Stck!' SECURITY INFORMATION 576?

StCK!' SECURITY INFORMATION 576?:

LOS ALAMOS SCIENTIFIC LABORATORY of the UNIVERSITY OF CALIFORNIA

Report written: May 1953

1 his document consists of _23_ pages No. 2 ^ of 30 copies, Series A

THE PREPARATION AND CRYSTAL STRUCTURE OF SOME INTERMBTALUC POLONIUM COMPOUNDS by ■ Willard G. Witte man Angelo L. Gtorgt t Dwayne T. Vlei_ CLASSIFICATION CANCELLED DATE------L-J tL r£J.- For I be Atomic Encigy Commission •14 & Chief, Declassification Branch0 ^ ^ _ 3

R E S T R I C DATA This document contains restricted data as the Atomic Energy Act of 1948. Its trans- mittal or the disclosure of Its contents In an to an unauthorised person Is prohibited.

SECURITptFORMATION

* ORMATION

ABSTRACT

A micro technique for the preparation of Inter metallic polonium compounds is described. Advantages of the technique are: 1. Polonium compounds can be prepared on a microgram scale and in a closed system. 2. Reaction temperatures can be se­ lected over a wide range. 3. The rate of reaction can be determined by gamma count­ ing. 4. The reaction product is formed in a quarts capillary of suitable dimensions for direct X-ray diffraction study. 5. The decomposition pressure of the compound can be determined as a function of temperature. 0. Stoichiometric ratios of the elements in the intermetallic compounds or solutions can be determined. Compounds of polonium with beryllium, calcium, magnesium, and nickel were p re­ pared by this technique ana their composition and crystal structure investigated by X-ray diffraction. The results obtained for the beryllium, calcium, and magnesium compounds are: S o Reaction Crystal Density, Compound Tem p., °C Form Type o «o<*> gm/cc BePo 600 cubic ZnS 5.838 a 0.006 7.31 CaPo 550 cubic NaCl 6.514 t 0.006 6.01 MgPo 450 hexagonal NIAs 4.345 t 0.010 7.077 i 0.020 6.72

Nickel and polonium apparently form compounds with a composition and a crystal struc­ ture which vary continuously between NlPo and NlPOg and between NIAs and Cd(OH)g structures, respectively. Attempts to correlate experimentally determined mole ratios and calculated lattice constants were unsuccessful. A sim ilar investigation of polonium with tantalum, wolfram, and molybdenum evidenced no reaction at 500°C. . ^

ACKNOWLEDGMENT

The authors are deeply grateful to W. H. Zacharlasen tor his assistance in determining the crystal structure of magnesium polonide. CONTENTS

Abstract 3 Acknowledgment 3 1. Introduction 5 3. Experimental Procedure 6 3.1 Purification of Polonium 0 2.2 Preparation and Loading of Quarts Capillary 0 2.3 Heating the Quarts Capillary 8 2.4 Preparation of 8ample for Calorimetry and X-ray Analysis 12 3.5 Calorimetry 12 3.6 X-ray Diffraction Procedures 13 3. Preparation of Individual Samples 13 3.1 Beryllium Polonlde 13 3.3 Calcium Polonlde 13 3.3 Magnesium Polonlde 13 3.4 Magnesium Teliurlde 13 3.8 Nickel Polonlde 13 3.0 Wolfram, Molybdenum, and Tantalum 14 4. X-ray Diffraction Results 14 4.1 Beryllium Polonlde (BePo) 14 4.2 Calcium Polonlde (CaPo) 17 4.3 Magnesium Polonlde (MgPo) 18 4.4 Nickel Polonlde (NlPo) 21 5. Conclusion 22 0. References 23 $E(tf

I. Introduction

Little Is known concerning high temperature reactions oi polonium metal with other metals. The early literature on this phase of polonium chemistry contains only fragmentary information of doubtful significance. More recently, in an Investigation of the action of mol* tea polonium on gold, platinum, nickel, and tantalum surfaces, Witte man and Vler* found tvi dance of compound or alloy formation with the first three metals, but found no reaction with tantalum. In addition, Goode2 has reported the preparation and Identification of tine polonide (ZnPo), lead polonide (PbPo), sodium polontde (Na^Po), platinum polonldt (PtPoj), and nickel polonide (NlPo).

The reaction of polonium with nickel, beryllium, tantalum, wolfram, molybdenum, mag­ nesium, and calcium Is described below. The reactions of polonium with nickel and beryllium were chosen since neutron sources manufactured by this laboratory are almost exclusively polonium-beryllium sources In all-nickel containers. Tantalum, wolfram, and molybdenum were selected for Investigation in a search for metals that could be used with polonium at high temperatures without reaction. The complete absence of reaction found with these metals indicates thetr usefulness at high temperatures and suggests that polonium deposits can be recovered most easily from them. The reactions with magnesium and calcium are among those which might be expected to lead to simple stable compounds with polonium. Savere limitations are imposed on experimental procedures that can be considered for 3 an investigation of high temperature reactions involving polonium metal. The high volatility and oxide-forming properties* of polonium metal require the reaction to be carried out In a closed system and either in an Inert atmosphere or in vacuum. The small quantities of polonium available, as well as the high specific alpha activity of polonium (32 curies of polonium weigh 7 mg and produce 1 watt of heat), exclude macro techniques. Finally, the radioactive decay of polonium to form lead at a rate of 1/2% per day^ requires both s purification of the polonium Immediately before use and a short period of Investigation with a particular sample. Because of these limitations, a method for studying these reactions was developed S te m SfCUKilY ItfOUWiON

which take* advantage of the high vapor pressure of polonium metal. The method Is de­ scribed la some detail sloe# It offers a micro method for studying reactions with any element or compound which has a diffidently high vapor preasure.

3. Experimental Procedure

The reaction chamber waa an evacuated and sealed quarts capillary with the metal to be investigated (referred to as "baa? metal" In subsequent discussion) In one end of the capillary and with a relatively large excess of freshly purified polonium metal In the other. The quarts capillary was positioned within a split furnace that maintained the base metal end of the cap­ illary at a temperature that was a selected number of degrees higher than the polonium end. Thus, the bane metal was exposed to polonium vapor at a pressure corresponding to the vapor pressure of polonium st the temperature of the colder end of the furnace and was fret of excels polonium. The reaction between the base metal and polonium was conveniently followed by count­ ing the gamma rays originating at the base metal end of the capillary. For this purpose, a Stllbene type scintillation gamma counter was used with a lead silt system (3/4" wide—6" long hole) which permitted the crystal to "see" only the base metal end of the capillary. Completion of the reaction was assumed when the gamma count reached a constant maximum value. Finally, the base metal end of the capillary was sealed off, and the sample obtained was cilorlmetered to determine the polonium content and was identified by X-ray diffraction photographs.

3.1 Purification of Polonium Polonium Is received by thie laboratory a t a metallic deposit on platinum gauxe. In this form, the polonium contains varying amounts of lead and, possibly, bismuth. The polo­ nium purification procedure consisted of the following steps:* a. The polonium was distilled from the platinum gauze onto a glass surface where it waa deposited as a polonium mirror. b. The polonium mirror waa dissolved In 15 ml of 7.8 N ftNO., and diluted to 78 ml 3 3 volume, and the polonium electroplated on a tantalum foil (0.010" thick). c. The polonium metal was distilled from the tantalum foil Into the quartz reaction chamber.

3.2 Preparation and Loading of Quarts Capillary The quarts capillary apparatus Is illustrated in the line drawing of Fig. 1.

- 6 - StCREl The procedure for the preparation and loading of the capillary follows. a. Quarts tubing (3 to 4 mm I.D .) was cleaned, drawn Into a capillary (A), and scaled

b. From 0.2 to 0.0 micromole of the base metal was weighed (or the weight obtained by using a known length of wire of known weight per unit length) and was placed at the ex­ treme end of the capillary at point (0 ). c. The tantalum foil (C) containing the polonium deposit was placed in the quarts tube. d. By use of Tygon tubing, the quarts tube was attached to a vacuum line and evacu­ ated to a pressure of less than 10'* mm of lig for a period of 1 hour. The quarts was then flame-sealed above the tantalum foil at point (D). e. The quartz clumber was wrapped in glass wool and placed in a horizontal chamber so that the capillary end extended approximately 1-1/4" beyond the edge of the furnace. The furnace was heated to 400°C for 18 hours. The temperature gradient caused the polonium to migrate and deposit as a narrow band ( F ) In the capillary. f. By means of a small flame, the capillary was separated from the tube at a point (G) between the polonium and the tantalum foil.

2.3 Heating the Quartz Capillary The capillary was positioned and heated within the capillary well of a "differential tem­ perature" furnace. The furnace asse '.bly was constructed In such a manner that a tempera­ ture differential at the extreme ends of the capillary could be maintained and controlled. Figure 2 Is a cross-sectional drawing showing the details of the furnace and its position relative to the scintillation counter.

2.3.1 Differential Temperature Furnace. A drawing giving the dimensions of the stain­ less steel furnace Is given in Fig. 3. A photograph Is also included (Fig. 4) which Illustrates the disassembled and assembled views of the fumace. The furnace was constructed from stainless steel rod and was made up of two sections (A) and (B ) ( Fig. 2). A capillary well (D) extended into both sections and was aligned In assembly by three studs (C). Two thermocouple wells (F ) were Included In each section. The temperature of the polonium section (B ) was controlled by a tube furnace (E). The base metal section (A) contained an Insulated heater winding (G) which kept this section at a fixed higher temperature. Temperatures were maintained within 10°C and a continuous record of the temperature of each furnace section was kept on a Brown Electronik 4-point recorder.

2.3.2 Heating Procedure. The procedure followed In the loading, assembly, and sub­ sequent heating of the differential temperature furnace Is outlined below.

SECRET Y INFORMATION 'Itottrtlryyr 1 i 1 F F F C ' m ■ T77 '///////////, V 7 Z ■ 7 3

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Fig. 2. Reaction furnace and counting assembly

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SECRET SECURITY SECRET SECURITY INFORMATION

a. The quartz capillary was oriented so that the end containing the base metal extended into section (A) of the furnace. b. By means of the three aligning studs (C), the two sections were carefully assem­ bled. c. This assembly was positioned In the tube furnace (E) so that the base metal sec­ tion was opposite the silt system In the lead shielding. d. Thermocouples were placed In wells (F ) for recording and controlling the tempera­ ture of each section. Section A can be maintained at a fixed higher temperature by a rea­ sonably constant current through the heater winding on that section. e. The temperature of the furnace was raised to the reaction temperature and main­ tained at that level until a constant maximum counting rate was obtained on the scintillation counter. f. After completion of the reaction, the furnace was allowed to cool. During the cool­ ing, a temperature differential was maintained. When the temperature of the polonium section was below 50°C, the two sections were carefully separated and the capillary removed.

2.4 Preparation of Sample for Calorimetry and X-ray Analysis The capillary was flame-sealed in the center to separate the excess polonium from the reaction product. The sample In this form was suitable for X-ray studies and calorimetry.

2.5 Calorimetry The amount of polonium In the reaction product was determined by means of a calorl- 7 meter which measured the heat given off by the sample.

2.6. X-ray Diffraction Procedures

2.6.1 X-ray Apparatus. All exposures were taken on Eastman, Type-A X-ray film In a North American Phillips 114.59 mm diameter powder camera. Copper radiation from a General Electric CA-6 X-ray diffraction tube was used in a General Electric XRD-1 unit. The films were measured without magnification with a pointer attached to a slide. A vernier scale permitted measurements of any one sharp line to within a possible error of tO. 1 mm.

2.6.2 Exposure Procedure. A nickel foil (0.001 cm thick) was used over the camera pinhole assembly to filter out the K-beta radiation from the copper. Since the high alpha activity of polonium caused the quartz capillary to fluoresce strongly, It was neceeary to cover the film in the camera with a sheet of black photographic paper (0.001 cm thick). This procedure proved to be completely effective In eliminating any fogging from the fluores­ cence and had no effect on the normal exposure time of 18 to 20 hours used in all runs. The samples were contained in quartz capillary tubes of approximately 0.5 mm diameter and 0.15

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mm wall thickness.

3. Preparation of Individual Samples

Using the technique described, reactions were carried out between polonium and beryl­ lium, calcium, magnesium, and nickel. Experiments were also conducted to study the effect of polonium on tungsten, molybdenum, and tantalum. The values reported for the vapor pressure of polonium at the various temperatures are 3 taken from a report by Brooks.

3.1 Beryllium Polonide An amount of 200 mesh Brush beryllium metal powder, estimated to be less than 1 pg, was heated to a temperature of 600°C in an atmosphere of polonium vapor; The polonium was held at 575°C to give a polonium vapor pressure of 10 mm of Hg. Reaction between the beryllium and polonium appeared complete after a period of 6-3/4 hours. A black powder resulted which contained 0.900 curie of polonium.

3.2 Calcium Polonide An amount of Merck and Co., Inc., calcium filings, estimated to weigh about 1 pg, was heated to a temperature of 550°C in polonium vapor. The polonium metal was held at a temperature of 525°C, to give a polonium vapor pressure of 4 mm of Hg. After heating 7-1/4 hours, a constant maximum gamma count was observed and the reaction was assumed to be complete. A grey product resulted which contained 1.314 curies of polonium.

3.3 Magnesium Polonide Approximately 1 pg of 100 mesh Brawn magnesium metal was heated to a temperature of 450°C In polonium vapor. The polonium was held at 425°C to give a polonium vapor pressure of 0.5 mm of Hg. Reaction was assumed to be complete after 5 hours of heating. The resulting black product contained 0.384 curie of polonium.

3.4 Magnesium Tellurlde To test the method of preparation, a reaction was carried out between magnesium metal and tellurium metal. For this experiment, an amount of 100 mesh Brawn magnesium metal, estimated to be less than 1 pg, was placed at one end of a quartz capillary tube. An excess of Elmer and Amend, C.P. grade, tellurium metal was positioned at the opposite end of the capillary. The magnesium metal was heated to 485°C in an atmosphere of tellurium vapor (0.02 mm of Hg) for 21 hours. A grey product resulted which was shown to be MgTe.

3.5 Nickel Polonide Grade A nickel wire (0.0015" diameter), produced by W. B. Driver Company, was used

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SECRET SECURITY INFORMATION SECRET SECURITY INFORMATION in the preparation of the nickel polonide samples. Several reactions were carried out in which known weights of nickel wire (determined by using known lengths of wire having a known weight per unit length) were heated at various temperatures between 300° and 600°C in polonium vapor. A metallic product resulted in each case. From the amount of polonium contained in each sample, the experimental mole ratios of polonium to nickel were calculated. Consistent mole ratios were not obtained, but in most cases the ratio of Po to Ni was between 1 and 2. It should be pointed out that the experimental mole ratios calculated for the polonide samples assumed that none of the nickel was lost by volatilization of the polonide. This assumption was not valid since nickel polonide is quite volatile and tended to distill toward the colder end of the capillary at temperatures above 500°C. The polonldes of the other metals which were investigated did not exhibit this phenomenon. An inspection of the nickel polonide samples under a microscope revealed crystal for­ mation at temperatures above 500°C. At these temperatures, the polonide tended to distill and recrysiallize, forming sharply defined crystal plates whose surfaces exhibited a high metallic lustre. A photomicrograph of such a sample is shown in Fig. 5. The \>olonide melted at 625 * 15°C.

3.6 Wolfram, Molybdenum, and Tantalum Approximately 0.5" length wire samples of 0.010" diameter Fansteel tantalum, Fansteel molybdenum, and Elmet wolfram were heated in polonium vapor for periods of 8 days, 6 days, and 5 days, respectively, at a temperature of 500°C. The polonium was held at 4.75°C to give a polonium vapor pressure oi 2 mm of Hg. At the end of this time, the samples were flame-sealed in the quartz capillaries and calorlmetered. The calculated mole ratio of polonium to metal was less than 4 x 10~* in all cases. The results indicate that, under the conditions of the experiments, there Is no reaction between polonium and these metals.

4. X-ray Diffraction Results

4.1 Beryllium Polonide (BePo) N The beryllium polonide sample prepared by the interaction of beryllium metal and polo­ nium vapor yielded an X-ray diffraction pattern which indexed completely to a face-centered- o cubic (f.c.c.) cell with the lattice constant, aQ * 5.838 t 0.006 A. Assuming a ZnS type structure, relative Intensities* were calculated' for the various reflecting planes and are

•The relative intensities of lines on the powder photographs were calculated by using the following formula: i 4 cos2 29 a sln2d cos#

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shown in Table 1. TADLE 1

DIFFRACTION ANGLES AND LINE INTENSITIES FOR BERYLLIUM POLON1DE

Indices Degrees Sin2 Q Relative Intensities hkl e Obs. Calc. •• Obs. Calc. 111 13.23 0.0524 0.0522 VS 1.00 200 15.33 0.0690 0.0606 MS 0.49 220 21.03 0.1395 0.1393 MS 0.42 311 25.03 0.1912 0.1915 MS 0.49 222 27.25 0.2097 0.2089 W 0.13 400 31.83 0.2781 0.2785 F 0.07 331 35.08 0.3303 0.3307 MW 0.19 420 36.15 0.3480 0.3482 MW 0.17 422 40.23 0.4171 0.4178 W 0.14 511-333 43.28 0.4700 0.4700 W 0.16 440 . - 0.5571 nil 0.05 531 51.38 0.6107 0.6093 MW 0.18 442-600 52.28 0.6260 0.6267 W 0 . 1 1 620 «• m . - . 0.6963 nil 0.09 533 . . 0.7485 nil 0.09 622 61.18 0.7676 0.7659. VW 0.10 444 - . 0.8356 nil 0.04 711-551 70.53 0.8889 0.8878 W 0. 30 •Intensities are given designations: VS * very strong, MS ■ moderately strong, MW « moderately weak, W * weak, VW * very weak, and F * faint. 9 0 ••Sin* 0 values calculated for a * 5.838 A. o

The very close agreement between the observed and calculated relative intensities indi­ cates that the f. c.c. cell is of the ZnS structure. The beryllium polonide is, therefore, lsostructural to the corresponding sulfide, selenlde, and tellurlde compounds, and the formula for the compound is BePo. A comparison of the sulfide, selenide, tellurlde, and polonide compounds of beryllium is presented in Table 2. The values for a (lattice constant), R-X (interatomic distance) +f 0 o and R ...X (ionic radii) are reported in A units.

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SECRET SEOIRTTY INFORMATION TABLE 2

COMPARISON OF BERYLLIUM SULFIDE, SELENIDE, TELLURIDE, AND POLONIDE

Compound Ionic Radii* • R-X RX ao Rn X'“ Obs. Calc. (CN-6) £ (CN-4)*** (CN-6)*** BeS 4.86* 0.30 1.00 2.10 2.20 BeSe 5.08* 0.30 2.02 2.20 2.32 BeTe 5.55* 0.30 2.22 2.41 2.52 BePo 5. 83q 0.30 2.30 2.53 2.60

•Data from Wyckoff. ® ••Data from Zacharlasen. ® •••CN-4 and CN-6 refer to coordination numbers 4 and 6, respectively.

The Interatomic distances observed are smaller than the calculated values In all cases. This Is to be expected because of the smaller coordination number.

4.2 Calcium Polonlde (CaPo) The X-ray diffraction pattern for the calcium polonlde sample Indexed completely to a f.c.c. cell with the lattice constant, aQ = 6.514 ± 0.006 2. Relative Intensities for the various reflecting planes were calculated on the basis of a NaCi type structure and these data are given in Table 3.

TABLE 3

DIFFRACTION ANGLES AND LINE INTENSITIES FOR CALCIUM POLONIDE

Indices Degrees Sin2 e Relative Intensities hkl e ObS. Calc.* Obs. Calc. 111 11.90 0.0425 0.0419 S .0.81 200 13.73 0.0563 0.0559 VS 1.00 220 19.63 0.1129 0.1118 S 0.77 311 23.15 0.1546 0.1538 MS 0.44 222 24.23 0.1684 0.1678 M 0.28 400 28.30 0.2248 0. 2237 MW 0.13 331 31.05 0.2660 0. 2656 M 0.19 420 31.98 0.2805 0.2796 MS 0.37 422 35.43 0.3361 0.3355 MS 0.25 511-333 37.90 0.3774 i 0.3775 M 0.14 440 42.00 0.4474 0.4474 vw 0.09 531 44.40 0.4895 0.4893 M 0.15 442-600 45.20 0.5035 0.5033 M 0.18 620 48.40 0.5592 0.5592 W 0.13 533 50.85 0.6014 0.6012 VW 0.06 622 51.65 0,6150 0.6151 w 0.13 •Sin2 Q values calculated for a0 * 6.514 A.

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A tabulation of the sulfide, selenide, tellurlde, and polonide compounds of calcium Is shown in Table 4.

TABLE 4

COMPARISON OF CALCIUM SULFIDE, SELENIDE, TELLURIDE, AND POLONIDE Compound Ionic Radii" R-X RX X~ Obs. Calc. (CN-6) . (CN-6) (CN-6) CaS 5.69* 0.94 1.90 2.84 2.84 CaSe 5.92* 0.94 2.02 2.96 2.96 CaTe 6.35* 0.94 2.22 3.18 3.1G CaPo 6.514 0.94 2.30 3. 26 3.24

•Data from Wyckoff.8 ••Data from Zacharlasen. ®

4.3 Magnesium Polonide (MgPo) The X-ray diffraction pattern obtained with the magnesium polonide sample conformed to a hexcgonal-close-packed cell. The calculated lattice constants for the unit cell are:

a„o ■ 4.345 ± 0.010 £ and c o ■ 7.077 ± 0.020 £. These values indicate that the unit cell of the magnesium polonide compound is smaller than the unit cell of magnesium tellurlde (MgTe) as reported by Zacharlasen.10 Since the radius of the polonium atom is larger than the radius of the tellurium atom, the unit cell would be expected tc be larger if the two compounds were lsomorphous. It would appear, therefore, that the magnesium polonide cell is not of the ZnO type. A magnesium tellurlde sample was prepared by a technique similar to the one employed in the preparation of the polonide. The Lattice constants determined from the diffraction pat­ tern agreed with the values reported by Zacharlasen^ and indicate that the compound MgTe was formed. Examination of the reflections from the 211 and 114 planes revealed a differ­ ence In Intensity of about a factor of 2 as predicted for the ZnO structure. One may con­ clude, therefore, that the difference between the structures of magnesium tellurlde and mag­ nesium polonide Is not due to the method of preparation. The diffraction angles and relative line Intensities observed for the sample of magne­ sium polonide are presented In Table S. For purposes of comparison, relative line intensi­ ties have been computed for a magnesium polonide structure of the ZnO type (MgTe type) and of a NIAs type. These data are included in Table 5.

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TABLE 5

DIFFRACTION ANGLES AND LINE INTENSITIES FOR MAGNESIUM POLONIDE

Relative Intensities Indices Degrees Sin2 B NlAs ZnO hkl e Obs. Calc.* Obs. • Calc. Calc. 100 11.80 0.0418 0.0419 MS 0.12 0.35 002 12.58 0.0474 0.0474 . MS 0.20 0.3 a 101 13.43 0.0539 0.0538 VS 1.00 l.o b 102 17.43 0.0897 0.0893 MW" 0.27 0.21' 110 20.75 0.1255 0.1257 S 0.27 0.32 103 22.63 0.1481 0.1486 M" 0.25 0.35 112 24.60 0.1733 0.1731 S 0.20 0.32 201 25.08 0.1797 0.1795 M 0.19 0.19 004 25.80 0.1894 0.1896 W 0.05 0.04 202 27.60 0.2161 0.2150 W 0.07 0.06 203 ' 31.58 0.2743 0.2743 MW 0.09 0.13 210 32.80 0.2934 0.2933 VW 0.04 0.04 211 33.53 0.3051 0.3052 M 0.15 0.16 114 34.15 0.3151 0.3153 M 0.12 0.09 105\ 0.3382 0.06 0.09 35.70 0.3405 W" 212/ 0.3407 0.06 0.06 300 37.85 0.3765 0.3771 MW 0.04 0.05 213 39.20 0.3995 0.4000 MW 0.10 0.14 302\ 0.4245 0.05 0.05 40.65 0.4244 MW" m l 0.4266 0.01 - 0.01 205\ 0.4639 0.05 0.05 42.95 . 0.4643 F 106/ 0.4685 0.02 0.02 220 45.20 0.5044 0.5028 F 0.03 0.04 222\ 0.5502 * 0.04 0.05 116 48.05 0.5531 0.5523 W" 0.04 0.04 • 311/ 0.5566 0.07 0.09 304 48.90 0.5679 0.5667 W 0.05 0.04 215\ 0.5896 0.06 0.09 312 50.20 0.5903 0.5921 W" 0.03 0.03 206/ 0.5942 0.01 0.01 313 53.85 0.6520 0.6511 VW 0.06 0.09 224 56. 50 0.6954 0.6924 VW 0.05 0.04

•Sin2 6 values calculated for a_ ■ 4. 345 k and c . 7.077 A. 0 0 ••These reflections appear as broad diffuse lines.

Except for the reflections from the 102 and 103 planes, the observed line Intensities arc In good agreement with the values calculated for the NlAs structure. Of special Interest are the reflections from the 211 and 114 planes. Here one would expect approximately equal Intensities from the NlAs type structure and a difference of almost a factor of 2 between the

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Intensities of the two lines from a ZnO type structure. A NIAs type structure Is suggested since the two lines are actually of about equal intensity. Although the lines from the 102 and 103 planes are less Intense than expected for a NIAs type structure, one may still accept this structure for magnesium polonlde since the lines are rather broad and diffuse and, hence, may be expected to appear less Intense. The diffuse nature of these lines suggests a possible "stacking disorder" along the c-axis. It is recognized that the agreements based on observed and calculated relative line intensities are not sufficiently conclusive to permit a definite selection of the structure of magnesium polonlde. Nevertheless, consideration of the radtt of magnesium and polonium atoms, together with the interatomic distances calculated for the two structures, renders the ZnO structure inherently unlikely and suggests that the magnesium polonlde has a NIAs type of structure. The formula for the compound Is MgPo. Different lattice parameters for the compounds MgS, MgSe, MgTe, and MgPo are tabulated in Tables 6 and 7. The NIAs type structure Is assumed for MgPo.

TABLE 6

LATTICE PARAMETERS OF MAGNESIUM SULFIDE, SELENIDE, TELLURIDE, AND POLON1DE

Lattice Constants ____ Ionic Radii •• a c Compound Type 0 0 R" X*' (CN) MgS NaCl 5.18 • - - 0.65 1.00 (6) 0 MgSe NaCl 5.45j* - - 0.65 2.02 (6) MgTe ZnO 4.52* 7. 33* 0.65 2.10 (4) MgPo NIAs 7.07, 0.65 2.30 (6)

•Data from Wyckoff.8 0 ••Data from Zacharlasen.

TABLE 7 COMPARISON OF MAGNESIUM SULFIDE, SELENIDE, TELLURIDE, AND POLON1DE RX R-X X-X Compound Obs. (CN) Calc,. (CN) Dlff. Obs. (CN) Calc. (CN) Overlap MgS 2.59 (6) 2.65 (6) 0.04 3.67 (8) 3.80 (6) 0.13 MgSe *.72 (6) 2.67 (0) 0.05 3.B5 (6) ' 4.04 (6) 0.19 MgTe 2.75 (4) 2.75 (4) nil 4.52 (4) 4.20 (4) neg. MgPo 3.07 (6) 2.05 (8) 0.12 4.35 (8) 4.60 . (8) 0.25 INFORMATION

From Table 7 It is apparent that In the proposed MgPo structure the observed Po-Po distance is smaller than the calculated value (double the tonic radius of Po~‘ ). This means that there is an actual overlap of the electron clouds surrounding adjacent polonium atoms. Further contraction of the lattice (increased overlap) is, presumably, resisted by coulombic repulsion, leaving the Mg-Po (R-X) distance larger than the sum of the radii of the Mg44 and Po‘“ ions. One might expect the MgPo to exist in a structure free from X-X overlap, e .g ., as in the MgTe structure where the coordination number Is 4. However, the measured lattice constants of MgPo are smaller than the lattice constants of MgTe and such a structure appears to be excluded. Further, the proposed structure and coordination number of 6 for MgPo are consistent with the structures of MgS and MgSe where coordination numbers of 6, X-X overlap, and expansion of the R-X distances are also observed.

4.4 Nickel Polonide (NiPo) The first sample of nickel polonide was in the form of rather large crystals and yielded an X-ray diffraction pattern comprised of a large number of spots rather than the usual diffraction lines. Nevertheless, it was possible to determine diffraction angles suf­ ficiently well to establish a hexagonal-close-packed structure for the nickel polonide with o o 0 aQ~ 3.95 A and cq ~ 5.68 A. The sample was heated above its melting point (about 625 C) and then cooled rapidly in air in an attempt to decrease the crystal siee. This treatment not only decreased the siie of the crystals ( as evidenced by definite lines in the diffraction pattern), but also Induced the following changes: a. Evidence of free polonium metal was found in the diffraction pattern. Thus, the "new” nickel polonide contained a lower Po Nl ratio than the original sample. b. The lattice constants had Increased In value, i.e., a — 3.96 A and c ^6.70 A. c. The order of the relative intensities of the diffraction lines had changed—suggesting a change in structure of the nickel polonide unit cell. Several additional samples of nickel polonide were prepared using known weights of nickel metal. Attempts were made to correlate mole ratios (Po Ni) with lattice constants determined from corresponding diffraction patterns. Variations in the lattice constants for the hexagonal cell were observed between the limits a * 3.05—c « 5.68 to a • 3.08— o o o cq « 5.7!. Because of uncertainties in the actual quantity of nickel in the final sample, the extent of the reaction between the polonium and nickel, and since excess polonium was present in some samples, a correlation of mole ratios with lattice constants was not achieved. How­ ever, the variations In the lattice constants suggest the formation of nickel-polonium com­ pounds oVyarytng composition. Further, the results obtained on the first nickel-polonium sample estamiaiLjp increase in lattice constants with a decrease in the Po to NI mole ratio.

SKRFT These observations Indicate that the Nl-Po system may be very similar to the Ni-Te sys­ tem. Kiemm and Fratlnl11 have established that nickel and tellurium form compounds of continuously varying composition between the limits NiTe and NlTSj and that the lattice con­ stants for ths hexagonal cell increase with a decrease in the Te to Ni mole ratio. A change in the structure of the unit cell (NiAs type for the NiTe and Cd(OH)g type for the NlTe^) was also observed.

5. Conclusion

A technique for the preparation of intermetalllc compounds of polonium with various metals has been described. Dy this method, polonides of beryllium, calcium, magnesium, and nickel have been prepared. The composition and crystal structure of these compounds were investigated with X-ray diffraction powder photographs. A summary of the results ob­ tained for the beryllium, calcium, and magnesium polonides is given In Table 8.

TABLE 8

SUMMARY OF MEASURED LATTICE CONSTANTS

... Lattice Constants ompound R-X T fc Ih» ao Co Obs. Calc.* BePo ZnS M 3 e 2.53 2.60 CaPo NaCl . 6 .514 3.28 3.24 MgPo NiAs 4.345 7.07«j 3.07 2.95

•Calculated from values obtnined from Zacharlasen. ®

Results obtained with the nickel polonlde samples indicate (hat (he nickel-polonium system forms compounds of continuously varying composition between the mole ratios (Po to Nl) of 1:1 and 2:1. Attempts to correlate the calculated lattice constants and the experimentally determined mole ratios were unsuccessful. The method developed for the preparation of the polonium compounds is considered to have the following outstanding features: a. Polonium compounds can be prepared from extremely small quantities of material. b. The temperature for the reaction can be selected over a wide range. c. The rate of the reaction can be determined by the relative gamma count. d. The reaction product la contained in a quarts capillary of suitable dimensions for direct X-ray diffraction study. It ts believed that this technique will also prove very useful In studies other than those of compound formation. The method could be adapted to the study of the dissociation temperatures of various polonium compounds. Studies might also be made of the solubility of polonium In various metals. Because of the general nature of the technique, It is not limited to studies on polonium but can be used with any gamma active material with suitable vapor pressures.

6. References

1. W. G. Wltteman and D. T. Vler, "The Action of Molten Polonium on Platinum, Gold, Tantalum, and Nickel," Los Alamos Scientific Laboratory Report LA-1546 (1053). 2. Pages 89 - 93 TID 5221 •'Polonium” July 1956, Office of Technical Services, Washington 25, D. C. 3. loc. cit. pages 192-199. 4. G. Moulton, J. Farr, and D. Vler, "Preparation and Reduction of Polonium Dioxide," Los Alamos Scientific Laboratory Report LA-1523 (1053). 5. William H. Beamer and V/lUlam Easton, J. Chem. Phya. 17j 1298 (1949). 8. M. G. Bowman and N. H. Krlkorlan, "Polonium Electroplating Procedures," Los Alamos Scientific Laboratory Report LA-1402 (1953). 7. R. B. Sevan and L. H. Trelman, Los Alamos Scientific Laboratory Report to be issued. 8. R. W. G. Wyckoff, Crystal Structures, Vol. 1, Interscience, New York (1951). 9. W. H. Zachariasen, personal communication. 10. W. H. Zachariasen, Z. Physik. Chem. 124, 277 ( 1928). 11. W. Klemm and N. Frattnl, Z. anorg. allg. chem. 25^ p. 222 (1943).

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