Anomalous elastic behavior in Cu2O under pressure

The elastic constants Cij of single-crystal Cu2O measured to 3 kbar show linear variation with pressure. The best-fit values of the Cij and dCij/dP are: C11 = 1228.8, C44 = 121.0, C′ = (C11–C12)/2 = 81.9 kbar, dC11/dP = 3.62, dC44/dP = −0.69, and dC′/dP = −0.63, respectively. The elastic behavior of Cu2O is found to be anomalous in that dC44/dP and dC′/dP are both negative, and dμ/dP, where μ is the isotropic shear modulus, is significantly negative (−0.67). This anomalous behavior suggests an instability of crystal structure. The high- and low-temperature limiting values of Gruneisen γ, γH and γL, computed from the dCij/dP are −1.98 and −3.59, respectively. The value of γH is in fairly good agreement with the reported Gruneisen γ value based on the nuclear quadrupole relaxation data. The negative Gruneisen γ values are consistent with the observed negative thermal expansion. The implication of γL being appreciable more negative than γH is that the coefficient of thermal expansion of Cu2O should become more negative at low temperatures. Die elastischen Konstanten Cij von Cu2O-Einkristallen zeigen bis 3 kbar eine lineare Abhangigkeit vom Druck. Die die beste Anpassung ergebenden Werte fur Cij und dCij/dP sind: C11 = 1228,8; C44 = 121,0; C′ = (C11–C12)/2 = 81,9 kbar; dC11/dP = 3,62; dC44/dP = = −0,69 und dC′/dP = −0,63. Es wird gefunden, das das elastische Verhalten von Cu2O anomal ist, indem dC44/dP und dC′/dP negativ sind, und dμ/dP, wobei μ der isotrope Schubmodul ist, betrachtlich negativ (−0,67) ist. Diese Anomalie weist auf eine Instabilitat der Kristallstruktur hin. Die aus dCij/dP berechneten Hoch- und Tieftemperaturgrenzwerte γH und γL des Gruneisenparameters γ betragen −1,98 bzw. −3,59. Der Wert von γH ist in recht guter Ubereinstimmung mit anderen Gruneisen γ-Werten, die auf Kernquadrupol-Relaxationsdaten basieren. Die negativen γ-Werte sind konsistent mit der beobachteten negativen thermischen Ausdehnung. Aus der Tatsache, das γL betrachtlich starker negativ ist als γH kann geschlossen werden, das der Koeffizient der thermischen Ausdehnung von Cu2O bei tiefen Temperaturen starker negativ wird.

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