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Abstract  

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 m 0(CaNb2O6, 298.15 K)=167.3±0.9 J K−1 mol−1 and S m 0(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.

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Heat capacity and phonon spectra of A IIIN

Experiment and calculation

Journal of Thermal Analysis and Calorimetry
Authors:
D. Sedmidubský
,
J. Leitner
,
P. Svoboda
,
Z. Sofer
, and
J. Macháček

Abstract  

The low temperature heat capacities of three A IIInitrides, A III=Al, Ga and In, were measured by relaxation method in the temperature range 2–300 K and the corresponding entropies at the reference temperature 298.15 K were evaluated from the experimental data. The lattice heat capacity at constant volume was also assessed theoretically within harmonic crystal approximation by direct method using a combination of VASP software package to obtain the Hellmann-Feynman forces and the Phonon program to calculate the phonon spectra. The experimental data were analyzed by means of a Debye-Einstein model taking use of the calculated heat capacity and involving additionally an anharmonic contribution.

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Journal of Thermal Analysis and Calorimetry
Authors:
M. Hampl
,
J. Leitner
,
K. Růžička
,
M. Straka
, and
P. Svoboda

Abstract

The heat capacity and the heat content of bismuth niobate BiNb5O14 were measured by the relaxation time method, DSC and drop method, respectively. The temperature dependence of heat capacity in the form C pm=455.84+0.06016T–7.7342·106/T 2 (J K−1mol−1) was derived by the least squares method from the experimental data. Furthermore, the standard molar entropy at 298.15 K S m=397.17 J K−1mol−1 was derived from the low temperature heat capacity measurement.

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Journal of Thermal Analysis and Calorimetry
Authors:
J. Venkrbec
,
J. Kousal
,
J. Štětina
,
J. Fiksa
,
V. Papež
,
V. Rosická
,
V. Knobloch
,
J. Leitner
, and
J. Kohout

Abstract  

This work contributes to the growth of bulk crystals where crystals are grown from a molten-solution zone (MSZ). Our original modifications ofTHM have been used for a crystallization of GaSb and of (Ga.In)Sb—the ternary Solid Solution (TSS). The crystallization process has been accelerated with a low frequency and low energy vibrational stirring (VS). Lately, the stirring has been combined with the magneto-hydrodynamical stirring (MHD-S) and applied on GaSb. The lattice parameter ‘a’ ofTSS crystals has been constant throughout the significant part of the ingot length. This approach has permitted the growth of these crystalline ingots with ‘a’ apriori chosen and calculated—having the deviation from its constancy less than 0.03% (0.2 pm) with a 75 mm length. Crystals can have a mosaic structure at this stage.

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Journal of Thermal Analysis and Calorimetry
Authors:
J. Leitner
,
M. Hampl
,
K. Růžička
,
M. Straka
,
D. Sedmidubský
, and
P. Svoboda

Abstract  

The heat capacity and the enthalpy increments of strontium metaniobate SrNb2O6 were measured by the relaxation method (2-276 K), micro DSC calorimetry (260-320 K) and drop calorimetry (723-1472 K). Temperature dependence of the molar heat capacity in the form C pm=(200.47±5.51)+(0.02937±0.0760)T-(3.4728±0.3115)·106/T 2 J K−1 mol−1 (298-1500 K) was derived by the least-squares method from the experimental data. Furthermore, the standard molar entropy at 298.15 K S m 0 (298.15 K)=173.88±0.39 J K−1 mol−1 was evaluated from the low temperature heat capacity measurements. The standard enthalpy of formation Δf H 0 (298.15 K)=-2826.78 kJ mol−1 was derived from total energies obtained by full potential LAPW electronic structure calculations within density functional theory.

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