Phasenumwandlungen von Lithiumnitrid unter Druck

Bereits bei 600 MPa und Raumtemperatur wandelt sich α-Li3N in β-Li3N um, dessen Rückumwandlung bei Normaldruck erst oberhalb 200°C einsetzt. β-Li3N kristallisiert im Na3As-Typ. Mit dieser Modifikation läßt sich Li3 N in die Struktursystematik der Alkalimetallverbindungen A3B mit Elementen der Stickstoffgruppe einreihen (A = Li, Na, K, Rb, Cs; B = N, P, As, Sb, Bi). Da der Phasenübergang bei relativ niedrigem Druck stattfindet, ist die Bildung geringer Anteile an β-Li3N beim Mörsern kaum auszuschließen, so daß Röntgenpulveraufnahmen von α-Li3N stets auch geringe Anteile an β-Li3N zeigen

Phase Transformations of Lithium Nitride under Pressure

The transformation of α-Li3N to β-Li3N occurs at 600 MPa and room temperature. The reverse transformation, however, only begins to take place above 200°C under normal pressure. β-Li3N crystallizes as the Na3As structure type. This modification allows Li3N to be placed systematically in the structural series of alkali-metal compounds A3B involving elements of the nitrogen group (A = Li, Na, K, Rb, Cs; B = N, P, As, Sb, Bi). Since the phase transition occurs at relatively low pressure, the formation of small amounts of β-Li3N upon grinding with a mortar and pestle cannot be ruled out. Indeed, X-ray powder diffractions of α-Li3N always reveal the presence of small amounts of β-Li3N

Electronic and structural transitions in dense liquid sodium

At ambient conditions, the light alkali metals are free-electron-like crystals with a highly symmetric structure. However, they were found recently to exhibit unexpected complexity under pressure 1-6. It was predicted from theory 1.2 - and later confirmed by experiment 3-5 - that lithium and sodium undergo a sequence of symmetry-breaking transitions, driven by a Peierls mechanism, at high pressures. Measurements of the sodium melting curve 6 have subsequently revealed an unprecedented (and still unexplained) pressure-induced drop in melting temperature from 1,000 K at 30 GPa down to room temperature at 120 GPa. Here we report results from ab initio calculations that explain the unusual melting behaviour in dense sodium. We show that molten sodium undergoes a series of pressure-induced structural and electronic transitions, analogous to those observed in solid sodium but commencing at much lower pressure in the presence of liquid disorder. As pressure is increased, liquid sodium initially evolves by assuming a more compact local structure. However, a transition to a lower-coordinated liquid takes place at a pressure of around 65 GPa, accompanied by a threefold drop in electrical conductivity. This transition is driven by the opening of a pseudogap, at the Fermi level, in the electronic density of states - an effect that has not hitherto been observed in a liquid metal. The lower-coordinated liquid emerges at high temperatures and above the stability region of a close-packed free-electron-like metal. We predict that similar exotic behaviour is possible in other materials as well