NMR study of hydrogen molybdenum bronzes: H1.71MoO3 and H0.36MoO3

Proton NMR relaxation times (T₂T₁, and T_1?) and absorption spectra are reported for the compounds H_1.71MoO₃ (red monoclinic) and H_0.36MoO₃ (blue orthorhombic) in the temperature range 77 K < T < 450 K. Rigid lattice dipolar spectra show that both compounds contain proton pairs, as OH₂ groups coordinated to Mo atoms in H_1.71MoO₃ and as pairs of OH groups in H_0.36MoO₃. The room temperature lineshape for H_1.71MoO₃ shows that the average chemical shielding tensor has a total anisotropy of 20.1 ppm. The relaxation measurements confirm that hydrogen diffusion occurs and give E_A = 22 kJ mole^?1 and $\text{τ}{}^0{}_{\mathrm{<mtext>C</mtext>}}\text{? 10}{}^{\mathrm{?13}}\text{sec}$ for H_1.71MoO₃ and E_A = 11 kJ mole^?1 and $\text{τ}{}^0{}_{\mathrm{<mtext>C</mtext>}}\text{? 3 × 10}{}^{\mathrm{?8}}\text{sec}$ for H_0.36MoO₃.     

Pulsed NMR study of NH4+ β-alumina

Proton NMR relaxation times T₂, T₁, and T_1? are reported for NH₄⁺ β-alumina powder in the temperature range 77 K < T < 500 K at 16 MHz. The measurements show that the NH₄⁺ ions both reorient and translate. The translational process can be characterized by the parameters E = 20 kJ mole^?1 and τ⁰_d = 3 × 10^?11 sec. Relaxation at high temperatures is dominated by dipolar coupling to paramagnetic impurities. Reasons for the different activation energies measured using NMR and other techniques for β-alumina compounds are discussed.     

NMR study of water reorientation in molybdic acids: MoO3 · 2H2O and yellow MoO3 · H2O

Proton NMR relaxation times (T₂, T₁, T_1?) are reported for powder samples of MoO₃ · 2H₂O and yellow MoO₃ · H₂O in the temperature range 150–325 K and at 20 and 60 MHz. No translation of hydrogen atoms is detected but the spin-lattice relaxation behavior indicates reorientation of H₂O molecules. The waters coordinated to Mo atoms undergo 180° flips (about their C₂ axes) with similar motional parameters in both compounds. The interlayer waters in MoO₃ · 2H₂O undergo 180° flips with different parameters. An assumed Arrhenius-type temperature dependence of correlation times leads to preexponential factors which are “anomalously” low. The possible involvement of temperature-dependent activation barriers is discussed.     

Ion-beam irradiation of lanthanum compounds in the systems La2O3-Al2O3 and La2O3-TiO2

Thin crystals of La₂O₃, LaAlO₃, La_2/3TiO₃, La₂TiO₅, and La₂Ti₂O₇ have been irradiated in situ using 1 MeV Kr²⁺ ions at the Intermediate Voltage Electron Microscope-Tandem User Facility (IVEM-Tandem), Argonne National Laboratory (ANL). We observed that La₂O₃ remained crystalline to a fluence greater than 3.1×10¹⁶ ions cm⁻² at a temperature of 50 K. The four binary oxide compounds in the two systems were observed through the crystalline-amorphous transition as a function of ion fluence and temperature. Results from the ion irradiations give critical temperatures for amorphisation (T_c) of 647 K for LaAlO₃, 840 K for La₂Ti₂O₇, 865 K for La_2/3TiO₃, and 1027 K for La₂TiO₅. The T_c values observed in this study, together with previous data for Al₂O₃ and TiO₂, are discussed with reference to the melting points for the La₂O₃-Al₂O₃ and La₂O₃-TiO₂ systems and the different local environments within the four crystal structures. Results suggest that there is an observable inverse correlation between T_c and melting temperature (T_m) in the two systems. More complex relationships exist between T_c and crystal structure, with the stoichiometric perovskite LaAlO₃ being the most resistant to amorphisation.     

NMR study of the order-disorder phase transitions of KHCO3 and KDCO3 single crystals

KHCO₃ and its deuterated analogue KDCO₃ are typical materials that undergo order-disorder phase transitions at 318 and 353 K, respectively. The spin-lattice relaxation times, T₁, spin-spin relaxation times, T₂, and the number of resonance lines for the ¹H, ²D, and ³⁹K nuclei of these crystals were investigated using NMR spectrometer. These materials are known to exhibit anomalous decreases in T₁ near T_C, which have been attributed to a structural phase transition. Additionally, changes in the symmetry of the (HCO₃)₂²⁻ (or (DCO₃)₂²⁻) dimers in these materials are associated with large changes in T₁, T₂, and the number of resonance lines. Here we found that the resonance lines for ¹H, ²D, and ³⁹K nuclei decrease in number as the temperature is increased up to T_C, indicating that the orientations of the (HCO₃)₂²⁻ (or (DCO₃)₂²⁻) dimers and the environments of the K ions change at T_C. Moreover, based on number of resonance lines, the results further indicate that the (HCO₃)₂²⁻ (or (DCO₃)₂²⁻) dimers reorientate to approximately parallel to the directions of the hydrogen bonds (or deuteron bonds) and the direction of the a-axis. The transitions at 318 and 345 K of the two crystals are of the order-disorder type. The present results therefore indicate that the orientations of the (HCO₃)₂²⁻ and (DCO₃)₂²⁻ dimers and the environment of the K ion play a significant role in these phase transitions.     

Thermally induced incomplete high-spin (5T2) ⇌ low-spin (1A1) transition in solid bis[2-(2-pyridylamino)-4-(2-pyridyl) thiazolato]iron (II)

The ⁵⁷Fe Mössbauer effect in two samples (A and B) of [Fe(papt)₂] and in its solvates with CHCl₃ and C₆H₆ has been studied between 4.2 and 343 K and clearly indicates a temperature induced high-spin (⁵T₂) ? low-spin (¹A₁) transition in these compounds [paptH = 2-(2-pyridylamino)-4-(2-pyridyl) thiazole]. At 343 K, sample B shows a doublet with ΔE_Q = 2.03 mm s^?1 and δIS = +0.87 mm s^?1, characteristic of a ⁵T₂ ground state. At 257 K, a second doublet, typical for a ¹A₁ ground state, is observed and its intensity increases as the transition progresses but levels off below ～ 100 K. At 4.2 K, 83% of the intensity is due to the ¹A₁ state, and ΔE_Q(¹A₁) = 1.56 mm s^?1 and δ^IS(¹A₁ = +0.32 mm s^?1. In an applied magnetic field, V_zz(¹A₁) < 0 and η ≈ 0.7 have been determined, whereas for the ^sT₂ ground state, V_zz(^sT₂) > 0, η ≈ 0.75, and an internal hyperfine field H_n ≈ ?13 kG have been observed. Similar results have been obtained with the other samples.Debye-Waller factors f5_T2 and f1_A1 were determined from the saturation corrected areas in the Mössbauer spectra, assuming Curie-Weiss dependence of the magnetic susceptibility for the ⁵T₂ and constant υ_cff for the ¹A₁ ground state. The temperature dependence of ?In f1_A1 closely follows the Debye model with Θ1_A1 = 165 K, whereas the same applies to ?ln f5_T2 only above ～ 210 K and Θ5_T2 = 134 K. The nature of the observed transition is discussed and the data presented are shown to be incompatible with a model based on a Boltzmann distribution between the two states.     

On the magnetic structure of DyNiO3

The crystallographic structure of DyNiO₃ has been investigated at T=200, 100, and 2 K from high-resolution neutron powder diffraction (NPD) data. We show that the structure is monoclinic, space group P2₁/n, from the metal-insulator transition temperature at T_MI=564 K down to 2 K. The Ni atoms occupy two different sites 2d (Ni1) and 2c (Ni2), whose valences, estimated from bond-valence consideration, are +2.43(1) and +3.44(1) at 2 K, respectively. This is interpreted as the result of a partial charge disproportionation of the type 2Ni³⁺→Ni1^(3−δ)++Ni2^(3+δ)+, with δ≈0.55 at T=2 K. The magnetic structure has been studied from a NPD pattern at T=2 K, well below the establishment of the antiferromagnetic (AFM) ordering at T_N=154 K, as well as from sequential data collected from 16 K down to 2 K. The magnetic order is defined by the propagation vector k=(1/2,0,1/2). Two possible magnetic structures are compatible with the magnetic intensities. In the second solution both Ni sublattices participate in the magnetic order, as well as Dy since it corresponds to a total disproportionation of Ni³⁺ to Ni²⁺ and Ni⁴⁺. In the second solution both Ni sublattices participate in the magnetic order, as well as Dy. The magnetic moments for Ni1 and Ni2 atoms at T=2 K are 1.8 (2) and 0.8 (2) μ_B, respectively. These values are also compatible with a partial charge disproportionation. Dy³⁺ ions exhibit long-range magnetic ordering below 8 K. An abrupt contraction of the unit-cell volume is observed at this temperature, due to a magnetoelastic coupling. The magnetic moment for Dy³⁺ at T=2 K is 7.87 (6) μ_B.     

Neutron powder diffraction on RbCrl3 and magnetic measurements on RbCrl3 and CsCrl3

β-RbCrI₃ (a = 13.772(3), b = 8.000(2), c = 7.069(2) Å β = 95.85(1)°, $\text{Z = 4,}\text{C2}\text{m}$ at 293 K) and γ-RbCrI₃ (a = 13.586(2), b = 7.923(2), c = 14.094(3) Å, β = 96.88(1)°, Z = 8, C2 at 1.2 K) are isostructural to β-RbCrCl₃ and γ-RbCrCl₃ and are both Jahn-Teller distorted BaNiO₃ structures. In both compounds elongated octahedra occur. γ-RbCrI₃ most probably has a magnetic spiral structure at 4.2 and 1.2 K. Theoretically, a spiral propagating along the b axis is expected. A model with k = 9/19b1 yielded the best result. However, no good fit was obtained possibly because of a misfit in k and canting of the magnetic moments due to anisotropy. χ vs T single-crystal measurements on β-CsCrI₃ are in accordance with its magnetic structure. The three-dimensional magnetic ordering temperature T_c is estimated as 27(1) K. From the χ vs T curves of γ-RbCrI₃, T_c could not be determined. From fits to χ vs T powder data $\text{J}\text{k}$ of CsCrI₃ and RbCrI₃ are estimated to be ?14(2) and ?11(1) K, respectively.     

Thermodynamic properties of BaCeO3 and BaZrO3 at low temperatures

Specific heat capacities (C_p) of polycrystalline samples of BaCeO₃ and BaZrO₃ have been measured from about 1.6 K up to room temperature by means of adiabatic calorimetry. We provide corrected experimental data for the heat capacity of BaCeO₃ in the range T < 10 K and, for the first time, contribute experimental data below 53 K for BaZrO₃. Applying Debye's T³-law for T → 0 K, thermodynamic functions as molar entropy and enthalpy are derived by integration. We obtain C_p = 114.8 (±1.0) J mol⁻¹ K⁻¹, S° = 145.8 (±0.7) J mol⁻¹ K⁻¹ for BaCeO₃ and C_p = 107.0 (±1.0) J mol⁻¹ K⁻¹, S° = 125.5 (±0.6) J mol⁻¹ K⁻¹ for BaZrO₃ at 298.15 K. These results are in overall agreement with previously reported studies but slightly deviating, in both cases. Evaluations of C_p(T) yield Debye temperatures and identify deviations from the simple Debye-theory due to extra vibrational modes as well as anharmonicity. The anharmonicity turns out to be more pronounced at elevated temperatures for BaCeO₃. The characteristic Debye temperatures determined at T = 0 K are Θ₀ = 365 (±6) K for BaCeO₃ and Θ₀ = 402 (±9) K for BaZrO₃.     

Thermal and physical properties of hollandite-type K1.3Mn8O16 and (K,H3O)xMn8O16

Two kinds of samples of cryptomelane: synthetic single crystals of K_1.33Mn₈O₁₆ (A) and (K,H₃O)_xMn₈O₁₆ powder prepared by aqueous chemistry (B) were studied by thermogravimetry, magnetic, and electrical (dc and ac) measurements. B loses water at 100–185°C. A and B are decomposed in the range 460–610°C into Mn₂O₃ in air and MnO, under vacuum. They are antiferromagnetic with T_N 18 K (A), 11 K (B). A is a semiconductor with σ(300 K) ～ 3 Ω^?1 m^?1 and E_σ = 0.38 eV. The ac measurements did not reveal any significant contribution of ionic conduction up to 740 K.     

Li+ ion conductivity in the system Li4SiO4Li3VO4

Two ranges of solid solutions were prepared in the system Li₄SiO₄Li₃VO₄: Li_4?xSi_1?xV_xO₄, 0 < x ? 0.37 with the Li₄SiO₄ structure and Li_3+yV_1?ySi_yO₄, 0.18 ? y ? 0.53 with a γ structure. The conductivity of both solid solutions is much higher than that of the end members and passes through a maximum at ～40Li₄SiO₄ · 60Li₃VO₄ with values of ～1 × 10^?5 ohm^?1 cm^?1 at 20°C, rising to ～4 × 10^?2 ohm^?1 cm^?1 at 300°C. These conductivities are several times higher than in the corresponding Li₄SiO₄Li₃(P,As)O₄ systems, especially at room temperature. The solid solutions are easy to prepare, are stable in air, and maintain their conductivity with time. The mechanism of conduction is discussed in terms of the random-walk equation for conductivity and the significance of the term c(1 ? c) in the preexponential factor is assessed. Data for the three systems Li₄SiO₄Li₃YO₄ (Y = P, As. V) are compared.     

Structures isomorphes MeX2O4—Evolution structurale entre 2 K et 300 K de l'antimonite FeSb2O4: Elasticité et ordre magnétique anisotropes

Neutron diffraction has been used to study the whole structural evolution of the antimony oxide FeSb₂O₄, from 2 to 300 K. The antiferromagnetic order has been investigated: at 2 K the magnetic moment is M = 3.8 μ_B. An extrapolated Néel temperature T_N = 45 ± 6 K is observed. The function M(T) below T_N is similar to that found in the isomorphous NiSb₂O₄. Magnetostrictive effects are observed. Above 70 K, the thermal expansion tensor is anisotropic with α_a ? α_c. Using the anisotropic temperature factors B_a (Ų), B_c (Ų) at 2 and 300 K, anisotropic Debye temperatures are calculated. Then, using simple approximations, mean force constants F_a, F_c are calculated; they allow to evaluate the anisotropic compressibility coefficients χ_a ～ 0.857 × 10^?11Pa^?1, χ_c ～ 0.467 × 10^?11Pa^?1; the value of the Grüneisen constant is $\text{γ}\text{= 0.33}$.     

Dynamics of superionic Rb3H(SeO4)2 single crystals studied by H and Rb spin-lattice relaxation

The ¹H and ⁸⁷Rb spin-lattice relaxation and spin-spin relaxation times in superionic Rb₃H(SeO₄)₂ single crystals grown by the slow evaporation method were measured over the temperature range 160-450 K. The temperature dependencies of the ¹H T₁, T_1ρ, and T₂ are measured. In the ferroelastic phase, T₁ differs from T_1ρ, which is in turn different from T₂, although these three relaxation times converge to similar values near 410 K. This transition seems to occur at temperature which is about 40 K lower than the superionic transition temperature. The observation of liquid-like values of the ¹H T₁, T_1ρ, and T₂ in the high temperature is compatible with the phase being superionic, indicating that the destruction and reconstruction of hydrogen bonds does indeed occur at high temperature. In addition, the ⁸⁷Rb T₁ and T₂ values at high temperature were similar (on the order of milliseconds), a trend that was also observed for ¹H T₁ and T₂. This behavior is expected for most hopping-type ionic conductors, and could be attributed to interactions between the mobile ions and the neighboring group ions within the crystal. The motion giving rise to this liquid-like behavior is related to the superionic motion.     

Neutron powder diffraction and magnetic measurements on RbTil3, RbVI3, and CsVI3

CsVI₃ (a = 8.124(1) c = 6.774(1)Å,Z = 2, P6₃/mmc at 293 K) adopts the BaNiO₃ structure. Three-dimensional magnetic ordering takes place atT_c = 32(1)K. At 1.2 K the magnetic moment is 1.64(5) μ_B and it forms a 120° spin structure in the basal plane. RbVI₃ (a = 13.863(2) c = 6.807(1) Å,Z = 6, P6₃cmor P3¯c1 at 293 K) and RbTiI₃ (a = 14.024(3) Å,c = 6.796(2) Å,Z = 6, P6₃cm orP3¯c1 at 293 K) adopt a distorted BaNiO₃ structure, probably isostructural with KNiCl₃.T_c of RbVI₃ is 25(1) K. At 1.2 K, RbVI₃ has a spin structure similar to the one of CsVI₃ with a magnetic moment of 1.44(6) μ_B. RbTiI₃ shows no magnetic ordering at 4.2 K. It is shown that a deviation from the 120° structure is expected for compounds with a distorted BaNiO₃ structure such as RbVI₃. The cell dimensions of CsTiI₃ are reported.     

Thermal expansion in (La0.62Pb0.38)MnO3: Possible evidence for a coupling between elastic and magnetic exchange forces

Using single-crystal, automated diffractometer techniques, the linear coefficient of thermal expansion has been determined for La_0.62Pb_0.38MnO₃ from 298 to 627 K. The linear coefficient of thermal expansion is observed to undergo a change from 7.2 × 10^?5 Å/K for T < T_c to 10.8 × 10^?5 Å/K for T > T_c. It is concluded that while the rhombohedral distortion in the (La, Pb)MnO₃ system can be understood qualitatively on the basis of ionic size and polarizability considerations alone, the quantitative systematics of the distortion parameters and the change in the linear thermal expansion coefficient at T_c indicate a significant coupling between the elastic and magnetic exchange forces.     

Energy transfer in linear chain manganese salts: Emission spectra of CsMnBr3, RbMnBr3 and CsMnI3 crystals doped with Er3+

The luminescence spectra of CsMnBr₃, RbMnBr₃ and CsMnI₃ crystals doped with Er³⁺ have been investigated in the 10–300 K temperature range. These linear chain manganese salts behave as “pseudo” one-dimensional antiferromagnets. The temperature dependence of luminescence from the doped materials indicates a rapid thermally activated transfer of excitation energy from the Mn²⁺ ions of the bulk crystal to the Er³⁺ impurity centers. Analysis of the data suggests that energy migration occurs by two distinct processes with activation barriers of: 200 and 500 cm^?1 for CsMnBr₃, 180 and 500 cm^?1 for RbMnBr₃ and 300 and 650 cm^?1 for CsMnI₃. The behavior of CsMn_xCd_1-xBr₃ crystals containing Er³⁺ clearly indicates that the process with the higher activation barrier corresponds to two- or three-dimensional exciton migration.     

Thermodynamic studies on SrFe12O19(s), SrFe2O4(s), Sr2Fe2O5(s) and Sr3Fe2O6(s)

The citrate-nitrate gel combustion route was used to prepare SrFe₂O₄(s), Sr₂Fe₂O₅(s) and Sr₃Fe₂O₆(s) powders and the compounds were characterized by X-ray diffraction analysis. Different solid-state electrochemical cells were used for the measurement of emf as a function of temperature from 970 to 1151 K. The standard molar Gibbs energies of formation of these ternary oxides were calculated as a function of temperature from the emf data and are represented as (SrFe₂O₄, s, T)/kJ mol⁻¹ (±1.7)=−1494.8+0.3754 (T/K) (970?T/K?1151). (Sr₂Fe₂O₅, s, T)/kJ mol⁻¹ (±3.0)=−2119.3+0.4461 (T/K) (970?T/K?1149). (Sr₃Fe₂O₆, s, T)/kJ mol⁻¹ (±7.3)=−2719.8+0.4974 (T/K) (969?T/K?1150).Standard molar heat capacities of these ternary oxides were determined from 310 to 820 K using a heat flux type differential scanning calorimeter (DSC). Based on second law analysis and using the thermodynamic database FactSage software, thermodynamic functions such as Δ_fH°(298.15 K), S°(298.15 K) S°(T), C_p°(T), H°(T), {H°(T)-H°(298.15 K)}, G°(T), free energy function (fef), Δ_fH°(T) and Δ_fG°(T) for these ternary oxides were also calculated from 298 to 1000 K.     

A method for obtaining potential parameters for helium from T1 measurements

The temperature dependence of T₁ for ³He gas in the range 0–4°K is calculated for a Lennard-Jones (12,6) potential. The relaxation of the nuclear spins is assumed to be due to a dipolar interaction between the nuclei. A minimum value in the relaxation time, T_1,min, is found to occur at a temperature denoted by T_min. By repeating the calculation for different pairs of values of the potential parameters ? and σ, we have found that for a density of 10^?2 g/cm³,σ²T_min = 13.0?1.12 × 10³ ?σ², T_1,min/σ²(T_min)^{$\text{1}\text{2}$} = 17.4?6.56 × 10² ?σ², with ?, σ, T_min and T_1,min in eV, Å, °K and minutes, respectively. From measurements of T_min and T_1,min, ? and σ can be determined.     

Thermodynamic Studies on NdFeO3(s)

The enthalpy increments and the standard molar Gibbs energy of formation of NdFeO₃(s) have been measured using a high-temperature Calvet microcalorimeter and a solid oxide galvanic cell, respectively. A λ-type transition, related to magnetic order-disorder transformation (antiferromagnetic to paramagnetic), is apparent from the heat capacity data at ∼687 K. Enthalpy increments, except in the vicinity of transition, can be represented by a polynomial expression: {H°_m(T)−H°_m(298.15 K)}/J·mol⁻¹ (±0.7%)=−53625.6+146.0(T/K) +1.150×10⁻⁴(T/K)² +3.007×10⁶(T/K)⁻¹; (298.15≤T/K ≤1000). The heat capacity, the first differential of {H°_m(T)−H°_m(298.15 K)} with respect to temperature, is given by C^o_{p, m}/J·K⁻¹·mol⁻¹=146.0+2.30×10⁻⁴(T/K)−3.007×10⁶(T/K)⁻². The reversible emf's of the cell, (−) Pt/{NdFeO₃(s) +Nd₂O₃(s)+Fe(s)}//YDT/CSZ//{Fe(s)‘FeO’(s)}/Pt(+), were measured in the temperature range from 1004 to 1208 K. It can be represented within experimental error by a linear equation: E/V:(0.1418±0.0003)−(3.890±0.023)×10⁻⁵(T/K). The Gibbs energy of formation of solid NdFeO₃ calculated by the least-squares regression analysis of the data obtained in the present study, and data for Fe_0.95O and Nd₂O₃ from the literature, is given by Δ_fG°_m(NdFeO₃, s)/kJ·mol⁻¹(±2.0)=−1345.9+0.2542(T/K); (1000≤T/K ≤1650). The error in Δ_fG°_m(NdFeO₃, s, T) includes the standard deviation in emf and the uncertainty in the data taken from the literature. Values of Δ_fH°_m(NdFeO₃, s, 298.15 K) and S°_m(NdFeO₃, s, 298.15 K) calculated by the second law method are −1362.5 (±6) kJ·mol⁻¹ and 123.9 (±2.5) J·K⁻¹·mol⁻¹, respectively. Based on the thermodynamic information, an oxygen potential diagram for the system Nd-Fe-O was developed at 1350 K.     

Rate coefficients for so reactions with O2 and O3 over the temperature range 230 to 420 K

SO was produced from SO₂ by pholodissociation with an ArFlaser (193 nm). SO₂ chemiluminescence from the SO + O₃ reaction was used to monitor the decay of SO and determine rate coefficients for SO reactions with O₂ and O₃ over the temperature range 230–420 K. The rate expressions are k_O2=(2.4^+2.6_?0.9) x 10^?13 exp[(?2370⁺²⁰⁰_?250)/T] and k_o3=(4.8^+1.6_?0.8) × 10^?12 exp[(?1170⁺⁸⁰_?120)/T] cm³ molecule^?1 s^?1.