ROLE OF HYDROGEN-SUBLATTICE ORDERING FOR MAGNETIC AND METAL-SEMICONDUCTOR TRANSITIONS IN BETA-HOH2+X

The electrical resistivity, ρ, of β-HoH2+x has been measured in the interval 1.3≤T≤330 K for the entire x range within the pure β phase, 0≤x≤0.14. It is shown that the excess hydrogens, x, on octahedral sites interact below ∼200 K with each other to form short-range-ordered (SRO) structures for x0.05, transforming into long-range-ordered (LRO) configurations for x0.1. Disorder introduced into these structures by quench from room temperature recovers in two stages, centered near Tp(1)∼150 K (for SRO) and near Tp(2)∼180 K (for LRO), with activation energies of Em(1)∼0.15 eV and Em(2)=0.26-0.27 eV, respectively. Several magnetic transitions are observed below ∼10 K (three for low x values, 0≤x≤0.03) attributed to commensurate and incommensurate antiferromagnetism; they decrease in T with increasing x due to weakening Ruderman-Kittel-Kasuya-Yosida interaction. For higher x (0.05), additional magnetic transitions are induced by the ordered hydrogen configurations acting upon the crystal-field symmetry. A tentative magnetic phase diagram is constructed from the present ρ results and published susceptibility, neutron-diffraction, and specific-heat data. Metal-semiconductor transitions are observed near 250-270 K (MS), attributed to the breakdown of a delocalized band when crossing the x ordering interval, and near 100-130 K (SM) assigned to carrier localization due to atomic disorder. © 1994 The American Physical Society.