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We use first-principles calculations to investigate the stability of bi-axially strained Pnma perovskite CaMnO3 towards the formation of oxygen vacancies. Our motivation is provided by promising indications that novel material properties can be engineered by application of strain through coherent heteroepitaxy in thin films. While it is usually assumed that such epitaxial strain is accommodated primarily by changes in intrinsic lattice constants, point defect formation is also a likely strain relaxation mechanism. This is particularly true at the large strain magnitudes (>4%) which first-principles calculations often suggest are required to induce new functionalities. We find a strong dependence of oxygen vacancy defect formation energy on strain, with tensile strain lowering the formation energy consistent with the increasing molar volume with increasing oxygen deficiency. In addition, we find that strain differentiates the formation energy for different lattice sites, suggesting its use as a route to engineering vacancy ordering in epitaxial thin films.

crease in structural relaxation time 2. When 2 exceeds the experimental observation time, the liquid properties are no longer observed in eauilibrium. and the svstem has passed into the glassy state. Thus, a L-L transition in a supercooled liquid may also be obscured by the glass transition.With these limitations in mind. what are the possibilities for observing liquid polymorphism? Among the most promising candidates to date are liquids having open molecular coordination environments at low pressure. Notably, these include liquids with a A t low temperatures, the thermodynamic implications for liquid and glass science, as locally tetrahedral molecular structure, such equilibrium state of a pure substance is an well as for the thermodynamics of systems as Si, Ge, C, Si02, Ge02, and H 2 0 itself. For ordered crystal. The crystalline portion of with fixed composition.liquid Si, observation of a first-order glass-tothe pressure-temperature phase diagram is If liquid-liquid (L-L) phase transitions liquid transition, consistent with an underlyusually subdivided among several chemically can occur for pure, isotropic substances, ing L-L transition, has been reported in flashidentical but thermodynamically distinct why are they not commonly observed? First, heating experiments (4) and computer simupolymorphs (I). Diamond and graphite are because the density will generally be an ap-lations (5). For H 2 0 (6), Si02 (7), andGe02 well-knownexamples. Such polymorphism is propriate order parameter for such transi-(8), liquid polymorphism is suggested from not restricted to crystals. Above the triple tions, the exploration of a wide range of glass studies at low temperatures. In all three point, in the regimes of liquid and gas, dis-pressure and temperature may be required cases, abrupt changes in density and glass tinct disordered phases are observed (see for their detection.structure with changing pressure closely refigure). Above the liquid-gas semble behavior associated critical temperature Tc, these with first-order phase transistates merge into a single tions and have been described "fluid" phase. In this sense, the as !'polyamorphic", that is, liquid and gas states can be reamorphous solid analogs of garded as "polymorphs" of the crystalline polymorphism (9). disordered fluid state (2). Although a glass is not an equiThe laws of thermodynamlibrium thermodynamic state, ics permit the liquid region of if its behavior parallels that of the phase diagram for a pure the corresponding supercooled substance to be subdivided into liquid, then the observation of distinct phases (3). The coex-1 --D e w -a polyamorphic transition in a istence of two distinct liquids is Schematic phase diagram of a pure substance exhibiting a liquid-liquid glass indicates the existence of common in multicom~onent phase transition. (A) S d~d lines locate the coexistence lines between the distinct liquid states. chemical systems and HIso in iiquid, gas, and tw~.crystal polymorphs. The liquid-gas coexistence line Theoretical and computer liquidcrystals...

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We report on the energetics of intercalation of lithium, sodium and potassium in graphite by density functional theory using recently developed van der Waals density functionals. First stage intercalation compounds are well described by conventional functionals like GGA, but van der Waals functionals is crucial for higher stage intercalation compounds and graphite, where van der Waals interactions are important. The vdW-optPBE functional gave the best agreement with reported structure and energetics 10 for graphite and LiC 6 and was further applied for intercalation of Na and K. The enthalpy of formation of LiC 6 and KC 8 were found to be -16.4 and -27.5 kJ/mol respectively. NaC 6 and NaC 8 were unstable with positive enthalpies of formation (+20.8 and +19.9 kJ/mol). The energetics of stacking of graphene and intercalant layers was investigated from first to fifth stage intercalation compounds. Higher stage compounds of Li and K were stable, but with smaller enthalpy of formation with increasing stage order. 15The higher stage Na compounds possessed positive enthalpy of formation, but less in magnitude than the energy difference of 0.6 kJ/mol between graphite with AB and AA stacking. The abnormal behaviour of the lower stage Na intercalation compounds were rationalized by the lower energy involved in the formation of the chemical bond between carbon Na relative to the corresponding bond with Li or K. The chemical bond between alkali metal and carbon is characterized by charge transfer from the alkali-metal 20 to carbon resulting in ionized alkali-metals. The intercalation induces only a subtle increase in the inplane C-C bond lengths, with longer C-C bonds in the vicinity of the alkali metals but without breaking the hexagonal symmetry.