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Preparation and properties of some uranium selenides and tellurides
J. I n o r g . N u cl. C h e m . , 1963, Vol. 25, pp. 795 t o 800. P e r g a m o n Press Ltd. P r i n t e d in N o r t h e r n I r e l a n d
P R E P A R A T I O N A N D PROPERTIES OF SOME U R A N I U M SELEN|DES AND TELLURIDES L. K. MATSOY, J. W. MOODY, R. C. H!MES Battelle Memorial Institute, Columbus, Ohio (Received 20 October 1962 : in revised/brm 10 January 1963) Abstract Selenides and tellurides of uranium have been prepared by solid-vapour reaction and densified by melting. Melting points were estimated and some structures were determined by X-ray analysis. Electrical resistivities, Hall coefficients, and thermoelectric powers of specimens of con> pounds were measured. The electrical properties indicate, generally, that these compounds are semimetallic conductors.
RESEARCH on the uranium selenides and tellurides was undertaken with the objective of developing a systematic body of knowledge on the preparation and properties of these compounds. Seven compounds within the two systems have been described in the literature: however, most of this past work was concerned with structural studies and few chemical- or physical-property data are given. Accordingly, an experimental programme was initiated to develop suitable laboratory-preparation techniques for the compounds and to determine certain of their basic properties. Particular emphasis was directed toward assay of properties of importance in electronics. The existence of USe, U2Se a, US% and a selenide with a higher Se content than US% has been reported in the uranium selenium system. ~al Uranium diselenide seems to occur in two modifications, while USe was reported to have a NaCI structure.
OF PREPARATION
The uranium compounds were prepared by reaction of chalcogen vapour with turnings of uranium metal at elevated temperatures. The tellurium employed was CP grade (about 99.99 per cent pure), slick tellurium obtained from the Fischer Scientific Co. The selenium was the American Smelting and Refining Company's rectifier-grade selenium shot of about 99.999 per cent purity. The uranium used had only 0-01 per cent spectrographically detectable impurities. The reactants, in selected stoicheiometric proportions, were contained in a simple evacuated Vycor capsule (designed as illustrated in Fig. 1). Oxides were removed from the surfaces of the uranium turnings by washing in dilute nitric acid and rinsing with distilled water and then acetone immediately before they were loaded in the capsule. An indentation in the bottom of the reaction tube served to prevent direct contact of the elements in the condensed state (and a possible violent reaction) and assured a smoothly-proceeding solid vapour reaction. The reactions were carried out in a resistance-wound tube furnace. The temperature of the furnace was increased gradually until the reaction appeared visually to be complete, when the tubes were inspected on trial withdrawal from the furnace. The temperature was then raised another 100:C and the reactants maintained at this temperature for about 24 hr. Final temperatures of approximately 700 and 875~'C were required for the selenides and tellurides, respectively. ~l, A. COLAN1, C.R. Acad. Sei., Paris 137, 382-383 (1903); Ann. Chim. 12, 85 (1907). ~:' R. FERRO, Z . Anorg. Chem. 275, 320-326 (1954). ,a, A. COLANI, C.R. Acad. Sci., Paris 137, 383 (1903); Ann. Chim. 12, 87 (1907). 795
L . K . MATSON,J. W. MOODYand R. C. HIMES
796
\ i~k
JI -"
Vycor tube Tantalum wire
o I J 00-.-Induc.on coil
0
o
o
I! II
FIG. 1.--Apparatus used to densify the uranium chalcogenides.
0
Tantalumcrucible - Granular reaction product
II/I-A, ,oi, ro°iation,,,e,, J-~'-Brass end
cap
I IJ~Argon inlet To mercury bubbler ~
0
HOII Coeffici'ent (cm3/coulomb t . . . . . a Resistivity (ohm cm) ,6 Thermoelectric power(,uV/°C
P"' ""'-
r
I
RH rO-I
P - - 10-3
I I I I I ! I
]FIG. 2.--Electrical properties of a P-type sample of USe as a function of reciprocal temperature.
j / 102 10-2
~ O
4
O; I I
101 0
2
4
6
103/T(°K)
8
IO
10-4
Preparation and properties of some uranium selenides and tellurides
797
In all cases, the reaction products were obtained in a granular form. These products were cast into ingots by melting under argon in a tantalum crucible in the apparatus illustrated in Fig. 1. The tantalum crucible was suspended from a Quartz hook by means of tantalum wire. Power for the induction heating employed was supplied by a 20 kW, Lepel r.f. generator. It was possible to estimate the melting points of these compounds by noting, as the temperature was increased slowly, (l) the highest temperature at which solid could be observed visually and (2) the lowest temperature at which liquid was definitely observed. This range of temperature which was measured with an optical pyrometer was relatively small in most cases and it is felt that the data give a reasonable estimate of the melting points of the compounds. (Decomposition of the compounds at the melting points appeared to be negligible.) The data are given in Table 1 along with some other TABLE I.--PHYSICAL PROPERTIESOF URANIUMSELENIDESAND TELLUR1DES Synthetic composition
Melting point (cC)
Crystal structure
Lattice constant (a 0)
USe
1850-2000 NaCI(B-I)
U2S% USe2
1560-1590 Weak lines* 13504 380 Weak lines, but different from U2S% 1550-1650 Th3P4 (major phase) NaCI (minor phase)
UTe UaTe4
1430-1470 Th3P4
U2Tea UTe2
1325-1355 Th3P4 1215-1235 Single phase but complext
Calculated density (g/cm a)
5.71 .
1I-3 .
.
.
. . 9.393 6.151
.
.
9"3980 [9-397 by ref (2)] 9"3960
Determi~aed density (g/cm a) 10.91 Ill.07 byreftl)] 9"43 9.0~,
8.81 10.43 9'81
9.6~ [10.37 by ref (3)] 9'4~
9"18
9.0,,
--
8"%
* An orthorhombic structure of Sb2S8 type (a = 11.33, b == 10.94, and c -- 4.06) was reported ~1~. + A tetragonal structure (a = 4"006, c / a z 1"865) which is neither of CaC2 or Cu2Sb type was reported ~. physical properties determined in the course of this work. It is interesting to note that the melting points of each series of compounds tend to decrease as the metalloid to metal ratio increases. As would be expected, a given selenide melts at a higher temperature than the corresponding telluride. To establish conditions suitable for handling the compounds, a few simple tests were made to determine their reactivity with common reagents and atmospheres. With but few exceptions, the selenides and tellurides of uranium react readily with both acids and bases. With acids, hydrogen selenide or hydrogen telluride is generated. However, UTe2 does not appear to react with concentrated hydrochloric acid. Neither USe nor U2Se3 was found to react with 3 N KOH. Except for UT%, the uranium telluride samples disintegrated in air in a matter of days. The uranium selenides do not show this tendency. (A surface reaction occurred at 300°C on USe.) CRYSTAL
STRUCTURES
A limited X - r a y analysis o f the v a r i o u s c o m p o s i t i o n s was u n d e r t a k e n as a p r o o f o f p r e p a r a t i o n . In s o m e cases, the p o w d e r d i a g r a m s c o u l d n o t be i n d e x e d with great a c c u r a c y and in o n e case ( U T e ) the p r e p a r a t i o n p r o v e d to be p o l y p h a s e . Thus, the s y m b o l i c r e p r e s e n t a t i o n used in d e s i g n a t i n g the m a t e r i a l s p r e p a r e d indicates the n o m i n a l c o m p o s i t i o n o f the m a t e r i a l but d o e s n o t necessarily i n d i c a t e t h a t the m a t e r i a l is a s i n g l e - p h a s e s p e c i m e n o f a c o m p o u n d h a v i n g precisely the d e n o t e d s t o i c h e i o m e t r y . T h e results o f the X - r a y d a t a are s u m m a r i z e d in T a b l e 1. T h e u r a n i u m m o n o selenide ( U S e ) p r e p a r a t i o n was f o u n d to h a v e the B-I t y p e ( N a C I ) structure, as r e p o r t e d in the literature. T h e s t r u c t u r e o f the sesqui- a n d diselenide c o m p o s i t i o n s c o u l d not be definitely d e t e r m i n e d . H o w e v e r , it was established that the X - r a y p a t t e r n s were dissimilar, which indicates the p r e s e n c e o f different phases at the two s t o i c h e i o m e t r i e s .
798
L.K. MATSON,J. W. MOODYand R. C. HIMES
The uranium telluride sample of over-all one-to-one composition was found to consist of a major phase with a Th3P a structure and a minor phase with a NaC1 structure. A ThaP 4 structure was found for both U3Te 4 and U2Te 3. It will be noticed that the lattice constant of U3Te 4 was larger than that for the Th3P4-structured phases of either the one-to-one composition or the two-to-three composition. Thus, it appears that vacancies occurring in either the cation or anion sublattice of the Th3P 4 structure of these latter phases lead to a decrease in the lattice constant. Uranium ditelluride was found to be single phase but it was not possible to index the complex X-ray pattern obtained for this composition. The USe ingot proved to be a large single crystal as cast. Densities of all the preparations were determined by comparing their weights in air and in carbon tetrachloride. These measured densities are given in Table 1 where they may be compared with theoretical densities of the compounds as calculated (when possible) from their unit-cell dimensions. It will be noted that the measured density of the UTe composition is intermediate between that calculated for the two structures found even though the Th3P 4 structure was the dominant phase found by X-ray analysis. E L E C T R I C A L PROPERTIES The samples prepared here were not subject to extensive purification or crystallization procedures and, thus, they were not in that condition of refinement and crystalline perfection generally desirable for studies of the electrical properties of materials. Even so, it was felt that a study of the electrical properties of these preparations might yield important information on the bonding in the compounds and provide guidance for future studies. Samples for electrical measurement were taken from the densified ingots. These samples were fashioned into small parallelepipeds onto which electrical leads were soldered with indium metal. Electrical resistivity, Hall coefficient, and thermoelectric power were measured at various temperatures. (Unfortunately, the tellurides disintegrated before their thermoelectric powers could be determined.) Representative data, obtained at r o o m temperature and liquid nitrogen temperature, are given in Table 2. All the compounds studied had rather high charge-carrier concentrations as evidenced by the low Hall coefficients. Carrier concentrations, TABLE 2 . - - E L E C T R I C A L PROPERTIES OF SOME URANIUM SELENIDES AND TELLURIDES
Synthetic composition
Temperature (°K)
Resistivity (ohm-cm)
Hall coefficient (cma/coulomb)
Thermoelectric power (~-v/°K)
USe
300 80 300 300 80 300 80 300 80 300 80
2.8 × 10-4 1.2 x 10-4 2.3 × 10-3 3.34 x 10-~ 5.7 × 10-1 1-3 × 10-3 1.2 × 10-a 5.5 × 10-3 7.0 × 10-3 1.2 × 10-~ 1.0 × 10-z
+0.008 +0-12 -0.009 +0-01
+38 +21 --2 +21
U2Se3 USe2 UTe U2Te3 UTe2
+0.2 +0.02 +0'31 --0.34
Preparation and properties of some uranium selenides and tellurides
799
calculated from the Hall coefficients, ranged from about 2 ~< 1019 to about 10'~1 carriers/cm ~. The very low values of the Hall coefficient preclude ascribing much precision to the values quoted; however, it is felt by the authors that the relative magnitudes are correct and meaningful. The signs of the Hall coefficient and thermoelectric power (where measured) were positive for all samples except U2Se~ and UTe a. Electrons appeared to be the dominant charge carriers in U2S% and UT%. The high carrier concentrations found immediately suggest that these compounds are semimetallic conductors. The electrical properties of the single crystal of USe are of particular interest. At room temperature, the thermoelectric evaluation factor (ZK, equal to thermoelectric power squared divided by the resistivity) is 5"6 :: 10 ~;watt per cm-deg. This is higher than for the other compounds studied and may be compared with an evaluation factor of about 3 ; 10.-5 forp-type Bi2Tea at room-temperature. At high temperatures, the thermoelectric properties of USe may compare even more favourably with the best known thermoelectric materials. The mobility of the charge carriers, as calculated from the product of the Hall coefficient and conductivity, is encouragingly high for such an unrefined material. Because of this, the electrical properties of USe were investigated in greater detail. In Fig. 2 are shown the Hall coefficient, resistivity and thermoelectric power as functions of reciprocal temperature. The general character of the curves shown have been confirmed by measurements on other specimens. It will be noted that the thermoelectric power is increasing in a region where the Hall coefficient is decreasing. The behaviour of the electrical properties of the sample over the temperature range of the measurements is consistent with semimetallic conduction and the notion of overlapping bands. At higher temperatures, the resistivity of the sample increased to a second maximum. Such behaviour is suggestive of some phase transition but much more data would be necessary to resolve this question. It was of interest to determine the effects of stoicheiometric deviations on the electrical properties of USe. Two additional samples were prepared from nominal compositions containing one atomic per cent excess uranium and one atomic per cent excess selenium, respectively. These changes had no significant effect on the electrical properties of the compound. It is interesting to note, however, that electrons, rather than holes, were the dominant carriers in the U2Sea sample. The dominant charge carriers in USe are holes, as evidenced by the positive Hall coefficients and thermoelectric powers of the samples. One may speculate that, if the bonding in the compound were predominantly ionic, then n-type (electron) conduction might be expected, since uranium has more valence electrons than are needed, formally, to satisfy the Se 2- ion. Thus, it appears that any suitable model of the conduction processes in USe must attribute importance to covalent bonds. Since the arrangement of atoms in the rock-salt structure involves octahedral coordination, the covalent bonds of selenium might be expected to be of the p3d2s hybrid type involving the 5s-orbital, since the 4s-orbital is at so much lower energy. Such hybridization is found in [SeBrG]2 .(41 The covalent bonds of uranium would probably be of thef3d2s type (as, for example in the compound UCIB), since the alternate structure, d~sp3, requires promotion of three pairs of electrons from the 5fto the 7p shellJ '~) m L. PAULINe, The Nature of the Chemical Bondpp. 184-185, Cornell University Press (1948). {~}J. C. EISENSTEIN,J. Chem. Phys. 25, 143-146 (1956).
800
L.K. MATSON,J. W. MOODYand R. C. HIMES
If the hypothesized covalent bonding does operate, each uranium atom would contribute six electrons to the bonding system. Each selenium atom would contribute only four electrons, holding an inert pair in the 4s-orbital. Twelve electrons would be needed to fill the six octahedral bonds to one atom in USe, and the proposed bonding system lacks two electrons. Thus, it seems reasonable to hypothesize that the p-type conductivity observed for USe is due to unsatisfied bonds. That the bonds are strongly covalent was also concluded by KHODADADc6) on the basis of oxidation studies of USe, U3Se4 and U2Se3 with Hg 2+ ions. On the basis of these considerations, one might postulate that PuSe would be a semiconductor, should it exist in the rock salt structure. Plutonium has two more electrons than does uranium which could be contributed to the bonding system. Acknowledgement--The authors wish to thank the Selenium and Tellurium Development Committee
whose sponsorship made this work possible and Dr. D. A. VAUGHNwho performed the X-ray analyses. Helpful discussions with Messrs. J. F. MILLERand R. T. BATEare also acknowledged. ~6~p. KHODADAD,C.R. Acad. Sci., Paris 253, 1575-1577 (1961).
P R E P A R A T I O N A N D PROPERTIES OF SOME U R A N I U M SELEN|DES AND TELLURIDES L. K. MATSOY, J. W. MOODY, R. C. H!MES Battelle Memorial Institute, Columbus, Ohio (Received 20 October 1962 : in revised/brm 10 January 1963) Abstract Selenides and tellurides of uranium have been prepared by solid-vapour reaction and densified by melting. Melting points were estimated and some structures were determined by X-ray analysis. Electrical resistivities, Hall coefficients, and thermoelectric powers of specimens of con> pounds were measured. The electrical properties indicate, generally, that these compounds are semimetallic conductors.
RESEARCH on the uranium selenides and tellurides was undertaken with the objective of developing a systematic body of knowledge on the preparation and properties of these compounds. Seven compounds within the two systems have been described in the literature: however, most of this past work was concerned with structural studies and few chemical- or physical-property data are given. Accordingly, an experimental programme was initiated to develop suitable laboratory-preparation techniques for the compounds and to determine certain of their basic properties. Particular emphasis was directed toward assay of properties of importance in electronics. The existence of USe, U2Se a, US% and a selenide with a higher Se content than US% has been reported in the uranium selenium system. ~al Uranium diselenide seems to occur in two modifications, while USe was reported to have a NaCI structure.
OF PREPARATION
The uranium compounds were prepared by reaction of chalcogen vapour with turnings of uranium metal at elevated temperatures. The tellurium employed was CP grade (about 99.99 per cent pure), slick tellurium obtained from the Fischer Scientific Co. The selenium was the American Smelting and Refining Company's rectifier-grade selenium shot of about 99.999 per cent purity. The uranium used had only 0-01 per cent spectrographically detectable impurities. The reactants, in selected stoicheiometric proportions, were contained in a simple evacuated Vycor capsule (designed as illustrated in Fig. 1). Oxides were removed from the surfaces of the uranium turnings by washing in dilute nitric acid and rinsing with distilled water and then acetone immediately before they were loaded in the capsule. An indentation in the bottom of the reaction tube served to prevent direct contact of the elements in the condensed state (and a possible violent reaction) and assured a smoothly-proceeding solid vapour reaction. The reactions were carried out in a resistance-wound tube furnace. The temperature of the furnace was increased gradually until the reaction appeared visually to be complete, when the tubes were inspected on trial withdrawal from the furnace. The temperature was then raised another 100:C and the reactants maintained at this temperature for about 24 hr. Final temperatures of approximately 700 and 875~'C were required for the selenides and tellurides, respectively. ~l, A. COLAN1, C.R. Acad. Sei., Paris 137, 382-383 (1903); Ann. Chim. 12, 85 (1907). ~:' R. FERRO, Z . Anorg. Chem. 275, 320-326 (1954). ,a, A. COLANI, C.R. Acad. Sci., Paris 137, 383 (1903); Ann. Chim. 12, 87 (1907). 795
L . K . MATSON,J. W. MOODYand R. C. HIMES
796
\ i~k
JI -"
Vycor tube Tantalum wire
o I J 00-.-Induc.on coil
0
o
o
I! II
FIG. 1.--Apparatus used to densify the uranium chalcogenides.
0
Tantalumcrucible - Granular reaction product
II/I-A, ,oi, ro°iation,,,e,, J-~'-Brass end
cap
I IJ~Argon inlet To mercury bubbler ~
0
HOII Coeffici'ent (cm3/coulomb t . . . . . a Resistivity (ohm cm) ,6 Thermoelectric power(,uV/°C
P"' ""'-
r
I
RH rO-I
P - - 10-3
I I I I I ! I
]FIG. 2.--Electrical properties of a P-type sample of USe as a function of reciprocal temperature.
j / 102 10-2
~ O
4
O; I I
101 0
2
4
6
103/T(°K)
8
IO
10-4
Preparation and properties of some uranium selenides and tellurides
797
In all cases, the reaction products were obtained in a granular form. These products were cast into ingots by melting under argon in a tantalum crucible in the apparatus illustrated in Fig. 1. The tantalum crucible was suspended from a Quartz hook by means of tantalum wire. Power for the induction heating employed was supplied by a 20 kW, Lepel r.f. generator. It was possible to estimate the melting points of these compounds by noting, as the temperature was increased slowly, (l) the highest temperature at which solid could be observed visually and (2) the lowest temperature at which liquid was definitely observed. This range of temperature which was measured with an optical pyrometer was relatively small in most cases and it is felt that the data give a reasonable estimate of the melting points of the compounds. (Decomposition of the compounds at the melting points appeared to be negligible.) The data are given in Table 1 along with some other TABLE I.--PHYSICAL PROPERTIESOF URANIUMSELENIDESAND TELLUR1DES Synthetic composition
Melting point (cC)
Crystal structure
Lattice constant (a 0)
USe
1850-2000 NaCI(B-I)
U2S% USe2
1560-1590 Weak lines* 13504 380 Weak lines, but different from U2S% 1550-1650 Th3P4 (major phase) NaCI (minor phase)
UTe UaTe4
1430-1470 Th3P4
U2Tea UTe2
1325-1355 Th3P4 1215-1235 Single phase but complext
Calculated density (g/cm a)
5.71 .
1I-3 .
.
.
. . 9.393 6.151
.
.
9"3980 [9-397 by ref (2)] 9"3960
Determi~aed density (g/cm a) 10.91 Ill.07 byreftl)] 9"43 9.0~,
8.81 10.43 9'81
9.6~ [10.37 by ref (3)] 9'4~
9"18
9.0,,
--
8"%
* An orthorhombic structure of Sb2S8 type (a = 11.33, b == 10.94, and c -- 4.06) was reported ~1~. + A tetragonal structure (a = 4"006, c / a z 1"865) which is neither of CaC2 or Cu2Sb type was reported ~. physical properties determined in the course of this work. It is interesting to note that the melting points of each series of compounds tend to decrease as the metalloid to metal ratio increases. As would be expected, a given selenide melts at a higher temperature than the corresponding telluride. To establish conditions suitable for handling the compounds, a few simple tests were made to determine their reactivity with common reagents and atmospheres. With but few exceptions, the selenides and tellurides of uranium react readily with both acids and bases. With acids, hydrogen selenide or hydrogen telluride is generated. However, UTe2 does not appear to react with concentrated hydrochloric acid. Neither USe nor U2Se3 was found to react with 3 N KOH. Except for UT%, the uranium telluride samples disintegrated in air in a matter of days. The uranium selenides do not show this tendency. (A surface reaction occurred at 300°C on USe.) CRYSTAL
STRUCTURES
A limited X - r a y analysis o f the v a r i o u s c o m p o s i t i o n s was u n d e r t a k e n as a p r o o f o f p r e p a r a t i o n . In s o m e cases, the p o w d e r d i a g r a m s c o u l d n o t be i n d e x e d with great a c c u r a c y and in o n e case ( U T e ) the p r e p a r a t i o n p r o v e d to be p o l y p h a s e . Thus, the s y m b o l i c r e p r e s e n t a t i o n used in d e s i g n a t i n g the m a t e r i a l s p r e p a r e d indicates the n o m i n a l c o m p o s i t i o n o f the m a t e r i a l but d o e s n o t necessarily i n d i c a t e t h a t the m a t e r i a l is a s i n g l e - p h a s e s p e c i m e n o f a c o m p o u n d h a v i n g precisely the d e n o t e d s t o i c h e i o m e t r y . T h e results o f the X - r a y d a t a are s u m m a r i z e d in T a b l e 1. T h e u r a n i u m m o n o selenide ( U S e ) p r e p a r a t i o n was f o u n d to h a v e the B-I t y p e ( N a C I ) structure, as r e p o r t e d in the literature. T h e s t r u c t u r e o f the sesqui- a n d diselenide c o m p o s i t i o n s c o u l d not be definitely d e t e r m i n e d . H o w e v e r , it was established that the X - r a y p a t t e r n s were dissimilar, which indicates the p r e s e n c e o f different phases at the two s t o i c h e i o m e t r i e s .
798
L.K. MATSON,J. W. MOODYand R. C. HIMES
The uranium telluride sample of over-all one-to-one composition was found to consist of a major phase with a Th3P a structure and a minor phase with a NaC1 structure. A ThaP 4 structure was found for both U3Te 4 and U2Te 3. It will be noticed that the lattice constant of U3Te 4 was larger than that for the Th3P4-structured phases of either the one-to-one composition or the two-to-three composition. Thus, it appears that vacancies occurring in either the cation or anion sublattice of the Th3P 4 structure of these latter phases lead to a decrease in the lattice constant. Uranium ditelluride was found to be single phase but it was not possible to index the complex X-ray pattern obtained for this composition. The USe ingot proved to be a large single crystal as cast. Densities of all the preparations were determined by comparing their weights in air and in carbon tetrachloride. These measured densities are given in Table 1 where they may be compared with theoretical densities of the compounds as calculated (when possible) from their unit-cell dimensions. It will be noted that the measured density of the UTe composition is intermediate between that calculated for the two structures found even though the Th3P 4 structure was the dominant phase found by X-ray analysis. E L E C T R I C A L PROPERTIES The samples prepared here were not subject to extensive purification or crystallization procedures and, thus, they were not in that condition of refinement and crystalline perfection generally desirable for studies of the electrical properties of materials. Even so, it was felt that a study of the electrical properties of these preparations might yield important information on the bonding in the compounds and provide guidance for future studies. Samples for electrical measurement were taken from the densified ingots. These samples were fashioned into small parallelepipeds onto which electrical leads were soldered with indium metal. Electrical resistivity, Hall coefficient, and thermoelectric power were measured at various temperatures. (Unfortunately, the tellurides disintegrated before their thermoelectric powers could be determined.) Representative data, obtained at r o o m temperature and liquid nitrogen temperature, are given in Table 2. All the compounds studied had rather high charge-carrier concentrations as evidenced by the low Hall coefficients. Carrier concentrations, TABLE 2 . - - E L E C T R I C A L PROPERTIES OF SOME URANIUM SELENIDES AND TELLURIDES
Synthetic composition
Temperature (°K)
Resistivity (ohm-cm)
Hall coefficient (cma/coulomb)
Thermoelectric power (~-v/°K)
USe
300 80 300 300 80 300 80 300 80 300 80
2.8 × 10-4 1.2 x 10-4 2.3 × 10-3 3.34 x 10-~ 5.7 × 10-1 1-3 × 10-3 1.2 × 10-a 5.5 × 10-3 7.0 × 10-3 1.2 × 10-~ 1.0 × 10-z
+0.008 +0-12 -0.009 +0-01
+38 +21 --2 +21
U2Se3 USe2 UTe U2Te3 UTe2
+0.2 +0.02 +0'31 --0.34
Preparation and properties of some uranium selenides and tellurides
799
calculated from the Hall coefficients, ranged from about 2 ~< 1019 to about 10'~1 carriers/cm ~. The very low values of the Hall coefficient preclude ascribing much precision to the values quoted; however, it is felt by the authors that the relative magnitudes are correct and meaningful. The signs of the Hall coefficient and thermoelectric power (where measured) were positive for all samples except U2Se~ and UTe a. Electrons appeared to be the dominant charge carriers in U2S% and UT%. The high carrier concentrations found immediately suggest that these compounds are semimetallic conductors. The electrical properties of the single crystal of USe are of particular interest. At room temperature, the thermoelectric evaluation factor (ZK, equal to thermoelectric power squared divided by the resistivity) is 5"6 :: 10 ~;watt per cm-deg. This is higher than for the other compounds studied and may be compared with an evaluation factor of about 3 ; 10.-5 forp-type Bi2Tea at room-temperature. At high temperatures, the thermoelectric properties of USe may compare even more favourably with the best known thermoelectric materials. The mobility of the charge carriers, as calculated from the product of the Hall coefficient and conductivity, is encouragingly high for such an unrefined material. Because of this, the electrical properties of USe were investigated in greater detail. In Fig. 2 are shown the Hall coefficient, resistivity and thermoelectric power as functions of reciprocal temperature. The general character of the curves shown have been confirmed by measurements on other specimens. It will be noted that the thermoelectric power is increasing in a region where the Hall coefficient is decreasing. The behaviour of the electrical properties of the sample over the temperature range of the measurements is consistent with semimetallic conduction and the notion of overlapping bands. At higher temperatures, the resistivity of the sample increased to a second maximum. Such behaviour is suggestive of some phase transition but much more data would be necessary to resolve this question. It was of interest to determine the effects of stoicheiometric deviations on the electrical properties of USe. Two additional samples were prepared from nominal compositions containing one atomic per cent excess uranium and one atomic per cent excess selenium, respectively. These changes had no significant effect on the electrical properties of the compound. It is interesting to note, however, that electrons, rather than holes, were the dominant carriers in the U2Sea sample. The dominant charge carriers in USe are holes, as evidenced by the positive Hall coefficients and thermoelectric powers of the samples. One may speculate that, if the bonding in the compound were predominantly ionic, then n-type (electron) conduction might be expected, since uranium has more valence electrons than are needed, formally, to satisfy the Se 2- ion. Thus, it appears that any suitable model of the conduction processes in USe must attribute importance to covalent bonds. Since the arrangement of atoms in the rock-salt structure involves octahedral coordination, the covalent bonds of selenium might be expected to be of the p3d2s hybrid type involving the 5s-orbital, since the 4s-orbital is at so much lower energy. Such hybridization is found in [SeBrG]2 .(41 The covalent bonds of uranium would probably be of thef3d2s type (as, for example in the compound UCIB), since the alternate structure, d~sp3, requires promotion of three pairs of electrons from the 5fto the 7p shellJ '~) m L. PAULINe, The Nature of the Chemical Bondpp. 184-185, Cornell University Press (1948). {~}J. C. EISENSTEIN,J. Chem. Phys. 25, 143-146 (1956).
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L.K. MATSON,J. W. MOODYand R. C. HIMES
If the hypothesized covalent bonding does operate, each uranium atom would contribute six electrons to the bonding system. Each selenium atom would contribute only four electrons, holding an inert pair in the 4s-orbital. Twelve electrons would be needed to fill the six octahedral bonds to one atom in USe, and the proposed bonding system lacks two electrons. Thus, it seems reasonable to hypothesize that the p-type conductivity observed for USe is due to unsatisfied bonds. That the bonds are strongly covalent was also concluded by KHODADADc6) on the basis of oxidation studies of USe, U3Se4 and U2Se3 with Hg 2+ ions. On the basis of these considerations, one might postulate that PuSe would be a semiconductor, should it exist in the rock salt structure. Plutonium has two more electrons than does uranium which could be contributed to the bonding system. Acknowledgement--The authors wish to thank the Selenium and Tellurium Development Committee
whose sponsorship made this work possible and Dr. D. A. VAUGHNwho performed the X-ray analyses. Helpful discussions with Messrs. J. F. MILLERand R. T. BATEare also acknowledged. ~6~p. KHODADAD,C.R. Acad. Sci., Paris 253, 1575-1577 (1961).