Concurrent Pathways in the Phase Transitions of Alloys and Oxides: Towards an Unified Vision of Inorganic Solids - Inorganic 3D Structures - page 132

EZKC Extended Zintl-Klemm concept
hcp fcc Hexagonal close-packed, face-centred cubic
HP

High pressure

HT High temperature

LP Lewis pair

LT Low temperature

RT Room temperature

TCTP Tri-capped trigonal prism

... La flamme ne subsiste point sans l'air; donc, pour connaıˆtre l'un, il faut connaıˆtre

l'autre Donc, toutes choses e´tant cause´es et causantes, aide´s et aidantes, me´diates et

inme´diates, et toutes s'entretenant par un lien naturel et insensible qui lie les plus e´loigne´es
et les plus diffe´rantes, je tiens impossible de connaı ˆ tre les parties sans connaı ˆ tre le tout, non
plus que de connaı ˆ tre le tout sans connaı ˆ tre particulie ` rement les parties.

...

... A flame can not exist without the air; therefore, to understand one, one must
comprehend the other.... Thus, since all things affect and are affected, help and are
helped, mediate and are mediated, and all is held together by an invisible web which
links even the most distant and different things, I hold that it is equally impossible to
comprehend the parts without understanding the whole, as it is to comprehend the whole
without understanding the parts individually.

Pense´es. Blaise Pascal (1623-1662)

1 Introduction

The similar structural consequences of oxidation and pressure were noted, as early
as 1994 [ 1 ]. It was deduced from the experimental observation of the pressure-
induced CrB CsCl transition in the BaSn alloy [ 2 ]. The unexpected result was

that the HP phase of BaSn (CsCl type) was identical to the cation array of the
BaSnO 3 perovskite. The simila ri ty applies not only to the structure (both cells
belong to the space group Pm3m) but also in the dimensions, with unit cell
parameters of both approximately 4.10 ˚ . The outcome was that the insertion of
three O atoms, in the BaSn alloy, produced the same effect as the application of an
external pressure. This idea was developed in an extensive review concerning the
role of cations in the structures of oxides [ 3 ]. It was later confirmed with more than
100 examples of similar structural identities [ 4 ] .
Although some isolated examples, such as the pairs Ca 2 Si/Ca 2 SiO 4 and Y 5 Si 3 /
Y 5 Si 3 O 12 F, had been considered by O'Keeffe and Hyde [ 5 ] , the importance of the
structures analysed by Vegas et al. [ 3 , 4 ] resides in the observation that the cation
subarray, in the oxide, maintains under ambient conditions the structure of the
parent alloy in some cases, whereas, in others, the cations adopt the structures of HP
phases of the same alloy.
This alternative behaviour was interpreted, in a qualitative way, assuming that
the pressure exerted by the O atoms could be proportional to the O-content.

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