SAV (Super Abundant Vacancies) was first conceptualized by the pioneer 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 still an open question and some reproducibility issues have been pointed (e.g. we have a long-experienced colleague, V.E. Antonov, who did not find any evidences of SAV with Pd-H using High P / High T) (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 is not clear ; ibid with the kinetics ; and only a few structural investigations have been made on Pd-H SAVs (and neutron diffraction still misses) ; so that it is maybe not so surprising that Pd-H 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) - and with its FCC alloys as well as with other FCC M-H systems.

(written in 2019).


(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
 DOI : 10.1143/JJAP.32.L1256
 (ii) [X414] Formation of Superabundant Vacancies in Pd Hydride under High Hydrogen Pressures
 Y. Fukai and N. Okuma
 DOI : 10.1103/PhysRevLett.73.1640

These two papers ((i), (ii)) do actually follow their previous and initial SAV investigation made on the Mn-H system ((iii) and (iv)) :
 (iii) [X1395] Some unsolved problems of Hydrogen in Metals
 Y. Fukai
 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.
 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. To find these papers : see X.2.a.1, X.2.a.2 and X.2.a.3 in the Pd-H page.

(2) See section 2.4 pp. 568-573 in A. Pundt & R. Kirchheim's review paper published in 2006.

(3) Ooooooooopssssss, it has now changed (written in 2021) :
Four recent experimental investigations (HighP/HighT) didn't find any evidences of SAV production (i.e. no apparent indication of SAV synthesis) :
T. Fedotenko et al., 2019 --> see section "III.B.1" within the paper.
B. Guigue et al., 2020 --> see section "III.B" within the paper.
T. Fedotenko et al., 2020 --> see section "3.3" within the paper.
Z.M. Geballe et al., 2021 --> see the second to last paragraph of section "III. Results and Discussion" within the paper.
Note however that the experimental conditions (P, T, duration, mode of heating, pressure transmitting media) used by these three independent teams are different than those used by Y. Fukai et al. in their original HighP/HighT experiment (see (i) and (ii)).