Spectroscopic investigation of [(NH4)xK1−x]2SnCl6 mixed crystals

Polycrystalline samples of the solid solution [(NH₄)_xK_1?x]₂SnCl₆(0?x? 1) have been investigated by DSC, X-ray diffraction and Raman scattering experiments. Substitution of K⁺ by NH⁺₄ depresses the phase transition temperature T₁. For 0? x ?0.05 a linear temperature coefficient $\text{d}\text{T}{}_1\text{dx=?5.16}\text{K/mol}$ % is obtained. The cubic lattice constant roughly obeys Vegard's law, whereas the linewidth of the SnCl₆^2?F_2g internal vibration displays a nonlinear dependence on composition.     

H nuclear magnetic resonance study of the ferroelastic and superionic phase transitions of (NH4)4LiH3(SO4)4 single crystals

We investigated the temperature dependences of the line shape, spin-lattice relaxation time, T₁, and spin-spin relaxation time, T₂, of the ¹H nuclei in (NH₄)₄LiH₃(SO₄)₄ single crystals. On the basis of the data obtained, we were able to distinguish the “ammonium” and “hydrogen-bond” protons in the crystals. For both the ammonium and hydrogen-bond protons in (NH₄)₄LiH₃(SO₄)₄, the curves of T₁ and T₂ versus temperature changed significantly near the ferroelastic and superionic phase transitions at T_C (=232 K) and T_S (=405 K), respectively. In particular, near T_S, the ¹H signal due to the hydrogen-bond protons abruptly narrowed and the T₂ value for these protons abruptly increased, indicating that these protons play an important role in this superionic phase transition. The marked increase in the T₂ of the hydrogen-bond protons above T_S indicates that the breaking of O-H?O bonds and the formation of new H-bonds with HSO₄^- contribute significantly to the high-temperature conductivity of (NH₄)₄LiH₃(SO₄)₄ crystals.     

Phase transitions and molecular motions in [Zn(NH3)4](BF4)2 studied by nuclear magnetic resonance, infrared and Raman spectroscopy

Middle infrared absorption, Raman scattering and proton magnetic resonance relaxation measurements were performed for [Zn(NH₃)₄](BF₄) in order to establish relationship between the observed phase transitions and reorientational motions of the NH₃ ligands and BF₄⁻ anions. The temperature dependence of spin-lattice relaxation time (T₁(¹H)) and of the full width at half maximum (FWHM) of the bands connected with ρ_r(NH₃), ν₂(BF₄) and ν₄(BF₄) modes in the infrared and in the Raman spectra have shown that in the high temperature phase of [Zn(NH₃)₄](BF₄)₂ all molecular groups perform the following stochastic reorientational motions: fast (τ_R≈10⁻¹² s) 120° flips of NH₃ ligands about three-fold axis, fast isotropic reorientation of BF₄⁻ anions and slow (τ_R≈10⁻⁴ s) isotropic reorientation (“tumbling”) of the whole [Zn(NH₃)₄]²⁺ cation. Mean values of the activation energies for uniaxial reorientation of NH₃ and isotropic reorientation of BF₄⁻ at phases I and II are ca. 3 kJ mol⁻¹ and ca. 5 kJ mol⁻¹, respectively. At phases III and IV the activation energies values for uniaxial reorientation of both NH₃ and of BF₄⁻ equal to ca. 7 kJ mol⁻¹. Nearly the same values of the activation energies, as well as of the reorientational correlation times, at phases III and IV well explain existence of the coupling between reorientational motions of NH₃ and BF₄⁻. Splitting some of the infrared bands at T_C2=117 K suggests reducing of crystal symmetry at this phase transition. Sudden narrowing of the bands connected with ν₂(BF₄), ν₄(BF₄) and ρ_r(NH₃) modes at T_C3=101 K implies slowing down (τ_R?10⁻¹⁰ s) of the fast uniaxial reorientational motions of the BF₄⁻ anions and NH₃ ligands at this phase transition.     

Magnetic susceptibility studies of (CH2)3(NH3)2FeCl2Br2 and (CH2)6(NH3)2FeCl2Br2

The magnetic susceptibility of the layered compounds (CH₂)₃(NH₃)₂FeCl₂Br₂ and (CH₂)₆(NH₃)₂FeCl₂Br₂ has been measured in the range 80 < T < 300 K. The results follow a Curie-Weiss behavior in the range 120 < T < 300 K but are field dependent for T < 120 K. The results are interpreted in terms of a two-dimensional antiferromagnetic interaction which is canted. A comparison with the corresponding pure chloride compounds is given.     

Polarisation remanente dans les varietes de basse temperature de (NH4) 3AlF6 ET (NH4) 3FeF6

Dielectric and thermocurrent measurements have been carried out on (NH₄) ₃AlF₆ and (NH₄) ₃FeF₆ ceramic samples. A maximum of permittivity is observed close to the transition temperature (T_{T(NH4) 3AlF6} = 217K; T_T(NH43FeF6 = 264K. In the low-temperature phase a polarization current of about 10^-9A is obtained and can be reversed when the sign of the polarization field is changed, a property which could correspond to a ferroelectric behavior. However, no pyroelectric current is detected when the temperature decreases from T_T. Another hypothesis, based on a field-induced polarization, has been considered : the depolarization current could be due to charge displacements from potential minima favored by rising temperature. In any way, the low-temperature phase is characterized by a remanent polarization.     

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.     

Two-dimensional critical behaviour in antiferrodistortive Al ur6(CIO4)3 and Ga ur6(ClO4)3

Critical behaviour with dimensionality d = 2 has been observed for the 300 K antiferrodistortive phase transition in Al ur₆(ClO₄)₃ and Ga ur₆(ClO₄)₃ by means of the temperature dependence of the ESR parameter D. The systems exhibited d = 2 behaviour in the static critical behaviour for T<T_c?40 K for T>T_c + 40 K. From the ESR data including line width measurements the local order parameter relaxation rate ω₁ has been obtained for various temperatures above T_c, with a lowest value of ω₁ = 150 MHz at T_c + 15 K     

Electron hopping mechanism in hematite (α-Fe2O3)

The frequency dependence of the real (?′) and imaginary (?″) parts of the dielectric constant of polycrystalline hematite (α-Fe₂O₃) has been investigated in the frequency range 0-100 kHz and the temperature range 190-350 K, in order to reveal experimentally the electron hopping mechanism that takes place during the Morin transition of spin-flip process. The dielectric behaviour is described well by the Debye-type relaxation (α-dispersion) in the temperature regions T<233 K and T>338 K. In the intermediate temperature range 233 K<T<338 K a charge carrier mechanism takes place (electron jump from the O²⁻ ion into one of the magnetic ions Fe³⁺) which gives rise to the low frequency conductivity and to the Ω-dispersion. The temperature dependence of relaxation time (τ) in the −ln τ vs 10³/T plot shows two linear regions. In the first, T<238 K, τ increases with increasing T implying a negative activation energy −0.01 eV, and in the second region T>318 K τ decreases as the temperature increases implying a positive activation energy 0.12 eV. The total reorganization energy (0.12-0.01) 0.11 eV is in agreement with the adiabatic activation energy 0.11 eV given by an ab initio model in the literature. The temperature dependence of the phase shift in the frequencies 1, 5, 10 kHz applied shows clearly an average Morin temperature T_Mo=284±1 K that is higher than the value of 263 K corresponding to a single crystal due to the size and shape of material grains.     

Vibrational properties of superionic glasses (AgI)x(Ag2O nB2O3)1-x

In this letter we refer on the Raman-scattering measurements in superionic glasses (AgI)_x(Ag₂O nB₂O₃)_1-x where 0 ? x ? 0.5. The behaviour of the low-frequency Raman spectra, Δν < 250 cm^?1, has been interpreted as due to a vibrational density of states mainly due to the silver halide. Nonlinear increase of the Raman efficiency with the increase of AgI concentration has been found: a phenomenological explanation is presented.     

PMR spin lattice relaxation by reorientational motions of NH3 and CH3 groups in some organic compounds

The spin-lattice relaxation times, T₁, of protons in o, m, p-phenylene-diamine dihydrochlorides C₆H₄(NH₂)₂·2HCl, phenylhydrazinium chloride C₆H₅NHNH₃Cl, hexaethylbenzene C₆(CH₂CH₃)₆, tetrabutylammonium bromide [CH₃(CH₂)₃]₄NBr, iodide [CH₃(CH₂)₃]₄NI, tetraheptylammonium bromide [CH₃(CH₂)₆]₄NBr and iodide [CH₃(CH₂)₆]₄NI powders have been measured between 400 and 100 K at 60MHz. The experimental results have been explained by considering the reorientational motions of ?NH₃⁺ and ?CH₃ groups about C₃ axes and their role of behaving as sinks to rapid spin diffusion of the ring protons of the phenylene and the methylene protons. The observed T₁, minima in all these substances turn out to be the measures of the ratios between the total number of protons and the number of reorienting ?NH₃⁺ or ?CH₃ protons. Therefore it has been concluded that the T₁, minima of ?NH₃⁺ and ?CH₃ groups, when obtainable can indicate their number present in a solid sample.     

Study of ionic motion in salts of the type (NH4)2MX6 by NMR relaxation

The proton spin-lattice relaxation time in the laboratory frame, T₁, and rotating frame T_1ρ for polycrystalline cubic (NH₄)₂SiF₆, (NH₄)₂SnBr₆ and (NH₄)₂SnCl₆ have been measured over a temperature range 60–500°K. Reorientation of the ammonium ion is generally the dominant relaxation mechanism and T₁ minima are observed in all samples. Activation energies are low in each case, being 2·2 Kcal/mole for the fluosilicate, 1·44 and 1·24 Kcal/mole for the bromo- and chloro-stannate respectively. For the bromostannate a λ-point occurs at 145°K above which the activation energy apparently decreases to 0·26 Kcal/mole. Anion reorientation is detected in the fluosilicate at high temperatures, the correlation time for this motion being obtained from T_1ρ measurements. There is also some evidence to suggest anion reorientation is becoming important in the stannihalides at high temperatures. The proton T_1ρ in the stannibromide is largely determined by the rapid quadrupolar controlled relaxation of the bromine nuclei. Values for the bromine T₁ are deduced and the quadrupolar relaxation mechanism discussed.     

The temperature dependence of the angular variation of,and the critical point exponent for the EPR linewidth in the two-dimensional canted antiferromagnetic (C2H5NH3)2MnBr4: Evidence for a structural phase transition

The angular variation of the EPR linewidths in single crystals of (C₂H₅NH₃)₂MnBr₄ has been measured as a function of temperature. The angular dependence is well characterized by δH(θ) = a + b(3 cos²θ ? 1) + c(3 cos²θ ? 1)². The temperature dependence of the expansion coefficients is reported, and the effect of critical point fluctuations near the Néel temperature as well as a linear temperature dependence at high temperature are observed. A sharp decrease in linewidths at 160°K is attributed to a structural phase transition. The Néel temperature is determined to be 46°K (± 1°) from linewidth measurements of a powder sample. The linewidths diverge exponentially near the Néel temperature with a critical point exponent of 1.5.     

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.     

Anelastic effects in (AgI)x(Ag2O·B2O3)1−x superionic glasses

We present here the first measurements of acoustic absorption in (AgI)_x(Ag₂O·B₂O₃)_1?x superionic glasses at ultrasonic frequencies (5–45 MHz) and in the 80–434 K temperature range. The attenuation shows, at decreasing temperature, an unusually high peak which seems to be attributed to a thermally activated relaxation process. A possible microscopic explanation of this anomaly is proposed, in connection with the presence of mobile ions in those materials.     

Transport properties of n-type ferromagnetic semiconductor HgCr2−xInxSe4(x = 0.100)

Measurements of the electrical conductivity, magnetoresistance, and Hall effect were performed on a n-type ferromagnetic semiconductor HgCr_2?xIn_xSe₄(x = 0.100) single crystal from 6.3 to 296 K in magnetic fields up to 1.19×l0⁶A/m. The conductivity decreases rapidly near the Curie temperatureT_c (≈120 K) as the temperature is raised. A large peak in the magnetoresistance is observed near T_c. The Hall effect measurements indicate that the temperature dependence of the conductivity and the magnetoresistance are due mostly to a change in electron mobility. The electron mobility is 1.2 × 10^?2 m²/V · s at 6.3 K, and decreases rapidly near T_c with the rise in temperature. Then it increases slowly from 5.5 × 10^?4 m²/V · s at 160 K to 7.5 × 10^?4 m²/V · s at 241 K. This temperature dependence of the electron mobility can be explained in terms of the spin-disorder scattering which takes into account the exchange interaction between charge carriers and localized magnetic moments.     

Magneto-optical measurement on the layer type magnet (CH2)2(ND3)2MnCl4

The linear birefringence (LB) of the antiferromagnet (CH₂)₂(ND₃)₂MnCl₄ has been measured as a function of temperature and in magnetic fields up to 100 kOe. The temperature dependence of the LB points to a pronounced two dimensional magnetic behaviour. No anomaly corresponding to the effect of three dimensional ordering could be detected at T_N. In theffield dependent measurements the spin flop at H_SF = 33.6 ± 1 kOe (T = 4K) could clearly be detected.     

B and Pt NMR study of the superconductors Li2(Pd1−xPtx)3B without inversion symmetry

We report the ¹¹B and ¹⁹⁵Pt NMR measurements in non-centrosymmetric superconductors Li₂(Pd_1−xPt_x)₃B (x = 0.0, 0.2, 0.5, 1.0). From the measurements of spin–lattice relaxation time (T₁), we found that there was a coherence peak (CP) just below superconducting transition temperature (T_c) for x = 0–0.5 but no CP in x = 1. We demonstrated that the system for x = 0–0.5 were BCS superconductors but there existed line node in the superconducting gap for x = 1.0. The ¹⁹⁵Pt Knight Shift in x = 0.2 decreased below T_c, indicating spin-singlet state. The results showed that BCS superconducting state evolves into an exotic state with line-nodes in the gap function when x is increased, as the spin–orbit coupling is enhanced.     

Hall effect in α-RuCl3

The Hall effect measurements performed in the layer compound α-RuCl₃ show that, in the sheets perpendicular to the c-axis, μ(300 K) ? 0.2 cm²V sec^?1; the temperature dependence of μ appears to be μ(T) = μ₀T^-n, with n = 2.3 ± 0.1, in the 180–320 K range. The transport appears to be due to electrons that move with a band type of mechanism, the main scattering process being due to the homopolar high energy phonons which modulate the thickness of the layers.     

Pressure dependence of the rotational ground state tunnel splitting in Ni(NH3)6I2

Inelastic neutron scattering techniques have been used to determine the rotational tunnel splitting of the librational ground state of ammonia molecules in Ni(NH₃)₆I₂ as a function of hydrostatic pressure up to 5.6 kbar at 8 K. The resulting exponential dependence of the tunnel splitting on pressure was interpreted with the help of a concurrent measurement of the compressibility to suggest a dependence on the interatomic distance as r^?9 of the orientational potential. The phase transition temperature at 5.6 kbar was determined to be 27 K compared with 19.9 K at atmospheric pressure.     

Crystal structure,NMR study,dielectric relaxation and AC conductivity of a new compound [Cd3(SCN)2Br6(C2H9N2)2]n

The crystal structure, the ¹³C NMR spectroscopy and the complex impedance have been carried out on [Cd₃(SCN)₂Br₆(C₂H₉N₂)₂]_n. Crystal structure shows a 2D polymeric network built up of two crystallographically independent cadmium atoms with two different octahedral coordinations. This compound exhibits a phase transition at (T=355±2 K) which has been characterized by differential scanning calorimetry (DSC), X-rays powder diffraction, AC conductivity and dielectric measurements. Examination of ¹³C CP/MAS line shapes shows indirect spin–spin coupling (¹⁴N and ¹³C) with a dipolar coupling constant of 1339 Hz. The AC conductivity of this compound has been carried out in the temperature range 325–376 K and the frequency range from 10⁻² Hz to 10 MHz. The impedance data were well fitted to two equivalent electrical circuits. The results of the modulus study reveal the presence of two distinct relaxation processes. One, at low frequency side, is thermally activated due to the ionic conduction of the crystal and the other, at higher frequency side, gradually disappears when temperature reaches 355 K which is attributed to the localized dipoles in the crystal. Moreover, the temperature dependence of DC-conductivity in both phases follows the Arrhenius law and the frequency dependence of σ(ω,T) follows Jonscher's universal law. The near values of activation energies obtained from the conductivity data and impedance confirm that the transport is through the ion hopping mechanism.