Zeitschrift für Kristallographie, Bd. 137, S. 24—34 (1973)

Al-Ga and Si-Ge diadochy in syntheticBaAl2Si208 and SrAl2Si208

By G. Gazzoni

III Sezione del Centro Nazionale di Cristallografia del C.N.R.Istituto di Mineralogia e Geochimica dell'Università di Torino

(Received 24 May 1972)

AuszugDer vollständige Ersatz von AI durch Ga und von Si durch Ge in den

synthetischen Verbindungen BaAl2Si208 und SrAl2Si20s wurde an polykri-stallinen Proben und einzelnen Kristallen mittels Röntgeninterferenz-Methodenuntersucht. Jede der Verbindungen hat entweder eine monokline oder einepseudorhombische stabile Modifikation ; für die Ba-Verbindungen ist die mono-

kline Phase die stabilere, für die Sr-Verbindungen die pseudorhombische.BaGa2Ge2C>8 und SrGa2Si208 kommen in beiden Modifikationen vor; die pseu-dorhombische ist jedoch stabiler. Die monoklinen Modifikationen sind isostruk-turell mit dem Ba-Feldspat Celsian und dem Sr-Feldspat, die pseudorhombischenmit dem Paracelsian. Die monoklinen Kristalle weisen stets eine Überstrukturmit c r~ 14 Â auf. Beide Modifikationen haben Gitterkonstanten, die mit einergeordneten Tetraederabwechslung vereinbar sind. Uber die Bestimmung derGitterkonstanten und der Raumgruppen wird berichtet.

AbstractThe complete substitution of Al-Ga and Si-Ge ions in synthetic BaAl2Si20s

and SrAl2Si20s was studied. Polycrystalline samples and single crystals were

investigated by x-ray diffraction methods. The compounds present either a mono-

clinic or a pseudo-orthorhombic stable phase; for Ba compounds the more

stable phase is the monoclinic, for Sr compounds the pseudo-orthorhombicphase; BaGa2Ge20g and SrGa2Si2Os exhibit both modifications, the pseudo-orthorhombic being the more stable. The monoclinic modifications are isostruc-tural with Ba feldspar (celsian) and Sr feldspar; the pseudo-orthorhombicmodifications are isostructural with paracelsian. Monoclinic crystals constantlyshowed a superstructure with c r~ 14 Â. Either modifications have unit-cellparameters compatible with an ordered alternation of tetrahedra. Unit-cellparameters and space group determination are reported.

Al-Ga and Si-Ge diadochy in synthetic BaAl2Si20s and SrAl2Si208 25

IntroductionA number of systematic investigations has been carried on during

the last few years, in this Institute, on the isopolymorphic relationships,and on the stability and the structural features of the feldspar phase,in the synthetic aluminosilicates having general formula RA^SiaOs.The crystallochemical effects due to a controlled substitution of thebivalent cation have been examined.

As regards the Sr and Ba compounds, it was possible to ascertainthat the diadochy Sr-Ba is complete in the feldspar phase (Bruno andGazzoni, 1969) and to confirm that also the monoclinic modificationof SrAl2Si2C>8 (Sr feldspar) has a superstructure with c ~ 14 À andspace group I2/c (Bruno and Gazzoni, 1970) as the correspondingmodification of BaAl2Si208 (Ba feldspar) (Gay, 1956; Nbwnham andMegaw, 1960).

We have now extended the investigation to the substitution ofAl-Ga and Si-Ge ions in the above two aluminosilicates. The studyof such compounds is not only intrinsically interesting for noticingthe effects due to a substitution in the anionic group on the stabilityof phases having a feldspar-like structural framework, but also forthe possibility of a deeper interpretation of order-disorder structuralphenomena. On this topic we remind the work of Goldsmith andLaves (1955) on the degree of Al-Si ordering in Ca feldspar revealedby the Al-Ga and Si-Ge substitution.

As quoted above, the peculiar feature of the feldspar phase ofBaAl2Si208 and SrAL^Os (and generally of aluminosilicates withAl/Si ratio of 2/2) is a superstructure leading to a c parameter 14 Àlong, namely to a unit cell which consists of two subcells (c ~ 7 â)differing in the detailed atomic coordinates and gives rise to weakersuperlattice reflections (type b reflections). It has not yet been ascer-

tained whether these differences in atomic coordinates are a conse-

quence of a thorough Al-Si ordering (Newnham and Megaw, I960)1.A limitation in determining the ordering degree of Al and Si by

an x-ray crystal-structure anatysis is set by the similarity of the atomicscattering factors of these two atoms which prevents a complete deter-mination of their relative positions. On the other hand, if the Al-Gaor the Si-Ge diadochy is possible, a crystal-structure determinationof the derivative compounds permits to detect their structural

1 A crystal-structure determination of synthetic Sr feldspar is being carriedon in this Institute.

26 G. Gazzoni

order-disorder and to infer considerations on the ordering state of theoriginal aluminosilicates. Several examples may be found in theliterature either of partial or total Al-Ga and Si-Ge substitution infeldspar, besides those of the already quoted Goldsmith and Lavesarticle (e.g. Goldsmith, 1950; Pentinghaus and Bambauer, 1971a,1971b); in particular, a complete miscibility inside the four-componentsystem of the barium alumino(gallo)silicates(germanates) was provedby Grebenshchikov (1963). In the present investigation these lattercompounds were re-examined, with the corresponding Sr terms, forcompletenes' sake and for a comparative discussion of the structuralaspects; the general crystal-chemical properties and preliminarystructural data are herein reported for the fully substituted compounds.

Experimental proceduresThe samples were sjmthetized both by reaction in the solid state

and by fusion and successive crystallization of stoichiometric mix-tures of analytically pure oxides and carbonates (SrCOs, BaO, AI2O3,Ga203, Si02, Ge02). The reactions in the solid state were carried outat graduated temperatures and time intervals; the products obtainedwere used for the identification and the definition of the relativestability fields of the principal polymorphic modifications of the singlecompounds. In the products of crystallization from the melt thepossible high-temperature metastable modifications were looked for,and from the polycrystalline bulks single-crystals of the variousmodifications were isolated. The syntheses were run in silicium-carbideresistivity ovens and the temperature measured with a Pt-PtRhthermocouple, embodied in the sample-holder crucible.

The volatilizability of Ge02 imposed severe restrictions abouttimes and temperatures of sjmthesis of Ge compounds, particularlyin the case of solid-state reactions. In the fusion syntheses, the highfluidity of the melt and the quick crystallization allowed good homo-geneous crystallizations even in very short times.

The x-ray investigations [A(Cu/iT«) = 1.54178] were performed, forpolycrystalline samples, with a Guinier-de-Wolff camera (Enraf-Nonius) ; the spectra were indexed by comparison with those of thecorresponding aluminosilicates; an evaluation of the degree of crys-tallinity and purity of the phases was also attempted.

Unit-cell parameters were computed and refined by a least-squaresprocedure; an adequate number of 6 values was measured on thepowder spectra of the best crystallized compounds using quartz as

Al-Ga and Si-Ge diadochy in synthetic BaAUSiäOs and SrAl2Si20s 27

internal standard. The possibility of parameter variations as a func-tion of the thermic state was not considered. The single-crystal x-rayanalyses were made using precession and Weissenberg methods; thespectra were analysed for checking the indexing of powder spectraand especially, in the case of monoclinic phases, for seeking thepresence of superlattice reflections and to confirm the choise of a unitcell with c~ 14Â. The determination of space groups completed theinquiry about the isostructurallity of several compounds.

Crystal dataTable 1 summarizes the results of our investigations on the poly-

morphism and the phase stability of the several compounds. Theresults, as regards the Ba compounds, are generally in agreement withGrebenshchikov (1963) remarks, although we did not obtain thereversion from the pseudo-orthorhombic to the monoclinic modifica-tion for BaGa2Ge2Ü8.

Under our experimental conditions, the monoclinic modificationboth of BaGa2Ge208 and SrGa2Si20s was obtained only from crys-tallization of the melt. This modification turned out to be metastable,

Table 1

compound synthesistemperature phase state

BaGa2Si308

BaAl2Ge208

BaGa2Ge208

SrGa2Si208

SrAl2Ge208

SrGaaGeaOs

1100 -f- 1420°C

1200 -r- 1550

cf. Table 2

cf. Table 2

1200 -r- 1520

1000 1300

monoclinic

monoclinic

monoclinic

monoclinicpseudo-orthorhombicmonoclinic

monoclinicpseudo-orthorhombicmonoclinic ?

orthorhombic

monoclinicpseudo-orthorhombic

stable

stable

metastable

stable

metastable

stable

metastable

stable

stable

28 G. Gazzoni

nevertheless the transition looks peculiar : on one hand the compoundscrystallize from the melt in the monoclinic phase and retain thisphase under whatever kind, even very slow, of cooling, on the otherhand the transition to the pseudo-orthorhombic phase occurs, quickly,on a subsequent heating of the sample. The most significant resultsdue to thermal treatments are reported in Table 2. By reaction in thesolid state a modification of SrA^GeaOs was obtained which shouldbe the monoclinic one; the crystallization was, however, not pureenough to permit a conclusive determination of this phase.

The data, from powder spectra, regarding the monoclinic modifica-tions are listed in Table 3 ; the presence of superlattice reflections oftype b, on single-crystal long-exposure photographs, confirmed the

Table 2

compound startingmaterial temperature tune phase

SrGa2Si208a

bc

de

f

Im

u

BaGa2Ge208a

bc

de

fgh

oxidesoxidesoxidesoxidesoxidesoxidesoxidesoxidesoxidesoxidesoxidesh,i,l,mh,i,l,m

oxidesoxidesoxidesoxidesoxidesoxidesoxidesoxidesoxidesoxidesg.h.ig>h>i

1200°C1300135014001200130013501400-* 251400^13001400->13001400^-100012001300

960960

11001260960

110012601300^- 251300^ 9601300-^1200960

1200

1 h111

1201201207265

1I1I

1212

(11

727272726121

pseudo-orthorhombicpseudo-orthorhombicpseudo-orthorh. + glass

pseudo-orthorhombicpseudo-orthorhombicpseudo-orthorh. -f glassmonoclinicmonoclinicmonoclinicmonoclinicpseudo-orthorhombicpseudo-orthorhombic

pseudo-orthorhombicpseudo

-

or thorhombicpseudo-orthorhombicpseudo-orthorh. + glasspseudo-orthorhombicpseudo-orthorhombicpseudo-orthorh. + glassmonoclinicmonoclinicmonoclinicpseudo-orthorhombicpseudo-orthorhombic

Al-Ga and Si-Ge diadochy in synthetic BaAl2Si20s and SrAl2Si208 29

Table 3. Monodinic phases

SrGa„Si„0Q BaGa_Si_0c

5.874.634.183.8273.8053.6453.5343.4853.312

3.2623.018

I ) 2.933

2.787

6.63

5.954.694.293.9583.8533.6753-5993.5233.3963.324

) 3.314

3-066

2.9612.816

6.7»

6.014.734.323.9973.8903.7143.6273.551

4.374.0363.9403.7573.6683.5943.469

3.136

3.026

2.874

3 1 42 4 2

1 1 4

3 1 0

T 5 2

3 3 2T 1 6

2261 5 2

0600063 1 62 4 2Ï 5 44 0 2T 5 4

2-5712.5052.4382.3732.338

2.2372.222

2.1262.097

BaGa„Si„0„ BaAl„Geo0o BaGanGe„0„

2.6372.622

2.5872.4552.4272.368

2.294

2,26l2.246

2.1602.141

2.655

) 2.643

2.6082.484

2.4502.385

2.314

2.2832.271

2.683

) 2.675

2.6372.5092.4762.415

2.342

2.3082.298

2.202

2. 18B

Table 4. Pseudo-orthorhombic and orthorhombic phases

SrAl„Ge„0D

4.003.963.7703.7513.6843.6543.5533.5263.2673. 1493.0933.058

[ ) 2.961

2.9382.9102.865

4.11 1

4.02 10

3.798 8

3.708 3

3.581 73.299 10

3.182 3

3.110 1

3.008 6

2.9572.899

SrGa„Geo0„

4.074.043.8313.8163.7513.7323.6193.5963-3313.2023.1533.123

3.040

3.014

2.9922.9712.925

4.234.14 )'0

3.8053.6963.6843.3993.2853.209

3.091

3.0523.0322.978

2 3 0

2 2 2T 1 32 2 21 l 33 2 02 3 12 3 1

Ï321 3 2

2 0 33 1 2

2 0 30 4 0

3 1 21" 2 31 2 32 1 3

SrGo„Si„0,

2.721 2

2.702 32.686 2

) 2.569 4

2.537 5

) 2.468 1

) 2.437 1

2.392 1

2.379 1

) 2.370 1

2.354 2

2.335 1

) 2.320 2

) 2.296 1

SrGa„Ge„0o

2.7402.721

2.562

2.485

2.4062.3882.392

2.356

2.335

2.7732.7612.741

I ) 2.620

2.588

2.520

2.480

2.4372.423

2.4092.380

2.3462.334

2.821

I ) 2.805

I 2"695 \

I ) 2.681'

2.635

2.570

2.539

2.479

I 2.471

2.458

) 2.429

2.410

| ) 2.396

correct indexing of powder spectra on the assumption of a doubleunit cell with c ^ 14 A. Likewise, Table 4 presents the measurements,done on powder spectra, regarding the orthorhombic and pseudo-orthorhombic modifications. The doubling of hkl, hkl reflections, dueto a deviation from strict orthorhombic symmetry, was distinctlyobserved in the spectra of SrGa2Si20s and SrGa2Ge208. The splittingin the case of BaGa2Ge20s was just within the separation power of thecamera while no doubling was noticed for SrAl2Ge20s- Comparison ofintensities, on single-crystal photographs, confirmed that (100) and

30 G. Gazzoni

Table 5

Compound formulaweight

SrAl3Si2081

SrGa2Si208metastable

BaAlsS^Og1

BaGa2Si208

BaAl2Ge208

BaGa2Ge208metastable

SrAl2Si2082SrGa2Si2Os

SrAl2Ge208

SrGa2Ge208

BaAl2Si2083

BaGa2Ge208

8.386Â0.008

8.4810.009

8.6380.002

8.7270.005

8.7990.005

8.8980.006

8.910

9.0100.005

9.1130.005

9.2100.008

9.0760.005

9.3490.006

12.972Â0.010

13.1330.010

13.0430.002

13.2400.006

13.3710.006

13.5280.006

9.343

9.4880.006

9.5590.005

9.6660.008

9.5830.005

9.9030.006

14.275A0.012

14.4800.011

14.4010.003

14.6080.006

14.7270.006

14.9060.006

8.345

8.4160.004

8.5150.004

8.5700.007

8.5780.005

8.7700.005

115.130.13

115.390.14

115.090.03

115.000.06

114.930.07

114.870.06

90.590.06

90.00

90.560.10

90.000.17

90.360.07

1406Â3

1457

1469

1530

1571

1628

719

742

763

812

3.08

3.75

3.40

4.00

3.93

4.49

3.11

3.80

3.71

4.35

3.35

4.50

325.77

411.25

375.50

460.98

464.50

549.98

411.25

411.77

500.25

549.98

1 Bruno and Gazzoni (1969); 2 Barrer and Marshall (1964).(1953).

P2i/a(Pnom)(Pna2i)

Smith

(001) planes are pseudo-symmetry planes for compounds SrGa2Si208,SrGa2Ge20"8 and BaGa2Ge20s. Attempts to isolate a single crystal ofSrAi2Ge208 were unsuccessful.

The strict analogy among powder spectra, as well as among single-crystal photographs, and the presence of the same classes of extintions,

Al-Ga and Si-Ge diadochy in synthetic BaAl2Si208 and SrAl2Si20s 31

gives evidence that all the monoclinic modifications are isostructuraland likewise that all the pseudo-orthorhombic modifications are

isostructural.In the spectra of the monoclinic crystals the reflections hkl with

h + k +1 odd and hOl with h and I odd are absent ; there is an ambiguitybetween space group Ic and I2/c.

The spectra of the pseudo-orthorhombic crystals do not presentthe following classes of reflections: hOl with h odd, OkO with k odd,001 with I odd. A few and very weak Okl reflections with k-\-l oddwere observed on log-exposure photographs. To these modificationsare therefore attributable space group P2ija and pseudo space groupPna2L or Pnam. A summary of crystal-data is reported in Table 5where, for an easy comparison, also the data regarding BaAl2Si208and SrAl2Si208 are included.

DiscussionThe inspection of our experimental data allows the deduction of

some general considerations on the trend of the chemico-physicalproperties of the compounds studied. The most interesting aspectsof the results concerns the isopolymorphic interrelations between thecompounds in question with the corresponding aluminosilicates and,at the same time, the stability fields of the phases (cf. Tables 1 and 5).

The crystals present either a monoclinic or an orthorhombic stablephase (for simplicity's sake we shall use the concise expression "or-thorhombic phase" for indicating compendiously true- and pseudo-orthorhombic phase). The monoclinic modifications are isostructuralwith the corresponding monoclinic forms of BaAl2Si20s (celsian)(Newrham and Megaw, 1960) and of SrAl2Si20s (Bruno and Gazzoni,1970), while the orthorhombic modifications are isostructural withparacelsian (Smith, 1953), orthorhombic metastable phase ofBaAl2Si208 (Lin and Foster, 1968)2.

Either modification has a feldspar-like structure; this morphotropicside of the problem has been stressed by Smith (1953) in his work on

paracelsian structure. The transition from one to another modificationcan be easily related to the unit cell: the idealized transformationfrom a monoclinic to an orthorhombic cell is sketched in Fig. 1 andthe parameters characterizing the common cell, for the several com-

pounds, are reported in Table 6.2 We must here remind that Babbeb and Marshall (1964) succeeded in

synthetizing hydrothermally an orthorhombic phase of SrAl2Si20s.

32 G. Gazzoni

In both structures the substitution Al-Ga and Si-Ge is diadochicand tend to shift the stability from the monoclinic to the orthorhombicphase. For Ba compounds the monoclinic phase is still the more stable,

Table 6

SrGa2Si20ä monoclinic

a = 13.278 Âb = 13.133c = 8.481ß = 99.86°

SrGa2Si208 pseudo-orthorhombica = 13.084 Ab = 13.084c = 8.416y = 92.96°<x = 89.59ß = 89.59

BaGa2Ge208 monoclinic BaGa2Ge208 pseudo-orthorhombica = 13.777 Â a = 13.619 Âb = 13.528 b = 13.619c = 8.898 c = 8.770ß = 101.00° y = 93.30°

<x = 89.75ß = 89.75

Transformation matrix [u v w]from common to monoclinic cell from common to pseudo-orthorhombic cell

1 0 Ï [ 1 1 00 10 1 I 01 0 0 0 0 Î

Al-Ga and Si-Ge diadochy in synthetic BaAl2Si20s and SrAl2Si20s 33

while for Sr compounds the orthorhombic phase is the more stable;compounds BaGa2Ge20s and SrGa2Si20s may be considered thetransitional terms in this exchange of stability field.

In crystallochemical words, the larger Ba ions should supporta stable monoclinic phase even with a framework expanded by theintroduction of Ga and Ge tetrahedra. With the smaller Sr ions, theframework should assume the orthorhombic symmetry in order tofulfil the requirements of having an energetically favourable structure.It follows therefore that the phase with a smaller specific volume ismore stable, in agreement with the known thermodynamio and struc-tural aspects of polymorphism.

The small difference between the specific volumes of the mono-

clinic and orthorhombic phases of BaGa2Ge20s, could possibly explainthe disagreement between the results of Grebenshchikov (1963) andours about the relative stability of these two phases.

As regards the ordering degree of the tetrahedra, only a detailedstructural analysis of the several crystals, which we have just initiated,shall permit to establish it definitively. Anyway, the single-crystalspectra of the monoclinic modifications analyzed by us constantlyshowed the superlattice reflections of type b and these crj^stals havetherefore the superstructure of the corresponding aluminosilicatesand a c parameter 14 â long; such a value is the necessary conditionfor having an ordered alternation of Al (or Ga) and Si (or Ge) tetra-hedra in the monoclinic unit cell (Newnham and Megaw, 1960). Onthe other hand, we remind that also the dimensions of the orthorhombicunit-cell parameters are comparable with an ordered disposition(Smith, 1953).

AcknowledgementsIt is a pleasure to thank Prof. G. Rigault for useful discussions.

I am indebted to Dr. E. Bruno, Dr. M. B. Calleri and Dr. G. Fer-raris for their assistance; in particular to Dr. N. Caldoro for hissupport in the synthesis and chemical-analysis works.

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