Europium titanate has the cubic perovskite structure containing divalent Eu (7 μB) and tetravalent Ti. From magnetic measurements we find that EuTiO3 is one of the few antiferromagnetic materials with a positive θ (TN=5.3°K, θ=3.8°K). At 1.3°K the magnetic moment (σ) increases linearly with field to 10 kOe; above 14 kOe the moment saturates and σ=156 emu/gm (6.93 μB) at 20 kOe. Powder neutron-diffraction work indicates that EuTiO3 has the Type G magnetic structure in which a given Eu++ has six nearest-neighbor europium ions antiparallel and 12 next-nearest-neighbor europium ions parallel. In a perovskite structure where only the 12-coordinated ion is magnetic, i.e., Eu++, the molecular field relations for a two sublattice model yield J1/k=−0.021°K, where J1 is the effective intersublattice exchange interaction, and J2/k=0.040°K, where J2 is the effective intrasublattice exchange interaction. The signs of J1 and J2 are opposite to those found in the europium chalcogenide series. The chalcogenides, however, have the rocksalt structure in which the number of 90° cation-anion-cation interactions differs from the perovskite structure.

Through single-crystal x-ray diffraction methods, the crystal structure of TiNi has been determined in the temperature range −70° to 900°C. Contrary to what has been assumed from previous work based on the powder pattern methods, the TiNi crystal structure is not a simple CsCl type. Rather, it has an a0=9Å superlattice and an a0=3Å sublattice with 54 atoms per unit cell complex structure. The 9Å superlattice undergoes, at about 166°C, a ``martensitic'' pseudo order-disorder transition which is not accompanied by a crystallographic transformation. Through the understanding of this unique transition the apparent contradicting observations made on TiNi by various past investigators can now be reconciled and the unusual physical properties associated with the alloy are explained qualitatively.

The magnetic structures in FeCr2S4 and FeCr2O4 have been investigated by the powder neutron diffraction technique. From room temperature diffraction patterns, it is established that both compounds are normal spinels with sulfur and oxygen parameters u=0.384 and 0.386, respectively. The FeCr2S4 specimen exhibits a Néel temperature of 180°K, with σ0=1.6 μB. The magnetic intensities of this compound at 4°K are in good agreement with those based on a simple Néel model with μFe=4.2 μB and μCr=2.9 μB. The magnetic properties of FeCr2O4 are, on the other hand, more complex. The moment σ0 is considerably lower than the value expected from the Néel model. Moreover, the crystal becomes tetragonal with c/a<1 around 135°K. The magnetic reflections at 4°K show satellite peaks characteristic of a cone spiral structure described in the theory of Menyuk, Dwight, Lyons, and Kaplan for tetragonal spinels. However, a quantitative comparison of observed and calculated intensities indicates that some modifications of the simple cone model are necessary. The satellite reflections disappear at 35°K, and the spin arrangement is collinear between this temperature and the Néel temperature at 80°K.

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