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Neutron diffraction study of ErPO4 and ErVO4
Volume
2. number
1
CHEMICAL
NEUTRON
PHYSICS
DIFFRACTION
May 1968
LETTERS
STUDY
OF
ErP04
AND
ErV04
E. PATSCHEKE Physikaliscires
Institut
der Unit~ersitZit KarLsmhe
and H. FUESS and G. WILL Eduard-Zintl-Znstitut
der TZi Darmstadt. Received
Lehrstiihl
12 March
fir
Strukturforschung
1968
The oxvfCen parameters x and t of ErPOa and ErVOa (I 4/amd) have been refined froa neutron diffraction powderdata by least squares analysis.=The P.-O aGd_V-O dikances in the PO:- and VOZ- tetrahedra are 1.54 and 1.72 A respectively. The bonding mechanism in the tetrahedra is discussed.
1. INTRODUCTION In the course of a study of the magnetic behavior of phosphates and van&&s of the rare earth elements we have refined the atomic crystal structure parameters of the oxygen ions in ErP04 and ErV04 from neutron diffraction powder data. ErP04 and ErV04 crystallize isomorphous with ZrSiO4 (zirconium) in the tetragonal space group I 4/amd-D:: (fig. 1) [1,2], with a unit cell at room tempezature [3] of a = b = = 6.863(3); c = 6.007(3) 4 for ErP04 and a = b = = ?.100(3); c = 6.279(3) A for ErV04 (standard deviations in parentheses). Er3+, P and V occupy special positions: Er3+: 4(a) 000, P, V: 4(b) OO$ and oxygen is in general position 0: 16(k) O.rz. Because of the comparatively small scattering amplitude of oxygen together with the high absorption of erbium for X-rays, the oxygen parameters cannot be determined reliably from X-ray data. The neutron scattering lengths of these elements [5] with b(Er) = 7.9 f, b(P) = 5.3 f, ii(V) = -0.5 f and b(O) = 5.77 f (If = = lo-l3 cm) on the other hand are of comparable magnitude and allow an accurate determination of the oxygen parameters in the presence of heavy elements.
speed of 1 deg/hour and at interlals of 0. lo in 20. A 235U fission chamber was used as a beam monitor, and the wave length was l-Q69 k Because of the high absorption cross section of erbium (u(Er) = 100 barn for 1.07 ii) a flat aluminum sample holder of 50 x 39 Y 3.4 mm3 was used. The absorption of the sample was determined by measuring the transmission of the direct beam through the sample, yielding coefficients of ~_rErP04) = 0.66 cm-1 and g(ErV04) = \ =0.87cm-. The refinement of the oxygen parameters was carried out by least squares methods [Sj with the 300 and 77OK data based on the structure factor F-values of 11 non-overlapping peaks in case of Table 1 Oxygen parameters x and z. and temperature factors of ErP04 at 300°K and of ErV04 at 300 and 77OK: standard deviations in parenthesis. ErP04
RESULTS
w(0) = y(O)
.
0.175 (2) 0.339
Z(O)
lo-l6
cm2
B(V)
R
=
F
hi
-
(3)
77%
0.1E7 (2)
0.1%
0.325 (3)
0.3% (21
(I)
0.G (23)
B(overal1)
B(O)
Neutron diffraction diagrams of powder ErP04 and ErV04, prepared by a sintering technique [3,4], were recorded at 300, 77 and 4.2”K at the
300’K
300%
B(Er)
2. EXPERIMENTAL
E A’04
FCTIC ‘
Fobs
0.037
0.37 (45:
0.27 (40)
0.53 (22)
c-54 (13)
0.54 (35)
0.64 (27)
0.027
0 -033
47
Volume 2, number 1
CHEMICAL PHYSICS LETTERS
Fig. I. Crystal structure of ErP04 and ErV04 (I 4/amd) and 8 peaks in case of ErV04. Individual temperature factors were used in the calculation, except for vanadium, where the initially determined overall temperature factor value was taken. The results are listed in table 1. Table 2 lists the C_iF2 values for ErP04 at 300’K and for ErVO4 at 77OK. The X-value in table 2 is defined as ErP04
with summation i over all planes contributing to the same reflection and summation k over all observed peaks. Table 3 lists some individual bond distances at 300°K for the tetrahedral coordination of P and V. and for the dodecahedral coordination of Er3+, which is coordinated to two sets of four equi-distant oxygen atoms at the six surrounding tetrahedra. 3. DISCUSSION The crystal structures of ErP04 and ErV04 consist of slightly distorted PO$’ and VOI- tetrahe’dra, with P or V in the center and Er3+ between the te$rahedra. The P-O bond length in ErPO4 is 1.54 A. This is in agreement with P-O distances found in other compounds containing PO4-tetrahedra: KH2PO4 with 1.538 A from neutron dif48
May 1968
The oxygen atoms are at the corners of the tetrahedra.
fraction [7,8]; and CaHP04 [9], CaHP04.2H20 [IO], HP04 [ll]. CalO!PP4)6(OH)2 1121, and LiMnPOq [13] with 1.54 A from X-ray diffraction. The bond shortening of 0.17 a for the phosphate ion from the single bond value of 1.71 A (calculated by the Schomaker-Stevenson correction [14]), has been explained by a partial doublebond character of the P-O bond 1151 and also by molecular orbital treatment of the X0”--tetrahedra (X = Si, P, S. or Cl) by Cruicks 41ank [ 161. In the PO;’ tetrahedral ion two strong n-bonding molecular orbitals are formed with the 3d,,2 I 2 2% and 3d72 orbitals of P and the appropriate and 2p’_’ orbitals of oxygen, using the oxygen combination 1/2@,1 +pg +p3 +p4) and l/2@\ +ph + + ,I& + pi). The four sp -hybrid orbitals of P over lap with the 2pu-orbitals of the oxygens forming covalent u-bonds. We have adopted this treatment tentatively for interpretation of the bonding mechanism in VO4tetrahedra, where four d3s-hybrid orbitals [ 171 1/2(s*~~~~f’~_,.*f~~~*) and I/~(.s*~~,,~~~~W~~) of the central V ion overlap with the 2Pu orbitals of the oxygens forming molecular o-orbitals. In general there is little known about the bonding mechanism in vanadium compounds. For the V-V sipgle bond length in the metal a value of 1.224 A is listed by Pauling 1151, expressing a
Volume 2. number I
CHEMICAL PHYSICS LETTERS
Table 2 Comparison of observed and calculated values
c
jF'
for ErP04 at 300°K and for ErV04 at 77OK. The asteriks mark reflections corrected for superimposed Alpeaks from the nluminum cryostat.
P.VO4 -
101
1
200
202
n. 0.
36
31
182
75
72 n. 0.
1968
Table 3 Main bond distances and angles at 300°K with standard deviations. ErP04 Distance (.&
3oooK
XrV04 770K
ErPOg 3oooh
hfay
P.V -01.2,3
1.54 (2)
ErVO4 Distance (b) 1.72 (2)
tetra-
Ol-
02
2.40 (2)
2.66 (2)
hedra
01 - 03
2.57 (2)
2.89 (2)
angle (deg)
angle (deg)
01 - P.V - 02
102.3 (9)
100.8 (81
01 - P.V - 03
113.2 (5)
124.0 (4)
Distance &
Distance (A)
2 11
57
52
2
112
92
88
5
4
220
0
n.0.
44
44
202
333
324
297
305-
301
294
282
476
468
Er - 04
2.29 (1)
2.27 (2)
Er - 01
2.36 (2)
2.44 (2)
103
209
218
389
386’
321
313
400
446
449
312
601
587
300
300
400
266
265
203
213
7
10
11
411
39
39
7
420
10
10
303
0
004
0
402
214
0 214
160
103
628_
469
429
401
615 290
204
120
50
422
19
80
501
14
431
333
413 224
30
94
2@4
200 469
224
294-
578
577
317
292
538
540
0 347
318
578 154
553
615
163 80
314
455
522
3
R
7
1
323
520
181
100
332
521
214
0.078
high d-character of the orbitals up to 40%. Due to the similarity
ence in electronegativity (~0 = 3.5; x 3+ = 1.4 [la]). With the observed value d = I.% A in ErV04 a bond shortening of 0.08 b is cakulated, which again shall be considered a consequence of the n-double-bond character of the V-O bond in the VO4-tetrahedra. In case of V04 we think of 3d_,_2_,)2and 3d,Z orbitals of V forming z-bonding m-olecular orbitals with the 2.&i and Zpi7’ orbitals of oxygen. The Er3+ ions between the oxygen tetrahedra have point symmetry Dw. They are surrounded by 8 qxygens at a mean distance of 2.33 A and 2.35 A for ErPO4 and ErV04 respectively_ The interaction of the ligand fieId with the 4f-shell of Er3+ has been estimated by Kuse and Jorgensen [19], using the angular overlap model 1201, with only u-anti-bonding on the basis of our oxygen parameters determineu by this neutron diffraction study.
REFERENCES
111W.O. Milligan. 0.037
in the metal
of
of the high dbonding character between the metal and the VOq-tetrahedra, we have taken this metallic bond length of V. With this assumption a V-O single bond length of 1.8q Ais calculated, using the oxygen value of 0.74 A, and applying the Schomaker-Stevenson correction for the differ-
L. M. Watt and H. H. Racheford, J. Phys. Chem. 53 (1949) 227. 121A.Durif. Acta Cryst. 9 (1956) 471. 131H. Schwarz. 2. anorg. allg. Chem. 323 (2963) 44. [41 G. MUller-Vogt. Diplomarbeit. Universit%t Karlsruhe (1968). ClarenI51 G. E. Bacon. Neutron Diffraction (&ford. don Press. 1962). [61 W. R. Busing. K.O. Martin and H.A. Levy. @RX, Report TM-305 (i962). !71 G. E. Bacon and R.S. Pease. Droc. Roy.Soc. (London) A220 (1953) 397. I81 G. E. Bacon and R.S. Pease. Proc. Rox.Soc. (London) A230 (1955) 359. 49
\SU”“,
[lo] Ill]
“I
Y.
C. A. Reevers, Acts Cryst. 11 (1958) 273. G. E. R.Schulze, Z.physik. Chemie Abt. B24 (1934) 215. 1121 A. S. Posner. A. Perloff and A. F. Diorio. Acta Cryst. 11 (1958) 308. 1131 S. Geller and J. L.Durand. Acta Cry&. 13 (1960) 325. 1141 V. Schomaker and D. P. Stevenson. 3. Am. Chem. sot. 63 (1941) 37. r151 L. PauIinir. The Nature of the Chemical Bond. 3rd ed. (Corn& University Press. Ithaca. New York, 1960).
50
ieioo. 1171 H.Z. Bchl?Jfer and G. Gliemann.
Ligandenfeldtheorie
(Akademische
EinfUhnmg in die Verlagsgesell-
schaft, Frankfurt a. M., 1967). [161 17. Gordy and W-3.0. Thomas. J. Chem. Phvs.
(1956) 439.
24
(191 D.Kuse and C.K. Jbrgensen. Chem.Phys. Letters 1 (1967) 314. [20] C. K. JBrgensen. R. Pappalardo and H.-H. Schmidt. ke. J. Chem. Phys- 39 (1963) 1422.
2. number
1
CHEMICAL
NEUTRON
PHYSICS
DIFFRACTION
May 1968
LETTERS
STUDY
OF
ErP04
AND
ErV04
E. PATSCHEKE Physikaliscires
Institut
der Unit~ersitZit KarLsmhe
and H. FUESS and G. WILL Eduard-Zintl-Znstitut
der TZi Darmstadt. Received
Lehrstiihl
12 March
fir
Strukturforschung
1968
The oxvfCen parameters x and t of ErPOa and ErVOa (I 4/amd) have been refined froa neutron diffraction powderdata by least squares analysis.=The P.-O aGd_V-O dikances in the PO:- and VOZ- tetrahedra are 1.54 and 1.72 A respectively. The bonding mechanism in the tetrahedra is discussed.
1. INTRODUCTION In the course of a study of the magnetic behavior of phosphates and van&&s of the rare earth elements we have refined the atomic crystal structure parameters of the oxygen ions in ErP04 and ErV04 from neutron diffraction powder data. ErP04 and ErV04 crystallize isomorphous with ZrSiO4 (zirconium) in the tetragonal space group I 4/amd-D:: (fig. 1) [1,2], with a unit cell at room tempezature [3] of a = b = = 6.863(3); c = 6.007(3) 4 for ErP04 and a = b = = ?.100(3); c = 6.279(3) A for ErV04 (standard deviations in parentheses). Er3+, P and V occupy special positions: Er3+: 4(a) 000, P, V: 4(b) OO$ and oxygen is in general position 0: 16(k) O.rz. Because of the comparatively small scattering amplitude of oxygen together with the high absorption of erbium for X-rays, the oxygen parameters cannot be determined reliably from X-ray data. The neutron scattering lengths of these elements [5] with b(Er) = 7.9 f, b(P) = 5.3 f, ii(V) = -0.5 f and b(O) = 5.77 f (If = = lo-l3 cm) on the other hand are of comparable magnitude and allow an accurate determination of the oxygen parameters in the presence of heavy elements.
speed of 1 deg/hour and at interlals of 0. lo in 20. A 235U fission chamber was used as a beam monitor, and the wave length was l-Q69 k Because of the high absorption cross section of erbium (u(Er) = 100 barn for 1.07 ii) a flat aluminum sample holder of 50 x 39 Y 3.4 mm3 was used. The absorption of the sample was determined by measuring the transmission of the direct beam through the sample, yielding coefficients of ~_rErP04) = 0.66 cm-1 and g(ErV04) = \ =0.87cm-. The refinement of the oxygen parameters was carried out by least squares methods [Sj with the 300 and 77OK data based on the structure factor F-values of 11 non-overlapping peaks in case of Table 1 Oxygen parameters x and z. and temperature factors of ErP04 at 300°K and of ErV04 at 300 and 77OK: standard deviations in parenthesis. ErP04
RESULTS
w(0) = y(O)
.
0.175 (2) 0.339
Z(O)
lo-l6
cm2
B(V)
R
=
F
hi
-
(3)
77%
0.1E7 (2)
0.1%
0.325 (3)
0.3% (21
(I)
0.G (23)
B(overal1)
B(O)
Neutron diffraction diagrams of powder ErP04 and ErV04, prepared by a sintering technique [3,4], were recorded at 300, 77 and 4.2”K at the
300’K
300%
B(Er)
2. EXPERIMENTAL
E A’04
FCTIC ‘
Fobs
0.037
0.37 (45:
0.27 (40)
0.53 (22)
c-54 (13)
0.54 (35)
0.64 (27)
0.027
0 -033
47
Volume 2, number 1
CHEMICAL PHYSICS LETTERS
Fig. I. Crystal structure of ErP04 and ErV04 (I 4/amd) and 8 peaks in case of ErV04. Individual temperature factors were used in the calculation, except for vanadium, where the initially determined overall temperature factor value was taken. The results are listed in table 1. Table 2 lists the C_iF2 values for ErP04 at 300’K and for ErVO4 at 77OK. The X-value in table 2 is defined as ErP04
with summation i over all planes contributing to the same reflection and summation k over all observed peaks. Table 3 lists some individual bond distances at 300°K for the tetrahedral coordination of P and V. and for the dodecahedral coordination of Er3+, which is coordinated to two sets of four equi-distant oxygen atoms at the six surrounding tetrahedra. 3. DISCUSSION The crystal structures of ErP04 and ErV04 consist of slightly distorted PO$’ and VOI- tetrahe’dra, with P or V in the center and Er3+ between the te$rahedra. The P-O bond length in ErPO4 is 1.54 A. This is in agreement with P-O distances found in other compounds containing PO4-tetrahedra: KH2PO4 with 1.538 A from neutron dif48
May 1968
The oxygen atoms are at the corners of the tetrahedra.
fraction [7,8]; and CaHP04 [9], CaHP04.2H20 [IO], HP04 [ll]. CalO!PP4)6(OH)2 1121, and LiMnPOq [13] with 1.54 A from X-ray diffraction. The bond shortening of 0.17 a for the phosphate ion from the single bond value of 1.71 A (calculated by the Schomaker-Stevenson correction [14]), has been explained by a partial doublebond character of the P-O bond 1151 and also by molecular orbital treatment of the X0”--tetrahedra (X = Si, P, S. or Cl) by Cruicks 41ank [ 161. In the PO;’ tetrahedral ion two strong n-bonding molecular orbitals are formed with the 3d,,2 I 2 2% and 3d72 orbitals of P and the appropriate and 2p’_’ orbitals of oxygen, using the oxygen combination 1/2@,1 +pg +p3 +p4) and l/2@\ +ph + + ,I& + pi). The four sp -hybrid orbitals of P over lap with the 2pu-orbitals of the oxygens forming covalent u-bonds. We have adopted this treatment tentatively for interpretation of the bonding mechanism in VO4tetrahedra, where four d3s-hybrid orbitals [ 171 1/2(s*~~~~f’~_,.*f~~~*) and I/~(.s*~~,,~~~~W~~) of the central V ion overlap with the 2Pu orbitals of the oxygens forming molecular o-orbitals. In general there is little known about the bonding mechanism in vanadium compounds. For the V-V sipgle bond length in the metal a value of 1.224 A is listed by Pauling 1151, expressing a
Volume 2. number I
CHEMICAL PHYSICS LETTERS
Table 2 Comparison of observed and calculated values
c
jF'
for ErP04 at 300°K and for ErV04 at 77OK. The asteriks mark reflections corrected for superimposed Alpeaks from the nluminum cryostat.
P.VO4 -
101
1
200
202
n. 0.
36
31
182
75
72 n. 0.
1968
Table 3 Main bond distances and angles at 300°K with standard deviations. ErP04 Distance (.&
3oooK
XrV04 770K
ErPOg 3oooh
hfay
P.V -01.2,3
1.54 (2)
ErVO4 Distance (b) 1.72 (2)
tetra-
Ol-
02
2.40 (2)
2.66 (2)
hedra
01 - 03
2.57 (2)
2.89 (2)
angle (deg)
angle (deg)
01 - P.V - 02
102.3 (9)
100.8 (81
01 - P.V - 03
113.2 (5)
124.0 (4)
Distance &
Distance (A)
2 11
57
52
2
112
92
88
5
4
220
0
n.0.
44
44
202
333
324
297
305-
301
294
282
476
468
Er - 04
2.29 (1)
2.27 (2)
Er - 01
2.36 (2)
2.44 (2)
103
209
218
389
386’
321
313
400
446
449
312
601
587
300
300
400
266
265
203
213
7
10
11
411
39
39
7
420
10
10
303
0
004
0
402
214
0 214
160
103
628_
469
429
401
615 290
204
120
50
422
19
80
501
14
431
333
413 224
30
94
2@4
200 469
224
294-
578
577
317
292
538
540
0 347
318
578 154
553
615
163 80
314
455
522
3
R
7
1
323
520
181
100
332
521
214
0.078
high d-character of the orbitals up to 40%. Due to the similarity
ence in electronegativity (~0 = 3.5; x 3+ = 1.4 [la]). With the observed value d = I.% A in ErV04 a bond shortening of 0.08 b is cakulated, which again shall be considered a consequence of the n-double-bond character of the V-O bond in the VO4-tetrahedra. In case of V04 we think of 3d_,_2_,)2and 3d,Z orbitals of V forming z-bonding m-olecular orbitals with the 2.&i and Zpi7’ orbitals of oxygen. The Er3+ ions between the oxygen tetrahedra have point symmetry Dw. They are surrounded by 8 qxygens at a mean distance of 2.33 A and 2.35 A for ErPO4 and ErV04 respectively_ The interaction of the ligand fieId with the 4f-shell of Er3+ has been estimated by Kuse and Jorgensen [19], using the angular overlap model 1201, with only u-anti-bonding on the basis of our oxygen parameters determineu by this neutron diffraction study.
REFERENCES
111W.O. Milligan. 0.037
in the metal
of
of the high dbonding character between the metal and the VOq-tetrahedra, we have taken this metallic bond length of V. With this assumption a V-O single bond length of 1.8q Ais calculated, using the oxygen value of 0.74 A, and applying the Schomaker-Stevenson correction for the differ-
L. M. Watt and H. H. Racheford, J. Phys. Chem. 53 (1949) 227. 121A.Durif. Acta Cryst. 9 (1956) 471. 131H. Schwarz. 2. anorg. allg. Chem. 323 (2963) 44. [41 G. MUller-Vogt. Diplomarbeit. Universit%t Karlsruhe (1968). ClarenI51 G. E. Bacon. Neutron Diffraction (&ford. don Press. 1962). [61 W. R. Busing. K.O. Martin and H.A. Levy. @RX, Report TM-305 (i962). !71 G. E. Bacon and R.S. Pease. Droc. Roy.Soc. (London) A220 (1953) 397. I81 G. E. Bacon and R.S. Pease. Proc. Rox.Soc. (London) A230 (1955) 359. 49
\SU”“,
[lo] Ill]
“I
Y.
C. A. Reevers, Acts Cryst. 11 (1958) 273. G. E. R.Schulze, Z.physik. Chemie Abt. B24 (1934) 215. 1121 A. S. Posner. A. Perloff and A. F. Diorio. Acta Cryst. 11 (1958) 308. 1131 S. Geller and J. L.Durand. Acta Cry&. 13 (1960) 325. 1141 V. Schomaker and D. P. Stevenson. 3. Am. Chem. sot. 63 (1941) 37. r151 L. PauIinir. The Nature of the Chemical Bond. 3rd ed. (Corn& University Press. Ithaca. New York, 1960).
50
ieioo. 1171 H.Z. Bchl?Jfer and G. Gliemann.
Ligandenfeldtheorie
(Akademische
EinfUhnmg in die Verlagsgesell-
schaft, Frankfurt a. M., 1967). [161 17. Gordy and W-3.0. Thomas. J. Chem. Phvs.
(1956) 439.
24
(191 D.Kuse and C.K. Jbrgensen. Chem.Phys. Letters 1 (1967) 314. [20] C. K. JBrgensen. R. Pappalardo and H.-H. Schmidt. ke. J. Chem. Phys- 39 (1963) 1422.