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Investigations on the high-pressure varieties of GaAsO4
Volume
IO, number
MATERIALS
1,2
Investigations
on the high-pressure
September
LETTERS
varieties
1990
of GaAsO,
S. Matar, M. Lelogeais, D. Michau and G. Demazeau Laboratoire de Chimie du Solide du C.N.R.S., UniversitP Bordeaux I, 35 I cows de la Lib&ration, 33405 Talence Cedex, France Received
11 June 1990
This work reports on (i) the stabilization pressure varieties for GaAsO,: monoclinic,
of a-SiO,-type GaAsO, and AlAsO, using gas-medium pressure orthorhombic (CrVO,-type) and tetragonal (rutile-type).
1. Introduction Ternary oxides of formulation ABO4 (A = Al, Ga, .... B=P, As, V, ...) have been extensively studied [ 1,2] due to the possible change of coordination number versus pressure inducing new structures of interest in the domains of geophysics and materials science. Nevertheless very few studies were devoted to the oxide GaAsO,. Investigations on GaAs04 at room temperature [ 3,4] did not report on the possibility of the existence of high-pressure varieties of this ternary oxide. GaAs04 has a hexagonal structure closely related to that of a-SiO*, with the lattice parameters ahex= 5.00 A and chex= 5.68 8, [ 11. On the other hand, a-SiO, presents a high-pressure form called stishouite [ 51 with a structure close to the rutile-type. The present work reports on the results of our study of the transformations under high pressure of aGaAsO,. Furthermore, Al (III) and Ga (III ) having close behaviors regarding their electronic configurations (closed-shell configurations: Al (III ) corresponds to Ne, Ga(II1) to Ar 3d”), the isostructural AlAsO, compound has also been studied.
2. Experimental The a-SiOz forms of GaAsO, and been prepared either starting from a mixture of the oxides Ga,O, and Asz03 Asz03 under a pressure of dry oxygen 0167-577x/90/$
03.50 0 1990 - Elsevier
AlAsO, have homogenized or A1203 and gas, or from
Science Publishers
and (ii) three high-
a homogenized mixture of the oxides GazOj and Asz05 or A1203 and As205 under a pressure of a mixture of dry oxygen-nitrogen ( 1:9) gases. The use of gas pressure involves two complementary aspects: - the thermodynamic parameter pressure (oxygen or nitrogen) is used to hinder the sublimation of arsenic oxide (sublimation point of As203 z 190°C) in order to preserve the stoichiometry Ga(or Al):As=l:l; - the use of oxygen gas pressure, besides its ability to increase the oxidation state, allows the oxidation of As( III) into As(V) and favors this unusual valency state. The reacting materials As20s, As205, Ga203 and A1203 of 99.9% purity were handled in a glove box (problems of toxicity of arsenic oxides, AsZOS being, moreover, moisture sensitive). The gallium or aluminium oxides mixed with arsenic oxide weighed in stoichiometric proportions were introduced in a gold tube (chemical inertia). The tube was then put into the high-pressure reaction vessel. The experimental P, T conditions were as follows: P=O. 1 GPa, T~800”C, t=4 h. The starting and reacted materials were characterized by X-ray diffraction (XRD) using Cu-anticathode (A= 1.5418 A). Since the phase transitions under pressure of the ABO, compounds occur for pressure values higher than 3 GPa [ 6 1, a belt-type press [ 7 ] was used for the investigations of the high-pressure transformations of a-GaAsO, and a-AlAsO,. In order to avoid the reduction of As( V ) under the high-pressure-high-
B.V. (North-Holland
)
45
Volume 10, number I,2
MATERIALS LETTERS
temperature conditions, KClO-, was used as an oxygen source [ 7 1,
3. Results and discussion 3.1. XRD characterization of ar-GaAs@ and cyAlASO, The XRD patterns of the reacted powders under gas pressure were indexed in the hexagonal system of a-Si&. Extra lines of weak intensity belonging to EGa203 were identified in some experiments showing a slight sublimation of arsenic oxide (found in the upper cold zone of the reaction vessel). Other extra lines not belonging to any of the starting materials and that could not be indexed in the a-SiO, type structure were detected in both patterns of GaAs04 and AlAsO,. The existence of the extra lines led to an indexation in a superstructure: a’ =a0 and c’ = 2~ ( a0 and co being the cell parameters of the uSiO,-type st~cture ). The obtained lattice parameters given below, are in fairly good agreement with data from the literature [ 31: for GaAsO, , a=5.000-t0.002
experiment was done with GaAs04 in the same pressure and temperature range in order to stabilize the high-pressure variety. After temperature quenching, two phases were identified by XRD: a rutile phase and another phase. Indexation of the rutile phase was done using remaining KC1 as internal standard. Such a pattern was indexed using a supercell with a doubled c parameter with respect to the simple rutile-type, i.e. a =4.367 20.003 A, c= 5.640~0.005 8. In order to compare its high-pressure behavior, a-AlAsO was investigated in the same pressure and temperature range (P;z: 9 GPa, Tx 9OO*C). The XRD lines of the rutile variety were indexed using a doubled c parameter with respect to the one proposed in the literature [9], i.e. a=4.359+0.003 A, c=5.637ItO.O05 A. In as far as the phase identified besides the rutile form did not result from a decomposition of the GaAsOo, phase, it could be assigned to a mediumpressure variety. As a result further investigations of GaAs04 and AlAsO, at lower pressures (P< 7 GPa ) were made. 3.3. Investigations in medium-pressure range
A,
c= 11.400 + 0.005 A ;
for AlAsO, , a= 5.037 * 0.002 A , c= 11.226 + 0.005 A .
Such a supercell could be the result of a cationic order due to the difference in size and charge for Ga(III)-Al(II1) and As(V) 3.2. High-pressure behavior of cu-GaAsO, and CYAlAsOd at 9 GPa Fukunaga and Yamaoka [ 8 ] had proposed a diagram relative to the transformations of the ABO, phases under high pressure, based on the ratios RJ RB and (R,+ RB)/2Ro, where R denotes the radius of A, B or 0 ion. According to that diagram, GaAsO, would have as high-pressure varieties the CrVO,- and rutile-type structures as pressure increases whereas AlAsO, would have the rutile-type structure only. a-AlAsO, being transformed under high pressure into a rutile-phase at 9 GPa and %900 oC [ 9 1, an 46
September 1990
The experiments were undertaken within the range of pressure 3~ P<7 GPa. Temperature was held around 650°C during 15 min at the required pressure. For sake of comparison GaAsO, and AlAsO were studied. The main results of these investigations can be summarized as follows: (i) There is a “pressure threshold” PS 3 GPa below which no tmnsfo~ation of the aquartz type can be detected. For GaAs04, the X-ray pattern of the product obtained after quenching from 3 GPa consists of a two-phase mixture between u-quartz-type and another phase whose lines could be indexed in a monoclinic system. Conversely, for the same pressure value (i.e. 3 GPa) no transformation was detected for AlAsO., (the two samples having been compressed in the same pressure cell). (ii) At higher pressures (4~ P< 7 GPa) a twophase mixture between the monoclinic phase and a phase belonging to o~horhombic s~rnet~ was observed for GaAsO, and between the hexagonal (uSiO&ype) phase and a phase belonging to orthorhombic symmetry for AlAsO,. Both orthorhombic
Volume
10, number
1,2
Table 1 Refined lattice parameters
MATERIALS
of the high-pressure
varieties
5.000 11.400 120 3 246.82 82.27
Table 2 Refined lattice parameters
Monoclinic (non-defined)
Orthorhombic (CrVO,-type)
7.830 8.464 5.114 110.5 4 317.42 79.36
5.532 8.284 6.025 90 4 276. I I 69.03
z-496
AVIV
of the high-pressure
varieties
1990
Tetrahedral (rutile-type) 4.367 5.640 90 2 107.56 53.78 c -22%
z-13%
of AlAsO,
a-AlAsO, a (A) b(A) c(A) P (deg) Z V(A3) V/Z (A’) AVIV
September
of GaAs04
a-GaAsO, (a-quartz) a (A) b(A) c(A) P(deg) Z V(A)) V/Z (A’)
LETTERS
Rutile-type
Orthorhombic
5.037
5.540 8.251 6.024 90 4 275.36 68.84
11.226 120 3 246.66 82.22 ~-16%
phases could be indexed in a structure derived from CrVO,-type. Tables 1 and 2 summarize the relined lattice parameters of the different varieties of both arsenates.
4. Conclusion Despite the numerous experiments covering the pressure range from 3 to 9 GPa and the evidence of the high-pressure varieties of GaAsO,, those could not be isolated. Two possible origins could be proposed for this feature. The first one is of a technical order: the quenching in pressure would be too slow in our experimental conditions. The second one is of a crystallographic order: in the a-GaAsO, structure, Ga( III) and As(V) which have isoelectronic configurations (Ar 3d’O) are in tetrahedral environments ( Td) whereas in the rutile structure the cation is six-coordinated. Given the electronic structures of GA( III) and As(V) and the relatively small size of
4.359 5.637 90 2 92.76 46.38 sz - 32%
As(V), the high-pressure, rutile-type variety of GaAs04 would be metastable. Differential thermal analysis (DTA) of the product obtained after the following treatment: Pz 8 GPa, Tz 650°C t z 15 min (i.e. yielding a two-phase mixture of GaAsO, (rutile-type and CrVO,-type ) showed the existence of an irreversible, complex in shape, exothermal peak at FZ260°C significant of a complete transition from the high-pressure forms to aGaAsO,. The rather low temperature points to a low transformation energy. The X-ray pattern of the powder after DTA was indeed that of a-GaAsO,.
Acknowledgement We wish to thank Dr. Louis Rabardel for having undertaken the DTA experiment on GaAsO+
47
Volume IO, number 1,2
MATERIALS LETTERS
References [ I] E. Shafer and R. Roy, J. Am. Ceram. Sot. 39 ( 1956) 330. [2] A. Goiffon, G. Bayle, R. Astier, J.C. Jumas, M. Maurin and E. Philippot, Rev. Chim. Miner. 20 (1983) 338. [ 31 A. Goiffon, J.C. Jumas, M. Maurin and E. Philippot, J. Solid State Chem. 61 ( 1986) 384. [4] M. Ronis, Compt. Rend. Acad. Sci. (Paris) C 270 ( 1970) 73.
48
September 1990
[S] S.M. Stishov and S.V. Popova, Geokbimiya 10 ( 196 1) 837. [6] 0. Muller and R. Roy, Z. Krist. 138 (1973) 237. [ 7 ] G. Demazeau, These, Universitd Bordeaux I ( 1973); M. Contre, These, Universite Bordeaux I ( 1985 ). [ 810. Fukunaga and S. Yamaoka, in: Phase transitions in ternary oxides: comprehensive rules of pressure-induced phase transformations in ternary oxides ( 1970) p. 407. 19lA.R. Young, C.B. Sclar and CM. Schwartz, Z. Krist. 118 (1963) 223.
IO, number
MATERIALS
1,2
Investigations
on the high-pressure
September
LETTERS
varieties
1990
of GaAsO,
S. Matar, M. Lelogeais, D. Michau and G. Demazeau Laboratoire de Chimie du Solide du C.N.R.S., UniversitP Bordeaux I, 35 I cows de la Lib&ration, 33405 Talence Cedex, France Received
11 June 1990
This work reports on (i) the stabilization pressure varieties for GaAsO,: monoclinic,
of a-SiO,-type GaAsO, and AlAsO, using gas-medium pressure orthorhombic (CrVO,-type) and tetragonal (rutile-type).
1. Introduction Ternary oxides of formulation ABO4 (A = Al, Ga, .... B=P, As, V, ...) have been extensively studied [ 1,2] due to the possible change of coordination number versus pressure inducing new structures of interest in the domains of geophysics and materials science. Nevertheless very few studies were devoted to the oxide GaAsO,. Investigations on GaAs04 at room temperature [ 3,4] did not report on the possibility of the existence of high-pressure varieties of this ternary oxide. GaAs04 has a hexagonal structure closely related to that of a-SiO*, with the lattice parameters ahex= 5.00 A and chex= 5.68 8, [ 11. On the other hand, a-SiO, presents a high-pressure form called stishouite [ 51 with a structure close to the rutile-type. The present work reports on the results of our study of the transformations under high pressure of aGaAsO,. Furthermore, Al (III) and Ga (III ) having close behaviors regarding their electronic configurations (closed-shell configurations: Al (III ) corresponds to Ne, Ga(II1) to Ar 3d”), the isostructural AlAsO, compound has also been studied.
2. Experimental The a-SiOz forms of GaAsO, and been prepared either starting from a mixture of the oxides Ga,O, and Asz03 Asz03 under a pressure of dry oxygen 0167-577x/90/$
03.50 0 1990 - Elsevier
AlAsO, have homogenized or A1203 and gas, or from
Science Publishers
and (ii) three high-
a homogenized mixture of the oxides GazOj and Asz05 or A1203 and As205 under a pressure of a mixture of dry oxygen-nitrogen ( 1:9) gases. The use of gas pressure involves two complementary aspects: - the thermodynamic parameter pressure (oxygen or nitrogen) is used to hinder the sublimation of arsenic oxide (sublimation point of As203 z 190°C) in order to preserve the stoichiometry Ga(or Al):As=l:l; - the use of oxygen gas pressure, besides its ability to increase the oxidation state, allows the oxidation of As( III) into As(V) and favors this unusual valency state. The reacting materials As20s, As205, Ga203 and A1203 of 99.9% purity were handled in a glove box (problems of toxicity of arsenic oxides, AsZOS being, moreover, moisture sensitive). The gallium or aluminium oxides mixed with arsenic oxide weighed in stoichiometric proportions were introduced in a gold tube (chemical inertia). The tube was then put into the high-pressure reaction vessel. The experimental P, T conditions were as follows: P=O. 1 GPa, T~800”C, t=4 h. The starting and reacted materials were characterized by X-ray diffraction (XRD) using Cu-anticathode (A= 1.5418 A). Since the phase transitions under pressure of the ABO, compounds occur for pressure values higher than 3 GPa [ 6 1, a belt-type press [ 7 ] was used for the investigations of the high-pressure transformations of a-GaAsO, and a-AlAsO,. In order to avoid the reduction of As( V ) under the high-pressure-high-
B.V. (North-Holland
)
45
Volume 10, number I,2
MATERIALS LETTERS
temperature conditions, KClO-, was used as an oxygen source [ 7 1,
3. Results and discussion 3.1. XRD characterization of ar-GaAs@ and cyAlASO, The XRD patterns of the reacted powders under gas pressure were indexed in the hexagonal system of a-Si&. Extra lines of weak intensity belonging to EGa203 were identified in some experiments showing a slight sublimation of arsenic oxide (found in the upper cold zone of the reaction vessel). Other extra lines not belonging to any of the starting materials and that could not be indexed in the a-SiO, type structure were detected in both patterns of GaAs04 and AlAsO,. The existence of the extra lines led to an indexation in a superstructure: a’ =a0 and c’ = 2~ ( a0 and co being the cell parameters of the uSiO,-type st~cture ). The obtained lattice parameters given below, are in fairly good agreement with data from the literature [ 31: for GaAsO, , a=5.000-t0.002
experiment was done with GaAs04 in the same pressure and temperature range in order to stabilize the high-pressure variety. After temperature quenching, two phases were identified by XRD: a rutile phase and another phase. Indexation of the rutile phase was done using remaining KC1 as internal standard. Such a pattern was indexed using a supercell with a doubled c parameter with respect to the simple rutile-type, i.e. a =4.367 20.003 A, c= 5.640~0.005 8. In order to compare its high-pressure behavior, a-AlAsO was investigated in the same pressure and temperature range (P;z: 9 GPa, Tx 9OO*C). The XRD lines of the rutile variety were indexed using a doubled c parameter with respect to the one proposed in the literature [9], i.e. a=4.359+0.003 A, c=5.637ItO.O05 A. In as far as the phase identified besides the rutile form did not result from a decomposition of the GaAsOo, phase, it could be assigned to a mediumpressure variety. As a result further investigations of GaAs04 and AlAsO, at lower pressures (P< 7 GPa ) were made. 3.3. Investigations in medium-pressure range
A,
c= 11.400 + 0.005 A ;
for AlAsO, , a= 5.037 * 0.002 A , c= 11.226 + 0.005 A .
Such a supercell could be the result of a cationic order due to the difference in size and charge for Ga(III)-Al(II1) and As(V) 3.2. High-pressure behavior of cu-GaAsO, and CYAlAsOd at 9 GPa Fukunaga and Yamaoka [ 8 ] had proposed a diagram relative to the transformations of the ABO, phases under high pressure, based on the ratios RJ RB and (R,+ RB)/2Ro, where R denotes the radius of A, B or 0 ion. According to that diagram, GaAsO, would have as high-pressure varieties the CrVO,- and rutile-type structures as pressure increases whereas AlAsO, would have the rutile-type structure only. a-AlAsO, being transformed under high pressure into a rutile-phase at 9 GPa and %900 oC [ 9 1, an 46
September 1990
The experiments were undertaken within the range of pressure 3~ P<7 GPa. Temperature was held around 650°C during 15 min at the required pressure. For sake of comparison GaAsO, and AlAsO were studied. The main results of these investigations can be summarized as follows: (i) There is a “pressure threshold” PS 3 GPa below which no tmnsfo~ation of the aquartz type can be detected. For GaAs04, the X-ray pattern of the product obtained after quenching from 3 GPa consists of a two-phase mixture between u-quartz-type and another phase whose lines could be indexed in a monoclinic system. Conversely, for the same pressure value (i.e. 3 GPa) no transformation was detected for AlAsO., (the two samples having been compressed in the same pressure cell). (ii) At higher pressures (4~ P< 7 GPa) a twophase mixture between the monoclinic phase and a phase belonging to o~horhombic s~rnet~ was observed for GaAsO, and between the hexagonal (uSiO&ype) phase and a phase belonging to orthorhombic symmetry for AlAsO,. Both orthorhombic
Volume
10, number
1,2
Table 1 Refined lattice parameters
MATERIALS
of the high-pressure
varieties
5.000 11.400 120 3 246.82 82.27
Table 2 Refined lattice parameters
Monoclinic (non-defined)
Orthorhombic (CrVO,-type)
7.830 8.464 5.114 110.5 4 317.42 79.36
5.532 8.284 6.025 90 4 276. I I 69.03
z-496
AVIV
of the high-pressure
varieties
1990
Tetrahedral (rutile-type) 4.367 5.640 90 2 107.56 53.78 c -22%
z-13%
of AlAsO,
a-AlAsO, a (A) b(A) c(A) P (deg) Z V(A3) V/Z (A’) AVIV
September
of GaAs04
a-GaAsO, (a-quartz) a (A) b(A) c(A) P(deg) Z V(A)) V/Z (A’)
LETTERS
Rutile-type
Orthorhombic
5.037
5.540 8.251 6.024 90 4 275.36 68.84
11.226 120 3 246.66 82.22 ~-16%
phases could be indexed in a structure derived from CrVO,-type. Tables 1 and 2 summarize the relined lattice parameters of the different varieties of both arsenates.
4. Conclusion Despite the numerous experiments covering the pressure range from 3 to 9 GPa and the evidence of the high-pressure varieties of GaAsO,, those could not be isolated. Two possible origins could be proposed for this feature. The first one is of a technical order: the quenching in pressure would be too slow in our experimental conditions. The second one is of a crystallographic order: in the a-GaAsO, structure, Ga( III) and As(V) which have isoelectronic configurations (Ar 3d’O) are in tetrahedral environments ( Td) whereas in the rutile structure the cation is six-coordinated. Given the electronic structures of GA( III) and As(V) and the relatively small size of
4.359 5.637 90 2 92.76 46.38 sz - 32%
As(V), the high-pressure, rutile-type variety of GaAs04 would be metastable. Differential thermal analysis (DTA) of the product obtained after the following treatment: Pz 8 GPa, Tz 650°C t z 15 min (i.e. yielding a two-phase mixture of GaAsO, (rutile-type and CrVO,-type ) showed the existence of an irreversible, complex in shape, exothermal peak at FZ260°C significant of a complete transition from the high-pressure forms to aGaAsO,. The rather low temperature points to a low transformation energy. The X-ray pattern of the powder after DTA was indeed that of a-GaAsO,.
Acknowledgement We wish to thank Dr. Louis Rabardel for having undertaken the DTA experiment on GaAsO+
47
Volume IO, number 1,2
MATERIALS LETTERS
References [ I] E. Shafer and R. Roy, J. Am. Ceram. Sot. 39 ( 1956) 330. [2] A. Goiffon, G. Bayle, R. Astier, J.C. Jumas, M. Maurin and E. Philippot, Rev. Chim. Miner. 20 (1983) 338. [ 31 A. Goiffon, J.C. Jumas, M. Maurin and E. Philippot, J. Solid State Chem. 61 ( 1986) 384. [4] M. Ronis, Compt. Rend. Acad. Sci. (Paris) C 270 ( 1970) 73.
48
September 1990
[S] S.M. Stishov and S.V. Popova, Geokbimiya 10 ( 196 1) 837. [6] 0. Muller and R. Roy, Z. Krist. 138 (1973) 237. [ 7 ] G. Demazeau, These, Universitd Bordeaux I ( 1973); M. Contre, These, Universite Bordeaux I ( 1985 ). [ 810. Fukunaga and S. Yamaoka, in: Phase transitions in ternary oxides: comprehensive rules of pressure-induced phase transformations in ternary oxides ( 1970) p. 407. 19lA.R. Young, C.B. Sclar and CM. Schwartz, Z. Krist. 118 (1963) 223.