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J. Chil. Chem. Soc. v.50 n.1 Concepción mar. 2005
J. Chil. Chem. Soc., 50, N 1 (2005)
Physical Properties of the Phases A0.7Bi1.1P2Se 6 (A = Cu, Ag) and CuBi1-XSbXP2Se 6
A. GALDÁMEZ, V. MANRÍQUEZ, D. RUIZ, R.E. AVILA2
1) Facultad de Ciencias Universidad de Chile, Casilla 653, Santiago, Chile, E-mail: email@example.com
The new selenophosphate phases A0.7Bi1.1P2Se 6 (A = Cu, Ag) and CuBi1-XSbXP2Se 6, stabilized by the chalcophosphate anions (P2Se6)4-, were prepared by molten polythiophosphate fluxes. The reaction products were characterized by atomic absorption (ICP) and Scanning Electron Microscopy (SEM-EDX), powder X-ray (XRD), Fourier transform infrared spectroscopy (FTIR) and a.c. and d.c. electrical conductivity measurements. The phases A0.7Bi1.1P2Se 6 and CuBi1-XSbXP2Se 6 are structurally related to AMP2Se6 and they possess two-dimensional (2D) structure. These phases are semiconductors, with values of electrical conductivity, sI, about 10-4 W-1cm-1 at room temperature.
Keywords: selenodiphosphates, electrical properties,
The chalcophosphates of the type AMxPyQz (A = alkaline metals, M = transition metals and Q = S, Se) present the anions (PyQz)n- coordinated to the metals A and M through the chalcogen atoms Q [1-2]. The morphology of these materials appears in one dimensional (1D) form, and as two- and three-dimensional (2D, 3D) crystalline structures.
The study of chalcophosphates of metals of the group 15 (M = Bi, Sb) presents great interest. Bismuth in oxidation state +3 can manifest its s2 electrons (effect of inert pairs) in the structure [3-5]. Thus, the electron pairs can be stereochemically active, presenting a distribution that is not symmetrically spherical, which produces a distortion in the coordination of the metallic ion. On the other hand, materials and alloys of chalcogens of bismuth and antimony, such as Bi2-XSbXTe3-YSe Y possess technological applications. These phases are used as thermoelectric materials at room temperature. It is also known that chalcogenes based on structural modifications of Sb2Q3 and Bi2Q3 (Q = S and Se) possess high electric conductivity and thermoelectricity [4,6,7].
The quaternary chalcophosphates phases of the type AM(PxQy) (A = alkaline metals, M = Sb, Bi; Q = S, Se) form a class of solid materials with interesting structural properties. The first quaternary chalcophosphates of Bi and Sb reported were A3M(PS4)2 (A = K, Rb, Cs; M = Sb, Bi), Cs3Bi2(PS4)3 , Na0.16Bi1.28P2S 6  and KMP2S6 (M = Sb, Bi) [9,10]. The quaternary Selenodiphosphates a-KMP2Se6, b-KMP2Se6 and Cs8M4(P2Se6)5 (M = Bi, Sb) have been synthesized at low and high temperature [11-13]. The solid phases a-KMP2Se6 (M = Sb, Bi) contain the anion (P2Se6)4- with layers MP2Se6, in a 2D structure, where the metal M is hexacoordinated. The b phases possess 2D structure and crystallizes in the quiral space group P21 with the ethane groups P2Se64-. The compound Cs8M4(P2Se6 )5 (M = Sb, Bi) possess 2D structure and present weak interactions MM , the anions (P2Se6)4- are linked together by the metal M. These layer compounds present channels, where the cesium atoms are located. Measures of electrical conductivity, carried out in Cs8M4(P2Se6 )5, indicate semiconductor behaviors with values of electrical conductivity, of 10-9 S/cm at room temperature.
In this work, we report the synthesis, characterization and electrical properties of the new phases A1-3XBi1+XP2Se 6 and CuBi1-XSbXP2Se 6, which are structurally related to AMP2Se6 (A = Cu, Ag; M = Bi, Sb) .EXPERIMENTAL SECTION
Synthesis. The preparation of compounds of the type A0.7Bi1.1P2Se 6 (A = Cu, Ag) was carried out by direct combination of the pure metals A and Bi with an excess of P2Se5 and Se, so that all of the metal reacted to give well-crystallized products. The species A, Bi, P2Se5, Se were mixed in molar ratios 0.7:1.1:1.2:1.5. The reacted matter was slowly cooled to room temperature at the rate of 6 K/h. The reaction mixture was washed with DMF/ethylenediamine (3:1) to remove the PySez and Se. The reaction products were washed and dryed with anhydrous ether. The solid solutions CuBi1-XSbXP2Se 6 were obtained by direct combination of the pure elements in stoichiometric proportions, with an excess of selenium and phosphorus (1% mass) to avoid the formation of impurity phases (Cu3PSe4, Ag4P2Se6 and M2Se3). In both, the reaction mixtures were sealed in evacuated quartz ampoules, and heated at 1033 K for one week. Homogeneous and well-crystallized materials were obtained after grinding and reheating at 1033 K for one more week. SEM-EDX analyses carried out on the samples confirmed their purity, homogeneity and stoichiometry (Table 1 and Fig.1).
Characterization. XRD data were collected at room temperature on a Siemens D 5000 powder diffractometer, with CuKa radiation in the range of 5 < 2q < 80. The lattice parameters were calculated by least-square fits using the Powder Diffraction Package (PDP) and Powder Pattern Lattice Parameter (PPLP) refinement routine of the NRCVAX program. For Electron microscopy, analyses were performed on a JEOL 6400 Scanning electron microscope (SEM) equipped with Oxford Link Isis energy dispersive X-ray (EDX) detector.
Differential Thermal Analysis (DTA) and Thermogravimetric Analysis (TG) were performed on a Rheometric Scientific STA 1500H/625 Thermal Analysis System. The DTA/TG curves were run simultaneously on each sample from room temperature to 1273K, in flowing nitrogen at a heating rate of 10K/min.
Infrared spectra, of the powders contained in polyethylene pellets, were recorded on a BRUKER VECTOR 22 with Fourier transform SJ-IR spectrometer in the spectral range of 250-600 cm-1.
The electrical conductivity was measured by ac and dc methods. The two opposite flat surfaces of the samples were sputtered with gold and sandwiched between the platinum electrodes of the conductivity cell. Impedance measurements were carried out with a Solartron SI-1260 impedance gain-phase analyzer in the frequency range of 0.1 Hz to 10 MHz with a signal level between 25 mV and 1 V, to probe the charge transport mechanism. Direct current (d.c.) measurements were performed with a Keithley 237 source-meter, to ascertain the existence of slow relaxation mechanisms that may not be accessible through the impedance measurements. The I-V curves were verified to be linear, supporting the Ohmic character of the contacts.
RESULTS AND DISCUSSION
The thermal analysis of the phases A0.7Bi1.1P2Se 6 and CuBi1-XSbXP2Se 6 present DTA/TGA curves very similar to the phases AMP2Se6 . The phases studied show decomposition in two steps. The first step, between 543 and 673 K, corresponds to a slight weight loss of approximately 4%, which may be assigned to partial selenium loss and a decomposition peak at 773 K with a weight loss of approximately 30%. The XRD of the residues indicated the presence of M2Se3 and A2Se. The presence of the phases PYSeZ was confirmed by FTIR.
The X-ray power diffraction (XRD) shows that the phases A0.7Bi1.1P2Se 6 and CuBi1-XSbXP2Se 6 are structurally related to ABiP2Se6 (Fig. 2).
The indexation in the space group C2/m, is consistent with a 2D structure and the 00l interlayer d-spacing shows that the phases correspond to unintercalated layer compounds. The infrared spectrum in the low area shows a strong absorption near 440cm-1 (Table 2). This vibration can be assigned to the asymmetric stretching (nPSe3), indicating the presence of P2Se6-4 units in the structure. Both IR and XRD results indicate that the metals A+1 and Bi+3 were incorporated, rather than intercalated, into the layer of the phases AMP2Se6. The phases A0.7Bi1.1P2Se 6 present a deficiency of Cu+1 and an increase of Bi3+ cations as compared to ABiP2Se6. Thus, these phases present empty spaces in the crystalline lattice and the most appropriate chemical formula is A0.7Bi1.10.2P 2Se6, where represents the empty sites (Fig. 3).
The direct current (d.c.) and a.c. impedance measurements indicate that the phases A0.7Bi1.1P2Se 6 and CuBi1-XSbXP 2Se6 are semiconductors, with electrical conductivity, s, of about 10-4 W-1cm-1 at 298 K (Table 3). These values are similar to those reported in the phases AMP2Se6 and much larger than those of the related quaternary compounds Cs8M4(P2Se6 )5 with s = 10-9 W-1cm-1 at room temperature . The obtained value of electric resistance of the curve I-V (d.c.) coincides with the value obtained by means of measures of impedance (a.c.). This allows to confirm that the electric conductivity of the material is electronic, discarding taxes of ionic conductivity in the material.
The a.c. impedance measurements give typical depressed semicircular plots (Nyquist diagrams), suggesting the presence of wide distributions of relaxation times. By fiting discrete elements models [16,17] it is possible to identify the resistance and effective capacitance corresponding to the single arc. The values of the effective capacitance Q = 0.2 nF, suggest that the electrical conductivity in the A0.7Bi1.1P2Se 6 phases is controlled by the intergranular transport. The Nyquist plot resulting from CuBi0.9Sb0.1P2Se 6 has been singled out in Fig. 4. The behavior of this phase is adjusted to a simple circuit that consist in the parallel combination of a resistance (Rg) and a constant phase element (Cgb). The resistance Rgi, is 2.5 kW and the value of the effective capacitance Q = 0.48 nF suggests control by intergranular transport, as in the other samples.
The electrical measurements of solid solutions CuBi1-XSbXP2Se 6 show an increase of the conductivity when increasing the value of x. The antimony incorporation in the phases allows that the values of electric conductivity increase in an order of magnitude. However, this value is not superior to the limit phases CuBiP2Se6 and CuSbP2Se6.
This work was supported by FONDECYT through operating grants N 1020683
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