Anomalous temperature dependence of the band gap in AgGaSe2 and AgInSe2

The lowest band gaps of AgGaSe₂ and AgInSe₂ single crystals in the temperature range from 90 to 300 K were determined from photoconductivity measurements. Below (above ≈ 120 K in AgInSe₂ and ≈ 125 K in AgGaSe₂ the temperature coefficient of the band gap is +5 × 10⁻⁴ eV/K (−1.5 × 10⁻⁴ eV/K) and +1.1 × 10⁻⁴ eV/K (−4.28 × 10⁻⁴ eV/K), respectively. The positive value is explained with the lattice dilation effect being the dominant mechanism for the band gap variation at the temperatures less than ≈ 120–125 K.     

Li and K NMR relaxation study of a LiKSO4 single crystal

The ⁷Li and ³⁹K NMR relaxations in a LiKSO₄ single crystal grown by the slow evaporation method were investigated by employing a pulse NMR spectrometer. From the experimental data, the quadrupole coupling constant and asymmetry parameter were determined at the temperatures of 180 and 300 K. The relaxation processes of ⁷Li and ³⁹K were studied for the LiKSO₄ crystal, and the relaxation times for the ⁷Li and ³⁹K nuclei exhibit remarkable changes near T_c2 (=190 K). The activation energies for ⁷Li and ³⁹K were determined in phases I and III. The large change in the activation energy at 190 K indicates that the Li and K ions are significantly affected during this transition. The correlation time of the ⁷Li calculated from the spin-lattice relaxation time and quadrupole parameters was larger than that of the ³⁹K calculated using the same method. The reason for this is that the Li ion undergoes molecular motion as in the LiO₄ groups.     

An acoustic study of the mixed-alkali-mixed phase β-aluminas. Part 1: Naβ″/βAl2O3, Kβ″/βAl2O3 and (Na0.6K0.4)β″/βAl2O3

The relative attenuation of compressional sound waves of frequencies 10–60 MHz in mixed alkali (Na/K) mixed phase (β″/β)-aluminas is reported for temperatures 80–550 K. The internal friction peaks shift to higher frequencies at higher temperatures and are attributed to Na⁺ interactions in Naβ″/β alumina and Na⁺ and K⁺ in NaK β″/β alumina. The broad attenuation peaks occuring at low temperatures (< 300 K) and at higher temperatures (> 400 K) suggest multi-relaxation processes giving a distribution of activation energies. The estimated average activation energy for Na⁺ diffusion in Naβ″/βAl₂ O₃ at low temperatures and high temperatures is 0.183 eV and 0.387 eV respectively. In the NaK β″/βAl₂o₃ samples, the Na ⁺ values were 0.239 eV and 0.386 eV, respectively. The estimated average activation energies for K⁺ diffusion at low and high temperatures in the Kβ″/β-alumina samples were 0.269 eV and 0.371 eV and for K⁺ in the NaK β″/β samples, 0252 eV and 0.339 eV, respectively. The low temperature attenuation peaks were interpreted in terms of ionic interaction in the bulk and the high temperature peaks were related to interactions in the grain boundaries. The measured activation energies confirmed these interpretations. A reversal of the temperature appearance of the Na⁺ and K⁺ high temperature peaks in the NaKβ″/βAl₂ O₃ is explained by the disorder at the grain boundaries.     

Thermodynamic properties of the CeSn3 mixed valence compound

We have measured on the CeSn₃ compound, the expansion coefficient between 80 and 800 K at normal pressure, the isothermal compressibility in the 0–8 GPa pressure range at room temperature and the heat capacity at constant pressure in the 60–300 K temperature range. The experimental data were compared with those previously found for the isomorphous LaSn₃ phase, assumed as a proper reference material for the study of the intermediate valency states in CeSn₃. Both the thermal expansion (3α) and the isothermal compressibility (k) of CeSn₃ show behaviours quite different from those of LaSn₃: for instance, in the standard conditions, 3α is 55 × 10^?6K^?1for CeSn₃ and 38 × 10^?6K^?1for LaSn₃; k is 15 × 10^?12 Pa^?1 and 12 × 10^?12 Pa^?1 respectively for CeSn₃ and LaSn₃. The thermal behaviour of the molar specific heat at constant pressure of CeSn₃ is similar to that of LaSn₃ for temperatures lower than 50 K. In the 70–300 K temperature range, the heat capacity of CeSn₃ is clearly higher than that of LaSn₃, ΔC_p being maximum near 150 K. The analysis of the calorimetric data show that the electronic coefficient γ of CeSn₃ is temperature dependent: its value varies from 53 mJ K^?2 mole^?1 at low temperature 24 mJ K^?2 mole^?1 at 300 K.     

Structural phase transitions in KCdF3 and K0.5Rb0.5CdF3

Successive structural transitions of KCdF₃ and K_0.5Rb_0.5CdF₃ have been studied by means of thermal diffusivity, polarized light scattering and X-ray diffraction. It is found that transitions occur at about 460 K and 475 K in KCdF₃, and at 222 K, 232 K, 250 K, 258 K and 268 K in K_0.5Rb_0.5CdF₃. The partial exchange between K⁺ and Rb⁺ ions gives rise to simultaneous condensation of the M^z ₃ and R^x ₂₅ soft phonons.     

Anomalous homogeneous optical absorption line broadening of Eu3+ in superionic PbF2

Homogeneous broadening of one of the ⁵D₁←⁷F₀ absorption lines of Eu³⁺ in PbF₂ was measured in the temperature range 300–800 K. Below 625 K the linewidth depends linearly on T; above 625 K it increases rapidly followed by a plateau beginning near 700 K. This behavior is quite distinct from that expected for the normal Raman process in crystalline materials and appears to be correlated with F^- ion motion responsible for the superionic conductivity of PbF₂.     

The properties of the (H2O)6 water cluster that depend on its density during temperature transitions

Water clusters (H₂O)₆ are simulated by the Monte Carlo method with the Metropolis function at various temperatures (T ₁ = 273 K, T ₂ = 298 K, and T′₁= 373 K) and densities (ρ₁ = 0.9998 g/cm³, ρ₂ = 0.9167 g/cm³, and ρ₃ = 0.00059 g/cm³) of the system. It is established that the number of retained most probable configuration types at ρ₁ = 0.9998 g/cm³ during temperature transitions from T ₁ = 273 K to T ₂ = 298 K and from T′₁ = 373 K to T ₂ = 298 K is smaller than at ρ₃ = 0.00059 g/cm³. This result was acquired on the background of the following invariable parameters of the system with the same temperature transitions for each of three values of density: (i) the average number of retained most probable configuration types, (ii) the average fraction of weight coefficients of the most probable configuration types, and (iii) the average potential energy. The configuration type that was retained among the most probable configuration types of the system for all values of density (ρ₁ = 0.9998 g/cm², ρ₂ = 0.9167 g/cm³, and ρ₃ = 0.00059 g/cm³) of the system for temperature transitions from T ₁ = 273 K to T ₂ = 298 K and from T′₁ = 373 K to T ₂ = 298 K was also revealed.     

Regeneration of K S o mesons on carbon in the coherent production model

The process K _L ^o CK _S ^o C at kaon momenta (10–50) GeV/c is calculated in the framework of the coherent production model. The ratio of the moduli of forward K _S ^o -regeneration amplitudes on carbon and on one of its nucleon equal to 7.8±0.4 is obtained. The result together with the published data for the coherent K _S ^o -regeneration on carbon and hydrogen gives the value of the forward regeneration amplitude on neutrons. The calculated shape of the differential cross section for the elastic K _S ^o -regeneration on carbon is being compared with the corresponding measurements.     

Electronic and structural transitions in stoichiometric and nearly-stoichiometric lanthanum trihydride and trideuteride. A 139La and 1H nuclear magnetic resonance study.

We report the results of NMR measurements of the ¹³⁹La and 1p resonances in stoichiometric and nearly-stoichiometric lanthanum trihydride, LaH_x, and trideuteride, LaD_x, 2.89?x?3.00, covering the temperature range, 4.2 K?T?350 K. The Knight shift of the ¹³⁹La resonance is consistent with metallic behavior for T?210 K and with semiconducting behavior characterized by a band gap, E_g=0.10 eV, for T<210 K. In contrast, the ¹H T₁ follows T₁T=constant, consistent with metallic behavior, at least down to 77 K. Both the ¹³⁹La lineshape at T<210 K and the second moment of the ¹H resonance at 4.2 K confirm that the octahedral site hydrogen (deuterium) are randomly displaced from the geometrical centers of the octahedral sites.     

Magnetic anisotropy of the organic conductors (TMTTF)2X

The anisotropy in the static magnetic susceptibility of some tetramethyltetrathiafulvalene salts (TMTTF)₂X (where X^- = PF^-₆, ClO^-₄, SCN^- and Br^-) has been investigated with a torque method between 3 K and 300 K for magnetic fields up to 11 kG. The experimental results show that the salts with X^- = SCN^- and Br^- become antiferromagnetic at about 8 and 13 K respectively, whereas for the two other salts above 1 kG no field dependence of the magnetic susceptibility is observed down to 3 K.     

DyNi2Mn—magnetisation and M?ssbauer spectroscopy

The physical properties of DyNi₂Mn doped with ⁵⁷Fe have been investigated by X-ray diffraction, magnetisation (10–300?K) and ⁵⁷Fe M?ssbauer spectroscopy measurements (5–300?K). DyNi₂Mn(⁵⁷Fe) crystallizes in the MgCu₂-type cubic structure (Fd^??3m space group). The ordering temperature is found to be T_C?=?99(2) K, much higher than those of DyNi₂ (～22?K) and DyMn₂ (～35?K). Analyses of isothermal M–H curves and the related Arrott plots confirm that the magnetic phase transition at T_C is second order. The magnetic entropy change around T_C is 4.0?J/kg?K for a magnetic field change of 0?T to 5?T. The spectra above T_C exhibit features consistent with quadrupolar effects while below T_C the spectra exhibit magnetic hyperfine splitting. The Debye temperature for DyNi₂Mn has been determined as θ_D?=?200(20) K from a fit to the variable temperature isomer shift IS(T).     

Mössbauer studies of57Fe atoms isolated in CH4 and CO2 matrices

The Mössbauer absorption spectra of matrix isolated⁵⁷Fe atoms have been measured in the inert matrices CH₄ and CO₂ with matrix temperatures between ～3.3 and ～46 K. The isomer shift of the observed resonances is (?0.79±0.02) mm/s and (?0.76±0.05) mm/s with respect to iron metal at 300 K for⁵⁷Fe in CH₄ and for CO₂ respectively. This is within the experimental errors the same isomer shift as that of rare-gas matrix isolated⁵⁷Fe atoms. All spectra show quadrupole doublets due to the noncubic point symmetry of the lattice site occupied by the⁵⁷Fe atoms. The effective Debye temperatures as obtained from the temperature dependence of the resonance absorption areas are (46±4) K for the CH₄ matrix and (121±6) K for the CO₂ matrix annealed at ～20 K.     

A new ferromagnetic thiospinel CuCrZrS4 with re-entrant spin-glass behaviour

A new thiospinel CuCrZrS₄ has been successfully synthesized by a solid-state chemical reaction. This CuCrZrS₄ exhibits ferromagnetic properties below the Curie temperature at T _c = 60 -2 K. The appearance of irreversible effect between field-cooled and zero-field-cooled magnetization is prominent below around 5 K in a magnetic field of less than 150 Oe. The ac susceptibility χ_AC shows a rapid decrease below about 10 K. This low magnetic-field behaviour indicates the existence of a re-entrant spin-glass phase below about 10 K. The dc magnetic susceptibility above 100 K shows Curie-Weiss behaviour with an effective magnetic moment of 3.61 μ _B, which is a little less than the spin-only value of 3.87 μ _B for the Cr³⁺ ion. The asymptotic Curie temperature θ _P is approximately 65 K, which is a little higher than T _c. The valence state is confirmed to be Cu⁺Cr³⁺Zr⁴⁺S₄ ^2? on the basis of magnetic properties. The electrical resistivity ρ shows a semiconducting temperature dependence over the temperature range from 4.2 to 280 K with an activation energy of 6.84 210^?3 eV in the higher temperature range from 50 to 283 K.     

Magnetism and the magnetocaloric effect in PrMn1.6Fe0.4Ge2

The magnetic and magnetocaloric properties of PrMn_1.6Fe_0.4Ge₂around the ferromagnetic transitions T _C ^inter ~ 230 K and T _C ^Pr ~ 30 K have been investigated by magnetisation, ⁵⁷Fe Mössbauer spectroscopy and electron paramagnetic resonance (EPR) measurements over the temperature range 5–300 K. The broad peaks in magnetic entropy around T_C ^inter (intralayer antiferromagnetism of the Mn sublattice to canted ferromagnetism) and T_C ^Pr (onset of ferromagnetic order of Pr sublattice in addition to ferromagnetically ordered Mn sublattice) are typical of second order transitions with maximum entropy values of -ΔS _M ~ 2.0 J/kg K and -ΔS _M ~ 2.2 J/kg K respectively for ΔB = 0–6 T. The EPR signal around T = 48 K of g value g ~ 0.8 is consistent with paramagnetic free ion Pr^3?+?. Below T_C ^Pr ~ 30 K the g value increases steadily to g ~ 2.5 at 8 K as saturation of the Pr^3?+? ion is approached. The EPR measurements indicate additional effects in this system below T ~ 20 K with the appearance of EPR signals of low g value g ~ 0.6.     

Ferri-para-magnetism of tungsten and its carbide cermets for elevated temperature sensor application

In routinely analyzing experimental results of low-temperature, 5?K?≤?T?≤?300?K, magnetic susceptibility χ(T) of fused tungsten and its fused carbide cermets, hereafter called “the materials”, by assuming the susceptibility satisfying the Néel ferrimagnetic formula, χ^?1?=?χ₀^?1?+?χ₁^?1?+?χ₂^?1, where χ₀^?1, χ₁^?1, and χ₂^?1 are respectively the inverses of the Pauli paramagnetic, χ₀?>?0, a Curie diamagnetic, χ₁?=?C/T < 0, and a lattice diamagnetic susceptibilities, χ₂?=?(–1/b)(T ? Θ_p)<?0, this study finds that the increase in susceptibility of the materials is principally due to the effect of the Curie diamagnetic term in the formula and might be caused by the demagnetizing field that is induced by the interstitial vacancy in, e.g., tungsten lattice. By extending the formula to the region of lower (<5?K) and higher (>300?K) temperatures, one finds two singular points in the susceptibility inverse curve χ^?1. One is at Θ_p?<?0, and the other Θ_a?=??C/χ₀?>>?0, e.g., Θ_p?=??1.73?K, and Θ_a?=??C/χ₀?=?+8655?K for tungsten. In view of their susceptibility precisely satisfying the Néel ferrimagnetic formula in the temperature range of 5?K and 300?K, they are called “ferri-para-magnetism” and are good for elevated temperature sensor application if the temperature extension is satisfied. As temperature exceeding an asymptotic temperature, Θ_a, the paramagnetic state turns to a diamagnetic state, suggesting that there might be a superconducting transition at this temperature for some of the materials with Θ_as that are lower than their melting points, e.g., Θ_a(C7M1)?=?+2735?K and Θ_a(-VC)?=?+2242?K.     

Heat capacity measurements on irradiated and non-irradiated Mo3Ge and Mo5Ge3

We have measured the heat capacity of irradiated and non-irradiated Mo₃Ge and Mo₅Ge₃ in the temperature range ≈ 1.6 to 10 K. An irradiation of 2.2 × 10¹⁹ neutrons cm^-2 results in an increase in the superconducting transition temperature T_c from < 1.6 K in the non-irradiated state to ～ 4 K after irradiation for Mo₃Ge and a corresponding change from < 1.6 K to ～ 3 K for Mo₅Ge₃. Our analysis shows that this change in T_c is not accompanied by a change in the electronic density of states (within experimental error) but rather a decrease in the Debye temperature from 392 to 322 K for Mo₃Ge and 377 to 320 K for Mo₅Ge₃.     

Electron spin-lattice relaxation of Yb3+ and Gd3+ ions in glasses

The electron spin-lattice relaxation rate (T ₁ ^?1) was measured in two glass samples: (i) a phosphate glass doped with 1 wt% Yb₂O₃ and (ii) a Li₂Si₄O₉ glass sample doped with 0.2 wt% Gd₂O₃. In the Yb³⁺-doped glass sample,T ₁ was measured by an electron-spin-echo technique from 4.2 to 6 K, by the modulation method from 10 to 26 K and by the EPR linewidth from 30 to 100 K. It was found thatT ₁ ^?1 αT ⁿ withn=9 in the range 4.2–6 K.n decreased gradually as the temperature was increased and tended towards 2 above 40 K. Over the entire temperature range 4.2–100 K,T ₁ ^?1 was fitted toAT+BT ⁹ J ₈ (Θ _D/T) (whereA andB are two temperature-independent constants,J ₈ is the well-known Van Vleck integral andΘ _D is the Debye temperature). The value ofΘ _D (=46.3±0.9 K) so determined is in good agreement with that of Stevens and Stapleton from theirT ₁ measurements in the range 1.5 to 7 K. In the Gd³⁺-doped glass, it was observed thatT ₁ ^?1 αT over the range 50–150 K. The data for Ye³⁺-doped glass sample were accounted for by assuming that the phonon modulation of the ligand field is the dominant mechanism, associated with a low Debye temperature in accordance with the published data obtained by using other techniques to study lattice dynamics. On the other hand, the data on the Gd³⁺-doped glass sample were explained to be predominantly due to a mechanism involving Two-Level-Systems (TLS)     

Negative molecular ion formation and work function changes: Experimental results for CO and CO2 scattering from nickel (+potassium) surfaces

The scattering of CO⁺ and CO⁺₂ at grazing incidence from Ni(111)+K and clean Ni(111), Ni(110) surfaces produces CO, CO₂ and dissociated species. The observation of negative species O⁻ and CO⁻₂ is strongly dependent on the K coverage or work function of the surface. The dissociation of CO⁺ (CO) is weakly changed by the presence of K, whereas in the CO⁺₂ (CO₂) case dissociation via CO⁻₂ → CO + O⁻ is strongly increasing with K coverage.     

Experimental observations of ferroelectricity in double pyrochlore Dy2Ru2O7

Double pyrochlore Dy2Ru207 is synthesized and its magnetism and ferroelectricity below the Ru4＋ spin ordering temperature （NI00 K） are investigated. The ferroelectric transition appears at -18 K, much higher than the Dy3＋ spin ordering point at -1.8 K and lower than the Ru4＋ spin ordering point at -100 K. The measured electric polarization at ,-2 K is as big as 145℃/m2 in the polycrystalline samples. It is argued that the ferroelectricity is possibly ascribed to the electric dipole ordering arising from the collective monopole excitations in the Dy3＋ tetrahedrons in prior to the Dy3＋ spin ordering into spin-ice like state below -1.8 K.     

Structural Phase Transition and the Dielectric Permittivity of the Model Lipid Bilayer [(CH2)12(NH3)2]CuCl4

Structural phase transitions in the lipid-like bilayer material [(CH₂)₁₂(NH₃)₂]CuCl₄ have been observed using differential thermal scanning. The compound shows an irreversible thermochromic transition at ? 465 K and three reversible transitions at T ₁ = 433 ± 4 K and T ₂ = 411 ± 2 K and T ₃ = 358 K. The transition at 350 K is ascribed to chain melting. The other two correspond to crystalline phase transformation.

Phase (IV) T₃ = 358 ± 2K Phase (III) T₂ = 411 ± 2K Phase (II) T₁ = 433 ± 4K Phase (I)

Dielectric permittivity is studied as a function of temperature in the range 300-440 K and frequency, range (60 Hz-100 kHz). It confirms the observed transitions. The dielectric permittivity reflects rotational and conformational transitions for the compound. The variation of the real part of the conductivity with temperature is thermally activated in the temperature range above 350 K, with frequency-dependent activation energy, the values of activation energy lie in the range of ionic hopping. The dependence of the conductivity on frequency follows the universal power law σ = σ₀ + A(T) ω ^{s ( T )} with 0<s<1. Comparison of this material with other members of the series is discussed