Advances in Materials Physics and Chemistry,2012, 2,25-27
doi:10.4236/ampc.2012.24B007 PublishedOnline December 2012 (http://www.SciRP.org/journal/ampc)
Thermoelectrical Investigation of RareEarth Sulfide
Materials
V. V. Sokolov1,V.V.Bakovetz
1,S.M.Luguev 2,N.V.Lugueva 2
1NikolaevInstitute of Inorganic Chemistry SB RAS, NIIC SBRAS, Novosibirsk, Russia
2AmirkhanovInstituteof Physic DSC RAS,IPh DSC RAS, Makhachkala, Dagestan. Russia
Received 2012
ABSTRACT
Results are presented on synthesis and crystal growthof Gd2S3-Dy
2S3solid solution sulfides and study of their thermoelectric
properties in the range of temperatures 80-400 K. Gd0.2Dy0.8S1.48 composition has the best values ofthermoelectric efficiency 0.39 x
10-3/K at 400 K.
Keywords:Gd2S3 -Dy2S3 SolidSolutionSulfides;Synthesis; Growth of Crystals; Thermoelectric Properties
1. Introduction
Interest in investigation of rare-earth Ln2S3 sulfides with the
structure ofThorium phosphide isbound with possible applica-
tion of compositions on their base as working elements of
thermoelectric energy convertersfor high temperatures. The
researches of GdSy (1.45≤y≤1.50) [1,2]showed that
some compositions at 1000 K havemore high thermoelectric
efficiency ofZ then Ge-Sisolid solution compositions. The
structure of Ln2S3 sulfides allows to fill vacancy with rare-
earth andother metals and to change their thermoelectric prop-
erties.Toincreasethermoelectric efficiency ofmaterial itis
usual todecrease its thermalconductivity creatingthere addi-
tional centers of phonon scattering. Such centers for the GdSy
system may bepresented bythe dopingparamagnetic ionsof
rare-earth elements scattering phonons and decreasing thermal
conductivity but not modifying electrical characteristicsof the
compound. The research ofGd1-xDyxS1.48 compositions con-
tainingparamagnetic Dy ions showed thatin thissystem near
Gd0.2Dy0.8S1.48 havethe minimumthermal conductivity[3].
Therefore is of interest to study dynamics of thermoelectric
properties ofthese compositions independence from tempera-
ture. In this work the synthesis, preparation ofcrystals in
Gd2S3 - Dy2S3 system and their thermoelectricproperties in
the range of temperatures 80-400 K are presented.
2. Experimental
2.1. Preparation o
f
Gd2S3 and D
y
2S3 Sulfides
SulfidesGd2S3and Dy2S3werepreparedfromhigh-purity
rare earthoxides (99.95%)by H2Ssulfidizingat 950– 10000C
[4].
The mixtures of sulfides forcrystallization were prepared
with 0.1 mol. step.
2.2. GrowthofGd2S3 - Dy2S3 SolidSolution Crystals
Directedcrystallization fromsulfide melts at 1700-20000Cin
carbon and glass-carbon containers with HF heating was used.
Crystallization of prepared compositions was carried out
under inert gas atmosphere for preparation of nonstoichiometric
with electronconductivity crystals [5]. Crystallization velocity
was 5 - 30 mm/h.The diameter of the obtainedcylindrical
samples of crystals was 10 mm and height - 1030 mm.
2.3. Methods ofCharacterization of Crystals
-XRD- parametersofcubicTh3P4type
- Gravimetric measurements of density
- Measurements of Seebeck coefficientat 200C
- Chemical and gas-chromatografic analysis of composition
[6]
2.4. Study of Thermoelectrical Properties
To study thermoelectricalproperties the samples werecut from
the central part of ingots that was the most uniform. At temperature
measurements the composition changeon sulfur therefore
uncertainty in an index at sulfur+-0.01 is possible.
Measurements of thermal conductivity coefficient were
performed with absolute stationary method.
The equipment simultaneously with measurementof

in
the same samplesallowedto measureelectricalconductivity
and Seebeck coefficient at 80-400 K.
3. Results and Discussion
3.1. Characterization o
f
Prepared Cr
y
stals
Results of characterization of prepared crystals of Gd2S3-
Dy2S3solid solution is presented inTable 1.
All crystalshavecubicstructure ofTh3P4typewithlinear
dependence of cell parameters from composition.
The same dependence is in density of crystals from composition.
Deviationof compositionfrom stoichiometry from Gd2S3to
Dy2S3agree with their phase diagrams.
Increasing ofSeebeck coefficient fromGd2S3to Dy2S3to

V. V. SOKOLOVET AL. 26
agree withdeviation composition from stoichiometry of
crystals.
3.2. Results of Thermoelectric Measurements
Temperature dependencesof thermal conductivity coefficient in
Gd2S3-Dy
2S3system for the seriesof compositions at80 -400
K are presented in Figure 1.
Up to Gd0.6Dy0.4S1.48thermal conductivity weakly depends
on the concentration of Dy ions and Gd0.2Dy0.8S1.490sample has
minimal thermalconductivity in this system (Figure 2).
Analysis of thermal conductivity is made in paper [3].
Substantial decrease of thermal conductivity coefficient is
observed for the compositions with x >0.6, and minimal value
κ

at x = 0.8.Suchdependence ofthermal conductivity
coefficientof thesamplesunusual forphononheattransfer is
results from phonon scattering by paramagneticions of
Dysprosium.
Results of Seebeck coefficient and electrical conductivity
measurements are presented on Figure 3 and 4.
Temperature dependence of Seebeck coefficient and
electrical conductivity has the same character that usual metals
or dopedsemiconductors.At constantconcentration ofcarriers
of acurrentanddecreaseoftheirmobilitywith growthof
temperature Seebeck coefficient linearly growsand electrical
conductivity decreases.
Table 1.Characteristics of crystals in Gd2S3- Dy2S3system.
Composition
mol. Gd2S3
Parameter
of cell, Ǻ
Indexyin
Gd1-xDyxSy
exp. calc.
Density d,
g/ cm3
exp. calc.
Seebeck coeff.
 mkV/ К
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
8.372
8.365
8.358
8.350
8.340
8.331
8.328
8.323
8.311
8.302
8.292
1.466 1.464
1.464 1.467
1.471 1.470
1.476 1.477
1.479 1.480
1.478 1.482
1.479 1.473
1.488 1.482
1.490 1.487
1.489 1.486
1.493 1.495
6.30 6.29
6.33 6.34
6.35 6.35
6.36 6.37
6.39 6.39
6.42 6.43
6.46 6.45
6.47 6.45
6.50 6.49
6.54 6.53
6.55 6.56
62
68
77
72
80
76
88
107
110
104
125
Figure 1. Temperature dependences of thermal conductivity
coefficient of GdS1.48 (1), Gd0.6 Dy0.4 S1.48 (2), DyS1.48 (3), Gd0.2
Dy0.8 S1.48 (4).
Figure 2. Concentration dependencesof thermalconductivity
coefficient of solid solutions Gd1xDyxS1.48 at 300(1) and400 K (2).
Figure 3.Temperature dependences ofSeebeck coefficient of
Gd0.3Dy0.7S1.48 (1), Gd0.2Dy0.8S1.48 (2), Gd0.1Dy0.9S1.48 (3), GdS1.48 (4).
Figure 2. Temperature dependences ofelectrical conductivity of
GdS1.48 (1), Gd0.1Dy0.9S1.48 (2), Gd0.2Dy0.8S1.48 (3), Gd0.3Dy0.7S1.48 (4).
Replacement of Gadolinium by Dysprosium asit was
established earlier [7], at 300 K changes of Seebeck coefficient
growth and electrical conductivity lowering no more than for
18 % within Gd1-xDyxS1.48compositions. At 80 K electrical
conductivity of Gd0.3Dy0.7S1.48is lower for 30%, than at GdS1.48.
On the basisof experimental dataof Seebeck coefficientand
electrical and thermalconductivity thethermoelectric efficiency
(Z = α2σ/κ) ofthe studied compositionswas calculated.
Gd0.2Dy0.8S1.48 composition has the best values of this
parameteramong thesamples investigatedin thiswork. Values
Z forthe studiedsamplesat 300and 400K arepresentedin the
Table 2.
Thus, studyofthermalconductivity,Seebeck coefficient and
electricalconductivity of Gd1-xDyxS1.48compositions inarange
of temperatures80-400Kshowed thatthemain contributionto
heat transfer tothem isbrought byfluctuations of acrystal
Table 2. Thermoelectric properties of some Gd1-xDyxS1.48

V. V. SOKOLOVET AL.
27
composition.
Composition
-,
mkV·K-1
300 К
,
Om-1cm-1
300 K
,
Wm
-1K-1
300 K
Z·103,K
-1
300КZ·103,K
-1
400K
GdS1.48 723041.040.16 0.27
Gd0.3Dy0.7S1.48 832620.800.23 0.34
Gd0.2Dy0.8S1.48 802780.740.24 0.39
Gd0.1Dy0.9S1.48 752980.840.20 0.32
lattice.It is established that replacement of atoms of a
Gadolinium byDysprosium reducesthermal conductivity with
growth of deficiency of a crystals and additional scattering of
phonon on paramagnetic Dy ions. On the basis of experimental
datainallstudied temperatureintervalthe Gd0.2Dy0.8S1.48
sample has maximal value of thermoelectric efficiency.
REFERENCES
[1]G.G. Gadzhiev, S.M.Luguev,V.V. Sokolov, B.N. Magdiev.,
N.V. Lugueva.Inbook.:Transfer ofcarriers ofa chargeand
heat in semiconductors. Makhachkala, 1986,page 87-93.(on
Russian)
[2]S.M. Luguev, V.V. Sokolov, N.V. Lugueva. Advanced Materials
and Proceessing.Proceedingsof Russia-JapanSeminar,
September 15-20, 2007. Novosibirsk, 2007, р.71-75.
[3]S.M. Luguev, N.V. Lugueva, V.V. Sokolov. Temperature and
concentration dependences of thermal conductivity of solid
solutions of gadolinium anddysprosium//Thermophysics and
Aeromechanics, vol. 19,No. 3,pp. 375-380, 2012.
[4]V.V. Sokolov, A.A. Kamarzin, L.N.Trushnikova,M.V.
Savelyeva Optical materials containing rare earth Ln2S3sulfides
// J. Alloysand Comp.vol.225, No. 2,.pp. 567570, 1995.
[5]A.A.Kamarzin,K.E.Mironov,V.V. SokolovGrowthand
Properties of Lanthanum and Rare Earth Metal Sesquisulfide
Crystals// J.CrystalGrowth. vol.52, pp.619 –622, 1981.
[6]L.S. Chuchalina, I.G. Vasilyeva, A.A. Kamarzin Non-direc
t
method ofgas-chromatografic analysisof determination of
lanthanum sulfide composition//Journal of Analytical
Chemistry vol. 33,n. 1, pp.190-192,1978. (on Russian)
[7]S.M. Luguev,N.V. Lugueva, V.V. Sokolov. Thermoelectricsand
their applications. Reports of theX-th interstateseminar. , Ioffe
PTI, pp.179-183,2007. (onRussian)