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1.
The heat capacities and enthalpy increments of strontium bismuth niobate SrBi2Nb2O9 (SBN) and strontium bismuth tantalate SrBi2Ta2O9 (SBT) were measured by the relaxation method (2–150 K), Calvet-type heat-conduction calorimetry (305–570 K) and drop calorimetry (773–1373 K). The temperature dependences of non-transition heat capacities in the form Cpm = 324.47 + 0.06371T − 5.0755 × 106/T2 J K−1 mol−1 (298–1400 K) and Cpm = 320.22 + 0.06451T − 4.7001 × 106/T2 J K−1 mol−1 (298–1400 K) were derived for SBN and SBT, respectively, by the least-squares method from the experimental data. Furthermore, the standard molar entropies at 298.15 K Sm°(SBN)=327.15±0.80 and Sm°(SBT)=339.23±0.72 J K−1 mol−1 were evaluated from the low-temperature heat capacity measurements.  相似文献   

2.
3.
Heat capacity and enthalpy increments of ternary bismuth tantalum oxides Bi4Ta2O11, Bi7Ta3O18 and Bi3TaO7 were measured by the relaxation time method (2-280 K), DSC (265-353 K) and drop calorimetry (622-1322 K). Temperature dependencies of the molar heat capacity in the form Cpm=445.8+0.005451T−7.489×106/T2 J K−1 mol−1, Cpm=699.0+0.05276T−9.956×106/T2 J K−1 mol−1 and Cpm=251.6+0.06705T−3.237×106/T2 J K−1 mol−1 for Bi3TaO7, Bi4Ta2O11 and for Bi7Ta3O18, respectively, were derived by the least-squares method from the experimental data. The molar entropies at 298.15 K, S°m(298.15 K)=449.6±2.3 J K−1 mol−1 for Bi4Ta2O11, S°m(298.15 K)=743.0±3.8 J K−1 mol−1 for Bi7Ta3O18 and S°m(298.15 K)=304.3±1.6 J K−1 mol−1 for Bi3TaO7, were evaluated from the low-temperature heat capacity measurements.  相似文献   

4.
The heat capacity and the heat content of bismuth niobate BiNbO4 and bismuth tantalate BiTaO4 were measured by the relaxation method and Calvet-type heat flux calorimetry. The temperature dependencies of the heat capacities in the form Cpm=128.628+0.03340 T−1991055/T2+136273131/T3 (J K-1 mol-1) and 133.594+0.02539 T−2734386/T2+235597393/T3 (J K-1 mol-1) were derived for BiNbO4 and BiTaO4, respectively, by the least-squares method from the experimental data. Furthermore, the standard molar entropies at 298.15 K Sm(BiNbO4)=147.86 J K-1 mol-1 and Sm(BiTaO4)=149.11 J K-1 mol-1 were assessed from the low temperature heat capacity measurements. To complete a set of thermodynamic data of these mixed oxides an attempt was made to estimate the values of the heat of formation from the constituent binary oxides.  相似文献   

5.
Heat capacity and enthalpy increments of calcium niobates CaNb2O6 and Ca2Nb2O7 were measured by the relaxation time method (2–300 K), DSC (260–360 K) and drop calorimetry (669–1421 K). Temperature dependencies of the molar heat capacity in the form C pm=200.4+0.03432T−3.450·106/T 2 J K−1 mol−1 for CaNb2O6 and C pm=257.2+0.03621T−4.435·106/T 2 J K−1 mol−1 for Ca2Nb2O7 were derived by the least-squares method from the experimental data. The molar entropies at 298.15 K, S m0(CaNb2O6, 298.15 K)=167.3±0.9 J K−1 mol−1 and S m0(Ca2Nb2O7, 298.15 K)=212.4±1.2 J K−1 mol−1, were evaluated from the low temperature heat capacity measurements. Standard enthalpies of formation at 298.15 K were derived using published values of Gibbs energy of formation and presented heat capacity and entropy data: Δf H 0(CaNb2O6, 298.15 K)= −2664.52 kJ molt-1 and Δf H 0(Ca2Nb2O7, 298.15 K)= −3346.91 kJ mol−1.  相似文献   

6.
The calcium mixed phosphate Ca8P2O7(PO4)4 has been synthesized by thermal decomposition of octacalcium phosphate previously prepared by precipitation in ammoniacal phosphate solution. The enthalpy of formation at 298.15 K referenced to β-tricalcium phosphate and calcium pyrophosphate is determined. β-Tricalcium phosphate was prepared by two methods: precipitation in ammoniacal aqueous medium and high temperature solid-state reaction. Calcium pyrophosphate was prepared by high temperature solid-state reaction. All the compounds are characterized by chemical analysis, X-rays diffraction and IR spectroscopy. The enthalpy of formation +10.83 ± 0.63 kJ mol−1 is obtained by solution calorimetry at 298.15 K in nitric acid.  相似文献   

7.
The enthalpies of reactions between alkaline-earth cuprates M2CuO3 (M = Ca, Sr) and hydrochloric acid were measured in a hermetic swinging calorimeter at 298.15 K. The M2CuO3 samples were prepared by solid-phase synthesis from calcium or strontium carbonate and copper oxide and characterized by X-ray powder diffraction, EDX and wet analysis. The standard enthalpies of formation obtained for the cuprates, −1431 ± 4 kJ mol−1 for Ca2CuO3 and −1374 ± 3 kJ mol−1 for Sr2CuO3, are discussed and compared with previous experimental and assessed values.  相似文献   

8.
X-ray photoelectron spectroscopy (XPS) measurements were carried out on a strontium pyroniobate (Sr2Nb2O7) powder sample, which was synthesized using standard solid-state method. The binding energy (BE) differences between the O 1s and cation core levels, Δ(O-Nb)=BE(O 1s)-BE(Nb 3d5/2) and Δ(O-Sr)=BE(O 1s)-BE(Sr 3d5/2), were used to characterize the valence electron transfer on the formation of the Nb-O and Sr-O bonds. The chemical bonding effects were considered on the basis of our XPS results for Sr2Nb2O7 and earlier published structural and XPS data for other Sr- or Nb-containing oxide compounds. The new data point for Sr2Nb2O7 is consistent with the previously derived relationship for a set of Nb5+-niobates that Δ(O-Nb) increases with increasing mean Nb-O bond distance, L(Nb-O). A new empirical relationship between Δ(O-Sr) and L(Sr-O) was also obtained. Interestingly, the correlation between Δ(O-Sr) and L(Sr-O) was found to differ from that between Δ(O-Nb) and L(Nb-O). Possible cause for the difference is discussed.  相似文献   

9.
Preparation of new solid solutions containing divalent europium have been tried in the systems Eu2Nb2O7Sr2Nb2O7 and Eu2Ta2O7Sr2Ta2O7. These solid solutions described as Eu2xSr2(1?x)M2O7 (M = Nb and Ta) exist in a pure orthorhombic phase in a limited region of x from 0 to about 0.5. The compounds with compositions close to Eu2M2O7 exist but techniques have not been found to prepare them in pure form.  相似文献   

10.
The heat capacity of Cr(C5H7O2)3 has been measured by the adiabatic method within the temperature range 5-320 K. An anomaly with a maximum at ∼60 K has been discovered which points to the phase transformation of the compound. Anomalous contributions to entropy and enthalpy have been revealed. The thermodynamic functions (entropy, enthalpy and reduced Gibbs energy) at 298.15 K have been calculated using the obtained experimental heat capacity data. The Raman spectra have been measured in the frequency range 60-400 cm−1 and in the temperature range 5-220 K. It has been discovered that a new line (109 cm−1) appears at ∼60 K. The nature of these peculiarities in heat capacity and in Raman spectra is discussed.  相似文献   

11.
The two alkaline earth niobates Sr2Nb2O7 and Ba0.5Sr0.5Nb2O6 have been prepared, their electronic properties measured, and their photoresponses compared. The indirect band gap in Sr2Nb2O7 is 3.86 eV compared with 3.38 eV for Ba0.5Sr0.5Nb2O6. Hence, photoanodes composed of Sr2Nb2O7 respond to much less of the “white” light spectrum than those made from Ba0.5Sr0.5Nb2O6. Nevertheless, their electrical outputs at an anode potential of 0.8 eV with respect to SCE in 0.2 M sodium acetate under “white” xenon arc irradiation of 1.25 W/cm2 are comparable.  相似文献   

12.
The heat capacity of LuPO4 was measured in the temperature range 6.51-318.03 K. Smoothed experimental values of the heat capacity were used to calculate the entropy, enthalpy and Gibbs free energy from 0 to 320 K. Under standard conditions these thermodynamic values are: (298.15 K) = 100.0 ± 0.1 J K−1 mol−1, S0(298.15 K) = 99.74 ± 0.32 J K−1 mol−1, H0(298.15 K) − H0(0) = 16.43 ± 0.02 kJ mol−1, −[G0(298.15 K) − H0(0)]/T = 44.62 ± 0.33 J K−1 mol−1. The standard Gibbs free energy of formation of LuPO4 from elements ΔfG0(298.15 K) = −1835.4 ± 4.2 kJ mol−1 was calculated based on obtained and literature data.  相似文献   

13.
The thermal conductivity and heat capacity of high-purity single crystals of yttrium titanate, Y2Ti2O7, have been determined over the temperature range 2 K?T?300 K. The experimental heat capacity is in very good agreement with an analysis based on three acoustic modes per unit cell (with the Debye characteristic temperature, θD, of ca. 970 K) and an assignment of the remaining 63 optic modes, as well as a correction for CpCv. From the integrated heat capacity data, the enthalpy and entropy relative to absolute zero, are, respectively, H(T=298.15 K)−H0=34.69 kJ mol−1 and S(T=298.15 K)−S0=211.2 J K−1 mol−1. The thermal conductivity shows a peak at ca. θD/50, characteristic of a highly purified crystal in which the phonon mean free path is about 10 μm in the defect/boundary low-temperature limit. The room-temperature thermal conductivity of Y2Ti2O7 is 2.8 W m−1 K−1, close to the calculated theoretical thermal conductivity, κmin, for fully coupled phonons at high temperatures.  相似文献   

14.
新铌酸盐Ba5NdTi3Nb7O30的合成与介电性能   总被引:6,自引:0,他引:6  
方亮  张辉  鄢俊兵  杨卫明 《无机化学学报》2002,18(11):1131-1134
The New Niobate Ba5NdTi3Nb7O30 was synthesized by solid state reaction at 1250℃ for 48h. The crystal structure and dielectric properties of Ba5NdTi3Nb7O30 were determined by X-ray powder diffraction and dielectric measurements. The results show that Ba5NdTi3Nb7O30 belongs to ferroelectric phase of tetragonal tungsten bronze structure at room temperature with unit cell parameters: a=1.24424(4)nm, c=0.39476(2)nm, calculated density 5.719g·cm-3. Ba5NdTi3Nb7O30 belongs to relaxor ferroelectrics. The phase transition temperature (Tc) of Ba5NdTi3Nb7O30 from ferroelectric to paraelectric is found to shift toward higher temperature side at higher fre-quency, and Tc is 90℃ at 1kHz. At room temperature, the dielectric constant (εr) and dielectric loss of Ba5NdTi3Nb7O30 decrease with the increase of frequency, and Ba5NdTi3Nb7O30 ceramic have high dielectric constant 489 at 1kHz.  相似文献   

15.
The molar heat capacities of 1-(2-hydroxy-3-chloropropyl)-2-methyl-5-nitroimidazole (Ornidazole) (C7H10ClN3O3) with purity of 99.72 mol% were measured with an adiabatic calorimeter in the temperature range between 79 and 380 K. The melting-point temperature, molar enthalpy, ΔfusHm, and entropy, ΔfusSm, of fusion of this compound were determined to be 358.59±0.04 K, 21.38±0.02 kJ mol−1 and 59.61±0.05 J K−1 mol−1, respectively, from fractional melting experiments. The thermodynamic function data relative to the reference temperature (298.15 K) were calculated based on the heat capacities measurements in the temperature range from 80 to 380 K. The thermal stability of the compound was further investigated by DSC and TG. From the DSC curve an intensive exothermic peak assigned to the thermal decomposition of the compound was observed in the range of 445-590 K with the peak temperature of 505 K. Subsequently, a slow exothermic effect appears when the temperature is higher than 590 K, which is probably due to the further decomposition of the compound. The TG curve indicates the mass loss of the sample starts at about 440 K, which corresponds to the decomposition of the sample.  相似文献   

16.
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 ΔfH°(298.15 K), S°(298.15 K) S°(T), Cp°(T), H°(T), {H°(T)-H°(298.15 K)}, G°(T), free energy function (fef), ΔfH°(T) and ΔfG°(T) for these ternary oxides were also calculated from 298 to 1000 K.  相似文献   

17.
A new cesium uranyl niobate, Cs9[(UO2)8O4(NbO5)(Nb2O8)2] or Cs9U8Nb5O41 has been synthesized by high-temperature solid-state reaction, using a mixture of U3O8, Cs2CO3 and Nb2O5. Single crystals were obtained by incongruent melting of a starting mixture with metallic ratio=Cs/U/Nb=1/1/1. The crystal structure of the title compound was determined from single crystal X-ray diffraction data, and solved in the monoclinic system with the following crystallographic data: a=16.729(2) Å, b=14.933(2) Å, c=20.155(2) Å β=110.59(1)°, P21/c space group and Z=4. The crystal structure was refined to agreement factors R1=0.049 and wR2=0.089, calculated for 4660 unique observed reflections with I?2σ(I), collected on a BRUKER AXS diffractometer with MoKα radiation and a CCD detector.In this structure the UO7 uranyl pentagonal bipyramids are connected by sharing edges and corners to form a uranyl layer corresponding to a new anion-sheet topology, and creating triangular, rectangular and square vacant sites. The two last sites are occupied by Nb2O8 entities and NbO5 square pyramids, respectively, to form infinite uranyl niobate sheets stacking along the [010] direction. The Nb2O8 entities result from two edge-shared NbO5 square pyramids. The Cs+ cations are localized between layers and ensured the cohesion of the structure.The cesium cation mobility between the uranyl niobate sheets was studied by electrical measurements. The conductivity obeys the Arrhenius law in all the studied temperature domains. The observed low conductivity values with high activation energy may be explained by the strong connection of the Cs+ cations to the infinite uranyl niobate layers and by the high density of these cations in the interlayer space without vacant site.Infrared spectroscopy investigated at room temperature in the frequency range 400-4000 cm−1, showed some characteristic bands of uranyl ion and niobium polyhedra.  相似文献   

18.
Hydrated strontium borate, SrB4O7·3H2O, has been synthesized and characterized by XRD, FT-IR, DTA-TG and chemical analysis. The molar enthalpy of solution of SrB4O7·3H2O in 1 mol dm−3 HCl(aq) was measured to be (21.15 ± 0.29) kJ mol−1. With incorporation of the previously determined enthalpies of solution of Sr(OH)2·8H2O(s) in [HCl(aq) + H3BO3(aq)] and H3BO3 in HCl(aq), and the enthalpies of formation of H2O(l), Sr(OH)2·8H2O(s) and H3BO3(s), the enthalpy of formation of SrB4O7·3H2O was found to be −(4286.7 ± 3.3) kJ mol−1.  相似文献   

19.
Two new quaternary strontium selenium(IV) and tellurium(IV) oxychlorides, namely, Sr3(SeO3)(Se2O5)Cl2 and Sr4(Te3O8)Cl4, have been prepared by solid-state reaction. Sr3(SeO3)(Se2O5)Cl2 features a three-dimensional (3D) network structure constructed from strontium(II) interconnected by Cl, SeO32− as well as Se2O52− anions. The structure of Sr4(Te3O8)Cl4 features a 3D network in which the strontium tellurium oxide slabs are interconnected by bridging Cl anions. The diffuse reflectance spectrum measurements and results of the electronic band structure calculations indicate that both compounds are wide band-gap semiconductors.  相似文献   

20.
The compound cesium niobate, Cs2Nb4O11, is an antiferroelectric, as demonstrated by double hysteresis loops in the electric field versus polarization plot. The crystal structure refinement by X-ray diffraction at both 100 and 297 K shows it to have a centrosymmetric structure in point group mmm and orthorhombic space group Pnna, which is consistent with its antiferroelectric behavior. The 100-K structure data is reported herein. The lattice is comprised of niobium-centered tetrahedra and octahedra connected through shared vertices and edges; cesium atoms occupy channels afforded by the three-dimensional polyhedral network. Antiferroelectricity is produced by antiparallel displacements of niobium atoms along the c-axis at the phase transition temperature of 165 °C. The critical field for onset of ferroelectric behavior in a single-crystal sample is 9.5 kV/cm at room temperature.  相似文献   

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