was observed in the Cd compound at T= 105.5 ± 0.1 K (ΔH_t= 1472.3 ± 0.1 J mol^?1; ΔS_t = 13.956 ± 0.001 J K^?1mol^?1), in addition to two transitions that have been observed previously by other techniques. The higher of these transitions(T = 178.7 ± 0.3 K; ΔH_t = 982 ± 4 J mol^?1 ΔS_t = 6.16 ± 0.02 J K^? mol^?1) is known to be between two orthorhombic structures, while the structural changes at the lower transition (T= 156.8 ± 0.2 K; ΔH_t = 598 ± 5 J mol^?1, ΔS_t = 3.85 ± 0.03 J K^?1 mol^?1) and at the new transition are not known. It is proposed that these two transitions correspond respectively to the tetragonal to orthorhombic and monoclinic to tetragonal transitions in the propyl Mn compounds.In addition to the structural phase transitions (CH₃CH₂CH₂NH₃)₂MnCl₄ magnetically orders at t? 130 K. The magnetic contribution to the heat capacity is deduced from the heat capacity of the corresponding diamagnetic Cd compound and is of the form expected for a quasi 2-dimensional Heisenberg antiferromagnet.     

Neutron diffraction determination of the magnetic structures of NpCo2Si2 and NpCu2Si2

A small polycrystalline ingot sample of NpCo₂Si₂ (weight ≈ 1.5 g) has been studied by neutron diffration between 2 and 160 K on the multi-detector D1B of ILL, Grenoble. At 100 K, the crystal structure is body-centered tetragonal (space group 14/mmm) with a = 3.886 Å and c =9.649 Å. Below T_N = (44 ± 2) K, seven superlattice lines are observed which correspond to a simple tetragonal lattice with lattice constants as above. They are consistent with a type I antiferromagnetic structure of the Np (2a) sublattice, with (001) ferromagnetic sheets coupled antiferromagnetically according to the sequence +-+-. At 6 K, the neptunium moment obtained from the diffracted intensities is: (1.48 ± 0.20)_μuB, and makes an angle 52° ± 15° with the c axis. The cobalt moment is certainly smallet than 0.3_μuB. The Np moment correlates well with the ²³⁷Np hyperfine field deduced from Mos?sbauer spectroscopy; the sublattice magnetization-temoperature curve follows very well the $\text{J=}\text{1}\text{2}$ brillouin curve. The magnetism is therefore probably of lovalized character in this compound. An isomorphous sample of NpCu₂Si₂ (a = 3.990 Å c = 9.920 Å) was shown to be ferromagnetic below (41 ± 2) K, with the Np moment [1.5 ± 0.2)_μuB] aligned along the c axis.     

Neutron diffraction study of magnetic ordering in Tm0.5Eu0.5Se

A single crystal of the magnetic semiconductor Tm_0.5Eu_0.5Se was studied by means of neutron diffraction in the temperature range from 1.8 to 293 K. Long-range magnetic order is detected at temperatures below T_c = (18.5±1) K. The measured ferromagnetic moment component of (2.12±0.05) μ_B per rare-earth ion at saturation in zero external magnetic field indicates approximately antiparallel alignment of Tm moment and Eu spin (mutual angle 134°). The experimentally determined neutron magnetic form factor confirms the divalent state of both Tm and Eu in Tm_0.5Eu_0.5Se.     

Electron paramagnetic resonance in Y1−cH1.92:Erc

We have investigated the EPR of isotopically enriched ¹⁶⁸Er³⁺ in Y_1?cH_1.92:Er_c where c = 100 and 1400 ppm, at both 1.4 and 9 GHz and between 1.5 and 50 K. Resonance lines were observed from Er³⁺ ions in both sites of cubic symmetry and sites of axial symmetry. We determine the numbers of Er³⁺ in cubic, and C_4v axial symmetry to be in the ratio 2:1. The cubic site resonance line is at g = 6.85 ± 0.07 and is attributed to a Γ₇ doublet. The linewidth has a linear thermal broadening of 3.9 ± 0.05 gauss K^-1 below circa 7 K. From the nonlinear thermal broadening above this temperature we determine the first excited state, in the cubic crystal field scheme, to be a Γ₈ at 35 ± 10 K above the Γ₇ ground state. We have investigated the origins of the (T = 0) residual linewidth for the ions in cubic symmetry, and conclude there to be a small but significant contribution due to unresolved transferred hyperfine structure from the surrounding hydrogen nuclei.     

Magnetic structure of the cubic U2Te3 with a non-stoichiometric Th3P4 type of structure

The neutron diffraction patterns of the cubic U₂Te₃ have been obtained at 15 and 150 K. There are no extrapeaks of magnetic origin below the Curie point T_c = 70 ± 5 K. Thus the ordering corresponds to a collinear ferromagnetic structure. The value of the magnetic moment is 1.78 μ_B which is close to those values determined for a few cubic uranium pnictides and chalcogenides.     

The phase transition of two-dimensional squaric acid H2C4O4 studied by neutron scattering

At T_c = 370 K squaric acid undergoes a phase transition, which was studied in detail by neutron scattering. The transition is essentially two-dimensional with β = 0.137 ± 0.010. Diffuse scattering above T_e is found to be anisotropic. From “diffuse crystallography” it is concluded that proton disorder is not the dominant source of diffuse scattering. Individual molecules retain their low-temperature shape in the high-temperature phase.     

Magnon Raman scattering in CoBr2

The one-magnon Raman spectrum of CoBr₂ has been investigated as a function of temperature, and peak frequency, integrated intensity and width parameters obtained. The results obtained for the band energy at low temperature (22.2 ± 0.2 cm^-1 at 5.7.K) are in good agreement with AFMR and neutron scattering results. The one-magnon energy renormalises relatively slowly with increasing temperature and is about 15 cm^-1 at T_N = 19 K, whereas the integrated intensity approaches zero like the magnetization at T_N and the width diverges. A low intensity band at 26.8 ± 1 cm^-1 (7.6K) may be due to two-magnon scattering from spin waves along the c-axis.     

The heat capacity of the layer compound tetrachlorobis (methylammonium) manganese—II (CH3NH3)2MnCl4, from 10 to 300k

The heat capacity of the layer compound, tetrachlorobis (methylammonium) manganese II, (CH₃NH₃)₂MnCl₄, has been measured over the range 10K <T<300K. In this region, two structural phase transitions have been observed previously by other techniques: one transition is from a monoclinic low temperature (MLT) phase to a tetragonal low temperature (TLT) phase, and the other is from TLT to an orthorhombic room temperature (ORT) phase. The present experiments have shown that the lower transition (MLT→TLT) occurs at T = 94.37±0.05K with ΔH_t = 727±5 J mol^?1 and ΔS_t = 7.76±0.05 J K^?1 mol^?1, and the upper transition (TLT→ORT) takes place at T = 257.02±0.07K with ΔH_t = 116±1J mol^?1 and ΔS_t = 0.451±0.004 J K^?1mol^?1. These results are discussed in the light of recent measurements on (CH₃NH₃)₂CdCl₄, and also with regard to a recent theoretical model of the structural phase transitions in compounds of this type.In addition to the structural phase transitions, (CH₃NH₃)₂MnCl₄ also undergoes magnetic ordering at T < 150K. The magnetic component to the heat capacity, as deduced from a corresponding states comparison of the heat capacity of the present compound with that of the Cd compound, is shown to be consistent with the behaviour expected for a quasi 2-dimensional Heisenberg antiferromagnet.     

Neutron diffraction study of Ho(Rh0.3Ir0.7)4B4

Using powder neutron diffraction techniques, we have examined the magnetic order of the pseudoternary compound Ho(Rh_0.3Ir_0.7)₄B₄ below the Néel temperature T_N=2.7K. The magnetic structure consists of stacked antiferromagnetic basal plane sheets forming a body centered tetragonal unit cell, with a sublattice magnetization corresponding to 9.6±0.6μ_B per Ho³⁺ion at 1.5 K. Magnetic intensity versus temperature measurements indicate that the transition is second order and reveal no anomalous effects when the compound becomes superconducting at T_c=1.34K.     

Synthese et etude des transitions de phases de ceramiques et de cristaux de composition Li0, 02Na0, 98NbO3

Ceramics and single crystals of composition Li_{0. 02}Na_{0. 98}NbO₃ have been synthetised and studied as a function of temperature by X-ray diffraction, microcalorimetry, dielectric and pyroelectric measurements. Optical observation of domains and birefringence properties have been also carried out. The material shows five transitions at T₁ = 163 ± 10K, T₂ = 540 ± 20K, T₃ = 640 ± 20K, T₄ = 825 ± 10K and T₅ = 940 ± 15K. This sequence is quite different from that previously observed for pure NaNbO₃, the Li doped material being ferroelectric below T₃ and ferroelastic below T₅.     

Neutron diffraction studies of the magnetic ordering in the rare earth compounds TbNi2Si2, HoCo2Si2 and TbCo2Si2

Neutron diffraction studies and magnetic measurements on the compounds TbNi₂Si₂ (1), HoCo₂Si₂ (2) and TbCo₂Si₂ (3) revealed a collinear antiferromagnetic order below T_N = 10 ± 1 K (1), T_N = 13 ± 1 K (2) and T_N = 30 ± 2 K (3) with the rare earths moments oriented along the c-axis [m₀ = 8.8 ± 0.2 μ_B (1), m₀ = 8.1 ± 0.2 μ_B (2), m₀ = 8.8 ± 0.2 μ_B (3)] and the corresponding wavevector are $\text{k}\text{= [}\text{1}\text{2}\text{1}\text{2}\text{0] (1)}\text{and}\text{k}\text{= [ 0 0 1] (2) (3)}$. The magnetic structure of the compounds HoCo₂Si₂ and TbCo₂Si₂ consists of ferromagnetic layers perpendicular to the c-axis coupled antiferromagnetically (+?+?) while for TbNi₂Si₂ the ordering within (0 0 1) plane is antiferromagnetic and the planes (0 0 1) are indeed decoupled.     

High-resolution heat capacity of SnCl2·2H2O single crystal

The heat capacity of SnCl₂·2H₂O single crystal was measured in the close vicinity of the phase transition temperature, T_c = 217.994 ± 0.01 K. Its anomalous part ΔC could be expressed as ΔC = A_± ⊥ (T ? T_c)/T_c⊥^-α_±, where α₊ = 0.492 ± 0.02, α_- = 0.492 ± 0.02, A₊ = 1.148 JK^?mol^?, and A_- = 1.155 JK^?mol^?. A quasi-isothermal absorption of the enthalpy amounting to 34 J mol^? was observed at T_c.     

Thermal evidences for successive CDW phase transitions in 1T-TaS2

The heat capacity of 1T-TaS₂ has been measured over the temperature range including the successive phase transitions (140 K–370 K) by an adiabatic calorimeter. There are three transitions in the measured temperature range, two first-order transitions (at about 226 K (T₁) and about 353.5 K (T₃)) and one small anomaly at about 283 K (T₂) with a broad peak. The transition enthalpies are as follows; ΔH₁=52±5 cal·mol^-1, ΔH₂=7.5±2 cal· mol^-1 and ΔH₃=122±8cal·mol^-1.     

Magnetic properties of amorphous ferromagnet Fe78Mo2B20

Low-field magnetic susceptibility and the magnetic field dependence of magnetization of Metglas 2605 A (Fe₇₈Mo₂B₂₀) were studied between 300 and 600 K and in fields up to 10kG. It is shown here that for an amorphous ferromagnetic alloy, the various methods of determination of Curie temperature T_c give the same value, which in this case is (564 ± 1) K. The critical exponent γ is 1.7 ± 0.1. Our low-field susceptibility measurements on Metglas 2605 (Fe₈₀B₂₀) gives a T_c of (634 ± 3) K while the reported high-field method value is 647 K. These results are discussed in terms of crystallization temperatures.     

Diffraction de neutrons dans CoCl2, 6D2O determination de ses structures cristallographique et magnetique

We have performed neutron diffraction experiments on a CoCl₂, 6D₂O monocrystal. We show that a crystallographic transition from the C 2/m monoclinic structure to the P $\text{1}$ triclinic structure occurs at T = Tmt = 73.5K., together with a twinning of the monocrystal. We describe the two possible twins (four types of domains) related to the loss of the initial monoclinic symmetry, say of the elements 2 and m : either m becomes twin plane (Albite twinning) or 2 becomes twin axis (Pericline twinning). Each domain has an antiferromagnetic structure P 2a T, with an easy magnetization axis tilted from the ac plane by an angle φ = (30 ± 4)°, its component on this plane making an angle φ = (3.5 ± 2)° with the c axis.     

Dielectric properties of CsH2PO4 and CsD2PO4

Orthorhombic CsH₂PO₄ undergoes a ferroelectric transition at T_c = ?119.5°C, whereas the ferroelectric transition temperature in isomorphous CsD₂PO₄ is T_c = ?5.55°C. The transitions are of first order in both cases. The rather large isotope effect demonstrates the importance of the OHO bonds in the transition mechanism.     

Upper critical fields of Nb3Ge thin film superconductors

The upper critical field H_c2(T) of the highest T_c(～23K) Nb₃Ge superconducting films has been found to be ≈370kG at 4.2K. Measurements on lower T_c films show very broad transitions reflecting nonuniformity. The H_c2(T) characteristics are consistent with other Nb₃X type II superconductors.     

The antiferromagnetic structure of ErCo2Si2 by neutron diffraction

The magnetic structure of the tetragonal ErCo₂Si₂ compound is determined by neutron diffraction on powder sample at 4.2 K. The magnetic ordering is connected with a symmetry lowering, magnetic space group P_2s$\text{1}$ (Sh⁷₂)k = 000. The structure is collinear antiferromagnetic with the erbium magnetic moments making an angle of 56.2° with the c axis. The magnetic moment value for erbium is 6.75μ_B.     

Phase transitions in [Ni(NH3)6](NO3)2

Electric permittivity ^* = ′ − i″ of nickel-hexammino nitrate (NHN) has been measured within the range of temperature from 9 to 300 K at a frequency of 8.8 GHz (X-band). It has been found that the phase transitions at T_k1 = 247 K and T_k2 = 90 K are discontinuous structural transitions between centrosymmetric phases, whereas the transition at T_c = 63 K is a continuous phase transition (glass?).