M. Sahlberg et al. / Journal of Solid State Chemistry 184 (2011) 104–108
105
˚
opening using CuK
a
1 radiation (
l
¼1.540598 A). Silicon was used as
internal calibration standard. Unit cell axis refinements were
performed using the programs CHECKCELL [14] and FULLPROF [15].
Microstructures and elemental analyses were obtained from a
LEO 440 scanning electron microscope equipped with an EDAX
energy dispersive detector. High resolution images were recorded
with a LEO 1550 scanning electron microscope equipped with an
in-lens detector. The samples were prepared by dispersing the
powder material on carbon tape.
2.3. Thermal desorption spectroscopy
Thermal desorption spectroscopy (TDS) was performed in an
ultra-high vacuum system equipped with a dynamic sampling
mass spectrometer. Prior to the measurements the reactor was
evacuated down to 10ꢀ6 Pa and flushed with nitrogen. The samples
were heated with constant temperature rates (1–15 1C/min) and
the partial pressures of hydrogen were recorded. The activation
energy was determined using the Kissinger method [16]. The
system was calibrated by desorbing a known standard hydride
(TiH2, Alfa Aesar) at different heating rates and different amounts.
Fig. 1. XRD pattern of Sc(Al1ꢀxMgx). From the top: x¼0, 0.15, 0.20. Silicon is internal
calibration standard. The inset shows the unit cell parameter as a function of the
magnesium content.
2.4. Cycling
Absorption–desorption cycling were performed in a furnace
system in the pressure range from ultra-high vacuum to a hydrogen
pressure of 100 kPa. The samples were first hydrogenated ex situ
at 5 MPa and 420 1C, and thereafter desorbed in UHV and
rehydrogenated at 70 kPa and 400 1C. After five desorption–
rehydrogenation cycles, TDS was performed on the samples for
the sixth desorption. A small exposure to air during loading was
unavoidable in the TDS experimental setup.
2.5. Pressure-composition-isotherms and kinetic isothermal
measurements
The pressure-composition-isotherm (PCI) measurements were
performed on the ScAl in powder form using the SETARAM PCTPRO
volumetric instrument. The absorption isotherm at 385 1C (71)
was recorded by stepwise increase of hydrogen pressure. The
absorption kinetics is very slow and the equilibrium time is set to
maximum 10 h.
Fig. 2. Thermal desorption spectra of hydrogenated Sc(Al1ꢀxMgx) for x¼0, 0.15 and
0.20 with the heating rate 5 1C/min. The thermal desorption spectra were recorded
after the first absorption cycle.
The isothermal kinetic measurements were performed on
powder samples using the SETARAM PCTPRO volumetric device.
Absorption isothermal measurements were carried out at 400 1C
(71) under hydrogen pressures between 0.15 and 4.4 MPa.
Desorption was performed under dynamic vacuum at 530 1C
(1 h). The hydrogen sorption performances were repeatable under
similar pressure and temperature conditions.
˚
refined to 4.106(2) A (x¼0.20). This value is, however, uncertain
because of both the overlap and the large broadening of the peaks.
For Al(Mg) in hydrogenated Sc(Al1ꢀxMgx) with x¼0.0 and 0.15 the
uncertainties in the Bragg peak positions were too large to obtain a
value of the unit cell parameter. The very broad diffraction peaks
indicated that the grains of the hydrogenated sample are
very small.
3. Results and discussion
3.1. Thermal desorption spectroscopy
According to the X-ray powder diffraction profiles shown in
Fig. 1, all mother compounds crystallised in the cubic CsCl-type
structure. The unit cell parameter varied with the magnesium
content in accordance to Vegard’s law (see Fig. 1). The refined unit
TDS was performed on the hydrogenated compounds. The
hydrogen desorption starts at ꢁ300 1C, and the maximum of
temperature desorption (Tm) is ꢁ500 1C. As seen in Fig. 2, for
xo0.20 there was one large peak at ꢁ500 1C and for x¼0.20 there
were two peaks, a small peak at ꢁ365 1C and a large peak at
ꢁ500 1C. The small peak at ꢁ365 1C probably originates from small
amounts of amorphous MgH2, since the maximum solid solubility
of magnesium in aluminium is ꢁ18 at% [17].
˚
cell parameters were 3.383(3), 3.399(1) and 3.404(1) A for x¼0.0,
0.15 and 0.20, respectively.
The hydrogenated samples contained two phases, ScH2 and
aluminium with magnesium in solid solution [13]. ScH2 crystal-
lised in the CaF2-type structure with the unit cell parameter
The activation energy of hydrogen desorption was determined
by the Kissinger analysis (Fig. 3). The activation energy was
calculated for the reaction:
˚
4.777(1) A in agreement with previously reported data,
˚
a¼4.7832(4) A [7]. Due to severe overlaps in the Bragg positions
of ScH2 and Al(Mg) the programme FULLPROF was used to refine
the unit cell axis of ScH2 and Al(Mg). The unit cell of Al(Mg) was
ScH2+Al0.8Mg0.2-ScAl0.8Mg0.2+H2