Study of the elastic properties of K₂SeO₄at the phase transition to the incommensurate and the commensurate phase by ultrasonic techniques and Brillouin spectroscopy

Pressure–temperature phase diagram of (N(CH3)4)2CuCl4 crystals birefringent and elastic properties

“…At high pressure (P > Pk) the direct transition (wave vector q = C*/3) from P to C phase takes place. It provides two negative contributions to the changes of C,, below To [16] where x Q Q is the susceptibility of the order parameter. Both contributions change as the square of the order parameter.…”

Ferroelastic crystals and their nonlinear elastic properties observed in frequency range from gigahertz to milihertz

“…However, in contrast to K 3 Na(SeO 4 ) 2 , in LCS the additional anomaly was observed in only one direction - [100]. The elastic properties of LCS seem to be closer to those of the K 2 SeO 4 crystal [71], for which the occurrence of an additional elastic anomaly related to the incommensurate phase was observed in only one direction too. Many crystals of the total formula A 0 A 00 BX 4 have been found to show the presence of the incommensurate phase, including the crystals K 2 SeO 4 , LiNH 4 SO 4 , LiCsCrO 4 [19] and LiRb 4 SO 4 [72].…”

“…Depending on the configuration used in the experiment (TPB or PPS), different temperature dependencies of E 0 or DE 0 were obtained (see, e.g. the results presented in Figures 9 and 10 as well as [45,[71][72][73]). In contrast to UWP, the DMA method does not allow introduction of a transverse wave to the sample.…”

Dielectric and light scattering spectroscopy of incommensurate phases in crystals

“…As the jump is experimentally smaller than -0.2 cm-' (it was not observed) it implies that the stability limit To of the C phase is very close t o q (T -To 5 0.5 K) (Sannikov and Levanyuk, 1978). From the proportionality oA -q i (classical approximation) it follows that the order-parameter critical index / 3 = 0.33 k 0.5. elastic constant recently Rehwald, Vonlanthen, Kriiger, Wallerius and Unruh (1980) and Hoshizaki, Sawada, Ishibashi, Matsuda and Hatta (1980) obtained also 2p N 0.6-0.7. We can summarize that, as usual with most structural transitions, the situation concerning the critical indices is not quite clear, but indicates probably a non-classical behaviour, in some experiments at least.…”