The Orbital Origins of Chemical Bonding in Phase-Change Materials

Layered phase-change materials in the Ge–Sb–Te-system are widely used in data storage and are the subject of intense research to understand the elusive quantum-chemical origin of their unique properties. To uncover the nature of the underlying periodic wavefunction, we study the interacting atomic orbitals including their phase information as revealed by crystal orbital bond index (COBI) and fragment crystal orbital (FCO) analysis. In full accord with previous and also new findings based on projected force constants (pFC), we demonstrate the decisive role of multicenter bonding along straight atomic connectivities such as Te–Ge–Te and Te–Sb–Te. While the here found multicenter bonding resembles well-established three-center four-electron bonding in molecules, its solid-state manifestation beyond a molecular motif leads to distinct longe-range consequences, thus serving to contextualize the aforementioned material properties usually termed “metavalent”. For example, we suggest multicenter bonding to be the origin of their astonishing bond-breaking and also phase-change behavior. As a hole-in-one, multicenter bonding immediately explains the too small “van der Waals” gaps between individual layers since multicenter bonding forces these gaps to shrink below the nonbonding Te–Te distances.

Longe Range Exchange Interactions in Sintered CuMn Alloys: A Monte Carlo Study

It is presented a Monte Carlo (MC) study of the dependency of the magnetization of sintered CuMn alloys with the temperature and applied magnetic field using a RKKY function to model the Mn-Mn magnetic interaction. Two kinds of simulations were done: (i) simulations at zero magnetic field, where the energy is obtained as a function of the temperature and (ii) simulations at constant temperature, varying the applied magnetic field. For both kinds of simulations, the variations of the total energy and the magnetization with respect to the concentration x and the range of the interaction (cutoff distance) were calculated. These simulations indicate that the range of the interaction among the magnetic atoms has a significant impact on the thermodynamic properties of the studied alloy.

A MONTE CARLO STUDY OF THE SPECIFIC HEAT OF DILUTED ANTIFERROMAGNETS

The three-dimensional diluted antiferromagnet in a magnetic field has a longe-range ordered state for sufficiently low temperatures and external fields. We study the irreversibilities at the phase transition from this state into the paramagnetic state at higher temperatures. Performing field cooling and zero field cooling simulation procedures we find a small irreversibility in the internal energy and the specific heat, although the behavior of the order parameter is irreversible and the phase transition is suppressed in the FC case. The specific heat shows a noncritical broad maximum above the transition temperature Tc. The results are compatible with recent experimental results obtained by Satooka et al., whereas our interpretation of the data is different.

The Nature of the Mn Moment in Laves Phase Compounds: Evolution of the Magnetic Order in Ho1–xYxMn2

Neutron powder diffraction has been used to study the evolution of long range magnetic order in the pseudobinary CI 5 Laves phase system Ho1-xYxMn2. Particular attention has been paid to the nature of the Mn moment. At Y-rich compositions (x > 0.9) an incommensurate antiferromagnetic structure, similar to that of YMn2 is observed. Transition to the ordered state, as in YMn2, is accompanied by a 5% expansion of the unit cell, identifying the Mn moments, of 2.7 μB, as intrinsic. The magnetic structure of compositions with x<0.7 resembles that of HoMn2 and DyMn2, with only one quarter of the chemically equivalent Mn sites possessing a moment of 0.6 μB induced by the local symmetry of the antiferromagnetically ordered Ho sublattice. Transition to the ordered state is not accompanied by a cell expansion. Between x = 0.7 and x = 0.9 there is no longe range magnetic order, nor is there an expansion of the unit cell, suggesting the total absence of either induced or intrinsic Mn moments. The results indicate that a critical Mn-Mn near neighbour distance of 2.663 Å is necessary to sustain an intrinsic Mn moment in these compounds.