Structural stability of BaMF₄(M = Mg, Zn and Mn) at high pressures

Abstract

Piezoelectric fluorides of the composition BaMF(4) (M = Mg, Zn, Mn) have been studied in situ at high pressures in diamond anvil cells with single-crystal x-ray diffraction and Raman spectroscopy. All three compounds crystallize in the acentric space group Cmc 2(1) at ambient pressure. BaMgF(4) undergoes a reversible second order phase transition to the paraelectric phase (space group Cmcm) at pressures between 5 and 6 GPa. BaZnF(4) undergoes a reversible first order phase transition to a monoclinic phase (space group P 11n). Both high- and low-pressure polymorphs coexist in the pressure range 5-7 GPa. BaMnF(4) maintains the Cmc 2(1) structure up to pressures of 4 GPa. Above this pressure the diffraction signal decreases rapidly and at 6 GPa no diffraction signal could be detected in our experiment. The compound does not recover its crystallinity on decompression. A comparison of the effects of external and chemical pressure is presented.

Twinning and pseudosymmetry under high pressure

“…Although a series of acentric structures have been determined at high pressures (see e.g. [12][13][14][15][16][17][18]) for most of them insufficient detail is given with respect to the presence or absence of inversion twinning or the determination of the absolute structure.…”

“…One example, where the center of symmetry is recovered at high pressures, is the compound BaMgF 4 [15] which is ferroelectric at ambient pressures (space group Cmc2 1 ) and transforms to the paraelectric parent phase (Space group Cmcm) between 4.75 and 6.08 GPa. At ambient pressure the absolute structure could be determined and the absence of inversion twinning for this compound could be confirmed [19].…”

“…At ambient pressure the absolute structure could be determined and the absence of inversion twinning for this compound could be confirmed [19]. In the acentric phase at high pressures, however, it was not possible to reliably determine the volume fractions of the two twin individuals [15], although a Hamilton test [20] clearly confirmed the structure to be acentric. However, this becomes understandable if one takes into account that the f 00 value for Ba in the high pressure data (λ ¼ 0:4 Å:…”

“…An example is found in BaZnF 4 [15]. The compound crystallizes orthorhombic at ambient pressure (space group Cmc2 1 ) and undergoes a first-order phase transition to a monoclinic phase (P11n) at pressures above 5 GPa.…”

Crystal structures and stability of trigonal KLnF4fluorides (Ln = Y, Ho, Er, Tm, Yb)

“…For all the synchrotron single-crystal measurements, the intensities were indexed and integrated using the program XDS. 42 Further treatment of the high-pressure single-crystal data was carried out according to the procedures described by Friese et al 43 and Posse et al 44 Structure refinements were carried out with the program Jana2006. 45 All the relevant experimental, structural, and geometrical details are given below and in the ESI.…”