Heat capacity measurement of ZrOx and (Zr1−yNby)Ox from 325 TO 905 K

Heat capacities of zirconium-oxygen alloys, ZrO_x (x=0.17, 0.20, 0.28 and 0.31), and those of niobium doped alloys, (Zr_1–yNb_y)O_x (x=0.17 and 0.28, y=0.005 and 0.01), were measured from 325 to 905 K by an adiabatic scanning calorimeter. Two kinds of heat capacity anomalies were observed for all samples. The anomaly at higher temperatures was assigned to be due to an order-disorder rearrangement of oxygen atoms. Another anomaly at lower temperatures was due to a non-equilibrium phenomenon. The entropy change due to the order-disorder transition for Zr-O solid solution at higher temperature obtained from this experiment was compared with the theoretical value. The transition temperature, transition enthalpy and entropy changes due to the order-disorder transition decreased with increasing niobium contents, indicating that arrangement of oxygen atoms in lower temperature phase may be partially disordered by the interaction between niobium and oxygen atoms.

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Zusammenfassung Mittels eines adiabatischen Scanning-Kalorimeters wurden im Temperaturbereich 325–905 K die Wärmekapazitäten von Zirkonium-Sauerstoff Legierungen ZrO, (mit x=0.17, 0.20, 0.28 und 0.31) und von mit Niob versetzten Legierungen (Zr_1–yNb_y)O_x gemessen. Für alle Proben konnten zwei Arten von Wärmekapazitätsanomalien beobachtet werden. Die Anomalie bei höherer Temperatur wird einer Ordnung-Unordnung-Umwandlung von Sauerstoffatomen zugeschrieben. Eine andere Anomalie bei niedrigerer Temperatur steht mit einer Nicht-Gleichgewichtserscheinung in Zusammenhang. Die Entropieänderung der Ordnung-Unordnung-Umwandlung für einen Zr-O Mischkristall aus diesem Experiment wurde mit dem theoretischen Wert verglichen. Umwandlungstemperatur, Umwandlungsenthalpie- und Entropieänderungen der Ordnung-Unordnung-Umwandlung nehmen mit steigendem Niobgehalt ab, was zeigt, daß die Anordnung der Sauerstoffatome in Phasen mit niedrigerer Temperatur partiell durch die Wechselwirkungen zwischen Niob- und Sauerstoffatomen gestört werden kann.

     

Phase transition in V5O9

Electrical resistivity, magnetic susceptibility, and heat capacity of V₅O₉ have been measured through its phase transition temperature (T_t) around 129 K. The associated changes in enthalpy and entropy were found to be 2095 J/mole and 19.24 J/mole · deg., respectively. A qualitative thermodynamic analysis has been attempted to correlate the crystal symmetry, electrical, magnetic, and heat capacity behavior at T_t. The metal-semiconductor transition appears as a consequence of the crystallographic order-disorder process, since the electrical and magnetic contributions to configurational entropy change are relatively small.     

Heat capacity of MnAs0.88P0.12 from 10 to 500 K: Thermodynamic properties and transitions

The heat capacity of MnAs_0.88P_0.12 has been measured by adiabatic shield calorimetry from 10 to 500 K. It is shown that very small energy changes are connected with two magnetic order-order transitions, indicating that these can be regarded as mainly “noncoupled” magnetic transitions. At higher temperatures contributions to the excess heat capacity arises from a magnetic order-disorder transition, a conversion from low- to high-spin state for manganese, and a MnP- to NiAs-type structural transition. The observed heat capacity is resolved into contributions from the different physical phenomena, and the character of the transitions is discussed. In particular it is substantiated that the dilational contribution, which includes magnetoelastic and magnetovolume terms as well as normal anharmonicity terms, plays a major role in MnAs_0.88P_0.12. The entropy of the magnetic order-disorder transition is smaller than should be expected from a complete randomization of the spins, assuming a purely magnetic transition. Thermodynamic functions have been evaluated and the respective values of C_p, {S^O_m(T) - S^O_m(0)}, and -{G^O_m(T) - H^O_m(0)}/T at 298.15 K are 68.74, 72.09, and 32.30 J K⁻¹ mole⁻¹, and at 500 K 56.05, 108.12, and 56.64 J K⁻¹ mole⁻¹.     

Beiträge zu den Eigenschaften von Titanaten mit Ilmenitstruktur. II. Zur Thermodynamik und elektrischen Leitfähigkeit von NiTiO3 und anderen oxidischen Phasen mit Ilmenitstruktur

Contributions to the Properties of Titanates with Ilmenite Structure. II. Study on the Thermodynamics and the Electrical Conductivity of NiTiO₃ and Other Phases with Ilmenite Structure NiTiO₃ shows a phase transition at high temperatures (T_c = 1290°C). The standard enthalpy and entrop of the reaction NiO + TiO₂ = NiTiO₃ was estimated for temperatures above and below the transition temperature using emf-measurements based on the following solid state galvanic cell: Ni,TiO₂, NiTiO₃|ZrO₂(+CaO)|Ni,NiO. The transition enthalpy was found to be 18 ± 2 kJmol^?1, The transition entropy is 12 ± 1 JK^?1mol^?1. This is in good agreement with the calculated entropy change for an order-disorder transition (11.5 JK^?1mol^?1). The influence of other cations like Mg²⁺ and Co²⁺ on the transition temperature was investigated by measurements of the electrical conductivity as a function of composition. Ni_1?xMg_xTiO₃ shows a strong shift of the transition to higher temperatures if a small part of the Ni²⁺ was replaced by Mg²⁺. A linear correlation between the temperature shift and the amount of Co²⁺ was found for Ni_1?xCo_xTiO₃. Thermoanalytical investigations reveal an endothermic peak during the heating period some degrees below the melting point of CoTiO₃. The substitution of Ge⁴⁺ for Ti⁴⁺ is without any influence on the transition temperature. By doping the NiTiO₃ with Ga₂O₃, the anomalous increase of the electrical conductivity with temperature is shifted to lower temperatures.     

Phase transition of negative thermal expansion Zr1-xHfxW2O8 solid solutions

A powder X-ray diffraction experiment was performed on cubic Zr_1-xHf_xW₂O₈ (x=0.25, 0.50 and 0.75) solid solutions from 90 to 560 K. The lattice parameters of Zr_1-xHf_xW₂O₈ at 121 K decreased linearly with increasing Hf contents, due to smaller ionic radius of hafnium than that of zirconium. Transition temperatures due to α-β structural phase transition increased with increasing Hf contents, reflecting the decrease of lattice free volume related to the orientation of unshared vertex of WO₄. Anomaly in the heat capacity of Zr_0.5Hf_0.5W₂O₈ was observed around 450 K which was 9 K lower than that by X-ray diffraction method. Transition entropy of Zr_0.5Hf_0.5W₂O₈ was 2.1 J mol^-1 K^-1, consistent with those of ZrW₂O₈ and HfW₂O₈. This consistent entropy supports that Zr_1-xHf_xW₂O₈ (x=0-1.0) has the same order-disorder phase transition mechanism. This revised version was published online in August 2006 with corrections to the Cover Date.     

Dispersed Fluorescence Spectroscopy and Transition Dipole Moment of HCO β 2A′-X̄2A′

Dispersed fluorescence spectra of β ²A′ - x? ²A′ transitions are measured on exciting HCO to the vibrational levels (0,0,0), (0,0,1), (0,1,0) and (1,0,0) of β ²A′ in a gas cell. Vibrational energies of bound and resonant states are determined for energy up to 17 500 cm^?1 from the ground vibrational level of the x? ²A′ state. Rotationally resolved spectra of the bands β ²A′ - x? ²A′ 2⁰₃3⁰₁, 2¹₄ and 2⁰₂3¹₂ were recorded and analyzed. Abnormal intensities in the P, Q and R branches are due to the mechanism of “axis switching”. From the ratios of emission intensity of a- and b-type transitions, without taking into account the varied lifetimes of rotational states of the upper electronic state, the directions of transition dipole moments of these bands are determined to be 39 ? 43° to the inertial a axis.     

Ferroelectric phase transitions in Pb2xSn2(1-x)P2Se6 system

Heat capacities of the Pb_2xSn_2(1-x)P₂Se₆ crystals (x=0, 0.098, 0.251, 0.402 and 1.0) were measured using an adiabatic calorimeter at temperatures between 10 and 350 K. In the crystal of x=0, two heat capacity anomalies corresponding to the ferroelectric commensurate - intermediate incommensurate(C-IC) phase transition temperature T _i, and the incommensurate - paraelectric (IC-N) phase transition temperature T _c, were observed at 193.24±0.10 and 220.07±0.15 K, respectively. The phase transition temperatures decreased with an increase in Pb²⁺ concentration. The anomaly at Ti disappeared at x=0.251 in the mixed systems of the Pb_2xSn_2(1-x)P₂Se₆. In the crystal of Pb₂P₂Se₆ (x=1.0), no phase transition was observed. The normal heat capacities for the mixed crystals were determined by least squares fitting of the Debye and Einstein functions to the experimental data. The anomalous heat capacities gave the phase transition entropies of 8.5 and 1.5 J mol^-1 K^-1 for x=0. The large transition entropies are consistent with an order-disorder mechanism in the ferroelectric-paraelectric phase transitions in x=0. This revised version was published online in August 2006 with corrections to the Cover Date.     

Heat capacity and dual spin-transitions in the crossover system [Fe(2-pic)3]Cl2·EtOH

The heat capacity of [Fe(2-pic)₃]Cl₂·C₂H₅OH Crystal (2-pic: 2-picolylamine) has been measured with an adiabatic calorimeter between 13 and 315 K. Two phase transitions centered at 114.04 and 122.21 K were observed. This finding accords with recent prediction of possible existence of two-step spin-conversion (H. Köppen et al., Chem. Phys. Lett., 91 (1982) 348). The total transition enthalpy and entropy amounted to ΔH = 6.14 kJ mol^?1 and ΔS = 50.59 J K^?1 mol^?1. The transition entropy consists of the magnetic contribution (13.38 J K^?1 mol^?1), the orientational order-disorder phenomenon of the solvate ethanol molecule (8.97) and the change in the phonon system, in particular the change in stretching and deformation vibrations of the metal-ligand (28.24).     

Phase formation of BICUVOX solid solutions

Phase formation of Bi₄(V_{1 ? x} Cu _x )₂O_{11 ? z} solid solutions (BICUVOX) with x = 0.00–0.20 and Δx = 0.02 was studied. The concentration stability ranges were determined for the α, β, and γ polymorphs of BICUVOX solid solutions at room temperature, and their unit cell parameters were revised. The following was found to occur as x rises: the α ai β phase transition temperature between the monoclinic and orthorhombic phases shifts down, the β ai γ phase transition temperature to the high-temperature tetragonal phase shifts down, and the order-disorder phase transition temperature between γ′ ai γ tetragonal phases shifts up.     

Heat capacity measurements of SnSe and SnSe2

The heat capacities of SnSe and SnSe₂ were measured in the temperature range 230–580 K using a computer interfaced differential scanning calorimeter. From these measurements, the Debye temperatures of SnSe and SnSe₂ were calculated as a function of temperature. An estimated Debye temperature of 220 K for SnSe was used to calculate the absolute entropy of SnSe at 298 K to be 85.2 ± 6.0 J K^?1 mole^?1. In the light of other work, the suitability of Debye temperatures for estimating low temperature heat capacities of SnSe₂ is questioned.     

Heat capacity and magnetic phase transition of two-dimensional metal-assembled complex (NEt<Subscript>4</Subscript>)[{Mn<Superscript>III</Superscript>(salen)}<Subscript>2</Subscript>Fe<Superscript>III</Superscript>(CN)<Subscript>6</Subscript>]

Summary Heat capacity measurements of the two-dimensional metal-assembled complex, (NEt₄)[{Mn^III(salen)}₂Fe^III(CN)₆] [Et=ethyl, salen= N,N’-ethylenebis(salicylideneaminato) dianion], were performed in the temperature range between 0.2 and 300 K by adiabatic calorimetry. A ferrimagnetic phase transition was observed at T_c1=7.51 K. Furthermore, another small magnetic phase transition appeared at T_c2=0.78 K. Above T_c1, a heat capacity tail arising from the short-range ordering of the spins characteristic of two-dimensional magnets was found. The magnetic enthalpy and entropy were evaluated to be ΔH=291 J mol^-1 and ΔS=27.4 J K^-1 mol^-1, respectively. The experimental magnetic entropy agrees roughly with ΔS=Rln(5·5·2) (=32.5 J K^-1 mol^-1; R being the gas constant), which is expected for the metal complex with two Mn(III) ions in high-spin state (spin quantum number S=2) and one Fe(III) ion in low-spin state (S=1/2). The heat capacity tail above T_c1 became small by grinding and pressing the crystal. This mechanochemical effect would be attributed to the increase of lattice defects and imperfections in the crystal lattice, leading not only to formation of the crystal with a different magnetic phase transition temperature but also to decrease of the magnetic heat capacity and thus the magnetic enthalpy and entropy.     

Substrate temperature effects on the structural,compositional, and electrical properties of VO2 thin films deposited by pulsed laser deposition

The vanadium dioxide (VO₂) thin films were deposited on silicon (100) substrate using the pulsed laser deposition technique. The thin films were deposited at different substrate temperatures (500°C, 600°C, 700°C, and 800°C) while keeping all the other parameters constant. X‐ray diffraction confirmed the crystalline VO₂ (B) and VO₂ (M) phase formation at different substrate temperatures. X‐ray photoelectron spectroscopy analysis showed the presence of V⁴⁺ and V⁵⁺ charge states in all the deposited thin films which confirms that the deposited films mainly consist of VO₂ and V₂O₅. An increase in the VO₂/V₂O₅ ratio has been observed in the films deposited at higher substrate temperatures (700°C and 800°C). Scanning electron microscope micrographs revealed different surface morphologies of the thin films deposited at different substrate temperatures. The electrical properties showed the sharp semiconductor to metal transition behavior with approximately 2 orders of magnitude for the VO₂ thin film deposited at 800°C. The transition temperature for heating and cooling cycles as low as 46.2°C and 42°C, respectively, has been observed which is related to the smaller difference in the interplanar spacing between the as‐deposited thin film and the standard rutile VO₂ as well as to the lattice strain of approximately −1.2%.     

Hydrothermal Synthesis and Crystal Structure of AgVMO5 (M = Se,Te)

Single crystals of AgVSeO₅ and AgVTeO₅ were obtained under hydrothermal conditions at 190 °C by reacting stoichiometric amounts of AgNO₃, NaVO₃, TeO₂ and SeO₂, respectively. AgVSeO₅ crystallizes in Pbcm with a = 418.14(3) pm, b = 2007.70(6) pm, c = 521.17(2) pm, V = 437.52(2) × 10⁶ pm³ and Z = 4, as red needles. The structure consists of VO₅ square pyramids, trigonal SeO₃ pyramids and AgO₈ polyhedra, as primary building units. The VO₅ square pyramids are linked to chains running along the c‐axis, by sharing oxygen atoms in the basal plane in cis‐position. The remaining basal O atoms of the VO₅ moieties are shared with two oxygen atoms of the SeO₃ units. The resulting polyanionic strands of composition [VSeO₅]^? are interconnected by silver atoms to form a three dimensional network. AgVTeO₅ crystallizes as yellow needles in P2₁/c with a = 586.59(1) pm, b = 1137.98(2) pm, c = 680.78(1) pm, β = 102.733(1)°, V = 443.26(1) × 10⁶ pm³ and Z = 4. The structure consists of VO₄ tetrahedra, Ψ‐trigonal‐bipyramidal TeO₄ units and AgO₈ polyhedra as primary building units. The TeO₄ groups form dimers by edge sharing, which are linked through vertices to the VO₄ tetrahedra. The resulting one dimensional polyanion is extending along [101]. The structural motifs and charge distribution according to Se⁴⁺/V⁵⁺, and Te⁴⁺/V⁵⁺ respectively, seem to allow for a reshuffling of the charge distribution, thus inducing interesting physical phenomena, at elevated temperatures or pressures.     

Low-temperature phases of the solid electrolyte Ag26I18W4O16

Single crystal X-ray diffraction photographs taken with a Buerger precession camera, at temperatures 250, 214, and 122 K, corroborate the existence of three low-temperature phases of Ag₂₆I₁₈W₄O₁₆. These phases are labeled α′, β, and γ in order of decreasing temperature. The α′ phase is monoclinic, space group P2₁, Z = 2; the β phase is triclinic, space group $\text{P}\text{1}$ or P1, Z = 2; and the γ phase is triclinic, space group P1, Z = 1. Lattice constants at the aforementioned temperatures are given. Twins in the β and γ phases are related by the albite and pericline laws, as are twins in the feldspars. The highest symmetry known to be attained by the (W₄O₁₆)^8? entity is 2(C₂), which, strictly, it must lose at the transition to the α′ phase.     

Thermodynamic and structural investigations of chlorides in the systems KCl/MgCl2 and KCl/MnCl2

By means of a galvanic cell, emf values were measured for the solid-state reactionsnKCl+ MCl₂ = K_nMCl_n+2 for all existing compounds in the pseudobinary systems withM = Mg and Mn. ThusΔG^r values could be calculated and, from their linear temperature dependence in the range 550–730 K, reaction entropies could be determined. EnthalpiesΔH^r were calculated using the Gibbs-Helmholtz relation; they are compared with values found by solution calorimetry at room temperature. The magnitude of the entropy term for the free enthalpy of the formation reactions is discussed for the different compounds. For the modifications ofKMCl₃ the lattice parameters for the cubic, tetragonal, and one of the orthorhombic phases were determined by X-ray photographs at varying temperatures. By DSC measurements the transition enthalpy for the tetragonal to cubic transition of KMnCl₃ at 659 K was found to be 0.20–0.4 kJ · mole^?1, compared to 4.6 kJ · mole for the transition of the stable room-temperature modification with the NH₄CdCl₃ structure to the metastable GdFeO₃ structure.     

Thermal properties of seed proteins

The sample of LiCoO₂ was synthesized, and the heat capacity was measured by adiabatic calorimetry between 13 and 300 K. The smoothed values of the heat capacity were calculated from the data. The thermodynamic functions, standard enthalpy, entropy and Gibbs energy, of LiCoO₂ were calculated from the heat capacity and the numerical values are tabulated at selected temperatures from 15 to 300 K. The heat capacity, enthalpy, entropy, and Gibbs energy at T=298.15 K are 71.57 J K^–1mol^–1, 9.853 kJ mol^–1, 52.45 J K^–1 mol^–1, –5.786 kJ mol^–1, respectively. This revised version was published online in August 2006 with corrections to the Cover Date.     

Ionic conductivity of double vanadate Ag3Sc2(VO4)3

Ionic conductivity of double vanadate Ag₃Sc₂(VO₄)₃ with the NASICON structure is studied by the method of impedance spectroscopy in the frequency range from 5 to 5 × 10⁵ Hz and in the temperature range of 300–827 K. The vanadate Ag₃Sc₂(VO₄)₃ is prepared in the form of fine crystalline powder by solid-state synthesis from V₂O₅, Sc₂O₃, and AgNO₃ at 1173 K. The conductivity of Ag₃Sc₂(VO₄)₃ ceramic samples σ = 8 × 10^?3 S/cm (at 563 K). The σ vs. T curve demonstrates an anomaly at 563–623 K which corresponds to thermal effects in this temperature range. The values of enthalpy of ion transport activation are ΔH = 0.40 ± 0.05 eV (T < 563 K) and ΔH = 0.30 ± 0.05 eV (T > 623 K). The ionic conductivity of Ag₃Sc₂(VO₄)₃ is due to Ag⁺ ions localized in channels of the framework structure of the NASICON type.     

Calorimetric study of phase transitions in the thiourea carbon tetrachloride inclusion compound

Heat capacities of the inclusion compound (thiourea)_3.00CCl₄ have been measured in the temperature range 15–300 K. A first-order phase transition was found at 41.3 K and a second-order transition at 67.17 K. The enthalpy and entropy of the transition are 149 J mol^–1 and 3.7 J K^–1 mol^–1 for the former, and 241 J mol^–1 and 3.9 J K^–1 mol^–1 for the latter. A divergent expression C = A{(T _c –T)/T _c} ^– was fitted to the excess heat capacity of the upper phase transition. The best-fit parameters wereA = 7.4 J K^–1 mol^–1,T _c = 67.166 K and = 0.31. Possible types of molecular disorder in the high temperature phase are discussed in relation to the transition entropy and the molecular and site symmetries of the guest molecule. The heat capacity of the lowest temperature phase was unusually large and may indicate the existence of very low frequency vibrational modes or labile configurational excitation of the guest molecule. Standard thermodynamic functions were calculated from the heat capacity data and are tabulated in the appendix.Contribution No. 11 from the Microcalorimetry Research Center.     

Mechanical loss processes in polysaccharides

Dynamic mechanical properties of cellophane, amylose, and dextran have been obtained over the temperature range 100–520°K and frequency range 10^?2 to 10⁺² Hz on specimens containing various amounts of water. Four mechanical transitions have been characterized. At about 180°K, there is a γ transition that has been assigned to rotation of methylol groups; no comparable transition was found to exist in dextran. At about 240°K, there is a β transition that has been assigned to rotation of methylol–water complexes, but the β transition in dextran appears to be due to some other kind of motion. In cellophane at about 450°K there is an α₂ transition which appears to have contributions from motion of chain segments in disordered regions. The α₁ transition for cellophane occurs at temperatures too high to measure and may be due to segmental motions in chains within crystalline regions. Dextran and amylose were found to have at these same temperatures α loss processes that probably correspond to glass–rubber transitions in amorphous material. The changes in these mechanical loss mechanisms due to moisture uptake suggest that sorbed water associates with glucose repeat units in ways ranging from those which stiffen molecular structure to those which allow greater freedom for other types of motion to occur.