Abstract
Precision x-ray measurements of the motions of the atoms in LaCo every 50°C from room temperature to 1000°C indicate that the crystal space group is below 375°C and above 375°C. In the symmetry there are two distinguishable octahedral cobalt positions, having larger crystalline fields than . Since high- and low-spin cobalt ions are simultaneously present, this indicates preferential long-range ordering of low-spin cobalt at sites, and of high-spin cobalt at sites for °C. Calorimetric data show a first-order transition at °C and a higher order transition in the temperature interval °C, which is also manifest in a large Debye-Waller factor in this interval and in a plateau in the curve of reciprocal susceptibility versus temperature. At about 650°C there is some evidence, from differential thermal-analysis data, of another higher order transition. The electrical conductivity increases with increasing temperature below 650°C, but much more rapidly in the interval °C than below 125°C. It is nearly temperature-independent in the interval °C and is continuous through the first-order transition. However, above 937°C the resistivity increases with temperature as in a metal. The space group remains and the pseudocubic cell edge is continuous through the first-order phase change, but the rhombohedral angle drops abruptly from 60.4° to 60° and the ions are shifted discontinously along the axis toward a ion. Similar -ion displacements occur in the temperature interval °C. The Debye-Waller factor decreases by an order of magnitude on going to the high-temperature phase. It is pointed out that crystal-field and band theory should describe two different thermodynamic states of electrons: localized and collective. The data are interpreted to indicate (1) that the first-order phase change at °C is a localized-electron collective-electron phase change for electrons in orbitals of symmetry, higher temperatures introducing a Fermi surface and partial disproportionation between high- and low-spin cations at and positions; (2) that the number of charge carriers is constant through the transition, because the number of localized charge carriers is saturated below and just above the bandwidth of the collective-electron states is ; (3) that the mobilities of the charge carriers are also continuous through the transition, the activation energy for a localized-electron hop becoming at °C; (4) that the higher-order transition in the interval °C represents a region of short-range order, the ordered phase occurring at higher temperatures where the populations of high- and low-spin cobalt ions approach one another; and (5) that exciton transfer is an important mechanism in LaCo.
- Received 1 August 1966
DOI:https://doi.org/10.1103/PhysRev.155.932
©1967 American Physical Society