Inverse isotope effect of ZrMnx (x=1.9 or 2.0)-Q2 (Q=H or D) system

•An inverse isotope effect of formation of ZrMn2Qx was found.•This inverse isotope effect merely appears in absorption pressure.•The inverse isotope effect showed good reproducibility.•The mechanism of the effect was discussed with thermodynamic properties

Pressure–composition curves of ZrMnx (x=1.9, 2.0)-Q2 (Q=H, D) were measured by a volumetric method. An inverse isotope effect of absorption pressure was found for the ZrMnxQy system, i.e. the hydrogen absorption pressure was found to be larger than that of deuterium. On the other hand, the hydrogen desorption pressure was the same as that of deuterium. Thus, an inverse isotope effect merely appears in the absorption pressures. The degree of the inverse isotope effect decreased with increasing temperature. This effect can be explained by the hysteresis factor which is induced by hydrogen isotopes dissolved in ZrMn1.9 or ZrMn2.0.