Electrical properties of Na2US3,NaGdS2 and NaLaS2

Materials Research Bulletin, Vol. 34, No. 8, pp. 1291–1300, 1999 Copyright © 1999 Elsevier Science Ltd Printed in the USA. All rights reserved 0025-5408/99/$–see front matter

PII S0025-5408(99)00125-7

ELECTRICAL PROPERTIES OF Na2US3, NaGdS2 AND NaLaS2

Hidetoshi Masuda, Takeo Fujino,* Nobuaki Sato, and Kohta Yamada Institute for Advanced Materials Processing, Tohoku University 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan (Communicated by M. Shimada) (Received September 3, 1998; Accepted October 9, 1998)

ABSTRACT The electrical properties of ternary mixed sulfides Na2US3, NaGdS2, and NaLaS2 were studied by measuring the electrical conductivity and Hall coefficient by the van der Pauw method in a temperature range of 17–300 K. These compounds have closely related crystal structures with nearly the same atom separations, but uranium is in a U4⫹ state in Na2US3 in contrast to Ln3⫹ ions in NaGdS2 and NaLaS2. The electrical conductivity was the highest for NaGdS2 (7.75 ⫻ 102 and 11.2 ⫻ 102 Sm⫺1 at 17 and 300 K, respectively) and the lowest for Na2US3 (0.98 ⫻ 102 and 1.14 ⫻ 102 Sm⫺1 at 17 and 300 K, respectively). They showed semiconductive behavior from the temperature dependence of the electrical conductivity. The Hall coefficient showed the dominant carriers to be electrons for NaGdS2 and holes for NaLaS2 and Na2US3. The carrier densities were not so apart in these compounds, i.e., 0.2– 0.3 ⫻ 1025 m⫺3 for NaGdS2 and ⬃0.1 ⫻ 1025 m⫺3 for Na2US3. The activation energies of conduction were very low for all three compounds, especially at low temperatures below 200 K. © 1999 Elsevier Science Ltd KEYWORDS: A. chalcogenides, C. X-ray diffraction, D. crystal structure, D. electrical properties, D. semiconductivity INTRODUCTION The preparation and crystal structure analysis of new uranium sulfides Li2US3 and Na2US3 have been studied in our preceding work [1]. These compounds are regarded as the actinide

*To whom correspondence should be addressed. Phone: ⫹81-22-217-5163. Fax: ⫹81-22-217-5164. E-mail: [email protected]. 1291

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homologue of ALnS2 (A ⫽ Li, Na and Ln ⫽ lanthanide elements), which have two crystal structures according to the crystal radius ratio of A⫹/Ln3⫹ [2,3]. If this ratio is small, the ALnS2 compounds are reported to crystallize in a cubic NaCl-type structure where A⫹ and Ln3⫹ ions statistically occupy the 4a site of Fm3¯m space group. On the other hand, if the ratio is large (Li⫹/Ln3⫹ ⬎ 0.856 and Na⫹/Ln3⫹ ⬎ 1.033), the compounds crystallize in hexagonal ␣-NaFeO2 structure (space group R3¯m), which can be understood as resulting from the ordering of the metal atoms in the above NaCl structure. Due to the tetravalency of uranium [1], the uranium compounds have formulas A2US3 (A ⫽ Li, Na), but they can also be written in the form of ALnS2 as A(A1/3,U2/3)S2. Their X-ray diffraction patterns are almost the same as those of hexagonal ALnS2, showing that the A(A1/3,U2/3)S2 crystals are basically hexagonal. There were, however, a few weak peaks at low diffraction angles in the observed patterns, the whole indexing of which could be made on the basis of a monoclinic cell (space group C2/m). Such a lowering of symmetry is due to the ordering of (A1/3,U2/3). The actual crystal is in a state of partial ordering. In this work, the electrical properties of Na2US3 (⫽ 3/2Na(Na1/3,U2/3)S2), NaGdS2, and NaLaS2 were studied by measuring electrical conductivity and Hall effect at temperatures ranging from room temperature to 17 K. Comparison of the electrical properties of these compounds in relation to valence and crystal structure is intriguing since Na2US3 is pseudohexagonal and the formal valency of uranium is regarded as U4⫹. NaGdS2 is hexagonal and gadolinium is in a Gd3⫹ state, while NaLaS2 is cubic and lanthanum is in a La3⫹ state. EXPERIMENTAL Preparation of Mixed Sulfides. Na2CO3 and CS2 used were of analytical grade obtained from Wako Pure Chemicals Industries, Ltd. Gd2O3 and La2O3 of 99.9% purities were obtained from Nihon Yttrium Co., Ltd. For preparing UO2, ammonium diuranate purified by the TBP extraction method [4] was first obtained, which was changed to UO3 by heating in air at 773 K. Stoichiometric UO2, the main impurities being 54 ppm Pd and 94 ppm Tm by the ICP-ES method, was prepared by heating the UO3 in a stream of H2 at 1273 K for 6 h. H2 gas (99.99999%) used was obtained by a Balston 75-33 hydrogen generator. Nitrogen of 99.999% purity was purchased from Nippon Sanso Co., Ltd. For preparing Na2US3, NaGdS2, and NaLaS2, each of the transition metal oxides (UO2, Gd2O3, or La2O3) was thoroughly mixed with a 10 mol% excess Na2CO3 in an agate mortar. The mixture, placed on a platinum sheet in a quartz boat, was set into the quartz reaction tube of a horizontal electric furnace. After evacuating the system, CS2/N2 gas, obtained by bubbling N2 through liquid CS2 at room temperature, was introduced. Subsequently, the furnace temperature was raised at a heating rate of 20 K/min to 1073 K and maintained at this temperature for 2 h. After the reaction, the sample was furnace-cooled. Structural Characterization. Analysis of microstructure phase and the determination of the elements of the products were carried out using a Hitachi X560 electron probe microanalyzer. X-ray diffraction (XRD) was performed with a Rigaku RAD-IC diffractometer using Cu K␣1 (␭ ⫽ 1.54056 Å) radiation at 40 kV and 20 mA monochromatized with curved pyrolytic graphite. The slit system was 0.5°-0.5°-0.05 mm-0.6 mm. The intensity and least-squares lattice parameter calculations were carried out with the LAZY-PULVERIX [5] and LCR2 [6] programs, respectively. The program RIETAN [7] was used for refining the crystal data by the Rietveld method.

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TABLE 1 Comparison of the EPMA Data for Na2US3, NaGdS2, and NaLaS2 with Theoretical Values Atom ratio (with respect to U, Gd, or La) Compound Na2US3 NaGdS2 NaLaS2

Value

Na

S

Measured Theoretical Measured Theoretical Measured Theoretical

2.03 2 1.07 1 1.05 1

2.85 3 2.17 2 1.91 2

Electrical Properties Measurement. The powder samples were pelletized into disks of 10-mm diameter and about 1-mm thickness by using a uniaxial press at 20 MPa. The pellets were then sintered at 1073 K for 12 h in a flow of CS2/N2 gas. The electrical conductivity was measured by the four-probe van der Pauw method [8]. At four diagonal points on the side of the sintered pellet, fine copper wires were connected by indium contacts. After the pellet was set to the measuring device, it was cooled to 17 K in vacuum by a Daikin UV202CL helium refrigerator. Subsequently, the temperature of the pellet was raised to room temperature with a tape heater at a rate of 0.5 K/min by a Chino KP-1000 digital programmer. During this process, the electrical conductivity data were collected with 1-min interval: While a constant current of 30 mA (Advantest TR-6143) was applied to the sample, the potential drop was measured by a Keithley 182 voltmeter of which the input impedance was ⬎10 G. The hysteresis in the measurements of electrical conductivity was checked by lowering the temperature from room temperature to 17 K at the same rate. The Hall coefficient was measured for the same pellets at several temperatures between room temperature and 17 K by the van der Pauw method [8]. A constant current of 30 mA and a magnet field of 0.58 T was applied to the sample. At each temperature, the magnetic field or the current was reversed in order to eliminate misalignment and thermal voltage effects. RESULTS AND DISCUSSION Compositional and Structural Characterization of the Compounds. Each element of the compounds was determined by EPMA using standard samples. From the obtained concentrations of the constituents, the atom ratios in the compounds were calculated as shown in Table 1. The errors in the ratios are 20% for Na, 6% for U, 1% for Gd, 1% for La, and 10% for S. Although these errors are not small, the atom ratios measured by EPMA are seen to be close to the theoretical atom ratios for Na2US3, NaGdS2, and NaLaS2. The observed X-ray diffraction patterns are compared with the calculated intensities in Figure 1. For Na2US3 (Fig. 1a), all the observed peaks have been indexed on the basis of a monoclinic system with space group C2/m. The calculated intensities below the observed

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FIG. 1 X-ray diffraction patterns for (a)Na2US3, (b)NaGdS2, and (c)NaLaS2 compared with calculated intensities. pattern are in satisfactory agreement with the observed peaks except for four weak peaks at 15.30, 17.16, 19.90, and 23.22° (2␪) in the calculated pattern. These peaks, together with (0 2 0), (2 2 1¯), (1 1 3¯), and (3 1 1¯) peaks in the observed pattern (Fig. 1a), are resulted to form by ordering of the (Na1/3,U2/3) atoms in Na(Na1/3,U2/3)S2. Therefore, the weakening or disappearance of such peaks is considered to be associated with partial ordering of (Na1/3, U2/3), which is actually realized by the coexistence of hexagonal and monoclinic phases with very close atom arrangement. The lattice parameters calculated by the Nelson-Riley leastsquares method on the LCR2 program, which are the same as those previously reported [1],

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TABLE 2 Crystal Data for Na2US3, NaGdS2, and NaLaS2 Source

Crystal system

a (Å)

b (Å)

c (Å)

␣ (°)

RI (%)

Na2US3a

This work

6.992 19.780 19.899 19.958

109.5

This work ref. 3 This work ref. 3

6.990 4.036 4.018 4.019 5.880 5.878

12.105

NaGdS2

Monoclinic Hexagonal Hexagonal

9.9 7.8 5.5

Compound

NaLaS2

Cubic

3.8

a

The molar ratio of monoclinic and hexagonal phase is 32.0 and 68.0 mol%, respectively.

are listed on Table 2. The monoclinic crystal structure of Na2US3 (⫽ 3/2Na(Na1/3,U2/3)S2) is very close to the hexagonal (R3¯m) NaGdS2 structure as shown in Figure 1b. There seems to be no significant preferred orientation of (00l) as reported in ref. 3 for NaGdS2, since the RI value was 5.5% including these lines in this work. The lattice parameters of NaGdS2 and NaLaS2 are given in Table 2, which are well in accord with those of Sato et al. [3]. The cubic NaCl-type crystal structure of NaLaS2 is regarded as formed by the disordering of Na and La atoms in the hexagonal structure [2,3,9]. The relation between the hexagonal and pseudocubic lattices is shown in Figures 2a and b, and that between the monoclinic and pseudohexagonal lattices in Figure 2c. The pseudocubic lattice parameters for NaGdS2 are a ⫽ 5.703 Å and ␣ ⫽ 89.58° as rhombohedral, but they

FIG. 2 Illustration of the crystal structures of (a)NaLaS2, (b)NaGdS2, and (c)Na2US3, showing their relations. Light Ln (larger crystal radius); heavy Ln (smaller crystal radius).

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FIG. 3 Electrical conductivity, ␴, as a function of temperature for Na2US3, NaGdS2, and NaLaS2. Filled and open circles indicate the p-type and n-type semiconductivities, respectively.

are very close to cubic structure (␣ ⫽ 90°). The corresponding cell for Na2US3 has a difference in the lattice parameters, i.e., a1 ⫽ 5.705, a2 ⫽ 5.710, a3 ⫽ 5.708 Å, ␣1 ⫽ 89.96, ␣2 ⫽ 90.01, and ␣3 ⫽ 89.98°, which are closer to the cubic symmetry. The pseudo-cubic lattice parameters, a, for NaGdS2 and Na2US3 are near the cubic lattice parameter of NaLaS2, a ⫽ 5.880 Å, indicating that the structural environment around a transition metal ion in the crystal is essentially the same for the present three compounds. Electrical Properties. The electrical conductivity, ␴, as a function of temperature is shown for Na2US3, NaGdS2, and NaLaS2 in Figure 3. The densities of the pellets were obtained to be around 75% T.D., as calculated from the dimensions of the pellet and the lattice parameters. Correction of porosity was not made to the conductivity data. Since no significant hysteresis was observed in the measured electrical conductivities in the heating and cooling processes, only the heating curves are depicted in the figure. The electrical conductivity for NaGdS2 is higher than that of Na2US3 by about one order of magnitude. It increases with increasing temperature from 7.75 ⫻ 102 (at 17 K) to 11.2 ⫻ 102 Sm⫺1 (at 300 K). The electrical conductivity of Na2US3 is low, and it increases only slightly from 0.98 ⫻ 102 to 1.14 ⫻ 102 Sm⫺1 as the temperature increases from 17 to 300 K. The behavior of NaLaS2 is between that of NaGdS2 and Na2US3. The temperature dependence of the electrical conductivity suggests that the three compounds are semiconductive. The plot of ln ␴ vs. 1/T shows nonlinear change in the temperature range of the measurements (Fig. 4). If the electrical conductivity is expressed as ␴ ⫽ ␴0exp[⫺Ea/kBT], where Ea is the activation energy of conduction, the Ea values obtained are very close to zero

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FIG. 4 The ln ␴ vs. 1/T plot for Na2US3, NaGdS2, and NaLaS2.

(⬃0.1 meV) at temperatures below 100 K. From the change of the slope above 200 K, another conduction mechanism appears to arise with the activation energies 1.87, 4.65, and 3.32 meV for Na2US3, NaGdS2, and NaLaS2, respectively. It is noteworthy that these values are still very low; for example, n-InSb has an Ea value of 180 meV and GaAs, 1400 meV [10]. The Hall coefficients, RH, measured at 17, 100, 150, 200, 250, and 300 K (and also at 20, 30 K for NaGdS2 and NaLaS2) are plotted in Figure 5. From the polarity of the Hall voltage, the carriers appear to be electrons for NaGdS2, whereas they are holes for Na2US3 and NaLaS2. The Hall coefficients for NaGdS2 and NaLaS2 are almost unvaried with temperature in the whole range of the measuring temperatures. The coefficient for Na2US3, on the other hand, decreased slightly from 12.0 ⫻ 10⫺6 (at 100 K) to 8.96 ⫻ 10⫺6 m3A⫺1s⫺1 (at 300 K) with increasing temperature. These results of polarity in the Hall effect are not consistent with the measured nonstoichiometry of the compounds (Table 1). This discrepancy may be attributed to the relatively large error in EPMA data, especially for sodium and sulfur. Figure 6 compares the carrier density, n, as a function of temperature. The n values for Na2US3 are below 0.1 ⫻ 1025 m⫺3, those for NaGdS2 are 0.2– 0.3 ⫻ 1025 m⫺3. Both of the carrier densities increase very slightly with increasing temperature. The carrier density for NaLaS2 are much higher than those of Na2US3 and NaGdS2, and it increases from 1.08 ⫻ 1025 at 17 K to 1.77 ⫻ 1025 m⫺3 at 300 K. The carrier densities per formula at 300 K are 0.10 ⫻ 10⫺3, 0.24 ⫻ 10⫺3, and 1.80 ⫻ 10⫺3 for Na2US3, NaGdS2, and NaLaS2, respectively. These values are low. Figure 7 shows the temperature dependence of the Hall mobility, ␮, as a function of temperature in a logarithm scale. At temperatures below 150 K, the mobility was almost

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FIG. 5 Hall coefficient, RH, as a function of temperature for Na2US3, NaGdS2, and NaLaS2.

FIG. 6 Change of carrier densities, n, with temperature for Na2US3, NaGdS2, and NaLaS2.

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FIG. 7 The ln ␮ vs. ln T plot for Na2US3, NaGdS2, and NaLaS2.

temperature-independent, showing that the dominant scattering mechanism was the neutral impurity scattering [11]. At temperatures higher than 150 K, the mobility decreased with increasing temperature (⬃T⫺1/3 slope for NaLaS2), although it did not follow the T⫺3/2 slope for electron–phonon scattering [12]. The electrical data at 300 K for Na2US3, NaGdS2, and NaLaS2 are summarized in Table 3. From this table, it is seen that the Hall mobility change is limited in a rather small range for the present three compounds, although the variation of the carrier density is larger, i.e., from 0.07 ⫻ 1025 of Na2US3 to 1.77 ⫻ 1025 m⫺3 of NaLaS2 (300 K). The important point related to this behavior is that the main carriers are holes for NaLaS2 and Na2US3 while they are electrons for NaGdS2. Since the atom separations of U–S, Gd–S, and La–S are close in Na2US3, NaGdS2, and NaLaS2, the electrical properties of these compounds can be discussed in terms of the charge and ordering of the transition metals in the crystal. In the case where the metals were trivalent and disordered, the carriers were holes of higher carrier densities.

TABLE 3 Electrical Data for Na2US3, NaGdS2, and NaLaS2 at 300 K Compound Na2US3 NaGdS2 NaLaS2

␴ (102 Sm⫺1)

RH (10⫺6 m3A⫺1s⫺1)

n (1025 m⫺3)

␮ (10⫺3 m2V⫺1 s⫺1)

1.14 11.2 5.85

8.96 ⫺2.42 0.35

0.07 0.26 1.77

1.02 2.71 2.06

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In the ordered state, the carriers were electrons. Even when the atoms were in an ordered state, however, the carriers were holes for tetravalent metals. The reason of the change of the carrier type is not clear, but it is possible that in Na2US3 some of the U4⫹ ions, which have a higher charge than the Ln3⫹ ions, are making a contribution to the conduction. Note that the activation energy of conduction is close to zero for all three compounds. This shows that the band gap is very shallow. Ruthenium sulfide selenide, RuS2⫺xSex, has been reported to have a low Ea value of 20 – 40 meV [13]. The Ea values of the present compounds above 200 K were lower than that by one order of magnitude. The lowest temperature of our measurements, 17 K, was supposed to be already in a saturation range of conduction. The effect of impurities on conduction was not determined in this work and will be the subject of further study. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

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