Properties of metal phosphates and phosphides in glass-ceramic waste forms

Journal of Non-Crystalline Solids 263&264 (2000) 395±408

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Properties of metal phosphates and phosphides in glass-ceramic waste forms S.V. Raman * Nuclear Engineering Department, Idaho National Engineering and Environmental Laboratory, Idaho Falls, ID 83402, USA

Abstract Condensed glass-ceramic waste forms were developed by partial vitri®cation in a hot isostatic press. The calcined nuclear waste simulants, inclusive of volatiles, are partitioned among a variety of crystalline phases and glass. Phosphorus occurs largely in orthophosphate and phosphide crystals. X-ray powder di€raction analysis reveals changes in the Ôc/aÕ lattice parameters coincident with cadmium and cerium substitutions for calcium in ¯uor-apatite. The chemical durability of ¯uor-apatite is not a€ected by the changes in the c/a ratio. The limited solubility of Ce and Cd in calcium ¯uor-apatite is attributed to di€erences in Cd±O and Ce±O hybrid bonding orbitals. Similarly, the covalent linkage between phosphorus and oxygen in the PO3ÿ 4 tetrahedra is conceptualized in terms of hybrid bonding. In addition to phosphates, metal phosphides occur dispersed in the glass matrix and their compositions vary with changes in partial pressure of oxygen. Hybrid electron orbitals are proposed for the coordinating linkage between metals (M) and phosphorus (P). In (Fe, Cr)P4 , (Cd, Zr)P4 and Zr3 P4 phases, the M and P hybrid orbitals overlap more than in the phase (B, Ca, Sr, Cs, K, Na)4 P and contribute to a greater durability for the former phases. In both phosphates and phosphides, the hybrid bonding concept leads to the speculation that non-bonding itinerant electrons originate from phosphorus and are expectably localized by the negatively charged oxygen barriers in the phosphate. Ó 2000 Elsevier Science B.V. All rights reserved.

1. Introduction The text book descriptions of Rankama and Sahama [1] and Goldschmidt [2] classify phosphorus as the most abundant trace element of the earthÕs crust. Its metal compounds are sensitive to redox potentials [3]. While the terrestrial compounds are predominantly metal orthophosphates [1,2], the extra-terrestrial materials contain metal phosphides [1,2]. An observation is the absence of phosphate glasses in the geochemical environments and also the negligible presence of P2 O5 in

*

Tel.: +1-208 526 3606; fax: +1-208 526 0425. E-mail address: [email protected] (S.V. Raman).

the natural silicate glasses [4]. Instead, phosphorus occurs as the primary phosphate and phosphide minerals in igneous rocks and meteorites [1,2,4]. Apatite (Ca5 (PO4 )3 (F, Cl, OH) composes 95% of the primary phosphate minerals [1,2] and schreibersite (Fe, Ni, Co)3 P is the predominant phosphide in sulphide containing meteorites [1,2]. These minerals have endured the geochemical environments and are capable of accommodating a variety of elements in their cationic sites [5]. It is of interest to consider their formation in nuclear waste forms for immobilizing phosphorus and associated elements. For containment of all the elements, including volatiles, it may become necessary to develop a waste form in a condensed system at a constant

0022-3093/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 9 ) 0 0 6 4 8 - 1

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S.V. Raman / Journal of Non-Crystalline Solids 263&264 (2000) 395±408

pressure [6]. This experimental approach was needed for treating the Idaho calcined wastes, because they contain crystal forming, glass forming and modifying and volatile components [6]. The radionuclides in this simulated calcine are represented by the stable isotopes of Cs and Sr for the ®ssion products and Ce for the ®ssile elements (Table 1). Preliminary experimental results indicated an option of developing glass-ceramic materials in a hot isostatic press to ensure larger waste loading [6,7]. The results also indicated that the addition of reducing agents would control the redox potential in the condensed system and limit the corrosion rates to less than 1g/m2 day [7]. Release rates of cadmium and chromium were also reduced in the redox controlled samples. While cadmium is immobilized as sulphide, phosphide, phosphate and cadmium metal [7], chromium is contained in phosphide and in zircon (ZrSiO4 ) [7]. The bonding of phosphorus to form compounds is a€ected by its range in electronic charge from 5+ to 3) [3]. In a 5+ valency state, its usual tetrahedral bonding to oxygen forms a PO4 complex of 3) charge which places restrictions on P±O±P bonds [3]. This constraint contrasts with

the relative ease of forming Si±O±Si bonds, where SiO4 complex has a 4) charge (see Refs. [8,9], and references cited therein) The charge reaching an O2ÿ ion from Si4‡ is 1‡ whereas the charge from P5‡ is 1:25‡. The 0.25 excess positive charge on oxygen is not compatible with the electronegativity of this element and in apatite phosphate the coordinating metal cations (M) do not often have oxygen charge compensation in the M±O±P linkage [5,10,11]. Although, the valence changes in the metal cation sites are compensated by charge balancing coupled cationic substitutions [10], or by the formation of compensating cationic vacancies envisaged in this paper. In a bonded con®guration, oxygen should be either charge balanced or have an excess negative charge to be consistent with its electronegativity [12,13]. Further, phosphorus may function as an electropositive or an electronegative element [14] which may induce additional variations to chemical bonding, structure, electromagnetic properties and chemical durability. In this paper, an exercise was undertaken to develop hybrid bonding orbitals along the principles of valency bond theory [12,15,16] with an intent to explain the chemical bonding

Table 1 Chemical compositions (wt%) of Idaho waste calcine, batch compositions of glass-ceramic waste forms gc17 and gc16, electron microprobe analysis of glass g17 in glass-ceramic waste form gc17, and batch composition of glass g9 Al2 O3 B2 O3 CaO Ce2 O3 CdO Cr2 O3 Cs2 O K2 O MgO Na2 O P2 O5 SiO2 SrO ZrO2 CaF2 Clÿ SO2ÿ 4 Al metal Si metal Loading wt%

Calcine

gc17

gc16

g17

g9

9.60 2.90 12.20 1.20 5.70 1.20 0.50 1.10 0.50 5.10 0.50 0.00 0.60 17.50 39.40 0.10 2.60 0 0 100

6.72 2.03 8.54 0.84 3.99 0.84 0.35 0.77 0.35 3.57 7.40 19.46 0.42 12.25 27.58 0.07 1.82 1 2 70

6.72 2.03 8.54 0.84 3.99 0.84 0.35 0.77 0.35 3.57 7.40 13.46 0.42 12.25 27.58 0.07 1.82 3 6 70

23.20 2.25 0.69 0.39 0.01 1.81 0.19 0.24 0.10 0.69 0.18 66.11 0.27 1.34 1.03 0.27 0.01 0 0 ±

7.07 13.32 5.14 0.51 2.40 0.51 0.21 0.46 2.03 16.67 0.21 27.23 0.25 7.38 16.61 0.04 1.09 0 0 42

S.V. Raman / Journal of Non-Crystalline Solids 263&264 (2000) 395±408

related properties of both phosphates and phosphides. These properties are: 1. Changes in the lattice parameters of apatite with Cd and Ce substitution for Ca and concurrent stoichiometric cation vacancies. 2. Excess electrostatic charge on oxygen in tetrahedral coordination with phosphorus in apatite and possible restrictions to itinerant electron mobility. 3. Compositional changes of phosphides from MP4 to M4 P and M3 P4 (M ˆ metal, P ˆ phosphorus) types and possible occurrences of itinerant electrons. 4. Variations in the leach rates of phosphides as a result of changes in the spatial overlap between metal and phosphorus bonding hybrid orbitals.

397

formation of crystalline phases of ¯uorite (CaF2 ), zircon (ZrSiO4 ), baddeyelite (ZrO2 ), greenockite (CdS), ¯uor-apatite (Ca5 (PO4 )3 F) and metal phosphides. With the exception of ¯uorite, which occurs segregated around glass islands, all the other grains occur disseminated in the glass matrix [7]. For both ¯uor-apatite and metal phosphide grains the crystalline morphology is apparent in Fig. 1. The incompatible oxide, Cs2 O, glass forming component, B2 O3 and glass modi®ers, CaO, MgO, SrO, Na2 O and K2 O, form the glass g17. The composition of this glass as shown in

2. Experimental procedures The waste forms were prepared in vacuum sealed stainless steel canisters. The details of hot isostatic pressing, glass preparation, MCC-1 [17] leach tests and electron microprobe analysis are described in Refs. [7,18]. X-ray powder di€raction pro®les were collected using a computer controlled di€ractometer (Siemens D5000). The operating conditions of the ®ne focus Cu tube  were set at 40 kV and 30 mA …Cu Ka ˆ 1:5406 A† with a ®xed 2° divergent and antiscatter slits, monochromator slit (0.2 mm), focusing graphite monochromator, detector slit (0.6 mm) and a scintillation detector. The scan range was 6±76° 2h with a 4 s counting time at every 0.04° step. The data were collected at room temperature from powdered waste forms. The spectra were analyzed using software Seimens Di€ract-AT. 3. Results 3.1. Waste loading The glass-ceramic compositions with 70 wt% loading of calcines could be developed in a hot isostatic press, without the occurrence of unreacted waste components (Table 1, gc17). The major components of the calcines are consumed in the

Fig. 1. Backscattered scanning electron micrographs of (a) metal phosphides and (b) apatite phosphate in glass matirx of glass-ceramic waste form gc17. 1: (Fe,Cr)P4 ; 2: glass; 3: (Cd,Zr)P4 ; 4: apatite; 5: CdS; 6: ZrO2 . CdS and ZrO2 have similar atomic number contrasts in this micrograph.

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Table 1 is dominated by Al2 O3 (23 wt%) and SiO2 (66 wt%). This composition is formed in the hot isostatic press at 1000°C. Such a suppression in its forming temperature results from the solubility of volatiles (S, Cl, F) and modifying components under an isostatic pressure of 138 MPa [7]. Under ambient atmospheric pressure, these volatiles would be released upon heating, delaying melting till a higher temperature of around 1400°C is reached. In contrast to the glass-ceramic waste form, the waste loading in a homogeneous alkaliborosilicate glass g9 is of the order of 42 wt% (Table 1, g9). This glass was formed at 1200°C under atmospheric pressure. The glass is chemically homogeneous as measured by electron microprobe point analysis, optically transparent and isotropic in a petrographic microscope and X-ray amorphous in the powder di€ractometer [18]. 3.2. E€ects of metal reductants Si and Al To note the e€ects of redox potential on the compositions of glass-ceramic phases, the reducing agents, Si and Al metals, were exchanged for SiO2 without varying the waste loading. The Al±Si concentration sets were 1wt%Al±2wt%Si in the batch composition gc17 (Table 1) and 3wt%Al± 6wt%Si in gc16 (Table 1). These variations a€ect the composition of ¯uor-apatite and metal phosphides. 3.2.1. Apatite composition Under a reducing condition of 1wt%Al± 2wt%Si, apatite contains more Ce2 O3 and when the reducing condition is increased to 3wt%Al± 6wt%Si more CdO appears to replace Ce2 O3 . Assuming trivalent and divalent states for Ce and Cd, respectively, the concentrations are 5.2 mol% Cd2‡ and 4.0 mol% Ce3‡ in samples of gc16 and gc17 (Table 2). The e€ect of these cations on interplanar spacings of apatite were observed in the X-ray powder di€raction patterns (Fig. 2). Since Fig. 2(b) and (c) contain X-ray lines of all the predominant phases in the waste form powders, a pure ¯uorapatite mineral standard was measured for its Xray powder pattern and is included in Fig. 2(a) for comparison. Both mineral standard and samples were examined under identical instrumental con-

Table 2 Electron microprobe analysis of apatite (wt%), in glass-ceramic waste forms gc16 and gc17 gc16

gc17

B2 O3 CaO Ce2 O3 CdO P2 O5 SrO F

0.93 46.86 2.24 5.93 38.15 1.04 4.27

0.48 48.59 6.24 0.00 39.39 1.10 3.98

Total

99.42

99.78

ditions. On these patterns, the X-ray lines from the JCPDS ®le 15-0876 are displayed for re¯ections of a ˆ …2 1 0†; b ˆ …2 1 1†; c ˆ …1 1 2†; d ˆ …30 0†; e ˆ …2 0 2†; f ˆ …3 0 1†; g ˆ …2 1 2† and h ˆ …3 1 0†. A few other intense lines of apatite that fall outside the 2h range of these ®gures are …0 0 2†, …2 2 2† and …2 1 3†. Their intensities in the JCPDS ®le are 40%, 25% and 30%. The match between the JCPDS ®le 15-0876 and ¯uor-apatite mineral is evident in Fig. 2(a). For detailed analysis, the more intense lines of 100% (2 1 1), 60% (3 0 0) and 55% (1 1 2) were considered. They are shown in Fig. 2(b) and (c) for samples of gc16 and gc17 that were formed under reducing conditions speci®ed by 3wt%Al±6wt%Si and 1wt%Al±2wt%Si, respectively. The corresponding apatite compositions that were determined by electron microprobe wave length dispersive analysis are shown in Table 2. The analysis of this table was chosen from electron probe point analysis of ®ve di€erent apatite grains in the same polished sections of samples gc17 and gc16. The analysis totals ranged from 98.70 to 100.3 and much of this variation was caused by ¯uctuations in ¯uorine and boron wt%. For the remaining elements of Table 2 the variations were within ‹0.10 wt%. The samples were also analysed for Cl, S, Na, Si and Mg. No traces for Cl and S were observed, although some grains contained 0.3±0.8wt%Si, 1±1.7wt%Na and 0.03±0.07wt%Mg in gc17 and gc16 samples. The elements B, Cl, S, Si, Na and Mg are largely concentrated in the coexisting glass g17 (Table 1). The analysis totals do not point to the presence of OHÿ ions by difference method [19] that may have otherwise arisen from the residual moisture in the batch. It is

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399

Fig. 2. X-ray powder di€raction patterns of (a) calcium ¯uor-apatite mineral standard, (b) waste form gc16, (c) waste form gc17; a ˆ …2 1 0†; b ˆ …2 1 1†; c ˆ …1 1 2†; d ˆ …3 0 0†; e ˆ …2 0 2†; f ˆ …3 0 1†; g ˆ …2 1 2† and h ˆ …3 1 0†.

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therefore contended that the grains in both gc16 and gc17 samples are of ¯uor-apatite and the shifts in interplanar spacings have resulted from metal cation exchanges. These shifts in Ôd Õ spacings as a function of Ce, Cd, Ca exchanges are evident in Fig. 2 when b, c and d X-ray re¯ections are compared with that of pure ¯uor-apatite (Ca5 (PO4 )3 F) and are larger with increase in Ce. 3.2.2. Phosphide compositions Electron microprobe analysis of phosphide grains are shown in Table 3. These compositions were calculated to the nearest possible stoichiometry of (a) (B, Ca, Sr, Cs, K, Na)4 P, (b) Zr3 P4 , (c) (Cd, Zr)P4 and (d) (Fe, Cr)P4 . The oxygen contents shown in Table 3(b)±(d) were ignored, assuming surface oxidation by the electron beam during analysis [20]. The volume abundance of these grains is smaller than other grains, as a result they were not quantitatively determined in the microstructures nor were they detected in the X-ray powder patterns for structural features. The electron microprobe analysis in Table 3 was obtained from grains that were larger than 5 lm. Each grain was analyzed at two separate microprobe points using 1.0 lm beam diameter. The analysis of these Table 3 Electron microprobe analysis of phosphides (wt%)a (a) Al B Ca Ce Cd Cs Cr Fe K Mg Na P Si Sr Zr O Total a

(b)

(c)

0.0 4.19 57.29 0.0 0.0 1.77 0.0 0.0 2.37 0.0 14.42 18.67 0.0 1.76 0.0 0.0

0.0 0.0 0.11 0.13 0.0 0.0 0.03 0.0 0.0 0.0 0.0 33.90 0.01 0.0 65.08 0.48

0.0 0.0 0.02 0.14 45.44 0.0 0.03 0.01 0.0 0.0 0.02 50.10 0.0 0.0 3.96 0.22

(d) 0.0 0.0 0.06 0.11 1.03 0.0 6.70 26.91 0.08 0.0 0.03 64.17 0.11 0.0 0.30 0.64

100.47

99.74

99.94

100.14

(a) (Ba, Ca, Sr, Cs, K, Na)4 P, (b) Zr3 P4 , (c) (Cd, Zr)P4 , (d) (Fe, Cr)P4 .

points showed no measurable di€erences and the variations of ‹0.05 wt% are attributed to instrumental ¯uctuations of the electron microprobe [20,21]. However, some compositional changes were observed with variations in the redox state. The most notable is the presence of (B, Ca, Sr, Cs, K, Na)4 P in the waste form containing the larger concentration of 3wt%Al±6wt%Si reductants and its absence when reductant level is 1wt%Al±2wt%Si. The phases Zr3 P4 , (Cd, Zr)P4 and (Fe, Cr)P4 were observed under both redox conditions. The last two phases are identi®ed in the backscattered scanning electron micrograph by di€erences in their atomic number contrast (Fig. 1(a)). The grains are dispersed within the glass matrix. Some irregular crystallographic outlines are visible for (Fe, Cr)P4 and (Cd, Zr)P4 which are indicative of crystalline habit. 3.3. Elemental concentrations in the leachate and leach rates The leaching data in Table 4 were collected following 14 day MCC-1 leach tests [17] of waste form monoliths in deionized water at 90°C. Two monoliths per sample were separately tested for leaching. In view of the cost and time involved, the concentrations reported in Table 4 are based on single leachate analysis per sample, so no error bars could be determined. However, the di€erences in leaching between samples gc16 and gc17 are larger than instrumental errors for some elements and greater normalized leach rates result from larger elemental releases. For example, samples gc17 and gc16 have the same initial boron concentrations (Table 1), but the release of boron in the leachate of tests with gc16 is much larger than in gc17. Accordingly, the normalized leach rate for boron in gc16 is 3.3 g/m2 day, whereas in gc17, it is 0.41 g/m2 day. Similarly, Na release (Table 4) in gc17 is less (3.20 ug/ml) than in g9 (9.0 ug/ml). Its normalized leach rates di€er marginally and are 0.76 and 0.52 g/m2 day for gc17 and g9, respectively, despite the di€erences in Na normalization mass of 1.32 wt% in gc17 and 6.18 wt% in g9 (Table 1). The leach rates of glass-ceramic gc17 and glass g9 are comparable with Cr, P, Mg and Ca showing a larger leach resistance in the glass-

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401

Table 4 Elemental concentrations in the leachate (ug/ml) and the corresponding 14 day normalized leach rates (g/m2 day) of glass-ceramic waste forms gc16 and gc17 and glass waste form g9

Al B Ca Cd Cs Cr K Mg Na P Si Sr Zr

ug/ml

g/m2 day

ug/ml

g/m2 day

ug/ml

g/m2 day

gc17

gc17

gc16

gc16

g9

g9

2.07 0.41 3.12 0.92 0.06 0.06 0.44 0.01 3.20 0.20 5.20 0.06 0.0

0.34 0.28 0.11 0.25 0.59 0.07 0.29 0.08 0.76 0.04 0.30 0.30 0.01

6.18 4.94 67.50 0.03 1.50 0.01 2.80 0.28 17.0 22.0 7.11 2.08 0.02

0.59 3.30 2.35 0.01 14.50 0.01 1.81 0.40 3.94 5.03 0.48 3.74 0.0

5.23 2.88 5.90 0.03 0.02 0.16 0.27 0.80 9.00 0.10 5.84 0.15 0.01

1.00 0.50 0.35 0.01 0.07 0.33 0.50 0.47 0.52 0.78 0.33 0.50 0.00

ceramic waste form. The much larger leach rates of B, Ca, Cs, K, Na, P and Sr in the glass-ceramic gc16 (Table 4) result from compositiona1 changes of 1wt%Al±2wt%Si±19.46wt%SiO2 in gc17 to 3wt%Al±6wt%Si±13.46wt% SiO2 in gc16. 4. Discussion 4.1. Redox reactions and formation of metal phosphides The ®rst half of oxidation reactions of 1wt%Al± 2wt%Si and 3wt%Al±6wt%Si yield 0.004 and 0.012 N electrons …N ˆ 6:023  1023 †, because of their ionization potentials of 6.0 eV for Al3‡ and 8.1 eV for Si4‡ [14]. The electrons thus liberated with increase in temperature would create a reactive electron atmosphere within the condensed system of the stainless steel container. Some electrons may be lost to the ground. The formation of metal phosphide phases is related to the availability of reaction energy as well as the electrons. Additional energy is produced by exothermic dissociation of P2 O5 to P5‡ and O2ÿ ions, which is known to occur around 400°C for pure P2 O5 [3]. The element phosphorus, by virtue of its intermediate electronegativity, would tend to absorb electrons into its orbitals, which is also an exothermic process [3,16]. Under these energetic conditions additional dis-

sociation of refractory oxides may be promoted. The oxygen ions released by these dissociation processes contribute to the completion of the second half of the reaction to form Al2 O3 and SiO2 that are eventually consumed in the formation of borosilicate liquid and silicate crystals. The larger Si/Al ratio would, by way of oxidation, make more silica available for glass formation. Some variation in metal phosphide composition is noted in response to the electron abundance. The phase (B, Ca, Sr, Cs, K, Na)4 P was only noted in the glass-ceramic system containing 3wt%Al±6wt%Si. Other compositions of Zr3 P4 , (Cd, Zr)P4 and (Fe, Cr)P4 are present under both redox conditions. There are several interactive processes in the development of phases in a polycrystalline glassceramic and the availability of species for the formation of a particular composition would depend on the crystallization chronology and mobility of the concerned elements. The phase (B, Ca, Sr, Cs, K, Na)4 P is enriched in Ca and possibly its formation under greater redox potentials is related to the greater stability of CaO [22]. 4.2. Coordination and bonding orbitals of metal phosphides The general formula for these metal phosphides is M4 P, MP4 and M3 P4 . The crystals of these compositions have a massive appearance

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(Fig. 1(a)), indicating that these structures are probably three-dimensional (3D) and are not of a layered type. Assuming that they are 3D, the simple coordination geometry to match the composition of M4 P involves eight metal atoms around phosphorus and two phosphorus around one metal. In MP4 the same coordination would follow with positions of M and P interchanged. In the case of M3 P4 there are eight P around a M and six M around a P. The bonding of M and P on the basis of electrostatic forces explains the stoichiometry of Zr3 P4 , where charges are Zr4‡ and P3ÿ . For M4 P and MP4 compounds the bonding using electrostatic forces is not energetically favored. These bond considerations are better explained along the principles of valency bond theory by means of hybrid orbitals [12,15,16]. In Table 5 the bonding orbitals for the corresponding coordinations in phosphides are summarized. Consider, the compound Ca4 P which is a simpli®ed version of (B, Ca, Sr, Cs, K, Na)4 P. Here P in the ground state has the electronic con®guration P ˆ ‰HeŠ2s2 2p6 3s2 3p3 3d0 . In the eight orbital excited state, its electronic con®guration by decoupling the electron pairs would be P ˆ ‰HeŠ2s2 2p2 3s1 3p3 3d5 . The decoupled orbitals of [sp3 d4 ]d1 are made available for hybrid bonding with eight calcium atoms. Each Ca is in twofold coordination with P and has the activated electron con®guration of [Ne]3s2 3p6 4s1 4p1 . While Ca has two sp hybrid orbitals, P orbitals contain higher energy d electrons and one of them (d1 ) may become itinerant [23±26] in an eightfold coordinating environment. A similar itinerance can occur if Zr3 P4 compound was analyzed from the point of

hybrid bonding. The activated eightfold electronic con®guration of Zr is [Ar]3d10 4s2 4p4 4d5 5s1 . This con®guration gives hybrid orbital of the type sp2 d5 for bonding with P. Phosphorus is in turn sixfold coordinated to zirconium. The six hybrid orbitals of P are [sp3 d2 ]d1 and one d1 orbital is rendered itinerant. Similarly in FeP4 , the hybrid Fe orbital for eightfold coordination is sp3 d4 . In this compound P is in twofold coordination with Fe. The low-energy orbital con®guration for phosphorus is [Ne]3s2 3p3 with three unpaired p electrons. In twofold coordination, two p electron orbitals of P will be used in hybrid bonding with Fe, while the third p1 electron is rendered itinerant. In the case of CdP4 , eight orbitals must originate from cadmium to hybridize with phosphorus. In the ground state the neutral cadmium has the electronic con®guration Cd ˆ ‰KrŠ4d10 4f 0 5s2 5p0 . In the decoupled state for eight orbitals the con®guration of Cd ˆ ‰KrŠ4d7 4f 1 5s1 5p3 . The orbitals available from Cd for hybridization with eight P are d3 s1 p3 f1 . Here too, P is in twofold coordination and has one unpaired itinerant electron (p1 ). These unpaired non-bonding electrons are likely to contribute to magnetic moment [25], electronic conductivity and superconductivity of phosphides [27±31]. The existence of hybrid bonds is related to the di€erence in the energy absorbed for decoupling electron pairs and the energy released by bonding electrons of opposing spins [3,12±16]. The bond will be favored when this di€erence is negative [3,12±16]. The strength of the bond is related to spatial overlap of orbitals [16]. The spatial extent of hybridization contributes to larger overlap among the orbitals and hence promotes stronger

Table 5 Decoupled and hybrid electron orbital con®gurations and covalent linkage in phosphides (d1 and p1 are non-bonding itinerant electrons) Phase Ca4 P FeP4 CdP4 Zr3 P4

Elements P Ca Fe P Cd P Zr P

Decoupled orbital con®guration 2

2

1

3

5

[He]2s 2p 3s 3p 3d [Ne]3s2 3p6 4s1 4p1 [Ne]3s2 3p6 3d4 4s1 4p3 [He]2s2 2p2 3s2 3p3 [Kr]4d7 4f1 5s1 5p3 [He]2s2 2p2 3s2 3p3 [Ar]3d10 4s2 4p4 4d5 5s1 [He]2s2 2p4 3s1 3p3 3d3

Hybrid orbital con®guration 3 4

1

[sp d ]d [sp] [sp3 d4 ] [p2 ]p1 [d3 s1 p3 f1 ] [p2 ]p1 [sp2 d5 ] [sp3 d2 ]d1

Covalent linkage 8 2 8 2 8 2 8 6

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403

tions [32]. If electrostatic balancing principles were applied, the positive charges reaching the divalent oxygen are more than O2ÿ can compensate for. Yet, atom to atom replacement in the calcium site occurs because of closely matching sizes [14]. The charge imbalance among divalent ions and PO3ÿ 4 can perhaps be addressed from the principles of valency bond theory [12,15,16] by constructing hybrid orbitals. One possible test for the existence of hybrid orbitals could arise from the calculation of c/a lattice parameter ratios, for hybrid orbitals will have an a€ect on the geometry of coordination [12,16]. Geometric deviations in hybrid orbitals can change the c/a aspect ratio of the unit cell. Thus, lattice parameters were calculated from the di€raction peaks b, c and d shown in Fig. 2(a)±(c) without undertaking the task of pro®le shape re®nement and assessment of cationic coordinate positions. These calculations follow from the relation between (h k l) re¯ections and the lattice parameters, as shown in several X-ray di€raction text books [33]. The calculations predict the literature ÔaÕ and ÔcÕ lattice parameters [5,11] for the Xray pattern of pure ¯uor-apatite mineral standard (Fig. 2(a) and Table 6). Deviations from these parameters for samples gc16 and gc17 occur in response to substitution of Ca by Cd and Ce (Table 6). Assuming a linear dependence, the parameters for the compositional end members in the

bonding [15,16]. For the compounds mentioned here, the orbitals with d electrons are spatially more extended than the sp orbital of Ca. This localization of sp orbital energy leads to smaller hybridization with the sp3 d4 orbital of phosphorus and hence to weaker bonds, which possibly decreases the durability of the compound (B, Ca, Sr, Cs, K, Na)4 P. 4.3. Compositions and lattice parameters of apatite phosphate In an ordered ¯uor-apatite the two distinct cationic coordinations with oxygen are tetrahedral for phosphorus and three dimensionally sixfold for calcium where one oxygen is replaced by ¯uorine. Oxygen in turn is coordinated to three calciums and one phosphorus. The three calciums are further triangularly linked to the central ¯uorine [32]. The cadmium and cerium contents shown in Table 3, are compatible substituents for calcium. The sizes of Ca2‡ , Cd2‡ and Ce3‡ are in the range of 1.0±0.95 A [14]. Charge balance requirements in the stoichiometry of Ca5 (PO4 )3 F require nearly 33% vacancy when Ce3‡ is substituted for Ca2‡ . Thus Ce3‡ may cause the P5‡ charge to be balanced by creation of four double bonds per 3ÿ …PO4 †3 complex. However, in Ca ¯uor-apatite, all the oxygens of PO3ÿ 4 tetrahedra are linked to ca-

Table 6 Crystallographic data of apatitea

d211 d112 d300  a (A)  c (A)

c/a Ca2‡ /total Ce3‡ /total Ce2‡ /total Cd2‡ /total

I

1

2

Standard

gc16

gc17

100 55 60

2.799 2.770 2.703 9.363 6.870 0.730 1.0

2.808 2.781 2.711 9.391 6.886 0.733 0.934 0.014

2.816 2.788 2.719 9.419 6.962 0.739 0.960 0.040

4

5

6

7

9.375 7.078 0.743 0.920

10.330 8.388 0.810 0.00 0.666 0.00 0.00

10.835 9.170 0.840

9.523 9.470 0.994

0.080

1.00

1.00  of (2 1 1), (1 1 2) and (3 0 0) X-ray re¯ections, a and c ± lattice parameters (A),  I ± relative d2 1 1 , d1 1 2 , d3 0 0 ± interplanar spacings (A) intensities from JCPDS ®le 15-0876 for calcium ¯uor-apatite, 1 ± pure mineral standard of calcium ¯uor-apatite, 2 and 3 ± apatite in gc16 and gc17 glass-ceramic waste form powders, 4±7 ± lattice constants and compositions from calculations assuming a linear composition dependence for experimental lattice parameters in gc16 and gc17 apatite, total ˆ Ca ‡ Cd ‡ Ce atom fractions.

a

0.052

3

404

S.V. Raman / Journal of Non-Crystalline Solids 263&264 (2000) 395±408

Ca±Cd, Cd±Ce and Ca±Ce joins can be calculated from the following expressions: agc17 ˆ ‰…agc16 ÿ agc17 †=…xCe;gc16 ÿ xCe;gc17 †ŠxCe;gc17 ‡ azCd ; where agc17 ; agc16 ; xCe;gc17 ; xCe;gc16 are ÔaÕ lattice parameter and atom fraction of Ce in samples gc17 and gc16 obtained from X-ray patterns of Fig. 2. azCd is the calculated lattice parameter with z atom fractions of Cd and zCd ˆ xCd;gc16 ‡ ‰xCd;gc16 =…xCe;gc16 ÿ xCe;gc17 †ŠxCe;gc16 ; axCd ˆ ‰…astandard ÿ azCd †=…xCd;standard ÿ zCd †ŠxCd ‡ astandard ; where xCd ˆ 1:0. axCe ˆ ‰…astandard ÿ agc17 †=…xCe;standard ÿ xCe;gc17 †ŠxCe ‡ astandard ; where xCe ˆ 1:0 for Ce2‡ and 0:66 for Ce3‡ . Similar expressions relate c lattice parameters and composition. Their magnitudes and the aspect ratios are shown in Table 6. In these calculations the concentration of 0.01 atom fraction for Sr was not considered because of its negligible e€ect on the lattice parameter ratio. The changes in c/a ratio indicate limited substitution by Cd and Ce in calcium ¯uor-apatite. The solid solution range for Cd is even more limited by its tendency to evolve the crystal symmetry towards isometric type, where c=a ˆ 0:994 (Table 6). These changes in the aspect ratios are attributed to di€erences in the hybrid orbitals of Ca, Ce and Cd.

4.4. Hybrid bonding orbitals and ionicity in apatite phosphate The various hybrid orbitals are summarized in Table 7. The table also shows the extent of ionic bonding in apatite as calculated from the electronegativity di€erences between bonding atoms using PaulingÕs empirical expression for ionic component of bonding [12] as 2

%I ˆ 16fxA ÿ xB g ‡ 3:5fxA ÿ xB g ; where %I is the percentage of ionicity and x is the measure of electronegativity of A and B bonding atoms. Presumably, both ionic and covalent bonds are resonant [13] in apatite and here the origin for covalent linkage is discussed in terms of hybrid orbitals of charged ions. In the activated sixfold coordination state, the decoupled electron con®guration may be constructed as: Ca2‡ ˆ ‰NeŠ3s2 3p3 3d2 4s1 , Cd2‡ ˆ ‰KrŠ4d7 4f 0 5s1 5p2 and Ce2‡ ˆ ‰PdŠ4f 2 5s2 5p3 5d2 5f 0 6s1 6p0 . The activated sixfold hybrid orbitals are Ca2‡ ˆ sp3 d2 ; Cd2‡ ˆ d3 sp2 ; Ce2‡ ˆ sp3 d2 . These orbitals are based on the probable energy levels such as 3d greater than 3p greater than 4s in Ca, 5p greater than 5s greater than 4d in Cd and 5d less than 6p in Ce [15,16]. The activated electronic con®guration of neutral oxygen is [He]2s2 2p3 3s1 and its four hybrid orbitals are sp3 . But the activated electronic con®guration as shown here for oxygen is known to require a large absorption of energy [15] for 2p4 to become uncoupled to 2p3 3s1 . An alternative low-energy con®guration would involve ®lling of 3s and 3p

Table 7 Cation±oxygen ionicity, decoupled and hybrid orbital con®gurations, and covalent linkage in ¯uor-apatite (d1 is a non-bonding itinerant electron) Element

Cation±oxygen ionic bond (%)

Decoupled orbital con®guration

Hybrid orbital con®guration

Covalent linkage

O2ÿ P5‡ P4‡ Ca2‡ Ce4‡ Ce3‡ Ce2‡ Cd2‡

± 30 30 62 58 58 58 40

[He]2s2 2p2 3s1 3p3 [He]2s2 2p2 3s1 3p3 [He]2s2 2p2 3s1 3p3 3d1 [Ne]3s2 3p3 3d2 4s1 [Pd]4f2 5s2 5p3 5d0 5f0 6s1 [Pd]4f2 5s2 5p3 5d1 5f0 6s1 [Pd]4f2 5s2 5p3 5d2 5f0 6s1 [Kr]4d7 4f0 5s1 5p2

[sp3 ] [sp3 ] [sp3 ]d1 [sp3 d2 ] [sp3 ] [sp3 d1 ] [sp3 d2 ] [d3 sp2 ]

4 4 4 6 4 5 6 6

S.V. Raman / Journal of Non-Crystalline Solids 263&264 (2000) 395±408

orbitals by electron absorption and decoupling of one 2p orbital to complete the formation of four sp3 hybrid orbitals as [He]2s2 2p2 3s1 3p3 . In this con®guration oxygen will have 2-charge and four valency orbitals. The electrons for absorption by oxygen will arise from the decoupling of P orbitals to form a …PO4 †3ÿ radical, as well as from ionization of Al and Si metal reductants. The activated con®guration for P is proposed as [He]2s2 2p2 3s1 3p3 3d5 . P will acquire a 5+ charge with loss of high energy d electrons which is among the sources for outer electrons in O2ÿ . The hybrid orbital of P5‡ is sp3 . The electrons with opposing spins in the geometrically identical sp3 orbitals of P and O will pair to achieve the P±O bonds in tetrahedral coordination, which would leave an energetically unfavorable 1/4th positive electrostatic charge on each tetrahedral oxygen of (PO4 )3ÿ . This destabilization e€ect of phosphorus tetrahedra is eliminated if one of the higher energy d oribitals is rendered itinerant with loss of four electrons to yield a hybrid con®guration of sp3 d1 for P4‡ . The bonding among sp3 orbitals forms 3ÿ …PO4 † which di€ers from the previous one by an itinerant outer electron orbital contribution from P. In this mechanism of bond formation, decoupling of p electrons from the octet shell in P will require an absorption of energy [12±16]. Simultaneously, the process of electron absoprtion by the outer shell of oxygen is expected to be exothermic

405

owing to transfer of electrons from 3d2 of phosphorus to 3s1 and 3p1 of oxygen [12±16]. Additional energy release would be expected [14±16,25] by the pairing of oppositely spinning electrons among the three sp3 hybrid lobes of oxygen and the orbitals of the surrounding cations Ca2‡ , Ce2‡ or Cd2‡ . The Cd±O hybrid orbital geometry is expected [12,16] to di€er from Ca±O or Ce±O types because of d3 sp2 con®guration for Cd2‡ compared to sp3 d2 for Ca2‡ and Ce2‡ . 4.5. Stoichiometry and electrostatic charge of oxygen in apatite phosphate In light of hybrid bonding, it is of interest to evaluate the charges reaching the O2ÿ ion from the neighboring positive ions. With one outer electron rendered non-bonding and itinerant in P5‡ , only 1+ charge is expected to compensate the 1) charge on O2ÿ . Considering a sixfold coordination by a metal cation (M) with immediate oxygen neighbors and threefold coordination to metal cations by each of the four phosphate oxygens, there are (3 ´ 4)/6 M sites per PO4 tetrahedra. This con®guration requires that the formula M2‡ 5 …PO4 †3 F contain 16.6% vacancies, which means of the 12 M sites 10 are occupied, as shown for compound six in Table 8. This occupation leads to two oxygens with charges balanced (zero charge) and two remaining oxygens with 0.33 negative charge

Table 8 Permissible stoichiometry in the ordered apatite structure MZx vac6ÿx …PO4 †3 Fa M4‡ 1 2 3 4 5 6 7 8 9 10 11 12 13 a

M3‡

M2‡

Vacancy (%)

Charge on PO4 oxygens

Occupancy of 4‡ , 3‡ , and 2‡ metal cations

1 2 2 1 1 1 3

1 2 3 4 5 0.5 1 2 1.5 2.5 3.5 0.5

58 50 41.6 33.33 25 16.66 50 41.66 33.33 41.66 33.33 25 41.66

)0.33, )0.33, )0.33, +0.32 )0.33, )0.33, )0.33, +0.32 0, 0, )0.33, )0.33 0, 0, )0.33, )0.33 )0.33, )0.33, )0.33, +0.32 0, 0, )0.33, )0.33 )0.16, )0.17, )0.33, )0.66 0, 0, )0.33, )0.33 )0.17, )0.17, )0.17, )0.17 0, )0.17, )0.17, )0.33 0, 0, )0.17, )0.50 0, )0.17, )0.17, )0.33 0, 0, )0.17, )0.50

4, 4, 4, 4, 4 4, 4, 4, 4, 2, 2 4, 4, 4, 2, 2, 2, 2 4, 4, 2, 2, 2, 2, 2, 2 4, 2, 2, 2, 2, 2, 2, 2, 2 2, 2, 2, 2, 2, 2, 2, 2, 2, 2 4, 4, 4, 3, 3, 2 4, 3, 3, 3, 3, 2, 2 3, 3, 3, 3, 2, 2, 2, 2 4, 4, 3, 3, 2, 2, 2 4, 3, 3, 2, 2, 2, 2, 2 3, 3, 2, 2, 2, 2, 2, 2, 2 3, 3, 3, 3, 3, 3, 2

2.5 2.0 1.5 1 0.5 1.5 0.5 1 0.5

M4‡ , M3‡ and M2‡ proportions, vacancy (vac) concentration, charge on oxygen ion of phosphate tetrahedron, and metal (M) site occupancy.

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S.V. Raman / Journal of Non-Crystalline Solids 263&264 (2000) 395±408

(compound 6 in Table 8). Similarly, it follows that for M4‡ 2:5 …PO4 †3 F compound the cation vacancy concentration would have to be 58% with the corresponding negative and positive charges on oxygen as shown in Table 8 for compound 1. When this exercise is carried further to account for mixing among M cations and vacancies, a total of 13 permissible phases of apatite are formed. They are distinguished by stoichiometric vacancies, electrostatic charge on oxygen and the number of 4‡ , 3‡ and 2‡ M cation occupancies around a PO4 tetrahedron as shown in Table 8. Departure from the permissible stoichiometry would result from random mixing among the 13 phases of Table 8, leading to variations in the PO4 ±M motif. The features of Table 8 are based on the ®rst coordinations depicted by the original structure of pure apatite [32]. The e€ects of impurities in natural apatite, may a€ect the vacancy concentration and metal site occupancy. Some increase in positive charge contribution to oxygen may also result from the increase in metal cation to oxygen link in the second coordination sphere of natural apatite [11]. In the pure ¯uor-apatite of present glass-ceramic, the compositional analysis has not detected OHÿ or Clÿ ions. These ions and Fÿ predominantly link the metal cation polyhedra [32] and are expected to have variations in positive charge depending on the valence state of the metal cations. In three-coordination [32] with divalent metal cations their charges will remain balanced. Barring such intricate complications in the structure, it will be noted that the negative charge on oxygens as shown in Table 8 are barriers to itinerant electron mobility. In phase 9 the itinerant electron will experience an isotropic Coulomb repulsion, while in others its mobility is directionally restricted by charge anisotropy. Therefore, unlike the metal phosphides, where itinerant electrons account for ferromagnetism and superconductivity [27±31], the apatite phosphate will not have such properties, despite the creation of itinerant electrons by phosphorus hybrid orbitals. However, the con®gurations of hybrid orbitals a€ect the extent of cation solubility in Ca ¯uor-apatite. As evident from X-ray di€raction, the nearly isometric c/a ratio of 0.994 with Cd substitution is attributed to d3 sp2 con®guration and hence the anticipated limited

solid solution. Instead, the sp3 d2 orbitals are likely to increase the mutual solubility between Ca2‡ and Ce2‡ . Accordingly the c/a lattice parameter ratios for the Ca2‡ and Ce2‡ hexagonal apatite are 0.73 and 0.84. The hybrid bonds often contribute to increase in bond strength [12,14,16] and here account for the durability of apatite. 4.6. Chemical durability In MCC-1 tests, the sample has a de®nite and simple geometry, as a result comparable experimental conditions can be readily maintained for di€erent samples. This consistency enables consistent comparison of results among various samples. Further exploration of the post leaching changes by electron beam and spectral techniques [18] is facilitated if the sample is an intact monolith. An intact monolith also minimizes the possibility of contamination. Post leaching measurements are necessary for exploring the mechanisms of element mobility in the sample [18]. MCC-1 tests however must be supplemented by other tests to cover the variables a waste form may encounter under actual repository conditions [34]. While some departure is expected from variations in the surface area of monoliths, di€erences in elemental concentrations in the leachate and the corresponding normalized leach rates are potential indicators of waste form durability. The results here indicate similar leach resistances for both the glass-ceramic and homogeneous glass waste forms (Table 4), despite the microstructural di€erences [7,18]. But a greater waste form volume reduction is expected from a higher waste loading in glassceramic than in glass, which in the present case is about 30 wt% greater in glass-ceramic sample gc17 relative to glass sample g9 (Table 1). Nevertheless, the durability of glass-ceramic is additionally affected by the structure and composition of crystals and grain boundaries. Changes are evident in the present glass-ceramics as a function of variations in the redox state. When the redox condition is varied from 1wt%Al±2wt%Si in sample gc17 to 3wt%Al±6wt%Si in sample gc16, the elemental releases are larger and accordingly the normalized leach rates are greater for B, Ca, Cs, K, Na, P and Sr (Table 4). The normalized leach rates in gc16

S.V. Raman / Journal of Non-Crystalline Solids 263&264 (2000) 395±408

are 3.3, 2.35, 14.50, 1.81, 3.94, 5.03 and 3.74 g/m2 day, respectively, for the above elements. Considering that the only di€erence as a function of redox state is the metal phosphide composition, these increases in leach rates point to the smaller durability of (B, Ca, Sr, Cs, K, Na)4 P phase (Table 3(a)). Conversely, by controlling the redox state, it is possible to form more durable metal phosphide containing glass-ceramics. The metal phosphides are useful for immobilizing the Idaho calcines because they are capable of containing the toxic and carcinogenic Cr and Cd, that are otherwise leachable. MCC-1 tests indicate that Cd is also contained in these glass-ceramics by sulphide (CdS) [7] and apatite phosphate. These sulphide and phosphate make little contribution to the elemental concentration in the leachate (Table 4) and hence the leach rates of Cd and P are 0.25 and 0.04 g/m2 day, respectively. Speci®c tests must be conducted to assess the dissolution kinetics of phosphates, phosphides and sulphides in oxidizing and corrosive environments. At present their durability is qualitatively supported by the occurrence in the terrestrial geochemical environments of compositionally and structurally similar minerals, such as greenockite (CdS), apatite (Ca5 (PO4 )3 F) and shreibersite (Fe, Ni, Co)3 P. 5. Conclusions The electron atmosphere is an important part of the condensed matter of a closed system in a hot isostatic press (HIP). Al and Si reducing agents are the primary source for electrons in the ceramic batch composition. Some of the a€ects of these electrons is to form metal phosphides in association with metal sulphides in the borosilicate glass-ceramic. The metal phosphide compositions vary in response to redox potentials within the HIP container. The variations in stoichiometry result from the variable valency states of phosphorus and indicate smaller di€erences among the energies of the electron orbitals. For compounds in which the metal and phosphorus electron orbitals overlap, the hybridization produces durable metal phosphides. The durability of metal phosphides is also evident from their association with metal sulphide phases

407

of extra-terrestrial materials. In the glass-ceramic waste forms the leach rates of Cd and Cr are attributed to the durability of CdP4 and CrP4 crystalline phases. Among all the phases present in the polycrystalline glass-ceramic, Cr is partitioned to a maximum waste loading in phosphides, while Cd is found in phosphates, phosphides and sulphides. An important crystalline phosphate phase for Cd is calcium ¯uor-apatite. Apatite crystals nucleate and grow in the borosilicate liquid of the HIP container and cause the coexisting glass to become depleted in phosphorus. The apatite phase also contains cerium, which was included as surrogate for actinides. Presumably, Ce is a better match for Pu and heavier actinides because the 5f orbitals are as localized in these actinides as are the 4f orbitals in the rare earth elements. The hybrid orbitals of Ce and Ca have similar con®guration, with di€erences in their electron orbital levels. However, the hybrid orbital of Cd signi®cantly departs from Ce and Ca. These orbital variations can e€ect changes in the c/a lattice parameter ratios with Ce and Cd substitutions for Ca. In response to these changes there is limited solubility of Cd and Ce in calcium site. This limits the waste loading when the waste compositions are enriched in calcium. In the apatite structure, tetrahedral coordinations of phosphorus to oxygen and oxygen to cations are explained by means of valency hybrid orbitals. Possibly, hybridization produces stronger bonds and hence the high durability of ¯uor-apatite is evident in leach tests and in terrestrial rocks. Acknowledgements The experiments of this study were conducted under the management of Dr D.A. Knecht and Mr J.H. Valentine. It is my pleasure to thank Ms Brenda Boyle for generating the X-ray di€raction patterns. References [1] K. Rankama, Th.G. Sahama, Geochemistry, University of Chicago, 1950. [2] V.M. Goldschmidt, Geochemistry, Oxford, 1962.

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[3] F.A. Cotton, G. Wilkinson, Advanced Inorganic Chemistry, Wiley, New York, 1988. [4] I.S.E. Carmichael, F.J. Turner, J. Verhoogen, Igneous Petrology, McGraw-Hill, New York, 1974. [5] W.A. Deer, R.A. Howie, J. Zussman, An Introduction to the Rock Forming Minerals, Longman, New York, 1967. [6] S.V. Raman, in: Proceedings of the Sixth International High Level Radioactive Waste Management (IHLRWM) Conference, Las Vegas, NV, 1995, p. 594. [7] S.V. Raman, J. Mater. Sci. 33 (1998) 1887. [8] P.C. Hess, in: R.B. Hargraves (Ed.), Physics of Magmatic Processes, Princeton University, Princeton, NJ, 1980, p. 3. [9] R.K. Brow, R.J. Kirkpatrick, G.L. Turner, J. Am. Ceram. Soc. 76 (1993) 919. [10] J.G. Ronsbo, Am. Mineral. 74 (1989) 896. [11] J.M. Hughes, M. Cameron, K.D. Crowley, Am. Mineral. 74 (1989) 870. [12] R.C. Evans, An Introduction to Crystal Chemistry, Cambridge University, Cambridge, 1964. [13] L.C. Pauling, in: An Introduction to Modern Structural Chemistry, Cornell University, New York, 1960. [14] F.D. Bloss, Crystallography and Crystal Chemistry, Holt, Rinehart and Winston, 1971. [15] Y.K. Syrkin, M.E. Dyatkina, Structure of Molecules and the Chemical Bond [English edition by M.A. Partridge, D.O. Jordan, Interscience, New York, 1950]. [16] E. Cartmell, G.W.A. Fowles, Valency and Molecular Structure, Butterworths, London, 1956. [17] MCC, Nuclear Waste Materials Handbook (Materials Characterization Center, Hanford, WA) US Report no. DOE/TIC-11400, 1983. [18] S.V. Raman, J. Mater. Res. 13 (1998) 8.

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