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 exponantially (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 controversed and
some reproducibility issues have been pointed (e.g. we have long-experienced colleagues 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, 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 artifacts ?
be frank, the "artifacts" 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 mechanisms of SAV formation/decomposition with
Pd-H(D) are quasi unknown (I think it is honest to state it) ; ibid with the kinetics ; and only a
very few structural investigations have been made on Pd-H(D) SAVs ; 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).
(1) Here are the two papers :
[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 follows their previous and initial SAV investigations with the Mn-H system :
[X1395] Some unsolved problems of Hydrogen in Metals Y. Fukai 1993
See page 454.
The paper can be downloaded in the blue box below.
[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) preliminar 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 (with the Pd-H system) containing abnormally large amount of vacancies (hence stabilized by H).
(2) I will put here some key reviews.
: a scientific organization dedicated to the study of basic aspects of Hydrogen in Metals and in other Materials.