First-principles study of the pressure dependence of the structural and vibrational properties of the ternary metal hydrideCa2RuH6

Abstract

The influence of pressure on the structural and vibrational properties of Ca 2 RuH 6 has been investigated using periodic density functional theory calculations performed at the local density approximation ͑LDA͒ and generalized gradient approximation ͑GGA͒ levels. At ambient pressure, the calculated structure and vibrational frequencies are in satisfactory agreement with experimental data. The calculated P-V curves could be fitted with the Vinet equation of state, yielding B 0 = 67.6 and B 0 = 58.5 GPa at the LDA and GGA levels, respectively, and B 0 Ј=4.0 at both theoretical levels. The unit cell parameter is found to decrease faster with increasing pressure than the Ru-H bond length. The calculated pressure dependence of the vibrational frequencies agrees well with experiment for 5 ͑T 2g ͒ but not for 9 ͑A 1g ͒.

Structural and vibrational properties of Ca2FeH6 and Sr2RuH6

“…Thus noticeably different values were predicted at the LDA and GGA levels for the librational frequency of Ca 2 RuH 6 [7]. This explains the discrepancy observed between experiment and theory for the librational frequency of Sr 2 RuH 6 , which is calculated at 478 cm À 1 and was observed at 561 cm À 1 by Barsan et al [8].…”

“…As recently shown in our study of Ca 2 RuH 6 [7], the frequency of the librational mode is particularly sensitive to the surroundings of the [MH 6 ] 4 À complex and to pressure, thus making its theoretical determination quite challenging. Thus noticeably different values were predicted at the LDA and GGA levels for the librational frequency of Ca 2 RuH 6 [7].…”

“…The calculated bulk moduli of Ca 2 FeH 6 and Ca 2 RuH 6 are very close. In the case of Ca 2 RuH 6 we had seen that under pressure the [RuH 6 ] 4À octahedron is not compressed very much compared to the entire lattice [7]. The bulk modulus reflects thus mainly the interaction between the Ca 2 + ions and the relatively rigid [MH 6 ] 4À anions.…”

“…We have previously [5] attempted to correlate Ru-H stretching frequencies with Ru-H bond length using Badger's rule, however the correlation was not very satisfactory. Using both experimental [4][5][6][7][8][9][10] and theoretical results from the literature and from this study, we have plotted IR and Raman active stretching modes against the M-H bond length in Fig. 3.…”

“…Later, however, it was shown to be of interest as a material in thermal heat energy storage systems [3]. As to the vibrational properties of the A 2 TH 6 series several members have been studied in the past, such as Mg 2 FeH 6 or Ca 2 RuH 6 [4][5][6][7][8][9]. A recent review [10] summarizes the data available so far.…”