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.     

Preparation and low field magnetoresistance of double layered manganite La1.4Ca1.6Mn2O7

Double layered manganite of La_1.4Ca_1.6Mn₂O₇ (DLCMO) was prepared using solid state reaction method and had a metal-insulator transition temperature (T_MI) of 125 K. The short range 2D-feerromagnetic ordering (T_C2) starts growing when T<168 K and it gets converted into 3D-ferromagnetic ordering (T_C1) at 114 K. Low field magnetoresistance (MR) behaviour of the DLCMO was investigated and compared with an infinite layered manganite La_0.7Ca_0.3MnO₃ (LCMO). For DLCMO, in the temperature range between T_C1 and T_C2, the MR showed a gradual increase with the magnetic field. The observed MR and R-T behaviour of double layered manganite for T_C1<T<T_C2 has been explained in the frame work of the two phase model [ferromagnetic (FM) domains and paramagnetic (PM) regions] and percolative behaviour of transport in FM-PM mixture.     

Charge density waves in layer structures: A NMR study on a 2HNbSe2 single crystal

We have studied by pulsed NMR a single crystal of 2HNbSe₂ at ambient pressure and under hydrostatic pressure of 21 kbar in the temperature range 4.2–273 K. Our results are consistent with the onset of incommensurate charge density waves (ICDW) at T_CDW = 33 K atP = 1 bar and 26 K at P = 21 kbar. Below T_CDW, the lineshapes of the (m → m ? 1) transitions agree with a local distribution of Knight shift and electric field gradients respecting the symmetry of a triple ICDW, while above T_CDW, pre-transitional broadening is observed. The product T₁T = 500 ± 100 msk was found constant in the temperature range 4.2–77 K and pressure independant.     

Low field magnetoresistance and conduction noise in layered manganite La1.4Ca1.6Mn2O7

Temperature dependence of conduction noise and low field magnetoresistance of layered manganite La_1.4Ca_1.6Mn₂O₇ (DLCMO) are reported and compared with the infinite layered manganite La_0.7Ca_0.3MnO₃ (LCMO). The double layered manganite was prepared using standard solid state reaction method and had a metal-insulator transition temperature (T_M-I) of 155 K. The temperature dependence of susceptibility showed evolution of ferromagnetic ordering at 168 K. The observed voltage noise spectral density (S_V) shows 1/f^α type of behaviour at all temperatures from 77 K to 300 K. In the ferromagnetic region (T<168 K), S_V/V² shows two peaks at 164 K and 114 K. The observed two peaks in normalised conduction noise of DLCMO is attributed to the excess noise generated due to setting up of short range 2D-ferromagnetic ordering and long range 3D-ferromagnetic ordering at two different temperatures T_C2 and T_C1. In temperature range between T_C1 and T_C2, the magnetoresistance (MR) showed a gradual increase with the magnetic field. The observed MR has been explained in the framework of the two phase model [ferromagnetic (FM) domains and paramagnetic (PM) regions].     

NMR spectral densities and two dimensional diffusion in TiS2(NH3)1.0 and TaS2(NH3)x

NMR measurements of proton spin-lattice relaxation times T₁ and T_1? in the layered intercalation compounds TiS₂(NH₃)_1.0 and TaS₂(NH₃)_x (x = 0.8, 0.9, 1.0) are reported as functions of frequency and temperature (100 K – 300 K). These observations probe the spectral density of magnetic fluctuations due to motions of the intercalated molecules at frequencies accessible to the T₁ (4–90 MHz) and T_1? (1–100 kHz) measurements. Since the average molecular hopping time (τ) can be changed by varying temperature, different regions of the spectral density can be examined. For T > 200 K, both T^?1₁ and T^?1_1? vary logarithmically with frequency, reflecting the two dimensional character of the molecular diffusion. The temperature dependence of T₁ suggests that a more accurate picture of the short time dynamics is required. No dependence of relaxation rate on vacancy concentration is found.     

Superionic transitions and conductivity anisotropy in Na4.6FeP2O8.6F0.4 single crystals

The cation conductivity in the directions parallel (σ_‖[001]) and perpendicular (σ_⊥[001]) to the [001] crystallographic direction of Na_4.6FeP₂O_8.6F_0.4 single crystals has been investigated at 293–734 K. The specific features of the ionic conductivity have been studied near two phase transitions at T _{tr, 1} ～ 450 K and T _{tr, 2} ～ 545 K. At T = T _{tr, 1}, the activation enthalpy for the dependences σ_‖[001](T) and σ_⊥[001](T) decreases from 0.45 ± 0.01 to 0.33 ± 0.02 eV, and the σ_‖[001](T) curve has a jump of the ionic conductivity by a factor of almost two at T = T _{tr, 2}; the jump is related to a manifestation of commensurate modulation of the crystal structure. In the Na_4.6FeP₂O_8.6F_0.4 crystals, the ionic transport is anisotropic with the ratio σ_‖[001]/σ_⊥[001] = 7.7, 5.2, and 6.6 at 293 K (T < T _{tr, 1}), 500 K (T _{tr, 1} < T < T _{tr, 2}), and 700 K (T < T _{tr, 2}), respectively. The mechanism of cation conductivity in the Na_4.6FeP₂O_8.6F_0.4 crystals is discussed.     

Brillouin scattering study of phase transitions in T1Ins2

The temperature dependence of the elastic coefficients of T1InS₂ has been investigated by Brillouin scattering measurements. For longitudinal phonons propagating in the (001) plane the variation of ΔC_∼∼(T_c−T) was found below the ferroelectric transition point (T_c=189 K). For propagation perpendicular to the (001) plane (C₃₃ mode) clear anomalies were observed at T=213 K and T=195 K. Between these temperatures the Brillouin peaks show broadening, which is attibuted to a coupling between the acoustic mode and an overdamped mode.     

Nuclear magnetic resonance studies of CePd2In and LaPd2In at low temperatures

We report on In nuclear magnetic resonance (NMR) and nuclear quadrupolar resonance (NQR) measurements on the new heavy-electron compound CePdZIn over a wide temperature range, from 45 mK up to 30 K. CePd₂In undergoes an antiferromagnetic (AF) phase transition at T _N = 1.23 K involving small localised Ce moments of 0.11 μ_B. In zero field, the spin-lattice relaxation rate T ₁ ^?1 (T) shows remarkable changes in its temperature dependence. Above 3 K, T ₁ ^?1 is constant and 850 sec^?1. Between T _N and 2T _N, (T ₁ T)^?1 = 330 (Ksec)^?1, but rapidly decreases below T _N. A Korringatype relaxation, characteristic for simple metals at low temperatures, with (T ₂ T)^?1 = 17(Ksec)¹ is resumed below 0.6 K. This value is an order of magnitude larger than (T ₁ T)^?1 for LaPd₂In and therefore is associated with low-energy excitations of the itinerant charge carriers with 4f symmetry. The T ₁ ^?1 data at various non-zero magnetic fields fall on a single curve when plotted as a function of (T/H) if H exceeds 3.5 T. Thus the AF ordering, the 4f moment fluctuations and the Kondo screening are drastically suppressed by the application of fields H of the order of 3.5 T.     

A novel double-peaked spin-glass susceptibility-temperature response in the ternary alloy Fe69Mn26Cr5

We have studied the low-field (B ≦ 10^?2 T) d.c. susceptibility χ of the austenitic stainless-steel alloy Fe₆₉Mn₂₆Cr₅ as a function of the magnetic field B and temperature T. χ(T) shows structure, strong B dependence, and typical irreversible effects. The range of temperatures studied comprises three distinct regions. In the high-temperature region (300 K ≦ T ≦ 380 K) a blunt peak in the susceptibility is noticed at T₂ = 340 K. T₂ was not sensitive to thermal cycling. χ(T) displayed a sharp cusp at T₁ = 200 K. This peak was sensitive to the thermal history of the sample and was strongly suppressed by B. Between T₁ and T₂ a shallow valley with some hysteresis was observed. We interpret this behavior to be due to a low-temperature pure spin-glass phase, a high temperature conventional paramagnetic phase, and coexisting antiferromagnetic and spin-glass phases between T₁ and T₂.     

Sb Magnetic Resonance as a Local Probe for the Gap Formation in the Correlated Semimetal FeSb2

We report on a comparative study of the narrow-band semimetals FeSb₂ and its structural homologue RuSb₂ by means of ^121,123Sb nuclear quadrupole (NQR) and nuclear magnetic resonance (NMR) spectroscopy. From NQR for both compounds two temperature regimes could be identified by use of ¹²³(1/T ₁) measurements. Above 40 K a conventional activated behavior (with Δ/k _B ? 400 K for FeSb₂) dominates in ¹²³(1/T ₁), whereas below 40 K in both systems an unconventional ¹²³(1/T ₁) behavior with a smooth maximum at around 10 K is observed. To analyze this behavior, we propose the presence of T-dependent in-gap states forming a narrow energy level of localized spins with S = ½ near the bottom of the conduction band. These states might have originated from an inherent Sb-deficiency in both compounds. This model enables us to fit the ¹²³(1/T ₁) data in the entire investigated temperature range (2–200 K) for FeSb₂. Ab initio band structure calculations reveal more than a factor of two larger Δ value for RuSb₂ as compared with FeSb₂. This results in dissimilar behavior of ¹²³(1/T ₁) in FeSb₂ and RuSb₂ above 40 K evidencing the inefficiency of thermal activation of electrons over the large energy gap at T ≤ 300 K in RuSb₂ and dominating of quadrupole relaxation channel in RuSb₂ in this temperature range caused by phonon relaxation involving two-phonon (Raman) scattering. In addition, extra wide range field-sweep NMR measurements are performed at various temperatures on FeSb₂ and RuSb₂. The complex broad spectra could be modeled and from the shift of the ¹²¹Sb central transition the 3d component of the shift K _3d (T) could be extracted.     

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.     

An ESR study of phase transitions in crystals of M(BF4)2·6H2O (M = Fe,Co, Ni,Zn)

From a temperature dependent ESR study of Mn²⁺-doped crystals of M(BF₄)₂·6H₂O, M Zn, Co and Ni, new structural phase transitions have been detected and studied. First order structural phase transitions occur in Co(BF₄)₂·6H₂O at T₁ ~ 281K, T₂~189 K and T₃~172K (during cooling), in Zn(BF₄)₂·6H₂O at T₁ ~ 286 K and in Ni(BF₄)₂·6H₂O at T₁ ~ 301 K. A continuous phase transition occurs in Co(BF₄)₂·6H₂O at T_p ~ 257 K, in Zn(BF₄)₂·6H₂O at T_p ~ 277 K and in Ni(BF₄)₂·6H₂O at T_p ~ 294 K. The ESR spectral characteristics suggest similarities in the structures of these fluoroborate compounds in the phase above T₁ with the room temperature structure of Mg(ClO₄)₂·6H₂O. All these compounds are found to have a tendency to crystallise in a triply-twinned pseudo-hexagonal form, although the unit cell above T₁ is found to be orthorhombic. The structural changes related to the water octahedron around the metal at T₁ were found to be very small and basically the same for these three compounds. Although the unit cell structure of Fe(BF₄)₂·6H₂O above the first order phase transition temperature T₁ was found to be similar to that of the other fluoroborate compounds, the structural changes occurring at T₁ appeared to be quite different. The low temperature thermal behaviour differs considerably in the Co, Fe and Zn compounds.     

Mössbauer and magnetization studies of the U1−xNpxO2 fluorites

²³⁷Np Mössbauer effect and magnetic susceptibility measurements of the U_1?Np_xO₂ fluorite solid solution have been performed in the composition range 0.15 ? x ? 0.75. For x = 0.15 and 0.25, the Np ions order magnetically at a lower temperature T₀ than the bulk material (T_N) (T₀ ～ 19 K, T_N ～ 27 K for x = 0.15). For x = 0.50, T₀ ～ 10 K (T_N ～ 12 K from recent neutron diffraction measurements). For x = 0.75, T₀ ～ T_N ～ 9 K. The Np (induced) ordered moment is ～ 0.5 μ_B. The ²³⁷Np Mössbauer isomer shift shows that the Np ions are in a IV charge state.     

Critical behavior of heat capacity in the vicinity of phase transitions in [NH2(CH3)2]5Cd3Cl11

The heat capacity of a [NH₂(CH₃)₂]₅Cd₃Cl₁₁ crystal was studied calorimetrically in the temperature interval 100–300 K. The C _p (T) dependence indicates that, as the temperature is lowered, phase transitions occur at temperatures T ₁ = 176.5 K and T ₂ = 123.5 K. The thermodynamic characteristics of this crystal were determined. It is shown that the transition at T ₂ = 123.5 K is an incommensurate-commensurate phase transformation and that the transition at T ₁ = 176.5 K is a normal-incommensurate phase transition.     

Picosecond dynamics of photoinduced electron transfer involving singlet and triplet states in nitroporphyrin molecules

The dynamics of relaxation processes in meso-ortho-nitrophenyl-substituted octaethylporphyrins OEP-Ph(o-NO₂) and PdOEP-Ph(o-NO₂) occurring with the participation of the S ₁ and T ₁ states is studied in polar (dimethylformamide) and nonpolar (toluene) media at 295 and 77 K using pico-and nanosecond laser kinetic spectroscopy. It is shown that, at 295 K, the steric interactions between β-alkyl substituents in porphyrins and ortho-nitro groups of meso-phenyl in these compounds create optimal conditions for overlapping of molecular orbitals of the donor and the acceptor, which ensures an efficient photoinduced electron transfer (PET). It is ascertained that, in free-base OEP-Ph(o-NO₂), the PET occurs only via the S ₁ state of porphyrin in time intervals of 40 ps (dimethylformamide) and 125 ps (toluene), whereas the competing intersystem crossing S ₁ ? T ₁ is unlikely. In the case of the metal complex PdOEP-Ph(o-NO₂), the PET involves both S ₁ and T ₁ states. In the PdOEP-Ph(o-NO₂) molecule, direct PET from the T ₁ state to the charge-transfer state also occurs in the picosecond range (20 and 46 ps for dimethylformamide and toluene, respectively, at 295 K), with the rate constants being 3–5 times smaller in comparison with the corresponding values found for the PET occurring via the S ₁ state of this compound. For both compounds, the experimentally observed long-lived component in the decay kinetics of the induced T ₁-T _n absorption (250–700 ns) is due to the recombination processes in ion-radical pairs whose lifetime decreases with an increase in the polarity of the medium and is almost independent of the presence of molecular oxygen in the solution. The PET is completely absent at 77 K in the compounds under investigation when they are dissolved in rigid solutions.     

Low energy electron diffraction beam profiles and phase transition at Si(111)-7 X 7 surface

Angular profiles of low energy electron diffraction (LEED) beams from Si(111)-7 × 7 are measured for various crystal temperatures T near the phase transition with apparent critical temperature T_c ≈ 1140 K. From analyses of the profiles it is concluded that (1) long range superstructure order persists for T up to at least 50 K above T_c and (2) with increasing T the correlation length characterizing the short-range order peaks for T ≈ T_c ? 100 K and decreases rapidly for T >T_c. Conclusion (1) is discussed with reference to a dislocation network model of Si(111)-7 × 7 reconstruction.     

Structural, magnetotransport and morphological studies of Sb-doped La2/3Ba1/3MnO3 ceramic perovskites

We report here the structural, magnetotransport and morphological studies of Sb-doped La_2/3Ba_1/3Mn_1−xSb_xO₃ perovskite manganites. Pristine material La_2/3Ba_1/3MnO₃ (LBMO) shows two insulator-metal (I-M) transitions in the electrical resistivity-temperature (ρ-T) behavior. While the higher temperature transition (T_P1) at ∼340 K is reminiscent of the usual I-M transition in manganites, the lower temperature transition (T_P2) at ∼250 K has been ascribed to the grain boundary (GB) effects arising out of the ionic size mismatch between the ions present at the rare-earth site (La³⁺ and Ba²⁺). With Sb-doping T_P1 shifts to lower temperatures while T_P2 remains invariant up to 3% and shifts to lower temperature for 5%. Room temperature electrical resistivity and the peak values also increase successively with Sb-doping. Scanning electron micrographs of the samples exhibit a gradual increase in their grain sizes with Sb indicating a gradual decrease in the GB density. Shift of T_P1 with doping is explained on the basis of a competition between double-exchange and super-exchange mechanisms. The overall electrical resistivity increases and the shift in the electrical resistivity hump (T_P2) with Sb-doping is found related to be gradually decreasing GB density and the ensuing lattice strain increase at the GBs. The intrinsic magnetoresistance (MR) gets suppressed and extrinsic MR gets enhanced with Sb-doping. At T>T_P1, the electrical resistivity is found to follow the adiabatic polaron hopping model whereas the electron-magnon scattering is found to dominate in the metallic regime (T<T_P1).     

An experimental study of the collisional broadening of the Na-D lines by Ar and N2 perturbers in flames and vapor cells—II. The line wings

In this paper, we report on measurements of the fluorescence-excitation profile of the red Na-D₁ line wing from about 1 cm^?1 to 1100 cm^?1 and of the blue Na-D₂ line-wing from about 1 cm^?1 to 600 cm^?1, with respect to the corresponding line centre. We have also measured the fluorescence-excitation profile of the red Na-D₂ and blue Na-D₁ inner (overlapping) line wings from 1 cm^?1 with respect to the centre of each of the lines. All these wings were measured in several premixed, laminar, shielded H₂O₂Ar and H₂O₂N₂ flames at 1 atm (1400 K ? T ? 2300 K). We also measured these wings in a vapor cell containing Ar or N₂ perturbers (T ～ 480 K, p ～ 0.4 atm) in order to determine the influence of temperature on these wings. Using a tunable CW dye laser as excitation source, we determined the fluorescence-excitation profiles by measuring the total fluorescence intensity while tuning the laser wavelength. In order to specify the contributions that the different kinds of major flame perturbers (Ar, N₂, H₂O) make to the wings, we compared the wing profiles measured in various flames of different flame-gas compositions. In this comparison, the wing profiles were normalized with respect to the line-centre. We compared our measurements of far red Na-D₁ and blue Na-D₂ line-wings ($\text{Δσ ? 30}\text{cm}{}^{\mathrm{?1}}$) specified to Ar perturbers (at T = 500 K and T = 2000 K) with the results derived from quasi-static theory using available interaction potential data. In the case of Ar perturbers at T = 500 K, we observed a satellite at about 8 cm^?1 from the line-centre on the red Na-D₁ wing: this satellite was absent at T = 2000 K. The position of this red satellite was explained with the help of a set of “modified” potentials, which were constructed for interpreting the collisional Na-D broadening- and shift-rates deduced from our line-core observations and which were reported in Part I of this paper.     

The pressure effect on the CDW-transition temperatures in 1T-TaS2

The pressure dependence of the two CDW transition temperatures (T_d1? 350 K and T_d2 ～ 190 K) in 1T-TaS₂ has been investigated by measuring the resistivity up to 5 kbar. It is found that the thermal hysteresis of the phase transition at T_d2 is very sensitive to the pressure and is interpreted by the phenomenological Landau theory including cubic term.     

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)