Abstract
A complete set of elastic constants C ij and piezoelectric coefficients e ij of a La3Ga5SiO14 (langasite) single crystal was determined from 5.5 to 275.5 K by resonant ultrasound spectroscopy. Unlike a conventional crystal, the elastic constants C ij of the langasite crystal showed three types of temperature dependence: (i) monotonic elastic stiffening upon cooling (C 44 and C 14); (ii) monotonic elastic softening (C 66); and (iii) a stiffening-to-softening transition below 150 K (C 11, C 12, C 13 and C 33). In addition, a strong correlation between C 66 and the piezoelectric coefficient e 11 was confirmed. Group theoretical lattice dynamics analysis revealed that the novel phenomena of elastic softening and strong correlation are explained on the basis of two types of optical mode internal displacements which have the totally symmetric A 1 and doubly degenerated E symmetries in the point group D 3.
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