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On the crystal structure of the compounds CaP, SrP, CaAs, SrAs and EuAs
Journul
Metals. 30 (1973) 21 l--216 S.A., Lausanne - Printed in The Netherlands
01 the Less-Common
(‘8 Elsevier
Sequoia
ON THE CRYSTAL STRUCTURE SrAs AND EuAs
A. IANDELLI fstituto
di Chimicu
(Received
and
OF THE COMPOUNDS
211
CaP, SrP, &As,
E. FRANCESCHI
Fisica,
Unioersitiidi
Geneva. Genoa (stall)
June 23. 1972)
The crystal structure of the compounds CaP, SrP, CaAs, SrAs and EuAs has been determined from powder data. These compounds crystallize with the Na,02 structure type, characterized by groups P, or As,. The magnetic susceptibility and the lattice constants of EuAs show that Eu is divalent in this compound. The corresponding phases BaP and BaAs do not exist.
INTROI)I!CTT0N
In the course of an investigation of the phases existing in the systems of Ca, Sr and Ba with P and As, some compounds with the MX composition were found and their crystal structure has been determined together with that of EuAs. In the latter compound Eu is divalent and therefore, with EuSb and EuBi (and YbBi), it represents an exception to the general behaviour of the rare earths in the MX pnictides, all of which are of the NaCl structure type. The system Eu-As has recently been studied by Ono, Hui, Despault, Calvert and Taylor ‘, and our results for the compounds EuAs are in agreement with theirs. EXPERIMENTAL
The preparation of the MX compounds was carried out from the elements, using commercial products with a claimed minimum purity 99.8 ‘/&The reaction was performed by heating turnings of the reactive metals, obtained under argon, and the corresponding quantities of red phosphorus or arsenic in Pyrex tubes, sealed in uucuo. The temperature was slowfy raised to 6~*~50°~ until the metalloid vapour had disappeared. The products so obtained were pressed into cylindrical tablets and heated for several days at higher temperatures in degassed alumina crucibles, sealed under vacuum in quartz tubes. In no case was the sintering temperature higher than 900°C. as at lOOO”-1100°C the samples appeared to decompose; attempts to obtain homogeneous melted products for CaP, CaAs and EuAs were unsuccessful. The samples obtained had a black, metallic appearance. They were very sensitive to attack by moist air and had to be handled under dry argon. Owing to the unavailability of single crystals the compounds were studied by the X-ray powder method, using CuKcc, and for EuAs, Fe Kcc radiations.
212
A. IANDELLI,
E. FRANCESCHI
RESULTS
The X-ray examination showed the isomorphism of CaP, CaAs, SrP, SrAs and EuAs. Photographs of samples of the compositions BaP and BaAs, and of similar compositions but slightly richer in P and As, were completely different from those of the live preceeding compositions and were similar to those obtained for the phases richer in X in the systems Sr-P, Sr-As and Eu-As. It follows that in the systems Ba-P and Ba-As the MX phase does not exist. This is in agreement with the results of the study of the Ba-P system by Maas’ who found the existence of a solid solution between 52.4 at.% P(BaP,,,) and 57.4 at. % P(BaP,,,,), but no BaP compound. In the Eu-As system’ the phase immediately following EuAs has the formula Eu,As,, corresponding to the composition 57.1 at. ‘ji$As. For the live isomorphous MX compounds, it was not possible to index the reflections of the powder photographs on the basis of a tetragonal cell, as was done by Pytlewski3. Good agreement for the positions and intensities of the main, but not the weak, reflections was obtained for a hexagonal cell, corresponding to an anti-isomorphous NiAs type, with a = 4.406 A, c = 5.73 1 A for CaP, and similar values for the other MX compounds. Taking a larger cell, with a’=aJ3, all the reflections could be indexed. The tinal values of the lattice constants are reported in Table I. TABLE LATTICE
I CONSTANTS
AND
PARAMETER
VALUES
FOR
THE
MX COMPOUNDS
MX
a(R) * 0.002
c(A) * 0.002
x
x’
z
Z’
CaP SrP CaAS SrAs Etis
7.632 8.040 7.858 8.269 8.150
5.731 6.031 5.921 6.201 6.135
0.30, 0.30, 0.31, 0.31, 0.30,
0.64, 0.636 0.64, 0.64, 0.64,
0.30, 0.30, 0.29, 0.29, 0.29,
0.19, 0.19, 0.21, 0.21, 0.20,
This larger cell, if the positions of the anti- NiAs type were maintained, would contain 3 + 3 M atoms on the sides, at two different heights, and 2 + 4 X atoms over the positions 0 0 and 3 t,+ 3. Deformation of the structure may be effected by bringing the X atoms nearer together along the z axis to form X, groups, and by displacing the M atoms on the sides. This may be achieved by taking the following positions in the P62m group (No189 of the International Tables4): 3 M, in 3(f): x 0 0, etc., 3 M,, in 3(g): x’ 0 5, etc., 2 X, in 2(e) :O 0 z, etc., 4 Xi, in 4(h) : 3 3 z’, etc. The preceeding arrangement corresponds to the NazO, structure type5, if z’ = + - z, and it remains to find the values of the parameters. These were derived approximately from dimensional considerations and enabled a fairly good agreement between calculated and observed intensities to be obtained. Subsequently, parameter refinements were attempted by calculating the intensities for different values of parameters, obtained through systematic displacements of the atoms around the positions dimensionally found. Ultimately, the values which gave the best agreement between
CaP. SrP, GAS, SrAs, EuAs CRYSTAL STRUC’iURES
213
calculated and observed intensities, with reasonable interatomic distances, were selected. They have been reported in Table I. Table II contains the calculated and observed intensities. Owing to the lack of single crystal photographs, some uncertainty remains, however, in the values of the parameters. A 001 projection of the cell is drawn in Fig. 1. The cell is characterized by groupsX? along the z axis, withX-X distances varying from one compound to another, surrounded by M, and M,, atoms in two different ways. The M atoms are disposed in a distorted hexagonal close-packed arrangement. Table III contains the values of the distances for the nearest neighbour atoms.
0
M in 31f)
M in S(g)
0
X in 2&J
X in
4th)
Fig. 1. 001 prqiection of the cell of the MX compounds.
EuAs has a different structure from the other R.E. monoarsenides, while it is isomorphous with CaAs and SrAs. Its molecular volume has the value of 58.8 A3, in comparison with the values of 51.9 and 50.3 A3, for SmAs and GdAs, respectively, which indicates the divalency of Eu in this compound. This may be checked by magnetic measurements. We have measured the magnetic susceptibility of EuAs in the range - 1SO”C/ i- 2OtY’Cand found that it follows the Curie-Weiss law, with xkQ8K= 24,400 x 10e6 e.m.u., pB= 7.46, Be= 12 K, in agreement with the known values for Eu’+.
214 TABLE
A. IANDELLI.
E. FRANCESCHI
II
CALCULATED
AND
OBSERVED
INTENSITY
VALUES
FOR
THE
MX COMPOUNDS
(continued)
CaP. SrP. CaAs. SrAs. EuAs CRYSTAL TABLE
STRUCTURES
215
II (continued)
CONCLUSIONS
From the study of a number of compounds between electropositive metals and electronegative elements, the structures appear to follow the rules of a mixed ioniccovalent valence. The atoms of the more electronegative partner are bound to each other by covalent links, taking some electrons from the more electropositive atoms. For example, in CaSi the Si atoms form zig-zag chains but in CaSi, double sheets, in which each Si atom is bound to three others. The two dispositions can be obtained through a formal transfer of two electrons from a Ca atom to one of Si (which becomes isoelectronic with S) in CaSi. or to two of Si (isoelectronic with P) in CaSi?. Numerous
216 TABLE
A. IANDELLI.
E. FRANCESCHI
III
INTERATOMIC
DISTANCES
IN THE
MX COMPOUNDS
MX
CaP
SrP
CaAs
SrAs
EuAs
MI-6 MI, -2 x, -4 Xn MI,-6 M, -2 x, -4 Xi, X,-3 M, -3 M,, -1 x, X11-3 M, -3 Ma -1 Xn
3.85 2.90 2.89 3.85 2.96 3.00 2.90 2.96 2.26 2.89 3.00 2.26
4.05 3.05 3.05 4.05 3.15 3.17 3.05 3.15 2.30 3.05 3.17 2.30
3.96 2.98 2.99 3.96 3.05 3.07 2.98 3.05 2.49 2.99 3.07 2.49
4.16 3.13 3.14 4.16 3.21 3.22 3.13 3.21 2.60 3.14 3.22 2.60
4.11 3.08 3.10 4.11 3.18 3.18 3.08 3.18 2.53 3.10 3.18 2.53
examples of such behaviour for compounds of alkali metals have been reported by Klemm and Busmann6. The preceeding formal rules may be applied to the monophosphides of Ca and Sr and to the monoarsenides of Ca, Sr and Eu : each P or As atom would become a P2- or As2- ion, able to form couples P, or As, .Of all the rare earths, Eu shows the greatest tendency to become divalent in its compounds; only if the other partner has a sufficiently high electronegativity may Eu behave as trivalent. This may explain the existence of EuP with ELI”‘,while the trivalency is not possible in the compound EuAs.
REFERENCES 1 S. Ono. F. L. Hui, J. G. Despault, L. D. Calvert and J. B. Taylor, J. Less-Common Metals, 25 (1971) 287. 2 K. E. Maas. Z. Anorg. Allgem. Chew 374 (1970) 1. 3 L. L. Pytlewski, The preparation and properties of calcium phosphides, Ph. D., Univ. Pennsylvania, 1960. 4 International Tables for X-Ray Crystallography, Vol. I, Kynoch Press, Birmingham, England, 1962. 5 H. Fdppl. Z. Anorg. Allgem. Chem., 291 (1957) 12. 6 W. Klemm and E. Busmann. Z. Anorg. A&em. C’hem., 319 (1963) 297.
Metals. 30 (1973) 21 l--216 S.A., Lausanne - Printed in The Netherlands
01 the Less-Common
(‘8 Elsevier
Sequoia
ON THE CRYSTAL STRUCTURE SrAs AND EuAs
A. IANDELLI fstituto
di Chimicu
(Received
and
OF THE COMPOUNDS
211
CaP, SrP, &As,
E. FRANCESCHI
Fisica,
Unioersitiidi
Geneva. Genoa (stall)
June 23. 1972)
The crystal structure of the compounds CaP, SrP, CaAs, SrAs and EuAs has been determined from powder data. These compounds crystallize with the Na,02 structure type, characterized by groups P, or As,. The magnetic susceptibility and the lattice constants of EuAs show that Eu is divalent in this compound. The corresponding phases BaP and BaAs do not exist.
INTROI)I!CTT0N
In the course of an investigation of the phases existing in the systems of Ca, Sr and Ba with P and As, some compounds with the MX composition were found and their crystal structure has been determined together with that of EuAs. In the latter compound Eu is divalent and therefore, with EuSb and EuBi (and YbBi), it represents an exception to the general behaviour of the rare earths in the MX pnictides, all of which are of the NaCl structure type. The system Eu-As has recently been studied by Ono, Hui, Despault, Calvert and Taylor ‘, and our results for the compounds EuAs are in agreement with theirs. EXPERIMENTAL
The preparation of the MX compounds was carried out from the elements, using commercial products with a claimed minimum purity 99.8 ‘/&The reaction was performed by heating turnings of the reactive metals, obtained under argon, and the corresponding quantities of red phosphorus or arsenic in Pyrex tubes, sealed in uucuo. The temperature was slowfy raised to 6~*~50°~ until the metalloid vapour had disappeared. The products so obtained were pressed into cylindrical tablets and heated for several days at higher temperatures in degassed alumina crucibles, sealed under vacuum in quartz tubes. In no case was the sintering temperature higher than 900°C. as at lOOO”-1100°C the samples appeared to decompose; attempts to obtain homogeneous melted products for CaP, CaAs and EuAs were unsuccessful. The samples obtained had a black, metallic appearance. They were very sensitive to attack by moist air and had to be handled under dry argon. Owing to the unavailability of single crystals the compounds were studied by the X-ray powder method, using CuKcc, and for EuAs, Fe Kcc radiations.
212
A. IANDELLI,
E. FRANCESCHI
RESULTS
The X-ray examination showed the isomorphism of CaP, CaAs, SrP, SrAs and EuAs. Photographs of samples of the compositions BaP and BaAs, and of similar compositions but slightly richer in P and As, were completely different from those of the live preceeding compositions and were similar to those obtained for the phases richer in X in the systems Sr-P, Sr-As and Eu-As. It follows that in the systems Ba-P and Ba-As the MX phase does not exist. This is in agreement with the results of the study of the Ba-P system by Maas’ who found the existence of a solid solution between 52.4 at.% P(BaP,,,) and 57.4 at. % P(BaP,,,,), but no BaP compound. In the Eu-As system’ the phase immediately following EuAs has the formula Eu,As,, corresponding to the composition 57.1 at. ‘ji$As. For the live isomorphous MX compounds, it was not possible to index the reflections of the powder photographs on the basis of a tetragonal cell, as was done by Pytlewski3. Good agreement for the positions and intensities of the main, but not the weak, reflections was obtained for a hexagonal cell, corresponding to an anti-isomorphous NiAs type, with a = 4.406 A, c = 5.73 1 A for CaP, and similar values for the other MX compounds. Taking a larger cell, with a’=aJ3, all the reflections could be indexed. The tinal values of the lattice constants are reported in Table I. TABLE LATTICE
I CONSTANTS
AND
PARAMETER
VALUES
FOR
THE
MX COMPOUNDS
MX
a(R) * 0.002
c(A) * 0.002
x
x’
z
Z’
CaP SrP CaAS SrAs Etis
7.632 8.040 7.858 8.269 8.150
5.731 6.031 5.921 6.201 6.135
0.30, 0.30, 0.31, 0.31, 0.30,
0.64, 0.636 0.64, 0.64, 0.64,
0.30, 0.30, 0.29, 0.29, 0.29,
0.19, 0.19, 0.21, 0.21, 0.20,
This larger cell, if the positions of the anti- NiAs type were maintained, would contain 3 + 3 M atoms on the sides, at two different heights, and 2 + 4 X atoms over the positions 0 0 and 3 t,+ 3. Deformation of the structure may be effected by bringing the X atoms nearer together along the z axis to form X, groups, and by displacing the M atoms on the sides. This may be achieved by taking the following positions in the P62m group (No189 of the International Tables4): 3 M, in 3(f): x 0 0, etc., 3 M,, in 3(g): x’ 0 5, etc., 2 X, in 2(e) :O 0 z, etc., 4 Xi, in 4(h) : 3 3 z’, etc. The preceeding arrangement corresponds to the NazO, structure type5, if z’ = + - z, and it remains to find the values of the parameters. These were derived approximately from dimensional considerations and enabled a fairly good agreement between calculated and observed intensities to be obtained. Subsequently, parameter refinements were attempted by calculating the intensities for different values of parameters, obtained through systematic displacements of the atoms around the positions dimensionally found. Ultimately, the values which gave the best agreement between
CaP. SrP, GAS, SrAs, EuAs CRYSTAL STRUC’iURES
213
calculated and observed intensities, with reasonable interatomic distances, were selected. They have been reported in Table I. Table II contains the calculated and observed intensities. Owing to the lack of single crystal photographs, some uncertainty remains, however, in the values of the parameters. A 001 projection of the cell is drawn in Fig. 1. The cell is characterized by groupsX? along the z axis, withX-X distances varying from one compound to another, surrounded by M, and M,, atoms in two different ways. The M atoms are disposed in a distorted hexagonal close-packed arrangement. Table III contains the values of the distances for the nearest neighbour atoms.
0
M in 31f)
M in S(g)
0
X in 2&J
X in
4th)
Fig. 1. 001 prqiection of the cell of the MX compounds.
EuAs has a different structure from the other R.E. monoarsenides, while it is isomorphous with CaAs and SrAs. Its molecular volume has the value of 58.8 A3, in comparison with the values of 51.9 and 50.3 A3, for SmAs and GdAs, respectively, which indicates the divalency of Eu in this compound. This may be checked by magnetic measurements. We have measured the magnetic susceptibility of EuAs in the range - 1SO”C/ i- 2OtY’Cand found that it follows the Curie-Weiss law, with xkQ8K= 24,400 x 10e6 e.m.u., pB= 7.46, Be= 12 K, in agreement with the known values for Eu’+.
214 TABLE
A. IANDELLI.
E. FRANCESCHI
II
CALCULATED
AND
OBSERVED
INTENSITY
VALUES
FOR
THE
MX COMPOUNDS
(continued)
CaP. SrP. CaAs. SrAs. EuAs CRYSTAL TABLE
STRUCTURES
215
II (continued)
CONCLUSIONS
From the study of a number of compounds between electropositive metals and electronegative elements, the structures appear to follow the rules of a mixed ioniccovalent valence. The atoms of the more electronegative partner are bound to each other by covalent links, taking some electrons from the more electropositive atoms. For example, in CaSi the Si atoms form zig-zag chains but in CaSi, double sheets, in which each Si atom is bound to three others. The two dispositions can be obtained through a formal transfer of two electrons from a Ca atom to one of Si (which becomes isoelectronic with S) in CaSi. or to two of Si (isoelectronic with P) in CaSi?. Numerous
216 TABLE
A. IANDELLI.
E. FRANCESCHI
III
INTERATOMIC
DISTANCES
IN THE
MX COMPOUNDS
MX
CaP
SrP
CaAs
SrAs
EuAs
MI-6 MI, -2 x, -4 Xn MI,-6 M, -2 x, -4 Xi, X,-3 M, -3 M,, -1 x, X11-3 M, -3 Ma -1 Xn
3.85 2.90 2.89 3.85 2.96 3.00 2.90 2.96 2.26 2.89 3.00 2.26
4.05 3.05 3.05 4.05 3.15 3.17 3.05 3.15 2.30 3.05 3.17 2.30
3.96 2.98 2.99 3.96 3.05 3.07 2.98 3.05 2.49 2.99 3.07 2.49
4.16 3.13 3.14 4.16 3.21 3.22 3.13 3.21 2.60 3.14 3.22 2.60
4.11 3.08 3.10 4.11 3.18 3.18 3.08 3.18 2.53 3.10 3.18 2.53
examples of such behaviour for compounds of alkali metals have been reported by Klemm and Busmann6. The preceeding formal rules may be applied to the monophosphides of Ca and Sr and to the monoarsenides of Ca, Sr and Eu : each P or As atom would become a P2- or As2- ion, able to form couples P, or As, .Of all the rare earths, Eu shows the greatest tendency to become divalent in its compounds; only if the other partner has a sufficiently high electronegativity may Eu behave as trivalent. This may explain the existence of EuP with ELI”‘,while the trivalency is not possible in the compound EuAs.
REFERENCES 1 S. Ono. F. L. Hui, J. G. Despault, L. D. Calvert and J. B. Taylor, J. Less-Common Metals, 25 (1971) 287. 2 K. E. Maas. Z. Anorg. Allgem. Chew 374 (1970) 1. 3 L. L. Pytlewski, The preparation and properties of calcium phosphides, Ph. D., Univ. Pennsylvania, 1960. 4 International Tables for X-Ray Crystallography, Vol. I, Kynoch Press, Birmingham, England, 1962. 5 H. Fdppl. Z. Anorg. Allgem. Chem., 291 (1957) 12. 6 W. Klemm and E. Busmann. Z. Anorg. A&em. C’hem., 319 (1963) 297.