Thermodynamic properties and phase transitions of Tutton salt (NH4)2Fe(SO4)2·6H2O from MAS NMR and single-crystal NMR

The thermodynamic properties and phase transitions of Tutton salt (NH₄)₂Fe(SO₄)₂·6H₂O were investigated using thermogravimetric analysis, differential scanning calorimetry, and nuclear magnetic resonance. The first mass loss occurs around 330 K (T _d), which is interpreted as the onset of partial thermal decomposition. Phase transitions were found at 387 K (=T _C1) and 500 K (=T _C2). The temperature dependences of the spin–lattice relaxation time in the rotating frame, T _1ρ, and that in the laboratory frame, T ₁, for the H nuclei change abruptly near T _C1. These changes are associated with changes in the geometry of the arrangement of octahedral water molecules and ammonium protons.     

Identifying transition temperatures in bloodmeal-based thermoplastics using material pocket DMTA

Bloodmeal can be used to manufacture thermoplastics, but requires water, urea, sodium sulphite, and sodium dodecyl sulphate to modify chain mobility. Transition temperatures of bloodmeal, modified bloodmeal, and processed bloodmeal-based thermoplastics were compared using material pocket dynamic mechanical thermal analysis. The glass transition temperature (T _g) of bloodmeal dropped from 493 to 263 K using only water as a plasticizer but was restored when freeze dried. Modifying bloodmeal lowered T _g to 193 K. This was raised by drying, but not to that of unmodified bloodmeal indicating a permanent change. Three additional transitions were identified above T _g, for modified bloodmeal between 300 and 480 K. These were thought to be transitions of dehydrated bulk amorphous regions, amorphous regions between crystallites and chains segments in crystallites and were also seen at lower temperatures when replacing some water with tri-ethylene glycol (TEG). Material pockets increased resolution in processed samples. One broad T _g was observed in consolidated bars, at 335 or 350 K with or without TEG. In material pockets, these resolved into three transitions, similar to those observed before processing. Changes in relative magnitudes suggested some chain rearrangement leading to more bulk amorphous regions. Differences were detected between onset of drop in storage modulus and peaks in loss modulus and tan δ in pockets or bars, but generally led to the same conclusions. For bar samples, it was helpful to compare natural and log modulus scales. Good practice would use all these techniques in parallel to correctly identify relaxation temperatures.     

Heat capacity, enthalpy, entropy, and gibbs energy of terbium diboride as determined from calorimetric measurements within 5–300 K

The heat capacity at constant pressure C _p (T) of terbium diboride synthesized from elements via an intermediate hydride phase was studied experimentally within 5–300 K. A ferromagnetic phase transition manifests itself in the C _p (T) dependence as a sharp maximum at 142.4 ± 0.1 K. The C _p (T) dependence was used to calculate the tempreature dependences of the enthalpy, entropy, and the Gibbs energy and to determine the parameters of the electronic, lattice, and magnetic contributions to the heat capacity of TbB₂.     

Structure,nonstoichiometry and magnetic properties of the perovskites Sr1−xCaxMnO3−δ

The structural, thermal and magnetic properties of the perovskite-type alkaline-earth manganites of the series Sr_1−xCa_xMnO_3−δ (0⩽x⩽1) were investigated. SrMnO_3−δ forms a hexagonal perovskite lattice and shows a first-order transformation to a highly defective cubic high-temperature modification. By substituting Ca for Sr (x>0.25) the hexagonal perovskite is suppressed and a cubic (or orthorhombic) lattice becomes stabilized for all temperatures. For x=0.5 and 0.75 cubic perovskites with a large nonstoichiometry (e.g., δ=0.25 for x=0.5) are obtained at 1400 °C. The defective perovskites are prepared by either quenching from high temperature or by cooling in an inert atmosphere. The oxygen vacancies are easily filled by subsequent reoxidation at low temperature (400–600 °C) and stoichiometric samples are obtained. Orthorhombic perovskites are formed at T⩽1200 °C with the nonstoichiometry δ increasing with increasing temperature (e.g., δ=0.06 at 1000 °C and δ=0.14 at 1200 °C for x=0.5). Slow cooling in air results in almost complete reoxidation (δ=0). CaMnO_3−δ is an orthorhombic perovskite with a large range of nonstoichiometry (0⩽δ⩽0.30). The cubic to hexagonal phase transformation of the Sr-rich samples is accompanied by a large expansion of the lattice that is reduced by Ca substitution. The Ca/Sr-manganites are antiferromagnets with T_N of 170 K for x=0.5 and δ=0.02 and 120 K for x=1 and δ=0.05.     

Oxydes de plomb: V. Etude de la texture des phases quadratique et orthorhombique de Pb3O4: Influence des defauts sur la transition de phase

Several samples of Pb₃O₄ have been prepared by oxidizing PbO in air at various temperatures in the range 705–815°K. A correlation is established between the nonstochiometry of the samples and their X-ray diffraction line profiles at 295°K which are characteristic of an orthorhombic distortion of the tetragonal lattice. In the high-temperature phase (T > 170°K), orthorhombic microdomains exist in the tetragonal matrix. The mean distortion increases with the nonstochiometry of the compound. Below 170°K Pb₃O₄ exhibits an orthorhombic phase with orthorhombic domains according to two orientation states, and para crystalline distortion. A model of texture is proposed and compared with the high-temperature one. The pretransitional effect which is observed between 250 and 170°K is correlated with the presence of orthorhombic microdomains in the high-temperature phase (tetragonal).     

Low-temperature heat capacities, and thermodynamic properties of solid–solid phase change material bis(1-octylammonium) tetrachlorocuprate

A new crystalline complex (C₈H₁₇NH₃)₂CuCl₄(s) (abbreviated as C₈Cu(s)) was synthesized by liquid phase reaction. Chemical analysis, elemental analysis, and X-ray crystallography were applied to characterize the composition and crystal structure of the complex. Low-temperature heat capacities of the complex were measured by a precision automatic adiabatic calorimeter over the temperatures ranging from 78 to 395 K, and two solid–solid phase changes appeared in the heat capacity curve. The temperatures, molar enthalpies and entropies of the two phase transitions of the complex were determined to be: T _{trs, 1} = 309.4 ± 0.35 K, Δ_trs H _{m, 1} = 16.55 ± 0.41 kJ mol^?1, and Δ_trs S _{m, 1} = 53.49 ± 1.3 J K^?1 mol^?1 for the first peak; T _{trs, 2} = 338.5 ± 0.63 K, Δ_trs H _{m, 2} = 6.500 ± 0.10 kJ mol^?1, and Δ_trs S _{m, 2} = 19.20 ± 0.28 J K^?1 mol^?1 for the second peak. Two polynomial equations of the heat capacities as a function of the temperature were fitted by least-square method. Smoothed heat capacities and thermodynamic functions of the complex relative to the standard reference temperature of 298.15 K were calculated based on the fitted polynomial equations.     

Temperature dependences of solid structure and properties of biodegradable poly(butylene succinate-co-terephthalate) (PBST) copolyester

In this paper, studies of the temperature dependence for spherulitic growth of PBST copolyester bearing 70 mol% butylene terephthalate units (named as PBST-70) ranged from 70 to 170 °C were first reported based on the Lauritzen–Hoffman secondary nucleation theory. The results showed that maximum spherulitic growth rate of PBST-70 was obtained under crystallization temperature of 90 °C, and more perfect spherulites were formed via increasing isothermal crystallization temperature by POM measurement. The classical regime I → II and regime II → III transitions occurred at the temperatures of 150 and 110 °C, respectively, using the empirical universal values of U* = 6300 J mol^?1 and T _∞ = T _g ? 30 K. Moreover, the effects of isothermal crystallization temperature on crystal lamellar thickness, thermal and tensile properties of PBST-70 were systematically investigated by small angle X-ray scattering, differential scanning calorimeter, and strength tester. The results indicated that the crystal lamellar thickness increased by increasing isothermal crystallization temperature. The endothermic peak shifted to higher temperature and the tensile properties of PBST-70 were enhanced under higher isothermal crystallization temperature.     

Thermodynamic properties of crystalline phosphate Ba0.5Zr2(PO4)3 over the temperature range from T → 0 to 610 K

The temperature dependence of the heat capacity of crystalline barium zirconium phosphate C _p ^o  = f(T) was measured over the temperature range 6–612 K. The experimental data obtained were used to calculate the standard thermodynamic functions C _p ^o (T), H°(T) ? H°(0), S°(T), G°(T) ? H°(0) over the temperature range from T → 0 to 610 K and standard entropy of formation at 298.15 K. The data on the low-temperature (6 ≤ T/K ≤ 50) heat capacity were used to determine the fractal dimension of Ba_0.5Zr₂(PO₄)₃. Conclusions concerning the topology of the structure of phosphate were drawn. Thermodynamic properties of M_0.5Zr₂(PO₄)₃ (M = Ca, Sr, Ba) were compared.     

Structural and magnetic properties of MnAs0.90P0.10

MnAs_0.90P_0.10 takes the orthorhombic MnP type structure at temperatures below the second-order transition temperature T_D = 449 ± 2 K, and the hexagonal NiAs type structure between T_D and the melting point T_m = 1189 ± 10 K. The structural changes, in positional parameters and unit cell dimensions (as well as interatomic distances), have been determined for the increasing distortion at temperatures below T_D by means of powder X-ray and neutron diffraction. At low temperatures two distinct incommensurate magnetic structures are found, respectively, over the temperature intervals 0 < T < T_S1 = 105 ± 5 K and T_S2 = 185 ± 5 < T < T_N = 237 ± 5 K, whereas a ferromagnetic spin arrangement prevails over the intermediate temperature range T_S1 < T < T_S2.     

Phase transitions, structural changes and molecular motions in [Zn(NH3)4](BF4)2 studied by neutron scattering, X-ray powder diffraction and nuclear magnetic resonance

Nuclear magnetic resonance (¹H NMR and ¹⁹F NMR) measurements performed at 90-295 K, inelastic incoherent neutron scattering (IINS) spectra and neutron powder diffraction (NPD) patterns registered at 22-190 K, and X-ray powder diffraction (XRPD) measurements performed at 86-293 K, provided evidence that the crystal of [Zn(NH₃)₄](BF₄)₂ has four solid phases. The phase transitions occurring at: T_C3=101 K, T_C2=117 K and T_C1=178 K, as were detected earlier by differential scanning calorimetry (DSC), were connected on one hand only with an insignificant change in the crystal structure and on the other hand with a drastic change in the speed of the anisotropic, uniaxial reorientational motions of the NH₃ ligands and BF₄⁻ anions (at T_C3 and at T_C2) and with the dynamical orientational order-disorder process (“tumbling”) of tetrahedral [Zn(NH₃)₄]²⁺ and BF₄⁻ ions (at T_C1). The crystal structure of [Zn(NH₃)₄](BF₄)₂ at room temperature was determined by XRPD as orthorhombic, space group Pnma (No. 62), a=10.523 Å, b=7.892 Å, c=13.354 Å and Z=4. Unfortunately, it was not possible to determine the structure of the intermediate and the low-temperature phase. However, we registered the change of the lattice parameters and unit cell volume as a function of temperature and we can observe only a small deviation from near linear dependence of these parameters upon temperature in the vicinity of the T_C1 phase transition.     

An inorganic–organic intercalated nanocomposite, BEDT-TTF into layered MnPS3

In this paper we report the synthesis and magnetic properties of an inorganic–organic hybrid, Mn_0.84PS₃(BEDT-TTF)_0.35 (BEDT-TTF = bis(ethylenedithio) tetrathiafulvalene), which is obtained by the intercalation of pre-intercalation compound Mn_0.90PS₃(Phen)_0.32 (Phen = 1,10-phenanthroline) with (BEDT-TTF)₂I_x. The lattice spacing expansion (Δd) of 4.0 Å compared with the pristine MnPS₃ indicates that the molecular plane of BEDT-TTF is arranged parallel to the host layer. From the magnetic measurements it was found that two magnetic phase transitions occur. Above 50 K it shows paramagnetism in well agreement with Curie–Weiss law. Around 40 K it exhibits spin-glass transition and at 5 K a ferrimagnetic phase transition occurs, which is confirmed by M–H at different temperatures.     

Neutron powder diffraction study of phase transitions in Sr2SnO4

The phase transitions in Sr₂SnO₄ at high temperature have been studied using high resolution time-of-flight powder neutron diffraction. The room temperature structure of Sr₂SnO₄ is orthorhombic (Pccn), which can be derived from the tetragonal K₂NiF₄ structure by tilting the SnO₆ octahedra along the tetragonal [100]_T- and [010]_T-axes with non-equal tilts. At the temperature of about 423 K, it transforms to another orthorhombic structure (Bmab) characterized by the SnO₆ octahedral tilt around the [110]_T-axis. At still higher temperatures (∼573 K) the structure was found to be tetragonal K₂NiF₄-type (I4/mmm).     

Pulsed NMR study of molecular motions and phase transitions in [N(CH3)4]PbX3 (XCl,Br, I)

Proton spin—lattice relaxation time (T₁) is measured in [N(CH₃)₄]PbX₃ (XCl, Br, I) from 300-77 K at 9.75 MHz. All the compounds show discontinuous changes in T₁ values (at 256, 270 and 277 K, respectively), indicating phase transitions. Single T₁ minimum is observed in all the cases and the T₁ variation is explained in terms of [N(CH₃)₄] and CH₃ group dynamics. The activation energy E_α decreases from chloride to iodide (from 4 to 2 kcal/mol). In bromide and iodide, T₁ is found to decrease with increase in temperature at higher temperatures, indicating the presence of spin—rotation interaction.     

Investigation of structural and dynamic properties of NH4[N(CN)2] by means of X-ray and neutron powder diffraction as well as vibrational and solid-state NMR spectroscopy

The crystal structure, spectroscopic and thermal properties of ammonium dicyanamide NH₄[N(CN)₂] have been thoroughly investigated by means of temperature-dependent single-crystal X-ray and neutron powder diffraction, vibrational and MAS-NMR spectroscopy as well as thermoanalytical measurements. The comprehensive elucidation of structural details is of special interest with respect to the unique solid-state transformation of ammonium dicyanamide into dicyandiamide. This reaction occurs at temperatures >80°C and it represents the isolobal analogue of Wöhler's historic transformation of ammonium cyanate into urea. NH₄[N(CN)₂] crystallizes in the monoclinic space group P2₁/c with lattice constants a=3.7913(8), b=12.412(2), c=9.113(2) Å, β=91.49(2)° and Z=4 (single-crystal X-ray data, T=200 K). The temperature dependence of the lattice constants shows anisotropic behavior, however, no evidence for phase transitions in the investigated temperature range was observed. The hydrogen positions could be localized by neutron diffraction (10-370 K), and the temperature-dependent behavior of the ammonium group has been analyzed by Rietveld refinements using anisotropic thermal displacement parameters. They were interpreted by utilizing a rigid body model and extracting the libration and translation matrices of the ammonium ion by applying the TLS formalism. The results obtained by the diffraction methods were confirmed and supplemented by vibrational spectroscopy and solid-state ¹⁵N and ¹³C MAS-NMR investigations.     

Effect of a porous medium on the phase transitions and mobility of cyclohexane molecules

The effect of a porous medium on the phase transitions and molecular mobility of cyclohexane at a liquid content corresponding to a monolayer is studied by pulsed NMR. The times of longitudinal T ₁ and transverse T ₂ magnetic relaxation of protons of cyclohexane introduced into granulated porous glasses of the Vycor type with average pore diameters of 4, 11, and 32 nm are measured in the temperature range of 128–293 K. In spite of a relatively low liquid content, two phase transitions are observed for all porous glass samples at temperatures lower than those inherent in pure cyclohexane. At low temperatures, nonfreezing cyclohexane volumes with characteristic times of T ₂ ～ 100–200 μs and relative populations of 5–10% remain preserved due to the presence of a small number of micropores commensurable with molecular sizes. The appearance of an additional component with T ₂ ～ 200 μs upon temperature elevation to 148 K attests to thawing out of some cyclohexane volumes, which begins long before the crystal-plastic crystal phase transition. The nonexponential character of the transverse magnetization decay of cyclohexane above the temperature of the plastic crystal-liquid phase transition in the porous glass with a pore diameter of 4 nm suggests the existence of barriers for rapid molecular exchange. The obtained experimental results are indicative of the cluster mechanism of cyclohexane adsorption in the studied porous glasses.     

Studying the Origin of the Antiferromagnetic to Spin‐Canting Transition in the β‐p‐NCC6F4CNSSN. Molecular Magnet

The crystal structure of the spin‐canted antiferromagnet β‐p‐NCC₆F₄CNSSN^. at 12 K (reported in this work) was found to adopt the same orthorhombic space group as that previously determined at 160 K. The change in the magnetic properties of these two crystal structures has been rigorously studied by applying a first‐principles bottom‐up procedure above and below the magnetic transition temperature (36 K). Calculations of the magnetic exchange pathways on the 160 K structure reveal only one significant exchange coupling (J(d1)=?33.8 cm^?1), which generates a three‐dimensional diamond‐like magnetic topology within the crystal. The computed magnetic susceptibility, χ(T), which was determined by using this magnetic topology, quantitatively reproduces the experimental features observed above 36 K. Owing to the anisotropic contraction of the crystal lattice, both the geometry of the intermolecular contacts at 12 K and the microscopic J_AB radical–radical magnetic interactions change: the J(d1) radical–radical interaction becomes even more antiferromagnetic (?43.2 cm^?1) and two additional ferromagnetic interactions appear (+7.6 and +7.3 cm^?1). Consequently, the magnetic topologies of the 12 and 160 K structures differ: the 12 K magnetic topology exhibits two ferromagnetic sublattices that are antiferromagnetically coupled. The χ(T) curve, computed below 36 K at the limit of zero magnetic field by using the 12 K magnetic topology, reproduces the shape of the residual magnetic susceptibility (having subtracted the contribution to the magnetization arising from spin canting). The evolution of these two ferromagnetic J_AB contributions explains the change in the slope of the residual magnetic susceptibility in the low‐temperature region.     

Thermodynamic properties of crystalline magnesium zirconium phosphate

Heat capacity $ C_{\text{p}}^{^\circ } $ (T) of crystalline magnesium zirconium phosphate was measured between 6 and 815 K. The experimental data obtained were used to calculate the standard thermodynamic functions $ C_{\text{p}}^{^\circ } $ (T), H°(T) ? H°(0), S°(T), G°(T) ? H°(0) over the temperature ranging from T → 0 to 810 K and standard entropy of formation at 298.15 K. The fractal dimension of Mg_0.5Zr₂(PO₄)₃ was calculated from experimental data on the low-temperature (6 ≤ T/K ≤ 50) heat capacity, and the topology of the phosphate’s structure was estimated. Thermodynamic properties of structurally related phosphates M_0.5Zr₂(PO₄)₃ (M = Mg, Ca, Sr, Ba, Ni) were compared.     

Temperature-Dependent Diffusion Coefficients from 1D Raman Spectroscopy

A setup for the measurement of temperature-dependent diffusion coefficients has been developed based on 1D Raman spectroscopy. The presented setup was used to measure binary diffusion coefficients for mixtures of monoethanolamine + water, cyclohexane + toluene, and methanol + toluene for temperatures between T = 298.15 and 330.15 K. The experimental diffusion coefficients agree well with available literature data. For the mixture methanol + toluene, literature data was only found for T = 298.15 K. The novel setup was therefore used to determine temperature-dependent diffusion coefficients of methanol + toluene up to T = 328.15 K. The experimental data are also compared to a temperature correlation for diffusion coefficients in concentrated solutions. While the correlation describes the temperature dependence well for simple systems, measurement techniques remain indispensable for diffusion in complex mixtures.     

Structural control of dithiazolyl radicals: Case studies on 3′- and 4′-cyano-benzo-1,3,2-dithiazolyl, NCC6H3S2N

Dithiazolyl radicals with π-stacking motifs have attracted particular interest because of their ability to exhibit spin-switching between diamagnetic distorted π-stacks and paramagnetic regular π-stacked structures through a solid state phase transition. Previous studies indicate that inclusion of electronegative heteroatoms into the backbone favours lamellar structures. This methodology has been extended to the synthesis and characterisation of the title compound, 4′-cyanobenzo-1,3,2-dithiazolyl (4-NCBDTA). Its electronic structure is probed through DFT calculations, cyclic voltammetry and EPR spectroscopy and its crystal structure determined by X-ray powder diffraction at room temperature. Variable temperature SQUID magnetometry reveals that 4-NCBDTA undergoes two phase transitions, each exhibiting bistability; a high temperature phase transition occurs at room temperature (T_C↓ = 291 K, T_C↑ = 304 K, ΔT = 13 K); whilst the low temperature phase transition occurs below liquid nitrogen temperatures (T_C↓ = 37 K, T_C↑ = 28 K;ΔT = 9 K).     

Thermal properties,phase transitions,vibrational and reorientational dynamics of [Mn(NH<Subscript>3</Subscript>)<Subscript>6</Subscript>](NO<Subscript>3</Subscript>)<Subscript>2</Subscript>

[Mn(NH₃)₆](NO₃)₂ crystallizes in the cubic, fluorite (C1) type crystal lattice structure (Fm \( \overline{3} \) m) with a = 11.0056 Å and Z = 4. Two phase transitions of the first-order type were detected. The first registered on DSC curves as a large anomaly at T _C1 ^h  = 207.8 K and T _C1 ^c  = 207.2 K, and the second registered as a smaller anomaly at T _C2 ^h  = 184.4 K and T _C2 ^c  = 160.8 K (where the upper indexes h and c denote heating and cooling of the sample, respectively). The temperature dependence of the full width at half maximum of the band associated with the δ_s(HNH)F_1u mode suggests that the NH₃ ligands in the high temperature and intermediate phase reorientate quickly with correlation times in the order of several picoseconds and with activation energy of 9.9 kJ mol^?1. In the phase transition at T _C2 ^c probably only a some of the NH₃ ligands stop their reorientation, while the remainders continue to reorientate quickly with activation energy of 7.7 kJ mol^?1. Thermal decomposition of the investigated compound starts at 305 K and continues up to 525 K in four main stages (I–IV). In stage I, ²/₆ of all NH₃ ligands were seceded. Stages II and III are connected with an abruption of the next ²/₆ and ¹/₆ of total NH₃, respectively, and [Mn(NH₃)](NO₃)₂ is formed. The last molecule of NH₃ per formula unit is freed at stage IV together with the simultaneous thermal decomposition of the resulting Mn(NO₃)₂ leading to the formation of gaseous products (O₂, H₂O, N₂ and nitrogen oxides) and solid MnO₂.