Temperature dependence of the hyperfine quadrupolar interaction in K2ZrF6

The thermal dependence of the hyperfine quadrupolar interaction of¹⁸¹Ta at Zr sites in K₂ZrF₆ has been studied by TDPAC between RT and 511 °C. Four phases of ever increasing symmetry have been found in the range RT-437 °C. The possible existence of a healing mechanism with an activation energy of 0.187±0.040 eV is discussed for the lattice recovery in the range 270 °C–335 °C. The results at temperatures above 437 °C can be understood assuming a thermal decomposition of the compound.     

Ultrasonic investigation of phase transitions in K2SnCl6

The velocities and the attenuation of ultrasonic waves have been investigated as a function of temperature (255KT350K) for the cubic phase of K₂SnCl₆ which undergoes a structural transition atT _c1263K. An anomalous decrease of the shear stiffness constantc=1/2(c ₁₁ –c ₁₂) of about 30% between room temperature andT _c1 is found in this cubic high temperature phase whenT _c1 is approached from above. Whereas the softening ofc extends over a substantial temperature range (about 50 K), the other shear constantc ₄₄ shows only a weak decrease, which is an order of magnitude smaller and restricted toT–T _c110 K. NearT _c1, a strong increase occurs in the attenuation of the shear acoustic wave propagated along the [111] direction, while such an anomalous attenuation is not observed for the transverse acoustic wave propagated along [100]. Furthermore, atT _c1 a hysteresis is detected for the longitudinal sound wave velocityv _L[100], which is restricted to the temperature region 262KT263.5 K. In addition, the room-temperature elastic constants of (NH₄)₂SiF₆ and the hydrostatic pressure derivatives of the elastic constants of K₂SnCl₆ and (NH₄)₂SiF₆ at room-temperature are presented and discussed in terms of mode softening behaviour.     

Temperature dependence of diffuse field phase

Diffuse fields, which have scattered from microstructure or reflected from walls so much as to prohibit conventional analyses, are usually examined by means of the time evolution of their ultrasonic spectral energy density. The phase information is usually discarded as resisting analysis. The phase, while unpredictable is, however, robust; according to theory it remains constant if source and receiver are not disturbed. Nevertheless, in practice we do observe slow drifts of phase over time scales of minutes. Here we examine the hypothesis that the phase drifts are due to temperature fluctuations. Temperature changes on cooling from 40 degrees C to room temperature were monitored and compared with changes in diffuse field phase. It was found that the reverberant ultrasonic field in a 7 cm aluminum block evolves with temperature in a manner that is in accord with published data on the temperature dependence of the ultrasonic velocities. Our 1 MHz transient source gives rise to a complex waveform that is observed to undergo an almost pure dilation. The precision with which this shift can be measured approaches 20 ns. This is remarkable when compared with the 100 ms travel time of the signal. Thus the temperature dependence of elastic wave speed is measured with a precision limited by the precision of one's thermometer. The signal is also found to suffer some distortion which, it is suggested, is related to the different rates of change of longitudinal and shear speeds. The corresponding prediction for the degree of distortion is found to be in accord with measurements.     

Inelastic neutron scattering by the quasi two-dimensional librational modes in K2SnCl6

The origin of the diffuse streaks observed in the high temperature phase of K₂SnCl₆ is explained in terms of a dynamical model in which [SnCl₆] octahedra are coupled by torsional springs. The magnitude and the temperature dependence of the torsional spring constants is derived from inelastic neutron scattering data. The librational modes are soft and overdamped at room temperature, and a central component is observed up to 170 K above the lattice instability which occurs atT _c1 =261.5 K.     

Temperature dependence of muonium spin exchange with O2 in the range 88 K to 478 K

Our group at TRIUMF reported earlier a study of the spin exchange reactions of Mu with O₂ and NO in the range 295 K to 478 K. We have extended the measurement with O₂ to a low temperature region down to 88 K. From 135 K to 296 K, the spin depolarization rate constantk _d(T) was found to vary according to the relative velocity of the colliding species,T ^1/2, which indicates that the spin exchange cross section of Mu-O₂ is temperature independent in this temperature range. The value ofk _d(T) at 296 K is in good agreement with our earlier study. However, it was found that below 105 K and above 400 K,k _d(T) tends to have stronger temperature dependences (T ⁿ , withn>1/2). This deviation fromT ^1/2-behavior can be attributed to the velocity (energy) dependence of the spin exchange cross section.     

Temperature dependence of Mn2+ EPR in Sn2P2S6 near the phase transition

The X-band EPR spectrum of Mn²⁺ in Sn₂P₂S₆ was studied in the temperature rangeT=223–363 K. At room temperature the spin-Hamiltonian constants areg=2.00±0.01,B ₂ ⁰ =(163±3)·10^?4 cm^?1,B ₂ ² =(159±3)·10^?4 cm^?1,A=?(75±1)·10^?4 cm^?1. The effect of the invariance in temperature of the resonance magnetic fields in the narrow temperature rangeT=337–340 K and the model of the paramagnetic centre are discussed. According to EPR data a phase transition occurs atT=337 K. This transition from the paraelectric phase to the ferroelectric one is accompanied by a dramatic change in value of the spin-Hamiltonian constantB ₂ ⁰ .     

Temperature dependence of the magnetic hyperfine interaction for K and Ar in Fe

By means of an integral IMPAD measurement, a temperature dependence of the angular distribution for⁴⁰K ions in magnetically saturated iron has been observed. This temperature dependence has been tentatively attributed to the annealing of defects close to the implanted potassium atoms, associated with an activation energy ofE _a=0.31 (5) eV. A similar temperature dependence has been observed for the system ArFe.Supported by Deutsches Bundesministerium für Forschung und Technologie     

Temperature dependence of the antiferromagnetic resonance linewidth in MnF2

The temperature dependent contribution to the antiferromagnetic resonance linewidth in MnF₂ has been measured at K-band frequencies (～ 23 GHz) between 5° and 40°K. It can be closely approximated by a T⁴ power law except at the highest temperatures.     

Temperature dependence of the luminescence of TiO2 powder

The temperature dependence of the luminescence quantum yield of powdered titaniumdioxide in vacuum, nitrogen, and water vapor, respectively, was measured in the temperature range 300 K to 800 K. The observed luminescence is attributed to luminescence from surface states. We find that the luminescence is thermally quenched, as predicted by a simple model. Furthermore, the luminescence is shifted towards shorter wavelengths with increasing temperature. We also observed that the luminescence in vacuum is weakly quenched in the presence of nitrogen or water vapor.     

Temperature dependence of the electrical resistivity of the nickel-platinum alloy system

The temperature dependence of the electrical resistivity of Ni_1?xPt_x (x = 1, 2, 3, 4, 5, 8, 10, 15, 20, 25 and 30 at.%) between 77 and 700 K has been investigated. The experimental results are interpreted according to the theoretical model developed by Mott. It is shown that for this system Matthiessen's rule is not obeyed.     

On the volume dependence of superexchange in K2NiF4

Ultrasonic velocity measurements in the two-dimensional antiferromagnet K₂NiF₄ as a function of temperature at several hydrostatic pressures show T_N to be very weakly pressure dependent. This result differs from previous estimates of the volume dependence of superexchange in similar compounds.     

Temperature dependence of phonon spectra and structural characteristics in multiferroic LuFe2O4 system

The phonon spectra and structural characteristics of the LuFe₂O₄ system were investigated using X‐ray diffraction as well as Raman and infrared (IR) spectroscopic techniques. Two step‐like anomalies of the wavenumbers of the peak in the Raman spectra and the peak in IR spectra, as well as some weak anomalies of the lattice parameters, were found around the ferrimagnetic and ferroelectric transition temperatures. The substitutions of Lu with Yb and Fe with Mn lead to the corresponding wavenumber shifts in the optical spectra, which can be attributed to the changes in the atomic mass and lattice parameters. Furthermore, the Raman intensity was found to be related to the changes in the local structure and electrical conductivity. These results illustrate that the structural fluctuations could effectively influence the phonon modes, and a possible interplay among the structural, magnetic, and charge‐ordering properties exists in this multiferroic system. Copyright © 2011 John Wiley & Sons, Ltd.     

Temperature dependence of the rotational life times in solid H2

We have studied the temperature dependence of the nuclear transverse and longitudinal relaxation times in solid H₂ for ortho concentrations 10^-3 < X < 10^-2 between 0.4 K and the triple point, 13.9 K. We find a striking temperature dependence in T₂ over the whole temperature range. This new effect is ascribed to a coupling between molecular rotation and lattice vibrations, which is brought into evidence by the narrow width of the spectral density of the rotational fluctuations at low X.     

Reactive scattering of alkali dimers: Product K atoms from K2+Br2, IBr,BrCN, SnCl4

Approximate angular distribution measurements of reactively scattered K atoms from a potassium dimer K₂ beam by cross beam molecules Br₂, IBr, BrCN, SnCl₄ are reported. Forward peaking in the centre of mass differential cross section for K₂+Br₂, IBr, BrCN unambiguously confirms that the reaction path at large impact parameters yields K + KX + Y products. Differential cross sections at wide angles for K compared with KX products, suggest that the reaction path yielding KX + KY products becomes important at small impact parameters. Comparison of the reactive K atom scattering with the KX scattering lends further support to a schematic mechanism of the reaction dynamics suggested previously. The K₂ + SnCl₄ reaction does not yield reactively scattered K atoms.     

Temperature dependence of AC-photoconductivity of HgI2 crystal

An ac impedance method has been used to study the electrical properties of an illuminated HgI₂ crystal as a function of temperature [10–350 K] and frequency [1–10₄ Hz]. The complex impedance plane plots enabled us to determine the bulk resistance of the crystal as a function of temperature. Activation energies of [0.08±0.005 eV] and [0.25 ±0.01eV] are then found; they are attributed to acceptor and donor trapping levels, respectively. At temperatures lower than 230 K, a weak temperature dependence of the bulk resistance is observed. This weak dependence is supposed to be due to photoconductivity.