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The preparation and properties of some hydroxy compounds of beryllium
J. Inorg. Nuel. Chem., 1966, Vol. 28, pp. 61 to 76. Pergamon Pre8 Ltd. Printed in Northern Ireland
THE PREPARATION A N D PROPERTIES OF SOME H Y D R O X Y C O M P O U N D S OF BERYLLIUM R. A. MERCER and R. P. MILLER National Physical Laboratory, Teddington, Middlesex (Received 4 June 1965)
Abstract--The conditions governing the precipitation of the polymorphic forms of beryllium hydroxide from sodium beryllate solutions have been established, and the preparation of a series of hydroxy complexesof beryllium with calcium and strontium is described. The thermal decomposition reactions of these compounds have been followedby thermogravimetric analysis (T.G.A.), differential thermal analysis (D.T.A.) and X-ray powder photography. INTRODUCTION IN AN EARLIERCOMMUNICATION(1) some aspects of the chemistry of sodium beryllate solutions were considered in relation to the hydrolytic equilibrium defined in terms of the Na20:BeO ratio. Decomposition reactions of these solutions were described, and it was shown that the OH:Be ratio influenced both the extent of thermal decomposition and the tendency to form newly-discovered insoluble complexes with silica and alkaline-earth salts. The precipitation of beryllium hydroxide by boiling sodium beryllate solutions is a process of significance in the hydrometallurgy of beryllium, cz~ Not only is the quantitative aspect of this hydrolysis of technological importance but, in view of past controversy concerning the conditions governing the genesis and stability of the polymorphic modifications of beryllium hydroxide, the nature of the precipitated hydroxide is of more fundamental interest. The first part of the work to be described here defines the solution conditions which control the crystalline species precipitated. The results are then described of a more detailed examination of the preparative and decomposition reactions of the hydroxy complexes of beryllium with calcium and strontium. Difficulties have been encountered in the unambiguous determination of the true chemical composition of these compounds, but consideration of chemical analysis, thermogravimetric analysis and X-ray powder data has enabled the composition range to be established within the formula type: x M(OH)z'y Be(OH)~.z HzO.
M = Ca or Sr.,
where x = 1 ; y varies between 2.5 and 9; z varies between 0 and 3. The high-temperature chemistry of the thermally-decomposed compounds is of interest insofar as mixed oxide structures appear under some conditions of heat treatment. The thermal decomposition reactions of these compounds have therefore been followed in some detail by T.G.A., D.T.A. and X-ray powder photography. This study has been supplemented by a separate investigation by high-temperature a) D. A. Evlm~r, 1L A. Mlmclm, R. P. MnJ.~! and G. L. MILWAIO, J. Inorg. NueL Chem. 24, 525 (1962). (l~ p. S. ~ , Extraction and Refining of the Rarer Metals, Instn. Min. Metall., London 310 pp. (1957). 61
62
R.A.M.wcm~ and R. P. Mmt.m~
microscopy of melt reactions in the CaO-BeO and SrO-BeO systems, the results of which have been described elsewhere. ~s~ EXPERIMENTAL Sodium beryllate solutions were prepared either by direct dissolution of pure Bc(OH), (>99.9~. Be(OH)s) in 10 N-NaOH or by similar dissolution of Be(OH)s freshly precipitated from a chloride solution prepared from spectrographically-puremetal. The quantities used depended on the desired NasO:BeO ratio and solutions were diluted according to the overall OH- concentration selected for study. The time allowed for thermal decomposition of the beryllates by boiling was standardized at 1 hr. The precipitated beryllium hydroxide was filtered and the extent of decomposition was determined by estimating the beryllium remaining in the filtrates. X-ray powder photographs were taken of all precipitates. The standard procedure used in the preparation of hydroxy complexes by double decomposition reactions involved precipitation by direct addition of alkaline-em~ chloride or hydroxide solution. The precipitates were digested for 1 hr at 60°C and then allowed to equilibrate for 24 hr at room temperature before filtration. Stock solutions of M CaCI~, 0.2 M SrCls, and 0.05 M Sr(OH)~ were prepared from pure salts. Other methods of preparing calcium complexes were (a) direct addition of freshly precipitated Ca(OI-I)ltoberyllate solution, and (b) caustic soda digestion of co-precipitated Ca(OH)~and Be(OH)~. The temperature and times of digestions were standardized as for the double decomposition preparations. The Na~O:BeO ratios referred to in the tables take account of the hydroxyl requirements of the added metal and, where necessary, adjustments were made by caustic soda addition. As far as possible all preparative work on beryllates was done under COs-freeconditions by use of Sofnollte traps. Standard analytical methods were used in determining calcium, strontium and beryllium. Thermogravimetrie analyses were made on a Stanton thermobalanee. The differential thermal analyser was a commercial instrument manufactured by G. Netzseh. X-ray powder photographs were taken with a Philips 11.46 em camera using cobalt K~ radiation. RESULTS
The preparation of the polymorphicforms of beryllium hydroxide from sodium beryllate solutions The boundary solution conditions defining which particular polymorphic species of Be(OH)z is precipitated on boiling are given in Table I. These data show that, irrespective of the Na~O:BeO ratio, only the orthorhombic variety is precipitated when the overall normality is above 1.0 N-NaOH. Increasing amounts of the tetragonal polymorph appear if this normality is decreased. Only a minor proportion of the orthorhombic form is detected when the sodium hydroxide normality falls below 0"5 N - N a O H and the Na20:BeO ratio is decreased to 1"5:1 or below. The extent o f thermal decomposition of sodium beryllate solutions is controlled by the Na~O:BeO ratio. For example, when this ratio is 8:1 and the overall norreality is 6 N - N a O H , no decomposition occurs on boiling. Halving the Na20:BeO ratio while maintaining the sodium hydroxide normality caused 37 ~o of the beryllium to be precipitated. This trend is consistent over the wide range of solutions chosen. The last two results recorded in Table 1 stress that the polymorphic form of precipitated beryllium hydroxide depends only on the solution conditions and not upon the structure or purity of the beryllium hydroxide used to prepare the solution.
csjR.
A. MERCERand R. P. MILLER,Nature 202, 581 (1964).
The preparation and properties of some hydroxy compounds of beryllium TAeLe 1.--THe
~
DECOMI'OSITION Ol~ s o D I U M BERYLLATE soLUTIONS
Concentration Overall of beryllate NatO:BeO normality ratio of NaOH m mols/l g/l BeO BeO
Extent of decomposition ~o Bee pptd
Polymorph of Be(OH)l pptd Tetragonal only Tetragonal + minor amts of orthorhombic Orthorhombic only Orthorhombi¢ only Orthorhombic only Orthorhombic only Orthorhombic ~ minor amts of tetragonal Orthorhombic Jc trace of tetragonal Orthorhombic only Orthorhombic only Orthorhombic only Orthorhombi¢ only Orthorhombic + minor amts of tetragonal Orthorhombi¢ d- trace of tetragonal Orthorhombic only Orthorhombi¢ only Orthorhombi¢ only -Orthorhombic only Tetragonal + minor amts of orthorhombic
83"5 167
3.59 7.18
1'5:1 1.5:1
0.25 N 0.5 N
>98 >98
668 250 375 1250 62"5
28.70 6.24 9.36 15.60 1"56
1.5:1 2:1 2:1 2:1 4:1
2.0 N 1.0 N 1.5 N 5-0 N 0"5 N
>98 98 98 70 98
125
3.12
4:1
1.0 N
93
187.5 4.68 250 6.24 500 12.48 750 18.72 31.25 0.78
4:1 4:1 4:1 4:1 8:1
1"5 N 2.0 N 4.0 N 6.0 N 0.5 N
88 85 49 37 90
1"56
8:1
1"0
80
93.75 2.34 125 3.12 250 6.24 375 9.36 500* 12"48 125" 3.12
8:1 8:1 8:1 8:1 2:1 2:1
1.5 2.0 4.0 6-0 2-0 N 0.5 N
74 53 26 nil 90 98
--
62.5
63
* In each of these tests three original beryllate solutions wore prepared by dissolving (a) orthorhombic, (b) tetragonal, and (¢) gelatinous Be(OH)t in NaOH. Within each series the results were identical with respect to the extent of decomposition and the polymorphic form precipitated.
The preparation and composition of the calcium and strontium hydroxy complexes of beryllium The preparative conditions used and the calculated molecular formulae of the two series of compounds are summarized in Tables 2 and 3. The precipitates are formed in varying degrees of crystallinity. Microscopic examination showed that they range f r o m highly-disperse powders characterized by broad diffuse X-ray powder spectra, to well-crystallized hexagonal platelets. The following reservations have to be made concerning the molecular formulae given for these compounds. An overall chemical analysis of a precipitated product obviously includes any uncombined hydroxides or carbonates o f the alkaline-earth metal. In some cases X-ray powder photographs showed that minor contamination of this kind had occurred. X-ray diffraction, however, does not give the amounts and will not detect any amorphous precipitates which could arise under m a n y of the conditions of preparation used. Thermogravimetric analysis can resolve this difficulty provided (a) that the
64
R.A.
M E ~ c l ~ and R. P. MmLm~
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p ~2, o o-
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Z
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i-.-.
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t.-
llw
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I
,.b
~
,,b
,.~
The preparation and properties of some hydroxy compounds of beryllium
0
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65
66
R.A.
M~RCER and R. P. M~LLER
© o "0
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T h e p r e p a r a t i o n a n d properties o f s o m e h y d r o x y c o m p o u n d s o f beryllium
67
decomposition temperature of the complex differs from those of any contaminating compounds, and (b) that such compounds are not decomposition products. Only under these conditions can the weights of uncombined hydroxides be determined and subtracted from the overall chemical analysis to give an unambiguous empirical formula. Unfortunately, the manner in which the compounds decompose thermally (see below) does not permit the assumption that the weights of uncombined contaminating hydroxides or carbonates of calcium or strontium can be calculated from weight losses occurring at their respective characteristic decomposition temperatures. TASL~ 4.--X-RAY P O W D ~ DATA FOR 2 C a ( O H ) , . 4 Be(OI-I)f.H,O (unit cell: h e x a g o n a l ; a = 9.68 A , c = 9.13 A )
I/1o
d
hkl
1/lo
d
hkl
vs m m ms vvw m m w m s w vw
8.34 6.15 4.82 4.27 4.16 3.99 3.80 3.17 2.98 2.86 2.79 2.66
100 101 110 111 200 102 201 210 211 103 300 301
m m m m w mw ms m mw vw ms vw
2.50 2.59 2.47 2"42 2"38 2"25 2"10 2.07 2.00 1.89 1"85 1.82
212 113 203 220 302 311 400 312 204 223 214 410
With regard to any free Be(OH)~ which could be a contaminant, there are grounds for concluding that negligible quantities have, in fact, been co-precipiated. It will be recalled that only the two crystalline modifications of Be(OH)z can normally be thrown out of beryUate solutions. The lines of either polymorph are absent on the powder photographs of the calcium and strontium complexes. That any contaminating Be(OH)~ would be crystalline, and hence detectable, is supported by the behaviour of beryUates during unsuccessful attempts to prepare magnesium and ferric iron hydroxy complexes of beryllium by similar double decomposition reactions. In one case orthorhombic Be(OH)a was precipitated with free Mg(OH)~ whereas the tetragonal polymorph was co-precipitated with iron hydroxide in the other. The greatest uncertainty, therefore, in the compositions given in Tables 2 and 3 lies in the Ca(OH)~ or Sr(OH)a content. Although their true composition may differ from that given, the wide stoichiometric range of these compounds is nevertheless evident, and differences are reflected in their X-ray spectra. The powder photographs of both series of complexes have many common fines. As previously reported, (1) the most characteristic line is a strong reflection at about 8.3 A. A conclusion from the earlier work was that calcium and strontium complexes having a Be:Ca or Sr ratio of approximately 4:1 had identical hexagonal unit cells (a ---- 14.9 A; c = 16.6 A.) It has become clear, however, from this more extensive survey that differences develop both within and between members of the calcium and strontium series as the ratio Be~+: M 2+ varies. It has not been found possible to index all the photographs.
68
R.A. MERCERand R. P. MILLER
The calcium complex 2 Ca(OH)2.4 Be(OH)~.H20 can be indexed (Table 4) on a hexagonal cell (a = 9.68 A; c --- 9.13 A) but as the B e : C a ratio increases in the series, extra lines appear which do not permit a fit on this cell. It was possible to index the compounds with high B e : C a ratio (2 Ca(OH)2"9 Be(OH)s'2 H20; (2 Ca (OH)z. 10 Be(OH)~.'3 H20 ) on a very large hexagonal unit cell where the ' a ' parameter is , ~ 30 A, but it is felt that this is most likely to be a pseudo cell, the true symmetry having been lowered. Similar difficulties were encountered in the strontium series. A representative sel of d values of the compound 2 Sr (OH)2.9 Be(OH)2 are given in Table 5. TABLE 5.~X-RAY POWDER DATA FOR 2
vw vvs s vs vs vs vvw vvw s s vvw vvs vvs s w vvw vvw s s vvw s s vvw vvw
Sr(OH)2"9 Be(OH)2
10.49 w 8.21 vsd 7-39 vsd 6-91 vvs 6.42 vw 6.10 s 5-50 vs 5"30 s 5-09 s 4-86 s 4"59 w 4.44 s 4.30 s 4.11 vvw 3.84 s 3-73 s 3.67 w 3.53 w 3-45 w 3.29 w 3.18 w 3"06 w 3-01 vw 2.93 + many other lines
2-86 2.78 , 2.73 2-68 2.61 2-50 2-44 2.38 2.31 2.24 2.19 2.12 2-10 2.08 2.04 2-01 1.89 1"85 1.83 1.82 1.73 1.70 1-62
Thermaldecompositionof the hydroxy complexes The pyrolysis curves and differential thermograms of the hydroxy complexes showed the stepwise nature of the dehydroxylation during heating. The features were similar within the calcium series and within the strontium series but differences were evident between the two sets of compounds. Typical curves representative of both types of complex are shown in Figs. 1, 2, 3 and 4. Both sets of compounds lost loosely-bound water up to 110°C but during the course o f further heating the calcium and strontium compounds showed marked differences in behaviour. T.G.A. (Fig. 1) shows that the calcium compounds lose water discontinuously. Between 100 ° and 200 °, 200 ° and 270 °, 270 ° and 300 °, inflections occur which indicate differences in the rate of change of weight loss. D.T.A. (Fig. 3) shows corresponding
The preparation and properties of some hydroxy compounds of beryllium
69
,.,.I
70
60
~d 50
.4 t ?
40 .~'
30
I
•
2O
,o
i.f* .,.-'T" I00
200
300
400
500
Tempers't'ure,
600 *C
700
800
900
10(30
FIo. 1.--Thermogravimetric analysis of 2Co(OH)~-4Be(OH),.H,O160.4nag).
endotherms in these temperature regions. At higher temperatures T.G.A. shows that weight losses occur corresponding to dehydroxylation of calcium hydroxide (400 °) and to decarboxylation of calcium carbonate (600°C). This last phase was always formed by uptake of atmospheric carbon dioxide. The pyrolysis curves (not reproduced) of both calcium hydroxide and strontium hydroxide showed the readiness with which this contamination occurs under the conditions of heating in the thermobalance. The calcium complexes show an additional endotherm during D.T.A. corresponding to dehydroxylation of calcium hydroxide, but no evidence of an endotherm associated with carbonate breakdown. It is assumed that the carbon dioxide uptake is negligible in the D.T.A. apparatus, where the samples are encased in a sheath. X-ray analysis of samples taken at various stages during T.G.A. showed that between 170 and 200 ° the calcium complexes were completely destroyed. The only crystalline products identified by X-rays at 200 ° were orthorhombic Be(OH)s, Ca(OH)z and CaCO3. 60
50 h~
_go +. 3 0
q
!
i
20
0
•
I00
200
300
400
500
Temperature,
600
700
800
900
I000
*C
FIo. 2.--Thermogravimetric analysis of 2Sr(OH)2.9Be(OH)I(115.3 mg).
70
R . A . M_~c~ and R. P. MnJ,~R
I<3
.)80
Temperoture,
°C
~o. 3.--Differential thermo~m of 2Ca(OH)a.4Be(OH)j.H=O.
~
0
j Temperature, °C
FIG. 4.--Differential thcrmogram of 2Sr(OH)=.9Be(OH)=.
The preparation and properties of some hydroxy compoundsof beryllium
71
90
80
/ 70
I'
J
6O
50
I
o
30
2O
/
~0
!. 1 300
400
500
Temperature,
600
700
800
°C
FIG. 5.--Thermogravimetricanalysis of tetragonal Be.(OH)2 (215 mg).
[
80
7O
1"/" / 60
{
i E o
/
I L
50
40
~ 30 20
I
l
I
I
I 0
I00
200
300
400
500
600
700
Temperature, °C
FIG. 6.--Thermogravimetricanalysis of orthorhombicBe(OH)s (169.6 mg). Dchydroxylation of orthorhombic beryllium hydroxide begins at 220°C. T.G.A. data on both polymorphs are separately shown in Figs. 5 and 6. It is of interest to note from both these sets of data that the initial decomposition of the tetragonal polymorph appears to be significantly lower than the stable orthorhombic variety, and that both forms are dehydroxylated in two distinct stages. The bulk of the water is quickly lost up to 280°C, when the overall composition approximates between 6 B eO.HsO and 7 BeO.HsO. Thereafter the remaining water is not eliminated until
72
R.A.
M E R C ~ a n d R. P. Mmt.~R
500°C. These results are in substantial agreement with an earlier pyrolysis curve recorded by Duval using an unspecified form of beryllium hydroxide.tS~ The origin of the endotherm beginning at 280°C (Fig. 3) on the differential thermogram of the calcium complex has not been fully resolved in this study. By analogy with the behaviour of the strontium compounds described below, the heat effect may be associated with the decomposition of an intermediate basic complex which in this case is amorphous and hence not detectable by X-rays. No further interaction occurs at higher temperatures between the B e t and C a t produced from the thermally decomposed hydroxy complex. In a separate experiment T.G.A. of intimately mixed Ca(OH)z and orthorhombic Be(OH)2 confirmed the independent dehydroxylation and non-interaction of the simple oxide products. T A m ~ 6.--X-RAY POWDER DATA ON INTERMEDIATE COMPLEX 2 SrO.9 BeO.7 H s O F O ~ D BY ~ N ¢ 3 2 Sr(OH)~'9 Be(OH)a TO 220°C (unit cell; hexagonal, a = 11.02 A, e = 8.54 A) d
X/Zo
d
(A)
hkl
I/I0
(A)
hkt
s
4"77
200
s
2.08
402 410
m w rn
4"29 4.17 3.37
002 201 112
ms s m
1-95 1.91 1.80
204 500 420
s
3'18
{300 202
w
1"69
332
s
2.65
310
vw
1-59
600
vw
2"45
203
w
1"51
/305 1521
m s
2"39 2"14
400 004
ms
1"42
006
The behaviour shown by strontium compounds on thermal decomposition was more complex. The compound chosen for systematic study was well-crystallized 2 Sr(OH)2"9 Be(OH)v After an initial loss of water no weight change occurs until 180°C. T.G.A. (Fig. 2) then shows a steep rise in the rate of decomposition which is accompanied by a large endotherm during D.T.A. Microscopic examination showed that the decomposition between 180°C and 250°C was attended by pronounced exfoliation. X-ray analyses at 220°C showed that all trace of the original complex had disappeared. The powder data, however, showed no evidence of Be(OH)s, Sr(OH)z or SrCO a. All lines of the product phase, which must be regarded as a new complex basic oxide of empirical formula 2 SrO.9 Bet.7 H~O, could be indexed satisfactorily on a hexagonal unit cell with a = 11"02 A, c = 8.54 A (Table 6). Between 250°C and 300°C a change in rate of dehydroxylation was accompanied by a small endotherm (Fig. 4). These effects were associated with the decomposition of the hexagonal phase. The only crystalline products identified between 300°C and 600°C were strontium carbonate and beryllium oxide. The carbonate decomposed at 830°C to give strontium oxide. ~ R. FmCKE a n d H. H f f M ~ , Z. anorg. Chem. 178, 406 (1929). ~5~ T. DOVAL a n d C. DucAL, Anal. Chim. Act. 2, 53 (1948).
The preparation and properties of some hydroxy compounds of beryllium
73
In contrast to the calcium systems, further interactions occurred at higher temperatures between these freshly-formed simple oxides. X-ray analysis of the product at the completion of T.G.A. to 1000°C showed that no free SrO or BoO was present. Interaction had occurred to give a new phase, crystallizing with a hexagonal unit cell, the parameters a----4.60 A and c = 8"94 A being consistent with the data (Table 7). This phase was produced by all the other strontium-beryllium complexes on heating to lO00°C. T A B L ~ 7.--X-RAY P O W D E R
DU~CTION
D A T A O N MIXED OXIDE PHASE
2 SrO.9 B o O FORMED BY HEATING 2 Sr(OH)z'9 Be(OI-I)s TO 1000°C
(unit cell; hexagonal a ----4.60 A., c = 8.94 A)
1]1o
d (A)
hkl
I]Io
(A)
d
hkl
rn ms s s s ms m vs w ms
4.46 3.98 3.63 2.97 2.38 2.30 2,23 2,04 1,99 1.94
002 100 101 102 103 110 111 112 200 201
m m m ms w ms m m s s
1.81 1.65 1.63 1.60 1.50 1.48 1.42 1-39 1.34 1.33
202 203 105 114 120 204 212 106 213 300
When a mixture of Sr(OH)~.8 H~O and orthorhombic Be(OH)~ were heated together under similar conditions to those described above, neither the hexagonal basic hydroxy complex nor the compound beryllium-strontium oxides were detected. This suggests that the formation of these phases from the original hydroxy complexes proceeds topochemically. As the mixture of hydroxides was progressively heated Sr(OH)~.8 HsO loses the eight molecules of water below 100°C. Be(OH)~ then decomposes at 220°C but additional lines to those of BeO and Sr(OH)~ were present on the X-ray photographs, these were not identified as belonging to any phase characterized in the decomposition of the hydroxycomplexes. Their origin was not investigated in view of the transient nature of their existence. The phase or phases responsible had decomposed by 500°C. At this temperature Sr(OH)a decomposes but carbonate is readily formed until its decomposition temperature of 830°C is reached. The rehydration of the oxide produced was so rapid that X-ray photographs of the ignited products showed only St(OH)2 and in some cases Sr(OH)~.8 H~O along with BeO. The X-ray powder data of anhydrous Sr(OH)2 is given in Table 8 as it is not included in the A.S.T.M. index and it does not appear to have been previously published. DISCUSSION
The polymorphic forms of beryllium hydroxide The methods of preparation and stability relationships between the polymorphic forms of beryllium hydroxide have been the subject of several studies. Until recently the structural differences have been obscure, and early literature has been confused by the indiscriminate use of ' £ and 'fl' in describing stable and metastable forms. Beryllium hydroxide freshly precipitated by addition of alkali to acid solutions is
74
R.A. MERCERand R. P. MILI2~
amorphous, but it was noted by early workers t4~ that transformation to differing crystalline forms occurred progressively on ageing. The process was accelerated by boiling. The first modification to be produced was called by these authors 'metastable ~t form'. This passed, on months of standing, to the 'stable fl form'. This last variety was also found to be precipitated from hot concentrated alkaline solutions of beryllium hydroxide. TABLE8.--X-RAY POWDERDATAOF SF(OH)II Ilia
d
I/Io
d
s s ms s ms vw ms ms ms vw
6.188 4.530 3.652 3.351 3.142 2.951 2.842 2.811 2.467 2"358
w w(d) m w w vw(d) mw w w w
1.736 1"691 1"622 1.602 1"582 1"547 1.529 1"480 1"427 1"405
s
2"288
w
1"378
s m vw(d) w m w
2.225 2.104 2.058 1"971 1"821 1"751
w w w w w(d)
1"341 1'311 1"284 1.265 1.234
The crystal structure of this form was solved by Seitz et al.te) It was shown to have an orthorhombic cell (a = 4.61 fl~; b = 7"02 A; c = 4.52 J~) in which the Be~- ions are surrounded tetrahedrally by O H - ions but, unlike the majority of the hydroxides o f bivalent metals, it is not a layered structure. In this respect it is analogous to Zn(OH)9, the stable form of which is isostructural with orthorhombic Be(OH)v More recently, ~7~the powder data of the metastable form, precipitated by hydrolysis o f beryllium chloride solutions with urea, has been indexed on a tetragonal cell (a = 10.88 A; c = 7"83) but no structure analysis appears to be available. The reported value ~s~ of the heats of formation of the two polymorphs imply that the energy differences between them are negligible: A H 0 (orthorhombic)---216.1 K cals/mol; A H 0 (tetragonal)--216.8 K cals/mol. It has been shown c~ that direct precipitation of the stable modification can be achieved from acid solutions of basic beryllium acetate by very slow hydrolysis involving a gradual increase o f p H from 2.7 to 5.3. The present work shows that either polymorph can be precipitated from alkaline solution by suitable choice of hydrolytic conditions. These facts dispel the view which has been advanced that the formation of the metastable form arises by inclusion of foreign anions which are substituted for O H - ions, so becoming an integral part of the structure. ~e~A. S~rrz, U. R0slam and K. SCHUBERT,Z. anorg. Chem. 261, 94 (1950). c7~W. J. KmKPATRICK,G. R. ANDERSONand E. S. FtmsTou, General Electric Report, APEX-684. TID-4500, UC-4 (Chemistry), (1961). n~ R. FItlCKEand B. WULLHORST,Z. anorg. Chem., 205, 127 (1932).
The preparation and propertiesof some hydroxycompoundsof beryllium
75
The practicalimplicationsof being able to choose the hydrolyticconditions which will precipitatethe orthorhombic form centre on the density differencebetween this form and the lightertetragonalvariety,which is more difficultto filter.Furthermore, ifthe berylliaformed by ignitingthe hydroxide isrequiredto be compacted for ceramic purposes, the denser form of the hydroxide is desirable. This arisesfrom the topotaxial nature of dehydroxylation~9)inasmuch that there is a structuraland orientational relationshipbetween products and reactants. In the case of hydroxides the loss of water tends to leave a porous pseudomorph. The volume of the intersticeswill obviously be larger if the originalhydroxide is of a more open variety. It has been pointed out that during sintering all voids between particlesbecome filledmore readilythan internalvoids,which tend to persist,in the individualparticles. The hydroxy complexes o f beryllium
The existence of such compounds was first established during earlier studies on the precipitation reactions of sodium beryUate solutions. CI) It was shown that they formed as double decomposition products when calcium or strontium chloride solutions were added to the beryUates. The extent of the reaction in terms of the quantity of beryllium precipitated as complex was shown to depend on the NasO: BeO ratio used in preparing the beryUate solutions. This ratio is a convenient way of expressing what is effectively the OH-:Be ratio in solution. It was shown that the properties of these solutions depended both upon this ratio and upon the overall OH- concentration. Their behaviour could be explained qualitatively if it was assumed that isopolyanions of the type
o.
7"1
_
_
existed in solution, where n depended upon the OH-: Bea+ ratio and OH- concentration. Thus at low Na20:BeO ratios (,~1-5:1) and below 1N overall NaOH concentration the solutions were unstable. They exhibited a Tyndall effect, implying that they were colloidal hydrosols, and they precipitated tetragonal Be(OH)~ on standing at room temperature. At high OH- :Be2+ ratios no hydroxy complexes are precipitated and the solutions are thermally stable (see Table 1). This stability was considered to coincide with the increasing predominance of the stable monomeric ion
o.\ /o. 1 oHj
On the basis of sodium beryllate solutions being an alkali-dependent polymerization system it is to be expected that some relationship will hold between the hydrolytic equilibrium and the composition and constitution of the precipitated complexes. ~*~L. S. DE~¢r-Gi.~s~R, F. P. G ~ R and H. F. W. TAYLOR, Q~rt. R~. Vol. X3/I, No. 4, 343 (1962).
76
R . A . MERCERand R. P. MILLER
Such a relationship is implied by the relatively wide stoicheiometric range of compounds produced, but the exact constitution o f both the beryllate ions in solution and the complexes themselves are still uncertain. The structure of the majority of the hydroxy salts of bivalent metals is based upon octohedral co-ordination of the metal ions in layered lattices, o°} Mg(OH)2 or Ca(OH)~ are typical end-member types. Modifications include double layering and the development of stacking faults which give rise to various deviations, but the basic structural principle is established. The crystal class into which such compounds crystallize is usually hexagonal but some are monoclinic or triclinic The powder photographs of all the compounds described in this study are basically similar, but the inability to index satisfactorily the calcium and strontium compounds of high Be:Ca or Sr ratio on hexagonal cells suggests that the excess of the small ion with its high polarizing power has distorted the structures to lower symmetry. It was suggested earlier{1} that hydroxy-bridged bands of beryllium ions could provide a possible back-bone of the structures, but single crystal studies would be necessary to substantiate this.
Acknowledgement--Theauthors wish to thank Mr. J. REEWfor his contributions to the experimental work. ct0} W. FErrgh~orr (Translated by F. HUDSWELL)A.E.R.E. Lib/Trans 622 (1956).
THE PREPARATION A N D PROPERTIES OF SOME H Y D R O X Y C O M P O U N D S OF BERYLLIUM R. A. MERCER and R. P. MILLER National Physical Laboratory, Teddington, Middlesex (Received 4 June 1965)
Abstract--The conditions governing the precipitation of the polymorphic forms of beryllium hydroxide from sodium beryllate solutions have been established, and the preparation of a series of hydroxy complexesof beryllium with calcium and strontium is described. The thermal decomposition reactions of these compounds have been followedby thermogravimetric analysis (T.G.A.), differential thermal analysis (D.T.A.) and X-ray powder photography. INTRODUCTION IN AN EARLIERCOMMUNICATION(1) some aspects of the chemistry of sodium beryllate solutions were considered in relation to the hydrolytic equilibrium defined in terms of the Na20:BeO ratio. Decomposition reactions of these solutions were described, and it was shown that the OH:Be ratio influenced both the extent of thermal decomposition and the tendency to form newly-discovered insoluble complexes with silica and alkaline-earth salts. The precipitation of beryllium hydroxide by boiling sodium beryllate solutions is a process of significance in the hydrometallurgy of beryllium, cz~ Not only is the quantitative aspect of this hydrolysis of technological importance but, in view of past controversy concerning the conditions governing the genesis and stability of the polymorphic modifications of beryllium hydroxide, the nature of the precipitated hydroxide is of more fundamental interest. The first part of the work to be described here defines the solution conditions which control the crystalline species precipitated. The results are then described of a more detailed examination of the preparative and decomposition reactions of the hydroxy complexes of beryllium with calcium and strontium. Difficulties have been encountered in the unambiguous determination of the true chemical composition of these compounds, but consideration of chemical analysis, thermogravimetric analysis and X-ray powder data has enabled the composition range to be established within the formula type: x M(OH)z'y Be(OH)~.z HzO.
M = Ca or Sr.,
where x = 1 ; y varies between 2.5 and 9; z varies between 0 and 3. The high-temperature chemistry of the thermally-decomposed compounds is of interest insofar as mixed oxide structures appear under some conditions of heat treatment. The thermal decomposition reactions of these compounds have therefore been followed in some detail by T.G.A., D.T.A. and X-ray powder photography. This study has been supplemented by a separate investigation by high-temperature a) D. A. Evlm~r, 1L A. Mlmclm, R. P. MnJ.~! and G. L. MILWAIO, J. Inorg. NueL Chem. 24, 525 (1962). (l~ p. S. ~ , Extraction and Refining of the Rarer Metals, Instn. Min. Metall., London 310 pp. (1957). 61
62
R.A.M.wcm~ and R. P. Mmt.m~
microscopy of melt reactions in the CaO-BeO and SrO-BeO systems, the results of which have been described elsewhere. ~s~ EXPERIMENTAL Sodium beryllate solutions were prepared either by direct dissolution of pure Bc(OH), (>99.9~. Be(OH)s) in 10 N-NaOH or by similar dissolution of Be(OH)s freshly precipitated from a chloride solution prepared from spectrographically-puremetal. The quantities used depended on the desired NasO:BeO ratio and solutions were diluted according to the overall OH- concentration selected for study. The time allowed for thermal decomposition of the beryllates by boiling was standardized at 1 hr. The precipitated beryllium hydroxide was filtered and the extent of decomposition was determined by estimating the beryllium remaining in the filtrates. X-ray powder photographs were taken of all precipitates. The standard procedure used in the preparation of hydroxy complexes by double decomposition reactions involved precipitation by direct addition of alkaline-em~ chloride or hydroxide solution. The precipitates were digested for 1 hr at 60°C and then allowed to equilibrate for 24 hr at room temperature before filtration. Stock solutions of M CaCI~, 0.2 M SrCls, and 0.05 M Sr(OH)~ were prepared from pure salts. Other methods of preparing calcium complexes were (a) direct addition of freshly precipitated Ca(OI-I)ltoberyllate solution, and (b) caustic soda digestion of co-precipitated Ca(OH)~and Be(OH)~. The temperature and times of digestions were standardized as for the double decomposition preparations. The Na~O:BeO ratios referred to in the tables take account of the hydroxyl requirements of the added metal and, where necessary, adjustments were made by caustic soda addition. As far as possible all preparative work on beryllates was done under COs-freeconditions by use of Sofnollte traps. Standard analytical methods were used in determining calcium, strontium and beryllium. Thermogravimetrie analyses were made on a Stanton thermobalanee. The differential thermal analyser was a commercial instrument manufactured by G. Netzseh. X-ray powder photographs were taken with a Philips 11.46 em camera using cobalt K~ radiation. RESULTS
The preparation of the polymorphicforms of beryllium hydroxide from sodium beryllate solutions The boundary solution conditions defining which particular polymorphic species of Be(OH)z is precipitated on boiling are given in Table I. These data show that, irrespective of the Na~O:BeO ratio, only the orthorhombic variety is precipitated when the overall normality is above 1.0 N-NaOH. Increasing amounts of the tetragonal polymorph appear if this normality is decreased. Only a minor proportion of the orthorhombic form is detected when the sodium hydroxide normality falls below 0"5 N - N a O H and the Na20:BeO ratio is decreased to 1"5:1 or below. The extent o f thermal decomposition of sodium beryllate solutions is controlled by the Na~O:BeO ratio. For example, when this ratio is 8:1 and the overall norreality is 6 N - N a O H , no decomposition occurs on boiling. Halving the Na20:BeO ratio while maintaining the sodium hydroxide normality caused 37 ~o of the beryllium to be precipitated. This trend is consistent over the wide range of solutions chosen. The last two results recorded in Table 1 stress that the polymorphic form of precipitated beryllium hydroxide depends only on the solution conditions and not upon the structure or purity of the beryllium hydroxide used to prepare the solution.
csjR.
A. MERCERand R. P. MILLER,Nature 202, 581 (1964).
The preparation and properties of some hydroxy compounds of beryllium TAeLe 1.--THe
~
DECOMI'OSITION Ol~ s o D I U M BERYLLATE soLUTIONS
Concentration Overall of beryllate NatO:BeO normality ratio of NaOH m mols/l g/l BeO BeO
Extent of decomposition ~o Bee pptd
Polymorph of Be(OH)l pptd Tetragonal only Tetragonal + minor amts of orthorhombic Orthorhombic only Orthorhombi¢ only Orthorhombic only Orthorhombic only Orthorhombic ~ minor amts of tetragonal Orthorhombic Jc trace of tetragonal Orthorhombic only Orthorhombic only Orthorhombic only Orthorhombi¢ only Orthorhombic + minor amts of tetragonal Orthorhombi¢ d- trace of tetragonal Orthorhombic only Orthorhombi¢ only Orthorhombi¢ only -Orthorhombic only Tetragonal + minor amts of orthorhombic
83"5 167
3.59 7.18
1'5:1 1.5:1
0.25 N 0.5 N
>98 >98
668 250 375 1250 62"5
28.70 6.24 9.36 15.60 1"56
1.5:1 2:1 2:1 2:1 4:1
2.0 N 1.0 N 1.5 N 5-0 N 0"5 N
>98 98 98 70 98
125
3.12
4:1
1.0 N
93
187.5 4.68 250 6.24 500 12.48 750 18.72 31.25 0.78
4:1 4:1 4:1 4:1 8:1
1"5 N 2.0 N 4.0 N 6.0 N 0.5 N
88 85 49 37 90
1"56
8:1
1"0
80
93.75 2.34 125 3.12 250 6.24 375 9.36 500* 12"48 125" 3.12
8:1 8:1 8:1 8:1 2:1 2:1
1.5 2.0 4.0 6-0 2-0 N 0.5 N
74 53 26 nil 90 98
--
62.5
63
* In each of these tests three original beryllate solutions wore prepared by dissolving (a) orthorhombic, (b) tetragonal, and (¢) gelatinous Be(OH)t in NaOH. Within each series the results were identical with respect to the extent of decomposition and the polymorphic form precipitated.
The preparation and composition of the calcium and strontium hydroxy complexes of beryllium The preparative conditions used and the calculated molecular formulae of the two series of compounds are summarized in Tables 2 and 3. The precipitates are formed in varying degrees of crystallinity. Microscopic examination showed that they range f r o m highly-disperse powders characterized by broad diffuse X-ray powder spectra, to well-crystallized hexagonal platelets. The following reservations have to be made concerning the molecular formulae given for these compounds. An overall chemical analysis of a precipitated product obviously includes any uncombined hydroxides or carbonates o f the alkaline-earth metal. In some cases X-ray powder photographs showed that minor contamination of this kind had occurred. X-ray diffraction, however, does not give the amounts and will not detect any amorphous precipitates which could arise under m a n y of the conditions of preparation used. Thermogravimetric analysis can resolve this difficulty provided (a) that the
64
R.A.
M E ~ c l ~ and R. P. MmLm~
o
o
!
0 0 0
0
o e~
e~ z
O
O
e~
O
M f.w o R
8
0
p ~2, o o-
o o~.o o~-o ~'~
~'~~~:.~o
o
o o
e~
z z
o~
Z
Z
Z
Z
i-.-.
,1~
i'..-,
t.-
llw
.o
ZN 1".1
I
,.b
~
,,b
,.~
The preparation and properties of some hydroxy compounds of beryllium
0
0
"0
A
a
a
.~
.~
o ~ g.=.==
* g .g~g
~ =
0
Z
. . t"q
.
Z
Z
Z
eq
oo t'q
. . ¢'I
~
~
I",1
.
.
.
g .=
* =
g .=
0
0
Z
Z;
. t"q
¢',1
. t"q
t",,1
65
66
R.A.
M~RCER and R. P. M~LLER
© o "0
o 0
0 r-i
el
~
e,l
e,i
e,i
~D
0 0
.o = .o~ ._
0
o =.i:
=o.
~
. '~
..~
:0~: 3, ~
0
0
=
0
~.
o~
Ho
O~
C~
~.~
o
C~
Z
Z
Z
Z
Z
Z
IN
t",.I
t'~
t',l
t",l
r'-I
.~
.~
9
.o
.o
9
IN
('N
o.
o.
o.
0o
o.
C~
0o
C~
o
o
T h e p r e p a r a t i o n a n d properties o f s o m e h y d r o x y c o m p o u n d s o f beryllium
67
decomposition temperature of the complex differs from those of any contaminating compounds, and (b) that such compounds are not decomposition products. Only under these conditions can the weights of uncombined hydroxides be determined and subtracted from the overall chemical analysis to give an unambiguous empirical formula. Unfortunately, the manner in which the compounds decompose thermally (see below) does not permit the assumption that the weights of uncombined contaminating hydroxides or carbonates of calcium or strontium can be calculated from weight losses occurring at their respective characteristic decomposition temperatures. TASL~ 4.--X-RAY P O W D ~ DATA FOR 2 C a ( O H ) , . 4 Be(OI-I)f.H,O (unit cell: h e x a g o n a l ; a = 9.68 A , c = 9.13 A )
I/1o
d
hkl
1/lo
d
hkl
vs m m ms vvw m m w m s w vw
8.34 6.15 4.82 4.27 4.16 3.99 3.80 3.17 2.98 2.86 2.79 2.66
100 101 110 111 200 102 201 210 211 103 300 301
m m m m w mw ms m mw vw ms vw
2.50 2.59 2.47 2"42 2"38 2"25 2"10 2.07 2.00 1.89 1"85 1.82
212 113 203 220 302 311 400 312 204 223 214 410
With regard to any free Be(OH)~ which could be a contaminant, there are grounds for concluding that negligible quantities have, in fact, been co-precipiated. It will be recalled that only the two crystalline modifications of Be(OH)z can normally be thrown out of beryUate solutions. The lines of either polymorph are absent on the powder photographs of the calcium and strontium complexes. That any contaminating Be(OH)~ would be crystalline, and hence detectable, is supported by the behaviour of beryUates during unsuccessful attempts to prepare magnesium and ferric iron hydroxy complexes of beryllium by similar double decomposition reactions. In one case orthorhombic Be(OH)a was precipitated with free Mg(OH)~ whereas the tetragonal polymorph was co-precipitated with iron hydroxide in the other. The greatest uncertainty, therefore, in the compositions given in Tables 2 and 3 lies in the Ca(OH)~ or Sr(OH)a content. Although their true composition may differ from that given, the wide stoichiometric range of these compounds is nevertheless evident, and differences are reflected in their X-ray spectra. The powder photographs of both series of complexes have many common fines. As previously reported, (1) the most characteristic line is a strong reflection at about 8.3 A. A conclusion from the earlier work was that calcium and strontium complexes having a Be:Ca or Sr ratio of approximately 4:1 had identical hexagonal unit cells (a ---- 14.9 A; c = 16.6 A.) It has become clear, however, from this more extensive survey that differences develop both within and between members of the calcium and strontium series as the ratio Be~+: M 2+ varies. It has not been found possible to index all the photographs.
68
R.A. MERCERand R. P. MILLER
The calcium complex 2 Ca(OH)2.4 Be(OH)~.H20 can be indexed (Table 4) on a hexagonal cell (a = 9.68 A; c --- 9.13 A) but as the B e : C a ratio increases in the series, extra lines appear which do not permit a fit on this cell. It was possible to index the compounds with high B e : C a ratio (2 Ca(OH)2"9 Be(OH)s'2 H20; (2 Ca (OH)z. 10 Be(OH)~.'3 H20 ) on a very large hexagonal unit cell where the ' a ' parameter is , ~ 30 A, but it is felt that this is most likely to be a pseudo cell, the true symmetry having been lowered. Similar difficulties were encountered in the strontium series. A representative sel of d values of the compound 2 Sr (OH)2.9 Be(OH)2 are given in Table 5. TABLE 5.~X-RAY POWDER DATA FOR 2
vw vvs s vs vs vs vvw vvw s s vvw vvs vvs s w vvw vvw s s vvw s s vvw vvw
Sr(OH)2"9 Be(OH)2
10.49 w 8.21 vsd 7-39 vsd 6-91 vvs 6.42 vw 6.10 s 5-50 vs 5"30 s 5-09 s 4-86 s 4"59 w 4.44 s 4.30 s 4.11 vvw 3.84 s 3-73 s 3.67 w 3.53 w 3-45 w 3.29 w 3.18 w 3"06 w 3-01 vw 2.93 + many other lines
2-86 2.78 , 2.73 2-68 2.61 2-50 2-44 2.38 2.31 2.24 2.19 2.12 2-10 2.08 2.04 2-01 1.89 1"85 1.83 1.82 1.73 1.70 1-62
Thermaldecompositionof the hydroxy complexes The pyrolysis curves and differential thermograms of the hydroxy complexes showed the stepwise nature of the dehydroxylation during heating. The features were similar within the calcium series and within the strontium series but differences were evident between the two sets of compounds. Typical curves representative of both types of complex are shown in Figs. 1, 2, 3 and 4. Both sets of compounds lost loosely-bound water up to 110°C but during the course o f further heating the calcium and strontium compounds showed marked differences in behaviour. T.G.A. (Fig. 1) shows that the calcium compounds lose water discontinuously. Between 100 ° and 200 °, 200 ° and 270 °, 270 ° and 300 °, inflections occur which indicate differences in the rate of change of weight loss. D.T.A. (Fig. 3) shows corresponding
The preparation and properties of some hydroxy compounds of beryllium
69
,.,.I
70
60
~d 50
.4 t ?
40 .~'
30
I
•
2O
,o
i.f* .,.-'T" I00
200
300
400
500
Tempers't'ure,
600 *C
700
800
900
10(30
FIo. 1.--Thermogravimetric analysis of 2Co(OH)~-4Be(OH),.H,O160.4nag).
endotherms in these temperature regions. At higher temperatures T.G.A. shows that weight losses occur corresponding to dehydroxylation of calcium hydroxide (400 °) and to decarboxylation of calcium carbonate (600°C). This last phase was always formed by uptake of atmospheric carbon dioxide. The pyrolysis curves (not reproduced) of both calcium hydroxide and strontium hydroxide showed the readiness with which this contamination occurs under the conditions of heating in the thermobalance. The calcium complexes show an additional endotherm during D.T.A. corresponding to dehydroxylation of calcium hydroxide, but no evidence of an endotherm associated with carbonate breakdown. It is assumed that the carbon dioxide uptake is negligible in the D.T.A. apparatus, where the samples are encased in a sheath. X-ray analysis of samples taken at various stages during T.G.A. showed that between 170 and 200 ° the calcium complexes were completely destroyed. The only crystalline products identified by X-rays at 200 ° were orthorhombic Be(OH)s, Ca(OH)z and CaCO3. 60
50 h~
_go +. 3 0
q
!
i
20
0
•
I00
200
300
400
500
Temperature,
600
700
800
900
I000
*C
FIo. 2.--Thermogravimetric analysis of 2Sr(OH)2.9Be(OH)I(115.3 mg).
70
R . A . M_~c~ and R. P. MnJ,~R
I<3
.)80
Temperoture,
°C
~o. 3.--Differential thermo~m of 2Ca(OH)a.4Be(OH)j.H=O.
~
0
j Temperature, °C
FIG. 4.--Differential thcrmogram of 2Sr(OH)=.9Be(OH)=.
The preparation and properties of some hydroxy compoundsof beryllium
71
90
80
/ 70
I'
J
6O
50
I
o
30
2O
/
~0
!. 1 300
400
500
Temperature,
600
700
800
°C
FIG. 5.--Thermogravimetricanalysis of tetragonal Be.(OH)2 (215 mg).
[
80
7O
1"/" / 60
{
i E o
/
I L
50
40
~ 30 20
I
l
I
I
I 0
I00
200
300
400
500
600
700
Temperature, °C
FIG. 6.--Thermogravimetricanalysis of orthorhombicBe(OH)s (169.6 mg). Dchydroxylation of orthorhombic beryllium hydroxide begins at 220°C. T.G.A. data on both polymorphs are separately shown in Figs. 5 and 6. It is of interest to note from both these sets of data that the initial decomposition of the tetragonal polymorph appears to be significantly lower than the stable orthorhombic variety, and that both forms are dehydroxylated in two distinct stages. The bulk of the water is quickly lost up to 280°C, when the overall composition approximates between 6 B eO.HsO and 7 BeO.HsO. Thereafter the remaining water is not eliminated until
72
R.A.
M E R C ~ a n d R. P. Mmt.~R
500°C. These results are in substantial agreement with an earlier pyrolysis curve recorded by Duval using an unspecified form of beryllium hydroxide.tS~ The origin of the endotherm beginning at 280°C (Fig. 3) on the differential thermogram of the calcium complex has not been fully resolved in this study. By analogy with the behaviour of the strontium compounds described below, the heat effect may be associated with the decomposition of an intermediate basic complex which in this case is amorphous and hence not detectable by X-rays. No further interaction occurs at higher temperatures between the B e t and C a t produced from the thermally decomposed hydroxy complex. In a separate experiment T.G.A. of intimately mixed Ca(OH)z and orthorhombic Be(OH)2 confirmed the independent dehydroxylation and non-interaction of the simple oxide products. T A m ~ 6.--X-RAY POWDER DATA ON INTERMEDIATE COMPLEX 2 SrO.9 BeO.7 H s O F O ~ D BY ~ N ¢ 3 2 Sr(OH)~'9 Be(OH)a TO 220°C (unit cell; hexagonal, a = 11.02 A, e = 8.54 A) d
X/Zo
d
(A)
hkl
I/I0
(A)
hkt
s
4"77
200
s
2.08
402 410
m w rn
4"29 4.17 3.37
002 201 112
ms s m
1-95 1.91 1.80
204 500 420
s
3'18
{300 202
w
1"69
332
s
2.65
310
vw
1-59
600
vw
2"45
203
w
1"51
/305 1521
m s
2"39 2"14
400 004
ms
1"42
006
The behaviour shown by strontium compounds on thermal decomposition was more complex. The compound chosen for systematic study was well-crystallized 2 Sr(OH)2"9 Be(OH)v After an initial loss of water no weight change occurs until 180°C. T.G.A. (Fig. 2) then shows a steep rise in the rate of decomposition which is accompanied by a large endotherm during D.T.A. Microscopic examination showed that the decomposition between 180°C and 250°C was attended by pronounced exfoliation. X-ray analyses at 220°C showed that all trace of the original complex had disappeared. The powder data, however, showed no evidence of Be(OH)s, Sr(OH)z or SrCO a. All lines of the product phase, which must be regarded as a new complex basic oxide of empirical formula 2 SrO.9 Bet.7 H~O, could be indexed satisfactorily on a hexagonal unit cell with a = 11"02 A, c = 8.54 A (Table 6). Between 250°C and 300°C a change in rate of dehydroxylation was accompanied by a small endotherm (Fig. 4). These effects were associated with the decomposition of the hexagonal phase. The only crystalline products identified between 300°C and 600°C were strontium carbonate and beryllium oxide. The carbonate decomposed at 830°C to give strontium oxide. ~ R. FmCKE a n d H. H f f M ~ , Z. anorg. Chem. 178, 406 (1929). ~5~ T. DOVAL a n d C. DucAL, Anal. Chim. Act. 2, 53 (1948).
The preparation and properties of some hydroxy compounds of beryllium
73
In contrast to the calcium systems, further interactions occurred at higher temperatures between these freshly-formed simple oxides. X-ray analysis of the product at the completion of T.G.A. to 1000°C showed that no free SrO or BoO was present. Interaction had occurred to give a new phase, crystallizing with a hexagonal unit cell, the parameters a----4.60 A and c = 8"94 A being consistent with the data (Table 7). This phase was produced by all the other strontium-beryllium complexes on heating to lO00°C. T A B L ~ 7.--X-RAY P O W D E R
DU~CTION
D A T A O N MIXED OXIDE PHASE
2 SrO.9 B o O FORMED BY HEATING 2 Sr(OH)z'9 Be(OI-I)s TO 1000°C
(unit cell; hexagonal a ----4.60 A., c = 8.94 A)
1]1o
d (A)
hkl
I]Io
(A)
d
hkl
rn ms s s s ms m vs w ms
4.46 3.98 3.63 2.97 2.38 2.30 2,23 2,04 1,99 1.94
002 100 101 102 103 110 111 112 200 201
m m m ms w ms m m s s
1.81 1.65 1.63 1.60 1.50 1.48 1.42 1-39 1.34 1.33
202 203 105 114 120 204 212 106 213 300
When a mixture of Sr(OH)~.8 H~O and orthorhombic Be(OH)~ were heated together under similar conditions to those described above, neither the hexagonal basic hydroxy complex nor the compound beryllium-strontium oxides were detected. This suggests that the formation of these phases from the original hydroxy complexes proceeds topochemically. As the mixture of hydroxides was progressively heated Sr(OH)~.8 HsO loses the eight molecules of water below 100°C. Be(OH)~ then decomposes at 220°C but additional lines to those of BeO and Sr(OH)~ were present on the X-ray photographs, these were not identified as belonging to any phase characterized in the decomposition of the hydroxycomplexes. Their origin was not investigated in view of the transient nature of their existence. The phase or phases responsible had decomposed by 500°C. At this temperature Sr(OH)a decomposes but carbonate is readily formed until its decomposition temperature of 830°C is reached. The rehydration of the oxide produced was so rapid that X-ray photographs of the ignited products showed only St(OH)2 and in some cases Sr(OH)~.8 H~O along with BeO. The X-ray powder data of anhydrous Sr(OH)2 is given in Table 8 as it is not included in the A.S.T.M. index and it does not appear to have been previously published. DISCUSSION
The polymorphic forms of beryllium hydroxide The methods of preparation and stability relationships between the polymorphic forms of beryllium hydroxide have been the subject of several studies. Until recently the structural differences have been obscure, and early literature has been confused by the indiscriminate use of ' £ and 'fl' in describing stable and metastable forms. Beryllium hydroxide freshly precipitated by addition of alkali to acid solutions is
74
R.A. MERCERand R. P. MILI2~
amorphous, but it was noted by early workers t4~ that transformation to differing crystalline forms occurred progressively on ageing. The process was accelerated by boiling. The first modification to be produced was called by these authors 'metastable ~t form'. This passed, on months of standing, to the 'stable fl form'. This last variety was also found to be precipitated from hot concentrated alkaline solutions of beryllium hydroxide. TABLE8.--X-RAY POWDERDATAOF SF(OH)II Ilia
d
I/Io
d
s s ms s ms vw ms ms ms vw
6.188 4.530 3.652 3.351 3.142 2.951 2.842 2.811 2.467 2"358
w w(d) m w w vw(d) mw w w w
1.736 1"691 1"622 1.602 1"582 1"547 1.529 1"480 1"427 1"405
s
2"288
w
1"378
s m vw(d) w m w
2.225 2.104 2.058 1"971 1"821 1"751
w w w w w(d)
1"341 1'311 1"284 1.265 1.234
The crystal structure of this form was solved by Seitz et al.te) It was shown to have an orthorhombic cell (a = 4.61 fl~; b = 7"02 A; c = 4.52 J~) in which the Be~- ions are surrounded tetrahedrally by O H - ions but, unlike the majority of the hydroxides o f bivalent metals, it is not a layered structure. In this respect it is analogous to Zn(OH)9, the stable form of which is isostructural with orthorhombic Be(OH)v More recently, ~7~the powder data of the metastable form, precipitated by hydrolysis o f beryllium chloride solutions with urea, has been indexed on a tetragonal cell (a = 10.88 A; c = 7"83) but no structure analysis appears to be available. The reported value ~s~ of the heats of formation of the two polymorphs imply that the energy differences between them are negligible: A H 0 (orthorhombic)---216.1 K cals/mol; A H 0 (tetragonal)--216.8 K cals/mol. It has been shown c~ that direct precipitation of the stable modification can be achieved from acid solutions of basic beryllium acetate by very slow hydrolysis involving a gradual increase o f p H from 2.7 to 5.3. The present work shows that either polymorph can be precipitated from alkaline solution by suitable choice of hydrolytic conditions. These facts dispel the view which has been advanced that the formation of the metastable form arises by inclusion of foreign anions which are substituted for O H - ions, so becoming an integral part of the structure. ~e~A. S~rrz, U. R0slam and K. SCHUBERT,Z. anorg. Chem. 261, 94 (1950). c7~W. J. KmKPATRICK,G. R. ANDERSONand E. S. FtmsTou, General Electric Report, APEX-684. TID-4500, UC-4 (Chemistry), (1961). n~ R. FItlCKEand B. WULLHORST,Z. anorg. Chem., 205, 127 (1932).
The preparation and propertiesof some hydroxycompoundsof beryllium
75
The practicalimplicationsof being able to choose the hydrolyticconditions which will precipitatethe orthorhombic form centre on the density differencebetween this form and the lightertetragonalvariety,which is more difficultto filter.Furthermore, ifthe berylliaformed by ignitingthe hydroxide isrequiredto be compacted for ceramic purposes, the denser form of the hydroxide is desirable. This arisesfrom the topotaxial nature of dehydroxylation~9)inasmuch that there is a structuraland orientational relationshipbetween products and reactants. In the case of hydroxides the loss of water tends to leave a porous pseudomorph. The volume of the intersticeswill obviously be larger if the originalhydroxide is of a more open variety. It has been pointed out that during sintering all voids between particlesbecome filledmore readilythan internalvoids,which tend to persist,in the individualparticles. The hydroxy complexes o f beryllium
The existence of such compounds was first established during earlier studies on the precipitation reactions of sodium beryUate solutions. CI) It was shown that they formed as double decomposition products when calcium or strontium chloride solutions were added to the beryUates. The extent of the reaction in terms of the quantity of beryllium precipitated as complex was shown to depend on the NasO: BeO ratio used in preparing the beryUate solutions. This ratio is a convenient way of expressing what is effectively the OH-:Be ratio in solution. It was shown that the properties of these solutions depended both upon this ratio and upon the overall OH- concentration. Their behaviour could be explained qualitatively if it was assumed that isopolyanions of the type
o.
7"1
_
_
existed in solution, where n depended upon the OH-: Bea+ ratio and OH- concentration. Thus at low Na20:BeO ratios (,~1-5:1) and below 1N overall NaOH concentration the solutions were unstable. They exhibited a Tyndall effect, implying that they were colloidal hydrosols, and they precipitated tetragonal Be(OH)~ on standing at room temperature. At high OH- :Be2+ ratios no hydroxy complexes are precipitated and the solutions are thermally stable (see Table 1). This stability was considered to coincide with the increasing predominance of the stable monomeric ion
o.\ /o. 1 oHj
On the basis of sodium beryllate solutions being an alkali-dependent polymerization system it is to be expected that some relationship will hold between the hydrolytic equilibrium and the composition and constitution of the precipitated complexes. ~*~L. S. DE~¢r-Gi.~s~R, F. P. G ~ R and H. F. W. TAYLOR, Q~rt. R~. Vol. X3/I, No. 4, 343 (1962).
76
R . A . MERCERand R. P. MILLER
Such a relationship is implied by the relatively wide stoicheiometric range of compounds produced, but the exact constitution o f both the beryllate ions in solution and the complexes themselves are still uncertain. The structure of the majority of the hydroxy salts of bivalent metals is based upon octohedral co-ordination of the metal ions in layered lattices, o°} Mg(OH)2 or Ca(OH)~ are typical end-member types. Modifications include double layering and the development of stacking faults which give rise to various deviations, but the basic structural principle is established. The crystal class into which such compounds crystallize is usually hexagonal but some are monoclinic or triclinic The powder photographs of all the compounds described in this study are basically similar, but the inability to index satisfactorily the calcium and strontium compounds of high Be:Ca or Sr ratio on hexagonal cells suggests that the excess of the small ion with its high polarizing power has distorted the structures to lower symmetry. It was suggested earlier{1} that hydroxy-bridged bands of beryllium ions could provide a possible back-bone of the structures, but single crystal studies would be necessary to substantiate this.
Acknowledgement--Theauthors wish to thank Mr. J. REEWfor his contributions to the experimental work. ct0} W. FErrgh~orr (Translated by F. HUDSWELL)A.E.R.E. Lib/Trans 622 (1956).