- Email: [email protected]
Single-crystal structural study of a natural (Zn, Mn) ferrite
Materials Research Bulletin, Vol. 31, No. 12, pp. 1587-1592, 19% Copyright 0 1996 Elsevis Scieme Ltd Printed in the USA. All rights reserved 0025-5408/96 $15.00 +.OO
Pergamon
PI1 SOOZS-5408(96)00156-O
SINGLE-CRYSTAL
STRUCTURAL
STUDY OF A NATURAL
(Zn, Mn) FERRITE
J.R. Marcano
UNEXPO, Section de Fisica, Puerto Ordaz, Venezuela A.E. Mora
Departamento de Fisica, Facultad de Ciencias, ULA, Merida, Venezuela 0. Odreman
Escuela de Ing. Geologica, Facultad de Ingenieria, ULA, Mdrida, Venezuela J.M. Delgado*
Departamento de Quhnica, Facultad de Ciencias, ULA, Apdo. 40, La Hechicera, Merida 525 1, Venezuela (Refereed) (Received February 27, 1996; Accepted March 20, 1996)
ABSTRACT
The structure of a natural ferrite with chemical composition Zn&vIn~.~sFe~5004has been determined using X-ray single crystal diffraction techniques. Unlike ZnFea04 and MnFe204, this material is tetragonal with space group 14&und, a = 5.964(2) A, c = 8.449(3) A, V = 300.5(2) A3, and Z = 4. The structure was solved using the Direct Methods facilities of SHELXS-86 and refined by least-squares using SHELXL-93. The final disagreement factors were R(F) = 0.0170, wR(@ = 0.0440 and S = 1.398 for all 82 independent reflections merged from the 476 reflections measured. KEYWORDS: A. oxides, C. X-ray diffraction, D. crystal structure INTRODUCTION
Using X-ray diffraction and electron microscopy techniques, a number of oxides and sulfides have been identified in minerals extracted from the mines of the Bailadores region *To whom correspondence should be addressed. 1587
1588
J.R. MARCANO et al.
Vol. 31,No.12
in Merida, Venezuela (1,2). According to the literature an important fraction of them displays interesting physical properties. For some of these materials, crystals suitable for single crystal X-ray diffraction studies have been isolated. One of such materials is a natural ferrite with chemical composition Zn0.,sMno75Fe15oO4.In the literature there are reports of two minerals with related compositions: franklinite (3,4) and jacobsite (4,5), ideally ZnFe204 and MnFe*O+ respectively. These minerals and their synthetic counterparts are reported to exhibit the spine1 structure, space group Fd3m mo. 227). As it is well-known, the AB2X4 spine1 structure can be viewed as a cubic close-packed array of anions in which one-eight of the tetrahedral and one-half of the octahedral sites are occupied by the cations. In this structure, a wide range of cation distributions among the available sites is possible, ranging from the so-called normal spine1 in which the A cations occupy the tetrahedral sites and the B cations the octahedral sites, to the inverse spine1 in which one-half of the B cations are in the tetrahedral sites while the other half and the A cations share the octahedral sites. The present single crystal structural study has been carried out in order to determine the structure a natural (Zn,Mn) ferrite and to compare it with the structure reported for other natural or synthesized ferrites. Ferrites are certainly one of the most extensively studied family of magnetic materials. They are been used in a number of technologically important applications ranging from microwave devices to magnetic and magneto-optic recording for electronic information mass storage (6). EXPERIMENTAL A number of dark single crystal fragments were isolated from the mineral sample and analyzed with a Buerger precession camera. The crystal which produced the best precession photographs was selected to carried out the structure determination process. Chemical analysis performed with a Kevex EDX equipped Hitachi S-2500 scanning electron microscope on these selected crystal fragments indicated the presence of Zn, Mn, and Fe in an atomic ratio of approximately 1: 1:2 (Zno ,sMno -rsFeI 500~). The precession photographs revealed that this material was tetragonal but with strong pseudo-cubic symmetry and systematic absences, hkl: h+ k+l = 2n + 1, hk0: h, k = 2n + 1, and hhl: 2h +I = 4n + 1. These absences uniquely determined the space group to be 14Jamd [No. 1411. Refined lattice parameters were determined from a least-squares refinement of the angular positions of 25 well-centered reflections in the range 20” I 28 < 30” using a PC-controlled Nicolet P3/F diffractometer. The data were collected, processed and corrected for absorption effects using programs of the UCLA package (7). Three standard reflections measured every 97 reflections during the course of the data collection showed variations in intensity less than 1%. The v-scans for several reflections revealed significant variation in intensity. Consequently, an absorption correction was applied. The crystal data, intensity data collection and refinement parameters are presented in Table 1. RESULTS AND DISCUSSION The initial positions for all the atoms were determined by direct methods in space group 14Jamd [No. 1411 with the program SHELXS-86 (8). The oxygen atoms defined a cubic close-packed array of atoms with the cations occupying tetrahedral and octahedral sites. The refinement of the structure was carried out by full-matrix least squares on F* using
SPINEL FERRITE
Vol. 31, No. 12
1589
TABLE 1 Crystal Data, Intensity Data Collection and Structure Refinement Parameters for Zno.&Ino.7sFel.so04
~
Intensity Data Collection and Refinement
Parameters
X-radiation
MoKa, (1L= 0.7107 A)
Refs. measured
476
Scan type
8-20
Ind. reflections
82
29 range (“)
4” 5 20 I 50”
0.0428; 0.0278
Scan range (“)
1.3 below Ka,
R(F); % (F) No. of variables
18
to 1.6 above KaZ
R,;wR* [I 2 20(I)]
0.0167; 0.0433
Scan speed (“hnin)
3
R,;wR~ [all data]
0.0170; 0.0440
Abs. correction
emp. ( yr-scan)
Goodness-of-fit
1.398
Normal. transm. fact.
1.000-0.714
SHELXL-93 (9). As a starting model Zn was placed in the tetrahedral sites and Fe in the octahedral sites. With anisotropic thermal parameters for all atoms, the refinement converged to R(F) = 0.0241, wR(p) = 0.1133, and S = 1.090 for the 82 unique data and R(F) = 0.0237, wR(p) = 0.1078 for 81 reflections with Y 2 20 (I). The thermal parameters were reasonable and the maximum and minimum electron density in the final difference map were +1.028 and -0.555 e/A’. In order to establish the cation ordering scheme, the cations were allowed to disorder over the two sites with the following constraints: (a) each atomic site must be Mollyoccupied, (b) all atoms must have the same thermal parameters at a given site, and (c) the total number of atoms distributed over the two cationic sites must be consistent with the composition of the material based on the chemical analysis (Zno.7sMno.7~Fel.so04). Since the atomic scattering factor for Fe was used for both Mn and Fe, because of their proximity in the periodic table, from the X-ray point of view the composition of this material is Zno 7sFez.zs04. The structure refinement converged to R(F) = 0.0170, wR(p) = 0.0440, andS = 1.398 for the 82 unique data and R(F) = 0.0167, wR(p) = 0.0433 for 81 reflections with I 2 20(I). The maximum and minimum electron density in the final difference map were +0.333 and -0.495 e/A3. No unusual trends were found in an analysis of variance in terms of F,. The structure was standardized using the program STRUCTURE TIDY (10). The final positional parameters and the cation ordering scheme are presented in Table 2. The bond distances and angles are given in Table 3. The strong pseudo-cubic symmetry observed in the precession photographs suggests a structural relation of this material with the spine1 structure type. This relation can be confirmed by comparing the atomic coordinates obtained in space group 14Jamd (No. 14 1)with those derived from the spine1 structure type (space group Fd3m, No. 227) using the matrix transformation: al, = -1/2a,, + 1/2a2,, azt =-1/2a,, - 1/2a2,, ct = a3c,which relates the two cells.
J.R. MARCANO et al.
1590
Vol. 31, No.12
TABLE 2
Coordinates, Occupancy Factors and Equivalent Isotropic Thermal Parameters for Zno &In0 75Fel5004 Site
Atom
Occup. fact.
X
Z
.F
v,
(A2y
0.0919(7)
Znl
0
4(b) Fe( Mn) 1
0.0331(7)
Zli2
0.0018(7) 8(c)
0.010(l)
0
0
0
0.009( 1)
0
0.5226(7)
0.2391(4)
0.010(2)
0.2482(7)
Fe( Mn)2
0.5000
16(h)
0
0.3750
0.2500
a I/, = l/3 of the trace of the orthogonalized
The refined values
for occupancies
Ii, tensor.
of the two cationic
sites showed
the following
distribution:
Tetrahedral site:
Zn : Fe(Mn):
0.0919(7) / [0.0919(7) + 0.0331(7)] = 0.735(5) 0.0331(7) / [0.0919(7) + 0.0331(7)] = 0.264(5)
Octahedral site:
Zn : Fe(Mn):
0018(7) / [0.0018(7) + 0.2482(7)] = 0.007(5) 0.2482(7) / [0.0018(7) + 0.2482(7)] = 0.993(5)
Thus,
the refined
cation
distribution
can be represented
in the AB2X4 form as
[ZIlo74(1,Fe(Mn)02~(1)][Z~ool(l,Fe(Mn)l ~(~~104.It should be noted that the presence of Zn in the octahedral site is statistically insignificant. The standard deviations for the occupancy factors obtained from the least-squares refinement are very small. However, one might expect larger values considering the accuracy of the chemical analysis and the assumption that Fe and Mn have the same scattering factor. In AB2X4 spinel-type of systems, the breaking off the cubic symmetry may be due to distortions of the AX4 tetrahedra and/or BXs octahedra. In the material under study, the
TABLE 3 Selected Bond Lengths (A) and Angles (“>for Zno T5Mno75Fel5004 Zn-0
1.990(3) x 4
o-Zrk-O
109.44(9) x 2
O-ZIP-O
109.5(2) x 2
0-Zn-0
109.44(9) x 2
Fe(Mn)-O
2.018(2) x 4
0-Fe(Mn)XI
180.0 x 3
Fe(Mn)-O
2.025(3) x 2
0-Fe(Mn)4I
84.47(13) x 2
0-Fe(Mn)XI
95.53(13) x 2
0-Fe(Mn)-O
95.44(12) x 4
0-Fe(Mn)-O
84.56(12) x 4
Vol. 31, No. 12
1591
SPlNEL FERRITE
69 Zn b
@
WW
00
FIG. 1 Unit cell of Zn(Fe,Mn)zOd. tetragonal distortion observed appears to be caused by the distortion of the Fe(Mn) octahedra. The tetrahedra formed by the Zn atoms are regular as shown by the bond lengths and angles displayed in Table 3. The deformation of the Fe(Mn) octahedra might be attributed to the Jahn-Teller effect. Since it is not expected any distortion with Zn2’, Mnzf, or Fe3+,because ZnF@04 and M.nFe20dare cubic (4), the distortion observed in Zn(Fe,Mn)zOd might be due to the presence of the high-spin d” Mn3’ ion. This phenomenon has been observed in several materials containing Mn3’: hausmannite, a mineral with composition Mn304 (1 l), CuMnz04 (12), and CdMnzO4 (13). These materials also exhibit tetragonal symmetry with space group 14r/amd (No. 141). The possibility that this material is a new mineral is currently being examined. Figure 1 shows the structure of Zno75Mno.75Fel 5004. CONCLUSIONS
Single crystal X-ray diffraction techniques were used in the present study to determine the structure of a natural (Zn,Mn)-ferrite with chemical composition Zno 7Mn0 75Fe,5004,which might be a new mineral. This material is tetragonal but may be considered as a spine1 derivative. The matrix transformation alt = -1/2a,, + 1/2az,, 82, = -1/2al, - 1/2a2,, ct = a3, relates the two cells. It was possible to establish the cation ordering scheme upon refinement of the structural parameters. ACKNOWLEDGMENTS
The authors wish to thank Karin Cenzual (U. of Geneva) for valuable discussions. This work was supported by CDCHT-ULA grant C-727-95-08-A. Acknowledgment is made to the Programa de Nuevas Tecnologias of CONICIT, through grant NM- 18, for the establishment of the Centro National de Difraccion de Rayos-X (CNDR-X).
1592
J.R. MARCANO et al.
Vol. 31, No.12
REFERENCES
2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
J.A. Henao, G. Diaz de Delgado, J.M. Delgado, F.J. Castrillo, 0. Odreman, Mater. Rex Bull. 29, 1121 (1994). J.M. Delgado, unpublished results. PDF, Card No. 22-1012. Powder Diffraction File (1994). International Centre for Diffraction Data. Swarthmore, PA. U. Koenig, G. Cho, J. Appl. Crystuilogr. I, 124 (1968). PDF, Card No. 1O-319. Powder DiITraction File (1994). International Centre for Diffraction Data. Swarthmore, PA. See for instance, MRS Bull. 15(3) (1990). C.E. Strouse, UCLA Crystallographic Package, University of California, Los Angeles, (1988). G.M. Sheldrick, SHELXS-86. Program for Crystal Structure Determination. University of Gottingen, Germany, (1986). G.M. Sheldrick, SHELXL-93. Program for Crystal Structure Refinement. University of Gottingen, Germany (1993). L.M. Gelato and E. Par-the,J. Appl. Crystallogr. 20, 139 (1987). D. Jarosch, Miner. Petr. 37, 15 (1987). M. Beley, L. Padel, C. Bemier, Ann. Chim. Fr. 429 (1978). N.K. Radhakrishnan, A.B. Biswas, Z. Kristuff. 117 (1975).
Pergamon
PI1 SOOZS-5408(96)00156-O
SINGLE-CRYSTAL
STRUCTURAL
STUDY OF A NATURAL
(Zn, Mn) FERRITE
J.R. Marcano
UNEXPO, Section de Fisica, Puerto Ordaz, Venezuela A.E. Mora
Departamento de Fisica, Facultad de Ciencias, ULA, Merida, Venezuela 0. Odreman
Escuela de Ing. Geologica, Facultad de Ingenieria, ULA, Mdrida, Venezuela J.M. Delgado*
Departamento de Quhnica, Facultad de Ciencias, ULA, Apdo. 40, La Hechicera, Merida 525 1, Venezuela (Refereed) (Received February 27, 1996; Accepted March 20, 1996)
ABSTRACT
The structure of a natural ferrite with chemical composition Zn&vIn~.~sFe~5004has been determined using X-ray single crystal diffraction techniques. Unlike ZnFea04 and MnFe204, this material is tetragonal with space group 14&und, a = 5.964(2) A, c = 8.449(3) A, V = 300.5(2) A3, and Z = 4. The structure was solved using the Direct Methods facilities of SHELXS-86 and refined by least-squares using SHELXL-93. The final disagreement factors were R(F) = 0.0170, wR(@ = 0.0440 and S = 1.398 for all 82 independent reflections merged from the 476 reflections measured. KEYWORDS: A. oxides, C. X-ray diffraction, D. crystal structure INTRODUCTION
Using X-ray diffraction and electron microscopy techniques, a number of oxides and sulfides have been identified in minerals extracted from the mines of the Bailadores region *To whom correspondence should be addressed. 1587
1588
J.R. MARCANO et al.
Vol. 31,No.12
in Merida, Venezuela (1,2). According to the literature an important fraction of them displays interesting physical properties. For some of these materials, crystals suitable for single crystal X-ray diffraction studies have been isolated. One of such materials is a natural ferrite with chemical composition Zn0.,sMno75Fe15oO4.In the literature there are reports of two minerals with related compositions: franklinite (3,4) and jacobsite (4,5), ideally ZnFe204 and MnFe*O+ respectively. These minerals and their synthetic counterparts are reported to exhibit the spine1 structure, space group Fd3m mo. 227). As it is well-known, the AB2X4 spine1 structure can be viewed as a cubic close-packed array of anions in which one-eight of the tetrahedral and one-half of the octahedral sites are occupied by the cations. In this structure, a wide range of cation distributions among the available sites is possible, ranging from the so-called normal spine1 in which the A cations occupy the tetrahedral sites and the B cations the octahedral sites, to the inverse spine1 in which one-half of the B cations are in the tetrahedral sites while the other half and the A cations share the octahedral sites. The present single crystal structural study has been carried out in order to determine the structure a natural (Zn,Mn) ferrite and to compare it with the structure reported for other natural or synthesized ferrites. Ferrites are certainly one of the most extensively studied family of magnetic materials. They are been used in a number of technologically important applications ranging from microwave devices to magnetic and magneto-optic recording for electronic information mass storage (6). EXPERIMENTAL A number of dark single crystal fragments were isolated from the mineral sample and analyzed with a Buerger precession camera. The crystal which produced the best precession photographs was selected to carried out the structure determination process. Chemical analysis performed with a Kevex EDX equipped Hitachi S-2500 scanning electron microscope on these selected crystal fragments indicated the presence of Zn, Mn, and Fe in an atomic ratio of approximately 1: 1:2 (Zno ,sMno -rsFeI 500~). The precession photographs revealed that this material was tetragonal but with strong pseudo-cubic symmetry and systematic absences, hkl: h+ k+l = 2n + 1, hk0: h, k = 2n + 1, and hhl: 2h +I = 4n + 1. These absences uniquely determined the space group to be 14Jamd [No. 1411. Refined lattice parameters were determined from a least-squares refinement of the angular positions of 25 well-centered reflections in the range 20” I 28 < 30” using a PC-controlled Nicolet P3/F diffractometer. The data were collected, processed and corrected for absorption effects using programs of the UCLA package (7). Three standard reflections measured every 97 reflections during the course of the data collection showed variations in intensity less than 1%. The v-scans for several reflections revealed significant variation in intensity. Consequently, an absorption correction was applied. The crystal data, intensity data collection and refinement parameters are presented in Table 1. RESULTS AND DISCUSSION The initial positions for all the atoms were determined by direct methods in space group 14Jamd [No. 1411 with the program SHELXS-86 (8). The oxygen atoms defined a cubic close-packed array of atoms with the cations occupying tetrahedral and octahedral sites. The refinement of the structure was carried out by full-matrix least squares on F* using
SPINEL FERRITE
Vol. 31, No. 12
1589
TABLE 1 Crystal Data, Intensity Data Collection and Structure Refinement Parameters for Zno.&Ino.7sFel.so04
~
Intensity Data Collection and Refinement
Parameters
X-radiation
MoKa, (1L= 0.7107 A)
Refs. measured
476
Scan type
8-20
Ind. reflections
82
29 range (“)
4” 5 20 I 50”
0.0428; 0.0278
Scan range (“)
1.3 below Ka,
R(F); % (F) No. of variables
18
to 1.6 above KaZ
R,;wR* [I 2 20(I)]
0.0167; 0.0433
Scan speed (“hnin)
3
R,;wR~ [all data]
0.0170; 0.0440
Abs. correction
emp. ( yr-scan)
Goodness-of-fit
1.398
Normal. transm. fact.
1.000-0.714
SHELXL-93 (9). As a starting model Zn was placed in the tetrahedral sites and Fe in the octahedral sites. With anisotropic thermal parameters for all atoms, the refinement converged to R(F) = 0.0241, wR(p) = 0.1133, and S = 1.090 for the 82 unique data and R(F) = 0.0237, wR(p) = 0.1078 for 81 reflections with Y 2 20 (I). The thermal parameters were reasonable and the maximum and minimum electron density in the final difference map were +1.028 and -0.555 e/A’. In order to establish the cation ordering scheme, the cations were allowed to disorder over the two sites with the following constraints: (a) each atomic site must be Mollyoccupied, (b) all atoms must have the same thermal parameters at a given site, and (c) the total number of atoms distributed over the two cationic sites must be consistent with the composition of the material based on the chemical analysis (Zno.7sMno.7~Fel.so04). Since the atomic scattering factor for Fe was used for both Mn and Fe, because of their proximity in the periodic table, from the X-ray point of view the composition of this material is Zno 7sFez.zs04. The structure refinement converged to R(F) = 0.0170, wR(p) = 0.0440, andS = 1.398 for the 82 unique data and R(F) = 0.0167, wR(p) = 0.0433 for 81 reflections with I 2 20(I). The maximum and minimum electron density in the final difference map were +0.333 and -0.495 e/A3. No unusual trends were found in an analysis of variance in terms of F,. The structure was standardized using the program STRUCTURE TIDY (10). The final positional parameters and the cation ordering scheme are presented in Table 2. The bond distances and angles are given in Table 3. The strong pseudo-cubic symmetry observed in the precession photographs suggests a structural relation of this material with the spine1 structure type. This relation can be confirmed by comparing the atomic coordinates obtained in space group 14Jamd (No. 14 1)with those derived from the spine1 structure type (space group Fd3m, No. 227) using the matrix transformation: al, = -1/2a,, + 1/2a2,, azt =-1/2a,, - 1/2a2,, ct = a3c,which relates the two cells.
J.R. MARCANO et al.
1590
Vol. 31, No.12
TABLE 2
Coordinates, Occupancy Factors and Equivalent Isotropic Thermal Parameters for Zno &In0 75Fel5004 Site
Atom
Occup. fact.
X
Z
.F
v,
(A2y
0.0919(7)
Znl
0
4(b) Fe( Mn) 1
0.0331(7)
Zli2
0.0018(7) 8(c)
0.010(l)
0
0
0
0.009( 1)
0
0.5226(7)
0.2391(4)
0.010(2)
0.2482(7)
Fe( Mn)2
0.5000
16(h)
0
0.3750
0.2500
a I/, = l/3 of the trace of the orthogonalized
The refined values
for occupancies
Ii, tensor.
of the two cationic
sites showed
the following
distribution:
Tetrahedral site:
Zn : Fe(Mn):
0.0919(7) / [0.0919(7) + 0.0331(7)] = 0.735(5) 0.0331(7) / [0.0919(7) + 0.0331(7)] = 0.264(5)
Octahedral site:
Zn : Fe(Mn):
0018(7) / [0.0018(7) + 0.2482(7)] = 0.007(5) 0.2482(7) / [0.0018(7) + 0.2482(7)] = 0.993(5)
Thus,
the refined
cation
distribution
can be represented
in the AB2X4 form as
[ZIlo74(1,Fe(Mn)02~(1)][Z~ool(l,Fe(Mn)l ~(~~104.It should be noted that the presence of Zn in the octahedral site is statistically insignificant. The standard deviations for the occupancy factors obtained from the least-squares refinement are very small. However, one might expect larger values considering the accuracy of the chemical analysis and the assumption that Fe and Mn have the same scattering factor. In AB2X4 spinel-type of systems, the breaking off the cubic symmetry may be due to distortions of the AX4 tetrahedra and/or BXs octahedra. In the material under study, the
TABLE 3 Selected Bond Lengths (A) and Angles (“>for Zno T5Mno75Fel5004 Zn-0
1.990(3) x 4
o-Zrk-O
109.44(9) x 2
O-ZIP-O
109.5(2) x 2
0-Zn-0
109.44(9) x 2
Fe(Mn)-O
2.018(2) x 4
0-Fe(Mn)XI
180.0 x 3
Fe(Mn)-O
2.025(3) x 2
0-Fe(Mn)4I
84.47(13) x 2
0-Fe(Mn)XI
95.53(13) x 2
0-Fe(Mn)-O
95.44(12) x 4
0-Fe(Mn)-O
84.56(12) x 4
Vol. 31, No. 12
1591
SPlNEL FERRITE
69 Zn b
@
WW
00
FIG. 1 Unit cell of Zn(Fe,Mn)zOd. tetragonal distortion observed appears to be caused by the distortion of the Fe(Mn) octahedra. The tetrahedra formed by the Zn atoms are regular as shown by the bond lengths and angles displayed in Table 3. The deformation of the Fe(Mn) octahedra might be attributed to the Jahn-Teller effect. Since it is not expected any distortion with Zn2’, Mnzf, or Fe3+,because ZnF@04 and M.nFe20dare cubic (4), the distortion observed in Zn(Fe,Mn)zOd might be due to the presence of the high-spin d” Mn3’ ion. This phenomenon has been observed in several materials containing Mn3’: hausmannite, a mineral with composition Mn304 (1 l), CuMnz04 (12), and CdMnzO4 (13). These materials also exhibit tetragonal symmetry with space group 14r/amd (No. 141). The possibility that this material is a new mineral is currently being examined. Figure 1 shows the structure of Zno75Mno.75Fel 5004. CONCLUSIONS
Single crystal X-ray diffraction techniques were used in the present study to determine the structure of a natural (Zn,Mn)-ferrite with chemical composition Zno 7Mn0 75Fe,5004,which might be a new mineral. This material is tetragonal but may be considered as a spine1 derivative. The matrix transformation alt = -1/2a,, + 1/2az,, 82, = -1/2al, - 1/2a2,, ct = a3, relates the two cells. It was possible to establish the cation ordering scheme upon refinement of the structural parameters. ACKNOWLEDGMENTS
The authors wish to thank Karin Cenzual (U. of Geneva) for valuable discussions. This work was supported by CDCHT-ULA grant C-727-95-08-A. Acknowledgment is made to the Programa de Nuevas Tecnologias of CONICIT, through grant NM- 18, for the establishment of the Centro National de Difraccion de Rayos-X (CNDR-X).
1592
J.R. MARCANO et al.
Vol. 31, No.12
REFERENCES
2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
J.A. Henao, G. Diaz de Delgado, J.M. Delgado, F.J. Castrillo, 0. Odreman, Mater. Rex Bull. 29, 1121 (1994). J.M. Delgado, unpublished results. PDF, Card No. 22-1012. Powder Diffraction File (1994). International Centre for Diffraction Data. Swarthmore, PA. U. Koenig, G. Cho, J. Appl. Crystuilogr. I, 124 (1968). PDF, Card No. 1O-319. Powder DiITraction File (1994). International Centre for Diffraction Data. Swarthmore, PA. See for instance, MRS Bull. 15(3) (1990). C.E. Strouse, UCLA Crystallographic Package, University of California, Los Angeles, (1988). G.M. Sheldrick, SHELXS-86. Program for Crystal Structure Determination. University of Gottingen, Germany, (1986). G.M. Sheldrick, SHELXL-93. Program for Crystal Structure Refinement. University of Gottingen, Germany (1993). L.M. Gelato and E. Par-the,J. Appl. Crystallogr. 20, 139 (1987). D. Jarosch, Miner. Petr. 37, 15 (1987). M. Beley, L. Padel, C. Bemier, Ann. Chim. Fr. 429 (1978). N.K. Radhakrishnan, A.B. Biswas, Z. Kristuff. 117 (1975).