- Email: [email protected]
Heat-capacity behavior of impure neptunium metal below 320°K
J. Phys.
Chem. Solids
Pergamon
Press 1965. Vol. 26, pp. 1075-1078.
HEAT-CAPACITY
Printed in Great Britain.
BEHAVIOR OF IMPURE NEPTUNIUM METAL BELOW 320°K* THOMAS
LOS Alamos Scientific Laboratory (Received
A. SANDENAW
of the University
9 October
of California,
Los Alamos, New Mexico
1964; in revised form 1 February
1965)
Abstract-The experimentally determined low temperature C, vs. T curve of impure neptunium metal was found to exhibit irregular behavior. A small but persistent peak was found at about 85OK. The heat capacity data had considerable scatter between 75’K and 140°K, and a change of slope was found in the C, vs. T curve at about 240’K. The effects noted were real and correlate with anomalies in resistivity and magnetic susceptibility observed by others; they exhibit some similarity to effects observed in a-U and CX-Pu. It is felt that the observed behavior is intrinsic to the neptunium metal, and that results observed between 75OK and 140’K would be clearer cut in pure metal.
1. INTRODUCTION
information available on uranium and plutonium, very little has been published concerning the low-temperature physical properties of neptunium metal except in a recent paper by G. T. MEADEN on the electronic properties of the actinide metals. Some relatively incomplete information about neptunium was presented at the Second Geneva Conference (1958) on The Peaceful Uses of Atomic Energy,(s) but most of the available information on the anomalous behavior of neptunium at low temperatures actually has appeared only in Meaden’s doctoral thesis.(s) This thesis emphasizes the apparent sensitivity of the physical properties of neptunium to its impurity content. When a specimen of neptunium metal became available to the author on a short-term loan, it was subjected to an intense study of low-temperature heat-capacity behavior. It was afterwards discovered that the metal used had high impurity content, and that its anomalous low-temperature heat-capacity behavior was reflecting only phenomena that had previously been observed. In spite of this, it was felt that publication of heat-capacity data might be useful in increasing interest in the study of the material, and that thermodynamic IN
values derived from it should not be in error by more than about one percent.
COMPARISON with
* Work performed under the auspices of the United States Atomic Energy Commission.
2. SPECIMEN
DESCRIPTION
The Np metal was prepared from available NpOs by the same process developed for preparation of plutonium metal. Actual reduction to metal was done by Group CMB-11 of the Los Alamos Scientific Laboratory, who also cast it into the shape of a cylindrical rod and machined it to size. The final machined rod was given no annealing treatment. The Np test specimen used for heat-capacity measurements had a nominal length of 1 in. and weighed 14.303 g. Metallographic analysis of the specimen showed it to be a very ‘dirty’ casting, with a eutectic structure throughout. Identification of the impurity phase or phases was not made. The major impurities listed for the Np metal (in per cent) were: Pusss, 0.03; Th, 0.01 to O-1 ; SC, Ti, La, Yb, Ce, Pr, Nd, Sm, Na, less than 0.5 each; Cr, Mn, Fe, Co, Ni, Y, 22, less than 0.05 each; Li, B, Zn, less than 0.4 each. The powder diffraction data for metallic Np have been interpreted in terms of an eight-atom orthorhombic unit. 3. EXPERIMENTAL
DETAILS
The low-temperature heat-capacity measurements were made in an adiabatic calorimeter, 1075
THOMAS
1076
A. SANDENAW
which has been described in detail in a report on the low-temperature heat capacity of nonstoichiometric niobium carbides.@) Temperature was measured with 2.1 o/ocobalt in gold vs. copper thermocouples. These thermocouples were calibrated against a platinum resistance thermometer (N.B.S. certification) in the region from 100°K to 300°K. The thermocouples were also calibrated at the h-point and boiling point of liquid helium, at the triple point and boiling point of equilibrium hydrogen and at the boiling point of oxygen. In addition the Co-Au vs. Cu thermocouples were calibrated against a copper vs. constantan thermocouple furnished by Dr. Robert H. Sherman, in order to complete the temperature scale between N 20°K and N 87°K. The latter thermocouple had also been calibrated against a platinum resistance thermometer having a N.B.S. certification. The Np metal rod was spring-loaded into a copper calorimeter can having a total weight of 20.991 g including the Be-Cu spring. The specific heat of the calorimeter can assembly was determined both before and after measurements were made on the combination of Np metal and calorimeter can, and again after making low-temperature heat-capacity measurements on a specimen of 1~235 of similar dimensions. Although radioactive Puss* was present, there was not enough self-heating of the test specimen to make possible heat-capacity measurements without an additional energy supply. The energy supplied to the Np specimen was varied in steps by factors of about 1, 2, 3 and 4 over most of the temperature region studied. The resulting experimental heat capacity (C,) vs. temperature (T) data fell into a narrow band having a width of approximately + 0.05 Cal/OK/gram atom. As long as the data, taken at any heating rate, fell within the limits of this band, it was felt that heat supplied by the small amount of Pusas present was insignificant by comparison with the electrical energy, and it was ignored in all calculations. The smoothed C, vs. T curve for Np metal was obtained by arbitrarily choosing that curve falling in the middle of the experimentally determined band of C, vs. T data. 4. EXF’EFUMENTAL
RESULTS
Heat-capacity results The C, vs. T data for the temperature
range
from -8°K to 75°K were very reproducible and normal. A very small peak appeared in the curve at N 85°K in four out of five runs, and only a hump in the same temperature region in the fifth run. The band of experimental C, vs. T data for the temperature range 75°K to 140°K was 2 to 3 times as wide as the band for the experimental data below 75°K. The experimentally determined C, vs. T curve changed slope at -240°K. The values for heat capacity (per gram atom) of Np metal as a function of temperature are given in Table 1. The values are for the smoothed curve. The gram atomic weight of Np was taken to be 237. The gram calorie (mean) was taken to be equivalent to 4.184 joules absolute. The band width of the data was determined to be + 1% up to 70”K, &-2.5% from 75°K to 140”K, and + 1% between 140°K and 320°K. Since the smoothed C, vs. T curve was chosen as the mid-band value along the entire curve, it is believed that within any temperature range the actual accuracy is much better than these limits given for the band width itself. Thermodynamic properties The value of Ssss for Np metal obtained by summing the actual heat-capacity data up to 298.15”K is 12.04 Cal/g atom/OK, and the value of Hess-Ho is 1624 Cal/g atom. If the C, vs. T data are summed along the smoothed (peak free) curve of Table 1 instead of along the actual curve (slight peak at ~85°K) representing experimental data, then the values of Ssss and Hsss- Ho become 12.02 Cal/g atom/OK and 1622 Cal/g atom. The differences in entropy and enthalpy at 298*15”K resulting from the arbitrary smoothing procedure are insignificant. It is believed that the values for Ssss and Hegs-HO given here represent maximum values, and that correct values may be up to O-70,6 lower. Electronic-contribution coe@cient and 00 The temperature range from4”Kto N 22.4”Kwas covered manytimes in heat-capacity measurements, in an effort to get an approximate value for the electronic contribution to heat capacity. A plot of the C,/T vs Tz data for the temperature region below 15°K gave a straight line with an intercept suggestcoefficient of ing an electronic-contribution 3.3 x 10-a Cal/g atom/“Ks. The slope of the line
HEAT-CAPACITY
BEHAVIOR
OF IMPURE
NEPTUNIUM
METAL
BELOW
320’K
1077
Table 1. He& capacity of ne~t~ni~rn metal at rounded temperatures (peg gram atomic weight) G
G
8 10 12.5 15 17.5 20 2.5 30 35 40 45 50 55 60 65 70 75
0.060 O=lOO 0.169 0.273 0.414 0.594 0.990 1.45 1.94 2.41 2.84 3.23 3-58 3.90 4.18 4.43 4.65
125 130 13.5 140 145 1.50 160 170
suggested contribution coefficient of 33 x 10-d for compares reasonably well with the 26 x 10-4 quoted for uranium metal (Ref. 5. DISCUSSION
4.84 5.01 5.16 5.30 5.43 5.55 5.66 5.76 5.86 5.95 6.04 6.12 6.20 6.27 6.34 6.46 6.56
180 190 200 210 220 230 240 250 260 270 273.15 280 290 298.15 300 310 320
6.64 6.72 6-79 6-85 6.91 6.99 7.07 7.16 7.28 7.41 7.45 7-56 7.72 7.87 7.90 8.10 8.32
_-.
~-suggested a en of N 190”K.The
CP
(Cal/OK)
(Cal/OK)
(Cal/OK)
electronic Np metal value of 3, p. 120).
OF RESULTS
The low-temperature heat-capacity behavior found for Np metal reflects previously observed phenomena. Magnetic susceptibility measurements of Gardner on one Np specimen showed anomalies at about 155°K (also given as ~123OK) and at 273”K.(s~s) The magnetic phenomenon originally observed at about 123°K was considered to represent a ferromagnetic Curie point.@) However, another Np specimen did not show any magnetic effects at the lower of these temperatures, so the conclusion was reached that the first specimen had contained a ferromagnetic impurity.@) E. KING (unpublished data)(sBa) has found small anomalies in the electrical resistivity vs. temperature curve of Np metal at about 155°K and 273°K. J. D. HILL (unpublished data)(ss) has found small thermal arrests at about the same temperatures. MEADEN found a hysteresis loop in the resistivity VS. temperature curve of an Np specimen at about 155”K, and a kink in the curve at 273°K. This previous work, then, suggests that the anomalous
heat-capacity behavior of impure Np metal observed at ~75°K to 140°K and above ~240’K in the present work, is a real effect. OLSEN and ELLIOTT(~) have studied the lowtemperature electrical-resistivity behavior of the same Np specimen here investigated through heat capacity measurements. They note a slight hump in their p/T vs. T plot at -SOoK, which coincides with the heat capacity behavior at 85°K. A hump in the p/T vs. T curve for d-phase uranium metal at -46°K has been shown by Berlincourt (as reported by FISHER and MCSKIMIN@). The latter workers(s) have also reported an unusual temperature dependence of elastic moduli at -43”K, and BARRETT et aZ.(7) have correlated the physical properties of c+U at 43°K with changes in atomic position and cell dimensions. Humping in the p/T vs. T curve of cc-phase plutonium metal at 50°K has been reported by OLSEN and EI.LIOTT,@) and an offset in the Young’s modulus vs. temperature curve of CC-Pu has also been noted by LAIIEivmNT(g) at about the same temperature. It appears possible, by analogy with U and Pu, that there is an anomalous thermalexpansion behavior in Np at about 85°K. This of course is only speculation. A sample of Us35 metal of the same nominal size as the Np specimen was
1078
THOMAS
A.
investigated by the author in the same calorimeter can used with the Np, and no anomalies were seen for Us35 in the temperature range 75°K to 140°K. The anomalies observed for Np, then, apparently did not originate in the apparatus or measuring techniques used. EVANS and MARDON have reported specificheat results for Np metal between 60°C and 207°C. Projecting their curve back to room temperature and converting their data to atomic heat gives a value at 298°K which is -12”,/0 below that given in Table 1. The C, vs. T values, for the room-temperature region, determined for Usas in our apparatus, confirmed the value found by JONES, GORDON and LONG for Ua3s almost within the limits of experimental error. We do not believe the results of Table 1, for room-temperature C, of Np metal, are in error by as much as 12%. Purity of material must play an important role. It is noteworthy that the entropy for Np metal (298°K) is so close to the value of Ssss reported for U metal by JONES et uZ.(l~) Acknowledgements-The writer wants to thank the following people for their assistance: F. W. SCHONFELD, CMF-5, for loan of the Np specimen; E. A. KMETKO,
SANDENAW CMF-5 and C. E. OLSEN, CMF-13 for results of chemical analysis; C. E. OLSEN, CMF-13, and R. 0. ELLIOTT, CMF-5 for making results of their research available prior to publication; and KAYE A. JOHNSON, CMF-5, for results of metallographic analysis.
REFERENCES 1. MEADEN G. T., Proc. Roy. Sot. A276. 553 (1963). 2. MCKAY H. A. C. and W~LDRON M. B:, addition to Paper No. 304. Second U.N. International Conference on thepeaceful Uses of Atomic Energy, United Nations, Geneva, Vol. 28, p. 306 (1958). 3. MEADEN G. T., Low temperature properties of transuranic and other heavy metals, Dr. of Phil. Thesis, University of Oxford, St. Peters’ College, (June 1961). 4. SANDENAWT. A. and STORMS E. K., to be published. 5. OLSEN C. E. and ELLIOTT R. O., Phys. Rev. In press. 6. FISHERE. S. and MCSKIMIN H. J., Phys. Rev. 124, 67 (1961). 7. BARRETT C. S., MUELLER M. H. and HITTERMAN R. L., Phys. Rev. 129, 625 (1963). 8. OLSEN C. E. and ELLIOTT R. O., J. Phys. Chem. Solids 23, 1225 (1962). 9. LALLEMENTR., Phys. Lett. 5, No. 3 (1963). 10. EVANS J. P. and MARDON P. G., J. Phys. Chem. Solids 10, 311 (1959). 11. JONES W. M., GORDON JOSEPH and LONG E. A., J. of Chem. Ph_vs. 20, 695 (1952).
Chem. Solids
Pergamon
Press 1965. Vol. 26, pp. 1075-1078.
HEAT-CAPACITY
Printed in Great Britain.
BEHAVIOR OF IMPURE NEPTUNIUM METAL BELOW 320°K* THOMAS
LOS Alamos Scientific Laboratory (Received
A. SANDENAW
of the University
9 October
of California,
Los Alamos, New Mexico
1964; in revised form 1 February
1965)
Abstract-The experimentally determined low temperature C, vs. T curve of impure neptunium metal was found to exhibit irregular behavior. A small but persistent peak was found at about 85OK. The heat capacity data had considerable scatter between 75’K and 140°K, and a change of slope was found in the C, vs. T curve at about 240’K. The effects noted were real and correlate with anomalies in resistivity and magnetic susceptibility observed by others; they exhibit some similarity to effects observed in a-U and CX-Pu. It is felt that the observed behavior is intrinsic to the neptunium metal, and that results observed between 75OK and 140’K would be clearer cut in pure metal.
1. INTRODUCTION
information available on uranium and plutonium, very little has been published concerning the low-temperature physical properties of neptunium metal except in a recent paper by G. T. MEADEN on the electronic properties of the actinide metals. Some relatively incomplete information about neptunium was presented at the Second Geneva Conference (1958) on The Peaceful Uses of Atomic Energy,(s) but most of the available information on the anomalous behavior of neptunium at low temperatures actually has appeared only in Meaden’s doctoral thesis.(s) This thesis emphasizes the apparent sensitivity of the physical properties of neptunium to its impurity content. When a specimen of neptunium metal became available to the author on a short-term loan, it was subjected to an intense study of low-temperature heat-capacity behavior. It was afterwards discovered that the metal used had high impurity content, and that its anomalous low-temperature heat-capacity behavior was reflecting only phenomena that had previously been observed. In spite of this, it was felt that publication of heat-capacity data might be useful in increasing interest in the study of the material, and that thermodynamic IN
values derived from it should not be in error by more than about one percent.
COMPARISON with
* Work performed under the auspices of the United States Atomic Energy Commission.
2. SPECIMEN
DESCRIPTION
The Np metal was prepared from available NpOs by the same process developed for preparation of plutonium metal. Actual reduction to metal was done by Group CMB-11 of the Los Alamos Scientific Laboratory, who also cast it into the shape of a cylindrical rod and machined it to size. The final machined rod was given no annealing treatment. The Np test specimen used for heat-capacity measurements had a nominal length of 1 in. and weighed 14.303 g. Metallographic analysis of the specimen showed it to be a very ‘dirty’ casting, with a eutectic structure throughout. Identification of the impurity phase or phases was not made. The major impurities listed for the Np metal (in per cent) were: Pusss, 0.03; Th, 0.01 to O-1 ; SC, Ti, La, Yb, Ce, Pr, Nd, Sm, Na, less than 0.5 each; Cr, Mn, Fe, Co, Ni, Y, 22, less than 0.05 each; Li, B, Zn, less than 0.4 each. The powder diffraction data for metallic Np have been interpreted in terms of an eight-atom orthorhombic unit. 3. EXPERIMENTAL
DETAILS
The low-temperature heat-capacity measurements were made in an adiabatic calorimeter, 1075
THOMAS
1076
A. SANDENAW
which has been described in detail in a report on the low-temperature heat capacity of nonstoichiometric niobium carbides.@) Temperature was measured with 2.1 o/ocobalt in gold vs. copper thermocouples. These thermocouples were calibrated against a platinum resistance thermometer (N.B.S. certification) in the region from 100°K to 300°K. The thermocouples were also calibrated at the h-point and boiling point of liquid helium, at the triple point and boiling point of equilibrium hydrogen and at the boiling point of oxygen. In addition the Co-Au vs. Cu thermocouples were calibrated against a copper vs. constantan thermocouple furnished by Dr. Robert H. Sherman, in order to complete the temperature scale between N 20°K and N 87°K. The latter thermocouple had also been calibrated against a platinum resistance thermometer having a N.B.S. certification. The Np metal rod was spring-loaded into a copper calorimeter can having a total weight of 20.991 g including the Be-Cu spring. The specific heat of the calorimeter can assembly was determined both before and after measurements were made on the combination of Np metal and calorimeter can, and again after making low-temperature heat-capacity measurements on a specimen of 1~235 of similar dimensions. Although radioactive Puss* was present, there was not enough self-heating of the test specimen to make possible heat-capacity measurements without an additional energy supply. The energy supplied to the Np specimen was varied in steps by factors of about 1, 2, 3 and 4 over most of the temperature region studied. The resulting experimental heat capacity (C,) vs. temperature (T) data fell into a narrow band having a width of approximately + 0.05 Cal/OK/gram atom. As long as the data, taken at any heating rate, fell within the limits of this band, it was felt that heat supplied by the small amount of Pusas present was insignificant by comparison with the electrical energy, and it was ignored in all calculations. The smoothed C, vs. T curve for Np metal was obtained by arbitrarily choosing that curve falling in the middle of the experimentally determined band of C, vs. T data. 4. EXF’EFUMENTAL
RESULTS
Heat-capacity results The C, vs. T data for the temperature
range
from -8°K to 75°K were very reproducible and normal. A very small peak appeared in the curve at N 85°K in four out of five runs, and only a hump in the same temperature region in the fifth run. The band of experimental C, vs. T data for the temperature range 75°K to 140°K was 2 to 3 times as wide as the band for the experimental data below 75°K. The experimentally determined C, vs. T curve changed slope at -240°K. The values for heat capacity (per gram atom) of Np metal as a function of temperature are given in Table 1. The values are for the smoothed curve. The gram atomic weight of Np was taken to be 237. The gram calorie (mean) was taken to be equivalent to 4.184 joules absolute. The band width of the data was determined to be + 1% up to 70”K, &-2.5% from 75°K to 140”K, and + 1% between 140°K and 320°K. Since the smoothed C, vs. T curve was chosen as the mid-band value along the entire curve, it is believed that within any temperature range the actual accuracy is much better than these limits given for the band width itself. Thermodynamic properties The value of Ssss for Np metal obtained by summing the actual heat-capacity data up to 298.15”K is 12.04 Cal/g atom/OK, and the value of Hess-Ho is 1624 Cal/g atom. If the C, vs. T data are summed along the smoothed (peak free) curve of Table 1 instead of along the actual curve (slight peak at ~85°K) representing experimental data, then the values of Ssss and Hsss- Ho become 12.02 Cal/g atom/OK and 1622 Cal/g atom. The differences in entropy and enthalpy at 298*15”K resulting from the arbitrary smoothing procedure are insignificant. It is believed that the values for Ssss and Hegs-HO given here represent maximum values, and that correct values may be up to O-70,6 lower. Electronic-contribution coe@cient and 00 The temperature range from4”Kto N 22.4”Kwas covered manytimes in heat-capacity measurements, in an effort to get an approximate value for the electronic contribution to heat capacity. A plot of the C,/T vs Tz data for the temperature region below 15°K gave a straight line with an intercept suggestcoefficient of ing an electronic-contribution 3.3 x 10-a Cal/g atom/“Ks. The slope of the line
HEAT-CAPACITY
BEHAVIOR
OF IMPURE
NEPTUNIUM
METAL
BELOW
320’K
1077
Table 1. He& capacity of ne~t~ni~rn metal at rounded temperatures (peg gram atomic weight) G
G
8 10 12.5 15 17.5 20 2.5 30 35 40 45 50 55 60 65 70 75
0.060 O=lOO 0.169 0.273 0.414 0.594 0.990 1.45 1.94 2.41 2.84 3.23 3-58 3.90 4.18 4.43 4.65
125 130 13.5 140 145 1.50 160 170
suggested contribution coefficient of 33 x 10-d for compares reasonably well with the 26 x 10-4 quoted for uranium metal (Ref. 5. DISCUSSION
4.84 5.01 5.16 5.30 5.43 5.55 5.66 5.76 5.86 5.95 6.04 6.12 6.20 6.27 6.34 6.46 6.56
180 190 200 210 220 230 240 250 260 270 273.15 280 290 298.15 300 310 320
6.64 6.72 6-79 6-85 6.91 6.99 7.07 7.16 7.28 7.41 7.45 7-56 7.72 7.87 7.90 8.10 8.32
_-.
~-suggested a en of N 190”K.The
CP
(Cal/OK)
(Cal/OK)
(Cal/OK)
electronic Np metal value of 3, p. 120).
OF RESULTS
The low-temperature heat-capacity behavior found for Np metal reflects previously observed phenomena. Magnetic susceptibility measurements of Gardner on one Np specimen showed anomalies at about 155°K (also given as ~123OK) and at 273”K.(s~s) The magnetic phenomenon originally observed at about 123°K was considered to represent a ferromagnetic Curie point.@) However, another Np specimen did not show any magnetic effects at the lower of these temperatures, so the conclusion was reached that the first specimen had contained a ferromagnetic impurity.@) E. KING (unpublished data)(sBa) has found small anomalies in the electrical resistivity vs. temperature curve of Np metal at about 155°K and 273°K. J. D. HILL (unpublished data)(ss) has found small thermal arrests at about the same temperatures. MEADEN found a hysteresis loop in the resistivity VS. temperature curve of an Np specimen at about 155”K, and a kink in the curve at 273°K. This previous work, then, suggests that the anomalous
heat-capacity behavior of impure Np metal observed at ~75°K to 140°K and above ~240’K in the present work, is a real effect. OLSEN and ELLIOTT(~) have studied the lowtemperature electrical-resistivity behavior of the same Np specimen here investigated through heat capacity measurements. They note a slight hump in their p/T vs. T plot at -SOoK, which coincides with the heat capacity behavior at 85°K. A hump in the p/T vs. T curve for d-phase uranium metal at -46°K has been shown by Berlincourt (as reported by FISHER and MCSKIMIN@). The latter workers(s) have also reported an unusual temperature dependence of elastic moduli at -43”K, and BARRETT et aZ.(7) have correlated the physical properties of c+U at 43°K with changes in atomic position and cell dimensions. Humping in the p/T vs. T curve of cc-phase plutonium metal at 50°K has been reported by OLSEN and EI.LIOTT,@) and an offset in the Young’s modulus vs. temperature curve of CC-Pu has also been noted by LAIIEivmNT(g) at about the same temperature. It appears possible, by analogy with U and Pu, that there is an anomalous thermalexpansion behavior in Np at about 85°K. This of course is only speculation. A sample of Us35 metal of the same nominal size as the Np specimen was
1078
THOMAS
A.
investigated by the author in the same calorimeter can used with the Np, and no anomalies were seen for Us35 in the temperature range 75°K to 140°K. The anomalies observed for Np, then, apparently did not originate in the apparatus or measuring techniques used. EVANS and MARDON have reported specificheat results for Np metal between 60°C and 207°C. Projecting their curve back to room temperature and converting their data to atomic heat gives a value at 298°K which is -12”,/0 below that given in Table 1. The C, vs. T values, for the room-temperature region, determined for Usas in our apparatus, confirmed the value found by JONES, GORDON and LONG for Ua3s almost within the limits of experimental error. We do not believe the results of Table 1, for room-temperature C, of Np metal, are in error by as much as 12%. Purity of material must play an important role. It is noteworthy that the entropy for Np metal (298°K) is so close to the value of Ssss reported for U metal by JONES et uZ.(l~) Acknowledgements-The writer wants to thank the following people for their assistance: F. W. SCHONFELD, CMF-5, for loan of the Np specimen; E. A. KMETKO,
SANDENAW CMF-5 and C. E. OLSEN, CMF-13 for results of chemical analysis; C. E. OLSEN, CMF-13, and R. 0. ELLIOTT, CMF-5 for making results of their research available prior to publication; and KAYE A. JOHNSON, CMF-5, for results of metallographic analysis.
REFERENCES 1. MEADEN G. T., Proc. Roy. Sot. A276. 553 (1963). 2. MCKAY H. A. C. and W~LDRON M. B:, addition to Paper No. 304. Second U.N. International Conference on thepeaceful Uses of Atomic Energy, United Nations, Geneva, Vol. 28, p. 306 (1958). 3. MEADEN G. T., Low temperature properties of transuranic and other heavy metals, Dr. of Phil. Thesis, University of Oxford, St. Peters’ College, (June 1961). 4. SANDENAWT. A. and STORMS E. K., to be published. 5. OLSEN C. E. and ELLIOTT R. O., Phys. Rev. In press. 6. FISHERE. S. and MCSKIMIN H. J., Phys. Rev. 124, 67 (1961). 7. BARRETT C. S., MUELLER M. H. and HITTERMAN R. L., Phys. Rev. 129, 625 (1963). 8. OLSEN C. E. and ELLIOTT R. O., J. Phys. Chem. Solids 23, 1225 (1962). 9. LALLEMENTR., Phys. Lett. 5, No. 3 (1963). 10. EVANS J. P. and MARDON P. G., J. Phys. Chem. Solids 10, 311 (1959). 11. JONES W. M., GORDON JOSEPH and LONG E. A., J. of Chem. Ph_vs. 20, 695 (1952).