Magnetic Properties of Substituted Manganese-Tin Spinels

In a ${\mathrm{Me}}_2^{\text{2+}}{\mathrm{Sn}}^{\text{4+}}{\mathrm{O}}_4$ spinel, Sn⁴⁺ is known to occupy octahedral sites. Therefore manganese‐tin spinel, $(\mathrm{Mn}\mathrm{)[}\mathrm{MnSn}]{\mathrm{O}}_4$ , would be antiferromagnetic because of the strong interaction between octahedral and tetrahedral ions characteristic of magnetic spinels. The magnetic exchange interaction in a (Mn²⁺)‐O²⁻‐[Mn²⁺] linkage is of particular interest because it is known to be much weaker than is the case for Fe³⁺ with which Mn²⁺ is isoelectronic; magnetic measurements on slightly oxidized Mn‐Sn spinel yield a Néel temperature of 58°K. When the initial equality of the sublattice magnetizations of Mn²⁺ ions in octahedral and tetrahedral sites is upset by substitution of nonmagnetic, divalent ions, Mg²⁺ and Zn²⁺, for Mn²⁺, spontaneous magnetization appears. The Néel temperature is decreased by these substitutions largely as a consequence of the smaller number of interactions between the reduced number of Mn²⁺ ions in the two different sites. For equal substitutions for Mn²⁺ in ${\mathrm{Mn}}_2{\mathrm{SnO}}_4$ , the moment changes more rapidly in the case of Mg²⁺ than Zn²⁺. The moment depends upon the size of the substituent ion because, in this case, the ions involved are spherical and of the same valence. A smaller size of these substituent ions leads to a greater predominance of occupation of the octahedral sites because substitution there increases the Madelung potential both by reduction of lattice constant and increase of oxygen parameter. The sequence, Mg²⁺, Zn²⁺, Mn²⁺, of increasing ionic sizes corresponds to that of increasing lattice constants, 8.60 8.67, 8.88 A, of the respective tin spinels. Substitution of Ge⁴⁺ for Sn⁴⁺ takes place principally in tetrahedral sites because of the very small size of Ge⁴⁺. Similarly, spontaneous magnetization appears with oxidation of ${\mathrm{Mn}}_2{\mathrm{SnO}}_4$ . The resultant oxidized manganese ions (presumably Mn³⁺) are of the same average valence, though of smaller size, than the octahedral ions and, therefore, also appear in octahedral sites. Because Sn⁴⁺ is rejected to maintain valence balance, the octahedral sublattice magnetization is increased by oxidation.