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Magnetic structure of CaMnSiO4
J. Phys. Chem. Solids
Pergamon
Press 1965. Vol. 26, pp. 927-930.
MAGNETIC L. G. CARON, Electrical
Engineering
STRUCTURE R. P. SANTORO
Dept., Massachusetts
Printed in Great Britain.
OF CaMnSi04*
and R. E. NEWNHAM
Institute of Technology,
Cambridge,
Mass.
(Received 5 October 1964)
Abstract--CaMnSiOs crystallizes in the olivine structure and is antiferromagnetic below 9°K; magnetic-susceptibility measurements gave 0 = 26’K and pen = 5.77 pi. The cation distribution and magnetic spin structure were established by neutron diffraction. Divalent manganese occupies the inversion centers with calcium in mirror plane positions, isostructural with CaMgSiO4. At low temperatures the Mn moments align in antiferromagnetic chains along b with spin directions collinear to c.
TRANSITION-metal ions occupy distorted octahedral sites in the olivine structure; some are located on inversion centers and others possess mirror-plane symmetry. In CrsBeO@) and the orthosilicates of Ni, Mn, Fe and Co,@) both sites are filled by magnetic ions. All five compounds are antiferromagnetic, and all except CosSiOs possess rather complicated spin structures. Partial substitution of nonmagnetic cations promises to yield useful information regarding the exchange interactions in the olivine lattice. In isomorphous lithiophilite, LiMnPO4, only the mirror-plane positions are occupied by magnetic ions.(a) It, too, is antiferromagnetic with a transition temperature near 35”K.(4) Complementary knowledge concerning the inversion-site exchange interactions can be obtained from the transition-metal calcium silicates. The magnetic properties of glaucochroite, CaMnSi04, are described in this paper. Calcium manganese orthosilicate was synthesized by sintering MnCOs, CaCOs, and finely divided SiOs (Cab-0-Sil, Cabot Corp.) in nitrogen at 105O’C. Accurate lattice parameters were determined from X-ray diffractometer patterns using slow scanning speeds and CrKo! radiation. Highangle data and least-squares refinement gave the orthorhombic cell dimensions a = 11.19 + O-01, b = 6.529 +0*005, and c = 4*944&O-004 A, close * Supported by the U.S. Air Force, Aeronautical Systems Division, under Contract AF 33(616)4X353, and by Advanced Research Projects Agency, Dept. of Defense, through Contract SD-90. 927
to the values reporJ$d by O’MARA.(~) The space group is Pnma = Di;l, with four formula-units per unit cell. Magnetic susceptibility data taken with a vibrating-sample magnetometer are shown in Fig. 1. The Curie-Weiss coefficients for the paramagnetic region arep,ff = 5.77 + O-07 PB and 8 = 26 + 2”K, with slight deviations below 50”K, and a small anomaly near 10°K. Neutron diffraction patterns of polycrystalline CaMnSi04 taken at room temperature and at 8°K are shown in Fig. 2. The calcium and manganese positions were determined from the nuclear intensities given in Table 1. These reflections are sensitive to the cation distribution because of the marked difference in scattering amplitude: Ca+O-49, Mn-O-37 x lo-12 cm. Calculations carried out with the fayalite coordinates@) show that about 90 per cent of the manganese occupies inversion centers. Glaucochroite is therefore isomorphous with monticellite (CaMgSiOd), in which Ca populates the mirror sites and Mg the inversion positions.(‘) The result is consistent with those obtained for LiMnPOa(a) and for Als-%(Fe, Cr),BeOa solid solutions.@) In each of the compounds the largest cation occupies the mirror-plane octahedra. Assuming Mn to be completely ordered in the inversion sites, the magnetic reflections in the lowtemperature neutron-diffraction pattern can be explained by the spin configuration shown in Fig. 3. Indexed on the chemical unit cell, the magnetic
928
L.
“0
G.
CARON,
20
40
R.
P.
60
and R. E. NEWNHAM
SANTORO
80
too
Temperature FIG. 1. Reciprocal
120
140
magnetic susceptibility of polycrystalline
intensities obey the selection rule h+ 1 odd, k odd, suggesting a collinear model with Mn moments at (O,O,O) and (B,&,$) antiparallel to those at (O,=&,O) and (&O,&). With c as the spin direction, cakulations based on this model gave excellent agreement with experiment (Table 2). The NCel temperature of CaMnSiO4 was determined by monitoring the intense 110 magnetic reflection as a function of temperature. It disappeared near 9”K, significantly less than that of LiMnPOe, TN = 35”K.(4) Judged by their transition temperatures, Mn mirror-plane interactions are stronger than those between inversion sites; Mn-0-Mn superexchange linkages are operative in LiMnP04 but not in CaMnSiO4. Two other antiferromagnetic interactions appear important in CaMnSi04. The chains of Mn ions parallel to b share octahedral edges, suggesting direct cationcation interactions via tze orbitals.(s) Neighboring
160
180
OK CaMnSiO4.
chains must couple through Mn-0-0-Mn other long-range exchange forces.
or
AcJsnowMgments-We take pleasure in thanking Prof. C. SHULL, Drs. A. WEDGWOOD, J. H. FANG, S. NOMURA and Messrs. M. REDMANand J. PEARSONfor their assistance. Numerical calculations were carried out at the M.I.T. Computation Center.
FIG. 3.
Magnetic structure of CaMnSiOa. Calcium, silicon and oxygen positions are omitted.
I
I
I
I 001
ooz DOE 3Oti
0 001
OOE
OOb
L.
930
G.
CARON,
R.
P.
SANTORO
and
R.
E.
NEWNHAM
Table 1. Comparison of observed and calculated nuclear intensities for several calcium-manganese distributions in CaMnSiOa. Percentages denote the fraction of Mn atoms occupying inversion sites. Calculated intensity Observed intensity
100%
90%
80%
50%
0%
110 950 540
202 980 405
114 960 540
38 905 710
144 455 1205
945 0 835
215
187
159
138
188
463
211 020 I
560
565
538
494
218
43
121 311 I
370
220
261
252
260
340
411 112 202 I
370
500
530
565
640
475
O-18
o-12
0.24
0.66
0.96
hkl 200 101 210 111 201 1
R
Table 2.
Comparison
of
observed and calculated magnetic intensities for three possible spin directions Calculated intensity
hkl 110 011
Observed intensity
a
b
c
580 50
476 263
217 224
597 89
226
438
R
REFERENCES 1. SANTOROR. P. and NEWNHAM R. E., J. Amer. Cer. Sot. 47,491 (1964). 2. NOMURA S., SANTOROR., FANG J. and NEWNHAM R., J. Phys. Chem. Solids 25,901 (1964). 3. GELL.ER S. and DURAND J. L., Actu Cryst. 13, 325 (1960).
0.38
0.69
0.11
4. MAYS J., Phys. Rev. 131, 38 (1963). 5. O’MARA J. A., Amer. Min. 36, 918 (1951). 6. HANKE K., N. Jb. Miner. Mh. 8, 192 (1963). 7. BROWN G. B. and WEST J., 2. Krist. 66, 154 (1927). 8. NEWNHAMR., SANTOROR., PEARSONJ. and JANSEN C., Amer. Min. 49, 427 (1964). 9. GOODENOUGHJ. B., Phys. Rev. 117, 1442 (1960).
Pergamon
Press 1965. Vol. 26, pp. 927-930.
MAGNETIC L. G. CARON, Electrical
Engineering
STRUCTURE R. P. SANTORO
Dept., Massachusetts
Printed in Great Britain.
OF CaMnSi04*
and R. E. NEWNHAM
Institute of Technology,
Cambridge,
Mass.
(Received 5 October 1964)
Abstract--CaMnSiOs crystallizes in the olivine structure and is antiferromagnetic below 9°K; magnetic-susceptibility measurements gave 0 = 26’K and pen = 5.77 pi. The cation distribution and magnetic spin structure were established by neutron diffraction. Divalent manganese occupies the inversion centers with calcium in mirror plane positions, isostructural with CaMgSiO4. At low temperatures the Mn moments align in antiferromagnetic chains along b with spin directions collinear to c.
TRANSITION-metal ions occupy distorted octahedral sites in the olivine structure; some are located on inversion centers and others possess mirror-plane symmetry. In CrsBeO@) and the orthosilicates of Ni, Mn, Fe and Co,@) both sites are filled by magnetic ions. All five compounds are antiferromagnetic, and all except CosSiOs possess rather complicated spin structures. Partial substitution of nonmagnetic cations promises to yield useful information regarding the exchange interactions in the olivine lattice. In isomorphous lithiophilite, LiMnPO4, only the mirror-plane positions are occupied by magnetic ions.(a) It, too, is antiferromagnetic with a transition temperature near 35”K.(4) Complementary knowledge concerning the inversion-site exchange interactions can be obtained from the transition-metal calcium silicates. The magnetic properties of glaucochroite, CaMnSi04, are described in this paper. Calcium manganese orthosilicate was synthesized by sintering MnCOs, CaCOs, and finely divided SiOs (Cab-0-Sil, Cabot Corp.) in nitrogen at 105O’C. Accurate lattice parameters were determined from X-ray diffractometer patterns using slow scanning speeds and CrKo! radiation. Highangle data and least-squares refinement gave the orthorhombic cell dimensions a = 11.19 + O-01, b = 6.529 +0*005, and c = 4*944&O-004 A, close * Supported by the U.S. Air Force, Aeronautical Systems Division, under Contract AF 33(616)4X353, and by Advanced Research Projects Agency, Dept. of Defense, through Contract SD-90. 927
to the values reporJ$d by O’MARA.(~) The space group is Pnma = Di;l, with four formula-units per unit cell. Magnetic susceptibility data taken with a vibrating-sample magnetometer are shown in Fig. 1. The Curie-Weiss coefficients for the paramagnetic region arep,ff = 5.77 + O-07 PB and 8 = 26 + 2”K, with slight deviations below 50”K, and a small anomaly near 10°K. Neutron diffraction patterns of polycrystalline CaMnSi04 taken at room temperature and at 8°K are shown in Fig. 2. The calcium and manganese positions were determined from the nuclear intensities given in Table 1. These reflections are sensitive to the cation distribution because of the marked difference in scattering amplitude: Ca+O-49, Mn-O-37 x lo-12 cm. Calculations carried out with the fayalite coordinates@) show that about 90 per cent of the manganese occupies inversion centers. Glaucochroite is therefore isomorphous with monticellite (CaMgSiOd), in which Ca populates the mirror sites and Mg the inversion positions.(‘) The result is consistent with those obtained for LiMnPOa(a) and for Als-%(Fe, Cr),BeOa solid solutions.@) In each of the compounds the largest cation occupies the mirror-plane octahedra. Assuming Mn to be completely ordered in the inversion sites, the magnetic reflections in the lowtemperature neutron-diffraction pattern can be explained by the spin configuration shown in Fig. 3. Indexed on the chemical unit cell, the magnetic
928
L.
“0
G.
CARON,
20
40
R.
P.
60
and R. E. NEWNHAM
SANTORO
80
too
Temperature FIG. 1. Reciprocal
120
140
magnetic susceptibility of polycrystalline
intensities obey the selection rule h+ 1 odd, k odd, suggesting a collinear model with Mn moments at (O,O,O) and (B,&,$) antiparallel to those at (O,=&,O) and (&O,&). With c as the spin direction, cakulations based on this model gave excellent agreement with experiment (Table 2). The NCel temperature of CaMnSiO4 was determined by monitoring the intense 110 magnetic reflection as a function of temperature. It disappeared near 9”K, significantly less than that of LiMnPOe, TN = 35”K.(4) Judged by their transition temperatures, Mn mirror-plane interactions are stronger than those between inversion sites; Mn-0-Mn superexchange linkages are operative in LiMnP04 but not in CaMnSiO4. Two other antiferromagnetic interactions appear important in CaMnSi04. The chains of Mn ions parallel to b share octahedral edges, suggesting direct cationcation interactions via tze orbitals.(s) Neighboring
160
180
OK CaMnSiO4.
chains must couple through Mn-0-0-Mn other long-range exchange forces.
or
AcJsnowMgments-We take pleasure in thanking Prof. C. SHULL, Drs. A. WEDGWOOD, J. H. FANG, S. NOMURA and Messrs. M. REDMANand J. PEARSONfor their assistance. Numerical calculations were carried out at the M.I.T. Computation Center.
FIG. 3.
Magnetic structure of CaMnSiOa. Calcium, silicon and oxygen positions are omitted.
I
I
I
I 001
ooz DOE 3Oti
0 001
OOE
OOb
L.
930
G.
CARON,
R.
P.
SANTORO
and
R.
E.
NEWNHAM
Table 1. Comparison of observed and calculated nuclear intensities for several calcium-manganese distributions in CaMnSiOa. Percentages denote the fraction of Mn atoms occupying inversion sites. Calculated intensity Observed intensity
100%
90%
80%
50%
0%
110 950 540
202 980 405
114 960 540
38 905 710
144 455 1205
945 0 835
215
187
159
138
188
463
211 020 I
560
565
538
494
218
43
121 311 I
370
220
261
252
260
340
411 112 202 I
370
500
530
565
640
475
O-18
o-12
0.24
0.66
0.96
hkl 200 101 210 111 201 1
R
Table 2.
Comparison
of
observed and calculated magnetic intensities for three possible spin directions Calculated intensity
hkl 110 011
Observed intensity
a
b
c
580 50
476 263
217 224
597 89
226
438
R
REFERENCES 1. SANTOROR. P. and NEWNHAM R. E., J. Amer. Cer. Sot. 47,491 (1964). 2. NOMURA S., SANTOROR., FANG J. and NEWNHAM R., J. Phys. Chem. Solids 25,901 (1964). 3. GELL.ER S. and DURAND J. L., Actu Cryst. 13, 325 (1960).
0.38
0.69
0.11
4. MAYS J., Phys. Rev. 131, 38 (1963). 5. O’MARA J. A., Amer. Min. 36, 918 (1951). 6. HANKE K., N. Jb. Miner. Mh. 8, 192 (1963). 7. BROWN G. B. and WEST J., 2. Krist. 66, 154 (1927). 8. NEWNHAMR., SANTOROR., PEARSONJ. and JANSEN C., Amer. Min. 49, 427 (1964). 9. GOODENOUGHJ. B., Phys. Rev. 117, 1442 (1960).