High-pressure structures and vibrational spectra of barium fluoride: Results obtained under nearly hydrostatic conditions

Abstract

Powder x-ray diffraction and Raman spectroscopy experiments, together with first-principles calculations, have been carried out to corroborate and clarify the pressure-dependent structural evolution of barium fluoride. X-ray diffraction experiments were performed both with and without a pressure-transmitting medium. The latter serve to reproduce past experimental results, while the former show a marked difference. Specifically, the experiments employing helium as a pressure-transmitting medium yield observed relative intensities and volume compression consistent with the proposed Ni 2 In structure of barium fluoride at pressures greater than 14 GPa. The Raman spectroscopy measurements corroborate the proposed Ni 2 In structure, as the spectra exhibit the two modes expected for the high-pressure phase. The experiments also demonstrate that barium fluoride remains an insulator up to at least 77 GPa, and the results of first-principles calculations suggest that a subsequent insulator-metal transition may not be expected well into the megabar regime.

Phase transitions and equations of state of alkaline earth fluoridesCaF2,SrF2, andDorfman1, Jiang2, Mao3 et al. 2010Phys. Rev. B

“…The observed transition pressure for our room-temperature compression of CaF 2 and SrF 2 is higher than that in other studies; 12,15,16,[24][25][26][27] this reflects variations in metastable behavior under different degrees of differential stress. During initial pressurization of the DAC, BaF 2 was compressed to 3.8 GPa, and the transition from fluorite to cotunnite-type was already completed at this pressure.…”

“…Alternatively, K 0 Ј may be fixed to a reasonable value based on previous work. Previous experimental and theoretical work on CaF 2 , SrF 2 , and BaF 2 found that K 0 Ј is 4.67-4.7 for all phases of BaF 2 , 12 4.75 for the fluorite phase of SrF 2 , 21 and 4.38-4.83 for all phases of CaF 2 , 11,22 suggesting that K 0 Ј may have little variation with structure or cation for the alkaline earth fluorides. We therefore chose a K 0 Ј of 4.7 when using a fixed value for this parameter in our fits.…”

“…58 It is also consistent with the volume difference between cotunnite and fluorite obtained in recent work on BaF 2 . 12 With a zero-pressure bulk modulus of 74͑5͒ GPa, the cotunnite phase of CaF 2 is slightly more compressible than the fluorite phase. The same relationship was recently found between the cotunnite and fluorite phases of BaF 2 by Smith et al 12 A previous static compression experiment 15 on CaF 2 yielded a much higher bulk modulus for the cotunnite phase ͑K 0 = 170 GPa͒.…”

“…[5][6][7][8][9][10][11][12][13][17][18][19] Theoretical studies have provided predictions of the phase transitions and equations of state for CaF 2 , 5,7,10,11,19 SrF 2 , 6,7,17 and BaF 2 . 8,9,12,18 Although the transition to the cotunnite-type phase at high pressure is well known in CaF 2 and SrF 2 , the compressibility of these materials is poorly constrained. For CaF 2 , theoretical predictions of the zero-pressure bulk modulus of the cotunnite phase span a wide range from 66 to 154 GPa, and thus it is not clear whether the transition from fluorite ͑K 0 = 82 GPa͒ to cotunnite involves a modest decrease or a large increase in the bulk modulus.…”

“…3,4 The high coordination of the alkaline earth fluorides at ambient conditions can be analogous to the structure of lower coordination AX 2 compounds at high pressure, including oxides relevant to geoscience and planetary science. 1 High-pressure studies on CaF 2 , SrF 2 , and BaF 2 have addressed band structure, [5][6][7][8][9][10][11] metallization, [5][6][7][8][9]11,12 optical properties, 5,6,9,11 melting behavior, 13 shear strength, 14 phase transitions, 5,6,10,11,[15][16][17][18] and elastic properties. 10,11,15,[19][20][21][22][23] The relative simplicity of the AF 2 system makes it well suited for evaluating differences between theoretical and experimental studies of these properties.…”