CORRELATION BETWEEN MAGNETISM AND LATTICE SPACING c IN COMPOUNDS WITH NiAs-TYPE STRUCTURES

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CORRELATION BETWEEN MAGNETISM AND

LATTICE SPACING c IN COMPOUNDS WITH

NiAs-TYPE STRUCTURES

T. Kamimura

To cite this version:

(2)

JOURNAL DE PHYSIQUE

Colloque C8, Suppl6ment au no 12, Tome 49, d6cembre 1988

CORRELATION BETWEEN MAGNETISM AND LATTICE SPACING

c

IN

COMPOUNDS WITH NiAs-TYPE STRUCTURES

College of General Education, Physics Department, Tohoku University, Kawauchi, Sendai 980, Japan

Abstract. - Among compounds with NiAs-type structure, those of transition elements lighter than Fe and those of Co and Ni have been classified into separate groups according t o their nature of magnetism and lattice spacing c. The strong correlation between these properties is discussed with referring to the results on (Fel-,Cox), Ses and (Mnl-,Tix) Sb.

It has been known that a number of compounds formed from 3d-transition elements (M) and the elements

(X) of Vb and VIb groups in the periodic table have NiAs-type or slightly distorted NiAs-type structures at or near the stoichiometric composition (1:1), and that they show interesting magnetic behavior. In the ideal NiAs structure, metal atoms occupy the octahedral sites in the close-packed hexagonal array of metalloid ions. At the non-stoichiometric composition, vacancies at unoccupied octahedral sites in MI-,X, and excess metal atoms at tetrahedral interstices in Ml+,X play an important role in the system. Some of Ml-,X exhi- bit ordered arrangements of vacant metal sites. MnS and also MnSe and MnTe at high temperature have NaCl type structures. In the same way as in the NiAs type structure, Mn atoms in these compounds occupy octahedral sites in the close-packed cubic array of metalloid ions.

The magnetism possessed by these compounds is classified into two categories depending upon their constituents [I]: the magnetic compounds with rela- tively high transition temperature are those of Cr, Mn and Fe. These compounds are referred t o as "strongly magnetic" in the following. Other compounds belong to a class of "weakly magnetic"

.

Both chalcogenides and pnictides of Co or Ni are weakly paramagnetic. a-NiS is Pauli-paramagnetic above

TN

= 265 K. The low temperature antiferromagnetic phase of this com- pound is an exception. The origin of this antiferroma gnetism is considered to be distinct from that of the strongly magnetic compounds mentioned above. Com- pounds of Ti and V are also weakly paramagnetic; non- stoichiometric V chalcogenides such as V3X4 or VzX3 were reported as typical weak-antiferromagnets. Mo-

tizuki et al. calculated an energy band structure of some of these compounds and pointed out that their magnetism had t o be understood from viewpoints of the itinerant electron model and the spin fluctuation theory [2].

On the survey of the published crystallographic data of these compounds [I, 31, the chalcogenides of elements lighter than Fe and those of Co or Ni fall, respectively, into separate groups according t o their lattice

spacing; though a of all these compounds are nearly equal, c of the compounds belonging t o the latter group (low c) are about 10 % smaller than those in the for- mer one (high c) (Fig. 1). Similar classification into two groups can be made on antimonides. As seen in the figure, FeTe (and FeSb) belong to the low c group or are at the boundary of two groups. Simple classification based on c of pnictides with orthorhombic distortion (MnP-type) can not be made because dis- placement parameters u, v, w and x become important in this case. Mn compounds have an anomalous large lattice spacing, which may be closely related to the transition into NaCl type structures~occurred at high temperature. This is likely due to ionic character of Mn in these compounds.

Fig. 1. - Lattice parameters of transition element chalcc- genides at or near the stoichiometric composition.

Discontinuity in the lattice spacing c of chalcogenides, as pointed out above, has a strong corres- pondence to their magnetism which changes noti- ceably as go from Fe to Co compounds: FeS and

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C8

-

192 JOURNAL DE PHYSIQUE

FeSe are strongly magnetic (TN N 600 K) whereas CoS and CoSe are weakly paramagnetic with nearly temperature-independent susceptibility. Transforma- tion from strong to weak magnets is attributed to the increasing metallic interaction between 3d wave func- tions along the c-axis as a consequence of large reduction in c. The interaction broadens the 3d band and this broadening causes collapse of the magnetic moment of metal atoms. In this connection, observations by King et al. 141 are interesting. They carried out crystallographic and Mossbauer measurements on FeS under high pressure. A transition from magnetic to non-magnetic FeS took place at 67 kbar, it being ac- companied by a volume change (9 % ) and a large decrease in c. The isomer shift relative t o Fe metal changed from 0.7 mm/s at 52 kbar to 0.05 mm/s at 69 kbar. This result indicates an appreciable increase of metallic character.

It has to be mentioned here that weak paramagne- tism of the Co or Ni compounds are caused not by their low c/a ratio but by their low c. In literature, magnetic properties are often discussed by referring to c/a ratio. However, this is inadequate in the pre- sent case, because the c/a ratio of CrSb, for example, is smaller than those of CoS or CoSe but CrSb is the most strongly magnetic (TN = 700 K) among the compounds with NiAs type structure.

On the other hand, Ti and V compounds, which are in high c state, are also weakly paramagnetic. It is pos- sibly a result of extended charge distribution of these elements and, consequently, of increasing metallic nature. A substantial deviation in the lattice parameter and some of other physical properties of monoxides of T i and V from those of other transition elements monoxides was interpreted on the same basis. Thus, the high c group may be divided into two groups with res- pect to magnetism, nemaly, high c-I (weakly magnetic) and high c-I1 (strongly magnetic).

The indicated strong correlation between magnetism and lattice spacing c has been further investi- gated by the measurements on pseudo-binary systems formed between members of different groups (strongly or weakly magnetic): in (Fel-,Co,),Seg formed between ferrimagnetic Fe7Ses (high c-11) and paramagnetic CogSes (low c), c exhibited a large reduction (10.8 % ) with increasing

x

from 0 to 1 and ferrima- gnetism disappeared a t x

=

0.6. This is most likely due to the moment collapse caused by the reduction of c [5]. On the other hand, in Mnl-,Ti,Sb formed between high c-I and high c-I1 members, Mn moments remained unchanged up to z N 0.9 and a spinglass like state was observed [6]. Behaviors at the magnetic critical concentration of some of pseudo-binary systems in published literature are summarized in table I. It can be seen that, though there are some exceptions ((Mn, Ni)Sb and (V, Cr)Se), almost all [high c-I]-[high

c-II] or [high oII]-[low c] systems given in the table are, in their magnetic nature, respectively in the same cate- gory represented by Mnl-,Ti,Sb or ( F e l - , c ~ , ) ~ Ses.

Table I. - Magnetic nature of pseudo-binary systems formed between strongly (high c-11) and weakly (high c-I or low c) magnetic compounds. Intercalated compounds of TiS2 were also listed: SG: Spinglass like behavior at magnetic critical concentration. nl: No measurement at low-temperature.

[high c - I] [high c - II] [low c]

v3s4

- v3s4 - Ti3Te4 - V3Te4 - Ti5S6 - TiS - VSe - TiSb - TiAs - - Cr3S4 - (Fe3S4) - Cr3Te4 - Cr3Te4 - Cr5S6 - CrS - CrSe - MnSb - MnAs FegSe4 FegSeg FeS1.15 FeS CrSb MnSb - - Co3Se4, (Ni3Se4) - - CogSeg - nl - C O S ~ . ~ ~ - ? - (NiS)

-

CoSb

-

SG - NiSb - M,TiS2 - SG - FeTiSz M,TiS2 - ferro. at 0.1 < x < 0.3- M = Co

In conclusion, disappearance of strongly magnetic nature in low c state is correlated to increasing of metallic bonding along the c-axis. The behavior a t magnetic critical concentration of pseudo-binary systems formed between strongly and weakly magnetic compounds is understood on the same basis.

[I] Landolt-Bornstein New Series, 17, Ed. 0. Man- delung (Springer-Verlag) 1988.

[2] Motizuki, K. and Katoh, K., J. Phys. Soc. Jpn 53 (1984) 735;

Motizuki, K., Katoh, K. and Yanase, A., J. Phys. C 19 (1986) 495.

131 Constitution of Binary Alloys, Ed. M. Hansen (McGraw-Hill) 1958; first supplement, Ed. P. R. Elliot (McGraw-Hill) 1965; second supplement, Ed. F. A. Sunk (McGraw-Hill) 1969.

[4j King, H. E., Jr. and Prewitt, C. T., Phys. Chem. Min. 3 (1978) 72;

King, H. E., Jr. and Prewitt, C. T., Acta Crys- tallogr. B 38 (1982) 1877.

[5] Sato, M., Kamimura, T. and Iwata, T., J. Appl. Phys. 57 (1985) 3244.