3.1. P-C Measurements
The reaction of hydrogen interactions with the Nd2
sample was studied in the present work at 250 and 300 °С. Absorption and desorption processes were carried out. In the Figure 1
, P-C (P- equilibrium hydrogen pressure, C = H/IMC) isotherms are shown for these processes.
As one can see on the obtained plots of the P = f(C) dependence, there is no plateau region, which is the characteristic feature of the formation of metal and IMC hydrides. We obtained the same result as a previous study [8
] for the hydrogen reaction with Nd2
at 200 °C, which are consistent with results shown in references [20
However, Figure 2
a,b shows the absorption and desorption isotherms collected at 200 and 250 °C, which do not coincide with the region of 1.8 < C < 4.3, while the absorption and desorption isotherms measured at 300 °C coincide completely (see Figure 2
In other words, in the Nd2
system, there is a small pressure hysteresis. Previously, we observed such hysteresis in the Sm2
system at 250 °C [10
] and Nd2
system at 200 °C [8
]. We obtained the following values of hysteresis for the Nd2
system at 200 and 250 °С at a hydrogen concentration С~3.6 ln(10/4.9) = 0.84 and ln(13/10) = 0.60, respectively.
Similar phenomena were observed previously in references [22
] for AB2
systems, including Zr(Fe0.75
systems. The authors explained this phenomenon by the formation of hydride phases.
Thus, the presented data permit us to suggest that in the Nd2Fe17-H2 system at 200 and 250 °C, the existence of hydride phases is possible.
3.2. Calorimetric Results
The calorimetric investigation of the hydrogen interactions with Nd2
in the present work was carried out at 250 and 300 °С. As a result, we obtained the dependences of the change in the partial molar enthalpy of absorption and desorption with hydrogen concentration C (C = H/ Nd2
) in the intermetallic compound ΔHabs. (des.) = f(C) (see Figure 3
a–c and Table 1
The data which we obtained earlier for the Nd2
system at 200 °С [10
] are presented in Table 1
Analyzing the data presented in the Table 1
and Figure 3
a–c, it should be noted that in the Nd2
system, there are two regions with constant values of ∆Habs. at 200 and 250 °С. The increase in reaction temperature results in shrinkage of a length of the part with the constant values of ∆Habs. At 300 °С, there is only one region with constant values of ∆Habs. (des.). Furthermore, it should be noted that an increase in the reaction temperature of hydrogen interaction with Nd2
results in the decrease in values of partial molar enthalpy in terms of absolute magnitude.
Previously, Isnard et al. [26
] investigated the hydride phases of R2
, where R is the light rare-earth element with rhombohedral structure R-3m, by means of neutron diffraction analysis and determined that hydrogen atoms in these intermetallic compounds occupied octahedral 9e and tetrahedral 18g sites. It was determined that 9e sites were filled completely and 18g sites were occupied partially.
In a previous study [4
], authors studied the hydrogen desorption from Nd2
by means of differential scanning calorimetry (DSC) and they determined that for the Nd2
sample, the DSC curve at a high hydrogen concentration (X = 5) had two peaks, which were namely high-temperature (HT) and low-temperature (LT). At the smaller hydrogen concentrations in metallic matrix (X = 1, 2, 3), there was one high-temperature peak in the DSC curve. The authors theorized that hydrogen atoms occupied one type of interstitial site in this case.
At X = 3 on the DSC curve, the clear shoulder appears. Based on these data, the authors concluded that hydrogen H1
occupied the pseudo-octahedral interstitial 9e site and hydrogen H2
occupies the tetrahedral 18g site. In addition, the authors noted that 9e sites that occupied hydrogen H1
possibly consisted of two energy non-equivalent interstitial sites, which experimentally verified the existence of the clear shoulder in the plots (see Figure 2
(top) and refer to a previous study [4
The calorimetric results that we obtained in the present work and in previous studies [8
] are consistent with the data obtained in reference [4
]. We may assume that hydrogen atoms fill 9e sites during absorption at 200 and 250 °C at the range of hydrogen concentration of 0 < C < 2.0 (ΔHabs. = −85.05 ± 0.65 kJ/mol H2
at 200 °C). Furthermore, at C > 2.0, the occupancy of 9e sites by hydrogen atoms results in less heat evolution (ΔHabs. = −80.64 ± 1.00 kJ/mol H2
at 200 °C). In other words, we may conclude that the 9e position consists of two non-equivalent energy sites. The increase in experimental temperature up to 300 °C results in the appearance of one region with the constant enthalpy values. In this case, we may assume that smoothing of two energy levels in the 9e site takes place at 300 °C.
At the hydrogen concentration C > 2.7 in the intermetallic compound, the values of partial molar enthalpy of hydrogen absorption decrease sharply in absolute magnitude when hydrogen atoms start to occupy 18g site (see Figure 3
and Table 1
). In the plots ΔHabs. = f(C), there are no regions with constant enthalpy values. It is difficult to assume that we deal with the formation of stable hydride in this case as suggested by a previous study [4
]. However, the P-C isotherms in Figure 2
a,b show some hysteresis in the range of 1.8 < C < 4.3.
Comparing results of the calorimetric study for the Nd2
and for Sm2
systems (presented in Table 2
made on the basis of the results in reference [10
]), one can notice that the values of ∆Habs. (des.) for the Nd2
system are higher in absolute value compared to the Sm2
This phenomenon may be explained by the fact that the samarium radius is less than the neodymium radius due to lanthanum contraction, which leads to a decrease in the volume of the hole available for filling by hydrogen. In other words, the stability of ternary hydrides in the range of 0 < C < 3.0 depends on the unit cell volume of the alloy. This suggestion was obtained from reference [21
Cuevas et al. [4
] investigated hydrogen desorption from Nd2
hydrides by the DSC method. It was determined that the heat of reaction of hydrogen desorption was equal to 29.2 ± 0.8 kJ/mol H2
for both R2
compounds. However, our results show that values of enthalpy absorption and desorption differ for the Nd2
systems and the enthalpy values change depending on reaction temperature and hydrogen concentration in the metallic matrix.
In a previous study [26
], the authors noted that both sites exhibit different behavior depending on the temperature. The structure analysis shows that D1 is the most thermally stable and the most occupied site at higher hydrogen concentrations.
The study of hydrogen with Nd2
B was carried out in the present work at 50 °С. The results of calorimetric study of hydrogen interaction with Nd2
B are presented in Figure 4
and in Table 3
As seen in Figure 4
in the plot of ΔHabs./des. = f(C) dependence, it is possible to mark three parts with constant values of enthalpy absorption and two parts with constant values of enthalpy desorption. The values of absorption and desorption enthalpy in these regions are presented in Table 3
As one can see from these data, the enthalpy values of absorption and desorption coincide at absolute values on the regions of 2.0 < C < 3.0 and 3.2 < C < 3.7.