Some Physicochemical Properties of Yb14MnSb11 and Its Solid Solutions with Gadolinium Yb14-xGdxMnSb11 Type

Data on structure, melting temperatures, density, thermal dilatation, kinetic of oxidation of Yb14MnSb11 and its solid solutions with gadolinium are presented in this article.All solid solutions are isostructural with Zintlcompound Ca14AlSb11 and crystallizes in tetragonal structure. Investigations melting temperatures, thermal dilatation, oxidation have shown that GdincorporatenearGd≈ 0.5 and increased melting point and thermal dilatation.


Introduction
Zintl phases form a large family of compounds whose electronic conform to 8-N or Zintl -Klemm concept. This class of materials has been identified as promising for thermoelectric(TE) applications.A new direction of thermoelectric materials began when Slack theorized the so-called "electron crystals-phonon glass" [1]. Slack considered that a good TE material should have the electronic structure of a heavily doped narrow-band-gap semiconductor and thermal conductivity like a glass. One of new class materials for utilization as high-temperature thermoelectric materials are Zintl phases that employ the pnictides of rare earth elements and have the Ca 14 AlSb 11 structure type.For the first time Zintl phase of Yb 14 MnSb 11 was obtained in 1998 [2]. Yb 14 MnSb 11 is high-temperature thermoelectricalmaterial [2][3][4][5].
In spite of numerous publications, there are no works on thermal properties, there are no works on chemical properties. From the position of thermoelectrical materials thermal stability of these materials is also functional property since the exploitation of the material is carried out under high temperatures, where diffusion, volatility, oxidation processes easily occur.
Due to this the results of melting temperature and thermal dilatation, kinetic of oxidation of Yb 14 MnSb 11 and its solid solutions with gadolinium are presented in the present work.

Synthesis
Crystals with the expected stoichiometry of Yb 14-x Gd x MnSb 11 were grown from a molten Sn flux (Yb 14-x Gd x Mn 6 Sb 11 Sn 86 ) in a glassy carbon crucible. The elements were divided into layers: antimony-manganese-Gd-ytterbium-tin and quartz cotton that plays the role of filter at centrifugation was placed above. The crucible was placed in quartz ampoule pump out and seal. Pumped out quartz ampoule is placed in to the furnace and heat up to 1000°С with the delay at this temperature for 6 hours. Cooling up to 700°С was made at rate 2°/hour. At700°С ampoule was taken out from the furnace and centrifuged for 5 minutes. Crystals are separated into dry box [2].

2.2Electron microprobe analysis
The samples analyzed using a Camera JXA-8100 (JEOL, Japan) Electron Probe Micro-analyzer equipped with a wavelength-dispersive spectrometer with 20 keV accelerating potential and 30nA beam current.Net elemental intensities were determined with respect to calibration standards of YbPO 4 , GdPO 4 , SnO 2 , Sb (high concentration relation to that in "14-1-11"), and Algarnet-Arizona ruby O-145 (here content of MnO was 9.9 mass.%). The composition of the major phase in each sample was determined by calculating averages and standard deviations from 15-30 random data points. The This is an Open Access article distributed under the terms of the Creative Commons Attribution License 4.0, which permits distribution, and reproduction in any medium, provided the original work is properly cited.

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Article available at http://www.matec-conferences.org or http://dx.doi.org/10.1051/matecconf/20164304001 composition of the minor phases was determined from 5-7 data points depending on the area of the phases and it was not sufficient for standard deviation (Table 1).

Powder X-ray diffraction
X-ray structural study of single samples was conducted on the apparatus DRON-UM (R=192mm, CuKDradiation, Ni-filter, scintillation detector with amplitude discrimination, step 0.02q 2T, impulse storage time in each point 3 s, room temperature) in the region of angles from 5 to 80q 2T. Total shooting time was 9 hours. After crystal pounding in the mortar with heptanes and suspension drying on the polished side of the standard quartz cavity, the sample was flat and thin (~ 100 μm) layer. All studied samples have tetragonal structure and obtained lattice parameters are given in the Table 2.

Thermal stability
Thermal analysis with 2-3 mg was repeated 3-5 times with each sample. Two procedures were developed to study thermal stability of the tested samples. First was heating with the rate of 3000°C/min up to 1900°C in apparatus with the He pressure equal to 7 atm. During heating, all variations with the sample were fixed and related to appropriating peaks on the heating curve. Second procedure was step-by-step heating with examination what happens with the sample being heated to a given temperature. Three temperatures were taken: up to 1500°C, then 1750°C and at last 1900°C to fix stability without any vaporization, melting, and incongruent evaporation of melt. The apparatus was calibrated by melting point of Pd (1554°C) used as standard and this action was performed before and after measurements of the sets of the samples.

Thermal expansion
Thermal expansionwasmeasured in cylindrical samples about 4 mm in diameter, 10-13mm long the temperature range 25-750°С using a high-temperature dilatometer which construction is described in [15].

Kinetica
Oxidation kinetics was studied using thermogravimetric method based on the continuous sample weighing at temperatures 773, 873 и 973°С. Oxidation was carried out in air atmosphere in alumina crucibles.

Results and discussion
The experimental data were treated mathematically and the compositions of the major and minor phases and their calculated errors are given in Table 1.
The powder X-ray diffraction patterns for some samples are shown in Figure1.

ICNMS 2016
All crystals have tetragonal structure and these lattice parameters are given Table2.  Table 3. Some thermal properties ofYb 14-x Gd x MnSb 11 .

Compounds
Тmelt  Thermal expansion coefficients presented in the table are found by slopes of straight lines.
To obtain information on chemical link stability characteristic temperature Debye and mean-square substitution in crystal were calculated.
In [16], equation (1) which links thermal expansion coefficient with characteristic Debye temperature, was obtained by superposition of the Lindermann -Borelius and Grueneizen expressions: where Ā-mean-square atomic weight; V-molecular volume; α-coefficient of thermal expansion.
Both thermal expansion coefficients, melting temperatures and Debye temperatures evidences of changes after composition of gadolinium≈ 0.5 characteristics. Probably it can be explained by that fact that solubility limit of gadoliniumin this compound switches to this concentration.
The oxidation rate increases in all samples with the temperature increase. Oxidation curves have parabolic form with intensive oxidation rate in the initial period.InFigures3-6 oxidation kinetic curves of solid solutions are presented. According to the kinetic curves built basing on change in sample weight depending on time, the oxidation rate was calculated at each temperature. True oxidation rate of the compounds calculated according to tangent, drawn from the coordinate start to the curves and calculated by formula: К= g/s•∆t, is presented in Table4. According to the direct dependence lgK-1/T, apparent activation energy was calculated(Table4).

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The maximum amount of each RE substituted for Yb was determined to be 0.7 for La, 0.6 for Ce, and 0.5 for Tm [5][6][7][8]. Microprobe analysis of the crystals of Yb 14-x Gd 0x MnSb 11 of which properties were measured provided the following compositions (Table 1) showing that the maximum amount of Gd that can substitute for Yb in Yb 14-x Gd x MnSb 11 is x = 0.48.As data show (Tables 2-4) maximum introduction of does not exceed х=0.5, that proves obtainedvalues of melting,temperature, coefficient of thermal expansion andactivation energy.
Obtained physic-chemical characteristics along with another property allow us to assume the nature of this new class of inorganic materials.