Understanding the Unusual Response to High Pressure in KBe2BO3F2

Abstract

Strong anisotropic compression with pressure on the remarkable non-linear optical material KBe2BO3F2 has been observed with the linear compression coefficient along the c axis found to be about 40 times larger than that along the a axis. An unusual non-monotonic pressure response was observed for the a lattice parameter. The derived bulk modulus of 31 ± 1 GPa indicates that KBe2BO3F2 is a very soft oxide material yet with stable structure up to 45 GPa. A combination of high-pressure synchrotron powder X-ray diffraction, high-pressure Raman spectroscopy, and Density Functional Theory calculations points to the mechanism for the unusual pressure response being due to the competition between the K-F bond length and K-F-K bond angle and the coupling between the stretching and twisting vibration modes.

Unexpected giant negative area compressibility in palladium diselenide

“…To date, there are three mechanisms known to occur in NAC materials, in which the Lifshitz mechanism is most established and dominantly governs NAC materials, including TlGaSe 2 [ 16 ], NaV 2 O 5 [ 17 ], KBe 2 BO 3 F 2 ( R 32 and R -3 c phases) [ 13 , 14 ], silver(I) tricyanomethanide [ 10 ] and Zn(CH 3 COO) 2 ·2H 2 O [ 11 ]. In this mechanism, NAC behavior can be generated by rigid flattening of ripples in corrugated 2D-like layers due to extrusion of neighboring counterparts under pressure [ 18 ].…”