Complex Hydrides Studied by Raman Spectroscopy and Thermal Conductivity Measurements under High Pressure
- PDF / 147,525 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 65 Downloads / 246 Views
0971-Z07-03
Complex Hydrides Studied by Raman Spectroscopy and Thermal Conductivity Measurements under High Pressure Bertil Sundqvist, Alexandr V. Talyzin, and Ove Andersson Department of Physics, Umeå University, Umeå, S-90187, Sweden
ABSTRACT The pressure-temperature phase diagrams of alkali metal alanates and borohydrides are of large current interest, and we have recently studied phase transformations under pressure in several of these materials. We here report Raman studies of KBH4 under pressure at room temperature, showing a phase transition near 6 GPa. Although no structural information is yet available, the similarity between KBH4 and NaBH4 suggests the new structure is orthorhombic. We also report studies on LiBH4 showing that the high pressure phase of this material is metastable to zero pressure below 200 K.
INTRODUCTION Alkali metal alanates and borohydrides contain very high weight fractions of hydrogen and have been suggested as suitable for hydrogen storage applications. Interest in the structural properties of these materials has therefore increased significantly in recent years. In particular, there has been a large number of theoretical predictions of pressure-induced structural phase transitions [1-5], and several of these transitions have also recently been found experimentally [6-16]. The materials studied under high pressure have mainly been the light alkali metal compounds, including LiAlH4 [6,8,12,13], NaAlH4 [7,8,14], LiBH4 [16], and NaBH4 [8-11,15]. Although we have studied all these materials over the last two years we concentrate in this report on new results on the borohydrides. All alkali metal borohydrides except LiBH4 crystallize in the face centered cubic Fm3m structure at room temperature and show phase transitions into lower symmetry phases at low temperatures, 190 K for NaBH4 and at 76 K for KBH4. Both low temperature structures are tetragonal with space groups P421c and P42/nmc, respectively [17]. For the heavier compounds RbBH4 and CsBH4 the low temperature structures are not yet known. The low temperature phase transitions involve orientational ordering/disordering of the BH−4 tetrahedral anions. NaBH4 has been studied by several groups under pressure and has been reported to exhibit two phase transitions, the first from the ambient pressure cubic phase to the tetragonal phase at 6.3 GPa [9,11] and the second into an orthorhombic Pnma phase at 9-11 GPa [8,10,11]. Because the transition from cubic to tetragonal is an order-disorder type transition corresponding to a relatively small change in the lattice structure there are only very small differences between their Raman spectra. For this reason only the second phase transition was observed in Raman data
[8,10] while the transition into the tetragonal phase was clearly seen in both diffraction studies [9,11] and thermal conductivity data [15]. Taking into account the close similarity between the NaBH4 and KBH4 structures at ambient conditions it should be anticipated that phase transitions can be found at high pressure als
Data Loading...
