Crystals and magnetic structure of RMn2Si2 (R = Pr, Nd, Y) and YMn2Ge2

Crystallographic and magnetic properties of PrMn₂Si₂, NdMn₂Si₂, YMn₂Si₂ and YMn₂Ge₂ intermetallics were studied by X-ray, neutron diffraction and magnetometric measurements. The crystal structure of all four compounds was confirmed to be body-centered tetragonal (space group I4/mmm). All were found to be antiferromagnetic with Néel points at 368, 380, 460 and 395 K respectively. Neutron diffraction results indicate that their magnetic structure consists of ferromagnetic layers composed of Mn ions piled up along the c-axis. Each layer is antiferromagnetically coupled to adjacent layer. The magnetic space group is I_p4/m′m′m′. No magnetic ordering of the R sublattice was observed at 1.8 K in the case of R = Pr and Nd.     

Electronic-excitation transfer collisions in flames—V. Cross sections for quenching and doublet-mixing of K(42P)-doublet by N2, O2, H2 and H2O

Weighted average cross sections for quenching of the K(4²P)-doublet by N₂, H₂, O₂ and H₂O, measured in flames, show no significant temperature dependence in the range from 1500 to 2500K. Doublet mixing cross sections for K(4²P$\text{3}\text{2}$?4²P$\text{1}\text{2}$) transitions were measured at 1720K for N₂, O₂, H₂O. The ratios of both mixing cross sections were measured independently and were found to agree with the detailed balance condition within 2 per cent. It is shown that an ionic intermediate-state model cannot explain the large magnitude of N_2? mixing cross sections.     

ESR and photoacoustic spectra of V2O5−MO2 (M = Se, Te, Ge) glasses

ESR spectra of V₂O₅?MO₂ (M = Ge, Se, Te) glasses are investigated in the range 298–498 K. The spectra at 298 K are characteristic of V⁴⁺ with the 3d¹ electron localized on a single ${}^{51}\text{V}\text{(I =}\text{7}\text{2}$) in the glass network. At higher temperature, the hyperfine structure progressively broadens, leading eventually to a broad, single ESR peak. These results are consistent with thermally-induced electron hopping from V⁴⁺ to V⁵⁺. Photoacoustic spectra of the glass at 298 K are characteristic of V⁴⁺ in a distorted octa environment. A correlation of ESR and PAS data suggests that covalency increased as M is charged from Ge through Te to Se.     

Excitation spectra and fluorescent lifetime measurements of Mn2+ in CaF2 and CdF2

The excitation spectrum of the Mn²⁺ emission has been measured in CaF₂ and CdF₂. The observed excitation bands have been assigned to transitions of the Mn²⁺ ions in a cubic environment. The calculated values for the crystal field (Dq) and Racah parameters (B,C) are Dq = 425 cm^-1 for CaF₂, Dq = 500 cm^-1 for CdF₂ and, B = 770 cm^-1 and C / B = 4.48 for both compounds. The lifetime of the fluorescent level ⁴T_1g(⁴G) has been measured in both compounds at different temperatures in the range from 10 to 500 K. The lifetime thermal dependence is explained taking into account different mechanisms (purely radiative, phonon assisted, and radiationless transitions) for the decay of excited Mn²⁺ ions.     

Measurement of the branching ratios for Kμ2+, Kπ2+, Ke3+ and Ke3+ decays using a magnetic spectrometer with streamer chambers

The momenta of ～30 000 charged particles from K⁺ decays were measured using a magnetic spectrometer with streamer chambers. The ratio R = Γ(K_π2⁺)/Γ(K_μ2⁺) = 0.3355 ± 0.0057 was obtained. Our values for the branching ratios are: (63.18±0.43)% for K_μ2⁺, (21.18±0.33)% for K_π2⁺, (3.33±0.51)% for K_μ3⁺, (4.99±0.54)% for K_e3⁺.     

Mössbauer study of Fe2+ ions in trigonal sites of spinels: GeFe2O4, GeCo2O4, GeNi2O4

We discuss here the results and the interpretation in the crystalline-field approach of some Mössbauer experiments on Fe²⁺ ions in the spinels GeFe₂O₄, GeCo₂O₄ and GeNi₂O₄. Once the sign of the quadrupolar interaction e²qQ has been deduced from a magnetic spectrum, the thermal variation of e²qQ may be used for determining the electronic level scheme of the Fe²⁺ ion (including the energies and wavefunctions of the levels). Then we may predict the form and magnitude of the spin hamiltonian, of the magnetic anisotropy and of the hypefine field tensor. Below T_N the experimental results are expressed in terms of a molecular-field, the eventual variations of which have been studied in magnitude and in orientation; by using the same calculation for the three compounds, we obtained a reasonable agreement between the experimental and calculated values of the hyperfine field at 0°K.     

Magnetic properties of NdCo2Ge2, ErCo2Ge2 and PrFe2Ge2 compounds

Magnetometric and neutron diffraction studies of polycrystalline NdCo₂GE₂, ErCo₂Ge₂ and PrFe₂Ge₂ compounds were carried out in the temperature range between 4.2 and 300 K. All samples are antiferromagnetic with Néel temperature 26.5, ～ 4.2 and 13 K, respectively. The RECo₂Ge₂ compounds have collinear antiferromagnetic order of +?+? type. For PrFe₂Ge₂ a sinusoidal magnetic structure is observed. Magnetic moment is localized on RE atoms only and is equal to that of RE³⁺ free ion value. In ErCo₂Ge₂ the magnetic moment of Er atoms is perpendicular to the c-axis, whereas for remaining compounds it is parallel to the c-axis.     

Magnetic properties of MgCo2, MgNi2 and Mg2Ni

The magnetic properties of MgCo₂, MgNi₂ and Mg₂Ni were investigated in the range 4–400 K. The compound MgCo₂ is ferromagnetic below T_c = 321 but gives rise to a first order ferromagnetic-antiferromagnetic transition near 45 K. The compounds MgNi₂ and Mg₂Ni are both Pauli paramagnetic; they have an almost temperature-independent magnetic susceptibility. For all compounds the lattice constants were determined.     

Magnetic properties of stuffed copper chromium tellurides Cu1+xCr2+yTe4 (x=0-1, y<0.3)

We present a detail study of the effect of excess metal atoms on the magnetic properties of Cu_1+xCr_2+yTe₄ at 2-400 K. With the increase in x=0-1 and y<0.3, these compounds retain metallic behavior, while ferromagnetic ordering temperature reduces from 325 to 160 K. Our low field susceptibility χ_ac measurements reveal a second transition on cooling below the ferromagnetic ordering; the transition at around 160-180 K intensifies with the excess amount of copper and chromium atoms. The value of spontaneous magnetization at 2 K remains between 2.6 and 2.9μ_B across all the compositions and it reduces with temperature as M(T)∼A₀T^3/2+A₁T^5/2, as expected for the excitation of Bloch's spin waves in a model of the Heisenberg ferromagnet. Our terminal composition Cu_1.9Cr_2.25Te₄ showed only second transition at 160 K with short range magnetic order much above the transition temperature and in the absence of the specific heat jump at this temperature. The magnetic properties are explained as a result of random magnetic anisotropy in the excess-metal compositions induced by the interstitial atomic defects in their parent spinel structure. The large stuffing of cations has been made possible in the telluride compounds because of the large size of tellurium and also by the covalent bonding that stabilizes the defect structure.     

Microwave spectrum of methylene fluoride-d2, CD2F2 in the excited vibrational states

The rotational spectra of ¹²CD₂F₂ in the ν₂, ν₃, ν₄, 2ν₄, ν₅, ν₇, ν₈, and ν₉ states were observed and assigned. Weak Coriolis interactions between ν₃ and ν₇, ν₃ and ν₉, and ν₅ and ν₇ were analyzed using approximate expressions for the rotational energy levels. The resonance between the ν₂ and the ν₈ state was found much stronger, and an effective two-dimensional Hamiltonian with the Coriolis term in the off-diagonal block was set up to analyze the spectra. The effect of the Fermi resonance between ν₃ and 2ν₄ was found to be very small.The ground-state spectrum of ¹³CD₂F₂ was observed and the rotational constants and the centrifugal distortion constants were determined. The data on ¹²CD₂F₂ and ¹²CDHF₂ were also improved very much in accuracy.The Coriolis coupling constants and the differences between two vibrational levels in resonance, which were determined by the analysis of the satellite spectra, are in good agreement with those obtained from vibrational spectra, except for the ν₂ band center, which is revised to 1170.3 cm^?1. The force constants were also checked using the centrifugal distortion constants of ¹²CD₂F₂, ¹³CD₂F₂, and ¹²CHDF₂.     

The crystal structures of the ion conductors (NH+4)Zr2(PO4)3and (H3O+)Zr2(PO4)3

The crystal structures of (NH⁺₄)Zr₂(PO₄)₃ and (H₃O⁺)Zr₂(PO₄)₃ have been determined from neutron time-of-flight powder diffraction data obtained at 15 K. Both compounds are rhombohedral, $\text{R}\text{3}\text{c}$, with cell parameters a=8.7088(1) and c=24.2197(4) Å for the ammonium compound and a=8.7528(2), c=23.6833(11) Å for the hydronium compound. In both cases the ions are completely localized in the type I cavities and hydrogen bonded to lattice oxygens. The measured unit cell parameters are relatively large for this class of compounds but the entrance ways into the cavities are still too small to allow for unrestricted movement of the ions. Thus the low conductivity of the hydronium ion is related to this and other structural features.     

Thermoelectric power of quasi-one-dimensional Nb3X4 with X=S, Se andTe

The thermoelectric power of linear chain synthetic metals Nb₃X₄ (with X=S, Se and Te) was measured from 5 to 300 K. The thermopower is negative indicating a dominant transport by electrons. Common to three compounds, in lower temperature regions the thermopower rises linearly with temperature but soon saturates. With respect to Nb₃S₄ and Nb₃Se₄ we have found no special anomaly of the thermopower except for a little higher magnitude.With respect to Nb₃Te₄ anomalies in the thermopower vs temperature appear at about 80 and 20 K which are explained in terms of the charge- density-wave phase transition from the simultaneous measurement of the resistivity and the observation of the electron diffraction patterns.     

Spectre d'absorption et effet Zeeman de l'ion Mn2+ dans un sulfate acide de manganese et de potassium MnK4H2(SO4)4, 2H2O a basse temperature

We present aborption spectra of an oriented single crystal of MnK₄H₂(SO₄)₄, 2H₂O at temperatures between 20 and 1.6 K. This salt has orthorhombic symmetry and gives, both in the visible and ultraviolet regions, an absorption spectrum which changes considerably when the temperature decreases from 20 to 4.2 K. The spectrum has sharp lines at 1.6 K in the two absorption regions corresponding to teh excited levels ⁴A₁⁴E(⁴G) and ⁴T₂(⁴D). A study has been made of the Zeeman splitting of these sharp lines; a discontinuity has been observed in the Zeeman splitting as a function of magnetic field at 1.6 K. This discontinuity is a consequence of a magnetically-ordered phase.     

On the mixed valence of the chromium ions in the noncollinear ferrimagnetic spinels Mn1−xCuxCr2S4 (Where x = 0.85, 0.90, 0.95)

For the polycrystalline samples of Mn_1?xCu_xCr₂S₄ (x = 0.85, 0.90, 0.95) the magnetization was measured in the temperature range between 77 K and the Curie temperature, T_C, using a magnetic balance (Faraday's method) and pulsed magnetic fields up to 2.0 T. The magnetic susceptibility was measured between T_C and about 600 K. The Curie temperatures were obtained using the kink point method.In the temperature range between 4.2 and 77 K the magnetization was measured in stationary magnetic fields up to 14 T. The data indicate a noncollinear ferrimagnetic structure. The compounds under investigation can be treated as CuCr₂S₄ slightly doped with Mn, with a valence distribution Mn²⁺_1?xCu¹⁺_xCr³⁺_2?xCr⁴⁺_xS^2?₄.     

Transition densities between the 01+, 21+, 41+, 02+, 22+, 11− and 31− states in 12C derived from the three-alpha resonating-group wave functions

All the multipole transition densities between the seven T = 0 states in ¹²C are calculated with the use of the microscopic 3α resonating-group wave functions which are obtained by dynamically solving the 3α relative motion with the total antisymmetrization taken into account exactly. The observed elastic and inelastic electron scattering form factors for the transition to the 2₁⁺, 4₁⁺, 0₂⁺, 1₁^?and 3₁^? states are well reproduced with no additional effective charge. The existence of a deformed intrinsic state for the 0₁⁺, 2₁⁺and 4₁⁺states is deduced by the analysis of the transition densities between them which are derived by the weak-coupling-type 3α wave functions; the intrinsic density distribution is illustrated. The monopole density distribution of the 0₂⁺, 2₂⁺ and 1₁^?, states is found to be much longer ranged than that of the 0₁⁺, 2₁⁺ and 4₁⁺ states; the 3₁^? state case is intermediate. On the basis of the transition densities between the 0₁⁺, 2₁⁺, 0₂⁺ and 2₂⁺ states, analysis is made of the transition between the shell-like states and the molecule-like states with a large spatial-structure change. Specific, effective nucleon-nucleon interactions are folded into the transition densities here obtained. The evident dependence of the radial shape of the folded nucleon-¹²C form factors on the choice of the interactions and the multi-step form factors for the excitation of the 0₂⁺, 1₁^? and 3₁^? states are discussed.     

The ν2, 2ν2, 3ν2, ν4, and ν2 + ν4 bands of 15NH3

The ir absorption of gaseous ¹⁵NH₃ between 510 and 3040 cm^?1 was recorded with a resolution of 0.06 cm^?1. The ν₂, 2ν₂, 3ν₂, ν₄, and ν₂ + ν₄ bands were measured and analyzed on the basis of the vibration-rotation Hamiltonian developed by V. ?pirko, J. M. R. Stone, and D. Papou?ek (J. Mol. Spectrosc.60, 159–178 (1976)). A set of effective molecular parameters for the ν₂ = 1, 2, 3 states was derived, which reproduced the transition frequencies within the accuracy of the experimental measurements. For ν₄ and ν₂ + ν₄ bands the standard deviation of the calculated spectrum is about four times larger than the measurements accuracy: a similar result was found for ν₄ in ¹⁴NH₃ by ?. Urban et al. (J. Mol. Spectrosc.79, 455–495 (1980)). This result suggests that the present treatment takes into account only the most significant part of the rovibration interaction in the doubly degenerate vibrational states of ammonia.     

Electronic transport properties of stuffed compositions of ferromagnetic copper chromium telluride: Cu1+xCr2Te4 (x=0-1)

We present here a detailed study of electronic transport properties of the metallic-ferromagnetic compounds Cu_1+xCr₂Te₄, having excess Cu atoms with x=0-1, from 2 to 400 K. The stuffing of the copper atoms in the parent structure reduces the ferromagnetic ordering temperature T_C from 325 to 156 K, while for the entire range the dependence of the electrical resistance and the thermopower with temperature and the anomalies in them on the magnetic ordering remain similar. All the compounds show a magnon-drag contribution in thermopower as a positive maximum around T_C/3, and a T² - dependence of resistivity at low temperatures. The increasing effects of the short range magnetic ordering in the paramagnetic resistivity are seen with the increase in the stuffing of atoms in these compounds. The transport properties are explained by the current carriers —the holes in a wide energy band dominated by the p-state of Te-atoms, which are scattered by the spindisorder in the paramagnetic phase and from the magnons in the ferromagnetic phase.     

The properties of five highly conducting salts: (TMTSF)2X, X = PF6-, AsF6-, SbF6-, BF4- and NO3-, derived from tetramethyltetraselenafulvalene (TMTSF)

The properties of five new highly conducting salts of TMTSF, (TMTSF)₂X, X = PF₆^-, AsF₆^-, SbF₆^-, BF₄^- and NO₃^- are reported. The measurements include d.c. and m.w. conductivity, thermopower, optical reflectivity, magnetic susceptibility and ¹⁹F-NMR. Preliminary structural data indicate uniform donor stacks. Metal to insulator transitions in four materials occur below 20 K, where specific conductivities higher than 10⁵ (Ω-cm)^?1 are observed for two salts (X = PF₆^- and NO₃^-).     

Polarization spectroscopy of the E0g+-B0u+ band system of Br2

The E-B (0_g⁺-0_u⁺) band system of Br₂ has been investigated at Doppler-limited resolution using polarization labeling spectroscopy. Merged E state data for the three naturally occurring isotopes in the range v_E = 0–16, expressed in terms of the constants for ⁷⁹Br₂, are (in cm^?1) Y_0,0 = 49 777.962(54), Y_1,0 = 150.834(22), Y_2,0 = ?0.4182(28), Y_3,0 = 6.6(11) × 10^?4, Y_0,1 = 4.1876(28) × 10^?2, Y_1,1 = ?1.607(16) × 10^?4, and Y_0,2 = 1.39(39) × 10^?8. The bond distance is $\text{r}{}_{\mathrm{<mtext>e</mtext>}}\text{= 3.194}\text{A}\text{?}$, and the diabatic dissociation energy to $\text{Br}{}^{\mathrm{+}}\text{(}{}^3\text{P}{}_2\text{) +}\text{Br}{}^{\mathrm{?}}\text{(}{}^1\text{S}{}_0\text{)}\text{is 34 700 cm}{}^{\mathrm{?1}}$.     

Emission and excitation spectra of Bi2Ge3O9:Eu3+

The emission and excitation spectra of the Bi₂Ge₃O₉:Eu crystal are observed at 77 K and 297 K. The spectra contain groups of sharp lines which are attributed to the transitions within 4f⁶ (Eu³⁺) configuration. The numbers of Stark splitting of terminal levels of transitions from ⁵D₀ and ⁷F₀ multiplets indicate that Eu³⁺ substitutes for Bi³⁺ in Bi₂Ge₃O₉. Tentative assignment of Stark levels of ⁷F_0-4 multiplets is made to crystal quantum numbers of C₃ symmetry which represents the site symmetry of Bi³⁺ in Bi₂Ge₃O₉. The following set of values of crystal field parameters of the C₃ point group is found to give the best overall agreement between the observed energy levels and the calculated levels: B²₀ = -533.84 cm^-1, B⁴₀ = 1085.99 cm^-1, Re(B⁴₃) = 327.57 cm^-1, Im(B⁴₃) = 75.209 cm^-1, B⁶₀ = 185.02 cm^-1, Re(B⁶₃) = - 68.475 cm^-1, Im(B⁶₃) = - 300.45 cm^-1, Re(B⁶₆) = 137.24 cm^-1 and Im(B⁶₆) = 882.29 cm^-1.