Journal of Alloys and Compounds460(2008)
安全生产标准化建设
职工食堂管理制度
155–159
Crystal structure determination of CoGeTe from powder diffraction data
F.Laufek a,∗,J.Navr´a til b,J.Pl´aˇs il c,T.Plech´aˇc ek b
a Czech Geological Survey,Geologick´a6,15200Praha5,Czech Republic
b Joint Laboratory of Solid State Chemistry of IMC ASˇCR and University of Pardubice,Studentsk´a
84,53210Pardubice,Czech Republic
c Faculty of Science,Charles University,Albertov6,12843Praha2,Czech Republic
Received24April2007;received in revid form8June2007;accepted13June2007
Available online17June2007
Abstract
The crystal structure of cobalt germanium telluride CoGeTe has been determined by direct methods
using integrate intensities of conventional X-ray powder diffraction data and subquently refined with the Rietveld method.The title compound was prepared by heating of stoichiometric amount of Co,Ge and Te in silica glass tube at670◦C.
CoGeTe adopts orthorhombic symmetry,space group Pbca with unit cell parameters a=6.1892(4)˚A,b=6.2285(4)˚A,c=11.1240(6)˚A, V=428.8(1)˚A3and Z=8.Its crystal structure is formed by[CoGe3Te3]octahedra sharing both edges and corners.CoGeTe reprents a ternary ordered variant of␣-NiAs2type structure.An important feature prent in CoGeTe is an occurrence of short Co–Co distance across the shared edge of[CoGe3Te3]octahedra.Differential thermal analysis(DTA)has revealed that CoGeTe melts incongruently at about725◦C;CoGeTe decompos into GeTe,CoGe and CoTe2.Temperature dependence of the electrical conductivity and value of Seebeck coefficient at300K are also reported.©2007Elvier B.V.All rights rerved.
Keywords:CoGeTe;Crystal structure;X-ray powder diffraction;Direct methods;Rietveld refinement
1.Introduction
The prent paper is a part of systematic investigations on the crystal structures of the MXCh compounds of cobalt-group metals(M=Co,Ir and Rh)and main groups IV and VI elements (X=Si,Ge a
nd Sn;Ch=S,Se and Te).With regard to ternary cobalt bearing germanide chalcogenides(Ch=S,Se and Te), the examples encountered so far are CoGe1.5S1.5,CoGe1.5Se1.5 [1,2]and CoGe1.5Te1.5[3].The crystal structures of the com-pounds can be viewed as a modification of skutterudite structure MX3(M=Co,Rh or Ir;X=P,As or Sb),where Ge and Ch atoms exhibit long-range ordering,which results in a lower-ing of the symmetry from original cubic to rhombohedral[2]. The phas are of interest in materials science becau of their possible thermoelectric applications.
In this work,we prent a detailed study of the crystal struc-ture of the new ternary compound CoGeTe,which is a distorted variant of␣-NiAs2,using conventional powder X-ray diffrac-
∗Corresponding author.Fax:+420251818748.
E-mail address:(F.Laufek).tion.Data obtained by differential thermal analysis are also reported.In spite of the relatively intensive rearch effort on the Co–Ge–Te system[3–5],it is surprising that this compound has not been previously reported.
2.Experimental
2.1.Synthesis,chemical composition and differential thermal analysis
The CoGeTe ternary compound was synthesid from the elements by high temperature solid-state reactions.Co powder wasfirst heated at900◦C for2h in H2/Ar(15:85)atmosphere to remove possible oxides.Stoichiometric amounts of Co(99.99%),Ge(99.99%)and Te(99.99%)were aled in evacuated quartz tubes and heated at1150◦C for4h.Following this,the sample was ground in argon atmosphere using the agate mortar and pestle,and cold presd into cylindrical pellet.The pellet was then aled into evacuated quartz tube and heated at550◦C for3days.The resultant material was once again ground and heated at670◦C for6days.After annealing,the sample was quenched in cold water.
The chemical composition of the CoGeTe was characterid in the polished ction by an energy-dispersive system Oxford Link ISIS300connected to the CamScan IV scanning electron microscope.The accelerating voltage was t to 20kV and the sample current to2.5nA.Data were collected from veral spots on veral different crystals and then averaged.As measured,the ratio of the
0925-8388/$–e front matter©2007Elvier B.V.All rights rerved. doi:10.1016/j.jallcom.2007.06.048
156F .Laufek et al./Journal of Alloys and Compounds 460(2008)155–159
elements Co:Ge:Te was 1:0.98:1.04(32.9(2)%Co,32.5(2)%Ge and 34.49(3)%Te)which is extremely clo to our final crystal structure solution of 1:1:1.
The differential thermal analysis (DTA)was carried out using the instrument R.M.I.-DTA 003(Electronic Measuring Instruments,Czech Republic)at a non-isothermal regime in the temperature range 25–800◦C.Small quartz ampoule with powdered sample (about 40mg weight)was evacuated down to 10−3Pa and heated with rate of 5K min −1.The calibration was made with the help of In,Al,Zn,Pb and Sn in order to eliminate the differences between the temperature of the thermocouples in the furnace and in the vicinity of the sample.Pure Al 2O 3was ud as a standard.
2.2.Structure determination and refinement
Preliminary scanning-electron obrvations showed that the crystals have plate-like habit,typically up to 5m,rarely up to 15m.Therefore,a single-crystal study was impracticable,and the crystal structure of CoGeTe was solved and refined from powder data ab initio.
下一个约定The X-ray diffraction pattern ud for the structure determination was col-lected in Bragg–Brentano g
eometry on X’Pert Pro PANalytical diffractometer,equipped with X’Celerator detector using CuK ␣radiation.The sample was found to be pha pure;no other compounds were detected.To minimize back-ground,the sample was placed on a flat low-background silicon wafer.The data were collected in the range between 13and 109◦2θ.The details of the data collection and basic crystallographic facts are given in Table 1.
The indexing of the X-ray powder pattern was performed using the DICLVOL04program [6].The first 20lines,with an absolute error of 0.03◦2θon peak positions,were indexed on the basis of the orthorhombic cell listed in Table 1.The figures of merit M 20[7]and F 20[8]for asssing the quality of the solution were M 20=69and F 20=79.9(0.0037and 68).The analysis of sys-tematic abnces pointed to the space group Pbca .The extraction of the integral intensities as well as the structure solution by direct methods was accomplished using the EXPO2004program [9].After the subtraction of the background scat-tering,the integral intensities up to 2θ=62◦were extracted with the Pearson VII function using the modified LeBail method.All three atoms were found from the first four peaks generated from an E-map with the highest figure of merit.All of them are in a general Wyckoff position 8c of the space group Pbca ;no special positions are occupied.
The structural model found by EXPO2004[9]was introduced into the Full-Prof program [10]for a Rietv
eld analysis.The Rietveld refinement was carried out in the angular range 13–109◦2θcontaining 268reflections.A pudo-V oight function was lected to describe individual line profiles,with a variation of the mixing factor ηdefining the Lorentzian and the Gaussian character of the peaks’shape.The corrections taking into account the asymmetry of the peaks caud by axial divergence were ud according to [11].The angular variation of thewps分页
Table 1
Data collection and Rietveld analysis Data collection
Radiation type,source X-ray,CuK ␣
Generator ttings
40kV ,30mA
Data collection temperature Room temperature Range in 2θ(◦)13–109Step size (◦)0.0167273Crystal data Space group Pbca (no.61)Unit cell content
CoGeTe,Z =8Unit cell parameters (˚A)
a =6.1892(4)
b =6.2285(4)
c =11.1240(6)Rietvel
d analysis No.of reflections
268No.of structural parameters 12No.of profile parameters 7R F 0.051R B 0.065R p 0.065R wp 0.083χ2
2.42Weighting scheme
1/y 0
R agreement factors defined according to McCuster et al.[23].
line width was accounted for by using the Cagliotti function [12].Intensities within 15times the full width at half maximum of a peak were considered to contribute to the central reflection.The background was determined by the linear interpolation between concutive breakpoints in the patte
rn.The convergence criterion,ε,forcing the termination of the refinement when the parameter shifts <ε×σ,was t to 0.1.The refinement involved the following parameters:12atomic parameters (including 3isotropic atomic displacement parameters),1scale factor,1zero-point,3cell parameters,3half-widths,2asymmetry param-eters,2variables for the angular variation of ηand parameter G1linked to the preferred orientation along [001]according to the March–Dolla [13]func-tion.The refinement converged to satisfactory residual factors R Bragg =0.065and R wp =0.082.The details of the refinement are reported in Table 1;Fig.1shows the final Rietveld plot.Atomic coordinates and isotropic displacement parameters are listed in Table 2
.
Fig.1.The Rietveld plot of CoGeTe.The upper trace shows the obrved data as dots and the calculated pattern is shown by solid line.The lower trace is a plot of the difference:obrved −calculat
ed.The vertical bars indicate the positions of Bragg peaks.
F.Laufek et al./Journal of Alloys and Compounds460(2008)155–159157 Table2
Atomic coordinates and isotropic displacement parameters for the CoGeTe compound
Atom x y z B iso(˚A2) Co0.0055(6)0.6069(6)0.1119(3) 1.41(6) Ge0.6633(4)0.7698(6)0.0710(2) 1.57(5) Te0.1157(2)0.9887(3)0.1740(3) 1.23(3)
2.3.Electrical conductivity
Electrical conductivity was measured with four-probe method using Lock-In Amplifier(EG&G model5209).The Seebeck coefficient was determined using temperature gradient on the samples not exceeding10K at the temperature 300K.
3.Results and discussion
3.1.Crystal structure
CoGeTe is the cond cobalt germanide telluride discov-ered in the Co–Ge–Te system,where11phas
es were known before.The crystal structure of CoGeTe is depicted in Fig.2. Each cobalt atom is octahedrally coordinated by three ger-manium and three tellurium atoms,with Co–Ge and Co–Te distances of2.385(3)–2.392(5)˚A and2.552(4)–2.602(3)˚A(e Table3),respectively,comparable to tho obrved in cobalt binary tellurides[14]and germanides[15].X–M–X(X=Te and Ge)angles in CoGeTe,ranging from79.8(1)◦to107.5(1)◦, exhibit a significant deviation from the ideal octahedral geome-try.When compared with the octahedra in binary tellurides such as Co0.63Te[16],the octahedra in CoGeTe are more distorted.In CoGeTe structure,one Ge–Ge edge of[CoGe3Te3]octahedron is shared with an adjacent[CoGe3Te3]octahedron,and all cor-ners are shared with two other[CoGe3Te3]octahedra.The Co atoms across the shared Ge–Ge and shared corners are parated by of2.823(4)˚A and by4.361(4)˚A,respectively.The structure of the CoGeTe compound displays the prence of Ge–Te anion pairs with the distance of2.713(3)˚A,so a Ge–Te single bond Table3
Selected bond distances(˚A)for CoGeTe and CoAsSe[21]
CoGeTe CoAsSe Co–Ch(i) 2.552(4) 2.358(7) Co–Ch 2.568(4) 2.375(6) Co–Ch(ii) 2.602(3) 2.37(1) Mean Co–Ch 2.574 2.367 Co–X(iii) 2.385(3) 2.34(1) Co–X(iv) 2.388(5) 2.348(6) Co–X 2.392(5) 2.331(6) Mean Co–X 2.388 2.339 Co–Co(v) 2.825(4) 3.50(1) Symmetry codes:(i)1/2−x,y
−1/2,z;(ii)−x,y−1/2,1/2−z;(iii)x−1/2, 1/2−y,−z;(iv)1/2−x,y−1/2,z;(v)−x,−y,−z.
can be suppod.Apart from this Ge–Te bond,each Ge and Te anion is also surrounded by three Co atoms showing distorted tetrahedral coordination.
The crystal structure of CoGeTe can be directly compared to that of PtSiSb which has been recently described by Wang et al.[17].As suggested by Wang et al.[17],the structures can be derived from that of␣-NiAs2,which is also known as a mineral pararammelsbergite[18].All compounds have the same space group(Pbca)and display a very similar structural arrange-ment.Fig.2b depicts the linking of[CoGe3Te3]octahedra along the c-axis.Analogically to the PtSiSb,CoGeTe can be viewed as a ternary ordered variant of␣-NiAs2,which is transitional between the marcasite-type and the pyrite-type structures.The major difference displayed by the three structures is the way the octahedra are connected together.Whereas,in pyrite
面粉做煎饼struc-Fig.2.Two reprentations of the CoGeTe structure:(a)ball and stick reprentation;(b)polyhedral reprentation showing the[CoGe3Te3]octahedra.
麻雀吃什么158F.Laufek et al./Journal of Alloys and Compounds460(2008)155–159
Table4
Selected distortion parameters and Co–Co distance(˚A)for CoGeTe compared with tho for CoAsSe[21],CoSbS[19],CoPSe[21]and␣-NiAs2[21]
QE OA V Co–Co distance CoGeTe 1.02275.59 2.825(4) CoAsSe 1.00828.68 3.50(1) CoSbS 1.01026.46 3.350(2) CoPSe 1.00928.58 3.448(3)
␣-NiAs2 1.00826.54 3.531(9)
OA V(octahedral angle variance)(◦)=
11
i=1
(θi−90)2/11[22];QE(quadratic elongation):=
6
i=1
(d i−d0)2/6(d0is the centre-to-vertex distance of a regular
octahedron of the same volume)[22].
ture the octahedra share only corners,in marcasite structure they share both edges and corners,forming chains of linked octa-hedra.The CoGeTe structure is compod of layers of atoms perpendicular to the c-axis with stacking alternating between pyrite-and marcasite-type of blocks,as shown in Fig.2b.In the CoGeTe structure,the alternating layers have every cond marcasite-type layer in the rever orientation.This arrange-ment has been described for the structure of PtSiSb and CoSbS by Wang et al.[17]and by Rowland et al.[19],respectively. Similarly to the PtSiSb,the centres of Co–Co pairs form a face centred cubic type arrangement elongated along c-axis.
An important feature prent in CoGeTe is a short Co–Co distance across the shared edge of[CoGe3Te3]octahedra. The Co atoms are displaced from the centre of the octahedra towards each other,reducing the Co–Co distance to the value of 2.825(4)˚A.This distance is somewhat longer than the Co–Co distance of2.506˚A obrved in elementary Co[20],but can be compared with the Pt–Pt distance of2.8565(8)˚A found in PtSiSb[17],which adopts also␣-NiAs2type structure.The metal–metal
distance of other␣-NiAs2-type compounds con-taining Co is significantly longer(e Table4);the difference between the Co–Co distance in CoGeTe and that in CoAsSe[21] is more than0.6˚A.
To asss the displacement of the metal atom from the centres of octahedra,we applied an approach of the so-called distortion parameters.Such distortion indices as octahedral angle variance and quadratic elongation[22]are commonly ud to characteri the departure of an irregular octahedron from the ideal regular geometry.Table4shows octahedral angle variance,quadratic elongation and Co–Co distance of Co-bearing NiAs2-type com-pounds.It is obvious that the[CoGe3Te3]octahedra are more deviated from their ideal geometry than octahedra in CoAsSe, CoSbS and CoPSe.This corresponds to the shortening of the Co–Co distance in CoGeTe,which,as a conquence,deforms the[CoGe3Te3]octahedron(more specifically,it lowers one of the angles of X–Co–X(X=Ge,As,Sb and P).Fig.3depicts both the contraction of the Co–Co distance and the deformation of the X–Co–X angle;moreover,it shows a comparison with CoAsSe and␣-NiAs2.
3.2.Differential thermal analysis
The DTA curve is reproduced in Fig.4.The sharp endother-mic peak at about725◦C reprents the incongruent melting of CoGeTe;it decompos into GeTe(which is prent in a liquid form),CoGe and
CoTe2.The phas were identified by XRD after heating the sample of CoGeTe in an evacuated silica glass tube at750◦C and conquently rapid quenching to the room temperature.
3.3.Electrical conductivity
Very weak temperature dependence of the electrical conduc-tivity of CoGeTe sample(e Fig.5)and quite high value of the Seebeck coefficient at300K(S300∼−180V K−1)sug-gests miconducting behaviour of the CoGeTe sample.
The Fig.3.Comparison of the edge-sharing octahedra in(a)CoGeTe;(b)CoAsSe[21];(c)␣-NiAs2[21].
F .Laufek et al./Journal of Alloys and Compounds 460(2008)155–159
159
Fig.4.Differential thermal curve for CoGeTe
compound.
Fig.5.Temperature dependence of the electrical conductivity of CoGeTe sample compared with the electrical conductivity of PtSiSb [17].卫浴洁具十大排名
opinion is supported by comparing the above-mentioned depen-dence with the one for isostructural compound PtSiSb [17],which was considered also as miconductor with small band gap (E g ∼0.6eV).4.Conclusion
To conclude,CoGeTe reprents a ternary ordered variant of ␣-NiAs 2structure,the most striking difference being the prence of metal–metal bonded pairs.As discusd in [17],
民族民间舞
with the exception of PtSiSb,all hitherto known ␣-NiAs 2type compounds have 20valence electrons per formula unit.As oppod to that,PtSiSb and CoGeTe have 19valence electrons per formula unit.This change in number of electrons per for-mula unit is accompanied by creating one Co–Co bond formed to each Co atom.It might be revealing to study the develop-ment of metal–metal interaction depending on the continual change of the electron count,for instance,in the solid solution Co x Ni 1−x GeTe.This investigation is currently in progress.Other solid solutions derived from a different ␣-NiAs 2-type could be an interesting subject to further analys.Acknowledgements
This work was supported by the internal project of Czech Geological Survey (project number 3230)a
nd by the Czech Science Foundation (project number 203/07/0267).References
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