Thermodynamic properties of LiZr2(PO4)3 crystal phosphate

The temperature dependence of the heat capacity of LiZr₂(PO₄)₃ crystal phosphate is studied in an adiabatic vacuum calorimeter in the temperature range of 6 to 358 K. A phase transition caused by the transition of a low-temperature (triclinic) modification to a high-temperature (rhombohedral) modification is observed in the temperature range of 290–338 K and its standard thermodynamic characteristics are estimated and analyzed. Standard thermodynamic functions are calculated from experimental data: heat capacity, enthalpy, entropy, and Gibbs function in the range of T → 0 to 358 K. Fractal dimensionality D is calculated from the data on low-temperature (20 K ≤ T ≤ 50 K) heat capacity and the topology of the phosphate’s structure is estimated.     

Temperature dependences of J(C,H) and J(C,D) in 13CH4 and some of its deuterated isotopomers

The nuclear spin—spin coupling constants J(C,H) and J(C,D) have been measured over the temperature range 200–370 K for the methane isotopomers ¹³CH₄, ¹³CH₃D, ¹³CHD₃ and ¹³CD₄. The coupling constants increase with increasing temperature for any one isotopomer and decrease with increasing secondary deuterium substitution at any one temperature. The results are entirely attributable to intramolecular effects and the data have been fitted by a weighted least-squares regression analysis to a spin—spin coupling surface thereby yielding a value for ¹J_e(C,H), the coupling constant at equilibrium geometry, and values for the bond length derivatives of the coupling. We find that ¹J_e(C,H) = 120.78 (±0.05) Hz which is about 4.5 Hz smaller than the observed value in ¹³CH₄ gas at room temperature. Results are also reported for J(H,D) in ¹³CH₃D and ¹³CHD₃ for which no temperature dependence was detected.     

Simultaneous Spin-Crossover Transition and Conductivity Switching in a Dinuclear Iron(II) Coordination Compound Based on 7,7′,8,8′-Tetracyano-p-quinodimethane

The reaction of Fe(OAc)₂ and Hbpypz with neutral TCNQ results in the formation of [Fe₂(bpypz)₂(TCNQ)₂](TCNQ)₂ ( 1 ), in which Hbpypz=3,5-bis(2-pyridyl)pyrazole and TCNQ=7,7′,8,8′-tetracyano-p-quinodimethane. Crystal packing of 1 with uncoordinated TCNQ and π–π stacking of bpypz⁻ ligands produces an extended two-dimensional supramolecular coordination assembly. Temperature dependence of the dc magnetic susceptibility and heat capacity measurements indicate that 1 undergoes an abrupt spin crossover (SCO) with thermal spin transition temperatures of 339 and 337 K for the heating and cooling modes, respectively, resulting in a thermal hysteresis of 2 K. Remarkably, the temperature dependence of dc electrical transport exhibits a transition that coincides with thermal SCO, demonstrating the thermally induced magnetic and electrical bistability of 1 , strongly correlating magnetism with electrical conductivity. This outstanding feature leads to thermally induced simultaneous switching of magnetism and electrical conductivity and a magnetoresistance effect.     

Variable temperature far and mid FT-IR as a valuable tool to determine the spin transition temperature of iron(II) spin-crossover coordination compounds

The thermally induced spin transition behaviour of selected iron(II) coordination compounds with substituted tetrazole ligands has been monitored using variable temperature FT-IR spectroscopy. The reliability of these results is discussed and compared with independent analytical techniques such as SQUID measurements and ⁵⁷Fe Mössbauer spectroscopy proofing variable temperature IR spectroscopy to be a valuable tool in the determination of the spin transition temperature T_1/2.     

1A1 5T2 Spin transition in the solid phases of FexNi1-x(ATr)3(NO3)2 (ATr = 4- amino- 1,2,4- triazole)

The effect of Fe²⁺ substitution by Ni²⁺ in the complex of iron(II) nitrate with 4- amino- 1,2,4- triazole Fe(ATr)₃(NO₃)₂ on the character of the¹A₁ ⁵T₂ spin transition (ST) is studied by magnetic susceptibility and calorimetry methods. Solid phases of Fe_xNi_{1- x}(ATr)₃(NO₃)₂ (0.1 ≤ x ≤ 0.9) were synthesized. The temperature dependences of the effective magnetic moment were measured in the range of 78– 360 K. Heat capacities were measured in the range of 210– 340 K for 0.1 ≤ x ≤ 0.5 and in the range of 230– 340 K for 0.6 ≤x ≤ 0.9. As x decreases, the transition temperature (T_C), hysteresis (δT_C, and transition enthalpy (δH) decrease and the ST is leveled. The results are compared with the data obtained previously for the solid phases of Fe_xZn_{1- x}(ATr)₃(NO₃)₂ (0.01 ≤ x ≤ 0.8). The dependence Μ_eff(T) is analyzed theoretically in terms of both the domain model and the spin equilibrium model. Translated fromZhumal Strukturnoi Khimii, Vol. 38, No. 4, pp. 696–703, July–August, 1997.     

Lattice trapping of metastable quintet state bis(2,6-bis(pyrazol-3-yl)pyridine)iron(II) bis(tetrafluoroborate), a spin crossover system,and kinetics of the quintet-singlet transformation

The⁵T₂ ⇌ ¹A₁ spin transition in [Fe(bpp)₂][BF₄]₂ (bpp = 2,6-bis(pyrazol-3-yl)pyridine) is abrupt and complete and shows hysteresis with the transition temperatures T_c↓ = 173 K and T_c↑ = 183 K. Rapid cooling of the sample causes the freezing-in of metastable quintet state species at low temperatures. Relaxation of the metastable quintet to singlet state species within the range 99–114 K follows simple first-order kinetics with an activation energy E_a = 19.5 kJ mol⁻¹.     

Magnetic transition in dimerized radical cation salt of (BPDT-TTF)2ICl2 studied by heat capacity measurements

Thermodynamic investigation using the relaxation calorimetry technique and the microchip calorimetry technique is performed to clarify low-temperature behaviors of a radical cation salt consisting of a donor molecule of bispropylenedithiotetrathiafulvalene (BPDT-TTF) and a linear anion of ${{\text{ICl}}_{2}^{-}}$ ICl 2 ? . This compound has a layered structure similar to numerous BEDT-TTF compounds. The donor molecules form a dimerized arrangement in the layer. Temperature dependence of heat capacity obtained by the relaxation technique shows a broad hump structure around 20–25 K corresponding to the temperature where the magnetic susceptibility shows a drastic decrease due to the formation of the singlet spin state. The microchip calorimetry technique detected a step-like anomaly around 23 K in the temperature dependence of C _p T ^?1 of which entropy is evaluated as only few % of Rln2 corresponding to the full entropy of localized π-electrons located on each dimer unit. The negligibly small T-linear term in the low-temperature heat capacity and absence of magnetic fields dependence below 3.2 K predict opening of rigid gap structure in the spin excitations, which is consistent with a spin-singlet formation due to the formation of spin-Peierls type ordering or charge ordering state.     

Temperature dependence of the thermodynamic parameters of cobalt(II) clathrochelate: X-ray diffraction and calorimetric characteristics of the low temperature structural phase transition

The temperature dependence of heat capacity of the polycrystalline sample of cobalt(II) clathrochelate in a range of 6–300 K is studied. Based on the smoothed dependence C _p(T), the entropy and enthalpy values in a temperature range of 8–300 K and their standard values at 298.15 K are calculated. In the C _p(T) curve in a range of 50–70 K, a process is recorded whose entropy and enthalpy are 1.2 J·(K·mol⁻¹) and 68 J·mol⁻¹ respectively. A comparison of the results with the data of a multitemperature X-ray diffraction study makes it possible to attribute this process to the structural phase transition.     

Thermodynamic investigation of several natural polyols (II)

The low-temperature heat capacity C _p,m of sorbitol was precisely measured in the temperature range from 80 to 390 K by means of a small sample automated adiabatic calorimeter. A solid-liquid phase transition was found at T=369.157 K from the experimental C _p-T curve. The dependence of heat capacity on the temperature was fitted to the following polynomial equations with least square method. In the temperature range of 80 to 355 K, C _p,m/J K⁻¹ mol⁻¹=170.17+157.75x+128.03x ²-146.44x ³-335.66x ⁴+177.71x ⁵+306.15x ⁶, x= [(T/K)−217.5]/137.5. In the temperature range of 375 to 390 K, C _p,m/J K⁻¹ mol⁻¹=518.13+3.2819x, x=[(T/K)-382.5]/7.5. The molar enthalpy and entropy of this transition were determined to be 30.35±0.15 kJ mol⁻¹ and 82.22±0.41 J K⁻¹ mol⁻¹ respectively. The thermodynamic functions [H _T-H _298.15] and [S _T-S 298.15], were derived from the heat capacity data in the temperature range of 80 to 390 K with an interval of 5 K. DSC and TG measurements were performed to study the thermostability of the compound. The results were in agreement with those obtained from heat capacity measurements.     

High-temperature spin crossover in the [Fe(L)2][Fe(L)(NCS)3](NCS)·2H2O complex, L = tris(pyrazol-1-yl)methane

A novel iron(II) coordination compound with tris(pyrazol-1-yl)methane (L) of the composition [FeL₂][Fe(L)(NCS)₃](NCS)·2H₂O has been synthesized. Employing the XRD technique, its crystal structure has been determined. The compound was studied with the help of IR and UV-Vis spectroscopy and static magnetic susceptibility methods. A magnetochemical study of the complex within the temperature range 78-400 K has demonstrated that the compound exhibits a high-temperature spin crossover (SCO) ¹А₁ ⇔ ⁵Т₂. The transition temperature amounts to 380 K.     

Thermodynamic properties of second generation poly(phenylene-pyridyl) dendrimer

The temperature dependence of heat capacity C _p ^o = f (T) of second generation hard poly(phenylene-pyridyl) dendrimer (G2-24Py) was measured by a adiabatic vacuum calorimeter over the temperature range 6–320 K for the first time. The experimental results were used to calculate the standard thermodynamic functions: heat capacity C _p ^o (T), enthalpy H ^o(T)–H ^o(0), entropy S ^o(T)–S ^o(0) and Gibbs function G ^o(T)–H ^o(0) over the range from T → 0 K to 320 K. The standard entropy of formation at T = 298.15 K of G2-24Py was calculated. The low-temperature heat capacity was analyzed based on Debye’s heat capacity theory of solids. Fractal treatment of the heat capacity was performed and the values of the temperature characteristics and fractal dimension D were determined. Some conclusions regarding structure topology are given.     

Study of Integer Spin S = 1 in the Polar Magnet β-Ni(IO3)2

Polar magnetic materials exhibiting appreciable asymmetric exchange interactions can potentially host new topological states of matter such as vortex-like spin textures; however, realizations have been mostly limited to half-integer spins due to rare numbers of integer spin systems with broken spatial inversion lattice symmetries. Here, we studied the structure and magnetic properties of the S = 1 integer spin polar magnet β-Ni(IO₃)₂ (Ni²⁺, d⁸, ³F). We synthesized single crystals and bulk polycrystalline samples of β-Ni(IO₃)₂ by combining low-temperature chemistry techniques and thermal analysis and characterized its crystal structure and physical properties. Single crystal X-ray and powder X-ray diffraction measurements demonstrated that β-Ni(IO₃)₂ crystallizes in the noncentrosymmetric polar monoclinic structure with space group P2₁. The combination of the macroscopic electric polarization driven by the coalignment of the (IO₃)⁻ trigonal pyramids along the b axis and the S = 1 state of the Ni²⁺ cation was chosen to investigate integer spin and lattice dynamics in magnetism. The effective magnetic moment of Ni²⁺ was extracted from magnetization measurements to be 3.2(1) µ_B, confirming the S = 1 integer spin state of Ni²⁺ with some orbital contribution. β-Ni(IO₃)₂ undergoes a magnetic ordering at T = 3 K at a low magnetic field, μ₀H = 0.1 T; the phase transition, nevertheless, is suppressed at a higher field, μ₀H = 3 T. An anomaly resembling a phase transition is observed at T ≈ 2.7 K in the C_p/T vs. T plot, which is the approximate temperature of the magnetic phase transition of the material, indicating that the transition is magnetically driven. This work offers a useful route for exploring integer spin noncentrosymmetric materials, broadening the phase space of polar magnet candidates, which can harbor new topological spin physics.     

The magnetic and structural elucidation of 3,5-bis(2-pyridyl)-1,2,4-triazolate-bridged dinuclear iron(II) spin crossover compounds

Variable temperature magnetic susceptibility, Mössbauer spectroscopic and X-ray crystallographic studies are described on two structurally similar families of dinuclear iron(II) spin crossover (SCO) complexes of formula [Fe(NCX)(py)]₂(μ-L)₂, where L is either a 3,5-bis(2-pyridyl)-pyrazolate bridging ligand, bpypz⁻, examples of which have been earlier reported by Kaizaki and coworkers, or a corresponding 3,5-bis(2-pyridyl)-1,2,4-triazolate, bpytz⁻. Compounds synthesised were [Fe(NCS)(py)]₂(μ-bpypz)₂ (1), [Fe(NCSe)(py)]₂(μ-bpypz)₂ (2), [Fe(NCS)(py)]₂(μ-bpytz)₂ (3), [Fe(NCSe)(py)]₂(μ-bpytz)₂ (4), [Fe(NCBH₃)(py)]₂(μ-bpytz)₂ (5). The crystal and molecular structures of 1 and 3 are very similar in their HS–HS forms (HS = high spin d⁶). In contrast to reported SCO behaviour for precipitated samples of 1, also repeated here, crystals of 1 show only HS–HS behaviour with no spin crossover transition. Complex 3 likewise displays HS–HS magnetism, with very weak antiferromagnetic coupling. Compound 5 displays a well resolved two-step, full spin transition from HS–HS to LS–LS states while compound 2 shows a one step transition. The Mössbauer data for 2 and 5 show unusual features at low temperatures.     

Charge–transfer-induced spin transition in K0.28Co1.36[Fe(CN)6] · XH2O with different annealing temperatures

The Prussian blue analog K_0.28Co_1.36[Fe(CN)₆]?·?XH₂O was prepared by standard chemical co-precipitation. The precipitate was filtered and dried in a vacuum oven at room temperature, 80°C, and 120°C. The powder X-ray diffraction measurement indicates a typical face-centered cubic pattern. The diffraction peaks show a slight shift to higher angle with increasing annealing temperatures, a signature of lattice contraction, which is mainly related to the inner charge transfer from Fe^III to Co^II. The value of χ?·?T is variable and dependent on temperature. The temperature dependence of χ ^?1 shows a large deviation from the Curie–Weiss law. The behavior could result from a charge-transfer-induced spin transition. Isothermal magnetization curves also suggest that the inner charge-transfer spin transition depends on the annealing temperature.     

5T2-1A1 Spin transition and residual paramagnetism in bis(2,4-bis(2-pyridyl)thiazole)iron(II) complexes: mössbauer effect and magnetism

The ⁵⁷Fe Mössbauer effect in [Fe(pythiaz)₂] (BF₄)₂ (I) and [Fe(pythiaz)₂] (C&O₄)₂ (II) has been studied between 298 and 4.2°K (pythiaz = 2,4-bis(2-pyridyl)thiazole). At 298°K compound I shows a doublet with ΔE_Q(⁵T₂) = 1.29 mm sec^?1 and δ^1S(⁵T₂) = +0.93 mm sec^?1 characteristic of a ⁵T₂ ground state. At 236°K, a second doublet, typical for a ¹A₁ ground state appears. The transition ⁵T₂ å ¹A₁ progresses as the temperature is lowered but levels off below ≈ 120°K. At 4.2°K, 59% of the intensity is due to the ¹A₁ state, and ΔE_Q(¹A₁) = 1.59 mm sec^?1 and δ^1S(¹A₁) = +0.26 mm sec^?1. In an applied magnetic field, V_zz(¹A₁) < 0 has been determined Similar results have been obtained with compound II.Debye-Waller factors f5_T2 and f1_A1. were determined from the Mössbauer spectra under the assumption of Curie-Weiss dependence of the magnetism for the ⁵T₂ and constant μ_eff for the ¹A₁ ground state. The resulting temperature dependence of f1_A1 is highly unusual thus suggesting complicated magnetic behaviour of both ground states in the transition region. Two mechanisms for the nature of the transition are discussed, a “spin-flip” mechanism being the physically more reasonable one. The assumption of a simple Boltzmann distribution (“spin equilibrium”) may be ruled out for the solid but could be encountered in solutions.     

Thermodynamics of hard poly(phenylene-pyridyl) dendrimers

The temperature dependences of heat capacity C ^○ _p = f(T) of hard poly(phenylene-pyridyl) dendrimers of the first and the second generations based on 1,3,5-triethynylbenzene were studied over the temperature range from 7–180 to 455–470 K for the first time. Over the range 290–350 K, the relaxation transition supposedly caused by sample devitrification was detected and characterized for the dendrimer of the first generation. The experimental results were used to calculate the standard thermodynamic functions, namely, heat capacity, enthalpy, entropy, and change in the Gibbs energy on heating. The standard entropy of formation of the compounds under study was determined at T = 298.15 K. The qualitative and quantitative dependences of the thermodynamic properties of the poly(phenylene-pyridyl) dendrimers on their composition and structure were revealed by comparison of the determined parameters with similar data for the earlier studied dendrimers of this series.     

On the role of intermolecular interactions on structural and spin-crossover properties of 2D coordination networks [Fe(bbtr)3]A2 (bbtr=1,4-bis(1,2,3-triazol-1-yl)butane; A=ClO4(-), BF4(-))

A series of complexes [M(bbtr)₃]A₂ (M=Fe^II, Zn^II; bbtr=1,4‐bis(1,2,3‐triazol‐1‐yl)butane; A=ClO₄^?, BF₄^?) and [Fe_xZn_1?x(bbtr)₃](ClO₄)₂ (0<x<1) dilute systems was synthesized and characterized. Earlier studies on [Fe(bbtr)₃](ClO₄)₂ ( 1?ClO₄ ), which crystallizes in space group P$\bar 3A series of complexes [M(bbtr)(3)]A(2) (M=Fe(II), Zn(II); bbtr=1,4-bis(1,2,3-triazol-1-yl)butane; A=ClO(4)(-), BF(4)(-)) and [Fe(x)Zn(1-x)(bbtr)(3)](ClO(4))(2) (0相似文献   

Brillouin scattering and rotation translation coupling of (KCN)xX1−x mixed crystals

Brillouin scattering is used to study the concentration and temperature dependence of longitudinal and transverse phonon frequencies of KCN_χBr_1?χ, KCN_χC1_1?χ, and KCN_0.85I_0.15 mixed crystals. Unlike pure KCN, mixed crystals with χ < 0.7 show no phase transition upon cooling, and the transverse acoustic frequency ω_TA(110) displays a minimum at some characteristic temperature T_i.T_i is below the phase transition of pure KCN and depends on the C1^? concentration. The Brillouin data are discussed in terms of coupling of the acoustic phonons with orientational motion of CN^? ions and mean field theory.     

Anomalous spin state of iron(II) in 57Fe Mössbauer emission spectra of 57Co-Labeled tris(2-methylphenanthroline) cobalt(II) perchlorate

⁵⁷Fe Mössbauer emission spectra of the ⁵⁷Co labeled complex compound [⁵⁷Co(2-CH₃-phen)₃] (ClO₄)₂·2H₂O have been measured as a function of temperature between 293 and 4.6 K. The spectra exclusively show high-spin iron(II) resonances beside a small fraction of an high-spin iron(II) species, whereas the corresponding iron(II) compound is known to exhibit thermally induced high-spin ⁵T_2g(O_h) ? low spin ¹A_1g(O_h) transition. The electronic nature of the anomalous spin state has been found to be ⁵A₁(D₃) by a theoretical treatment of the temperature dependence of the quadrupole splitting. The results are in good agreement with those obtained from Mössbauer absorption measurements on [⁵⁷Fe_0.01Co_0.99(2-CH₃-(phen)₃] (ClO₄)₂·2H₂O.     

Magneto-thermal and dielectric properties of biferroic YCrO3 prepared by combustion synthesis

Microstructural, magnetothermal and dielectric properties of YCrO₃ powders prepared by combustion and solid state methods have been studied by a combination of XRD, specific heat, magnetization and permittivity measurements. The TEM and XRD characterization confirm that the combustion powders are amorphous plate-like agglomerates of nano-sized crystalline particles. A more uniform grain size along with an increase of the relative density is observed by SEM in the sintered samples prepared by combustion route with respect to those produced by solid state reaction. Similar to the material obtained through solid state synthesis, the material prepared by the combustion method also shows spin canted antiferromagnetic ordering of Cr⁺³ (S=3/2) at ∼140 K, which is shown by magnetization as well as λ-type anomaly in the total specific heat. Furthermore, the magnetic contribution to the total specific heat reveals spin fluctuations above T_N and a spin reorientation transition at about 60 K. Both YCrO₃ compounds show a diffuse phase transition at about 450 K, typical of a relaxor ferroelectric, which is characterized by a broad peak in the real part of the dielectric permittivity as a function of temperature, with the peak decreasing in magnitude and shifting to higher temperature as the frequency increases. The relaxor dipoles are due to the local non-centrosymmetric structure. Furthermore, the high loss tangent in a broad range of temperature as well as conductivity analysis indicates a hopping mechanism for the electronic conductivity as we believe it is a consequence of the outer d³-shell, which have detrimental effects on the polarization and the pooling process in the YCrO₃ bulk material. The more uniform particle size and higher density material synthesized through the combustion process leads to an improvement in the dielectric Properties.