SAV (Super Abundant Vacancies) was first conceptualized by the pionneer work of Yuh Fukai in 1993 (with M-H systems such as Pd-H) (1). Since, SAV have been found/verified with many other M-H systems (2), hence evidencing their generality. As for the Pd-H system (and its alloys), the amount of published experimental investigations made on the SAV question has grown almost exponentially (from 1993 to 2019), initially with fundamental-research-Labs and now even with applications-oriented-research-Labs (such as membranes science). Therefore, presented as it, this subject does not seem to be debated, and exploring the literature does not seem to really dispute this statement.

However, this subject is controverted and some reproducibility issues have been pointed (e.g. we have a long-experienced colleague who did not find any evidences of SAV with Pd-H using one of the most known of the many current techniques leading to SAV : v.z. HighP/HighT) (another example : superlattice reflections, which are a signature of SAV presence and of H-M-Vac - Vac-M-H ordering, are not always observed). Regretfully and unfortunately, the unsuccessful results are not systematically published (3), so that it does not help to find out what is the truth : are we facing a new kind of state (rending the initial widely-known-state as being metastable as Yuh Fukai suggested) or are the results be explained by a collection of artefacts ?

To be frank, the "artefacts" possibility is hard to swallow (to us) just because many various techniques were/are used to evidence SAVs with many other M-H systems, directly or indirectly, using e.g. XRD/ND, TDS, SEM, PAS, etc ... etc ... . Nevertheless, the very mechanism(s) of SAV formation/decomposition with Pd-H(D) is not clear ; ibid with the kinetics ; and only a few structural investigations have been made on Pd-H(D) SAVs (and neutron diffraction still misses) ; so that it is maybe not so surprising that Pd-H(D) SAVs are not always observed (and controlled). Besides, there are various kind of Pd-H SAV phases and their T-P stability domains are quasi unknown.

Consequently, it is one of our current experimental projects to explore in details the SAV question with the Pd-H(D) system, using the various syntheses techniques (with the aim to shed light on it) - with its FCC alloys as well as with other FCC M-H systems (such as Ni-H).

Nico.




(1) Here are the two papers :
(i) [X488] Evidence of copious vacancy formation in Ni and Pd under a high Hydrogen pressure
Y. Fukai and N. Okuma
1993
DOI : 10.1143/JJAP.32.L1256
(ii) [X414] Formation of Superabundant Vacancies in Pd Hydride under High Hydrogen Pressures
Y. Fukai and N. Okuma
1994
DOI : 10.1103/PhysRevLett.73.1640

These two papers ([X488] and [X414]) do actually follow their previous and initial SAV investigation with the Mn-H system :
(iii) [X1395] Some unsolved problems of Hydrogen in Metals
Y. Fukai
1993
See page 454.
The paper can be downloaded in the blue box below.
(iv) [X1360] High-pressure synthesis and magnetic properties of cubic Manganese Hydride
Y. Fukai, H. Ishikawa, T. Goto, J'I. Susaki, T. Yagi, J.L. Soubeyroux, D. Fruchart.
1989
DOI : 10.1524/zpch.1989.163.Part_2.0479

One has to mention that before Fukai's conceptualization of SAV, some few earlier (70's) experimental results made by other investigators were already present in the literature (but only scarcely known). Those results pointed out structural indications of new phases (in the Pd-H system) containing abnormally large amount of vacancies (hence stabilized by H). They will be cited here soon.

(2) I will put here some key reviews.

(3) Ooooooooopssssss, it has now changed :
Three recent experimental investigations (HighP/HighT) didn't find any evidences for SAV synthesis :
T. Fedotenko et al., 2019 --> https://doi.org/10.1063/1.5117786 see section III.B.1.
B. Guigue et al., 2020 --> https://doi.org/10.1063/1.5138697 see section III.B.
T. Fedotenko et al., 2020 --> https://doi.org/10.1016/j.jallcom.2020.156179 see section 3.3
Note however that experimental conditions (P, T , duration, mode of heating, pressure transmitting media) used by these two independent teams are different than those used by Y. Fukai in his original HighP/HighT experiment.