Phase transformation of AnX compounds under high pressure (An ≡ Np, Pu; X ≡ Sb, Te)

Journal of the Less-Common

35

Metals, 160 (1990) 35-52

PHASE TRANSFORMATION OF AnX COMPOUNDS PRESSURE (An= Np, Pu; X= Sb, Te) S. DABOS-SEIGNON*,

U. BENEDICT, S. HEATHMAN

UNDER HIGH

and J. C. SPIRLET

Commission of the European Communities, Joint Research Centre, European Institute for Transuranium Elements, Posrfach 2340, D-7500 Karlsruhe I (F.R.G.)

M. PAGES lnstitut Curie, Section de Physique et Chimie, Physicochimie des Ekments Transuraniens, U. R.A. C.N. R.S. 448, I I rue Pierre et Marie Curie, F- 75231 Paris, Cedex 05 (France)

(Received June 3, 1989)

Summary High pressure X-ray diffraction studies were performed on AnX compounds (An = Np, Pu; X= Sb, Te) in the pressure range up to about 57 GPa, at room temperature, using a diamond anvil cell in an energy-dispersive X-ray diffraction facility. These AnX compounds have an f.c.c., NaCl-type structure at ambient pressure and undergo first-order phase transitions under pressure. Whereas NpTe and PuTe transform to a primitive cubic cell of the CsCl type at 13 GPa and 15 GPa respectively, a tetragonal structure, space group P4/mmm, is observed for NpSb above 10 GPa. This tetragonal structure can be regarded as a “distorted” CsCl-type structure, the atomic positions being the same as in the cubic CsCl cell. In PuSb two transitions are detected: the CsCl-type structure is observed above 17 GPa; then PuSb transforms to the tetragonal P4/mmm structure at about 42 GPa. In addition, the compressibility of the low pressure phase was determined from the V (p) data for each compound.

1. Introduction The contribution of the 5f electrons to the chemical bond in solids containing an actinide element is central to understanding the physical properties of these materials. Crystal structure, lattice parameters and elastic properties are related to the localized or itinerant behaviour of the 5f electrons. The degree of localization of the 5f electrons depends on the actinide element, the actinide environment in a compound and the interatomic distances. As Hill pointed out [ 11, one of the crucial parameters is the interactinide spacing which determines the 5f-5f overlapping

*Also at Institut Curie. 0022-5088/90/$3.50

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36

and/or 5f-7s6d hybridization. Hybridization of 5f electrons can also occur with the d or p orbitals of the non-actinide element. At ambient pressure, actinide monopnictides and monotellurides (AnX) have the cubic face-centred structure of the NaCl type, except ThTe which is CsCl-type [2]. In this series, the interactinide spacing is above the Hill limit [3] and the degree of delocalization of the 5f electrons is weak enough for them to be considered as not contributing to the chemical bond. Applying pressure leads to structural and electronic changes, and may induce phase transitions. These phase transformations can be easily detected by X-ray diffraction. For actinides and their compounds, because of the correlation between the crystal structure and the behaviour of the 5f electrons, the observed phase transition under pressure may indicate the onset of the delocalization of the 5f electrons. This delocalization is brought about by the decrease in the interatomic distances due to the effect of pressure. The ThX [4, 51 and UX [6-l l] series have been extensively studied by X-ray diffraction under pressure. A few high pressure X-ray diffraction results are reported for neptunium and plutonium compounds, e.g. NpAs [ 121 and PuAs [ 131. According to ref. 14, the localization of 5f electrons is higher with a heavy pnictogen or chalcogen element: antimony and tellurium. However, the pnictogen element gives rise to higher delocalization than does the chalcogen element of close ionic radius, probably because the energy between 5f and X-p orbitals is larger in monochalcogenides than in monopnictides. In addition, the localized character of the 5f electrons increases with the atomic number of the actinide element in these series, from uranium to plutonium. From these points of view, studying the monotellurides and monoantimonides of neptunium and plutonium under pressure would allow us to separate the influence of the ligands (tellurium and antimony, of comparable ionic radius) of the actinide element, of valency and high pressure crystal symmetry on the understanding of itinerant vs. localized behaviour of the 5f electrons.

2. Experimental

procedure

The synthesis of actinide monochalcogenides and monopnictides is described in detail in ref. 15. The compounds were obtained by reaction of the elements which are heated in a vacuum-sealed quartz tube to about 600 “C. The resulting powder was then transformed into single crystals by mineralization. The pure single crystals were finely ground to obtain the polycrystalline sample needed for our high pressure experimental work. The lattice parameters of the samples studied are reported in Table 1. They were determined by precision X-ray diffractometry (NpTe and PuTe) or from a Debye-Scherrer pattern (NpSb and PuSb), and are in good agreement with the literature values [2]. High pressure studies were performed at room temperature using a diamond anvil cell of Syassen-Holzapfel type. The powdered sample was loaded in a hole of 0.2 mm diameter of an Inconel gasket inside the pressure cell. The pressure was measured in situ according to the ruby fluorescence method [ 161 using a small ruby

37

splinter placed within the sample. Silicone oil was added as a pressure-transmitting medium to allow for hydrostatic conditions. X-ray diffraction analysis was performed in an energy-dispersive X-ray facility. In our equipment, the use of a double conical-slit assembly fixes two Bragg angles at about 5” and 7”. This se&-up allows the simultaneous collection of data under these two angles. Accurate values of these angles were determined from the diffraction pattern of a reference sample of UC. The high efficiency of this doublecone set-up comes from the increased number of available diffraction lines in the same energy range, according to the Bragg equation. Details of this equipment are given by Benedict and Dufour [ 171. In the present study, the fluorescence lines of antimony and tellurium, lying in the energy range 26-32 keV (Table 2, [IS]), led to some overlapping with the diffraction lines of the sample at high pressures. Combining the 5” and 7” data for indexing the spectra minimizes this drawback. For each pressure step, the lattice parameters were determined from the diffraction pattern using the lattice constant refinement program of Williams [ 191.

3. Results 3.1. Monotellurides: NpTe and PuTe 3.1.1. NpTe Diffraction spectra of neptunium monotelluride were obtained up to 5 1 GPa in 15 increasing and three decreasing pressure steps. Figure 1 shows the variation TABLE 1 Lattice parameters of the ambient pressure phase of AnX compounds (An = Np, Pu; X = Te, Sb) AnX

a (pm) This work

a (pm) Literature [2]

NpTe PuTe NpSb PuSb

620.21(5) 617.74(5) 625.17(5) 623.75(5)

619.8 618.3 624.85 623.96

TABLE 2 Fluorescence

Sb Te

lines of antimony and tellurium [ 181

26.36 27.47

26.11 27.20

29.72 30.99

30.39 31.70

38

in the interplanar distances with pressure. From this graph it appears that the NaCl structure is conserved up to about 12 GPa. At this pressure new lines start to grow, indicating a phase transition. Up to 20 GPa, both phases coexist, as some weak lines of the low pressure phase persist. Above 20 GPa, the high pressure phase is observed as a single phase. This high pressure phase is characterized by the appearance of the strong diffraction line, lying between the 200 and 220 of the f.c.c. phase. This new phase was unambiguously identified as CsCl type. Table 3 gives the observed lattice parameters of both phases at the transition pressure. On releasing pressure, the inverse transformation (CsCl-to-NaCl-type) shows a strong hysteresis down to 2 GPa, at which pressure the 110 line of the CsCl-type structure is still visible. At ambient pressure the compound recovers a single-phase NaCl-type structure. The compression curve, relative volume V/V, (the index 0 refers to the ambient pressure parameter) vs. pressure, is shown in Fig. 2, where the observed transition is first order. The volume decrease at the phase transition is 7%. 3.1.2. PuTe The plutonium monotelluride was studied up to 47 GPa in 18 increasing and six decreasing pressure steps. As described for NpTe, a first-order phase transition

Fig. 1. NpTe: variation in interplanar pressure.

distances

vs. pressure. 0, Increasing pressure;

0, decreasing

39

to the C&l-type structure is observed. The 110 diffraction line of the CsCl structure starts to grow at 15 GPa, and the high pressure phase was pure at 20 GPa. The observed lattice parameters at the phase transition are given in Table 3. On releasing pressure, a strong hysteresis of the inverse transformation down to 2 GPa is again noticeable. Figure 3 shows the interplanar spacings vs. pressure. The volume collapse associated with the phase transition is 9% as shown in Fig. 4 where the relative volume V/V, vs. pressure data are reported.

3.2. Monoantimonides: NpSb and PuSb 3.2.1. NpSb We studied this compound up to 52 GPa in 32 increasing and eight decreasing pressure steps. Figure 5 shows the variation in the interplanar distances with pressure. Up to 10 GPa, the X-ray diffraction patterns show the characteristic diffraction lines of the NaCl-type structure. Above 10 GPa, the spectra reveal a new set of lines, indicative-of a phase transformation. Both sets coexist in the TABLE 3 Lattice parameters at the phase transition of AnX compounds (An = Np, Pu; X = Sb, Te)

NaCl phase

CsCl phase

Tetragonal

pdPa)

a (pm)

a (pm)

a (pm) c (pm)

NpTe PuTe NpSb

13 15 12

589( 1) 580(l) 599.3( 7)

361.2(6) 357.0( 7)

PuSb

17 42

589( 1)

366( 3) 356( 1)

AnX

0

20

30

376.1(4) 328.0( 7)

60

361(2) 322(3)

60 P.m

Fig. 2. NpTe: relative volume V/v, vs. pressure. Increasing pressure: 0, NaCl-type; Decreasing pressure: + .

X, CsCI-type.

180 170 160 150

PuTe

IL0 130 120 110 100 90 0

10

20

30

LO

50

a,: 0

10

20

30

LO

50

P,@

Fig. 3. PuTe: variation in interplanar pressure.

P.W

distances

VS.pressure. 0, Increasing pressure;

Fig. 4. PuTe: relative volume V/y, VS.pressure. Increasing pressure: 0, NaCl-type; Decreasing pressure: +.

0,

decreasing

X,

CsCl-type.

pressure range lo-18 GPa, above which the high pressure phase is pure. On releasing pressure, the inverse transformation showed a strong hysteresis down to 6 GPa where the low pressure phase starts to reappear, being pure at 2 GPa. From Fig. 5, it immediately appears that instead of one diffraction line lying between the 200 and 220 lines, as for the monotellurides, there are two lines. This suggests that the observed phase is a distorted CsCl structure. The observed interplanar distances could be indexed by a tetragonal structure, space group P4/mmm with the atoms located in the following sites: Np: la:O, 0,O and Sb: lb::, :, :. To check the validity of this indexation, we used the ENDIX program [20] which simulates theoretical diffraction patterns according to the space group and the atomic positions. Excellent agreement between the theoretical and experimental spectra was found. The lattice parameters observed at the phase transition in both phases are given in Table 3. This phase transition of NaCl to distorted CsCl type is of the first order, as shown by the relative volume V/ V, compression curve in Fig. 6. The volume collapse is 11.5% at the phase transition. It is the first time that such a phase transition has been reported for an actinide AnX, NaCl-type

41

I

0

60

lo

M

30

10

Fig. 5. NpSb: variation in interplanar pressure.

distances

Fig. 6. NpSb: relative volume V/f$ vs. pressure. P4m3. Decreasing pressure: 0.

compound, pounds.

50

p.GPo

~,@=a

VS.pressure.

0, Increasing pressure; 0, decreasing

Increasing pressure: 0, NaGtype;

the usual one being the NaCl-to-C&l-type

structure

A, tetragonal,

for such com-

Figure 7 shows the variation in the interplanar distances vs. pressure. The sample was investigated up to 57 GPa in 32 increasing and six decreasing pressure steps. From this graph we observe that PuSb undergoes two phase transitions. The first tr~sition takes place in the pressure range 17-20 GPa. This transition is identical with that observed for the monotellurides: the high pressure phase could be indexed by a CsCl-type structure as described for PuTe. Up to about 42 GPa this structure is maintained, whereas above this pressure, we observed splitting of some of the di~raction lines - mainly the 110 and 2 11 of the CsCl structure indicating that another phase transition is occurring. This new structure is similar to that observed for NpSb and could be indexed in the tetragonal structure space group P4/mmrn. The ENDIX program [20] allowed us to confirm the validity of this indexation. When releasing pressure, strong hystereses were observed down to

FuSb

Fig. 7. P&b: variation in interplanar pressure.

distances

vs. pressure. 0, Increasing pressure;

0, decreasing

33 GPa for the tetragonal to CsCl-type phase and down to 4 GPa where the compound recovered its low pressure NaCl-type phase. The relative volume V/V, VS.pressure plot in Fig. 8 shows that both phase transitions are first order. Table 3 reports the lattice parameters observed at each phase transition in both phases. The volume differences associated with these transformations are 4% and 5% respectively. These values are particularly small compared with the values in the range of 7%-11% usually observed for such transitions.

4. Discussion 4.1. Phase transition: influence of the Sf electrons. As mentioned in the introduction, the degree of localization of the 5f electrons is related to the interactinide spacing and to the crystal symmetry: delocalized 5f electrons favour distorted, low symmetric structure, whereas d

43

electrons prefer high symmetry structures. The interatomic distances have been determined for each compound and in each phase at the transition pressure. Tables 4 and 5 show the interactinide spacings and the An-X spacings respectively. Corresponding values calculated from refs. 4, 10 and 11 for analogous thorium and uranium compounds are reported for comparison. The critical Hill limit is in the range 305-320 pm for neptunium and 330-340 pm for plutonium [3]. From Table 4, it appears that all the compounds

0.7-

0.60

10

M

30

LO

50

60 P.ti

Fig. 8. PuSb: relative volume P/y, vs. pressure. Increasing pressure: 0, NaCl-type; X, CsCl-type; tetragonal, P4m3. Decreasing pressure: 0, NaCl-type; +, CsCl-type; V, tetragonal, P4/mmm.

A,

TABLE 4 lnteractinide spacings at ambient pressure and at the phase transition of NpTe, PuTe, NpSb and PuSb, and homologous thorium [4] and uranium [lo, 1 l] compounds AnX

UTe[ll] NpTe PuTe ThSb [4] USb[IO] NpSb

PuSb

NaCl phase

Ambient 15 Ambient 13 Ambient 15 Ambient 10 Ambient 10 Ambient 12

435.30 416.16 438.68(5) 416( 1) 436.81(5) 41O(lj 446.70 433.75 438.69 422.02 442.06( 5) 424( 1)

Ambient 17 42

441.06(5) 417( 1)

CsCl phase

Tetragonal

362.90 361.2(6) 357.0(7) 374.54 362.90 376.1(4 328.0(7 366( 3) 356(l)

361(2) 322(3)

44

TABLE 5 An-X spacing at ambient pressure and at the phase transition of NpTe, PuTe, NpSb and PuSb, and homologous thorium [4] and uranium [ 10,l l] compounds AnX

P (GM

NaCl phase

UTe[ll]

Ambient 15 Ambient 13 Ambient 15 Ambient 10 Ambient 10 Ambient 12 Ambient 17 42

306.6 294.27 310.19(5) 294( 1) 308.87(5) 290( 1) 315.90 306.67 310.20 298.41 312.58(5) 300( 1)

NpTe PuTe ThSb [4] USb[lO] NpSb PuSb

311.87(5) 294.9( 5)

C&l phase

Tetragonal

313.76 312.8(6) 309.2(8) 324.36

314.28 312(l) 317(3) 308.8( 1)

302.1(5)

analysed in the present work have an interactinide spacing far above the critical Hill value at ambient pressure. At the phase transition, this interactinide spacing which has decreased because of the pressure, still remained above the limit, in the low pressure f.c.c. phase as well as in the high pressure phase, CsCl-type (NpTe, PuTe and PuSb) and distorted CsCl-type (NpSb). 4.1.1. NaCl-to-CsCl-type transition NpTe, PuTe and PuSb undergo an NaCl-to-CsCl transition similar to that described for ThSb [4], USb [6-8, lo] and UTe [7, 9, 111. Comparisons with the behaviour under pressure of isostructural thorium compounds are helpful for understanding the influence of the 5f electrons on the phase transition, as thorium’s 5f levels are unoccupied. From crystallographic considerations, a given crystal structure type can be formed only in a corresponding range of cation-to-anion radius ratio, and is characteristic of a given valency state. The NaCl-CsCl transition increases the coordination number from 6 to 8. A corresponding increase in the ionic radius is expected [2 11, leading to an increase in the interatomic An-X distance. Both cation and anion size vary with coordination number [21]. Unfortunately, because no value of the ionic radius variation of plutonium and neptunium is reported in the literature, it is difficult to determine the expected increase in the An-X spacing. Table 6 gives the variation observed in the interatomic distances at the phase transition. Comparing this work with the recent one performed on homologous thorium and uranium compounds, ThSb [4], USb [lo] and UTe [ 111, we found a

45

TABLE 6 Variation in interatomic distances at the phase transition for AnX isostructural compounds (An = Th [4],U[lO, ll],Np,Pu;XrTe,Sb) AnX

NaCI to tetragonal

NaCI to CsCl AAn-An (pm)

AAn-X (pm)

UTe [ll] NpTe PuTe ThSb [4] USb [lo] NpSb

-54 -55 -53 -59 -59

19 19 19 18 16

PuSb

-51

AAn-An (pm)

AAn-X (pm)

-48 -96

12

CsCl to tetragonal AAn-An (pm)

22

AAn-X (pm)

5 -34

-7

similar behaviour under pressure: the same phase transition from NaCl to CsCl, a volume collapse at the transition in the range of 7%-lo%, except for PuSb which has a decrease of only 4% at the NaCl-to-CsCl transition. For the monotellurides the decrease in the interactinide spacing is in the same range: UT’e, - 54 pm, NpTe, - 55 pm and PuTe, - 53 pm; with the monoantimonides the same decrease of - 59 pm is observed for ThSb and USb, whereas for PuSb a decrease of only - 5 1 pm is observed. This small decrease might be related to the small volume decrease observed at the phase transition which can in turn be related to the particular behaviour under pressure of PuSb which undergoes two phase transitions. In addition, this value is the same as that observed at the phase transition of PuAs [ 131. The increase in the An-X distance at the phase transition is also the same for monotellurides, i.e. 19 pm, whereas monoantimonides exhibit more scatter in the increase in this An-X distance: ThSb, 18 pm, USb, 16 pm and PuSb, 22 pm. In addition, we note in Table 5 that the An-X spacing in the CsCl-type phase is larger than in the NaCl-type phase at ambient pressure. As a consequence of the increase in the An-X distance, the electronic d-p or f-p interaction is weakened; in contrast, the decrease in the interactinide spacing may induce a 5f-5f overlapping or a 5f-6d hybridization. Because the interactinide spacings are above the Hill limit at the transition pressure, we can assume that the NaCl-to-CsCl transition is mainly governed by 7s6d electrons. This point of view is supported by the similar behaviour of isostructural thorium and uranium compounds. These results correspond to the pressure-coordination rule [22]. 4.1.2. Distorted CsCl phase NpSb is the first actinide NaCl-type compound for which the transition from NaCl to distorted CsCl is observed. This transition has already been mentioned by Leger et al. [23] for rare earth monoantimonides, LaSb and CeSb at 11 GPa and

46

12 GPa respectively. These researchers assume that the 4f electrons have no influence on the high pressure phase because the behaviours of LaSb and CeSb are identical. They also conclude that, because of the short distance between the two antimony atoms, a covalent Sb-Sb bonding is likely to occur, which increases the bulk modulus. They observed a volume contraction of 10.5% at the transition. This value is comparable with that of 11.5% observed for NpSb. This phase transition leads to two interneptunium distances of 376 and 328 pm, whereas the Np-Sb distance increases from 300 to 312 pm. The shortest interneptunium distance in the high pressure phase is close to the Hill limit, but still above it. The coordination number has increased from 6 (NaCl phase) to 8 in the high pressure phase, as the shortest interatomic distance corresponds to the Np-Sb spacing. In this case, we believe that the existence of this tetragonal phase might be correlated to the cation size, and might indicate the influence of an Sb-Sb covalent bonding due to the short Sb-Sb spacing. The increase in the Np-Sb distance weakens the d-p interactions whereas the existence of two different Sb-Sb distances would favour Sb-Sb p interactions. On the contrary, the behaviour of plutonium monoantimonide under pressure is completely different. It is the first time that two phase transitions have been observed in the pressure range O-57 GPa. It is also the first time we have observed the following sequence in the phase transition: NaCl to CsCl to tetragonal, distorted CsCl. These two phase transitions are accompanied by small volume decreases of 4% and 5% respectively, compared with the 7%-10% usually observed for these transitions. The NaCl-to-CsC1 transition has been described above. The second phase transition leads to two different interplutonium distances, as described for NpSb. At the transition pressure, we observed Pu-Pu spacings of 322 and 361 pm compared with the 356 pm observed in the CsCl phase (Table 4). The critical Hill limit is in the range 330-340 pm for plutonium [3]; it appears that the two interplutonium distances are on each side of this limit. This would mean that the 5f electrons are likely to enter the conduction band, leading to a 5f-6d hybridization and/or a 5f-5f overlapping. In the same way as for NpSb, the Sb-Sb p interaction is reinforced. The decrease in the Pu-Sb spacing of 7 pm at the phase transition indicates that an increase in the electronic interaction d-p and/or f-p is also reinforced. In addition, the coordination number remains at 8: the Pu-Sb spacing has decreased by 7 pm from 309 to 302 pm, which is the smallest interatomic distance in this high pressure phase. As there is no change in the coordination number, the pressure-coordination rule [22] is not followed in this case. From this, we can assume that this phase transition is related to the onset of 5f electron itinerancy. This phase transition which does not obey the pressure-coordination rule might also be explained by considering only the covalent Sb-Sb bonding and by a steric effect, whereas the CsCl structure can exist only in a certain range of cation-to-anion radius ratios. The problem is to understand why the behaviours under pressure of these neptunium and plutonium monocompounds with the same NaCl-type structure at ambient pressure are different. The valence state of the actinide element has been well established to be + 3 in monopnictides: from neutron magnetic form factor [24] and X-ray photoelectron spectroscopy [25] measurements, USb has the

47

definite valency of + 3; Mossbauer spectroscopy on NpSb reported an isomer shift which agrees with a trivalent neptunium [26]; magnetic measurements on NpSb [27] are also in good agreement with Np 3+ . Neutron form factor measurement on PuSb showed unambiguously an oxidation state of + 3 for PuSb [28-301. The situation is not so clear with monotellurides. A trivalent state is the most credible for UTe, as partial delocalization of the 5f electrons is likely to occur because of metal-metal (5f-6d) hybridization [31]. Mossbauer spectroscopy on NpTe also suggested a trivalent state [26]1 Resistivity measurements suggested a Kondo behaviour and probably a heavy fermion system for NpTe [32]. The problem of valency in PuTe is very puzzling: its lattice parameter is too small to be Pu* +, and no magnetic order is observed to confirm Pu3+. Neutron diffraction experiments [33] revealed that the magnetization density is much reduced below that expected in a free Pu3+ but, as quoted in ref. 33, the measurements are not in favour of Pu 2+. A band system was suggested for this compound [33]. Susceptibility measurements on PuTe show that the magnetic susceptibility is independent of temperature for T>50 K [34]. The almost independent paramagnetism is difficult to understand with Pu3+, but is incompatible with a mixed valence compound, whereas resistivity and specific heat measurements are in contradiction with a band system. From theoretical calculation it was suggested that PuTe is a relativistic semiconductor [35]. The theoretical value of the bulk modulus should be 23 GPa, according to ref. 35. It is also reported that PuTe is probably a heavy fermion system [36]. Theoretical electronic calculation and the equation of state of PuTe have been compared with our preliminary high pressure diffraction studies [37]. These calculations reproduce in very good agreement our preliminary high pressure results: the volume collapse at the phase transition is 8% [37], compared with our 9%. The researchers concluded that the NaCl-to-CsCl transition should be accompanied by a semiconductor-tometal transition which can also be reflected by the low value of the bulk modulus. The semiconductor-to-metal transition can be experimentally checked by high pressure resistivity measurement. The antimony and tellurium atoms have close ionic radii; thus their actinide monocompounds are expected to behave in the same way under pressure. The observed differences might be explained in the following way: antimony and tellurium have close ionic radii, but with the pnictogen element the 6p band is larger than the p orbital of the telluride. As a consequence, hybridization of the 7s6d electrons of the actinide element (An) with p orbitals of the non-actinide element (X), as well as X-X interactions, will be easier with antimonides than with tellurides. So a distorted CsCl structure is observed and f-p mixing is favoured in NpSb, but not in NpTe. 4.2. Bulk mod& The bulk modulus and its pressure derivative were determined for the low pressure phase, by fitting the v(p) data to the Murnaghan [38] and Birch [39] equations. The results are given in Table 7 where the bulk modulus and pressure derivative of the homologous thorium and uranium compounds are also reported [4, 10, 111. ThTe is not mentioned as it already has the CsCl-type structure at

48 TABLE 7 Bulk modulus and pressure derivative for the AnTe and AnSb series (An = Th [4], U [ 10,111, Np, Pu) AnX

4, (GW

B’,,

UTe[ll] NpTe (B) (M)h (A)’ PuTe (B) (M) (A) ThSb [4] USb [lo] NpSb (B) (M) (A) PuSb (B) (M) (A)

480) 61(2) Q(2)

4.9 2.1 1.6 1.8(2) 14 9.0 12(8) 5.2 4.0 9.0 7.5 8(l) 3.5 3.0 3.3( 3)

61.5(2.0) 34(2) 39(2) 37(2) 84(8) 62(3) 54(2) 55(2) 54.5(2.0) 67(2) 69(2) 68(2)

“Birch equation. hMumaghan equation, ‘Average. am

-

-

.

-

*

6LO 620

-

\I.

bwib

..__A---. 0

600B,.GPa’ 90 60 70 -

.‘\

‘\

‘\

60!%ILO -

B’0 10 6 6 L 2 l-n

m

u

1 Np

PJ

Fig. 9. Lattice parameter at ambient pressure, bulk moduli and pressure derivative vs. An elements: l, AnTe; X, AnSb.

ambient pressure. In Fig. 9 the variation in the lattice parameter at ambient pressure in the AnX series is compared with the bulk modulus and the pressure derivative. In the AnTe series, B, increases with the lattice parameter. With the AnSb compounds, the variation is different (NpSb has a lower bulk modulus than

49

USb and PuSb, but its lattice parameter is bigger). This is probably due to its peculiar behaviour under pressure while it undergoes a transition from NaCl to distorted CsCl, and which involves an increase in Sb-Sb covalent mixing. This singular high pressure behaviour also finds expression in the high value of its pressure derivative. The low value of the bulk modulus of PuTe (37 GPa) is not far from the predicted value of 23 GPa [35]. A log-log plot of II, vs. unit cell volume is given in Fig. 10. Anderson and Nafe first established the bulk modulus-specific volume relationship, which has the following expression: ln B, = - k In V, + constant [40]. For ionic crystals, the slope k is - 1; for covalent solids it is - 4/3; for oxide compounds k is nearly - 4. Thorium monopnictides [4] have a slope of - 2; monochalcogenides a slope of - 1.8 5 [ 51; for uranium compounds, a slope of - 5/3 was found for monopnictides [lo] and - 2 for monochalcogenides [ 113. Jayaraman et al. [41] studied the bulk modulus scaling vs. volume for rare earth monochalcogenides; a slope of - 1 was found for semiconducting R2 +X and for metallic R3 +X compounds. The slope agrees with that of alkali halides, indicating that the compounds are predominantly ionic. In Fig. 10, the straight lines represent the trends of uranium and thorium monochalcogenides and monoantimonides according to refs. 4, 5, 10 and 11. The sodium halide line is given too. We have also reported the B, values of NpAs [ 121 and PuAs [ 131. From our data we can observe that NpAs and PuAs fit on the uranium monochalcogenides line, whereas NpTe, NpSb and PuSb are closer to the uranium monopnictides line. For PuTe, the small bulk modulus does not fit with any of the series and is below the uranium monochalcogenides. However, NpSb and NpAs fit on the same line, for which the slope k is - 1.2. This value is close to

Fig. 10. Bulk modulus scaling vs. cell volume per formula unit for thorium and uranium monopnictides and monochalcogenides and for NpAs [ 121 and PuAs [ 131, NpSb, PuSb, NpTe, PuTe. 1: Thorium monopnictides [4], 2: thorium monochalcogenides [5], 3: uranium monopnictides [ 101, 4: uranium monochalcogenides [ 1 I], 5: sodium halides.

50

-4/3 and indicates that these compounds are covalent solids, according to Anderson and Nafe [40]. In addition, PuAs and PuSb have the same bulk modulus, so a connecting line would have a slope of zero. When going from thorium to neptunium and plutonium isostructural monocompounds, the slope k tends to decrease to unity. 5. Conclusion This paper reports compression studies on AnSb and AnTe compounds (An = Np, Pu) which have the NaCl-type structure at ambient pressure. The monotellurides transform to a CsCl-type structure at 13 GPa for NpTe and 15 GPa for PuTe. This phase transition is first order and is governed mainly by 7s6d electrons, and the 5f electrons are not believed to delocalize. With NpSb, a first-order phase transition to a tetragonal, distorted CsCl structure is observed at about 10 GPa. It is the first time that such a phase transition has been observed for an actinide monocompound. This distorted phase might be explained by the covalent Sb-Sb bonding which does not exist with the tellurides. In addition, this phase transition might be related to the ratio of the cation-to-anion radius which probably does not allow the CsCl-type structure to be formed. Two first-order phase transitions occur for PuSb at 17 GPa and 42 GPa respectively. The first transformation is of the NaCl-to-CsCl type and the second one is the CsCl to tetragonal, distorted CsCltype structure, similar to that observed for NpSb. The transition from CsCl to distorted CsCl-type phase is not accompanied by any change in the coordination number. At both phase transitions for PuSb the volume decrease is very small (4%-5%). It is the first time that these two transitions have been observed for an NaCl-type structure compound. In this case, we conclude that the 5f electron delocalization is likely to occur. Future measurements under pressure such as electrical resistivity, optical reflectivity and X-ray absorption spectroscopy must be performed on these compounds because they would add further information about the role of the 5f electron during the transition. References 1 H. H. Hill, The early actinides: the periodic system’s f electron transition metal series. In W. M. Mimer (ed.), Plutonium, Met. Sot., AIME, New York, 1970, pp. 2- 19. 2 U. Benedict, Structural data of the actinide elements and of their binary compounds with nonmetallic elements. J. Less Common-Met., 128 (1987) 7-45. 3. J. M. Fourier and L. Manes, Actinides solids: 5f dependence of physical properties. In L. Manes (ed.), Actinide Chemistry and Physical Properties, Structure and Bonding, Vols. 59160, Springer, Berlin, 1985, pp. l-45. 4 L. Gerward, J. Staun Olsen, U. Benedict, S. Dabos, H. Luo, J. P. ItiC and 0. Vogt, Bulk moduli and high-pressure phases of ThX compounds: I. The thorium monopnictides, High Temp. High Pressures, 20(1988) 545-5.52. 5 J. Staun Olsen, L. Gerward,

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